Novel chemoselective reduction of the tetrahydro-4-pyridyl versus indole moiety governed by electron donation: first X-ray evidence for indolopiperidyl–borane complexation

A series of amino–borane complexes of structure type 2 are the key intermediates in the preparation of 3-(piperidyl) indole derivatives. The selective reduction of the tetrahydro-4-pyridyl double bond via 2 under strongly acidic conditions is feasible only when the pyridyl double bond is conjugated with electron rich substituents, such as indoles. This reduction methodology allows the presence of reducible and hydrogenolizable functional groups. An improved process to prepare 2 by treatment of the 3-(tetrahydro-4-pyridyl) indole derivatives with NaBH4 in THF under acidic conditions (AcOH or CF3COOH) is also described.