Thiazolylketoses: a new class of versatile intermediates for glycoside synthesis

An account is provided on the very recent work carried out in the authors' laboratory dealing with the preparation of thiazolylketose acetates and their use as effective glycosyl donors in reactions with oxygen, carbon, nitrogen and phosphorus nucleophiles. These reagents are formed by addition of 2-lithiothiazole to sugar lactones followed by acetylation of the resulting thiazolylketoses. Coupling reactions, promoted by TMSOTf in CH2Cl2, are described with primary and secondary sugar alcohols, trimethylsilyl azide, triethyl phosphite and various C-nucleophiles. Suitable transformations of the resulting glycosides are carried out owing to the ready conversion of the thiazole ring into the formyl group followed by reduction to alcohol and/or oxidation to carboxylic acid. Thus, anomeric glycosyl amino acids, ketosyl and ulosonyl disaccharides and phosphonates have been prepared. Special applications include the synthesis of O-glycosyl calix[4]arene derivatives (calixsugars) and a cyclic ketotrisaccharide. The triethylsilane reduction of thiazolylketose acetates leads to thiazolyl C-glycosides that once subjected to the thiazole-to-formyl conversion afford formyl C-glycosides. These anomeric sugar aldehydes are convenient starting reagents for the synthesis of various C-glycosides and C-glycoconjugates, such as (1 --> 6)-disaccharides and oligosaccharides, glycosyl aminoacids (glycine, serine, asparagine), and a galactocerebroside. Formyl C-glycosides participate in the azomethine ylide 1,3-dipolar cycloaddition reaction to C-60 fullerene to give sugar fullerenes. The cover illustration shows some products of these reactions and a view of Palazzo dei Diamanti (Palace of Diamonds), a beautiful and unique building in the architectural context of Ferrara, constructed in the first half of 1500. The facade of the Palace is decorated with more than eight thousand five-hundred diamond shaped small blocks of marble.