Add time:08/07/2019         Source:sciencedirect.com

The positions of conformational equilibria due to the double ring inversion in cis-decahydroquinazoline (7 ⇄ 8) and cis-decahydro-3-methylquinazoline (9 ⇄ 10) were determined by 13C n.m.r. spectroscopy in the range 215-225 K. In both cases there is a very strong preference (-ΔG7→8o 1.08 kcal mol-1; -ΔG9→10o 1.10 kcal mol-1) for the conformation which would allow the lone pairs on N(1) and N(3) to lie on the hindered 'inside' face of the molecules. However, the values of 3J(CHNH) coupling constants for both cis-decahydro-3-methylquinazoline (10) and cis (4aH, 8aH) cis (2H, 8aH)-decahydro-2,3-dimethylquinazoline (14) show that the N-inversion equilibrium at N(1) prefers the conformation in which N(1)-H is 'inside' (axial), and N(1)-lone pair equatorial, thus demonstrating the ability of the anomeric effect to outweigh steric repulsions.

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