M. Alajarin et al., Surpassing torquoelectronic effects in conrotatory ring closures: Origins of stereocontrol in intramolecular ketenimine-imine [2+2] cycloadditions, CHEM-EUR J, 5(3), 1999, pp. 1106-1117
Intramolecular ketenimine-imine [2+2] cycloadditions leading to 1,2-dihydro
azeto[2,1-b]quinazolines occur in a notably stereocontrolled manner along t
he newly formed C-C single bond when prochiral ketenimine and imine fragmen
ts are combined. Computational studies support stepwise mechanisms for thes
e reactions. In sharp contrast with other well-known [2+2] cycloadditions,
the first step of the reaction determines its stereochemical outcome, there
by surpassing the low stereocontrol induced by torquoelectronic effects.