Anhydro sugars as electrophilic donors

Of equal significance is the ability to control the anomeric stereochemistry. Isomerisation can be initiated by protection of the alkyne as the dicobalthex-acarbonyl complex (51) subsequent epimerisation with catalytic amounts of trillic acid affords the 3-anomer (52). 

More recent work by van Boom and co-workers has seen the use of this method in the synthesis of cis- and tran -fused bicyclic ethers via a ring closing metathesis 

Stereoselective synthesis of a-C-(alkynyl)-glycosides via ring opening of a-1,2-anhydrosugars [65, 66]. 

Notes and discussion. Ring-opening of an a-1,2-epoxide function in sugars with lithium alkynyl derivatives, in the presence of zinc chloride, has been shown to proceed with retention of configuration to afford a-C-alkynyl glycosides in good yields. 

FuUy benzylated methyl 6,7-dideoxy-a-D-gluco-hept-6-ynopyranoside (48) [68] (1 equiv.) 

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