Enantioselective intramolecular [2 + 2 + 2] cycloaddition of 1,4-diene-ynes: a new approach to the construction of quaternary carbon stereocenters.

The intramolecular [2 + 2 + 2] cycloaddition of various 1,4-diene-ynes was examined using a chiral rhodium catalyst. In the case of 1,4-diene-ynes with a substituent at the 2-position of the 1,4-diene moiety, tricyclic compounds possessing a strained bicyclo[2.2.1]heptene skeleton with two quaternary carbon stereocenters were obtained in high enantiomeric excess. On the other hand, in the case of 1,4-diene-ynes with no substituent at this position, bicyclic cyclohexa-1,3-dienes with a quaternary carbon stereocenter were obtained probably by carbon-carbon bond cleavage of the reaction intermediate.