2,3 -dihydropyrazine amidation

It should be noted that, as all carbon positions in pyrazine are identical, the locant 2- in a monosubstituted derivative is unnecessary. All possible reduced derivatives of pyrazine 1, and several of those of its benzo analogues quinoxaline 2 and phenazine 3, are known. There are four dihydropyrazines, the 1,2-, 2,3-, 1,4-, and 2,5-isomers, two tetrahydropyrazines, the 1,2,3,4- and 1,2,3,6-, and hexahydropyrazine or piperazine, the last of which is omitted in this chapter. The reduced quinoxalines are the 1,2- and 1,4-dihydro compounds and 1,2,3,4-tetrahydroquinoxaline. The only known reduced phenazine is 1,4-dihydrophenazine. Hydroxypyrazine 4 and hydroxyquinoxaline 6 have been shown to exist in the tautomeric amide form by spectral studies, and therefore they are formulated as 2(1//)-pyrazinone 5 and 2(l//)-quinoxalinone, respectively. In contrast, aminopyrazine and aminoquinoxaline exist as described in the amino rather than the imino forms (Figure 1). 

The earliest report on such lactim ether formation was from Sammes [72JCS(P1)2494], who converted piperazine-2,5-dione to 2,5-diethoxy-3,6-dihydropyrazine (173) with an excess of triethyloxonium fluoroborate. Subsequently, Rajappa and Advani (73T1299) converted proline-based piperazine-2,5-diones into the corresponding monolactim ethers. The starting material was a piperazinedione in which one of the amino acid units was the secondary amino acid proline, and the other a primary amino acid. This naturally led to the regiospecific formation of a monolactim ether (169) (on O-alkylation) from the secondary amide, whereas the tertiary amide remained intact. This was later extended to piperazine-2,5-diones in which the secondary amino acid was sarcosine [74JCS(P 1)2122], leading to the monolactim ethers (170). 

Treatment of diphenylacetonitrile in toluene with sodium amide and 2-chloro-pyrazine gave 2-(C-cyano-C,C-diphenylmethyl)pyrazine (1021), and 2-vinylpyrazine with phenylacetonitrile and sodium heated at 120-130° for 10 minutes gave 2-(3 -cyano-3 -phenylpropyl)pyrazine (731). 2-Amino-5-bromomethyl-3-cyano-pyrazine with sodium hydride and methyl cyanoacetate in tetrahydrofuran formed the dialkylated product (56) (1031). 2-Amino-3-mercapto-5,6-dimethylpyrazine in methanol with potassium hydroxide and chloroacetonitrile gave 2-amino-3-cyanomethyIthio-5,6-dimethylpyrazine (1229), and 2-carboxypyrazine refluxed with chloroacetonitrile and triethylamine in ethyl acetate for 45 minutes gave the cyanomethyl ester (1317). 2-Hydroxy 5-methyl-3-propylpyrazine with cyanogen halides in aqueous sodium hydroxide-dimethylformamide at 0-5° gave l-cyano-5-methyl-2-oxo-3-propyl-l, 2-dihydropyrazine (1123). 

The reactions of pyrazine with sodium amide in liquid ammonia to give the anion of 2-amino-1,2-dihydropyrazine (608), and of 3-substituted 1-methylpyrazinium ions with liquid anunonia to give 3(or 5)-substituted 2-amino-1-methyl-1,2-dihydro-pyrazines (609) have been described in Section IV. 1C. Similar reactions of pyrazine with phenyllithium at —45° in tetrahydrofuran (720a) and of 1,2,5-trimethyl-pyrazinium salts in liquid ammonia with nitromethide and ethanethiolate anions (721) have been described in Section IV.2C(8). 

Reactions of piperazine-2,5-diones with phosphorus pentachloride and phosphorus pentabromide have been described in Sections V.ID and V.IF, respectively. Aromatic aldehydes condense with 3-methylpiperazine-2,5-dione in the presence of acetic anhydride to form mainly mono-A -acetyl derivatives of trans-3-arylidene-6-methylpiperazine-2,5-diones (e.g., 96, R = Ac) (1066). In these products the acetyl group was shown to be attached to position 1 and the 4,5-amide group was found to be sterically hindered. Photolysis formed the cis isomers. Both isomers were deacetylated with methanolic potassium hydroxide (1066). Condensation of 1,4-diacetylpiperazine-2,5-diones with aldehydes has been applied to the synthesis of unsymmetrical 3,6-diarylidenepiperazine-2,5-diones and the reaction has been extended to l,4-diacetyl-3,6-dimethylpiperazine-2,5-diones (1624). Treatment of (96, R = H) with triethyloxonium tetrafluoroborate in dichloromethane gave the monoimino ether, 5-benzylidene-6-ethoxy-3-hydroxy-2-methyl-2,5-dihydropyrazine (97) (1066). l-Methylpiperazine-2,5-dione similarly treated gave 5-ethoxy-l-methyl-2-oxo-l,2,3,6-tetrahydropyrazine (which was condensed with anthranilic acid at 150° to 2-methyl-l,2-dihydropyrazino[2,l-fi]quinazoline-3(4/0.6-dione (98) (1625), and l,4-dimethylpiperazine-2,5-dione gave 5-ethoxy-l,4-dimethyl-2-oxo-1,2,3,4-tetrahydropyrazine and 5,5-diethoxy-l,4-dimethylpiperazin-2-one (1626). 

Partial reductions of qnatemary salts to dihydro-componnds can be achieved with borohydride, bnt snch processes are much less well studied than in pyridininm salt chemistry (8.6). 1,4-Dihydropyrazines have been produced with either silicon or amide protection at the nitrogen atoms, and all the diazines can be rednced to tetrahydro derivatives with carbamates on nitrogen, which aids in stabilisation and thns allows isolation. 2-Amino-pyrimidines are rednced to 3,4,5,6-tetrahydro derivatives with triethylsilane in trifluo-roacetic acid at room temperature, the products thus retaining a gnanidine nnit. ... 

A typical tautomerism is represented by the equilibrium between hydroxypyrazine (4) or (20) and 2(177)-pyrazinone (6) or (21), in which the latter amide form predominates over the former. A similar situation exists in hydroxyquinoxaline (5). An IR study has been conducted on the tautomerism between hydroxy and keto forms in 2,5- and 2,6-dihydropyrazines which were found to exist in 5- and 6-hydroxy-2(177)-pyrazinone forms by comparison with the carbonyl absorption of the corresponding methoxypyrazinones <65JCS6681>. In addition to IR evidence predicated on the carbonyl absorption, the predominance of the amide form is proved by the H NMR spectra of 2(177)-pyrazinones, in which the ring protons appear to higher field by 0.4-0.9 ppm than those of... 

H. Mofakham, Z. Hezarkhani, A. Shaabani, J. Mol. Cat. 2012, 360, 26-34. CeUulose-SOjH as a biodegradable solid acid catalyzed one-pot three-component Ugi reaction synthesis of a-amino amide, 3,4-dihydroquinoxalin-2-amine, 4H-benzo [f>][l,4]thiazin-2-amine and l,6-dihydropyrazine-2,3-dicarbo-nitrile derivatives. 

The oxidation of two unusual enediamide motifs (3,4-dihydropyrazin-2(lf7)-ones), where two amides flank a double bond, led to a ring-opened product from the cleavage of the double bond in one case and a di-dioxygenated compound from the other. The formation of these products is rationalized on the basis of calculated free energies of intermediates." ... 

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