Influence of Remote Picolinyl and Picoloyl Stereodirecting Groups for the Stereoselective Glycosylation

Abstract

Sugar compounds generally consist of several hydroxyl groups, responsible for the generation of complex oligosaccharides and are discriminated against or manipulated by using protecting groups. The protecting groups in carbohydrate chemistry adopt a central position in controlling stereoselectivity of glycosylation reaction. In all, picoloyl (Pico) and picolinyl (Pic) group are scrutinize as admirable protecting groups for the evaluation of efficient and convenient glycosyl donors which in turn, synthesize both 1,2‐cis‐ and 1,2‐trans‐linked complex and challenging oligosaccharides stereoselectively either through hydrogen bond mediated aglycone delivery (HAD) or by neighbouring group participation. This review explores the recent advances in the use of picoloyl (Pico) and picolinyl (Pic) as stereo‐directing groups for high facial α‐ or β‐stereoselectivity and its applications in the synthesis of biologically important oligosaccharides.

Chemical Synthesis of β-l-Rhamnose Containing the Pentasaccharide Repeating Unit of the O-Specific Polysaccharide from a Halophilic Bacterium Halomonas ventosae RU5S2EL in the Form of Its 2-Aminoethyl Glycoside

“…5 Hz and at 100.9 ppm (C-1′) with J CH 153. 8 Hz confirmed the formation of the desired disaccharide acceptor 5β. Glycosylation reaction of the rhamnosyl donor 6 with the disaccharide acceptor 5β using NIS in the presence of TMSOTf at −5 °C afforded the trisaccharide 23 in 76% yield.…”

“…The crude mixture thus obtained was purified by flash chromatography using n-hexane−EtOAc (12:1 → 7:1 → 4:1) as the eluent to afford a white sticky compound 19 (1.4 g, 95%). 2-Azido Ethyl 2,3-Di-O-benzyl-β-L-rhamnopyranoside (8). To a solution of compound 19 (1.32 g, 2.4 mmol) in CH 2 Cl 2 /H 2 O = 9:1 (20 mL), DDQ (600 mg, 2.6 mmol) was added, and the mixture was stirred at 25 °C for 2 h when TLC (n-hexane−EtOAc; 1.5:1, R f = 0.5) showed complete conversion of the starting material to a slower moving spot.…”

“…The presence of two consecutive 1,2- cis linked rhamnose and a 1,2- cis linked glucose is the prominent synthetic challenge toward this particular total synthesis. For the 1,2- cis linked rhamnose units, picoloyl-induced hydrogen-bond-assisted aglycon delivery (HAD) − was used successfully, whereas for the 1,2- cis linked glucose moiety, remote participation of the carbonyl functional group was utilized . 2-Aminoethyl glycoside was chosen, leaving the scope of further glycoconjugate formation using the terminal free amine without hampering the stereochemistry at the reducing end.…”

Thiocarbonyl as a Switchable Relay-Auxiliary Group in Carbohydrate Synthesis

“…A strategy for stereocontrolled glycosylation is to utilize the auxiliary participation effect. 19 The 1,2-trans glycoside 7 can be obtained via a C2-neighboring-group participation (Scheme 1B). Its mechanism involves an anomeric stabilization from the C2-acyl group, subsequent to which the acceptor undergoes a 1,2-trans nucleophilic addition.…”

“…A strategy for stereocontrolled glycosylation is to utilize the auxiliary participation effect . The 1,2- trans glycoside 7 can be obtained via a C 2-neighboring-group participation (Scheme B).…”