Highly b-stereoselective nucleosidation from a-D-xylo- and a-D-ribo-furanose 1,2-thiocarbonates

Cyclic 1,2-thiocarbonates of a-D-xyloand a-D-ribo-furanoses were found to be excellent glycosyl donors in mild NISmediated nucleophilic substitution reactions, affording b-nucleosides with complete stereoselectivity and moderate to high yields after treatment with persilylated pyrimidinic bases. The nucleophile is believed to open the thiocarbonate ring at the anomeric position presumably via an SN2 mechanism. Participation of the nucleobase silylating agent [N,O-bis(trimethylsilyl)acetamide] in the mechanism of the nucleosidation step was shown, where a large excess of it has been proven to be necessary in order to achieve high yields. Absolute configurations at C-10 were ascertained by chemical correlation synthesizing the corresponding 2,20-anhydronucleosides. 2004 Elsevier Ltd. All rights reserved.