Abstract
An analysis of the potential involvement of alicyclic cations in biogenetic-like olefin cyclizations is presented. Application of the concepts of conformational analysis and transition state structures to the intramolecular attack of an olefinic double bond on a conformationally rigid cyclohexyl cation leads to the conclusion that such reactions should proceed with high stereoselectivity. A number of examples of cationic olefin cyclizations, including some undertaken specifically to test the theoretical analysis, are reviewed and discussed in relation to the potential involvement of alicyclic cations as intermediates in these cyclizations. This review illustrates some of the problems encountered in attempting to study mechanisms of olefinic cyclizations related to those involved in in vivo terpenoid biogenesis. The intervention of cyclohexenyl intermediates is implicated as a major cause of formation of large amounts of cis products in cationic olefin cyclizations. The results serve to demonstrate that intrinsic steric factors as well as stereoelectronic factors will favor stereoselective cyclization of polyolefins in enzymatically controlled olefin cyclizations as well as in vitro olefinic cyclizations.
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