Synthesis of (1 → 6)-carbodiimide-tethered pseudooligosaccharides via aza-Wittig reaction

The synthesis of unsymmetrical sugar carbodiimides through aza-Wittig coupling of sugar derived iminophosphoranes and isothiocyanates is reported. The reaction is particularly fast and efficient in the case of phosphoranylide derivatives of 6-azido-6-deoxysugars and glycosyl isothiocyanates, and has been exploited in the one-pot preparation of (1 → 6)-linked pseudodi- and trisaccharides incorporating carbodiimide bridges as convenient precursors for the corresponding ureido, thioureido, and guanidino derivatives. Kinetic considerations suggest that sugar phosphazides, formed in the early stages of the Staudinger reaction of triphenylphosphine and sugar azides, are the reactive intermediates.