Reactions with diazines

There are several other methods for generation of (per)fluoroalkyl radicals in reactions with diazines. In particular, photochemical decomposition of perfluoroalkyl mercury derivatives was used for perfluoroalkylation of uracils 568 (Scheme 120) [297]. Whereas for the parent uracil the method gave satisfactory results (30-54 % yields), the procedure was unfruitful for the sugar-modified derivatives (6-11 % yields). 

Ortho-dxKCXed lithiation followed by reaction with TSN3 and reduction has led to an improved, general synthesis of aminodiazines 3a-c <96S838>. In some cases, tetrazolo-fused diazines could be isolated as intermediates. 

Unsaturated 5(4//)-oxazolones derived from aromatic and heterocyclic aldehydes including phthalic anhydride/ antipyrine/ " chromone/ indoles/ pyridines/" ° quinolines/" diazines/" benzoxazoles/" and benzimidazoles " " have been prepared. Reaction with nitrogen nucleophiles and subsequent cycliza-tion leads to the expected 5(477)-imidazolones. 

Pyrroles, indoles and benzo[ft]thiophene act as good dienophiles in inverse electron demand Diels-Alder reactions with 1,2-diazines, 1,2,4-triazines and sy/n/n-tetrazines. This is examplified by the formation of compounds (189) in excellent yields on interaction of indoles and benzo[c]thiophene with dimethyl l,2,4,5-tetrazine-3,6-dicarboxylate (87JOC4610 90JOC3257). There are also many examples of such intramolecular reactions, e.g. (190 — 191). 

The most interesting compounds described in this paragraph are meso-ionic naphtho[reactions with olefinic compounds. For instance, the reaction with acetylenedicarboxylates gives rise to adducts 458, which are dehydrogenated spontaneously to the new heteroaromatic systems 459 with the aromatic 147r-electron contour [75JCS(PI)556]. 

Use of noncoordinated donor centers of diazines, for instance in 775 [493], in reactions with Lewis acids (3.224) is widespread. A bridge joining the metal -carbonyl group, for example 776, as well as formation of the metal-cycle 777 was observed in these processes ... 

Reactions with electrophilic reagents become successively more difficult than with pyridine, both at nitrogen (weakened basicity) and at ring carbon atoms (no reaction at all without activation, even in diazines). 

Successive introduction of nitrogen atoms into benzene causes a gradual reduction in aromatic stabilization. The diazines still show typical aromatic behavior in that in most of their reactions they revert to type. However, with the triazines and tetrazines, decreasing aromaticity increases the ease of both thermal and photochemical fragmentations and rearrangements, and of cyclic transition state reactions with other reagents. 

Diazines, polyazines, and azapyrylium salts participate in inverse electron demand DielsAlder/retro-DielsAlder-type reactions with electron-rich dienophiles leading to pyridines . 1,2,4-Triazines react with enamines and enol ethers to give pyridines as illustrated in Scheme 162 <2004T6021, 2004CC508>. 

Oximes, hydrazones, semicarbazones, diazines and carbodiimides all undergo reactions involving addition to a carbon-nitrogen double bond (A, D], but these reactions are of limited value, with the exception of the reaction with ketoximes. This last reaction, with an excess of Grignard at elevated temperatures, is a useful route to aziridines, (4), although yields are rarely high. In some cases, reduction of the intermediate azirine (2) leads to an alternative aziridine (5). 

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