Substitution and elimination reactions of 3- and 4-alkyl-2H-1-benzopyrans with organoaluminium and organomagnesium reagents

Abstract

2,3-Dialkyl- and 2,2,3-trialkyl-2H-1-benzopyrans react with triethylaluminium or ethylmagnesium bromide yielding substituted o-allylphenols in good yield. The reactions of 2,3-dialkyl-2H-1-benzopyrans are stereoselective. Either the E or Z isomers can be formed preferentially depending upon the experimental conditions; the stereoselectivity increases with the bulk of the alkyl substituent at C-3. However 2,4-dialkyl- and 2,2,4-trialkyl-2H-1-benzopyrans lead to o-allylphenols only with ethylmagnesium bromide; the mixture of E and Z isomers thus formed was found to be very difficult to separate.