LiBF4: A mild lewis acid for intramolecular diels-alder reactions.

The lithium salts, LiClO4 and LiBF4, have been investigated for their potential use as Lewis acid catalysts in the intramolecular Diels-Alder reaction. No cycloaddition of the trienone, 1, is observed when LiClO4 is used. LiBF4 provides quantitative yield of the cis-fused cycloadduct in 72 hours at room temperature. The catalysis is ascribed to the slow release of BF3 rather than to the lithium cation.