Pyridine carbohydrate substrates

V-(4,6-0-Benzylidene-l-0-methyl-3-oximino-a-D-r bo-hexopyranos-2-yl)pyridin-ium p-Toluenesulfonate. A Novel Versatile Carbohydrate Substrate, W. A. Szarek, B. T. Lawton, and J. K. N. Jones, Tetrahedron Lett., (1970) 4867—4870. 

Surprisingly, the introduction of the pyridine ring not only influences the velocity of the enzymatic transformations, but also induces promising stereochemical effects (Table 1). For instance, at 40% conversion (R)-phenylethanol is obtained from the pyridyl acetate 25 with 73 % ee, whereas the value for the corresponding phenylacetate is only 28%. Also, the secondary alcohol liberated from the ester 26 displays 98% ee at 40% conversion, whereas the respective phenylacetate leads to 1-phenylpropanol with 94% ee but at a conversion rate of 12% only [19,20]. These results demonstrate that the stereoselecting properties of penicillin acylase may be enhanced by appropriate engineering of the substrate. This is of particular interest since this enzyme has already been used for the kinetic resolution of various chiral alcohols [21-24], e.g. furyl alkyl carbinols [24], which are valuable precursors for the de novo synthesis, with moderate to high ee values, of carbohydrates. 

In the context of heterogeneous palladium catalysts, the previously mentioned Pd/C catalysts are commonly used for water-soluble substrates, i.e., carbohydrates [91]. Other examples of heterogeneous Pd catalysts are rare. Recently it was shown that in addition to ruthenium, palladium can also be introduced in the brucite-layer of the hydrotalcite [92]. As with Ru/Co-hydrotalcite (see above), apart from benzylic and allylic also aliphatic and cyclic alcohols are smoothly oxidized using this palla-dium-hydrotalcite. However a major shorthcoming is the necessity of at least 5 mol% catalyst and the co-addition of 20-100 mol% pyridine. 

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