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Keywords:

  • aldol reaction;
  • aldolases;
  • asymmetric synthesis;
  • biotransformations;
  • lyases

Abstract

A novel serine hydroxymethyl transferase from Streptococcus thermophilus (SHMT) and a L-threonine aldolase from Escherichia coli (LTA) were used as stereocomplementary biocatalysts for the aldol addition of glycine to N-Cbz amino aldehydes and benzyloxyacetaldehyde (Cbz=benzyloxycarbonyl). Both threonine aldolases were classified as low-specific L-allo-threonine aldolases, and by manipulating reaction parameters, such as temperature, glycine concentration, and reaction media, SHMT yielded exclusively L-erythro diastereomers in 34–60 % conversion, whereas LTA gave L-threo diastereomers in 30:70 to 16:84 diastereomeric ratios and with 40–68 % conversion to product. SHMT is among the most stereoselective L-threonine aldolases described. This is due, among other things, to its activity–temperature dependence: at 4 °C SHMT has high synthetic activity but negligible retroaldol activity on L-threonine. Thus, the kinetic L-erythro isomer was largely favored and the reactions were virtually irreversible, highly stereoselective, and in turn, gave excellent conversion. It was also found that treatment of the prepared N-Cbz-γ-amino-β-hydroxy-α-amino acid derivatives with potassium hydroxide (1 m) resulted in the spontaneous formation of 2-oxazolidinone derivatives of the β-hydroxyl and γ-amino groups in quantitative yield. This reaction might be useful for further chemical manipulations of the products.