Structure of columbin, a diterpenoid furanolactone from Tinospora cordifolia Miers

(1S,4R,5R,8S,10R,12S)-4-Hydroxy-15,16-epoxycleroda-2,12(16),14- trieno-17,12: 18,1-biscarbolactone, C20H22O6, Mr = 358.2, m.p. = 453-454 K, orthorhombic, P2(1)2(1)2(1), a = 7.3869 (6), b = 11.986 (1), c = 19.896 (2) A, V = 1761.65 A3, Z = 4, Dx = 1.351, Dm(by flotation) = 1.349 g cm-3, lambda(Cu K alpha) = 1.5418 A, mu = 8.36 cm-1, F(000) = 760, T = 295 K, R = 0.0432 for 1662 observed reflections. Two terpene rings, two delta-lactones, two methyl groups, a tertiary hydroxyl group and a beta-substituted furan ring are present in the structure. The H atoms at C(12) and C(8) are alpha- and beta-oriented. The terpene ring A is locked into a boat conformation by the C(1)-C(4) lactone bridge. The furan ring is attached equatorially at atom C(12). The hydroxyl group is involved in intramolecular hydrogen bonding.