Cycloadditions of 6H‐1,3,4‐Oxadiazin‐6‐ones (4,5‐Diaza‐α‐pyrones). 14. Reactions of 6H‐1,3,4‐Oxadiazin‐6‐ones with Norbornene: Non‐catalysed and Catalysed by Trifluoroacetic Acid
In 11 out of 13 non‐catalysed reactions of oxadiazinones 1 with norbornene, γ‐oxoketenes 4 were observed and could be isolated in three cases (4b,d,m). Except for 4m, all γ‐oxoketenes isomerised to enollactones of type 5 on thermolysis. However, 4b furnished the cyclobutene derivative 10 as the major product. No γ‐oxoketenes were detected in the reactions of 1k and 1l, which gave rise to the formation of 5k and the enollactone 19, respectively. The latter was converted into 5l on treatment with trifluoroacetic acid (TFA). Four oxadiazinones 1 were allowed to react with norbornene in the presence of TFA. Three of them (1a,b,l) afforded mixtures of enollactones of type 5 and their diastereomers 12. Heating of 4 in the presence of norbornene led to the formation of the symmetrical δ‐lactones 6. This process was observed to be efficient only where the conversion 4 → 5 is slow (4b) or inoperative (4m). In five cases, the treatment of the components with boron trifluoride etherate proved to be a useful preparative alternative (6a,d,g,h,m). On treatment of 4a with methanol, the pseudoester 7a was formed, whereas 4b gave mixtures of 7b and the esters 8b and 15. Hydrogen chloride converted 4b into the pseudochloride 14, which furnished pseudoester 7b on methanolysis. Reaction of 4b,m with formic acid gave rise to pseudoanhydrides of type 13. From 4b and TFA a mixture of the enollactones 5b and 12b was obtained. Boron trifluoride etherate transformed 4i to enollactone 12i; analogously, 12m resulted on treatment of 4m with trifluoromethanesulfonic acid.
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Cycloadditionen von 6H-1,3,4-0xadiazin-6-onen (4,5-Diaza-a-pyronen), 1 5L01 Reaktionen von 6H-1,3,4-0xadiazin-6-onen mit Norbornadien. Ein neuer Weg zu 3,6-disubstituierten a-Pyronen* Upon reaction with norbornadiene, ten disubstituted 1,3,4-strates in the absence of boron trifluoride-ether, was aloxadiazin-6-ones l were converted into 3,6-disubstituted a-lowed to react with the Lewis acid. Without use of an acid, pyrones 6. For this purpose, solutions of the substrates were only 6f could be obtained free from further compounds, treated with boron trifluoride-ether or trifluoroacetic acid. whereas in the other cases enol lactones and 1 : 2 products of The smooth formation of 6 was also observed when a y-0x0 the types 8 and 7 were formed additionally. Oxadiazinone ketene, initially generated by heating a solution of the sub-1 j gave enol lactone 10 in the noncatalysed reaction.
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