Abstract
The structures and relative stereochemistry of nine new briaran -based diterpenes from a Briareum species have been deduced from 2-D n.m.r. experiments at high field, and nuclear Overhauser effect experiments. The new compounds are (1R*2R*,3R*,5Z,7S*,8S*,9S*,10S*,11R*,-12S*,14S*,17R*)-2,3,14-triacetoxy-8,17:11,12-bisepoxy-9-hydroxybriar-5-en-18-one (5), (1R* ,2R* ,3R* ,5Z*,7S*,8S* ,9S*,10S*,11R*,12S*, 14S*, 17R*)-3,14-diacetoxy-2-butyryloxy-8,17:11,12-bisepoxy-9-hydroxybriar-5-en-18-one (6),(1R*,2R*,3R*,5Z,7S*,8S*,9S*,10S*,11R*,12S*,14S)-2,3,14-triacetoxy-11,12-epoxybriara-5,8(17)-dien-18-one(17),(1S*,2S*,4R*,5Z,7S*,8S*,9S*,10S*,2,4,9.12-tetraacetoxy-8,17-epoxy-11-hydroxybriara-5,13-dien-18-one (8),(1S*, 2S*,4R*, 5Z,7S*,8S*,9S*,10S*, 11S*, 12R*,13Z,17R*)-2,4,9-triacetoxy-8,17-epoxy-11,12-dihydroxybriara-5,13-dien-18-one (9), (1S *,2S*,4R*,5Z,7S*,8S*,9S*,10S*, 11S *,12R*,13Z,17R*)-2,4,9,12-tetraacetoxy-8,17-epoxy-11-hydroxybriara-5,13-dien-18-one(10),(1Si,2S*,5Z,7S',8S*,-9S*,10S*,11R*,12R*,13Z,17R*)-2,12-diacetoxy-8,17-epoxy-9-hydroxybriara-5,13-dien-18-one(ll), (1R*,2R*,3Si,5Z,7S* ,8S* ,9S* ,10S* ,11Z,14S*,17R*)-2,14-diacetoxy-3-butyryloxy-8,17-epoxy-9-hydroxybriara-5,11-dien-18-one (12) and (1R *,2R*,3S*,5Z,7S*,8S*,9S*,10S*,11Z,-14S',17R*)-2,3,14-triacetoxy-8,17-epoxy-9-hydroxybriara-5,11-dien-18-one (13). In addition to the nine new briaran derivatives, the same coral afforded two cladiellane derivatives (1R*,4R*,5S*,6R*,8R*,12R*,13R*,14R*)-cladiellane-4,8,12-triol (14) and its 4-acetoxy derivative (15). Aknowncembranoid diterpene (1R *,4R*,ZE,7E,l1E)-cembra-2,7,11-trien-4-ol(21) wasalso isolated together with an unprecedented perhydrophenanthrene-based diterpene (20): (1R *, 1aR*,4S*,4aR*,5aR*,8aR*)-4-isopropyl-l,8a-dimethyl-5-methylenetetradecahydrophenanthren-1-ol. The extraordinary biosynthetic ability of soft corals from the genus Briareum is once more revealed.