Pyrido pyrazines derived from

Further work is required to establish the orientation of two other fused systems derived from pyrido[2,3-h]pyrazines. As the assignment of the phenacyl compound 137 as the 3-oxo derivative is uncertain, the structure of the furo compound 138, obtained on treatment of compound 137 with phosphorus pentoxide in concentrated sulfuric acid at 100° must also... 

Pyrido[2,3]pyrazines can be prepared by thermal cyclization of enaminones derived from 2-aminopyrazines (Table VIII, entry 5). 

Reaction of 3-amino-4-imino-4//-pyrido[l,2- ]pyrazine 306 (R = 4-MeC6H4) with ketenes 328, prepared in situ from the appropriate acetyl chloride with NEt3, yielded tricyclic derivatives 329 (Scheme 29) <1999JPR332>. Pyridine-3,4-dicarboxylates 330 were obtained from 4//-pyrido[l,2- pyrazines 307 with DMAD in refluxing toluene. 

Derivatives of pyrido[2,3-f>]pyrazine were prepared from pyridyl-substituted AAs. Reductive cyclization afforded 115 (79CZ387), whereas cyclization with potassium carbonate gave the dione 116 (87TL6363). 

Pyrazine 168 underwent cross-coupling with propyne in the tri-o-tolylphosphine, and copper(l) iodide to provide 170. The isocyanate or methyl chloroformate and sodium hydride to give An isolated example of the synthesis of chiral pteridines from a (Scheme 33). 2-Isothiocyanatopyrazine-3-carboxylates have been isothiocyanatopyrazine-2-carboxylate 172 reacted with R)- —) provided the pteridine derivative 173 and uncyclized pyrazine with pyridine precursors afforded pyrido[2,3 Pytitnidines. 

Interaction of substituted dienediamines with ethylenediamine yields im-idazo[3,2-n]pyridine derivatives (95JHC477). 3-Arylaminoindoles, with distinct enamine properties, are the starting materials for the synthesis of indolo[3,2-b]quinoline derivatives [96KFZ(7)42]. a-Carboline derivatives can be obtained from enamine-based 3-dimethylamino-2-indolinones [96KFZ(9)35, 96KFZ(10)32], The synthesis and transformation of enamines based on the pyrido[l,2-n]pyrazine ring system into imidazo[l,2-n]pyridine and imidazo[l,2-a]pyrimidine derivatives have been reported... 

Reduction of 5-methyl-l-oxo-2,3,4,6,7,8-hexahydro-l//-pyrido[l,2-ajpyrazinium iodide, its 2-benzyl derivative and cis- and trans-5-methylperhydropyrido[l,2-a]pyrazin-l-ones with Li in liquid NH3 gave 1-methyldecahydro-l,5-diazecin-5-one (73CPB1248). Under the same conditions, 2-benzoyl-5-methylperhydro-pyrido[l,2-a]pyrazinium iodide afforded 2-benzylperhydropyrido[l,2-a]pyrazine. The 10-membered l-methyldecahydro-l,5-diazecin-5-one was also obtained from trans-5-methylperhydropyrido[l,2-fl]pyrazin-l-one by treatment with sodium amalgam in aqueous EtOH. 

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