SYNTHESIS AND ABSOLUTE-CONFIGURATION OF THE C-12-TERPENOID DEHYDROGEOSMIN FROM THE FLOWER SCENTS OF REBUTIA-MARSONERI AND DOLICHOTHELE-SPHAERICA (CACTACEAE) - A NEW APPROACH TO BIS-ANGULARLY SUBSTITUTED TRANS-DECALINS

Optically pure(+)-(4S,4aS,8aS)-1,2,3,4,4a,5,8,8a-octahydro-4,8a-dimethylnaphthalen-4a-ol ((+)-1; dehydrogeosmin) is released from flower heads of the two cactaceae Rebutia marsoneri and Dolichothele sphaerica. The absolute configuration of (+)-1 is identical with that of the known microbial metabolite geosmin (-)-2. The key reactions of the synthesis of 1 are the kinetically controlled transesterification of the primary alcohol 4 using a lipase from Candida cyclindracea and the stereo- and regiospecific angular alkylation of a cis-decalin skeleton by a Lewis -acid-assisted ring opening of the quaternary epoxy-alcohol 3 with MeMgBr/Cul. The sequence provides a new entry into the class of bis-angularly substituted trans -decalins.