Intramolecular Lewis acid catalyzed ene reaction of aldehyde 3 gives the (E)-ene adduct 18 with 85-90% selectivity. Intramolecular ene reactions of aldehydes 6, 10, and 13 proceed with 88-100% selectivity for the isomer with an equatorial methyl group and an axial hydroxyl group. Intramolecular type-I1 ene reaction of allenic aldehyde 17 occurs either thermally or with Lewis acid catalysis to give a mixture of the expected ene adduct, bis ...
