EXPERIMENTAL STUDIES OF THE ANOMERIC EFFECT. PART 2 RING INVERSION AND NITROGEN INVERSION EQUILIBRIA IN CIS-DECAHYDROQUINAZOLINES

Article abstract of DOI:10.1016/S0040-4020(01)85926-3

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 (13)C n.m.r. spectroscopy in the range 215-225 K.In both cases there is a very strong preference (-ΔG⁰_7-->8 1.08 kcal mol^-1; -ΔG⁰_9-->10 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.