1.2- Diamines pyrazines, 2-amino

One of the few examples of formation of the pyrazine ring as the last step in the synthesis of imidazo[4,5-fe]pyrazines (504) involves the reaction of biacetyl with the diamine (503) generated in situ from the 4-nitro-5-amino compound (502) (70TL1013>. Another synthesis involving the use of an imidazole is the condensation of ethylenediamine (505) with (506) to give the perhydro derivative (507). 

To access a pyrazine in this way one needs a 1,2-diamine and a 1,2-dicarbonyl compound, and a subsequent oxidation, but if both components are unsymmetrical, mixtures are formed. The dimerisation of 2-aminocarbonyl compounds also generates symmetrically substituted dihydro-pyrazines - perhaps the best known examples of such dimerisations involve the natural amino acids and their esters, which dimerise to give dihydropyrazine-2,5-diones - diketopiperazines . 

An ingenious modification of the general method uses 5,6-diaminouracil as a masked unsaturated 1,2-diamine the products can be hydrolysed with cleavage of the pyrimidinone ring finally arriving at 2-amino-pyrazine-3-acids as products. ... 

The condensation of pyridine-2,3-diamine with diethyl oxalate is reported to give, regardless of the amount of ester applied, a mixture of pyrido[2,3-6]pyrazine-2,3(1 f/,4//)-dione (12 25%) and ethyl /V-(3-amino-2-pyridyl)oxamate (11 75%). Sublimation of the latter compound at 3 Torr affords the cyclization product 12.14... 

The reaction between pyridine-2,3-diamine and oxalyl chloride dioxime affords 2,3-bis(hydroxy-amino)pyrido[2,3-A]pyrazine (27).30... 

Amino-3-pyridyl)oxamic acid (1), prepared by reacting pyridine-3,4-diamine with dimethyl oxalate in 1 M hydrochloric acid, is converted into pyrido[3,4-6]pyrazine-2,3(l.//,4//)-dione (2) in 61% yield by heating at 230 C.97... 

A methyl group at one of the amino substituents of pyridine-3,4-diamine influences the regiochemistry of the cyclization. Thus, the reaction of A -methylpyridine-S,4-diamine and ethyl pyruvate yields 49% of l,3-dimethylpyrido[3,4-6]pyrazin-2(l//)-one (14), whereas N3-methylpyridine-3,4-diamine gives the same yield of 2,4-dimethylpyrido[3,4-/j]pyrazin-3(4f/)-one (15).97... 

The most common way to construct the pyrazine ring is the condensation of 1,2-diamines with 1,2-dicarbonyl compounds followed by the aromatization. Pritchard and co-workers developed an efficient procedure to prepare pyrazines, quinoxalines and 1,2,4-triazines by such condensations <01JCS(PI)668>. Dicarbonyl component 193, prepared by coupling amino acid 192 with (terf-butoxycarbonylmethylene)triphenylphosphine using DCC-DMAP, underwent ozonolysis to give 194. Then, 194 was treated with 1,2-ethylenediamine followed by aromatization with Pd-C to give pyrazine 195 in good yield. 

Synthetic studies on the preparation of polyimides via the imidization route together with characterization studies have been made on systems derived from bis(4-amino-3-methoxyphenyl)methane and pyrazine tetracarboxylic acid, m- and p-carboranylenediamines and tetracarboxylic acids, and aromatic diamines and a new class of dianhydride monomers. Copolymers have also been prepared from the reaction of 3,3, 4,4 -benzophenone tetracarboxylic acid dianhydride with 3,3 -sulphonyl bis(phenylisocyanate) and 4,4 -sulphonyl bis(phenylisocyanate). ... 

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