Reactions of three [c]annelated 2-aminothiophenes with electron poor olefins

A series of electron poor alkenes [maleic acid derivatives 3a-d, dimethyl maleate (4), ethyl acrylate (5a), phenyl vinyl sulfone (5b)] undergo [4+2]-cycloaddition to the thiophene moiety of benzopyrano[3,4-c]annelated 2-aminothiophenes 1a,b and 2. The primary adducts tend to release hydrogen sulfide, in this way the fused thiophene ring is replaced by a fused benzene ring bearing the amino group and any new substituents introduced by the dienophile. With maleic anhydride (3a) and dichloromaleic anhydride (3b) acylation of the amino group competes with the [4+2]-cycloaddition to the thiophene ring. The cycloaddition of the monosubstituted alkenes 5a, b follows a head-to-head regioselectivity as predicted from FMO-considerations.