The Reaction of Dimeric Tri-O-acetyl-2-deoxy-2-nitroso-α-D-hexopyranosyl Chlorides with Alcohols

Abstract

The kinetics of the reactions of dimeric tri-O-acety~-2-deoxy-2-nitroso-cc-~-g~ucopyranosy chloride (1) with simple aliphatic alcohols (4 mol equiv) in dimethylformamide to form alkyl tri-0-acetyl-2-oximinoa-D-arabino-hexopyranosides (5) were investigated by n.m.r. and optical rotation. It is concluded that the conversion of 1 to 5 proceeds basically by way of three s~tccessively formed intermediates, namely, the monomeric form (2) of 1, the tri-0-acetyl-2-oximino-a-D-arabino-hexopyranoy chloride (3), and the tri-O-acetyl-l,5-anhydro-2-deoxy-2-nitroso-~-arabino-hex-I-enitol(4). Direct evidence was obtained only for the intermediacy of 3 and the transient existence at low concentration of an uncharacterized intermediate which may be the p-anomer of 5. The intermediate 4 could form directly from 2 and thereby provide a mechanism for the formation of 5 which by-passes 3. However, the kinetic investigations showed this route to be of very minor, if any, importance. In all cases, the rate of the disappearance of 1 was virtually the same and within experimental error the same as for the isomerization of 1 to 3 in the absence of the alcohol for which the rate-controlling stage is dissociation of 1 to 2. Methanol reacts with 4 only about 5 times faster than does r-butanol.The utility of the reaction for the preparation of oximinoglycosidic structures related to disaccharides is illustrated by glycosidations of 1,2;3,4-di-0-isopropylidene-r-D-galactopyranose 1,2;5,6-di-0-isopropylidene-a-D-glucofuranose both by 1 and by the dimeric tri-O-acetyl-2-deoxy-2-nitroso-r-o-galactopyranosyl chloride (6) and of 2,3;5,6-di-0-isopropylidene-a-~~n~annofuranose by 1. The reaction of 6 with 1,2;3,4-di-0-isopropylidene-cc-D-galactopyranose provided an 8.7% yield of the p-oximinoglycoside along with an 80% yield of the a-anomer.Les cinktiques de reaction entre le chlorure de tri-0-acetyl deoxy-2 nitroso-2-a-D-glucopyranosyle dimere (1) et les alcools aliphatiques simples (4 mol equiv) dans le dimethylformamide pour conduire aux alkyles tri-0-acetyl oximino-2-a-D-arabino-hexopyranosides (5) ont Cte CtudiCes par r.m.n. et rotation optique. On a conclu que la conversion de 1 a 5 procede surtout par la formation successive de trois intermediaires a savoir, la forme monomere (2) de 1, le chlorure de tri-0-acetyl oximino-2-cc-D-arabinohexopyranosyle (3) et le tri-0-acetyl anhydro-1,5 deoxy-2 nitroso-2-D-arabino-hexenitol (4). Une preuve directe a ete obtenue seulement pour l'intermtdiaire 3 de meme que I'existence transitoire, A faible concentration, d'un intermediaire non caracterisk qui pourrait Ctre I'anomere p de 5. L'intermediaire 4 peut se former directement a partir de 2 et de ce fait, fournit un mecanisme pour la formation de 5 sans passer par 3. Les etudes cinitiques montrent que cette voi est cependant d'importance mineure. Dans tous les cas, la vitesse de disparition de 1 reste virtuellement la mCme; de mCme que pour I'isomerisation de 1 en 3, dans les limites de I'erreur experimentale, en I'absence d'alcool pour laquellel'ktape det...

2-Oxoglycosyl (“Ulosyl”) and 2-Oximinoglycosyl Bromides: Versatile Donors for the Expedient Assembly of Oligosaccharides with β-d-Mannose, β-l-Rhamnose, N-Acetyl-β-d-mannosamine, and N-Acetyl-β-d-mannosaminuronic Acid Units

“…In the case of oximinolactosulosyl bromide 207 , reagents and conditions for accessing α-glycosidulose oximes have been studied in greater detail. , Surprisingly, high α-selectivity is already achieved by refluxing in methanol, yet not only α-glycosidation occurs but cleavage of the oxime ester function as well, to afford the α- d -lactosuloside oxime 225 (Scheme ). Of the several reactions involved in this conversion, methanolysis is thought to be the first, since the resulting α-bromo oxime is expected to dehydrobrominate to a nitrosoglycal intermediate of type 223 , which is known to add alcohols with high α-preference. This course is substantiated by isolation of the anticipated intermediate and its conversion into the α-glycoside: treatment of 207 with base ( s -collidine) in N , N -dimethylformamide readily delivered the 2-nitrosolactal ester 223 in dimeric form (67%) and was cleanly converted into α-lactosuloside oxime 225 (69%) on refluxing with methanol.…”