Construction of the benzindenoazepine skeleton via cyclopentannulation of Fischer aminocarbene complexes: Total synthesis of bulgaramine

摘要

A total synthesis of bulgaramine has been accomplished with a longest linear sequence of eight steps and an overall yield of 23% from commercially available 3,4-dimethoxyphenethyl alcohol. An intramolecular cyclopentannulation reaction of a Fischer aminocarbene complex provided the key step and occurred under significantly milder conditions and in higher yields than those of other reported examples of this reaction type. The reaction solvent was a critical factor in the cyclopentannulation reaction, with measurable amounts of the desired product observed only when THF was utilized. The product yield could be further enhanced by the addition of two-electron donor ligands, demonstrating the first example of this effect on the thermal reaction of aminocarbene complexes with alkynes.