Tulearins A, B, and C; structures and absolute configurations

The relative configuration of tulearin A (1) is determined by X-ray diffraction analysis of a cyclic carbonate derivative 2 and the absolute configuration (2R,3R,5S,8S,9S,15R,17S) from the 9-MTPA-esters 1R and 1S is determined using the modified Mosher’s method. A mechanism for the unexpected formation of carbonate 2 is suggested. Two N-phenyltriazolinedione derivatives 3 and 4 are also prepared. Two additional tulearins, B and C (5 and 6) are isolated in very small amounts and their structures are elucidated by spectroscopic means.