ENANTIOSELECTIVE AND DIASTEREOSELECTIVE PROTONATION OF PHOTODIENOLS - TOTAL SYNTHESIS OF (R)-(-)-LAVANDULOL

The total synthesis of (R)-(-)-lavandulol 1 has been achieved by asymmetric protonation of photodienols obtained from the irradiation of prochiral alpha,beta-unsaturated esters. The photodeconjugation of ethyl 5-methyl-2-(1'-methylethylidene)-4-hexenoate (3a), carried out in the presence of catalytic amounts of a beta-amino alcohol prepared from (+)-camphor, gives the beta,gamma-unsaturated isomer 2a in good yields but with moderate enantioselectivities (40% ee). In contrast, irradiation of the corresponding ester 3b, bearing the 1,2:5,6-di-O-isopropylidene-D-glucose group as a chiral alkoxy moiety, affords the deconjugated product 2b in high de (>95%). Simple reduction of the ester function with LiAlH4 gives (R)-(-)-lavandulol (1) without loss of optical purity.