Total Synthesis of (−)-Himgaline

The first total synthesis of (−)-himgaline and a highly enantioselective synthesis of its congener (−)-GB 13 are described. Decarboxylative aza-Michael reaction of the hexacyclic lactone precursor under acidic conditions, followed by basic workup, yielded (−)-GB 13 in 80% yield. Cyclization of (−)-GB 13 to oxohimgaline under acidic conditions, followed by internally coordinated sodium triacetoxyborohydride reduction, gave (−)-himgaline as the exclusive product.