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Dynamic NMR Analysis of Conformational Isomerism in a Diterpenoid Carboxamide
Author(s) -
Cambie Richard C.,
Taylor Michael J.,
Rutledge Peter S.,
Stevenson Ralph J.,
Woodgate Paul D.
Publication year - 1996
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/(sici)1097-458x(199605)34:5<360::aid-omr887>3.0.co;2-4
Subject(s) - chemistry , carboxamide , activation energy , hydrogen bond , stereochemistry , carbon 13 nmr , crystallography , molecule , organic chemistry
Conformational changes in N , N ‐diethyl‐14‐hydroxy‐12,19‐dimethoxypodocarpa‐8,11,13‐triene‐13‐carboxamide were studied by variable‐temperature 1 H and 13 C NMR over the range 228–328 K using three hydrogen and six carbon indicator sites. The parameters Δ H ≠ = 48.7±2.0 kJ mol −1 , Δ S ≠ = −23±7 J K mol −1 and Δ G 298 ≠ = 55.6±4.3 kJ mol −1 were obtained, which were attributed to restricted rotation of the C‐CONEt 2 bond. Restricted C—N rotation was also detected for which the free energy of activation at 250 K was ca . 53 kJ mol −1 . Hydrogen bonding of the carboxamide to the adjacent hydroxy group can account for the low value of the C—N activation energy.