5. Target glycosides synthesized by thermophilic glycosidases

The knowledge of the selectivity of the thermophilic glycosidases allows planning of chemoenzymatic synthesis of target useful glycosides.Interesting glycosides containing a double bond can be synthesized by thermophilic glycosidases. The importance of these compounds relies on the possibilities of transforming their double bond chemically, for the production of useful substrates for other enzymatic reactions Ref.21 or for the synthesis of modified polymers possessing hydrophilic properties Ref.6-16. Different models of unsaturated substrates have been used for the high-yield (60-80%) synthesis of corresponding beta- (Sulfolobus) and alfa- (Thermus) glycosides: allyl and propargyl alcohols (2-3, Fig. 1C) and cis-2-butene-1,4-diol (4, Fig. 1C).

Isolated yields of allyl--D-gluco- and galactosides are very competitive with respect to those obtained using mesophilic enzymes Ref. 6 (9-13%) probably due to the high resistance of the thermophilic catalyst to denaturing agents such as unsaturated alcohols Ref.21. Chemical modification of cis-2-butene-4-hydroxy-1-O--D-glucoside, and cis-2-butene-4-hydroxy-1-O-alafa-D-glucoside, results in interesting compounds such as glucosyl-epoxydes or selectively glucosylated erythritol diastereomers which, as peracetylated derivatives, can be substrates for other biocatalysts such as lipases.


The use of protected derivatives of polyols Ref.21, such as compounds (5, and 6, Fig. 1C) is another interesting aspect in view of preparing target glycosides precursor for natural and modified glycolipid synthesis Ref.22.Example of preparation of naturally occurring glycosides is the synthesis and stereochemical determination of aleppotrioloside Ref.20 (compounds 15-18, Fig. 1B). In this example the spectacular selectivity toward primary hydroxyl group (100%) observed permitted a simple chemo-enzymatic synthesis avoiding chemical protection/deprotection procedures.


The use of aromatic compounds bearing phenolic and aliphatic hydroxyl groups Ref.23 as substrates for glycosylation is interesting since many natural biologically active plant glycosides contains such aglycones. Compound 7 (Fig. 1C), the aglycone of a natural glycoside found in Betula platyphylla var. japonica Ref. 24, was used as starting material with various mesophilic and thermophilic enzymatic sources in different reaction conditions for the synthesis of corresponding glycoside at aliphatic hydroxyl group. Reaction conditions (high excess of acceptor) for obtaining high yield of the desired product were suitable for thermophilic -glycosidase from Sulfolobus Ref. 23.


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