Absolute structures and conformations of the spongian diterpenes spongia-13(16),14-dien-3-one, epispongiadiol and spongiadiol.

The absolute configurations of spongia-13(16),14-dien-3-one [systematic name: (3bR,5aR,9aR,9bR)-3b,6,6,9a-tetramethyl-4,5,5a,6,8,9,9a,9b,10,11-decahydrophenanthro[1,2-c]furan-7(3bH)-one], C(20)H(28)O(2), (I), epispongiadiol [systematic name: (3bR,5aR,6S,7R,9aR,9bR)-7-hydroxy-6-hydroxymethyl-3b,6,9a-trimethyl-3b,5,5a,6,7,9,9a,9b,10,11-decahydrophenanthro[1,2-c]furan-8(4H)-one], C(20)H(28)O(4), (II), and spongiadiol [systematic name: (3bR,5aR,6S,7S,9aR,9bR)-7-hydroxy-6-hydroxymethyl-3b,6,9a-trimethyl-3b,5,5a,6,7,9,9a,9b,10,11-decahydrophenanthro[1,2-c]furan-8(4H)-one], C(20)H(28)O(4), (III), were assigned by analysis of anomalous dispersion data collected at 130 K with Cu Kalpha radiation. Compounds (II) and (III) are epimers. The equatorial 3-hydroxyl group on the cyclohexanone ring (A) of (II) is syn with respect to the 4-hydroxymethyl group, leading to a chair conformation. In contrast, isomer (III), where the 3-hydroxyl group is anti to the 4-hydroxymethyl group, is conformationally disordered between a major chair conformer where the OH group is axial and a minor boat conformer where it is equatorial. In compound (I), a carbonyl group is present at position 3 and ring A adopts a distorted-boat conformation.