Pyranosides hydrolysis

If in /3-D-glucopyranoside the linkage between glucon and aglucon is effected by S instead of by 0, as is the case in thiophenyl 8-D-gluco-pyranoside, hydrolysis by /5-glucosidase does not take place.42 This... 

With this ortho ester good selectivity for the axial alcohol is achieved in the acidic hydrolysis of a pyranoside derivative." ... 

Cyclic sulfates rapidly react with the fluoride ion sources to give monofluoro derivatives Thus, the 2,3-cyclic sulfate of methyl-4 6 O benzylidene p D manno pyranoside cleanly reacts with tetramethylammonium fluoride to give methyl 4,6-0-benzylidene 2 deoxy 2 fluoro-p-D-glucopyranoside-3-sulfate Acid hydrolysis followed by acetylation gives 2 deoxy 2 fluoro-P-D-glucopyranoside triacetate in 84% isolated yield [5S] (equation 38)... 

A number of 3-deoxyglyculosonic acids have been identified. These substances are acid-labile and are decomposed on hydrolysis with acid under normal conditions, and have therefore often escaped detection in the past. The simplest member of this class, 3-deoxy-L- /yccro-pentulosonic acid (26), occurs as terminal groups in the capsular polysaccharide from Klebsiella K38. Pyranosidic 3-deoxy-D-r/ircohexulosonic acid is a component of the Vibrio parahaemolyticus 07 and 012 LPS. The same acid, as )3-py-ranosyl groups, is also present in the extracellular polysaccharide from Azo-tobacter vinelandii. ... 

Duroux, L. et al.. Insight into napthoquinone metabolism 3-glucosidase-catalysed hydrolysis of hydrojuglone 3-pyranoside, Biochem. J., 333, 275, 1998. 

In their study of Bronsted acid induced cleveage of a [with axial C(l)-OMe] and (3 [with equatorial C(l)-OMe] glycopyranosides, Fraser-Reid et al.46 demonstrated that the (3/a rate ratios for hydrolysis of methyl pyranosides (Table 5) can be explained by the different intermediates and transition-state structures through which proceed the heterolysis of a and [3 isomers (Fig. 18). 

Table 5 p/a rate ratios for hydrolysis of methyl pyranosides... 

The implications of the above observations may be important, especially if similar trends are observed in pyranose anomers. For example, with respect to the mechanism of acid-catalyzed hydrolysis of pyranosides, endocyclic C-0 bond cleavage (preceeded by 05 protonation) may be assisted in P-anomers in which the Cl-01 bond is equatorial, since the 04-Cl bond may already be extended in these anomers. By a similar argument, exocyclic C-0 scission (preceeded by 01 protonation) may be assisted in the hydrolysis of a-pyranosides in which the Cl-01 is axial and extended, thus resembling the transition state. Post and Karplus have recently suggested that enzyme-catalyzed glycoside hydrolysis of P-pyranosides may indeed take place by ring oxygen protonation, followed by endocyclic C-0 bond scission. 

In the mannosidase series, two GH 38 enzymes, that of jack bean and also almond a-mannosidase, were evaluated with deoxy derivatives of 4-nitrophenyl a-D-manno-pyranoside.126 For both enzymes, it was found that scarcely any hydrolysis took place with the 2-deoxy, 3-deoxy, or 4-deoxy sugars, but the 6-deoxymannopyranoside was turned over at about the same rate as the parent compound, indicating that a strict conservation of functional groups is necessary for productive enzymatic action. 

Fig. 1. —Reaction Mechanism for the Acid Hydrolysis of(a) 2-Aminoethy] /3-d-G1uco-pyranoside and (b) Methyl 2-Amino-2-deoxy-/3-D-glucopyranoside.

On treatment of the labile ethyl 2,3,4-tri-O-methyl-a-D-giuco-hex-odialdo-l,5-pyranoside (76) with sodium methoxide, ethyl 4-deoxy-2,3-di-0-methyl-a-L-threo-hex-4-enodialdo-l,5-pyranoside (77) was formed in good yield 135 this product seems to react further on prolonged treatment with base. Preliminary studies indicated that compound 77 yields the furan 78 on mild, acid hydrolysis, with simultaneous release of the substituents at C-l, C-2, and C-3 (see Scheme 10). These reactions are thus analogous to those of the es-terified glycosiduronic acids (see Section IV,2, p. 214). 

An entropy value for the acidic hydrolysis of ethyl 0-D-galactofuranoside was reported by Overend and coworkers.136 The negative value (—7.1 e.u.) was interpreted as being diagnostic of an A-2 mechanism, in contrast to the positive values (+13.7 e.u., mean value) and A-l mechanism found for a large number of pyranosides. The proposed intermediate is shown in formula (34). It was considered that the planar, furanoid ring is capable of... 

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