Deuterium labelling experiments and nmr studies indicate that cyclisations of N-acyl-N-(2- phenylethyloxy) nitrenium ions occur via direct attack at the ortho position to give 34-dihydro- 1 H-21-benzoxazines. In contrast N-acyl-N-(3-phenylpropyloxy) nitrenium ions cyclise to 1345-tetrahydro-21-benzoxazepines through ipso attack followed by 12-carbon migration. In both cases hydrogen circumambulation occurs in the sigma complex before aromatisation.
