Huperzine A--a potent acetylcholinesterase inhibitor of use in the treatment of Alzheimer's disease.

Huperzine A, 9-amino-13-ethylidene-11-methyl-4-azatricyclo [7.3.1.0(3,8)]trideca-3(8),6,11-trien-5-one, C15H18N2O, Mr = 242.32, monoclinic, P2(1)/n, a = 8.8574 (6), b = 12.1833 (7), c = 12.4278 (7) A, beta = 99.956 (5) degrees, V = 1320.9 (1) A3, Z = 4, Dx = 1.22 g cm-3, lambda(Cu K alpha) = 1.54178 A, mu = 5.75 cm-1, F(000) = 520, T = 296 K, RF = 6.30% for 1402 reflections with Fo greater than or equal to 5 sigma(Fo) and 183 parameters. The pyridone ring is planar and the stereochemistry of the C(11)--C(12) double bond is E.