Radikalische Cyclisierung von Dienen, V. Substratkontrollierte asymmetrische Synthese von (−)‐Hirsuten und (−)‐3‐Hydroxyhirsuten aus (R)‐(−)‐Carvon

Radical‐Type Cyclization of Dienes, V. – Substrate‐Controlled Asymmetric Synthesis of (−)‐Hirsutene and (−)‐3‐Hydroxyhirsutene from ( R )‐(−)‐Carvone (3a R ,6a R )‐(−)‐6a‐Methyl‐3,3a,4,6a‐tetrahydro‐2 H ‐cyclopenta‐ [ b ]furan‐2‐one ( 6 ) and (1 R ,4 S )‐(−)‐3‐[4‐(but‐3‐inyl)‐3‐methyl‐cyclopent‐2‐enyl]‐2,2‐dimethylpropan‐1‐ol ( 11 ) are key compounds in the synthesis reported in this paper. Enantiomerically pure 6 was obtained in five straightforward steps from the inexpensive precursor ( R )‐(−)‐carvone ( 1 ). Compound 11 was prepared from 6 via an S N 2′ reaction, by analogy with the synthesis reported by Curran et al. The linear triquinanes 13 and 15 were obtained in gram quantities in two additional steps. The utilisation of ( S )‐(+)‐carvone ( ent ‐ 1 ) allows access to the other enantiomeric forms. X‐ray analysis of 15a confirmed the structure of 15 and, by correlation, that of 13 .