Chiral induction in intramolecular rhodium-catalyzed [2+2+2] cycloadditions of optically active allene-ene/yne-allene substrates

摘要

Allene-yne-allene and allene-ene-allene N-tosyl-linked substrates with two chiral centres in the α-position of the allene moiety were satisfactorily prepared starting both from racemic and chiral propargylic alcohols. The Wilkinson's complex-catalyzed [2+2+2] cycloaddition reaction of these substrates was evaluated. In the case of enantiomerically pure bisallenes, perfect stereoselectivity was observed, giving a diastereomerically pure cycloadduct. The chirality of starting bisallene substrates can be completely transferred to the cycloadducts, representing an atom-economical and enantiospecific process for the construction of fused polycycles. However, when reacting an oxygen-linked allene-ene-allene substrate, the stereoselectivity decreased and two diastereoisomers were formed. A detailed characterization study of the resulting cycloadducts allows us to identify the enantioisomer generated in the cycloaddition.