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Stereocontrolled Synthesis of α- C -Galactosamine Derivatives via Chelation-Controlled C -Glycosylation

1998; American Chemical Society; Volume: 63; Issue: 8 Linguagem: Inglês

10.1021/jo971727h

1520-6904

Dominique Urban, Troels Skrydstrup, Jean‐Marie Beau,

Chemical Synthesis and Analysis

The samarium diiodide-promoted reduction of 2-deoxy-2-acetamidogalactosyl pyridyl sulfone alpha-5 with ketones or aldehydes under Barbier conditions led unexpectedly to the stereoselective synthesis of alpha-C-galactosamine derivatives in good yields. With carbonyl substrates, alpha:beta selectivities ranged from 20:1 to 5:1, and with aldehydes a stereoselectivity of approximately 5:1 was observed at C7 in favor of the S-isomer. The stereochemical preference of these C-glycosylation reactions is explained by the intermediacy of an alpha-oriented anomeric glycosyl samarium(III) compound that is stabilized via chelation of the metal ion to the C2-acetamido group.