Metal hydride mediated reduction of 3-(alkylthio)oxindoles containing other potentially reducible groups

Abstract

The reduction of several 3-(methylthio)oxindoles bearing ester groups on the benzene ring has been studied. The reaction is very dependent on the substitution of the oxindole, and the position of the ester group. Deprotonation of the C3 center by the metal hydride is the major initial pathway. This deprotonation plays a role in the reduction of the pendant ester group. Ester groups ortho, and presumably para, to C3 are very difficult to reduce, reaction only occurring with excess LiAlH4 at elevated temperatures. Once reduction starts, it is very difficult to stop, with reduction of the ester to a methyl group being observed. When deprotonation at this center is blocked, ester reduction becomes straightforward and can be accomplished at room temperature with LiEt3BH. Keywords: oxindole, reduction, anion.