Characterisation and synthesis of Bactrocera fruit fly pheromones

Abstract

The major component of the rectal gland secretions of adult male cucumber flies (Bactrocera cucumis) has been shown to be the (2S, 6R, 8S) enantiomer of 2,8-dimethyl-1,7-dioxaspiro[5.5]undecane with no detectable amounts of the antipode. The major hydroxyspiroacetal component has been identified as the EEE diastereoisomer of 2,8-dimethyl-1,7-dioxaspiro[5.5]undecan-3-ol and specifically identified as the (2S, 3R, 6R, 8S) enantiomer with no sign of the antipode. Minor hydroxyspiroacetal components were identified as the (2S, 3S, 6R, 8S) enantiomer of 2,8-dimethyl-1,7-dioxaspiro[5.5]undecan-3-ol and the (2R, 6R, 8S) enantiomer of 2-hydroxymethyl-8-methyl-1,7-dioxaspiro[5.5]undecane.The major components of the rectal glandular secretion of male Bactrocera (Bactrocera) umbrosus (jack fruit fly), B. (Bactrocera) nigrotibialis, B. (Bactrocera) albistrigatus, B. (Zeugodacus) tau and B. (Zeugodacus) sp. (taxonomically similar to B. tau) have been identified by combined gas chromatography-mass spectrometric comparison with authentic synthesised compounds. Alcohols, spiroacetals, and amides were the dominant components. The major component in the rectal gland secretion of B. (Zeugodacus) cucurbitae (melon fly) has been shown to be ethyl 4-hydroxybenzoate and not 2-ethoxybenzoic acid. The rectal gland secretions of a number of species of the Bactrocera dorsalis complex have also been examined and the chemical findings are in agreement with taxonomic revisions based on other criteria.Ethyl (S)-(+)-lactate has been used as the chiral starting material in the divergent synthesis of the (2S, 6R, 8S) enantiomer of 2,8-dimethyl-1,7-dioxaspiro[5.5]undecane and 2-ethyl-8-methyl-1,7dioxaspiro[5.5]undecane along with the (2R, 6R, 8S) enantiomer of 2-hydroxymethyl-8-methyl-1,7-dioxaspiro[5.5] undecane. Gas-chromatographic analysis of these samples and the rectal gland secretion of male B. nigrotibialis, using a cyclodextrin based stationary phase, demonstrated that the (2S, 6R, 8S)-stereoisomers of 2,8-dimethyl- and 2-ethyl-8-methyl-1,7-dioxaspiro[5.5]undecane were the natural products, with no detectable level of the antipodes.The synthesis of all the possible EE isomers of the monohydroxy 2,8-dimethyl-1,7dioxaspiro[5.5]undecanes has been completed and with the exception of the axial alcohol isomer of the 3-hydroxy isomer, have all been obtained pure and their stereochemistry determined by 1H NMR spectroscopy. Two dimensional NMR spectroscopy enabled complete assignment of the 1H NMR chemical shifts.