Ring size and substituent effects in oxyanion-promoted cyclizations of enyne-allenes: observation of a Myers-Saito cycloaromatization at cryogenic temperature.

A series of acetoxy-substituted enyne-allenes, fused to cyclopentene and cyclohexene ring systems, were synthesized and treated with methyllithium to generate the corresponding enolates. It was found that whereas the cyclohexannulated examples underwent either C2−C7 (Myers−Saito) cycloaromatization or C2−C6 (Schmittel) cyclization depending on their terminal subsituents, the cyclopentannulated examples either failed to cyclize altogether or underwent C2−C7 cyclization. Both of these results lie in contrast to the behavior of their benzannulated analogues, which underwent exclusive C2−C6 cyclization independent of substituents. These findings are rationalized on the basis of both ring strain effects and the steric encumbrance of the terminal alkynyl and allenyl subsituents.