Carbocations or cyclopropyl gold carbenes in cyclizations of enynes

Theoretical and experimental studies on the gold(I)-catalyzed formal [4+2] cycloaddition of dienynes are consistent with a mechanism that proceeds by means of cyclopropyl gold(I) carbenes instead of simple carbocations. This work shows that homoallylic stabilization is significant even for systems in which the tertiary carbocation is stabilized by two methyl groups.