Pyridazinium-5-olates

Methylation (Mc2S04 or MeOTs) of pyridazin-3-ones (138) followed by deprotonation of the salts (139) gives good yields of the crystalline pyridazinium-3-olates(137 R = Me)(Vc=o 1550 cm ). These betaines (137 R = Me) appear to be unreactive toward dipolarophiles.  

Photolysis of l,6-dimethylpyridazinium-3-olate (137 R = R = Me, r3 = R = H) in water gives 2,6-dimethylpyridazin-3-one (140), whereas irradiation in acetonitrile solution gives the fused diaziridine 141 which is converted into compound 140 by water. This transformation (141 - 140) probably involves initial hydrolysis of the amide bond (see Section III,B,7).  

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Deprotonation of 7-hydroxypyrido[2,l-a]phthalazinium perchlorate with NEt3 in ethanol gave pyrido[2,l-a]phthalazinium-7-olate (20) (92CB929). 4-Hydroxy-2-phenyl and 2-hydroxy-4-phenylpyrido[l,2-6]pyridazinium perchlorates with aqueous KOH afforded 73 and 75 (90JHC1673). 

Photolysis of the zwitterionic pyrido[l,2-h]pyridazines (89) and their 5,6-benzo and 2,3-tetramethylene derivatives afforded 6,7,8,9-tetrahydro-4//-pyrido[l,2-fl]pyrimidin-4-ones (90) and their 8,9-benzo [77H(8)377] and 2,3-tetramethylene derivatives (75JOC2201). Photoinduced ring transformation of 2-phenylpyrido[l,2-fe]pyridazinium-4-olate (73) was investigated in methanol (94T4699). 

A reaction of particular interest is the photochemical rearrangement of compound 243 to 3-methyl-4(3//)-quinazolinone (246). This transformation (243 -> 246) (Scheme 10), which takes place in high yield in ethanol, has been applied to several other derivatives and is clearly analogous to the photorearrangement of pyridazinium-5-olates to 4-pyrimidones (Section in,A,6). The mechanism of the reaction is not clear. 

Dieckmann condensation of the diester 523 gives compound 524 which is readily oxidized to the pyridazinium-5-olate 525 (R = COjEt). This betaine (525 R = COjEt) has been converted into a number of derivatives (525 R = H, CO2H, Br, NHPh, SCH2Ph). Photolysis of the betaines 525 (R = H, CO2Et) results in rearrangement to isomeric 4(3//)-pyrimi-dinones. Full details of another study of this type of photochemical rearrangement of pyridazinium-5-olates (142) have now been published. ... 

Reaction of the 2-acetoxy-3(2//)-furanones (526) with monosubstituted hydrazines gives good yields of the pyridazinium-5-olates (527) together with varying amounts of isomeric products. Alkyl derivatives (527 R = alkyl) have also been prepared by base-catalyzed alkylation (Mel, Me2SO4, PhCH2Cl) of 3-methyl-6-phenyl-5-ethoxycarbonyl-4( 1 //)-pyridazinone. Reduction of the diphenyl compound 527 (R = Ar = Ph) by zinc and hydrochloric acid gives 3-ethoxycarbonyl-5-hydroxy-5-methyl-l,2-diphenyl-2-pyrrolin-4-one (528 R = Ar = Ph) (Scheme 21... 

Pyridazinium-5-olate betaines are isomerized photochemically to corresponding pyrimidin-4-ones by a similar path (Scheme 12). 

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