Novel Me3Si-mediated intramolecular cyclisation/[4+2] cyclofragmentation of beta-substituted gamma-nitro ketones

5-Nitro-4-phenylpentan-2-one (1) was found to undergo hitherto unknown Me3S iBr/Et3N-promoted cyclisation into the intermediate six-membered cyclic nit ronate 5. Subsequent silylation of 5 led to the corresponding 2-trimethylsi lyloxy-2H-[1,2]oxazine C, which undergoes a facile [4 + 2] cyclofragmentati on to give 2-phenylpropenal oxime trimethylsilyl ether (6) in moderate yiel d. The intermediacy of the open-chain trimethylsilyl nitronate 2 and the cy clic nitronate 5 was confirmed by independent experiments. The transformati on of the bicyclic nitronate 8 into the 8-trimethylsilyl-oxyimino-7-phenylo ct-6-enoic acid trimethylsilyl ester (9) upon silylation proved the silylat ion/cycloreversion to be a key step in 1,3-azadiene moiety formation. The p resented sequence of reactions could be carried out in one pot in low yield by a Me3SiOTf/Et3N-promoted hetero-[4 + 2] cycloaddition of 2-(trimethylsi lyloxy)propene to (E)-2-nitro-1-phenyl-ethene, followed by silylation and f ragmentation of the intermediate C.