Dynamic NMR study of cyclic derivatives of pyridoxine

Abstract

A series of pyridoxine derivatives was investigated by (1) H and 2D nuclear overhauser enhancement spectroscopy (NOESY) NMR. The free energies of activation for the pyridyl-oxygen rotation of the 2,4-dinitrophenyl ether of the seven-membered acetals of pyridoxine were measured by dynamic NMR. A conformational exchange between the chair and twist forms of the seven-membered acetal ring was confirmed by dynamic NMR and STO3G computations.

Stereodynamics of some pyridoxine derivatives

“…The results are presented in Table . Obtained energy barriers are in good agreement with literature data and the values of previously studied pyridoxine derivatives …”

“…The comparison of obtained results with our previous studies reveals the significant influence of dynamics of the adjacent conformational processes on each other. It was found that an increase in the seven‐membered cycle transformation rate leads to the raise of activation energies for the 2,4‐dinitrophenyl group rotation (50–53 kJ/mol for acetal derivatives, 46 kJ/mol for compound III , and 41–44 kJ/mol for ketals). The rotation of 2‐nitrophenyl group around one single bond is less dependent on the changes of distant substituents, and its activation barriers remain almost the same for all studied compounds.…”

“…Obtained energy barriers are in good agreement with literature data and the values of previously studied pyridoxine derivatives. [11,10] Semi-empirical calculations PM3 calculations of rotational barriers using the HyperChem 8 software were carried out in order to relate experimental activation barriers with the conformational processes. During the calculations of the rotation of 2-nitrophenyl moiety, values of the torsion angle φ1 (Fig.…”

“…One is a transformation of seven‐membered cyclic fragment between the chair and the twist forms. Another is a rotation of 2,4‐dinitrophenyl fragment of the molecules around a rather flexible connection to the pyridine ring by two single bonds . Correlations between the steric structure and reactivity of such compounds were also revealed …”

“…Another is a rotation of 2,4-dinitrophenyl fragment of the molecules around a rather flexible connection to the pyridine ring by two single bonds. [10][11][12] Correlations between the steric structure and reactivity of such compounds were also revealed. [13] In this paper, we present the results of dynamic NMR study of three new pyridoxine derivatives, a potent nonlinear optical materials [14] : 3-isopropyl-8-methyl-6-(2-nitrophenyl)-1,5-dihydro- [1,3]dioxepino [5,6-c]pyridine-9-ol (I), 3,3,8-trimethyl-6-(2-nitrophenyl)-1,5-dihydro- [1,3]dioxepino [5,6-c]pyridine-9-ol (II), and 9-(2,4dinitrophenoxy)-3,3,8-trimethyl-6-(2-nitrophenyl)-1,5-dihydro- [1,3] dioxepino [5,6-c]pyridine (III) (Fig.…”