Biomimetic cationic polyene cyclization

Stereoisomeric alcohols (93) and (94) yielded identical ring-expansion products [e.g. (97)] on formation of carbocations.168 This is evidence of a stepwise reaction in sterol biosynthesis, whereby a tertiary cation [e.g. the model (95)] rearranges to a secondary cation (96)-an anti-Markovnikov rearrangement . The synthetic aspects of biomimetic cyclizations of isoprenoid polyenes were reviewed.169 Included was a detailed discussion of carbenium ion-initiated cyclizations, with a discussion of the different mechanisms that have been proposed. A novel biomimetic carbocation polyene cyclization of a daurichromenic ester was reported an unusual 2 + 2-carbocation cyclization occurred as a side reaction.170... 

Johnson, W.S., Lindell, S.D., and Steele, J., Rate enhancement of biomimetic polyene cyclizations by a cation-stabilizing auxiliary, 7. Am. Chem. Soc., 109, 5852, 1987. 

Electrophilic Addition Reactions. Further work by Johnson on the biomimetic polyene olefin cyclization reactions has shown that asymmetric induction by a chiral centre remote from the initiating cationic centre is possible. This was shown by the preparation of 11 a-methylprogesterone and a stereospecific total synthesis of racemic 11 a-hydroxyprogesterone via this route (Scheme 48). > ... 

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