Synthesis of the 5-6-7 Tricyclic Core of Daphnicyclidin-Type Alkaloids via a Tiffeneau-Demjanov Ring Enlargement Strategy.

An expedient construction of the 5-6-7 tricyclic core of daphnicyclidin-type alkaloids is described. The synthetically challenging cycloheptanone C ring was constructed via a Tiffeneau-Demjanov ring enlargement reaction from a 5-6-6 tricyclic precursor commonly found in calyciphylline A-type alkaloids. Other key transformations included Davis oxidation, 1,2-addition, oxidation, and dehydration to elaborate the essential cyclcohept-2-enone motif.