Substituent interactions in slow-inverting aziridines

Series of aziridine esters bearing 2,3-dihydro-2-oxobenzoxazol-3-yl (II), phthalimido (III), 3,4-dihydro-2 methyl-4-oxoquinazolin-3-yl (IV), and 1,2-dihydro-2-oxoquinolin-1-yl (V) groups as N-substituents have been synthesised. All these aziridines show slow inversion on the n.m.r. time scale at room temperature and an explanation is offered for an unexpected change in the invertomer ratio as the size of the ester alkyl group is increased. The invertomer ratios of 3-(2-methyl-2-vinylaziridin-1-yl) benzoxazol-2-(3H)-one and 3-(trans-2-methyl-3-vinylaziridin-1-yl) benzoxazol-2-(3H)-one are compared.