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

Article abstract of DOI:10.1002/prac.199533701140

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 pseudoahydrides 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.