New Syntheses of rac-Huperzine A and its rac-7-Ethyl-Derivative. Evaluation of Several Huperzine A Analogues as Acetylcholinesterase Inhibitors

rac-Huperzine A, its rac-7-ethyl-derivative and two regioisomeric analogues have been prepared through synthetic sequences involving the elaboration of the pyridone ring in a late stage, by reaction of an intermediate pyrrolidine enamine with propiolamide, which gave mixtures of regioisomeric pyridone derivatives. The acetylcholinesterase inhibitory activity of these and two other recently described 11-unsubstituted huperzine A analogues was determined, the rac-7-ethyl analogue of huperzine A being the most active compound, although it is about 12-fold less active than (−)-huperzine A.