Diels-Alder reactions / Ketenes, y-0x0-1 1,2-Cyclopentanedione derivatives J Adipic acid, substituted 2-0x0-dimethyl esters Cycloadditions of 6H-1.3,4-0xadiazin-6-ones (4,5-Diaza-a-pyrones). 12 -Dieckmann Condensations without Bases Prepared by treatment of methyl 6-oxo-5-phenyl-6H-l,3,4-oxadiazine-2-carboxylate (1) with norbornene, norbornadiene, and styrene, respectively, the y-oxoketenes 2, 5, and ? were allowed to react with methanol. Inter alia, the semiacetals 3, 6, and 10 of 1,2-~yclopentanedione derivatives were formed. The identity of 3 has been established by an X-ray structure analysis. The pathway to these compounds seems to be closely related to the mechanism of the Dieckmann condensation. By using [D,]methanol, it has been shown that the semiacetal methoxy group underwent an intramolecular [1,2] migration.
Methanol was eliminated from semiacetal 10 by chromatography on silica gel with formation of the enol 11. In addition to 3 and 10, the dimethyl adipates 4 and 9 were observed as the expected methanolysis products of the y-oxoketenes 2 and 7, respectively. The diesters 9 were obtained on a second route from ?. Treatment of ? with trifluoroacetic acid and subsequent hydrolysis gave rise to the monoesters 12, which were converted into the diesters 9 by conventional esterification.