Total synthesis and structural elucidation of spongosoritin A
Two putative structures of spongosoritin A, with syn (6R,8R) and anti (6S,8R) configurations, were each synthesised in a total of 11 linear steps with only 8 purification procedures. The key steps in our strategy included Evans alkylation and olefin dihydroxylation to install the C8 and C6 stereocentres, a transacetalisation/dehydration cascade to construct the furanylidene core, and chromatographic separation of 9E- and 9Z-isomers of the final compounds with silver nitrate impregnated silica. Comparison of the 1H and 13C NMR data for the synthetic syn- and anti-isomers to that reported for the natural product revealed that the relative configuration of spongosoritin A is syn. The absolute stereochemistry was also confirmed as 6R,8R based on optical rotation measurements where the synthetic syn (6R,8R) and natural product had the same sign of optical rotation (negative).