Cycloaddition of acylnitroso compounds and nitrosobenzene with the pyrrolidine dienamine of Pummerer's ketone: X-ray crystal structure of a bridged 1,2-oxazine derivative
When the pyrrolidine dienamine 2 of Pummerer's ketone 1 was treated with nitrosocarbonylmethane 4, generated in situ by periodate oxidation of acetohydroxamic acid, the major product was a bridged 1,2-oxazine, shown to have the structure 10 by X-ray crystallographic analysis. This structure incorporates 2 mol equiv. of nitrosocarbonylmethane with, remarkably, reductive loss of 1 nitroso oxygen, despite the oxidising reaction conditions. The same product was accompanied by a pyrrolidino enone 13 and an acetamido enone 8 when the nitroso dienophile was liberated in situ by retro-Diels–Alder cleavage of the corresponding 9,0-dimethylanthracene cycloadduct 15. The dienamine 2 reacted with nitrosobenzene under various conditions to afford a bridged oxazine 17 incorporating 1 mol equiv. of nitrosobenzene, a phenylamino enone 19, and a bridged amino ketone 20 incorporating 2 equiv. of nitrosobenzene. The last two compounds had also undergone, formally, reductive loss of nitroso oxygen. However, the major product was a nitrogen-free, caged dimer 18 of the 4-oxo derivative of Pummerer's ketone.