Lumazines, 7-aryl

Lumazine, 7-amino-l,3-dimethyl-acylation, 3, 290 quatemization, 3, 305 reactions, 3, 294 reactivity, 3, 305 Lumazine, 6-aryl-1,3-dimethyl-synthesis, 3, 317... 

Another ring contraction is observed during 1,3-dipolar cycloadditions of 6-aryl-1,3-dimethyl-lumazine 5-oxides with dimethyl alkynedicarboxylate, forming 8-aryltheophyllines and 1,3-... 

An unusual O-arylation occurs when the amino-nitroso-uracil (i63) is treated with diphenyliodonium chloride the diazabutadiene (364) is formed in 96% yield. This undergoes regiospecific addition with enamines to form lumazine derivatives, e.g. (365) (Scheme 140). The different methods for ring-closing 3-aminopyrazine-2-carboxamide to 4-pteridones have been compared and evaluated. No one reagent is superior in all cases. 

Substituted lumazines can be arylated at the 6-position by reacting them with aryl-diazonium chlorides in alkaline solution this being a new method for the introduction of the aryl group to pteridines (Scheme 8) <97H(45)765>. 

Similar intermediates are probably involved in the thionyl-chloride-promoted cyclization of 6-(benzylidene-l-methylhydrazino)-l,3-dimethyl-uracils (246 R = Me R = H) to pyrazolo[3,4-ff]pyrimidinediones (247). Less easily explained is the cyclization of hydrazones (246 R = Me R = H) to the oxazolo [5,4-f/]pyrimidines (245 X = 0) by sodium nitrite in boiling acetic acid. Undoubtedly, nitrosation at the 5-position is the initial step in these reactions, but thereafter events remain unclear. Interestingly, the ketone hydrazones (246 R = H, R = Me), on similar treatment, followed by reduction with sodium dithionite, furnish 6-aryl-l,3-dimethyl-lumazines (248) in low yields (10—15%). ... 

These results shown in Schemes 3.106 and 3.107 indicate that the cyclization process depends strongly on the energetics of aryl ring formation. Thus, the reaction of 1,2-diethynylbenzene to give naphthalene is energetically less favorable. Apparently, the electronic situation for 1,2-bis(phenylethynyl)arenes is critical. Indeed, the triple bond in lumazine... 

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