Abstract
1α-[19- 13C]Hydroxyvitamin D 3 ( 6) was synthesized and its conformational equilibrium was investigated by means of variable temperature 13C NMR spectroscopy. The A-ring of 6 was found to exist in an equilibrium between two conformations in the ratio 52:48 at −100°C. The rate constants of the equilibrium were estimated by signal shape analysis according to the Gutowsky-Holm formulation. The thermodynamic parameters Δ G 188 K ≠ (coalescence temperature), Δ H ≠, and Δ S ≠ were deduced from the exchange rate to be 8.0 kcal/mol, 6.3 kcal/mol, and −9.0 cal/mol · deg, respectively.
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