Insect Pheromones Synthesized by Oxidative Transformations of Natural Monoterpenoids

Abstract

The literature on oxidative transformations of natural monoterpenoids to synthesize insect pheromones was reviewed.The literature on pheromone chemistry is broad and reflects the consistent interest in this area over the last 20 years in the form of monographs [1-7] and reviews [8][9][10][11][12][13][14][15].Insect pheromones known today are rather simple molecules (less than four asymmetric centers and four functional groups). Therefore, the "ideal" substrate (chiral or achiral) for most such structures is a moderately functionalized molecule, in particular, hydroxy-and amino-acids in addition to monoterpenoids. The last class of compounds is the most accessible for this series and is especially convenient for the synthesis of molecules with a branched C skeleton, primarily isoprenoid pheromones.Oxidative methods of transforming monoterpenoids are the most convenient and widely used methods for carrying out various transformations of starting molecules and introducing most known functional groups. Considering this aspect, articles on the synthesis of insect pheromones that for one reason or another did not appear in previous reviews are reviewed herein. ALLYLIC OXIDATION BY SELENIUM DIOXIDEOne of the most common methods for functionalizing the C skeleton of unsaturated monoterpenoids containing an isopropylidene group is regio-and stereoselective oxidation at the allylic position, for example, by SeO 2 , of geraniol (1) and its derivatives or similar compounds. This method enables introduction into molecules of hydroxy-or oxo-functions that can be used in further transformations.For example, treatment of geranylacetate (2) with a stoichiometric or catalytic (with t-BuOOH) amount of SeO 2 followed by hydride reduction gives unsaturated hydroxyacetate 3, which is widely used to synthesize insect pheromones [16][17][18][19].In particular, 3 was oxidized to aldehyde 4, which was then converted to the olefin by n-propylidenephosphorane [16], to prepare racemic 5,9-dimethylheptadecane (7), the sex pheromone of the pear leaf blister moth (Leucoptera scitella Zeller). Organocuprate coupling of the resulting allylic acetate 5 with n-hexylmagnesium bromide led to 5,9-dimethyl-3,5,9-heptadecatriene (6), exhaustive hydrogenation of which gave the desired pheromone 7.Condensation of bromoacetate 8 wth sodium malonic ester was used [17] to synthesize 10-hydroxy-4,8-dimethyl-4E,8E-decadienoic acid (11), an acyclic precursor of ferrulactone I (12), which is the principal component of the aggregation pheromone of the rusty grain beetle (Cryptolestes ferrugineus Stephen). Decarboxylation of the resulting triester 9 and subsequent saponification gave the key hydroxyacid 11, lactonization of which by bis-(4-t-butyl-N-isopropylimidazol-2-yl)disulfide gave the desired 12.