CYCLOADDITIONS OF 1,3,4-OXADIAZIN-6-ONES (4,5-DIAZA-ALPHA-PYRONES) .9. METHYL 6-OXO-5-PHENYL-1,3,4-OXADIAZIN-2-CARBOXYLATE - SYNTHESIS AND REACTIONS WITH NORBORNENE, NORBORNADIENE, CYCLOPROPENES, CYCLOBUTENE, AND BENZVALENE

Cycloadditions of 1,3,4‐Oxadiazin‐6‐ones (4,5‐Diaza‐a‐pyrones), 9. — 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‐1,3,4‐oxadiazin‐2‐carboxylate (7) was prepared by treatment of hydrazone 6 with dicyclohexylcarbodiimide. The reactions of 7 with norbornene and norbornadiene afforded the Diels‐Alder adducts 8 and 9, respectively, which decomposed in solution at 20°C to give mainly the γ‐oxoketenes 10 and 12 and small amounts of the β‐lactones 11 and 13, respectively. The stable γ‐oxoketenes 10, 12, and the bis(γ‐oxoketene) 14 were obtained directly from solutions of 7 and the respective olefin. Cyclopropene, 1‐methylcyclopropene, and cyclobutene were converted by 7 mainly into the oxepin derivatives 15, 17, 18 and the oxocin derivative 19, respectively. Benzvalene and 7 provided the tetracyclo[3.3.0.02,8.04,6]octanone 21. In these reactions, small quantities of β‐lactones were formed, too, which together with the β‐lactones 11 and 13 give evidence for the dihydropyryliumolates 24 as intermediates in the thermal denitrogenation of the Diels‐Alder adducts of 7, e.g. 8 and 9. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim