Quinoxalines s. a. Pyrazines

The chemistry of the pyrido[2,1 -c][ 1,4]oxazines (1), pyrido[2,l-c][l,4]thi-azines (2), and pyrido[2,1 -a]pyrazines (3) (Scheme 1) and their benzologues (4)-(18) (Schemes 2-4) has not been systematically reviewed. Only Mosby s book in 1961 treated the early articles on pyrido[2,l-c][l,4]thi-azines (61CH1182), pyrido[2,1 -ajpyrazines (61CH1188), 5//-pyrido[l,2,3-de -, 4-benzoxazines (61CH1180), pyrazino[l,2-a]quinolines (61CH1192), pyrido[l,2-n]quinoxalines (61CH1191), and pyrido[ 1,2,3-de]quinoxalines (61CH1193). 

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... 

Analogous to Hinsberg s quinoxaline synthesis31 (cf. Section 6.3.2. Vol. E9b), a most versatile method uses the condensation of pyridazine-2,3-diamines with oxalic acid and its derivatives, cz-oxo acids and derivatives, and 1,2-dioxo compounds, to synthesize pyrido[2,3-A]-pyrazines 28. The regioselectivity of these reactions has been discussed.122... 

Polyfluorenes are described in Chapter 5. Various PPPs with heterocyclic side rings (e.f., poly(quinoxaline-5,8-diyl)s with a pyrazine side rings) have also been synthesized. These polymers will be described in Chapters 14 and 18. This chapter also deals with some PPPs with heterocyclic side rings. 

Angew Chem Int Ed 44(2) 214—231. doi 10.1002/anie.200460441 Shvedov VI, Altukhova LB, Grinev AN (1970) A new method for the synthesis of pyrrolo[l,2-a] pyrazines and pyrrolo[l,2-a]quinoxalines. Chem Heterocyclic Compd 6(8) 975-977 Silvestri R, Pifferi A, Martino G, Massa S, Satumino C, Artico M (2000) Reductive smiles rearrangement of l-[(5-chloro-2-nitrophenyl)sulfonyl]-lff-p5rtrole-2-caibohydrazide to l-amino-6-chloro-2-(lW-pyrrol-2-yl)benzimidazole. Heterocycles 53(10) 2163-2174. doi 10. 3987/COM-00-8970... 

A great number of quinoxalines fused with other heteroaromatics have been reported, especially of interest for their potential use in fighting various pathophysiological conditions like epilepsy, Parkinson s, and Alzheimer s diseases. These compounds as well as fused pyrazines are important for pharmaceutical agents, but they are excluded here since they are beyond the scope of this chapter. 

The introduction of pterin- and quinoxaline-substituents on dithiolenes induces reactivity at the pyrazine N atoms. In both Cp2Mo(quinoxalyldithio-lene) (61, in reaction A) and Tp MoO(quinoxalyldithiolene) (62, in reaction B) complexes, electrophilic attack at atom N1 of the pyrazine ring results in an intramolecular cyclization producing a pyrrole-like ring (Scheme 2.40). X-ray structures of both 61 and 62 reveal that the dithiolene chelate is asymmetrically bound, exhibiting asymmetric Mo-S and S-C bond lengths. 

Kumbhar A, Kamble S et al (2012) Bronsted acid hydrotrope combined catalyst for environmentally benign synthesis of quinoxalines and pyrido[2,3-b]pyrazines in aqueous medium. Tetrahedron Lett 53 2756-2760... 

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