Statine and its derivatives. Conformational studies using nuclear magnetic resonance spectrometry and energy calculations

Abstract
The conformations of derivatives of 3(S)-hydroxy-4(S)-amino-6-methylheptanoic acid (statine) and its analogs were studied by MMR in CHCl3 and in dimethyl sulfoxide, and by molecular mechanics calculations. The coupling constant between NH and C4H was large, suggesting that the dihedral angle (.theta.) is near 165.degree. or 0.degree.. The coupling constant between C4H-C3H was small, indicating a vicinal bond angle of .apprx. 90.degree.. The H-2H exchange rate or statine amide protons was slow; the rate was dependent upon the electron withdrawing substituents adjacent to the amide NH''s. Intramolecular H bonds involving the NH of the statine amide group did not stabilize conformations of single amino acid derivatives. Based on the MMR results, 4 possible conformations of tert-butyloxycarbonyl-statine-O methylester in solution are possible. Molecular mechanics calculations indicated 1 conformation is especially likely.