Enzymatic and chemical synthesis of oligosaccharide analogs.

摘要

To explore the possibility of enzymatic glycosylation of analogs in which the C-bonded hydrogen atom is modified (termed carbon-branched sugars), a series of carbon-branched acceptor analogs was chemically synthesized. These analogs had the C-H bond of the ring carbon bearing the OH groups undergoing glycosylation replaced with a C-alkyl bond. Surprisingly, four glycosyltransferases: α1,3/4-fucosyltransferase, α1,3-galactosyltransferase, and the blood group A and B glycosyltransferases were found to be able to recognize the carbon-branched sugar analogs and catalyze their glycosylation. Efficient enzymatic synthesis of glycosides of the complex tertiary sugar alcohols was thus accomplished for the first time.; The glycosides of the tertiary alcohols produced by this chemoenzymatic approach were used as probes for carbohydrateprotein recognition. Analog 83 was found to be a kinetically competent substrate for α-fucosidase which cleaves the fucose residue in the Lewis X structure. Analog 86 , however, had much weaker binding affinity with E-selectin than the natural sialyl Lewis X. Analogs 87 and 89 displayed two and four times stronger binding affinity with the Griffonia simplicifolia I B₄ isolectin than their parent trisaccharides. The increased binding affinity may result from a less negative entropic contribution since the analogs may be conformationally more restricted due to the introduction of the bulky methyl group at the glycosidic linkage.; The second part of the thesis focused on new methods for the stereoselective synthesis of glycosides. An efficient methodology for the synthesis of β-glycosides of 2-amino-2-deoxy sugars was developed employing the p-nitrobenzyloxycarbonyl (PNZ) protecting group. The PNZ group was shown to be an orthogonal amine protecting group which can be readily removed either by hydrogenolysis or by sodium dithionite under neutral conditions.; Also described in this thesis is the exploratory work on the development of a method to make hemithio-orthoesters of complex sugar alcohols, which can be further reduced to give β-glycosides with high stereoselectivity. The preliminary results obtained suggest that the synthesis of β-glycosides from the hemithio-orthoesters will not be practical until better thionation chemistry is developed.