Synthetic studies on spongistatins: synthesis of the C29–C44 fragment

A convergent synthesis of the C29–C44 fragment, the common subunit of the spongipyran macrolides is described. The key step of the synthesis is the C-glycosidation reaction which is based on coupling of the lithiated F-ring sulfone with the E-ring aldehyde, and subsequent reductive desulfonylation to afford the E–F bis (pyran) with high stereoselectivity at the anomeric carbon.