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Synthesis of both enantiomers of the Aspidiotus nerii sex asymmetric centers. Stereoselective synthesis of a number of other trisubstituted cyclobutane derivatives also proves the pheromone and their diastereomers has been achieved using, as a key step, an intramolecular ester enolate versatility of the methodology used for the synthesis of the Aspidiotus nerii sex pheromone. alkylation reaction for the formation of the cyclobutane ring with a good control of the relative configurations of the Aspidiotus nerii (Homoptera, Diaspididae) is an endemic pest in southern Europe. This highly polyphagous [1] scale insect attacks olive, citrus fruits, plum and various other trees, shrubs, low-growing and ornamental plants such as oleander. The life cycle of the insect has, in the case of the adult female, two nymphal instars before molt and, in the case of the adult male, four. The adult female is immobile, but the male has two wings and is therefore able to fly. The Figure 1. Four enantiomers of Aspidiotus nerii sex pheromone damage caused by this piercing and sucking insect consists of a weakening of the plant, leaf fall, drying of shoots and resolution and/or sophisticated reactions in order to be able deformation of fruits. This scale insect usually has three to scale up the synthesis for biological field assays. An generations per year. These consequences have led us to inabundant monoterpene, carvone, commercially available vestigate the structure of the female sex pheromone proeither as a racemate or enantiomerically pure, was used as duced by this species as it is a potentially decisive factor starting material. The initial strategy was based on alkyin survey and control strategies. The structure of the sex lation of cyclobutane A, as K. Mori et al. described for the pheromone of Aspidiotus nerii [2] [3] has been elucidated by total synthesis of (ϩ)-grandisol [(1S,2R)-(ϩ)-2-isopropenyl-1 H-and 13 C-NMR experiments and mass spectrometry. 1-methylcyclobutaneethanol]; [10] the pheromone compo-However, these spectrometric methods did not allow the nent of the boll weevil Anthonomus grandis (see Scheme 1) relative and absolute configurations of the two stereogenic and some other beetles. [11] centers of the molecule to be determined. A synthetic sample of the pheromone with a known absolute configuration would therefore be useful as a reference material in order to assign the absolute configuration of that occurring naturally. Accordingly, one must compare the synthetic sample with the natural pheromone by physical and/or biological methods. Hence the problem of synthesizing the four stereomers (1S,2S)-and (1R,2R)-1 and (1R,2S)-and (1S,2R)-2 (see Figure 1) was addressed. This work presents the studies required to obtain these four stereomers, involving stereoselective synthesis of various cyclobutane derivatives. This class of compounds is involved in many organic transformations, [4Ϫ6] their skeleton constituting the basic Scheme 1. Strategy based on alkylation of cyclobutane A structure of several natural products [7Ϫ9] and the design of a versatile methodology for their synthesis is of topical
The starting ester 3 was synthesized from carvone in four interest.
steps according to a standard procedure [12,13] [34% overall The goal was to design an enantiospecific synthesis of yield (3 ϫ) in our hands]. Treatment of 3 with PTSA in a all these diastereomers without having recourse to optical mixture acetone/water gave the formyl ester 4, which was immediately reduced with sodium tetrahydridoborate to [a] Unite ´de Phytopharmacie et Me ´diateurs Chimiques, I.N.R.A., furnish the hydroxy ester 5 in an overall yield of 64%. Es-
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