From citronellal to iridoids: asymmetric synthesis of iridoids and their analogues via organocatalytic intramolecular Michael reactions

Abstract

A series of iridoids, including iridomyrmecin A, B, C′, D′, (−)-isoiridomyrmecin, (+)-7-epi-boschnialactone, and the inside-yohimbine analogues have been synthesized from readily available, naturally occurring (−)-citronellal via the key step reaction of metathesis, organocatalysis, and subsequent transformations, such as reduction, lactonization, alkylation, Pictet–Spengler reaction and lactamization. Notably, the use of DBU as an additive in the organocatalytic intramolecular Michael reaction of an aldehyde ester with Jørgensen–Hayashi catalysts resulted in better stereoselectivity than the conditions using acetic acid as an additive. The structures of three products have been unequivocally established with single-crystal X-ray crystallographic analyses.