Conformational Analysis of Indole Alkaloids Corynantheine and Dihydrocorynantheine by Dynamic 1H NMR Spectroscopy and Computational Methods: Steric Effects of Ethyl vs Vinyl Group

H-1 NMR (400 MHz) spectra of the indole alkaloid dihydrocorynantheine recorded at room temperature show the presence of two conformers near coalescence. Low temperature H-1 NMR allowed characterization of the conformational equilibrium, which involves rotation of the 3-methoxypropenoate side chain. Line-shape analysis yielded enthalpy of activation DeltaH(double dagger) = 71 +/- 6 kJ/mol, and entropy of activation DeltaS(double dagger) = 33 +/- 6 J/mol(.)K. The major and minor conformation contains the methyl ether group above and below the plane of the ring, respectively, as determined by low-temperature NOESY spectra, with free energy difference DeltaG degrees = 1.1 kJ/mol at -40 degreesC. In contrast to dihydrocorynantheine, the corresponding rotamers of corynantheine are in the fast exchange limit at room temperature. The activation parameters determined for corynantheine were DeltaH(double dagger) = 60 +/- 6 kJ/mol and DeltaS(double dagger) = 24 +/- 6 J/mol(.)K, with DeltaG degrees = 1.3 kJ/mol at -45 degreesC. The difference in the exchange rates of the rotamers of corynantheine and dihydrocorynantheine (respectively, 350 s(-1) and 9 s(-1) at 0 degreesC) reflects the difference in the steric bulk of the vinyl and the ethyl group. The conformational equilibria involving the side chain rotation as well as inversion of the bridgehead nitrogen in corynantheine and dihydrocorynantheine was studied by force-field (Amber(*) and MMFF) and ab initio (density-functional theory at the B3LYP/6-31G(*) level) computational methods, the results of which were in good agreement with the H-1 NMR data. However, the calculations identified the rotamers as essentially isoenergetic, the experimental energy differences being to small to be reproduced exactly by the theory. Comparison of density-functional and force-field calculations with experimental results identified Amber(*) as giving the most accurate results in the present case.