Conformation of 5-Amino-5-deoxypentonolactams

Four configuration isomers of 5-amino-5-deoxy-D-pentonolactam 1a-4a and their tri-O-acetyl derivatives 1b-4b were studied using NMR and CD spectroscopy. For all compounds chemical shifts of the 1H and 13C nuclei as well as of vicinal coupling constants were obtained. Comparison of the observed 3J(H,H) with those calculated for various conformations by a modified Karplus relationship led to the assignment of predominant conformation 3H4(D) or 4H3(D) to the lactams 1a-4a and 1b-4b in solution. The most important factor for determining the conformation seems to be the pseudoequatorial position of the substituent on the carbon next to the carbonyl group. The results of the CD spectra of the lactams 1a-4a in water, interpreted according to the currently used rules, agreed with the NMR results.

Conformation of 6-amino-6-deoxyhexonolactams

Eight configurational isomers of 6-amino-6-deoxyhexonolactam Ia-VIIIa and their tetra-O-acetyl derivatives Ib-VIIIb were studied using NMR and CD spectroscopy. For all the compounds most values of chemical shifts of the 1H and 13C nuclei as well as of vicinal coupling constants were obtained. Comparison of the observed values with 3J(H,H) values, calculated for various conformations of the studied compounds by a modified Karplus relationship, led to assignment of predominant conformation 1,NC4(D) or 4C1,N(D) to the lactams Ia-VIIIa and Ib-VIIIb in solution. For the measured set of compounds, the decisive conformation-determining requirement seems to be the equatorial position of substituent on the carbon next to the carbonyl group. The CD spectra of the lactams Ia-VIIIa in water, interpreted according to the currently used rules, in six cases agreed and in two cases disagreed, with the NMR results. The reasons of this discrepancy are discussed.

Conformational analysis in carbohydrate chemistry. I. Conformational free energies. The conformations and α : β ratios of aldopyranoses in aqueous solution

The relative free energies of the aldopyranoses in aqueous solution have been calculated, taking non-bonded interaction energies and the anomeric effect into account. It is shown that the calculated free-energy values correctly predict the predominant conformation of the α- and β-pyranose forms of each aldose. The α- to β-pyranose ratios of the aldoses in aqueous solution, calculated from these values, are in reasonable agreement with those determined experimentally.