CONFIGURATIONAL ANALYSIS OF NEW FURANOSESQUITERPENES FROM DYSIDEA-HERBACEA - ASSIGNMENT OF ABSOLUTE STEREOCHEMISTRY

Two new marine furanoterpenes, (+)-(4aS,6R,8aS)-4,4,7-trimethyl-4,4a,5,6,8,8a,9-hexahydronaphtho[2,3-b]furan-6-yl acetate (1) and (+)-(4aS,7R,8aS)-6,9,9-trimethyl-4,4a,7,8,8a, 9-hexahydronaphtho[2,3-b]furan-7-yl acetate (2) , were isolated from a sample of Dysidea herbacea, collected from Harrier Reef on the Great Barrier Reef, Australia. The enantiomer of 1, (-)-(4aR, 6S, 8aR)-4,4,7-trimethyl-4,4a,5,6,8,8a, 9-hexahydronaphtho[2,3-b]furan-6-yl acetate (17) was found in another sample of D. herbacea collected nearby from Norman Reef. The structures were determined by spectroscopic methods, in particular 2D NMR, and the absolute configurations of the parent rings of 1 and 2 defined by chemical correlation and application of Kakisawa's modification of the Mosher ester method. Compounds 1 and 2 are diastereomers of acetates 5 and 6, reported earlier from D. herbacea, but have opposite configuration with respect to the ring junctions. These results and others suggest D. herbacea may possess a dual capacity for antipodal cyclization pathways from a common achiral terpenoid precursor.