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.