Magnetic Resonance in Chemistry p. 495 - 504 (1996)
Update date:2022-08-25
Topics:
Palmer, Robert B.
Andro, Tina M.
Natale, Nicholas R.
Andersen, Niels H.
A series of 14 4-(3′,5′-disubstituted isoxazolyl)-1,4-dihydropyridine calcium channel antagonists were examined using variable-temperature proton nuclear magnetic resonance spectroscopy and nuclear Overhauser effect (NOE) experiments. Two of these compounds, the 1,4-dihydro-2,6-dimethyl-4-[5′-methyl-3′-(4″-fluorophenyl) isoxazol-4′-yl]-3,5-pyridinedicarboxylic acid dimethyl ester (3a) and 1,4-dihydro-2,6-dimethyl-4-[5′-methyl-3′-(4″-bromophenyl) isoxazol-4′-yl]-3,5-pyridinedicarboxylic acid dimethyl ester (5a), were synthesized to assist in the clarification of ambiguities discovered in the low-temperature spectra of 1,4-dihydro-2,6-dimethyl-4-(5′-methyl-3′-phenylisoxazol-4′yl)- 3,5-pyridinedicarboxylic acid diethyl ester (2b). The solid-state structure of 3a is also reported. The solution-state rotameric preferences of the 14 compounds are reported and compared with those calculated at the AM1 level. C-4 - C-4′ bond rotation barriers were also calculated at the AM1 level for nine of the systems examined. Several species failed to display temperature-dependent signals associated with hindered rotation owing to highly biased rotameric equilibria at the temperatures required to freeze out the rotation. The seven experimental rotation barriers (ΔG≠ from ≤26 to 40.4 kJ mol-1) reported are 1-6.8 kJ mol-1 higher than ΔH≠ calculated at the AM1 level).
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