NMR and theoretical study on the structure of O,O-dimethyl-1,4-oxoazetin-2-ylphosphonate

Enantio convergent biotransformation of O,O-dimethyl-4-oxoazetidin-2-ylphosphonate using fungal cells of Penicillium minioluteum and purified enzymes

This report presents the bioconversion of O,O-dimethyl-4-oxoazetidin-2-ylphosphonate 1 performed in two ways: with the enzymatic system of P. minioluteum and with the application of purified enzymes: penicillinase and two proteases of different origin. Recorded NMR spectra allowed confirming the reaction progress and also postulating possible mechanism of conversion. The path of bioconversion was defined as enantio convergent process for both modes of applied biocatalysts. This means that kinetically driven resolution of racemic mixture of the substrate leads to the one enantiomer of the product. The bioconversion started from ester bond hydrolysis (equally in both enantiomers) with the conversion degree from 30% (whole-cell) to 35% (isolated enzymes) and with the production of optically pure monoester (compound 2; 100% of e.e). For whole-cell bioprocess it was the initiative step for the enantioselective amide bond hydrolysis, what resulted in synthesis of desired product 3-amino-3-phosphonopropanoic acid 4. However, the most effective enzymatic hydrolysis of ester bond performed with penicillinase from Enterobacter cloacae led only to the monoester product 2.

Conformational analysis of the chemical shifts for molecules containing diastereotopic methylene protons

Quantum chemistry SCF/GIAO calculations were carried out on a set of compounds containing diastereotopic protons. Five molecules, including recently synthesized 1,3-di(2,3-epoxypropoxy)benzene, containing the chiral or pro-chiral center and the neighboring methylene group, were chosen. The rotational averages (i.e. normalized averages with respect to the rotation about the torsional angle τ with the exponential energy weight at temperature T) calculated individually for each of the methylene protons in 1,3-di(2,3-epoxypropoxy)benzene differ by ca. 0.6 ppm, which is significantly less than the value calculated for the lowest energy conformer. This value turned out to be low enough to guarantee the proper ordering of theoretical chemical shifts, supporting the interpretation of the (1)H NMR spectrum of this important compound. The rotational averages of chemical shifts for methylene protons for a given type of conformer are shown to be essentially equal to the Boltzmann averages (here, the population-weighted averages for the individual conformers representing minima on the E(τ) cross-section). The calculated Boltzmann averages in the representative conformational space may exhibit completely different ordering as compared to the chemical shifts calculated for the lowest-energy conformer. This is especially true in the case of molecules, for which no significant steric effects are present. In this case, only Boltzmann averages account for the experimental pattern of proton signals. In addition, better overall agreement with experiment (lower value of the root-mean-square deviation between calculated and measured chemical shifts) is typically obtained when Boltzmann averages are used.