Leucascandrolide B, a new 16-membered, extensively methyl-branched polyoxygenated macrolide from the calcareons sponge Leucascandra caveolata from northeastern waters of New Caledonia

We report a novel, extensively methyl-branched and polyoxygenated 16-membered macrolide, leucascandrolide B (= (1R*,5S*,7R*,9S*,13R*:,14R*,15R*,11Z)-5-[(E)-1,4-dimethy lhept-5-enyl]-1,7,9-trihydroxy-11,14-dimethyl-3-oxo-4,17-dioxabicyclo[ 11.3.1]heptadec-11-en-15-yl] carbamate; 2), isolated from the calcareous sponge Leucascandra caveolata BOROJEVIC and KLAUTAU from the northeastern coast of New Caledonia. The NMR structural assignments were corroborated by semisynthetic derivatives of 2, the functional groups and atom connectivity by derivatives 4-6 (obtained by acetylation, oxidation, and elimination-solvation, reap.), and the relative configurations by derivative 7 (obtained by carbonation). The preferred conformation of 2, centered on a chair-like tetrahydro-2H-pyran ring formed by intramolecular acetalization, was derived from molecular mechanics and semiempirical calculations which were in agreement with all NMR data. A dilactone `dimeric' structure 3, indistinguishable from 2 on the basis of all above data, was ruled out by tandem MS-MS experiments.