Glycosidation of sugars. IV. Methanolysis of D-glucose, D-galactose, and D-mannose

Abstract

Rates of rnethanolysis reactions of D-glucose, D-galactose, and D-mannose have been determined. The results confirm those found earlier for pentoses that the glycosidation reaction proceeds to equilibrium through four distinguishable competing reactions: (I) hexose -P furanosides, (2) anomerization of furanosides, (3) furanosides + pyranosides, and (4) anomerization of pyranosides. The glycoside compositions at equilibrium are interpreted in terms of conformational analysis for furanoid and pyranoid rings. The energy differences between the ground states also contribute to a n important degree in determining the relative rates of reaction.Canadian Journal o f Chemistry, 46, 3085 (1968) Earlier papers (1, 2) in this series described a study of the kinetics of methanolysis of the four D-pentoses and summarized the literature on this topic. The present paper reports the extension of these studies to the three hexoses, Dglucose, D-galactose, and D-mannose.The conditions cited for the reactions were carefully controlled and products were analyzed by gas-liquid chromatography of their fully trimethylsilylated derivatives as described previously (3). Products in one of the reactions in the D-galactose series (Table VII) were analyzed methanol. The equilibrium constants (K) in Table I were obtained from glycoside compositions after prolonged reaction periods (Table 11).Reliable rate data for reaction 1 (Table V) could only be obtained with D-glucose; Dgalactose and D-mannose were not sufficiently soluble in methanol at the concentrations used.No detailed interpretation of reaction 1 can be offered on the evidence available; the reaction was examined only t o establish the identities of the initial products. Qualitative examination c o l o r i~e t r i c a~l~ (4) after separation by paper of the products from methanolysis of galactose chromatography (5).confirmed the results obtained with D-glucose As with the pentoses, it was found that meth-that furanosides were the initial products in anolysis proceeds to equilibrium through four this reaction. For D-mannose, the initial proddistinguishable competing reactions and it was uct of methanolysis was methyl p-D-mannopossible to establish conditions under which a pyranoside. This result is similar to that found single reaction predominated. Since the observations were made at constant hydrogen ion concentrations, the bimolecular reaction could be expressed in terms of first-order rate kinetics. Compositions of reaction products at different times under specific conditions are given in Tables V-VIII (see Experimental). These results show that single reactions predominated during the early periods and were characterized earlier for the stereochemically equivalent lyxose which also gave pyranosides as initial products in methanolysis (2). Two recent reports (6,7) have discussed the role of dimethyl acetals as intermediates in the formation of furanosides from glycoses during methanolysis. The amounts of dimethyl acetals found were small and from rate studies it was concluded...

Complexity in molecular recognition

“…Methanolysis experiments of glucose indicate the same preference. 54 Again, the OPLS-AA, MP2 and the CCSD(T) results are in very good agreement.…”

Efficient Preparation of Octyl α‐D‐Glucopyranoside Monohydrate: A Recirculation Procedure Involving Water Removal by Product Crystallisation

“…In contrast to the methanolysis of D-glucose [28], no accumulation of furanoside intermediates was observed during the conversion of D-glucose in 1-octanol. Initially, I a and 1 p were formed in comparable amounts, followed by a relatively slow equilibration to 1 a:I = 2.3:l. Although the solubility of Dglucose in 1-octanol at 90°C is only -1.6 gll, a suspension of Dglucose (72 g per 1 1-octanol) did not show D-glucose particles anymore after -6 h reaction.…”

Concerning the Problem of Stereospecific Glycosylation. Synthesis and Methanolysis of some 2-O-Benzylated D-Galactopyranosyl and D-Galactofuranosyl Halides

“…Treatment of 9 with anhydrous hydrogen bromide and chloride, respectively, produced the substituted P-D-galactofuranosyl halides 11 and 12 as amorphous solids which exhibited strong levorotation but did not mutarotate. As is also the case with alkyl P-D-galactofuranosides (22), these 6-halides are expected to be the thermodynamically more stable anomers because they are free from the unfavorable, 1,2-cis substituent interaction which exists in the a-anomers ; therefore a-halides, if initially formed from 9, would be liable to rapid anomerization during the reaction. The triester presumed to have structure 10 gave a dextrorotatory bromo sugar, but further examination of this product was postponed.…”