Enzyme-Mediated Preparation of (+)- and (-)-cis-α-Irone and (+)- and (-)-trans-α-Irone

Abstract

The preparation of (−)- and (+)-trans-α-irone (1a and 1b, resp.) and of (+)- and (−)-cis-α-irone (1c and 1d, resp.) from commercially available Irone alpha ® is reported. The relevant step in the synthetic sequence is the initial chromatographic separation of crystalline (±)-4,5-epoxy-4,5-dihydro-cis-α-irone ((±)-5) from oily (±)-4,5-epoxy-4,5-dihydro-trans-α-irone ((±)-4). The latter was subsequently converted, after NaBH4 reduction, into the crystalline 3,5-dinitrobenzoate ester (±)-8, thus allowing a complete separation of the two corresponding diastereoisomeric alcohol derivatives. Suitable enantiomerically pure precursors of the desired products 1a – d were obtained by kinetic resolution of the racemic allylic alcohols derived from (±)-5 and (±)-8, mediated by lipase PS (Amano). The last steps consisted of MnO2 oxidation and removal of the epoxy moiety with Me3SiCl/NaI in MeCN. External panel olfactory evaluation showed that (−)-cis-α-irone (1d) has the finest and most distinct `orris butter' character.