Enantioselective synthesis of a key tricyclic intermediate en route to (+)-gelsemine

The efficient synthesis of an enantiopure tricyclic lactam starting from (S )-5-isopropoxypyrrolin-2-one is described. The racemate of this tricyclic l actam is an intermediate in our previously published total synthesis of rac emic gelsemine, so that the present work paves the way for the synthesis of the enantiopure alkaloid. The synthetic route includes a Sonogashira coupl ing, a highly selective Diels-Alder reaction and a trimethylsilyl triflate- mediated N-acyliminium ion cyclization as key steps. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.