Organocatalytic formal [3 + 3] cyclization of α-(6-indolyl) propargylic alcohols.

With the aid of acetic acid, a 1,10-conjugate addition-mediated formal [3 + 3] cyclization of alkynyl indole imine methides formed in situ from α-(6-indolyl) propargylic alcohols with 1,3-dicarbonyl compounds such as 4-hydroxycoumarins and cyclohexane-1,3-diones was developed, which provided robust access to a wide range of pyranocoumarin and pyran derivatives containing an indole skeleton with high efficiency under mild conditions.