A general strategy for the synthesis of indoloquinolizine alkaloids: Via a cyanide-catalyzed imino-Stetter reaction

A new strategy applicable to the synthesis of indoloquinolizine natural products has been developed. A cyanide-catalyzed intramolecular imino-Stetter reaction of aldimines, derived from 2-aminocinnamic acid derivatives and 2-pyridinecarboxaldehydes, provided indole-3-acetic acid derivatives bearing a pyridyl ring at the 2-position. Reduction of the carboxylic acid moiety to an alcohol followed by activation of the resulting alcohol with Tf₂O or TsCl generated indoloquinolizinium salts, which were utilized as precursors for indoloquinolizine natural products. The advantage of this protocol was successfully demonstrated in the total syntheses of arborescidine A and nauclefidine.