Abstract
Most proteins found in mammalians are glycoproteins. Their carbohydrate side chains play important roles in the biological selectivity, in particular, in recognition processes on membranes. All glycoproteins contain glycosidic linkages between the carbohydrate side chains and the peptide backbone. Therefore, the synthesis of glycopeptides requires methods for the stereoselective formation of these glycosidic linkages and selective protecting techniques which do not affect the glycosidic bonds. The benzylic protecting groups and their hydrogenolytic removal fulfil these demands. Since these protections, however, often are required for the blocking of the carbohydrate hydroxy functions, the functional groups of the peptide portion have to be protected with groups which are selectively removable and orthogonally stable towards the benzylic groups. In this sense, the fluorenylmethoxycarbonyl (Fmoc) group and its removal with the weak base morpholine and the allyloxycarbonyl (Aloc) groups removable by palladium (0) catalyzed allyl transfer revealed to be valuable amino protecting groups in glycopeptide chemistry. The allyl ester using the analogous palladium (0) chemistry and the acidolytically cleavable tert-butyl ester proved to be useful for the carboxy protection. The benzylic and the allylic (HYCRAM™) ester principle are successfully demonstrated as the anchoring systems in solid phase syntheses of glycopeptides.
