Synthetic Approach to Diterpene Alkaloids : Construction of the Bridged Azabicyclic Ring System of Kobusine

Abstract

A synthetic study was carried out on the 1-azabicyclo [2.2.1] heptane partial structure (D/E ring system) of an aconite alkaloid, kobusine. A 1-azabicyclo [2.2.1] heptane system, (±)-6, 15, 16-iminopodocarpane-8, 11, 13-triene (3), was synthesized from the 4a-nitromethylhydrophenanthrone (7), which was obtained from the tetrahydrophenanthol (4) through an abnormal Reimer-Tiemann reaction. The bridged nitrogen structure was formed from the bicyclic chloramine (23) by means of the Hofmann-Loffler reaction. The structure of 3 was confirmed by X-ray diffraction analysis.