Rh(I)-catalyzed [(3 + 2) + 1] cycloaddition of 1-yne/ene-vinylcyclopropanes and CO: homologous Pauson-Khand reaction and total synthesis of (+/-)-alpha-agarofuran.

A novel Rh(I)-catalyzed [(3 + 2) + 1] cycloaddition, which can be regarded as a homologous Pauson-Khand reaction, was developed to synthesize bicyclic cyclohexenones and cyclohexanones, enabling a new approach for synthesis of six-membered carbocycles ubiquitously found in natural products and pharmaceutics. The significance of the Rh-catalyzed [(3 + 2) + 1] cycloaddition has been demonstrated by the total synthesis of a furanoid sesquiterpene natural product, alpha-agarofuran, in which the bicyclic skeleton was constructed by the [(3 + 2) + 1] reaction of 1-yne-VCP and CO.