Synthesis of 5-Substituted Indole Derivatives, Part 5. A Synthesis of 5-Formyl-1H-indole-2-carboxylates: The CH2SO3H Functionality as a Masked Formyl Group

New synthetic intermediates, ethyl 5-formyl-1H-indole-2-carboxylates (4) were prepared from 2-ethoxycarbonyl-1H-indole-5-methanesulfonic acids (1). The transformation of the sulfomethyl group to formyl function was accomplished through elimination of SO 2 to yield ethyl 5-chloromethyl-1H-indole-2-carboxylates (2), hydrolysed to ethyl 5-hydroxymethyl-1H-indole-2-carboxylates (3), then oxidized to aldehydes (4).