1.3.4- Oxadiazin-5 -ones, tautomerism

Oxadiazine-3,5-diones are acidic the parent compound has pKa 7.6.5 These compounds arc included in this section because of formal tautomerism with unsaturatcd dihydroxy forms such as 5. However, there is no evidence for the existence of such tautomers. The oxadiazinediones exhibit two NH absorptions in the region 3200 3100 and two carbonyl bands, one at ca. 1750 and the other at 1710-1670 cm 1. The, 3C NMR spectra of oxadiazine-dione and its 2-substituted derivatives are informative C3 resonates at ca. 5 = 150, C5 at ca. 5 = 169 and C6 at ca. <5 = 70 ppm. An X-ray analysis of the parent compound confirmed its dione structure the structure shown gives bond lengths (pm) and angles of 2//-l,2,4-oxadi-azine-3,5(4/7,6//)-dione.6... 

MNDO calculations have been used to determine the relative stabilities of the tautomeric 1,3,4-oxadiazin-5-ones (9) and (10) and the dipolar form (11), and also of the three isomeric 1,3,4-oxadiazin-2-ones (12)-(14) <88CB887>. Calculated AHf values are appended to the diagrams. 

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