1,2,3,5-Oxathiadiazine

Diaryl-1,2,3,5-oxathiadiazines (277 from sulfur trioxide and aryl isocyanates) with (3-diketones yield pyrimidines (278). s-Triazine reacts with RCH2CN to give 4-aminopyrimidines (279 see Section 3.2.1.6.1 for a similar reaction), and with electron-rich alkenes and alkynes to yield pyrimidines such as (280) from EtC = CMe (Section 3.2.1.10.2). 

Photolysis of the 1,2,3,5-oxathiadiazine (132) in the presence of ethanol leads to the quinazolone (133). The product is not formed in the absence of ethanol, and the proposed mechanism involves electrocyclic ring opening, addition of ethanol, photochemical diradical formation and closure on to the C-6 phenyl ring followed by aromatization as shown in Scheme 8 (79JOC4435). 

An analogous product (252 X = Cl) is formed in the reaction of sulfur trioxide with cyanogen chloride (68AG(E)172). Aryl cyanates also condense with sulfur trioxide to give 1,2,3,5-oxathiadiazines analogous to those formed from dialkylcyanamides (67CB3736). 

Oxathiadiazine-2,2-dioxides, 1,4,3,5-oxathiadiazine-4,4-dioxides, and l,2,4,5-oxadithiazine-2,2,4,4-tetraoxides formed from cyano compounds and S03 93MI22. 

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-1,2,3,5-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). 

Irradiation of cither A -benzoyl-7V-tosylbenzimidamide in dichloromethane or 4,6-diphenyl-2,2-dioxo-1,2,3,5-oxathiadiazine in dichloromethane containing 5% by volume of ethanol or 2-methylpropan-2-ol at / > 290 nm produces 2-phenylquinazolin-4(3//)-one in 40-71% yields. 

The crystal structure of the 1,2,3,5-oxathiadiazine 2,2-dioxide (13) has been reported to be triclinic with the space group PI with z = 2 <9lJOU759>. 

The crystal structure of 1,2,3,5-oxathiadiazine-2,2-dioxide 24 was mentioned briefly in CHEC-II(1996) <1996CHEC-II(6)967>. The S-O bond length of 1.632 A is significantly longer than the analogous bond (1.610 A) in the cyclic trimer SO3 and this is consistent with the observation that this bond, rather than the C-O bond, ruptures in reactions of 24 with nucleophiles (see Section 9.14.5) <1997CHE1028>. 

Oxathiadiazine 2,2-dioxides 74 react readily with aqueous acid to produce (depending on substituents and conditions) A -acylamidines 104 or imides 105 (Scheme 8). Reaction of 74 (R = R = aryl) with alcohols in benzene in the presence of a small amount of water affords A -sulfamidines 106 (Scheme 8). Treatment of 74 with amines results in rupture of the C-O bond and formation of products such as 107 and 108 (Scheme 8) <1997CHE1028>. 

At room temperature and below, hydrolysis of oxathiadiazines 111 with water bound to 7-alumina affords 1,2,3,5-oxathiadiazine 2,2-dioxides 24, CX3CO2H, and ammonia (path 1), while at 30-50 °C, elimination of the CX3CO group is accompanied by a rearrangement (path 2) to furnish the 1,2,4,6-thiatriazine 1,1-dioxides 72 (Scheme 9) <2000RJ01229>. 

Oxathiadiazines. Reaction of cyano-containing compounds with 1,3,2,4,5-dioxadithiazine 2,2,4,4-tetraoxides bearing a donor or weak acceptor substituent (e.g., alkyl, tolyl) with a strong acceptor substituent, the isomeric... 

An unexpected transformation of 2,4,6-trisubstituted 1,2,3,5-oxathiadiazine 2-oxide derivative 18 on alumina, yielding 1,2,4,5-thiatriazine 1,1-dioxide derivatives 19, has been reported . 

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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