Conformational behavior of methyl (3R)-3-{[(3'R)-3'-hydroxybutanoyl]oxy}butanoate in solutions: Effect of intramolecular hydrogen bond

Methyl (3R)-3-{[(3'R)-3'-hydroxybutanoyl]oxy}butanoate (MHBOB) has been prepared as a model compound of the hydroxy terminal part of poly[(R)-3-hydroxybutanoate] [P(3-HB)]; the NMR assignments and conformational analyses of the compound are reported. The NMR assignments of MHBOB were made by means of two-dimensional pulsed field gradient H-1-H-1 COSY and H-1-C-13 HMBC (heteronuclear multiple-bond correlation) spectroscopy. The conformational behavior of MHBOB generated by rotation about two CH2-CH bonds was investigated in chloroform and in aqueous. solution at various temperatures by analysis of vicinal coupling in the 500-MHz (HNMR)-H-1 spectra. In both chloroform and aqueous solution, the monomer unit adjacent to the methoxy group was found to adopt a similar conformational distribution to that of the P(3-HB) polymer backbone, in which the trans and gauche conformers were predominant, while the another gauche conformer was suppressed to almost zero. On the other hand, the monomer unit adjacent to the hydroxy group had a different conformational distribution relative to that of the other monomer unit, due to the formation of an intramolecular hydrogen bond between hydroxy and carbonyl groups. In chloroform, only the gauche conformer was predominant at temperatures between -60 and 57 degrees C, and even the fraction of another gauche conformer was higher than that of trans conformer at low temperatures below 0 degrees C. In aqueous solution, the fraction of gauche conformer was also higher than that of trans conformer, suggesting the formation of the intramolecular hydrogen bond. However, the formation of the intramolecular hydrogen bond in aqueous solution was not so strong as that in chloroform.