4. Stereochemistry of the transfer reaction using simple glucose mimicking molecules

1,2-propanediol (1, Fig. 1B) is a simple diol molecule used as model substrate to investigate the stereochemistry of thermophilic glycosidases (Table 1) from different sources in the transglycosylation reaction using as donors aromatic glycosides such as alfa - or beta- gluco- galacto- and fuco- pyranosides. The overall results listed in Table 1 suggest, for the enzymes analyzed,

a similar situation regarding the selectivity between primary and secondary hydroxyl groups. Differences are encountered in the enantioselectivity for the transfer to the secondary hydroxyl group; at least two thermophilic enzymes (Thermus and Sulfolobus) possess high enantioselectivity, moderate enantioselectivity is encountered by using Caldocellum. Mesophilic -galactosidase from E. coli is not enantioselective Ref.18. These data probably reflect the different conformation of the active sites of the analyzed enzymes. Some of these results are obtained using crude homogenates and it is probable that the use of pure enzyme (Pyrococcus) will result in an increase of selectivity. The stereochemistry of the transfer of -galactose with thermophilic enzyme from Sulfolobus, studied with compounds 2-9 of Fig. 1B Ref.19 is similar to that of 1,2-propanediol.

Interesting is the functionalization of diol 8 in forming the monogalactosylated product useful for successive synthetic elaboration Ref.9.Increasing the complexity of the molecule being glycosylated as for example in compounds 15-19 of Fig. 1B we observed both an increase of regioselectivity (100% towards primary hydroxyl group in structures 15-17 of Fig. 1B) and the emergence of a certain degree of diastereoselection also for the transfer to primary hydroxyl group Ref. 20. The remaining compounds 10-14 and 18-19 (Fig. 1B) are less hindered molecules and for this reason are susceptible of glycosidation at each hydroxyl group, nonetheless they present interesting results when comparing the different proportions of diasteromers formed Ref. 21. The body of stereochemical results obtained for these compounds indicates that the primary hydroxyl groups are always favoured compared to the secondary hydroxyl groups, although the overall regioselectivity of the reaction depends upon the complexity of the molecule. Yields of reaction follow similar trend; compounds containing primary hydroxyl groups give higher yields with respect to secondary alcohols. Exceptional high yields 80-90% are however encountered for compounds 10-14 (Fig. 1B) and this result has been related to the particularly favourable interactions of these glucose mimicking molecules in the active site of the enzyme Ref. 21.

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