Nazarov Cyclizations of an Allenyl Vinyl Ketone with Interception of the Oxyallyl Cation Intermediate for the Formation of Carbon−Carbon Bonds

Treatment of an allenyl vinyl ketone with BF3·Et2O leads to a cyclic oxyallyl cation by a Nazarov reaction, and when this reaction is conducted in the presence of an acyclic diene, [4 + 3] and [3 + 2] products are obtained efficiently with high regio- and stereoselectivity. The proportion of [4 + 3] to [3 + 2] product depends on the substitution on the diene. Cyclic dienes react with the oxyallyl cation by forming only one carbon−carbon bond, but the site of bond formation can be affected by steric hindrance. Electron-rich alkenes intercept the allyl cation by forming one carbon−carbon bond, or two carbon−carbon bonds through [3 + 2] cyclization. In some instances, further treatment of the initial products with BF3·Et2O leads to equilibrated products in good yield.