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DOI: 10.1039/D2NP00068G (Review Article) Nat. Prod. Rep., 2023, 40, 866-889

Recent advances in the synthesis of insect pheromones: an overview from 2013 to 2022

João P. A. Souza a, Pamela T. Bandeira ac, Jan Bergmann *b and Paulo H. G. Zarbin *a
aLaboratório de Semioquímicos, Departamento de Química, Universidade Federal do Paraná, UFPR, Caixa Postal 19020, Curitiba 81531-990, PR, Brazil. E-mail: pzarbin@ufpr.br
bInstituto de Química, Pontificia Universidad Católica de Valparaíso, Avda. Universidad 330, Valparaíso, Chile. E-mail: jan.bergmann@pucv.cl
cDepartamento de Química, Universidade Federal de Santa Maria, Avda. Roraima, 1000, Santa Maria, RS, Brazil

Received 26th September 2022

First published on 23rd February 2023

Abstract

Covering: 2013 to June 2022

Pheromones are usually produced by insects in sub-microgram amounts, which prevents the elucidation of their structures by nuclear magnetic resonance (NMR). Instead, a synthetic reference material is needed to confirm the structure of the natural compounds. In addition, the provision of synthetic pheromones enables large-scale field trials for the development of environmentally friendly pest management tools. Because of these potential applications in pest control, insect pheromones are attractive targets for the development of synthetic procedures and the synthesis of these intraspecific chemical messengers has been at the core of numerous research efforts in the field of pheromone chemistry. The present review is a quick reference guide for the syntheses of insect pheromones published from 2013 to mid-2022, listing the synthesized compounds and highlighting current methodologies in organic synthesis, such as carbon–carbon coupling reactions, organo-transition metal chemistry including ring-closing olefin metathesis, asymmetric epoxidations and dihydroxylations, and enzymatic reactions.

image file: d2np00068g-p1.tif

João P. A. Souza

João Pedro de Albuquerque Souza studied chemistry (teaching stream program) at the Federal University of Technology – Paraná, Brazil, in 2016. He has an MSc degree in Organic Chemistry from the Federal University of Paraná, Brazil, in 2018. He is currently a PhD student in Organic Chemistry at the Federal University of Paraná under the supervision of Dr Paulo Zarbin and the main focus of his research is the identification and synthesis of insect pheromones.

image file: d2np00068g-p2.tif

Pamela T. Bandeira

Pamela T. Bandeira is a Professor in the Department of Chemistry at the Federal University of Santa Maria, Brazil. She received her PhD in organic chemistry from the Federal University of Paraná, Brazil, in 2019. From 2020 to 2021, she was a postdoctoral fellow in the research group of Professor Paulo H. G. Zarbin at the Federal University of Paraná, Brazil, working on pheromone synthesis.

image file: d2np00068g-p3.tif

Jan Bergmann

Jan Bergmann is a Full Professor at the Chemistry Institute of Pontificia Universidad Católica de Valparaíso in Valparaíso, Chile. His main research interest is the identification and synthesis of volatile semiochemicals. Current research projects deal with the identification of insect pheromones and the development of applications in pest control, as well as the study of volatile plant compounds and their roles in the evolution of different pollination syndromes.

image file: d2np00068g-p4.tif

Paulo H. G. Zarbin

Paulo H. G. Zarbin is a Full Professor in the Department of Chemistry at the Federal University of Parana (UFPR) in Brazil. He obtained his PhD in 1998 from the Federal University of São Carlos, with part of his thesis developed at the National Institute of Sericultural and Entomological Science in Tsukuba, Japan. He is presently the head of the Laboratory of Semiochemicals at UFPR. He is a member of several scientific societies, having been president of the International Society of Chemical Ecology (2011–2012) and the Latin American Association of Chemical Ecology (2016–2018). His main research interest is focused on the identification, synthesis, and biosynthesis of insect pheromones and other semiochemicals, as well as the understanding of the chemical ecology of insect–plant interactions.

1. Introduction

Since the discovery of the first pheromone in 1959,1 the synthesis of these intraspecific chemical messengers has been at the core of many research efforts in the field of pheromone chemistry. Pheromones are usually produced by insects in sub-microgram amounts, which prevents the elucidation of their structures by nuclear magnetic resonance (NMR) spectroscopy. Instead, a synthetic reference material is needed to confirm the structure of the natural compounds. Furthermore, the provision of synthetic pheromones enables large-scale field assays for the development of environmentally friendly pest management tools. Due to the potential applications in pest management, insect pheromones have become attractive targets for synthetic process development throughout the decades, and so the field of pheromone synthesis also reflects the advancement of organic synthesis.2 The synthesis of pheromones has been reviewed repeatedly by the late Professor Kenji Mori, covering the periods from 1960–1979,3 1979–1989,4 and 1990–2003.5 Our group discussed some highlights of insect pheromone synthesis in the periods 2002–2004 (ref. 6) and 2005–2007,7 and described all the syntheses carried out by Brazilian research groups.8 In addition to the chronological coverage used in the above-mentioned articles, Mori reviewed the synthesis of optically active pheromones9 and repeatedly emphasized the importance of organic synthesis in pheromone science, covering different aspects.10–15 Another review by Aleu et al. dealt with the use of biocatalysis in pheromone synthesis.16 The chemistry, including synthetic procedures, of chiral methyl-branched pheromones,17 lactones used in chemical communication18 and pheromones of mealybug and scale insects19 were reviewed as part of a Natural Product Reports special issue on chemical ecology. The identification of insect pheromones from 2001 to 2018 has also recently been revised by Zarbin and Vidal.20

The present review provides, on the one hand, a quick reference guide to the syntheses of insect pheromones published in the period from 2013 to mid-2022, giving an overview of the methodologies currently used in pheromone synthesis. We have classified each of the pheromone compounds based on the presumed biogenesis into fatty acid/polyketide-derived, isoprenoids and nitrogenous compounds. The first section is subdivided into smaller groups according to their chemical structure. For each group, the structures of the compounds are shown, and the main carbon–carbon coupling reaction(s) used in the respective synthesis is (are) indicated. On the other hand, we present selected syntheses, with an emphasis on how more recent developments have found their way into the synthesis of these chemical messengers. In the reaction schemes, we have color-coded the building blocks to facilitate the recognition of the key steps involving carbon–carbon coupling reactions. Due to the number of syntheses published in the period, neither the selection nor the discussion of the procedures can be comprehensive. Instead, we have selected examples that we think are representative of the respective class, have employed modern synthetic approaches, or are noteworthy for another reason.

A search in the Web of Science Core Collection using the parameters “pheromone AND synthesis*” in “All Fields” and “2013–2022” in “Year Published” returned 824 results. Out of those, we selected 244 articles that reported syntheses of insect pheromones, excluding simple one-step transformations into the target compounds, like acetylation and reduction reactions.

In the revised period, the number of articles reporting pheromone syntheses fluctuated between ca. 20–40 per year, without showing a clear trend over time. In 2022 there were fewer articles but only the first 6 months were considered. The pheromones of insects from Lepidoptera (Lep.) and Coleoptera (Col.) were most frequently synthesized (Fig. 1), which is not surprising since these orders are widespread and highly diverse and, most importantly, many agricultural and forestry pest insects belong to one of these two orders. Consequently, other syntheses targeted the pheromones of Hemiptera (Hem.), Hymenoptera (Hym.), and Diptera (Dip.), which are also of economic concern in many parts of the world. Syntheses of the pheromones of species from the orders Blattaria (Bla.), Neuroptera (Neu.), Orthoptera (Ort.), Strepsiptera (Str.), Thysanoptera (Thy.), and Trichoptera (Tri.) were also reported.


image file: d2np00068g-f1.tif
Fig. 1 Publications on pheromone synthesis from 2013 to mid-2022 (bars) and insect order (pie chart).

2. Fatty acid/polyketide-derived pheromones

Many of the pheromones used in insect communication are produced by the fatty acid pathway. The basic precursor is acetyl or malonyl-CoA, of which several units are condensed in subsequent steps to produce chains with an even number of carbon atoms.21 Modification of the chain and/or the functional group produces a wide variety of unbranched acyclic or cyclic compounds. When propionyl or methylmalonyl-CoA units (one or more) are incorporated into the growing chain, methyl-branched compounds are produced. In this review, this group of compounds are subdivided into acyclic unbranched, acyclic branched, and cyclic compounds.

2.1. Synthesis of acyclic unbranched compounds

2.1.1. Unsaturated hydrocarbons. The unbranched hydrocarbon pheromones synthesized are shown in Fig. 2, comprising the pheromones of Chauliognathus fallax (Col., Cantharidae): 1;22Chrysopa formosa (Neu., Chrysopidae): 2;23Susuacanga octoguttata (Col., Cerambycidae): 3;24Oxelytrum discicolle (Col., Silphidae): 4;25Oxelytrum erythrurum (Col., Silphidae): 5;26Semiotthisa spp. (Lep., Geometridae): 6; lymantrids (Lep., Lymantriidae): 7 and 8; Anticarsia gemmatalis (Lep., Noctuidae): 9; Utetheisa ornatrix (Lep., Erebidae): 10; Jodis lactearia (Lep., Geometridae): 11 and Maxates versicauda microptera (Lep., Geometridae): 12;27Naxa seriaria (Lep., Geometridae): 13;28Drosophila melanogaster (Dip., Drosophilidae): 14 and 15.29 The pheromones of this group are alkenes with a different number, position and stereochemistry of double bonds. The synthetic approaches employed included Wittig's olefination reaction (1, 6–12), selective reduction of alkynes (2, 4, 5) and the transformation of commercially available fatty acids (3, 13).
image file: d2np00068g-f2.tif
Fig. 2 Chemical structures of unbranched hydrocarbons synthesized as pheromones.

The synthesis of (7Z,11Z)-pentacosa-7,11-diene (14) and (7Z,11Z)-nonacosa-7,11-diene (15), attractants of the fruit fly Drosophila melanogaster, was reported by D'yakonov and coworkers using a cyclometallation reaction involving two allenes as the key step (Scheme 1).29 The titanium-catalyzed intermolecular cross-cyclomagnesiation reaction of unfunctionalized and oxygenated 1,2-dienes upon treatment with EtMgBr in the presence of magnesium and Cp2TiCl2 afforded the magnesacyclopentanes (highlighted in the Scheme 1), which were converted to the target (Z,Z)-dienes 14 and 15 in four steps. This approach was also successfully applied in the preparation of other insect pheromones containing the (1Z,5Z)-diene moiety.


image file: d2np00068g-s1.tif
Scheme 1 Synthesis of (7Z,11Z)-pentacosa-7,11-diene (14) and (7Z,11Z)-nonacosa-7,11-diene (15), attractants of the fruit fly Drosophila melanogaster, reported by D'yakonov et al. (2017). Reagents and conditions: (a) EtMgBr, Mg, Cp2TiCl2 (10 mol%), Et2O, rt, 6 h; (b) 5% HCl in H2O, rt (n:12, 92%; n:16, 94%); (c) pTSA, MeOH, CHCl3, 55 °C, 2 h (n:12, 89%, n:16, 86%); (d) MsCl, Et3N, CH2Cl2, 0 to 30 °C, 4.5 h; (e) LiAlH4, Et2O, 0 to 30 °C, 1.5 h (14, n:12, 90%; 15, n:16, 88% over two steps).29
2.1.2. Epoxy compounds. Fig. 3 shows the structures of the fatty acid-derived epoxides synthesized during the selected time period. All epoxides were identified from the order Lepidoptera, namely, Lymantria monacha (Lep., Erebidae): 16; L. dispar (Lep., Erebidae): 17;30–33Ectropis obliqua (Lep., Geometridae): 18;34–37Itame argillacearia (Lep., Geometridae): 19;38Thyrinteina arnobia (Lep., Geometridae): 20;39Lymantria mathura (Lep., Erebidae): 21;40 and Estigmene acrea (Lep., Erebidae): (9S,10R)-22.41 The epoxy functional group was obtained by intramolecular nucleophilic displacement reactions (16, 17), epoxidation of alkenes with reagents like mCPBA,34,39 Ti(i-PrO)4[thin space (1/6-em)]37,40 (18, 20), or Sharpless asymmetric dihydroxylation (AD-mix),37,38 followed by intramolecular epoxide ring-closing (18, 19). Five epoxides presented an unsaturated aliphatic chain with Z-configuration, obtained by stereoselective alkyne reduction (Lindlar37,39,41 and P2-Ni38,40) or stereoselective Wittig olefination.
image file: d2np00068g-f3.tif
Fig. 3 Chemical structures of epoxides synthesized as pheromones.

6,7-Epoxyoctadeca-3,9-diene (18) was identified as the sex pheromone of the tea geometrid Ectropis obliqua. The gram-scale total synthesis of enantiopure (3Z,9Z,6S,7R)- and (3Z,9Z,6R,7S)-18 from 2-pentyn-1-ol was described by Xu et al. (Scheme 2).37 This synthetic procedure was based on the Sharpless asymmetric dihydroxylation of (2E)-oct-2-en-5-yn-1-ol (step d, Scheme 2) to afford optically active syn-diols as key intermediates. The target pheromone was obtained in an 8% overall yield over 8 steps.


image file: d2np00068g-s2.tif
Scheme 2 Synthesis of epoxydiene 18, the sex pheromone of the tea geometrid Ectropis obliqua reported by Xu et al. (2017). Reagents and conditions: (a) TsCl, KOH, Et2O, rt, 3 h (98%); (b) propargyl alcohol, K2CO3, CuI, TBAI, DMF, rt, 8 h (90%); (c) LiAlH4, Et2O, 0 °C to rt, 18 h (80%); (d) AD-mix-α, MeSO2NH2, t-BuOH[thin space (1/6-em)]:[thin space (1/6-em)]H2O (1[thin space (1/6-em)]:[thin space (1/6-em)]1), 0 °C, 72 h (70%); (e) NaH, TIPS-imidazole, THF, 0 °C, 3 h (42%); (f) dec-1-yne, BuLi, BF3·Et2O, THF, 78 °C, 1 h (52%); (g) K2CO3, MeOH, rt, 3 h (80%); (h) H2, Lindlar catalyst, MeOH, rt, 1 h (93%).37

Another example was the identification and synthesis of the major component of the sex pheromone of the eucalyptus brown looper Thyrinteina arnobia, (6Z,9Z)-3,4-epoxyheneicosa-6,9-diene (20), described by Moreira et al. (Scheme 3).39 Four stereoisomers of 20 were synthesized using enantioenriched epoxyalcohols as key intermediates, which were prepared by two asymmetric epoxidation approaches: the Sharpless epoxidation of an allylic alcohol or organocatalytic epoxidation of an enal (step a, Scheme 3). The next steps involved the C–C sp–sp3 coupling between the corresponding epoxytriflates and an appropriate diyne (step c, Scheme 3), followed by stereoselective partial hydrogenation of the triple bonds (step d, Scheme 3) to afford the target epoxydiene 20.


image file: d2np00068g-s3.tif
Scheme 3 Synthesis of epoxydiene 20, the sex pheromone of the eucalyptus brown looper Thyrinteina arnobia reported by Moreira et al. (2013). Reagents and conditions: (a) (i) For Sharpless asymmetric epoxidation: Ti(i-PrO)4, (+)-L or (−)-D-DIPT, TBHP, DCM, −20 °C, 24 h (70–76% yield, 80–90% ee). (ii) For organocatalytic synthesis: Proline-based organocatalyst (10 mol%), EtOH:H2O (3[thin space (1/6-em)]:[thin space (1/6-em)]1), H2O2, NaBH4, 16 h (79–81% yield, 95–96% ee). (iii) For racemic synthesis: mCPBA, DCM, (98%); (b) Tf2O, 2,6-lutidine, DCM (98%). (c) BuLi, Et2O (80%); (d) H2, quinoline, Pd-CaCO3, THF (96%).39
2.1.3. Alcohols, carboxylic acids, and their esters. In this group, we find mainly primary and secondary alcohols and their acetates. In a few cases, esters are derived from unbranched carboxylic acids and iso-branched short-chain alcohols (e.g.30–33, 64) and are included here. Fig. 4 includes the pheromones of Formica polyctena (Hym., Formicidae) 23,42 longhorn beetles (Col., Cerambycidae): 24–26,43Haplodiplosis marginata (Dip., Cecidomyiidae): (R)-27, (R)-28, and (R)-29,44Bactrocera oleae (Dip., Tephritidae): 30–39,45Cnaphalocrocis medinalis (Lep., Pyralidae): 40 and 41,46 moth (Lep.): 42,47Plutella xylostella (Lep., Plutellidae): 43,48Trichoplusia ni (Lep., Noctuidae): 44,49Tortrix viridana (Lep., Tortricidae): 45,50Grapholita molesta, G. funebrana, and Hedya nubiferana (Lep., Tortricidae): (E)- and (Z)-46,51Plutella xylostella (Lep., Plutellidae): 47,52 moth (Lep.) 48–52,47Keiferia lycopersicella (Lep., Gelechiidae): 53,53Anthistarcha binocularis (Lep., Gelechiidae): 54 and 55,54Illiberis pruni (Lep., Zygaenidae): 56 and 57,55Rhyacionia buoliana (Lep., Tortricidae): 58,50Phyllonorycter blancardella (Lep., Gracillariidae): (Z)-59,56Ostrinia furnacalis (Lep., Crambidae): (E)- and (Z)-60,57Melanotus communis (Col., Elateridae): 61,58Dasineura pyri (Dip., Cecidomyiidae): 62,59Ithomia salapia (Lep., Nymphalidae): 63 and 64,60Cydia pomonella (Lep., Tortricidae): 65,61,62Stathmopoda masinissa (Lep., Stathmopodidae): 66 and 67,63Lobesia botrana (Lep., Tortricidae): 68,64–66Malacosoma neustria and Dendrolimus punctatus (Lep., Lasiocampidae): 69, Spodoptera littoralis and S. litura (Lep., Noctuidae): 70,67Thaumetopoea bonjeani (Lep., Notodontidae): 71 and 72,68Acanthoscelides obtectus (Col., Chrysomelidae): 73,69Plodia interpunctella hb. (Lep., Pyralidae): 74,70Pectinophora gossypiella (Lep., Gelechiidae): 75,29,71Spodoptera exigua (Lep., Noctuidae): 76,72Phyllonorycter ringoniella (Lep., Gracillariidae): 77,73,74Loepa sakaei (Lep., Saturniinae): 78,75Symmetrischema tangolias (Lep., Gelechiidae): 79,76Tuta absoluta (Lep., Gelechiidae): 80 and 81,50,77,78Conopomorpha cramerella (Lep., Gracillariidae) 82 and 83,79Ectropis obliqua (Lep., Geometridae): 84,27Cylas formicarius (Col., Brentidae): 85,48Acanthoscelides obtectus (Col., Chrysomelidae): 86 and 87,80,81Cactoblastis cactorum (Lep., Pyralidae): 88–93,82Drosophila spp. (Dip., Drosophilidae): 94–100,83Oleria onega (Lep., Nymphalidae): 101–109,84Limonius canus and Limonius californicus (Col., Elateridae): 110.85
image file: d2np00068g-f4.tif
Fig. 4 Chemical structures of unbranched alcohols, carboxylic acids and their esters as pheromones.

Two pheromone esters with conjugated E,Z-configured double bonds, (5E,7Z)-dodeca-5,7-dienyl acetate (69) and (9Z,11E)-dodeca-9,11-dienyl acetate (70), were synthesized via a cross-metathesis reaction by Luo et al. (Scheme 4).67 The Z-selective cross-metathesis of (3E)-1,3-dienes and terminal alkenes mediated by ruthenium-benzylidene Grubbs catalyst A afforded the target E,Z-pheromones 69 and 70 in a single convergent step with 30 and 56% yields, respectively.


image file: d2np00068g-s4.tif
Scheme 4 Synthesis by Luo et al. (2016) of E,Z-diene 69, the pheromone of Spodoptera littoralis and Spodoptera litura, and 70, pheromone of Malacosoma neustria and Dendrolimus punctatus. Reagents and conditions: (a) Ru-catalyst (4 mol%), DCE (1 mol L−1), Ar (constant flow), 24 h.67

Stereoselective synthesis (4E,6E,10Z)-82 and (4E,6Z,10Z)-83, pheromone components of the cocoa pod borer moth Conopomorpha cramerella was reported by Huang et al. (2017).79 The key steps are a Sonogashira coupling (step g, Scheme 5) and subsequent stereoselective partial hydrogenation of the resulting enyne, leading to the formation of the (4E,6Z)- or (4E,6E)-conjugated double bond systems, as shown in Scheme 5. Target pheromones 82 and 83 were obtained in 8 steps in 27% and 30% overall yields, respectively.


image file: d2np00068g-s5.tif
Scheme 5 Synthesis of (4E,6E,10Z)-hexadeca-4,6,10-trien-1-ol (82) and (4E,6Z,10Z)-hexadeca-4,6,10-trien-1-ol (83), pheromone components of the cocoa pod borer moth Conopomorpha cramerella, published by Huang et al. (2017). Reagents and conditions: (a) HCl/H2O, rt, 5 h; (b) hexyltriphenylphosphonium bromide, KHMDS, THF, −78 °C, 12 h; (c) PCC/Celite®, DCM, rt, 5 h; (d) PPh3, CBr4, DCM, 0 °C, rt, 12 h; (e) BuLi, THF, −50 °C, 2 h. (f) NaBH4, MeOH, 0 °C, 3 h; (g) A, Pd(PPh3)4, CuI, Et2NH, rt, 12 h; (h) LiAlH4, diglyme, reflux, 24 h; (i) Zn, 1,2-dibromoethane, LiBr, CuBr, EtOH, reflux, 4 h.79
2.1.4. Aldehydes and ketones. Fig. 5 shows the pheromones of Xylotrechus spp. (Col., Cerambycidae): 111,86Cnaphalocrocis medinalis (Lep., Pyralidae): 112, 113,46Chilo suppressalis (Lep., Pyralidae): 114, 115,87Actias luna (Lep., Saturniidae): 112, 116 and 129,88Aromia bungii (Col., Cerambycidae): 117,89Trogoderma spp. (Col., Dermestidae): (E)- and (Z)-118,90Carposina niponensis (Lep., Carposinidae): 119, 120,91,92Diatraea saccharalis (Lep., Crambidae): 121,93Chlorida festiva and C. costata (Col., Cerambycidae): 122,94Cameraria ohridella (Lep., Gracillariidae): 123,95,96 notodontid moths (Lep., Notodontidae): 124,68Micromelalopha siversi (Lep., Notodontidae): 125,97Teia anartoides (Lep., Erebidae): 126,98Corimelaena extensa (Hem., Thyreocoridae): 127,99Chilecomadia valdiviana (Lep., Cossidae): 128,100,101Rhodinia fugax (Lep., Saturniidae): 129,75Phyllocnistis citrella (Lep., Gracillariidae), 130 and 131,29,102Elaphidion mucronatum (Col., Cerambycidae): 132,103Callosamia promethea (Lep., Saturniidae): 133,104Apomyelois ceratoniae (Lep., Pyralidae): 134,105Phyllocnistis citrella (Lep., Gracillariidae): 135.74 The most commonly used approach to obtain aldehydes was the partial oxidation of primary alcohols using periodinanes (DMP or IBX) and even more commonly, chromium-based oxidants (PCC or PDC).
image file: d2np00068g-f5.tif
Fig. 5 Chemical structures of unbranched aldehydes and ketones as pheromones.

Chourreu et al. (2020) reported a short and scalable synthesis of (8E,10Z)-tetradeca-8,10-dienal (123), the sex pheromone of the horse chestnut leaf miner moth Cameraria ohridella.95 The natural product was obtained in 6 steps and 40% yield. The key step in this synthesis was an eco-friendly sp2–sp3 Fe-catalysed Kumada cross-coupling (step f, Scheme 6) between (1E,3Z)-hexa-1,3-dien-1-yl diethyl phosphate (A, Scheme 6) and a Grignard reagent obtained from dimethyl pimelate (steps c–e, Scheme 6).95


image file: d2np00068g-s6.tif
Scheme 6 Synthesis of (8E,10Z)-123, the sex pheromone of the horse-chestnut leaf miner moth Cameraria ohridella, by Chourreu et al. (2020). Reagents and conditions: (a) (i) t-BuOK, THF/NMP, −45 °C, 30 min, (ii) ClP(O)(OEt)2, −45 °C, 2 h, (80%, 1E,3Z/1E,3E = 7[thin space (1/6-em)]:[thin space (1/6-em)]3); (b) Maleic anhydride, methylcyclohexane, 70 °C to rt, overnight (54%); (c) Red-Al, 2-Me-THF, 45 °C, 3 h (98%); (d) HCl, Toluene, 110 °C, 20 h (87%); (e) BuMgCl, Mg, THF, −10 to 70 °C (90%); (f) A, Fe(acac)3, THF, −5 to 20 °C (89%); (g) (i) TEMPO, CAN, 20 °C, 30 min; (ii) PhI(OAc)2, 10 to 20 °C (82%).95

Dethe et al. (2022) employed a Ru-catalysed direct oxidative coupling reaction between vinyl ketones to obtain the sex pheromone of the painted apple moth, Teia anartoides. (8E,10E)-126 was obtained in 48% isolated yield in only one step (Scheme 7).98


image file: d2np00068g-s7.tif
Scheme 7 Synthesis of (8E,10E)-126, a component of the sex pheromone of the painted apple moth Teia anartoides, performed by Dethe et al. (2022). Reagents and conditions: (a) [RuCl2(p-cymene)]2, AgSbF6, Cu(OAc)2.H2O, 1,2-DCE, 24 h, 80 °C (48%).98

Stanton et al. (2016) identified and reported (5Z,8Z)-127 as the male-emitted pheromone of Corimelaena extensa. The synthesis was based on a palladium-free Sonogashira coupling (step a, Scheme 8) and a Z-selective reduction of a diyne with P2-nickel catalysis (step b, Scheme 8), and gave an overall yield of 32% over a total of 4 steps (Scheme 8). Field assays showed that plants perfumed with synthetic 127 attracted significantly more C. extensa than control plants.99


image file: d2np00068g-s8.tif
Scheme 8 Synthesis of (5Z,8Z)-127, the aggregation pheromone of C. extensa, reported by Stanton et al. (2016). Reagents and conditions: (a) CuI, NaI, K2CO3, DMF (87%); (b) P2-Ni, H2, (63%); (c) (i) TBAF, THF (95%); (ii) DMP, DCM (62%).99

The sex pheromone of the giant silkworm Actias luna was identified from female glands by Millar et al. (2016) as a mixture of (11Z)-11-octadecenal (112), (6E)-6-octadecenal (116), and (6E,11Z)-6,11-octadecadienal (129). The synthesis of (6E,11Z)-129 was achieved by the stereoselective reduction of an alkyne to the Z-alkene intermediate (step d, Scheme 9) and sp2–sp3 coupling using an organozirconium intermediate and Pd(acac)2 catalyst (step g, Scheme 9). The pheromone was obtained in ∼20% overall yield over 9 steps (Scheme 9).88


image file: d2np00068g-s9.tif
Scheme 9 Synthesis of (6E,11Z)-129, the aggregation pheromone of C. extensa, reported by Millar et al. (2016). Reagents and conditions: (a) DHP, pTSA; (b) BuLi, hexyl iodide, THF, 50 °C; (c) pTSA, MeOH (76% from starting material); (d) P2-Ni, ethylenediamine, H2, EtOH (89%); (e) (i) MsCl, Et3N, DCM, (ii) LiBr, acetone (89%); (f) Zr(Cp)2HCl, THF, NMP; (g) LiBr, Pd(acac)2, A (65%); (h) PDC, DCM (50%).88

2.2. Synthesis of acyclic branched compounds

2.2.1. Methyl-branched hydrocarbons. The synthesized methyl-branched hydrocarbons (Fig. 6) are pheromones of Alabama argillacea (Lep., Erebidae): (S)-136,106Neoclytus acuminatus acuminatus (Col., Cerambycidae): (S)-137 and (S)-138,107Cacopsylla pyricola (Hem., Psyllidae): 139,108Lyonetia prunifoliella (Lep., Lyonetiidae): (5S,9S)-140,109Galleria mellonella (Lep., Pyralidae): 141,110,111Leucoptera sinuella (Lep., Lyonetiidae): 142,112Eurytoma maslovskii (Hym., Eurytomidae): 143, 144, 145, 146, and 147,113Antitrogus parvulus (Col., Melolonthidae): 148,114Eurytoma maslovskii (Hym., Eurytomidae): 149 and 150,110,111Psacothea hilaris (Col., Cerambycidae): 151,115Lyonetia clerkella (Lep., Lyonetiidae): (S)-152,116–120Lyonetia prunifoliella (Lep., Lyonetiidae): (10S,14S)-153,121Trichogramma turkestanica (Hym., Trichogrammatidae): (2E,4E,6S,8S,10S)-154,122 and Tetrastichus planipennisi (Hym., Eulophidae): 155.123 Generally, methyl branching is introduced by (asymmetric) alkylation reactions (often using Evans oxazolidinones) or by using a methyl-branched precursor such as citronellol, a natural product commonly used as a building-block for total synthesis.
image file: d2np00068g-f6.tif
Fig. 6 Chemical structures of methyl-branched hydrocarbons as pheromones.

The enantioselective gram-scale synthesis of (R)- and (S)-13-methylheptacosane (139), the sex pheromone of the pear psylla Cacopsylla pyricola was described by Yuan et al. (2021) starting from tetradecanoic acid (Scheme 10).108 The key step was asymmetric methylation using Evans-type oxazolidin-2-one chiral auxiliaries (step b, Scheme 10), followed by Wittig reaction of the chiral methyl-branched alkyl triphenyl phosphonium salt with tridecanal (step e, Scheme 10) to give the target sex pheromone (R)- or (S)-139 in high enantiomeric purity.


image file: d2np00068g-s10.tif
Scheme 10 Synthesis of (R)- and (S)-139, the sex pheromones of the pear psylla Cacopsylla pyricola, by Yuan et al. (2021). Reagents and conditions: (a) (i) (COCl)2, DMF(cat.), DCM, 0 °C, 1 h; (ii) (R)- or (S)-4-benzyloxazolidin-2-one, NaI, THF, rt, 3 h (for R: 94%; for S: 97%); (b) CH3I, NaHMDS, THF, −78 °C, 2 h (for R: 85%, >99% ee; for S: 87%, >99% ee); (c) NaBH4, THF, H2O, rt, 2 h (for R: 85%, >99% ee; for S: 88%, >99% ee); (d) (i) CBr4, PPh3, DCM, 0 °C to rt, 12 h; (ii) PPh3, CH3CN, 85 °C, 24 h (for R: 49%; for S: 51%); (e) (i) BuLi, tridecanal, 1,2-dimethoxyethane, −40 °C to rt; (ii) H2, Pd/C, EtOH, AcOH, rt, 24 h ((R)-139: 67%, >99% ee; (S)-139: 88%, >99% ee).108

Another example is the synthesis of 4,6,8,10,16-pentamethyldocosane (148) performed by Basar et al. This natural product was isolated from the cuticular extract of the cane beetle Antitrogus parvulus and the stereoselective total synthesis of two stereoisomers of 148 was reported (Scheme 11).114 The key step in this approach involved a reaction between the chiral aldehyde A, obtained in four steps from (S)-citronellol (steps a–d, Scheme 11), and (4R)-5-benzyloxy-2,4-dimethylpent-2-enyl bromide mediated by bismuth(III) iodide (step e, Scheme 11) to give the 1,5-anti-product. Stereoselective hydrogenation and SN2-type methylation with inversion of the chiral center at the hydroxy group-bearing carbon (steps f and g, respectively, Scheme 11) established the configurations of the chiral carbons 6 and 8 in the final product. This synthesis allowed the assignment of the absolute configuration of the natural product as (4S,6R,8R,10S,16S)-148 by a systematic comparison of 13C NMR spectra.


image file: d2np00068g-s11.tif
Scheme 11 Synthesis of (4S,6R,8R,10S,16S)-148, a cuticular hydrocarbon of the sugarcane beetle Antitrogus parvulus, published by Basar et al. (2021). Reagents and conditions: (a) TBDMSCl, imidazole, THF, rt, 16 h (98%); (b) (1) O3, DCM, MeOH, −78 °C, 2 h; (2) NaBH4, −78 °C to rt, 3 h (98%); (c) (i) I2, Ph3P, imidazole, DCM, rt, 2 h; (ii) t-BuOK, THF, rt, 16 h; (iii) HCl (4 mol L−1), THF, rt, 3 h; (iv) H2, Pd/C, THF, rt, 16 h (60%); (d) DMP, DCM, NaHCO3, rt, 30 min; (e) BiI3, Zn powder, THF, rt, 2 h, add A and (R,E)-5-(benzyloxy)-1-bromo-2,4-dimethylpent-2-ene, THF, reflux, 2 h (60%); (f) (i) 4-NO2-C6H4CO2H, Ph3P, DIAD, THF, rt, 16 h (70%), (ii) NaOH (2 mol L−1), THF, 50 °C, 1 h (70%); (iii) Naphthalene, Li, THF, −25 °C, 2 h, 78%; (iv) TIPSCl, imidazole, THF, rt, 16 h (94%); (v) [Rh(NBD)Diphos-4]BF4(cat.), DCM, H2, 950 psi, rt, 5 h (70%); (g) (i) TsCl, DMAP, THF, rt, 16 h (93%); (ii) CuI, MeLi·LiI, 0 °C to rt, 16 h (21%); (iii) HCl (4 mol L−1), dioxane, THF, rt, 16 h (91%); (iv) TsCl, DMAP, DCM, rt, 16 h (85%); (v) NaI, acetone, reflux, 16 h (90%); (h) BuSO2Ph, THF, DMPU, BuLi, −40 °C, 30 min, C, −40 °C to rt, 16 h (95%); (i) (1) O3, DCM, MeOH, −78 °C; (2) NaBH4, −78 °C to rt, 16 h (85%); (j) (i) Na/Hg, MeOH, rt, 16 h; (ii) TsCl, DMAP, DCM, rt, 16 h; (iii) NaI, acetone, reflux, 2 h (65%); (k) MeSO2Ph, THF, BuLi, −40 °C, 30 min, add D, rt, 16 h (73%); (l) (i) BuLi, hexane, THF, DMPU, −40 °C, 30 min, add B, −40 °C to rt, 16 h; (ii) Na/Hg, MeOH, rt, 16 h, ((4S,6R,8R,10S,16S)-148: 37%).114
2.2.2. Methyl-branched alcohols, acids, and their esters. Fig. 7 shows the methyl-branched pheromones of Tenebrio molitor (Col., Tenebrionidae): (R)-156,124–128Oryctes rhinoceros (Col., Dynastidae): 157,128–131Kheper nigroaeneus (Col., Scarabaeidae): (R)-158,126,128,132Oryctes rhinoceros (Col., Dynastidae): 159,133Euproctis pseudoconspersa (Lep., Erebidae): 160,134,135Miltochrista calamina (Lep., Erebidae): (5R,7R)-161136,137 palm weevils (Col., Curculionidae): (4S,5S)-162 and (4S,5S)-163,138Rhynchophorus ferrugineus (Col., Dryophthoridae): 164, 165,133,139Scolytus multistriatus (Col., Curculionidae): 166,140Sitophilus granarius (Col., Dryophthoridae): 167,141Diabrotica longicornis (Col., Chrysomelidae): (2S,8R)-168,142,143Bicyclus spp. (Lep., Nymphalidae): 169,144Cryptothelea variegata (Lep., Psychida): (3R,13R,3′S)-170,145,146Eurytoma maslovskii (Hym., Eurytomidae): (2S,10R)-171, (2S,8S)- and (2S,8R)-172,147Pellaea stictica (Hem., Pentatomidae): 173,148,149Edessa meditabunda (Hem., Pentatomidae): 174,150Trichogramma turkestanica (Hym., Trichogrammatidae): 175,151Margarodes prieskaensis (Hem., Margarodidae): 176, 177.152
image file: d2np00068g-f7.tif
Fig. 7 Chemical structures of methyl-branched alcohols, acids, and their esters synthesized as pheromones.

The asymmetric synthesis of 10,14-dimethyl-1-pentadecyl isobutyrate (160), the sex pheromone of the tea tussock moth Euproctis pseudoconspersa, was performed by Zhang et al. (Scheme 12).135 Using a chiral lactone that is widely available from the recycling of industrial wastewater as the starting material, the two key steps of this synthesis were Julia-Lythgoe olefinations between an aldehyde and a sulfone (steps e and i, Scheme 12) to finally afford the precursor alkene with an E/Z ratio of 2[thin space (1/6-em)]:[thin space (1/6-em)]1, which was hydrogenated to the target pheromone 160 (Scheme 12). Based on this method, the chiral center of the starting material was completely retained and the desired product was obtained in an overall yield of 33% in 10 steps.


image file: d2np00068g-s12.tif
Scheme 12 Zhang et al. (2018) synthesis of 160, the sex pheromone of the tea tussock moth Euproctis pseudoconspersa. Reagents and conditions: (a) NaOH, BnCl; (b) H2SO4, EtOH (75% over two steps); (c) LiAlH4, 0 °C to rt (80%); (d) NaIO4, H2O, 0 °C (93%). (e) NaHMDS, 5-(isobutylsulfonyl)-1-phenyl-1H-tetrazole, −78 °C to −50 °C (87%); (f) Pd/C 10%, H2, MeOH, rt (86%); (g) 2-Mercaptobenzothiazole, Ph3P, DIAD, THF, 0 °C to rt (83%); (h) mCPBA, DCM, rt (99%); (i) NaHMDS, THF, −78 °C to −50 °C (79%); (j) Pt/C 10%, H2, MeOH, rt (96%).135

The enzymatic synthesis of all four possible stereoisomers of 4-methylheptan-3-ol (166), the pheromone of the bark beetle Scolytus multistriatus, was described by Brenna and coworkers (Scheme 13).140 The remarkable step is the formation of two stereogenic centres in a sequential one-pot bioreduction of the C[double bond, length as m-dash]C and C[double bond, length as m-dash]O double bonds of 4-methylhept-4-en-3-one catalysed by ene-reductases and alcohol dehydrogenases, respectively. Stereoisomers of the target pheromone 166 were obtained in 72–83% isolated yields with excellent stereoselectivity. The α,β-unsaturated ketone used as the starting material was obtained in 56% isolated yield by aldol condensation of propanal and 3-pentanone.


image file: d2np00068g-s13.tif
Scheme 13 Synthesis of 166, the pheromone of the bark beetle Scolytus multistriatus, by Brenna et al. (2017). Reagents and conditions: (a) KOH, MeOH, rt, overnight (56%). (b) NADP+, ene-reductases (OYE), glucose dehydrogenase (GDH), glucose, DMSO (1%), potassium phosphate buffer pH 7.0; then alcohol dehydrogenases (ADH), NAD+ or NADP+, GDH, glucose, 24 h (for 3R,4R-65: ee = 99%, de = 99%; for 3S,4R-65: ee = 92%, de = 99%; for 3R, 4S-166: ee = 99%, de = 92%; for 3S,4S-166: ee = 99%, de = 94%).140
2.2.3. Methyl-branched aldehydes and ketones. Fig. 8 shows the pheromones of Eburodacrys vittata (Col., Cerambycidae): 178,153Eburodacrys spp. (Col., Cerambycidae): 179,154Sitona discoideus (Col., Curculionidae): 180 and (4S,5S)-181,155Rhinostomus barbirostris (Col., Curculionidae): (4S,5R)- and (4S,5S)-181,156Sitophylus oryzae and S. zeamais (Col., Dryophthoridae): 181,157Diabrotica undecimpunctata howardi (Col., Chrysomelidae): 182,140,158Crematogaster spp. (Hym., Formicidae): 183,159Hesperophylax occidentalis (Tri., Limnephilidae): 184,154Rhynchophorus ferrugineus (Col., Dryophthoridae): 185,160,161Tribolium spp. (Col., Tenebrionidae): 186,162–164Diabrotica balteata (Col., Chrysomelidae): 187,165Pallantia macunaima (Hem., Pentatomidae): 188,166Lasioderma serricorne (Col., Anobiidae): 189,167Stylops mueller (Str., Stylopidae): 190,168Dociostaurus maroccanus (Ort., Acrididae): 191,169,170Xenos peckii (Str., Xenidae): 192,171,172Matsucoccus thunbergianae (Hem., Margarodidae): 193.173
image file: d2np00068g-f8.tif
Fig. 8 Chemical structures of methyl-branched carbonyl compounds synthesized as pheromones.

An ingenious stereoselective synthesis of (3R,5S,9R,7E,11E)-3,5,9,11-tetramethyl-7,11-tridecadienal (192), the sex pheromone of the strepsipteran Xenos peckii, and stereoisomers of 192 were developed by Zhai et al. (2016).172 The bis-methyl ester obtained from 2,4-dimethylglutaric anhydride was desymmetrized by an α-chymotrypsin-catalysed hydrolysis reaction followed by the selective reduction of the carboxylic acid function with BH3·DMS (step a, Scheme 14). This intermediate was converted into the iodide A by halogenation, reduction of the ester, and TBDMS-protection of the hydroxyl group (steps b and c, Scheme 14), or to the enantiomer ent-A by reversing the order of these reactions (not shown). In parallel, (R)-2,4-dimethyl-4-pentenal was converted to the vinyl iodide B by Takai olefination (step d, Scheme 14). The key step was then the Suzuki–Miyaura coupling reaction between fragments A and B after the conversion of A to the corresponding borate (step e, Scheme 14). The resulting alcohol was chain-extended by conversion to a nitrile, which was reduced to the target compound 192. Following this sequence employing different combinations of A, ent-A, B, and ent-B afforded different stereoisomers of 192. Only the (3R,5S,9R)-isomer attracted males in field experiments.172


image file: d2np00068g-s14.tif
Scheme 14 Synthesis of (3R,5S,9R,7E,11E)-3,5,9,11-tetramethyl-7,11-tridecadienal (192), the Xenos peckii sex pheromone, reported by Zhai et al. (2016). Reagents and conditions: (a) (i) α-Chymotrypsin, phosphate buffer (pH 7.5–7.8), H2O; (ii) BH3·DMS, THF, 0 °C (87%); (b) (i) I2, PPh3, imidazole, DCM; (ii) DIBALH, DCM, −78 °C to 0 °C (84%); (c) TBDMSCl, imidazole, DMAP(cat.), DCM (94%); (d) CHI3, CrCl2, THF (85%); (e) (i) A, t-BuLi, Et2O/THF, −78 °C, then 9-MeO-9-BBN, −78 °C to rt; (ii) AsPh3, Cs2CO3, Pd(dppf)Cl2, B, DMF; (iii) HCl in Et2O (64%, d.r.: 9[thin space (1/6-em)]:[thin space (1/6-em)]1); (f) (i) TsCl, Et3N, DMAP(cat), DCM; (ii) NaCN, DMSO, 50 °C (60%); (f) DIBALH, DCM, −10 °C (92%).172

Matsuone, (2E,4E)-4,6,10,12-tetramethyl-2,4-tridecadien-7-one (193), is the sex pheromone of bast scales of the genus Matsucoccus, including M. matsumurae, M. resinosae, and M. thunbergianae. Lee et al. (2019) reported a new method for the synthesis of racemic matsuone (193) (Scheme 15) and evaluated the catches of M. thunbergianae in the field. The LDA base-catalysed elimination reaction leading to the (3E,5E)- and (3E,5Z)-configured ester intermediates was the key step (step c, Scheme 15). The target pheromone was obtained from the ester by alkylation via the Weinreb amide yielding a 7[thin space (1/6-em)]:[thin space (1/6-em)]3 mixture of (2E,4E)- and (2Z,4E)-193 with an overall yield of about 30% in 6 steps.173


image file: d2np00068g-s15.tif
Scheme 15 Synthesis of matsuone, (2E,4E)-193, Matsucoccus spp. sex pheromone, published by Lee et al. (2019). Reagents and conditions: (a) Zn, THF, 70 °C (86%); (b) MsCl, Et3N, DCM, 0 °C (93%); (c) LDA, THF, −45 °C, (64%); (d) KOH, MeOH (78%); (e) DCC, N,O-dimethylhydroxylamine hydrochloride, Et3N, DCM (85%); (f) 1-bromo-2,4-dimethylpentane, Mg, THF, −78 °C (86%).173

2.3 Cyclic compounds

2.3.1. Lactones. Fig. 9 shows the pheromones of Osmoderma eremita (Col., Cetoniidae): 194,174,175Nasonia vitripennis (Hym., Pteromalidae): 195,176Bactrocera tsuneonis (Dip., Tephritidae): 196,177Popillia japonica (Col., Scarabaeidae): (R)-197 and Anomala osakana (Col., Scarabaeidae): (S)-197,178Rhagoletis batava (Dipt., Tephritidae): 198,179Macrocentrus grandii (Hym., Braconidae): 199,180Vespa orientalis (Hym., Vespidae): 200,181–184Culex spp. (Dip., Culicidae): 201,185–189Phoracantha synonyma (Col., Cerambycidae): 202,190Silvestritermes minutus (Iso., Termitidae): 203,191Heliconius erato phyllis (Lep., Nymphalidae): 204 (anti-aphrodisiac).192Agrilus planipennis (Col., Buprestidae): 205,193,194Oryzaephilus surinamensis (Col., Silvanidae): 206 and 207,195Cryptoletes pusillus (Col., Staphylinidae): 208,47Oryzaephilus surinamensis (Col., Silvanidae): 209.196 The lactone function was obtained mainly by intramolecular esterification of a hydroxy acid, e.g., using Yamaguchi lactonization conditions (202) or BOP-Cl lactonization (204), and by ring-closing metathesis reactions (200, 203, 206–209).
image file: d2np00068g-f9.tif
Fig. 9 Chemical structures of lactones synthesized as pheromones.

(R)-4-Decalactone (194) is a sex pheromone of the scarab beetle Osmoderma eremita and Zhou et al. (2018) reported a synthesis by gold-catalysed stereoselective cycloisomerization of allenoic acids. The pheromone was obtained in 4 steps and 43% overall yield (Scheme 16).175


image file: d2np00068g-s16.tif
Scheme 16 Synthesis of (R)-4-decalactone (211), a sex pheromone of the beetle Osmoderma eremite, reported by Zhou et al. (2018). Reagents and conditions: (a). (i) Ethyl pent-4-ynoate, (R)-2-pyrrolidine-1,1-dimethylmethanol, CuBr2, dioxane, 120 °C; (ii) LiOH·H2O, EtOH/H2O, 90 °C (45% in 2 steps, 98% ee); (b). AuCl(LB-Phos)/AgOTs, CHCl3 (99%, 97% ee, (R,E):(S,Z) = 98[thin space (1/6-em)]:[thin space (1/6-em)]2); (c) Pd/C, H2, EtOAc, rt (98%, 94% ee).175

An enantioselective synthesis of the pheromones of Popillia japonica and Anomala osakana (R)- and (S)-japonilure (197), respectively, was performed by Xu et al. (2014). The authors used the stereoselective addition of a terminal alkyne ester (methyl propiolate) to undec-2-ynal using Zn-ProPhenol catalysis (steps c and f, Scheme 17), followed by stereoselective P2-Ni reduction and lactonization (Scheme 17).178


image file: d2np00068g-s17.tif
Scheme 17 Synthesis of japonilure enantiomers ((R)- and (S)-197), a sex pheromone of Popillia japonica and Anomala osakana, reported by Xu et al. (2014). Reagents and conditions: (a) BuLi, 1-bromooctane, HMPA, THF, −78 to −30 °C to rt (90%); (b) DMP, DCM, 0 to 5 °C (95%); (c) Methyl propiolate, Me2Zn, (R,R)-ProPhenol, toluene, 4 °C (80%, 95% ee); (d) Ni(OAc)2·4H2O, NaBH4, ethylenediamine, EtOH, H2, 25 °C (80%, 95% ee); (e) pTSA, DCM, 25 °C (81%, 93% ee for (R)-205 and 72%, 93% ee for (S)-205); (f) Methyl propiolate, Me2Zn, (S,S)-ProPhenol, toluene, 4 °C (81%, 93% ee).178

Hötling et al. (2015) identified cucujolide XI (209) from the frass of the storage beetle Oryzaephilus surinamensis. The single stereocenter was carried through from the starting product (R)-epichlorohydrin to the final product. One of the two C–C double bonds was established by Z-selective Wittig reaction (step e, Scheme 18), while the other was obtained by stereoselective Lindlar hydrogenation (step h, Scheme 18). The key step, however, was the ring-closing metathesis with a terminal alkyne using the recently developed 2,4,6-trimethylbenzylidine molybdenum complex (step g, Scheme 18). Bioassays proved the biological activity of (R)-209.196


image file: d2np00068g-s18.tif
Scheme 18 Synthesis of the cucujolide XI (209), a pheromone of the beetle Oryzaphilae surinamensis, by Hötling et al. (2015). Reagents and conditions: (a) (i) Jones reagent, (ii) BF3·OEt2, MeOH, (iii) K2OsO4, NaIO4 (60% over 3 steps); (b) C2H5MgBr, CuCN; (c) (i). KOH, (ii) lithium acetylide, EDA complex (38% over 3 steps). (d) (i) PPh3, I2, imidazole, (ii) PPh3; (e) (i) NaHMDS, A, (ii) KOH (50% over 2 steps); (f) EDC, DMAP, B (77%) (g) [MesCMo{OC(CF3)2Me}3], toluene, rt (97%); (h) Lindlar, H2, (91%, 99% ee).196
2.3.2. Acetals. Fig. 10 summarizes the acetals as pheromones of Dendroctonus ponderosae (Col., Curculionidae): 210,197D. brevicomis (Col., Curculionidae): endo- and exo-211,197–200D. frontalis (Col., Curculionidae): 212,201Taphrorychus bicolor (Col., Curculionidae): 213,202Cantao parentum (Hem., Scutelleridae): 214,203Pityogenes chalcographus (Col., Curculionidae): 215,204 and Dacus oleae (Dip., Tephritidae): 216.205,206 In all cases, the acetal function was obtained via the corresponding dihydroxyketones. Five of the seven synthesized acetals were identified from Curculionidae.
image file: d2np00068g-f10.tif
Fig. 10 Chemical structures of acetals as pheromones.

A stereoselective synthesis of brevicomin (211), the pheromone of Dendroctonus brevicomis was reported by Fernandes et al. (2014) (Scheme 19). The Pd-catalysed syn to anti epimerization of a precursor lactone derived from D-glucono-D-lactone was the key step (step d, Scheme 19). The syn- and anti-lactones were separated by column chromatography and further converted to (+)-exo- and (+)-endo-brevicomin (211), respectively (Scheme 19).198

2.3.3. Other cyclic pheromones. Syntheses were reported for the cyclic pheromones of Homalinotus depressus (Col., Curculionidae): 217,207Neozeleboria cryptoides (Hym., Tiphiidae): 218,208Macropophora accentifer (Col., Cerambycidae): 219,209 (Fig. 11).
image file: d2np00068g-f11.tif
Fig. 11 Chemical structures of cyclic compounds synthesized as pheromones.

The identification and synthesis of the major constituent of the pheromone of the beetle Homalinotus depressus, 4,4,6-trimethyl-7-oxa-bicyclo[4.1.0]heptan-2-ol (217, named homalinol), was reported by Vidal and co-workers (Scheme 20).207 The synthesis of the enantiomers of cis-epoxyalcohol 217 in high enantiomeric excess was achieved by enzymatic kinetic resolution (EKR) of the racemate of cis-217 obtained from isophorone by non-stereoselective reduction of the carbonyl group followed by stereoselective epoxidation of the double bond. This biocatalytic approach allowed the assignment of the absolute configuration of the natural product as (1R,2R,6S)-217.


image file: d2np00068g-s19.tif
Scheme 19 Synthesis of (+)-exo- and (+)-endo-brevicomins (211), Fernandes et al. (2014). Reagents and conditions: (a). TBDMSCl, imidazole, DCM, 0 °C to rt, 12 h, (86% and 88%); (b) (i) DIBAL-H, DCM, −78 °C, 45 min, (ii) 1-(Triphenylphosphoranylidene)-2-propanone, DCM, reflux, 30 h, (iii) H2, Pd(OH)2/C, i-PrOH/EtOAc, rt, 24 h, (68% and 71%); (c) HCl(aq), MeOH, rt, 2.5 h (88% and 86%); (d) Pd(OAc)2 (10 mol%), PPh3, THF, rt, 6 h (88%).198

image file: d2np00068g-s20.tif
Scheme 20 Vidal et al. (2019) synthesis of 217, a pheromone component of the beetle Homalinotus depressus. Reagents and conditions: (a) LiAlH4, THF, 0 °C, 5 h (95%); (b) MPPA, NaHCO3, H2O, rt, 1 h (82%); (c) vinyl acetate, lipase AK “Amano” 20, THF, 37 °C, 24 h (46%, ee of (1R,2R,6S)-217: 98.8%).207

3. Isoprenoids

Another important class of natural products used in insect communication are the isoprenoids or terpenes. In insects, a key intermediate in their biosynthesis is mevalonic acid, hence the pathway is often referred to as the mevalonate pathway. The basic building blocks are the isomeric five-carbon units isopentenyl pyrophosphate and dimethylallyl pyrophosphate.21

During this period, the synthesized isoprenoid pheromones were identified from Necrodes surinamensis (Col., Silphidae): 220, Maconellicoccus hirsutus (Hem., Pseudococcidae): 221, Anthonomus rubi (Col., Curculionidae) and Planococcus ficus (Hem., Pseudococcidae): 222,210,211Thrips palmi (Thy., Thripidae): 223,212Planococcus kraunhiae (Hem., Pseudococcidae): 224,213Dysmicoccus neobrevipes (Hem., Pseudococcidae): 225,214Leptinotarsa decemlineata (Col., Chrysomelidae): 226,215–220Lasius meridionalis (Hym., Formicidae): 227,221,222Ips confuses (Col., Curculionidae): 228 and 229,223Urolepis rufipes (Hym., Pteromalidae): 230,224Solenopsis invicta (Hym., Formicidae): 231,225Solenopsis invicta (Hym., Formicidae): 231 and 232;226Astyleiopus variegatus (Col., Cerambycidae): 233 and 234,227Amblypelta lutescens (Hem., Coreidae): 235,228Monomorium pharaonis (Hym., Formicidae): 236 and 237,222Aonidiella aurantia (Hem., Diaspididae): 238229Pseudococcus calceolariae (Hem., Pseudococcidae): 239,230Anthonomus grandis (Col., Curculionidae): 240,231–233Aspidiotus nerii (Hem., Diaspididae): 241,234Pseudococcus longispinus (Hem., Pseudococcidae): 242,235–237Dysmicoccus brevipes (Hem., Pseudococcidae): 243,238,239Nipaecoccus viridis (Hem., Pseudococcidae): 244,240,241Pseudococcus baliteus (Hem., Pseudococcidae): 245,242Dolichovespula maculate (Hym., Vespidae): 246,243Tibraca limbativentris (Hem., Pentatomidae): 247,244Oebalus poecilus (Hem., Pentatomidae): 248,245Murgantia histrionica and Halyomorpha halys (Hem., Pentatomidae): 249,245,246Anoplophora malasiaca (Col., Cerambycidae): 250–252,247Anastrepha suspensa and A. zudens (Dip., Tephritidae): 253248 (Fig. 12).


image file: d2np00068g-f12.tif
Fig. 12 Chemical structures of isoprenoids as insect pheromones.

Lavandulol (220) is a constituent of lavender oil and is used as a perfume additive. It is also a defensive pheromone of Necrodes surinamensis, and some lavandulol esters (221, 222, and 223) have been identified as insect pheromones. Bhosale and Waghmode (2017) synthesized the enantiomers of lavandulol (220) starting from an enantioselective organocatalytic α-aminooxylation of an achiral aldehyde and further conversion to a mono-protected chiral triol (step a, Scheme 21). The next key step was a Claisen rearrangement with partial chirality transfer from the asymmetric carbon created in the organocatalysis step to the chiral carbon generated during the rearrangement (step e, Scheme 21). After the reduction of the initial aldehyde rearrangement product, the diastereomeric alcohols were separated by chromatography and further converted into the final product (Scheme 21). (R)- 220 and (S)-220 were obtained in 16% overall yield, as shown in Scheme 21.211


image file: d2np00068g-s21.tif
Scheme 21 Synthesis of lavandulol (220), published by Bhosale and Waghmode (2017). Reagents and conditions: (a) (i) D-proline, PhNO, CH3CN, −20 °C then MeOH, NaBH4, (ii) CuSO4, MeOH, 0 °C, 12 h (66%); (b) (i) cyclohexanone, pTSA, DMSO, 12 h, (ii) Pd/C, MeOH, overnight (91%); (c) DMSO, (COCl)2, DCM, −78 °C, 1 h, then Et3N, −78 °C, 30 min; (d) allyoxy methylene triphenylphosphonium chloride, t-BuOK, rt, 2 h (85%, Z/E ∼2[thin space (1/6-em)]:[thin space (1/6-em)]1); (e) neat, 180 °C, 10 min or benzene, 80–85 °C, 24 h, (f) NaBH4(aq), MeOH, 0 °C, 45 min; (g) BnBr, NaH, TBAF(cat.), 0 °C, 6 h (92%); (h) O3, DCM, −78 °C, PPh3 (94%); (i) i-PrPPh3Br, BuLi, Et2O, −5 °C, 3 h (85%); (j) (i) AcOH (80% aq.), 18 h, (ii) Silica supported NaIO4 (1[thin space (1/6-em)]:[thin space (1/6-em)]4), DCM, rt, 2 h (85%); (k) MeMgI, Et2O, 0 °C, 3 h (94%); (l) (i) DMSO, (COCl)2, DCM, −78 °C, 1 h then Et3N, −78 °C, 30 min, (ii) MePPh3I, BuLi, THF, −5 °C, 3 h (83%); (m) Li-naphthalide, THF, −25 °C, 1.5 h (82%).211

Ramesh et al. (2015) envisioned an inexpensive and scalable synthesis for the sex pheromone of the mealybug Pseudococcus longispinus. Starting from (+)-bornyl acetate, the corresponding 5-oxoacetate was subjected to Baeyer–Villiger oxidation, yielding a rearranged hydroxylactone (step a, Scheme 22) that, after oxidation, was subjected to transesterification and elimination in a one-pot reaction (step c, Scheme 22). Luche reduction and deoxygenation with BF3·Et2O and NaBH3CN (steps d and e, Scheme 22) afforded regioisomeric cyclopentenes, which were separated by chromatography and further converted into the final product (242). The unnatural enantiomer was obtained in the same way starting from (−)-bornyl acetate.237


image file: d2np00068g-s22.tif
Scheme 22 Synthesis of sex pheromone 242 from the mealybug Pseudococcus longispinus, by Ramesh et al. (2015). Reagents and conditions: (a). (i) H2O2, H2SO4, AcOH, (ii) K2CO3, MeOH (50%); (b) PDC, DCM, (89%); (c) pTSA, MeOH (88%); (d) NaBH4, CeCl3·7H2O, MeOH, 0 °C (>99%); (e) NaBH3CN, BF3·OEt2, THF, reflux (89%); (f) LiAlH4, THF (95%); (g) Ac2O, Et3N, DCM (86%).237

One component of the sex pheromone of Anthonomus grandis, (+)-grandisol (240), was synthesized by Bartlett et al. (2022) in six steps with an overall yield of 22%.233 A photoinduced intramolecular [2 + 2] cycloaddition catalysed by copper(I) triflate was used as the key step to obtain the cis-configured cyclobutane scaffold (step c, Scheme 23). The synthetic grandisol can be oxidized to the corresponding aldehyde (grandisal) and carboxylic acid (grandisoic acid), both previously described as pheromones of other insects.233


image file: d2np00068g-s23.tif
Scheme 23 Synthesis of (±)-grandisol (240), the sex pheromone of Anthonomus grandis and other weevils, by Bartlett et al. (2022). Reagents and conditions: (a). (i) NBS, H2O, (ii) K2CO3, 200 °C (46%); (b) methallyl alcohol, TFA, pentane, 0 °C (61%); (c) Hanovia medium-pressure Hg lamp, Et2O, CuOTf (75%); (d) TiCl3, BuLi, TMEDA, PhMe, THF (83%); (e) Ph3P, DEAD, ZnI2, THF (83%); (f) (i) t-BuLi, Et2O, −130 °C, (ii) ethyl chloroformate, (iii) LiAlH4 (70%).233

The sesquipiperitol (247) was identified as a minor component in the male-produced sex pheromone of Tibraca limbativentris, and stereoselective synthesis was reported by Lancaster et al. (2018) and Khrimian et al. (2020).244,249 To determine the absolute configuration of the pheromone components, Khrimian et al. (2020) started from a 4-substituted cyclohex-2-enone, and the key step was the rhodium-catalysed asymmetric 1,2-addition of trimethylaluminum to the carbonyl group to furnish 1,10-bisaboladien-3-ol (zingiberenol) with defined configurations at C-6 and C-7 (step a, Scheme 24). Oxidative rearrangement of the tertiary allylic alcohol with pyridinium chlorochromate (PCC) (step b, Scheme 24) led to sesquipiperitone with the retention of the configuration at both stereogenic centers, which was further reduced with lithium aluminium hydride (LAH) to give the desired diastereomeric sesquipiperitols 247 (Scheme 24).244 Analysis of the absolute configuration of the pheromone components showed that T. limbativentris produced a mixture of (1R,6S,7R)- and (1S,6S,7R)-isomers.244


image file: d2np00068g-s24.tif
Scheme 24 Synthesis of sesquipiperitol 247 as pheromone component of T. limbativentris, published by Khrimian et al. (2020). Reagents and Conditions: (a) trimethylaluminum, [Rh(cod)Cl]2 (0.05 equiv.) and (S)-BINAP (0.12 equiv.) or MeLi/Et2O, −25 °C (for non-selective synthesis); (b) PCC/NaOAc/DCM, 0 to 25 °C; (c) LAH (1.3 equiv.), EtOH/Et2O, −30 °C.244

4. Nitrogenous compounds

Fig. 13 shows the nitrogenous compounds reported as pheromones of Vespula vulgaris (Hym., Vespidae): 254,250Oleria onega (Lep., Nymphalidae): 255,84Eurytoma maslovskii (Hym., Eurytomidae): 256–259,251Locusta migratoria (Orth. Acrididae): 260 and 261.252
image file: d2np00068g-f13.tif
Fig. 13 Chemical structures of nitrogenous compounds as insect pheromones.

Mori and Yang (2017) synthesized the four pyrazines 256–259 contained in the female-produced volatiles of Eurytoma maslovskii. 2,5-Dimethylpyrazine was oxidized to the N-oxide, followed by reaction with POCl3 to give the intermediate chloropyrazine (steps a and b, Scheme 25). Pd-PEPPSI-catalyzed cross-coupling was used for the synthesis of vinylpyrazine 256 and Fe-catalysis for alkylpyrazines 257–259, as shown in Scheme 25 (step c).251


image file: d2np00068g-s25.tif
Scheme 25 Mori and Yang's (2017) synthesis of pyrazines 256–259. Reagents and conditions: (a) NaBO3·4H2O, AcOH, 80 °C, 6 h (60%); (b) POCl3, 60 °C, 30 min (71%); (c) CH2CHBF3K, Pd-PEPPSI-i-Pr, K2CO3, MeOH, reflux, 1 h (74%), or RMgBr, Fe(acac)3, THF/NMP (R = 2-methylpropyl, 82%; R = 2-methylbutyl, 82%; R = 3-methylbutyl, 82%).251

5. Conclusions

This review covers the synthesis of 268 different insect pheromone compounds described in 244 articles from 2013 to mid-2022. Esters were the largest class with 74 compounds, followed by isoprenoids with 35 compounds, and alcohols and aldehydes with 27 compounds each. Among the esters, 44 compounds were identified from 34 different Lepidoptera species and 8 compounds from 6 different Coleoptera species. Interestingly, the order Hemiptera was the most abundant among the synthesized isoprenoids, with 15 compounds, 9 of which were identified as pheromones of mealybugs of the family Pseudococcidae.

Natural products used as pheromones in insect communication exhibit a wide diversity of chemical structures with a large number of functional groups, which ensures that species-specific signals can be generated. Additionally, chirality and geometric isomerism are structural features of great importance for the biological activity of the compounds. To decipher these structural intricacies, and to provide synthetic pheromones for field applications, the development of procedures for the (stereo)selective synthesis of pheromones is needed. New synthetic procedures developed in the last few decades are now being widely used in the synthesis of pheromones. An example of these modern procedures is the use of chiral auxiliaries (Scheme 1) or chiral catalysts for the enantioselective synthesis of chiral compounds by asymmetric induction. Enzymes as biocatalysts and chiral catalysts based on transition metals, such as those employed in the Sharpless asymmetric epoxidation reaction, can nowadays be regarded as established synthetic methodologies for the selective synthesis of chiral molecules (Schemes 4–7). The most recent development in the field of asymmetric synthesis is organocatalysts, for which the Nobel Prize in Chemistry was awarded in 2021, and which, as shown by some of the examples presented in this review, have already found their way into pheromone synthesis (Schemes 4 and 20). Pheromone synthesis has also benefitted from the progress made in methodologies for carbon–carbon coupling reactions. Organometallic compounds such as Grignard reagents, organocuprates, organostannanes, or organozinc reagents are often employed (e.g., Schemes 3, 13, 14, 16–18, 20, and 24). Transition metal catalysts, in particular Cu- and Pd-based complexes, are nowadays indispensable tools for the construction of new bonds (e.g.Schemes 3, 8, 11, 13, 19, 23 and 24). Some of these methodologies are also useful for the stereoselective generation of C–C double bonds, but older methods such as the Wittig reaction (Schemes 1, 18 and 20) and its variations, or the selective reduction of triple bonds (Schemes 4, 5, 8 and 18) are still being widely used for this purpose. As can be seen from the examples in this review, another important reaction for the formation of C–C double bonds is olefin metathesis (Schemes 9 and 18).

One of the challenges to be overcome in the development and establishment of pheromone-based methods for crop protection is the supply of pheromones on a large scale. Many of the syntheses developed in research laboratories are not necessarily scalable to produce larger amounts. Cost-effective, eco-friendly, and large-scale syntheses with high yields, atom-economy, and few byproducts are needed, and this challenge is generally being faced by the chemical industry. Naturally, the size of the potential market is a crucial factor in the decision if a company starts to develop a synthesis to provide large amounts of pheromones at a low price. While there are successful examples of the use of synthetic pheromones in the management of widespread pests, their use in local or regional insect pest populations depends on whether a local player can provide the required amount of pheromones at a convenient cost. These are often not the research laboratories where the syntheses reviewed in this article have been developed.

We hope that this review shows that pheromone chemistry is still a thriving area of research. Certainly, efforts in this field are a fundamental contribution to our understanding of chemically mediated interactions in nature. For the establishment of pheromone-based methods for crop protection, efforts need to be made to develop a scalable synthesis, especially for local or regional problems.

6. Author contributions

Zarbin, P. H. G conceptualization, writing – review & editing, supervision Bergmann, J.: conceptualization, writing – review & editing Bandeira, P. T. investigation, writing – original draft, writing – review & editing Souza, J. P. A. investigation, writing – original draft, writing – review & editing.

7. Conflicts of interest

There are no conflicts to declare.

8. Acknowledgements

The authors gratefully acknowledge financial support by Coordenação de Aperfeiçoamento de Pessoal de Nível superior (CAPES, Brazil), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Brazil), Instituto Nacional de Ciências e Tecnologia de Semioquímicos na Agricultura (INCT, Brazil), and Vicerrectoría de Investigación y Estudios Avanzados of PUCV (VRIEA-PUCV).

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