STRUCTURE OF (1-ALPHA-H,6-BETA-H,7-ALPHA-H,8-ALPHA-H)-1-ALPHA,10-ALPHA-EPOXY-8-BETA-HYDROXY-GERMACR-2,11(13)-DIEN-6,12-OLIDE (INCOMPTINE-B) - A SESQUITERPENOID LACTONE

C15H18O4 is monoclinic, P2(1). Unit-cell dimensions at 293 K are a = 15.048(5), b = 9.756(3), c = 15.815(5) angstrom, beta = 114.18(2)-degrees, V = 2118(1) angstrom3, D(x) = 1.23 g CM-3 , and Z = 6. The structure has been determined from single-crystal data collected with a four-circle diffractometer and refined from 2816 observed reflections to R = 0.054. The structural features determined from chemical and spectroscopic studies am confirmed and extended. The incomptine B occurs as three independent molecules with different conformations. The stereochemistry at C(6), C(7), and C(8) is 6S, 7R, and 8R. The 10-membered ring has an approximate C2 symmetry for the A, B, and C molecules. The alpha-methylene gamma-lactone ring adopts a half-chair conformation for molecule A, and a conformation intermediate between beta-envelope and half-chair for the B and C molecules, respectively. The structures are stabilized by a three-dimensional network of 0-H...0 hydrogen bonds and several C-H...0 interactions.