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

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.