Pyrazines and Piperazines

Carboxylic acid, 161, also serves as starting material for a substituted pyrazine that has proven to be an important diuretic agent. As the first step in the synthesis the acid is converted to the corresponding amide (165). Treatment with a single equivalent of hypobromous acid effects Hoffmann rearrangement of only one of the amide groups. Ethanolysis of the intermediate carbamate leads directly to the amino ester (166). Exposure of the 

Reaction of w-methylpiperazine with phosgene affords the carbamoyl chloride (170). Treatment of this intermediate with diethylamine affords the antiparasitic agent diethyl carbamazine (171) 

Treatment of a mixture of ortho anisidine and bis(2-hydroxy-ethyl) amine with hydrogen chloride affords the aryl-substituted piperazine, 171. (The first step in this reaction probably consists in conversion of at least one hydroxyl group to the chloride this then serves to alkylate the aromatic amine.) Alkyla- 

Alkylation of the monocarbamate of piperazine with the halide, 173, affords 174 after removal of the protecting group by saponification. Alkylation of the amine with the chloroamide, 175 (obtained from amine, 176, and chloroacetyl chloride) gives the local anesthetic lidoflazine (177).  

Condensation of p-chlorobenzaldehyde with 3-mercaptopropionic acid in the presence of ammonium carbonate leads to the thiazi-none, 179. The reaction very probably proceeds by the intermediacy of the carbonyl addition product, I7S lactamization completes formation of the observed product. Oxidation of 179 to the sulfone by means of potassium permanganate in acetic acid gives chlormezanone (180), a minor tranquilizer with muscle-relaxant properties. 

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A number of reductive procedures have found general applicability. a-Azidoketones may be reduced catalytically to the dihydropyrazines (80OPP265) and a direct conversion of a-azidoketones to pyrazines by treatment with triphenylphosphine in benzene (Scheme 55) has been reported to proceed in moderate to good yields (69LA(727)23l). Similarly, a-nitroketones may be reduced to the a-aminoketones which dimerize spontaneously (69USP3453279). The products from this reaction are pyrazines and piperazines and an intermolecular redox reaction between the initially formed dihydropyrazines may explain their formation. Normally, if the reaction is carried out in aqueous acetic acid the pyrazine predominates, but in less polar solvents over-reduction results in extensive piperazine formation. 

In the aliphatic series, hydrogenation of y-nitroketones gives pyrrolidines in good yields (52) and tx-nitroketones give a mixture of pyrazines and piperazines (i5). 

Preparation of Pyrazines and Piperazines (1,4-Diazines), and of their Fused Derivatives... 

Pyrazines and piperazines are obtained by catalytic reduction of a-nitroketones thus, reduction of l-nitro-2-butanone in the presence... 

Apart from the pyrolysis of 2-tert-butylsulfonylpyrazine to afford pyrazine (1) [in 49% yield with loss of sulfur dioxide and unsaturated ( ) hydrocarbon]239 and the reduction of pyrazine to piperazine (2) (in 76% yield by treatment of an alkaline solution with Ni—A1 alloy),479 no new or improved routes to pyrazine or piperazine appear to have been reported in recent years nor has any di- or tetrahy-dropyrazine been prepared. Both pyrazine and piperazine are now available commercially at modest cost. 

These (substituted-amino)pyrazines and piperazines have proved to be useful intermediates for subsequent cyclizations and other reactions. Their formation from aminopyrazines and a few cyclizations are illustrated in the following examples ... 

Some very clever syntheses of pyrazines were reported. Tandem Mn02-mediated oxidation followed by in situ trapping with aromatic or aliphatic 1,2-diamines was shown to give rise to quinoxalines, dihydropyrazines, pyrazines, and piperazines without the need to isolate highly reactive 1,2-dicarbonyl intermediates <03CC2286>. A new intramolecular cyclization route to highly substituted chiral 6,7-dihydro-5//-imidazo[l,5-a]pyrazin-8-ones like 157 from Meldrum s acid was developed <030L3907>, and 5-chloropyrido[3,4-6]pyrazines were prepared from 1,2-dicarbonyl compounds and 2-chloro-3,4-diaminopyridine <03H(60)925>. A synthesis of... 

The aeylative cyclization of phenol with acetic anhydride was carried out over CeY type zeolites. The reaction mechanism is given in Fig. 4. At 380°C, the yield of 4-methyl coumarin was 75 % at 81% conversion of phenol. N-Methylpyrrolidine was synthesized from 1,4-butanediol and methylamine over Cr ZSM-5 and modified ZSM-5 catalysts at 300 C. The reaction mechanism is given in Fig. 5. The synthesis of a number of five- and six- membered heterocyclics have been depieted in Table 1. The reaction was carried out at 250-400°C at 30-80 hydrogen atm., under down-flow fixed bed conditions. The yield of N-methyl piperazine was 90% at 95% conversion over ZSM-5 catalysts. Similarly 2-methyl pyrazine and piperazine were synthesized from propylene glycol and ethylenediamine over HZSM-5 (Fig. 6). 

Classic A/-heterocychc ligands, eg, bipyridyl (bipy), terpyridyl, imidazole, pyrazine, phenanthroline, piperazine (including alkyl- and aryl-substituted derivatives), and polypyrazol-l-yl-borates (bis, tris, and tetra), have all been found to coordinate Th(IV) chlorides, perchlorates, and nitrates. The tripodal hydrotris(pyrazolyl)borates, HBPz, have been used to stabilize organometaHic complexes (31). Bis-porphyrin Th(IV) "sandwich" complexes have been... 

It was suggested that a-amino carbonyls such as 3-amino-butane-2-one formed a dihydropyrazine which was subsequently oxidized to a pyrazine (30, 311. The conversion of dihydropyrazine to pyrazine occurs with or without oxygen. There are two possible ways to convert dihydropyrazine into pyrazine without oxygen. One is the disproportionation of dihydropyrazine to give pyrazine and tetrahydropyrazine or piperazine. The other is the dehydration of hydroxy dihydropyrazine (32). Recently, a dialkylpyrazine radical was reported as an intermediate of pyrazine formation (33). However, this simple step from dihydropyrazine to pyrazine is not yet thoroughly understood. 

Aromatic sextets are not essential for the stability of heterocyclic rings and saturated and partially-saturated rings occur widely. These are usually named as the corresponding dihydro or tetrahydro derivatives, e.g., 37-39. The fully-saturated derivatives of pyridine and pyrazine are commonly referred to as piperidine 40 and piperazine 41. The antihypertensive calcium channel antagonist nifedipine 42 is a 1,4-dihydropyridine derivative , and sildenafil 43, which contains a piperazine ring, is a phosphodiesterase inhibitor used to treat erectile dysfunction . 

This chapter covers the preparations, physical properties, and reactions of pyrazine and its C-alkyl, C-aryl,. V-alkyl, or A/ -aryI derivatives as well as their respective di-, tetra-, and hexahydro derivatives (the last usually known as piperazines). In addition, it includes methods for introducing alkyl or aryl groups (substituted or otherwise) into pyrazines and hydropyrazines already bearing substituents and the reactions specific to the alkyl or aryl groups in such products. For simplicity, the term alkylpyrazine in this chapter is intended to include alkyl-, alkenyl-, alkynyl-, cycloalkyl-, and aralkylpyrazines likewise, the term arylpyrazine includes both aryl- and heteroarylpyrazines. 

The parent compound of this series, pyrazine, was first prepared in trace amounts by Wolff (30) by heating aminoacetaldehyde diethyl acetal [H2NCH2CH(OEt)2] with anhydrous oxalic acid at 110-190°C, and later in better yield by heating the mercuric or platinic chloride double salts (of the aminoacetaldehyde acetal) with hydrochloric acid (31) it was also obtained from aminoacetaldehyde with mercuric chloride in sodium hydroxide (23). Wolff in 1893 (22) also prepared pyrazine by decarboxylation of the tetracarboxylic acid, obtained by oxidation of tetramethyl-pyrazine and Stoehr (32) prepared it by the distillation of piperazine with lime and zinc dust. Brandes and Stoehr (33) in 1896 described the preparation of pyrazine by heating glucose with 25% aqueous ammonia at 100°C. 

Normal nucleophilic substitution reactions of alkyl and aryl chloropyrazines have been examined as follows 2-chloro-3-methyl- and 3-chloro-2,5-dimethyl(and diethyl)pyrazine with ammonia and various amines (535, 679, 680) 2-chloro-3(and 6)-methylpyrazine with methylamine and dimethylamine (681, 844), piperidine and other amines (681, 921) 2-chloro-5(and 6)-methylpyrazine with aqueous ammonia (362) alkyl (and phenyl) chloropyrazines with ammonium hydroxide at 200° (887) 2-chloro-3-methylpyrazine with aniline and substituted anilines (929), and piperazine at 140° (759) 2-chloro-3-methyl(and ethyl)pyrazine with piperidine (aqueous potassium hydroxide at reflux) (930,931) [cf. the formation of the 2,6-isomer( ) (932)] 2-chloro-3,6-dimethylpyrazine with benzylamine at 184-250° (benzaldehyde and 2-amino-3,6-dimethylpyrazine were also produced) (921) 2-chloro-3,5,6-trimethylpyrazine with aqueous ammonia and copper powder at 140-150° (933) and with dimethylamine at 180° for 3 days (934,935) 2-chloro-6-trifluoromethylpyrazine with piperazine in acetonitrile at reflux (759) 2-chloro-3-phenylpyrazine with aqueous ammonia at 200° (535) 2-chloro-5-phenylpyrazine with liquid ammonia in an autoclave at 170° (377) 2-chloro-5-phenylpyrazine with piperazine in refluxing butanol (759) but the 6-isomer in acetonitrile (759) 5-chloro-2,3-diphenylpyrazine and piperidine at reflux (741) and 5-chloro-23-diphenylpyrazine with 2-hydroxyethylamine in a sealed tube at 125° for 40 hours (834). 

This volume summarizes published pyrazine chemistry with emphasis on syntheses, properties, and reactions of pyrazines and pyrazine iV-oxides (Chapters 1-X). Treatment of theoretical aspects is minimal. Although not strictly relevant. Chapter XI is presented as a summary of earlier reviews and more recent literature of reduced pyrazines (including piperazines). The literature recorded in Beilstein to 1929 and Chemical Abstracts through 1978 (Volume 89) has been covered together with selected references to 1980. Whereas every reasonable effort has been made to incorporate most significant material, no attempt has been made to include all relevant data. Tables have been incorporated in the text to extend the range of examples. 

These compounds have been referred to as tetrazolo[l,5-c]pyrazines, and reduced derivatives have been named tetrazolo[l,5-a]piperazines. This ring system has attracted attention mainly in the study of tetrazolo azide tautomerism and has been used to generate nitrenopyrazine. Although tetrazolo derivatives of pyridazine and pyridine are stable in solution, tetrazolo[l,5-a]pyrazine isomerizes readily in chloroform or trifluoracetic acid. Although an earlier report indicated that no azide... 

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