Glucosyl donor

Various strains of oral streptococci produce D-glucosyltransferases which utilize sucrose as a o-glucosyl donor in the production of soluble and insoluble D-glucans. Consequently, it may be expected that some deoxyfluoro derivatives of sucrose function as competitive inhibitors for the dextransu-crases of tooth bacteria, thus preventing decay, or at least may be used as active-site probes for the enzymes. Another aim of these researches is to find non-metabolizable sweeteners. 

Polymer-supported glucal 37 was converted to the protected thioethyl glucosyl donor 39 as outlined in Scheme 2.11. Compound 37 was first epoxidized by the action of DMDO. The resulting 1,2-anhydrosugar was opened by a mixture of ethanethiol and dichloromethane (1 1) in the presence of a trace of trifluoroacetic acid. Polymer-bound 38 was thus obtained in 91% yield. This was a substantial improvement over the 78% yield obtained by the same protocol in solution. Protection by reaction with pivaloyl chloride occurred in quantitative yield to furnish 39a. 

Scheme 2.11 Synthesis of polymer-bound thioethyl glucosyl donor.

In contrast to animals, bacteria such as E. coli synthesize glycogen via ADP-glucose rather than UDP-glucose.88 ADP-glucose is also the glucosyl donor for synthesis of starch in plants. The first step in the biosynthesis (Eq. 20-18) is catalyzed by the enzyme ADP-glucose pyrophosphorylase (named for the reverse reaction). 

Incorporation of D-glucosyl branches that modify part of the D-galac-tosyl residues of the main chains in polymers 10b, lib, and 12 occurs with polyprenyl D-glucosyl monophosphate (2) as the D-glucosyl donor.60-62,286,289,310... 

Next, pyralomicin 2c (38) was synthesized from 32 and 37 (16). The glucosyl donor 37 was prepared from benzyl a-D-glucopyranoside by methoxymethylation followed by hydrogenolysis. The stereoselective N-glycosylation of 37 with 32 was effectively accomplished by using Mitsunobu conditions to give 38, after acidic deprotection. This was identical in all respects with the natural product 38 (16). 

Reaction of this fucose derivative with the glucosyl donor 54 (oe 3 2 1) in the presence of catalytic amounts of BF3 Et20 cleanly provided the desired p-configurated disaccharide 55. Deprotection followed by acylation of the resulting diol 55a with 6-heptenoic acid in the presence of DCC and DMAP... 

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