Oxadiazine ring opening

Oxadiazines herbicidal activity, 3, 1085 reactions with bases, 3, 1060 ring contraction, 3, 1065 ring opening, 3, 1061... 

Selective reduction of the 7-oxo group in pyrido[23-synthetic approach to 5,10-dideazatetrahydrofolic acid <00H(53)1207>. Cycloaddition of pyrimido[4,5-c][l,2,5]oxadiazine 96 with 2,3-dihydrofuran affords a new synthesis of dimethyllumazine derivative 97 which undergoes a ring-opening reaction to give pyrazine derivative 98 <00JHC419>. 

Electrocyclic ring opening of benzoxadiazines (56) is thought to be the first step in their ring contraction to benzoxazoles (75CC962). The benzoxadiazines are prepared by oxidation of the appropriate benzamidoximes, and ring-chain tautomerism between the intermediate nitrosoimine (57) and the oxadiazine (58) has been clearly demonstrated (75CC914). 

The 1,2,5-oxadiazine (62) is reported to be in tautomeric equilibrium with the ring opened nitrone (63). The nitrone is the only species observed in aqueous solution (69ZOR355). The 1,3,4-thiadiazine (64), which can be regarded as a stable tetrahedral intermediate, readily ring opens to (65), which is favoured in ethanol solution (77ACH(94)391). 

Aqueous permanganate converts l,3-dibenzyl-2-phenylimidazolidine into a mixture of benzamide and benzoic acid. When l-alkyl-3-phenylimidazolidines are subjected to N-oxidation the unstable JV-oxides rearrange to give tetrahydro-l,2,5-oxadiazines (180) via the ring-opened product (Scheme 89) (77LA956). 

Treatment of l-amino-4-phenylimidazoI-2-one (676) with a-haloketones, for example (677), produced (678). Ring opening of the oxadiazine occurred readily with aqueous HBr (70CB272, 62ZC153). 

Cu(hfacac)2-catalyzed elimination of N2 from aroyl(a-diazoacyl)hydra-zines 653 yields 2//-l,3,4-oxadiazin-2-ones 654 (70%). The reaction occurs consecutively via intramolecular carbonyl ylide formation, 1,3-cyclization into oxirane 655, and ring opening by the carbon-oxygen bond followed by a C- 0 shift with ring expansion (88CB887). 

Cyclobutane formation, occasionally in fairly low yield due to ring-opening reactions, also occurred by photochemical [2 - - 2] cycloaddition of vinylcyclopropane derivatives to an a, -unsaturated ketone moiety, " a cyclobuta-1,3-diene, a 1,4-benzoquinone, 2-acylthiophenes, and on irradiation of e t/o,enr/o-2,4-bis[( )-2-(methoxycarbonyl)vinyl]bi-cyclo[1.1.0]butane. A formal [2-1-2] cycloaddition also occurred on reaction of tricyclo[3.1.0.0 ]hex-3-ene with methyl 6-oxo-5-phenyl-l,3,4-oxadiazine-2-carboxylate to give methyl 9-oxo-10-phenyl-8-oxapentacyclo[4.4.0.0 ". 0 .0 °]decane-7-carboxylate, albeit in very low yield, after nitrogen extrusion. ... 

Two routes to the previously unknown l,2,4-oxadiazin-6(5//)-ones have been described. The first involves hydrolytic ring opening of 4-ethoxycarbonylmethyl-l,2,4-oxadiazol-5(4//)-ones (151 R = Ph, PhCHj, Ph(CH2)2) to the amidoxime derivatives (152), which on treatment with dicyclo-hexylcarbodiimide undergo cyclodehydration to the 4/f-l,2,4-oxadiazin-6(5//)-ones (153) in moderate yields (50-71%) (Scheme 20) <84CB2999>. 

Fully saturated 1,2,5-oxadiazines (82) are accessible as stable colourless oils from condensation of aldehydes and ketones with the j8-aminoethylhydroxylamine (81) (Equation (12)) which is derived by ring opening of 1-methylaziridine with A-methylhydroxylamine <79JCS(P2)993>. Yields are excellent (80 90%) with aldehydes (R = = H and R = H, R = Me) but only moderate (43%) with... 

Not surprisingly, the 2-ethoxy-5,6-dihydro-4//-l,3,4-oxadiazine (78) (Section 6.17.7.2.2.1) is very susceptible to hydrolysis and ring-opens to the bis-l,2-(ethoxycarbonyl)hydrazine (79) during column chromatography on silica or on crystallization from wet solvents <84JCS(P1)1423>. 2-(Alkylimino)-3,4-dimethyl-3,4-dihydro-2//-l,3,4-thiadiazin-5(6//)-ones (80) in ethanolic... 

Analogous ring-opening reactions are noted with 1,3,4-oxadiazin-6-ones and a /, 2 of 1 h has been measured for 2,5-diphenyl-l,3,4-oxadiazin-6-one in a mixture of carbon tetrachloride and methanol at 20°C <85CB2940>. 

When 3-(4-pyridylcarbonylhydrazino)-2-(tetra-2,3,4,5-tetrafluorobenzoyl)acrylate 35 was refluxed in acetonitrile in the presence of potassium fluoride for 4 h, cyclization to the intermediate l,3,4-oxadiazino[6,5,4-/ ,y Jquinoline 36 took place. Ring opening of the 1,3,4-oxadiazine and pyridine ring of 36 and recyclization led to the 4,5-substituted pyrazole 37, which upon hydrolysis affords compound 38 (Scheme 3) <2001CHE1278>. 

The cycloaddition of l,3,4-oxadiazin-6-ones 97 with norbornene proceeds via 7-oxoketenes 100 by loss of N2 from the initial adducts 98 and subsequent ring opening of intermediates 99 (R =Ar, Pr R = Ar, Me, C02Me) <1995JPR659, 1996LA853>. Isomerization of 100 (R = R = Ph) in hot carbon tetrachloride yields the enol lactone... 

The l,3,4-oxadiazin-6-one (240) undergoes cycloaddition followed by a remarkable rearrangement to give the triazole A(-imine 241 and an open-chain product (136). Cycloadditions have also been carried out with the following ring systems 1,2-dihydroisoquinoline (242) (137) dihydro-1,3-oxazine (243) (138,139), 2H-, 3-benzothiazine (244) (140,141), and 27/-l-pyran-2-thione (245) (142). 

Cyclopentanone (403) and cyclohexanone enamines react with symmetric and non-symmetric diimides of type Et02CN=NAr, ArCON=NCOAr and ArN=NCOAr (404) to give 1,3,4-oxadiazine derivatives 405 or open-chain Michael-type compounds 406 (equation 85)243-245. The formation of the former products seems to be favored by the presence of acceptor substituents on the aromatic ring of the diazene, and their stability is highly dependent on the ring size of the enamine employed. 

The ring of 1.2,4-oxadiazin-6-ones is also opened by nitrous acid with the formation of /Y-acyl-a-amino acids 3.12... 

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