jo951194h_si_001.pdf (286.1 kB)
Chemistry of 1-Alkoxy-1-glycosyl Radicals: The Manno- and Rhamnopyranosyl Series. Inversion of α- to β-Pyranosides and the Fragmentation of Anomeric Radicals
journal contribution
posted on 1996-01-26, 00:00 authored by David Crich, Sanxing Sun, Jarmila BrunckovaThe formation and stereoselective quenching of 1-mannopyranosyl
radicals by a tributyltin hydride-mediated intramolecular 1,5-hydrogen abstraction sequence is described.
A competing process is
1,4-hydrogen atom abstraction leading principally to
glucopyran-2-ulosides. Fragmentation of the
anomeric radical resulting in the formation of ring opened products is
a problem in certain series.
The chemistry is dictated to a considerable extent by the nature
of the protecting groups employed
with the 4,6-benzylidene series and, for rhamnose, the Ley
3,4-dispiroketal, being particularly
susceptible to the 1,4-hydrogen atom abstraction but less to the
fragmentation. Photochemical
conditions are described, in which these side reactions are practically
eliminated, and applied to
the inversion of an α- to a β-mannoside in a
disaccharide.