Oxyanion-accelerated C-2-C-6 cyclization of benzannulated enyne-allenes

The cyclization of oxyanion-substituted, benzannulated enyne-allenes was fo und to proceed rapidly and efficiently at room temperature, producing subst ituted indanones and fluorenones through a C-2-C-6 cyclization pathway. The se reactions bear close resemblance to thermal C-2-C-6 cyclizations of enyn e-allenes previously reported by Schmittel and others, though the oxyanion- substituted cases cyclize far more rapidly, and stand in noteworthy contras t to the C-2-C-7 (Myers) cyclization of (Z)-1,2,4-heptatrien-6-yne, the par ent enyne-allene. The rate of reaction was found to be sensitive to the siz e of the alkyne and allene substituents, though the electronic effects of s ubstitution are not known. The acceleration imparted by the oxyanion substi tuent is consistent with the electronic stabilization of a proposed diradic aloid transition state for the C-2-C-6 cyclization, but the available evide nce does not permit the distinction between concerted and stepwise mechanis ms. Studies are ongoing to further elucidate the mechanism and expand the s cope of these transformations.