Interaction of Grignard reagents with coumarins. Part I. Novel 1,4-addition

Abstract

Ethyl-, isopropyl-, and cyclohexyl-magnesium bromide underwent 1,4-addition to 4-substituted coumarins, best in tetrahydrofuran, to yield 4,4-disubstituted 3,4-dihydrocoumarins. This novel mode of addition was encouraged by –/ substituents in the 4-position, discouraged by a 4-methyl group, unaffected by a 5-hydroxy-group, and not apparently subject to steric effects from the 4-substituent or the Grignard reagent. Methyl- and phenyl-magnesium bromide underwent normal 1,2-addition to the 4-substituted coumarins, except that exclusive 1,4-addition occurred with a 4-(2-pyridyl)coumarin.