Chemoselective biocatalytic reduction of conjugated nitroalkenes: new application for an Escherichia coli BL21(DE3) expression strain.

Chemoselective reduction of activated carbon-carbon double bond in conjugated nitroalkenes was achieved using Escherichia coli BL21(DE3) whole cells. Nine different substrates have been used furnishing the reduced products in moderate to good yields. 1-Nitro-4-phenyl-1,3-butadiene and (2-nitro-1-propenyl)benzene were successfully biotransformed with corresponding product yields of 54% and 45% respectively. Using this simple and environmentally friendly system 2-(2-nitropropyl)pyridine and 2-(2-nitropropyl)naphthalene were synthesized and characterized for the first time. High substrate conversion efficiency was coupled with low enantioselectivity, however 29% enantiomeric excess was detected in the case of 2-(2-nitropropyl)pyridine. It was shown that electronic properties of the aromatic ring, which affected polarity of the double bond, were not highly influential factors in the reduction process, but the presence of the nitro functionality was essential for the reaction to proceed. 1-Phenyl-4-nitro-1,3-butadiene could not be biotransformed by whole cells of Pseudomonas putida KT2440 or Bacillus subtilis 168 while it was successfully reduced by E. coli DH5α but with lower efficiency in comparison to E. coli BL21(DE3). Knockout mutant affected in nemA gene coding for N-ethylmaleimide reductase (BL21ΔnemA) could still catalyze bioreductions suggesting multiple active reductases within E. coli BL21(DE3) biocatalyst. The described biocatalytic reduction of substituted nitroalkenes provides an efficient route for the preparation of the corresponding nitroalkanes and introduces the new application of the strain traditionally utilized for recombinant protein expression.