Stereochemical Analysis by Solid-State NMR:  Structural Predictions in Ambuic Acid

Relative stereochemistry is predicted for ambuic acid using a novel solid-state NMR approach. This NMR technique entails a comparison of measured shift tensor principal values with computed values for all diastereomers, allowing the selection of a best-fit structure. The proposed method extends previous solution NMR structural data by simultaneously modeling with high statistical probability hydrogen-bonding arrangements and molecular conformation at two positions. A dimeric structure is proposed for ambuic acid based on the initial poor fit of the carboxyl carbon tensors to a monomeric model. The dimer model, consisting of hydrogen bonding between pairs of neighboring carboxyl groups, reduces the root mean square error at the carboxy tensor by a factor of 2.7. Lattice details are thus also described by the proposed approach. The structural characterization method presented is of general applicability and may be especially useful for characterizing difficult to crystallize or hydrogen-poor materials.