Ring Contraction During the Lead Tetraacetate Oxidation of Reducing Sugars

Abstract

The rate of oxidation of D-glucose by lead tetraacetate in acetic acid is affected little by a large change in concentration of reactants, showing that the glycol cleavage step is not ratecontrolling. Oxidation of 3-0-methyl-6-0-trityl-D-glucose yields 3-0-formyl-2-0-methyl-5-0-trityl-D-arabinose; the latter is not derived by production initially of a 4-0-fori11yl derivative that undergoesacyl migration. I t is probable that theoxidation of D-glucoSe proceeds with conversion of the pyranose sugar into a furanose intermediate. By contrast, D-mannose appears t o be oxidized mainly in the pyranose form, a s shown by the oxidation patterns of D-mannose-1-and -2-14C and 5-0-methyl-D-mannofuranose, and the effects of concentration change on the reaction of D-mannose and derivatives. The proximity of the ring oxygen atom t o the 1,2-diol group may contribute t o the unusually high reactivity of D-mannopyranose as compared with other types of monocyclic six-membered ring vic-diols.Intermediate proportions of furanose and pyranose pathways characterize the lead tetraacetate oxidation of the other six aldohexoses. Similarities in the behavior of glucose and idose, gulose and mannose, and allose and talose may originate in steric similarities between these pairs of sugars.T h e lead tetraacetate oxidation (1, 2) of reducing sugars in acetic acid involves primarily the cleavage of a-hydroxy hemiacetal glycol groups, and results in a stepwise shortening of the carbon chain (3,4,5). However, the uptake of oxidant in these reactions is lower than might be anticipated. For example, D-glucose (I) consumes only 2 inoles of lead tetraacetate per mole rather than the 3 moles theoretically possible for the 1,2,3,4-tetrol grouping of the pyranose sugar (I), yielding a diformate ester of D-erythrose rather than of D-glycerose. A possible explanation for these results is t o assume that a complex (6) between the tetravalent lead and the 1,2-diol group of a-D-glucofuranose (11) is a n intermediate in the reaction. This complex could be formed directly from the pyranose sugar, by analogy with the formation of D-glucofuranose 1,2-acetals and borate complexes (7,8), or from the furanose sugar if present in solution even as a minor constituent.Dissociation of the con~plex would give 3-0-formyl-D-arabinose, which, oxidized as a furanose sugar (111), would yield 2,3-di-0-formyl-D-erythrose (IV). Thus, involve~llent of a reactive structure such as I1 accounts for the high rate of the reaction (5, 9), as well as the consuinption of only 2 moles of oxidant per mole. Consistent with these possibilities is the fact that derivatives of D-glucose that are prevented from going over t o a furanose form by substituents a t C-4 are oxidized much more slowly than D-glucose itself, whereas the rate is essentially unaltered by a substituent a t C-3 or C-6 (10).

Kinetics of Oxidation of D-Galactose by Pyridinium Chlorochromate

“…In the equilibrium of anomeric form of D-Galactose it is the â-galactose with equatorial orientation of glycosidic hydroxyl group that predominantly exist (scheme 1) and is supposed to be more reactive species 18 .…”

Kinetics and mechanism of oxidation reaction of lactose by N-bromophthalimide: Micelles used as a catalyst

“…As far as the conformation is con H + cerned, the anomeric protons in α and β glucose are present in equatorial and axial positions, respectively [27]. It has also been suggested that the β anomer should be more reactive than the α anomer [28]. It has been reported earlier that NBP can act as sta ble oxidizing and brominating agent because of large polarity of N-Br bond.…”

Kinetics and Mechanism of Micellar Catalyzed Oxidation of Dextrose by N-Bromosuccinimide in H2SO4 Medium

“…Monosaccharides exist mainly as pyranoid and furanoid forms, the former being more stable [21]. On the other hand, NBS exists in the following equilibria [22].…”