A chemo-enzymatic route to diastereoisomers of 2-methyl-1-phenyl-1,3-butanediol: the dual role of microorganisms

Assigning stereochemistry to single diastereoisomers by GIAO NMR calculation: the DP4 probability

GIAO NMR shift calculation has been applied to the challenging task of reliably assigning stereochemistry with quantifiable confidence when only one set of experimental data are available. We have compared several approaches for assigning a probability to each candidate structure and have tested the ability of these methods to distinguish up to 64 possible diastereoisomers of 117 different molecules, using NMR shifts obtained in rapid and computationally inexpensive single-point calculations on molecular mechanics geometries without time-consuming ab initio geometry optimization. We show that a probability analysis based on the errors in each (13)C or (1)H shift is significantly more successful at making correct assignments with high confidence than are probabilities based on the correlation coefficient and mean absolute error parameters. Our new probability measure, which we have termed DP4, complements the probabilities obtained from our previously developed CP3 parameter, which applies to the case of assigning a pair of diastereoisomers when one has both experimental data sets. We illustrate the application of DP4 to assigning the stereochemistry or structure of 21 natural products that were originally misassigned in the literature or that required extensive synthesis of diastereoisomers to establish their stereochemistry.

Assigning the stereochemistry of pairs of diastereoisomers using GIAO NMR shift calculation

GIAO NMR chemical shifts have been calculated for a set of 28 pairs of diastereoisomers in order to test the ability of NMR shift calculation to distinguish between diastereomeric structures. We compare the performance of several different parameters for quantifying the agreement between calculated and experimental shifts from the point of view of assigning structures and introduce a new parameter, CP3, based on comparing differences in calculated shift with differences in experimental shift, which is significantly more successful at making correct structure assignments with high confidence than are the currently used parameters of the mean absolute error and the correlation coefficient. Using our new parameter in conjunction with Bayes' theorem, stereostructure assignments can be made with quantifiable confidence using shifts obtained in single point calculations on molecular mechanics geometries without computationally expensive ab initio geometry optimization.