Enol lactones, a$-unsaturated ~ Cycloadditions of 1.3.4-Oxadiazin-6-ones (4.5-Diaza-a-pyrones). 9 'I. -Methyl 6-0xo-5-phenyl-1,3,4-oxadiazin-2-carboxylate -Synthesis and Reactions with Norbornene, Norbornadiene. Cyclopropenes, Cyclobutene. and Benzvalene Methyl 6-oxo-5-phenyl-l,3,4-oxadiazin-2-carboxylate (7) was pene, and cyclobutene were converted by 7 mainly into the prepared by treatment of hydrazone 6 with dicyclohexylcar-oxepin derivatives 15, 17, 18, and the oxocin derivative 19, bodiimide. The reactions of 7 with norbornene and norbor-respectively. Benzvalene and 7 provided the tetracynadiene afforded the Diels-Alder adducts 8 and 9, respectively, clo[3.3.0.02~a.04~6]octan~ne 21. In these reactions, small quantiwhich decomposed in solution at 20°C to give mainly the y-ties of p-lactones were formed, too, which together with the oxoketenes 10 and 12 and small amounts of the p-lactones 11 p-lactones 11 and 13 give evidence for the dihydropyryliumand 13, respectively. The stable y-oxoketenes 10, 12, and the olates 24 as intermediates in the thermal denitrogenation of bis(y-oxoketene) 14 were obtained directly from solutions of 7 the Diels-Alder adducts of 7, e.g. A. 6-Oxo-5-phenyl-l,3,4-oxadiazin-2-carbonsauremethylester (7) Der Weg zu 7 ist dem von Steglich et al.3b,4) Chem.