A-glucosyl fluoride

Maltose and maltotriose were synthesized from a-glucosyl fluoride by all the a-amylases. Maltotetraose and sizeable amounts of the pentaose... 

The kinetic isotope effects Might/Mieavy, for the hydrolysis at pH 6.0 of a-glucosyl fluoride at 80 °C and /J-glucosyl fluoride at 50 °C, isotopically substituted as shown in structure 352, have been found381 to be as follows ... 

BEBOVIB-IV-TS structure for a-glucosyl fluoride hydrolysis involving the ring in a flattened 4C chair conformation and water and fluoride ion about the anomeric centre has been presented381,382. The conformations based on this structure reproduced well the a-D, j8-D, 5-D and [5-180] experimental KIE, but not the [1-13C] KIE (theor. KIE = 1.023 compared with 1.032 for exp. KIE). 

The dotted lines are meant to convey a minimal degree of covalent bonding kinetic isotope effect data for hydrolysis of a-glucosyl fluoride were fitted to a transition state structure with bond orders to leaving group and nucleophile of only 0.001. 

Figure 3.37 Internal return in the solvolysis of a-glucosyl fluoride and in a classical example from solvolysis chemistry, the acetolysis of exo-norbornyl p-toluenesulfonate.

H. P. Wessel and N. Ruiz, a-Glucosylation reactions with 2,3,4,6-tetra-0-benzyl-/j-D-glucopyranosyl fluoride and triflic anhydride as promoter, J. Carbohydr. Chem., 10 (1991) 901-910. 

A small piece of Na was added to MeOH (5 mL) and the resulting solution was added to tetra-O-acetyl-a-D-glucopyranosyl fluoride (130 mg, 0.37 mmol) and 4,6-di-6>-acetyl-2-deoxy-2-fluoro-3-0-(tetra-6>-acetyl-/3-D-glucopyranosyl)-/ -D-glucosyl fluoride (44 mg, 0.07 mmol) in MeOH (5 mL) and the solution stirred (1 h) before being neutralized with Amberlite resin IR-120 (H+), fdtered and concentrated to afford a colourless residue. 

Table I (104) shows the yields of products from maltose to malto-pentaose recovered from digests of 0.2 M crystalline a-D-glucosyl fluoride with crystalline a-amylase preparations from six different biological sources. The digests were incubated at 30 °C for 10 minutes, heat inactivated, and chromatographed for product isolation and analysis.

Table I. Maltosaccharide Synthesis from a-D-Glucosyl Fluoride...

Table II (104) shows the results of an experiment to learn the proportion of a-D-glucosyl fluoride that is hydrolyzed vs. that used for saccharide synthesis by the different crystalline a-amylases and by diluted saliva. The substrate was again 0.2 M and the enzymes at one-tenth to one-one hundredth the concentrations of the preceding experiment. These digests were analyzed directly for amounts of fluoride (104, 107) and D-glucose (glucose oxidase method) liberated during 2 hours at 30°C.

All the amylases caused release of more fluoride than D-glucose, both expressed as percent of substrate. The difference between them gives the percent substrate converted to saccharides. The main point is that with six of the seven a-amylases two to twenty times more a-D-glucosyl fluoride was converted to oligosaccharides than hydrolyzed to D-glucose. The great discrepancy was readily confirmed by direct visualization of the products by chromatography. In this experiment, preparations of so-... 

Table II. Saccharide Synthesis vs. a-D-Glucosyl Fluoride Hydrolysis by Different a-Amylases °...

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