Glycosyl fluoride hydrolysis

The proposed mechanism of a- and /3-glycosyl fluoride hydrolysis (55) is based on the assumption that catalysis requires a general acid and anionic base on opposite faces of the substrate-reactive center. The assumption is fully consistent with the operative mechanism of lysozyme, modified to catalyze a reaction with anomeric inversion. To continue the example of /3-xyIosidase, hydrolysis of the usual anomeric substrate (/3-D-xylosyl fluoride) involves water attack at the reactive carbon from the si face or below C-1. The general acid, situated above C-1 on the re face, protonates the fluoride (or glycoside oxygen) as it departs from the /3-position (57, 92) [Eq. (1)]. 

Evidence has been provided for a covalently bound intermediate in a j9-glucosidase mediated glycosyl fluoride hydrolysis. 2-Deoxy-2-fluoro-j8-D-glucopyranosyl fluoride becomes bound to the enzyme and then the 2-fluoro group considerably slows what is normally a fast hydrolysis step to regenerate the enzyme and produce the hydrolysed sugar. The adduct formed between 2-deoxy-2-fluoro- >D-mannopyranosyl fluoride and the enzyme was shown... 

Glycosyl fluorides generally resist hydrolysis (or solvolysis) under basic conditions. For example, 150,151 (see Section 11,3 and Table II), and the deprotected product (153) from 151 were unreactive to sodium methox-ide in refluxing methanol (overnight), only 152 giving the correspond-... 

Other inverting glucosidases which conform to the pattern of direct hydrolysis of glycosyl fluorides having the correct anomeric configuration, and transglycosylation with inversion if the anomeric configuration is opposite to that of the natural substrates are trehalase from rabbit renal cortex and from the yeast Candida tropicalis, and ) -D-xylosidase from Bacillus pu-milis. ... 

Evidence for a glycosyl-enzyme intermediate of finite lifetime with inverting a-D-glycosidases, and details of its reaction, came from studies with 2,6-anhydro-l-deoxyhept-l-enitols and glycosyl fluorides. - Analysis of hydration and hydrolysis products on the one hand, and of glycosyla-tion products on the other, indicated an intermediate that could be approached by water from the yff-face only of the ring, and by other glycosyl acceptors only from the a-face (see Schemes 4 and 5 This can be considered a proof of the principle of microscopic reversibility of chemical reactions. 

In the first case, typical reactions are the formation of the methyl fi-D-glucoside from a-D-glucopyranosyl fluoride and of its 2-amino-2-deoxy derivative from 2-amino-2-deoxy-a-D-glucopyranosyl fluoride, on treatment with sodium methoxide in methanol. The products from the reaction of aqueous bases with the glycosyl fluorides depend on the concentration of alkali. At low concentrations, the normal hydrolysis products are formed. At higher concentrations of base, if the proper (trans) steric relation exists between C-6 and the fluorine atom at C-1, anhydro compounds are formed, as in (21) from (20). 

Buten-2-yl glycosides can also be transformed to glycosyl fluorides and trichloroacetimi-dates by hydrolysis to the corresponding hemiacetal (HgO, HgBr2, aq. acetone) followed by standard treatment (CCI3CN, DBU, CH2CI2 or DAST, THF, respectively) [183]. 

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