On Nitrogen Coordination in Vincadifformine-Type Alkaloids
2017
Adizov, Sh. M. | Tashkhodzhaev, B. | Bruskov, V. P. | Talipov, S. A. | Yuldashev, P. Kh | Malikov, V. M.
X-ray crystal structure analyses (XSAs) were performed for the alkaloid (±)-vincadifformine (racemate) isolated from Vinca minor and (–)-vincadifformine methyliodide and the natural 5-oxo derivatives of (–)-vincadifformine ervinidinine and ervincinine isolated from Vinca erecta. The XSA results showed that tetrahedral N4 inverted on going from the base to the salt form of vincadifformine. The calculated heats of formation of the alkaloid in the two states indicated that these inverted forms were similar with a low (6 kcal/mol) barrier to inversion. The unshared electron pair of N4 was conjugated to the π-electrons of the neighboring ketone and N4 adopted trigonal-planar coordination in the 5-oxoderivatives ervinidinine and ervincinine. Conjugation of the unshared electron pairs on N1 and N4 was one of the main reasons for the alkaloids forming salts under ordinary conditions.
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