1,2,3,5-Oxathiadiazine heterocyclics

There are two important general types of reaction by which six-membered heterocycles containing two or more heteroatoms can be transformed into other six-membered heterocyclic systems, namely reactions which involve an ANRORC mechanism, and reactions which proceed by a Diels-Alder/retro-Diels-Alder type of mechanism. Transformation of 1,3-oxazinones and -thiazinones into pyrimidones (equations 193 and 194) has been extensively used, especially in the conversion of isatoic anhydride into quinazolinones (e.g. equation 195). 2,4-Diaryl-l,2,3,5-oxathiadiazines, which are readily accessible by reaction of sulfur trioxide with aryl isocyanates, are useful precursors to pyrimidines and 1,3,5-triazines (equation 196). 

These heterocycles include thiatriazine 1,1-dioxides, which have been the most studied six-membered heterocycles containing other heteroatoms besides the sulfamide moiety. The herbicidal activity claimed in a number of patents for 1,2,4,6-thiatriazine 1,1-dioxides has prompted the preparation and study of several of its derivatives. The less reported oxathiadiazine 4,4-dioxides and dithiadiazine 1,1-dioxides are also included in this section. 

Pyrimidines. 4,6-Diphenyl-l,2,3,5-oxathiadiazine 2,2-dioxide added to a suspension of ethyl sodiomalonate in dioxane, and heated 1 hr. at 100° after the slightly exothermic reaction has subsided 4-hydroxy-2-6-diphenyl-5-ethoxycarbonylpyrimi-dine. Y 82%. - Various other heterocyclics can be easily prepared from 1,2,3,5-oxathiadiazine 2,2-dioxides. F. e. s. H. Weidinger and H. J. Sturm, A. 716, 143 (1968). 

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