A fast assembly of pentacyclic benz[f]indolo[2,3-a]quinolizidine core by tandem intermolecular formal aza-[3 + 3] cycloaddition/Pictet-Spengler cyclization: a formal synthesis of (+/-)-tangutorine.

We have described a concise construction of the pentacyclic benz[f]indolo[2,3-a]quinolizidine intermediate 3 (with an overall yield of 54% for three steps), featuring a tandem intermolecular formal aza-[3 + 3] cycloaddition/Pictet-Spengler cyclization. The present work can be considered as a formal synthesis of beta-carboline alkaloid (+/-)-tangutorine. Our strategy disclosed herein constitutes a new effective general synthetic approach toward the indoloquinolizidine family of alkaloids.