Synthesis of 3-Deoxy-d-manno-oct-2-ulosonic Acid (KDO) and Pseudaminic Acid C-Glycosides.

The preparation of glycosyl dibutyl phosphates in the 3-deoxy-d-manno-oct-2-ulosonic acid (KDO) and pseudaminic acid series and their application to the formation of C-glycosides are described. Both donors were obtained from the corresponding thioglycosides by treatment with dibutylphosphoric acid and N-iodosuccinimide. As with the thioglycosides, both donors adopted very predominantly the strongly electron-withdrawing tg conformation of their side chains, which is reflected in the excellent equatorial selectivity of both donors in the formation of exemplary O-glycosides. With respect to C-glycoside formation on the other hand, contrasting results were observed: the KDO donor was either relatively unselective or selective for the formation of the axial C-glycoside, while the pseudaminic acid donor was selective for the formation of the equatorial C-glycoside. These observations are rationalized in terms of the greater electron-withdrawing ability of the azides in the pseudaminic acid donor compared to the corresponding acetoxy groups in the KDO series, resulting in a reaction through tighter ion pairs even at the SN1 end of the general glycosylation mechanism. The contrast in the axial versus the equatorial selectivity between C- and O-glycosylation cautions against the extrapolation of models for SN1-type glycosylation with weak nucleophiles for the explanation of O-glycosylation.