Reactions of Aminopyrazines

Some reactions of aminopyrazines have been discussed already the conversion of primary amino- into halogenopyrazines (Sections 4.1.4 and 4.3.2) and the conversion of aminopyrazines into pyrazinones (Section 5.1.1). The many remaining reactions are covered in the subsections that follow. 

The major methods for the preparation of this ring system involve formation of the imidazo ring, either by cyclization of appropriately substituted aminopyrazines or by the reaction of aminopyrazines with bifunctional reagents. 

The initial syntheses of Cypridina luciferin and its analogues were performed in low yields by reaction of appropriate 2-aminopyrazines with a-keto acids, followed by reduction with aluminum amalgam or catalytic hydrogenation, and treatment of the product with dicyclohexyl-carbodiimide. For example, 2-amino-5-phenylpyrazine (36) on reaction with pyruvic acid gave the product 37, which was reduced to give an intermediate formulated as 38, which was then cyclized to give the 0x0 compound 39 in 7% yield. It was later discovered that these products could be obtained in high yield in one step by reaction of aminopyrazines such as 36 with a-keto aldehydes such as pyruvaldehyde (MeCOCHO). Condensation of the appropriate aminopyrazine with... 

The only generally useful method of preparation of members of this ring system involves reaction of aminopyrazines 4 with nitriles in the presence of aluminum chloride to give the intermediates 5. Treatment of these compounds with lead tetraacetate gives 2-substituted derivatives of the heterocycle 6. The method has been successfully applied using phenyl cyanide and in spite of an earlier report to the contrary, methyl cyanide has also been used. - ... 

Broadly speaking, nucleophilic substitution may be divided into (a) the direct displacement of hydrogen and (b) the displacement of other substituents. Displacements of type (a) are rare and are typified by the Tschitschibabin reaction. Pyrazine reacts with NaNHa/NHs to yield 2-aminopyrazine, but no yield has been quoted (46USP2394963). Generally, the synthesis of aminopyrazines, aminoquinoxalines and aminophenazines is more readily accomplished by alternative methods, particularly displacement of halogen from the corresponding halo derivatives, which are themselves readily available. 

Coelenteramide and coelenterazine. The structure of AF-350 contains the same aminopyrazine skeleton as in Cypridina etioluciferin and oxyluciferin (Fig. 3.1.8), suggesting that the bioluminescence reaction of aequorin might resemble that of Cypridina luciferin. To investigate such a possibility, we prepared the reaction product of aequorin luminescence by adding Ca2+ to a solution of aequorin. The product solution (blue fluorescent) was made acidic, and extracted with... 

Substituted pyrimidine N-oxides such as 891 are converted analogously into their corresponding 4-substituted 2-cyano pyrimidines 892 and 4-substituted 6-cya-no pyrimidines 893 [18]. Likewise 2,4-substituted pyrimidine N-oxides 894 afford the 2,4-substituted 6-cyano pyrimidines 895 whereas the 2,6-dimethylpyrimidine-N-oxide 896 gives the 2,6-dimethyl-4-cyanopyrimidine 897 [18, 19] (Scheme 7.6). The 4,5-disubstituted pyridine N-oxides 898 are converted into 2-cyano-4,5-disubsti-tuted pyrimidines 899 and 4,5-disubstituted-6-cyano pyrimidines 900 [19] (Scheme 7.6). Whereas with most of the 4,5-substituents in 898 the 6-cyano pyrimidines 900 are formed nearly exclusively, combination of a 4-methoxy substituent with a 5-methoxy, 5-phenyl, 5-methyl, or 5-halo substituent gives rise to the exclusive formation of the 2-cyanopyrimidines 899 [19] (Scheme 7.6). The chemistry of pyrimidine N-oxides has been reviewed [20]. In the pyrazine series, 3-aminopyrazine N-ox-ide 901 affords, with TCS 14, NaCN, and triethylamine in DMF, 3-amino-2-cyano-pyrazine 902 in 80% yield and 5% amidine 903 [21, 22] which is apparently formed by reaction of the amino group in 902 with DMF in the presence of TCS 14 [23] (Scheme 7.7) (cf. also Section 4.2.2). Other 3-substituted pyrazine N-oxides react with 18 under a variety of conditions, e.g. in the presence of ZnBr2 [22]. 

The reaction of 3-aminopyrazine-2-carboxylic acid and EMME at 160-I80°C for 2 hr, or in diphenyl ether at 200°C for 2 hr, afforded 2-pyrazinylaminomethylenemalonate in 60% yield (70T3069 71IJC201) (Scheme 13). 

The direct reaction of a-hydroxyiminoketones with aminoacetonitriles was found to be a good route to pyrazine derivatives . However, in some cases the yields of products were not high. Recently, it was described that the reaction of oximes 338 with aminoacetonitriles was catalyzed by FeCls. In this case 2-aminopyrazines 339 were obtained in 55-80% yields (equation 147) . ... 

The direct alkylation of aminopyrazines is usually unsatisfactory as a synthetic method because it mainly takes place at the most basic ring nitrogen. However, 3,6-diamino-2,5-dicyanopyrazines are successfully alkylated by treatment with alkyl iodide or bromide in protic solvent in the presence of alkali such as NaOH in dimethylacetamide (DMA) to form bis(dialkylamino)pyrazines <1998DP(39)49>. Reaction of 2,6-diamino-3,5-diarylpyrazine with methylglyoxal in aqueous HCl-ethanol led to -alkylation but no formation of the expected bicyclic imidazolo[l,2- z]pyrazine <2001S768>. 

Construction of pyrazine rings from a-amino nitriles has been sometimes completed through multistep reactions. For example, 2-aminopyrazine 1-oxide 163 is synthesized via amide intermediate 162 formed by reaction of methyl a-aminocyanoacetate with a-oximino carboxylic acid (Scheme 45) <1994H(38)1581>. 

Published routes to pyrazino[2,3-3][l,4]thiazines, as reviewed in CHEC-II(1996) <1996CHEC-II(7)737>, involved the reaction of a 3-aminopyrazine-2-thiol with a biselectrophile and of a 3-chloropyrazin-2-amine with thioglycolate. In more recent work, the system is formed by condensation of a dichloropyrazine with a 2-aminothiophenol (Equation 182) <2002CPB922>. The same approach, using either 2-aminothiophenol or 2-aminoethanethiol, has been used to form oxadiazole-fused systems (Scheme 100) <1997CHE1352>. In studies toward the synthesis of ER-49890 151, benzannulated pyrazino[2,3-/ ][l,4]thiazines have also been accessed by an indirect sequence (Scheme 101)<2005H(65)403>. 

Pyrazino[l, 2-a ]pyrimidines are most commonly prepared from 2-aminopyrazines. Reaction of 2-aminopyrazine (274) with ethyl ethoxymethylenemalonate followed by heating in... 

Aminopyrazines are conveniently prepared from carboxamido-pyrazines by application of the Hofmann reaction (see Section V,B). Thus, Camerino and Palamidessi prepared aminopyrazine in 80% yield from carboxamidopyrazine.312 Aminopyrazine may also be prepared from the reaction of pyrazine with sodamide in liquid ammonia,313 and 3-amino-2,5-dimethylpyrazine is the product of amination of 2,5-dimethylpyrazine with sodamide in dimethyl-aniline.311 The ammonolysis of halopyrazines also represents a useful preparative procedure for aminopyrazines (see Section V,C). This reaction proceeds most easily in the case of fluoro compounds for example, fluoropyrazine is converted into aminopyrazine in 70% yield by treatment with concentrated aqueous ammonia at room temperature for 3 days,299 whereas the corresponding reaction with chloropyrazine has been carried out in a sealed tube at 150°.147 Alkaline hydrolysis of 2,4-dihydroxypteridines followed by decarboxylation yields aminopyrazines 315 thus, high-temperature alkaline hydrolysis of 7-methyl-2,4-dihydroxypteridine (7-methyIlumazine) gives, after decarboxylation of the intermediate pyrazinecarboxylic... 

Comparison of the ultraviolet absorption and ionization constants of aminopyrazine, methylaminopyrazine, and dimethylaminopyra-zine indicates that aminopyrazine exists as such and not in the tautomeric imino form.147 Aminopyrazines, like their pyridine analogs, form diazonium salts, which readily decompose to the corresponding pyrazinones. For example, nitrous acid treatment of aminopyrazine318 and 2-aminopyrazine-5-carboxylic acid210 gives the corresponding pyrazinones in 30 and 59% yield, respectively. The diazonium salt from aminopyrazine cannot be converted into bromo-pyrazine under the conditions of the Sandmeyer reaction. 

Two very simple precursors combine to produce 2-arylpyrrolo[2,3-6]pyrazines. The anion derived from 2-methylpyrazine (173) adds to a benzonitrile (e.g. (174)) to give the product (175) (Equation (59)). This reaction is not the favored pathway displacement of chlorine is preferred <89TL935> although a similar reaction without the chloro substituent does provide the pyrrolopyrazine as the major product <8iTLi2i9>. However, this reaction does not seem to have been investigated for additional examples. In another simple reaction, 2-aminopyrazine (176) reacts with ethyl chloro-acetoacetate to give ethyl 6-methyl-5//-pyrrolo[2,3-6]pyrazine-7-carboxylate (177) (Equation (60)) <89BSF467>. 

The thermal Curtius reaction of a 3-aminopyrazine-2-carboxylic acid azide proceeds with intramolecular cyclization to provide a versatile synthetic route to a wide variety of 1,3-dihydro-2tf-imidazo[4,5-i]-pyrazin-2-ones. Many compounds in this series are potent hypotensive agents in animals they are also inhibitors of the enyzme cyclic AMP phosphodiesterase in vitro. 

Very few imidazo[4,5-b] pyrazines have been reported in the literature1-3 and no good general method has been available for preparing compounds in this most interesting heterocyclic class. We have found that the Curtius reaction of a 3-aminopyrazine-2-carboxylic acid azide (Scheme I) proceeds with intramolecular cyclization to provide, in good yield, a wide variety of the subject compounds. We will report here only the 6-substituted 5-chloro-l, 3-dihydro-2tf-... 

The direct and indirect conversion of azido- into aminopyrazines has been covered in Section 7.3.1. The remaining reactions of azidopyrazines are illustrated in the following examples ... 

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