Direct umpolung of glycals and related 2,3-unsaturated N-acetylneuraminic acid derivatives using samarium diiodide.

The umpolung of glycals with samarium diiodide offers a simple route to novel carbohydrate-derived nucleophilic reagents in a single step using a readily available reductant. The corresponding allyl samarium reagent that arises from the hexose series reacts with ketones at the C3 position with high stereoselectivity; carbon-carbon bond formation takes place only anti to the substituent at the C4 position of the dihydropyran ring. For the sialic acid series, the completely regio- and stereoselective coupling process of the samarium reagent occurs at the anomeric carbon atom and provides a new approach to the α-C-glycosides of N-acetyl neuraminic acid.