The Lewis acid-mediated cyclisations of sugar-derived anomeric sulfone-enol ethers 5 and 6 to give respectively the bicyclic ketooxetanes 11/12 and 16/17 are reported. 0 1997 Elsevier Science Ltd.
Over the past several years we have been seeking to develop a general strategy for stereoselective Cglycosidation.
Underpinning our approach is the idea that stereoselectivity in the crucial anomeric C-C bondforming step may be achieved by tethering a functional group which possesses a carbon nucleophile to a template containing the electrophilic anomeric cation. We have shown that by using ether linkages as the tethers, four-,1 five-2 and six-membered3 rings may be made, often with high stereoselectivities.4
For the synthesis of simple, model bicyclic ketooxetanes using this approach,' it was necessary to start from protected 'sugars' 1, and to incorporate the requisite ketonic side-chain using an allylation-ozonolysis sequence, which gave 2. Hydrolysis of the glycosidic linkage, activation of the anomeric centres by S-glycosidation, and silyl enol etherification completed the synthesis of the substrates 3, which were formed as anomeric mixtures (Scheme 1).