Cycloadditionen von 6 H ‐1,3,4‐Oxadiazin‐6‐onen (4,5‐Diaza‐α1‐pyronen), 12. Dieckmann‐Kondensationen ohne Basen

Cycloadditions of 6 H ‐1,3,4‐Oxadiazin‐6‐ones (4,5‐Diaza‐α1‐pyrones), 12 [1] . – Dieckmann Condensations without Bases Prepared by treatment of methyl 6‐oxo‐5‐phenyl‐6 H ‐1,3,4‐oxadiazine‐2‐carboxylate ( 1 ) with norbornene, norbornadiene, and styrene, respectively, the γ1‐oxoketenes 2, 5 , and 7 were allowed to react with methanol. Inter alia, the semiacetals 3 , 6 , and 10 of 1,2‐cyclopentanedione 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 4 ]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 γ1‐oxoketenes 2 and 7 , respectively. The diesters 9 were obtained on a second route from 7 . Treatment of 7 with trifluoroacetic acid and subsequent hydrolysis gave rise to the monoesters 12 , which were converted into the diesters 9 by conventional esterification.