Dynamic NMR study of dinitrophenyl derivatives of seven‐membered cyclic ketals of pyridoxine

Abstract

Two pyridoxine derivatives containing a dinitrophenyl moiety were investigated by (1)H NMR spectroscopy. Conformational dynamics in solution were studied for each compound using dynamic NMR experiments. It was shown that both compounds studied are involved into two conformational exchange processes. The first process is a transformation of the seven-membered cycle conformation between the enantiomeric P-twist and M-twist forms, and the second is a rotation of the dinitrophenyl fragment of the molecules around the C-O bond. Energy barriers of both conformational transitions were determined.

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 …”

“…Taking into account the nonlinear temperature dependence of the exchange rate constant for the CH 2 ‐4 signal (Fig. ), an approach based on the decomposition of lines corresponding to two processes in the program Origin 7.5 was used for the calculation . The results are presented in Table .…”

“…The activation barrier for the R′ process for compound III was calculated as described in our previous paper by independent rotating of torsion angles φ 2 and φ 3 . The resulting potential energy surface is shown in Fig.…”

“…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.…”

“…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 …”