N-alkenyl nitrone dipolar cycloaddition routes to piperidines and indolizidines. Part 7. Hydroxylamine–alkyne cyclisations. Formation of cyclic nitrones and application to the synthesis of the proposed structure for (±)-acacialactam

The cyclisation of the alkynylhydroxylamines 2, 13, 14, 19, 27, 34, 39, 46 and 47 to give five-, six- and seven-membered cyclic nitrones is described. A concerted intramolecular ene-like pathway is proposed for the addition of the N–O–H group across the triple bond. Using the nitrone 48 as the starting material, the seven-membered lactam structure 49 proposed for the natural product acacialactam was prepared and was found to be incorrect.