Formation of 3,4-dimethyl-2-pyrones from allene carboxylates and 2-silyloxydienes via 3-carboethoxyethylidene cyclobutanols

The 3-carboethoxyethylidene cyclobutanols 4 are prepared in two steps via [2+2] cycloaddition of the 2-silyloxydienes 1 and the allene carboxylate 2 followed by acidic hydrolysis. Treatment of these cyclobutanols 4 with various bases affords good yields of the substituted 3,4-dimethyl-2-pyrones 6. The proposed mechanism involves ring opening of the metal alkoxide 7 to give the carbanion 8, which undergoes proton transfer to give the more stable carbanion 9 and double bond isomerization to give the enolate 10, which then forms the pyrone ring 6 via attack on the ester via 11.