The influence of the functional groups present in the glycosyl donor on the stereoselective glycosylation of 2‐azido‐2‐deoxy sugar thioglycosides under the N‐iodosuccinimide (NIS) and triflic acid (TfOH) mediated glycosylation conditions has been studied in detail. It was observed that beta glycosides of 2‐azido‐2‐deoxy sugar are formed mainly via a stable alpha‐glycosyl triflate intermediate followed by SN2 substitution of the triflate group by the acceptor. The presence of an electron withdrawing functional group such as O‐picoloyl (Pico) in the glycosyl donor facilitates the formation of stable α‐glycosyl triflate to furnish β‐glycosides, whereas the presence of an electron donating hydrogen bond mediating functional group such as p‐methoxybenzyl (PMB) in the glycosyl donor directs the formation of 1,2‐cis glycosides via the glycosyl oxycarbenium ion intermediate with B2,5 and 4H3 conformation. Excellent yields of the β‐glycosides were obtained in case of 2‐azido‐2‐deoxy‐d‐mannose and 2‐azido‐2‐deoxy‐d‐galactose thioglycosides, and 2‐azido‐2‐deoxy‐d‐glucose thioglycosides furnish moderate yields of the β‐glycosides.
A pentasaccharide repeating unit corresponding to the cell wall O-antigen of Salmonella enterica O55 containing a rare sugar, 3-acetamido-3-deoxy-d-fucose has been synthesized as its p-methoxyphenyl glycoside using a sequential stereoselective glycosylation strategy. A suitably functionalized 3-azido-3-deoxy-d-fucose thioglycoside derivative was prepared in very good yield and used in the stereoselective glycosylation reaction. Functionalized monosaccharide intermediates were prepared judiciously and stereoselectively assembled to get the desired pentasaccharide derivative in excellent yield.
A significantly fast one step reaction condition has been developed for the synthesis of unsymmetrical anomeric glycosyl disulfide derivatives in excellent yield by the treatment of anomeric glycosyl bromides with a combination of carbon disulfide (CS 2 ) and sodium sulfide nonahydrate (Na 2 S • 9H 2 O) in the presence of symmetrical alkyl, aryl and glycosyl disulfides at room temperature. Exclusive formation of anomerically betadisulfides was observed under the reaction conditions. Most of the reactions are high yielding and suitable for scale-up synthesis.
Organocatalytic coupling of glycosyl azides with enolates of active ketones and esters through azide-enolate [3 + 2] cycloaddition in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) furnished 1,4,5-trisubstituted-glycosyl-1,2,3-triazole derivatives in excellent yield. The reaction condition is simple and can be scaled-up.
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