Enantioselective Total Synthesis of (+)- and (−)-Nigellamine A₂

The nigellamine alkaloids are dolabellane diterpenes displaying potent lipid metabolism-promoting activity. Total synthesis of (+)- and (−)-nigellamine A₂ has been accomplished. Absolute stereochemistry of synthetic nigellamine A₂ was established through an intramolecular asymmetric allylic alkylation using a Pd(phosphinooxazoline) catalyst. Other notable transformations include a radical alkynylation, a diastereoselective Nozaki−Hiyama−Kishi cyclization, and a regio- and stereoselective catalytic epoxidation. On the basis of X-ray crystallographic analysis of an optically active intermediate, we have confirmed the assigned absolute stereochemistry of the natural product. Minor modifications of the synthetic sequence outlined here should provide access to the other nigellamine alkaloids.