Abstract
The use of a bifunctional cinchona/thiourea organocatalyst for the direct and α-stereoselective glycosylation of 2-nitrogalactals is demonstrated for the first time. The conditions are mild, practical, and applicable to a wide range of glycoside acceptors with products being isolated in good to excellent yields. The method is exemplified in the synthesis of mucin type Core 6 and 7 glycopeptides.
Highlights
Stemming from our interest in the development of novel catalytic systems for the stereoselective synthesis of glycosides,[17−20] we decided to focus our efforts on the organocatalytic preparation of 2-deoxy-2-amino-O-galactosides
Organocatalysis has been successfully applied to the synthesis of oligosaccharides.[15,18−25] Our team reported the use of Schreiner’s thiourea to promote the highly α-selective glycosylation of galactals with a number of OH nucleophiles.[20]
The organocatalytic enantioselective addition of soft carbon nucleophiles and thiols to nitroalkenes has been documented by various research groups.[26−33] A common feature in all the catalytic systems employed in these conjugate addition reactions is the use of bifunctional catalysts that contains both a thiourea and a basic amine group
Summary
Stemming from our interest in the development of novel catalytic systems for the stereoselective synthesis of glycosides,[17−20] we decided to focus our efforts on the organocatalytic preparation of 2-deoxy-2-amino-O-galactosides. It is proposed that while the thiourea functionality coordinates to the nitro group, and increases the electrophilicity of the nitroalkene, the pendant amine activates and directs the addition of the nucleophile into the prochiral alkene (Figure 1).[26,31,34] We hypothesized that such bifunctional organocatalysts would be ideally suited as mild promoters for the stereoselective glycosylation of 2-nitroglycals.
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