The behaviour of 2H-1-benzopyrans towards trialkylaluminium reagents

Abstract

2H-1-Benzopyrans react with trialkylaluminium compounds by alkyl- and hydrogen-transfer to C-2 and C-4 to give the o-allylphenols (2), (3), and the (E)-o-propenylphenols (4) and (5). The site of attack and the ratio of alkyl- to hydrogen-transfer depend on the alkylaluminium used and on the size of the substituent at C-2 of the pyran. A mechanism involving reaction of the alkylaluminium with the quinone methanide (6), which is in equilibrium with the pyran (1), is in accord with all experimental results.