Oxazolo pyridinium perchlorates

Babaev and co-workers prepared and isolated several oxazolo[3,2-a]pyridi-nium perchlorates (Scheme 1.284). For example, using Bradsher and Zinn s protocol,the authors treated l-phenacyl-2-pyridone 1077 (R = X = H) with concentrated H2SO4 to produce an intermediate sulfo betaine (not isolated), which was heated in 70% HCIO4 to afford 2-phenyloxazolo[3,2-a]pyridinium perchlorate 1078 (R = X = H) in good yield. 

Heating 5-methyl-2-(4-nitrophenyl)oxazolo[3,2-a]pyridinium perchlorate 1078 (R = CH3, X = NO2) with ammonia gave the expected 5-methyl-2-(4-nitrophenyl)... 

Novel photocyclizations of l-vinyl-2-pyridones (74) in aqueous perchloric acid to oxazolo[3,2-a]pyridinium perchlorates (75) have been reported. A reaction pathway involving reversible electrocyclization of the singlet excited vinyl-pyridone to the highly resonance-stabilized azomethine ylide (76) is favoured. For example, photolysis of the pyridones in aqueous solution permits isolation of the alcohols (77 R = Me) and (77 R = H) in 87.9% and 81% yield respectively. 

A variation of the classical procedure utilizes 2-methoxy-6-methylpyridine as a starting material and leads eventually to 5-aminoindolizines the intermediate oxazolo[3,2-fl]pyridinium salts are isolated as perchlorate salts (CAUTION). ... 

Alkylation of 2-methoxypyridine with a phenylacyl bromide or bromoacetone gives Mphenacyl- or 7V-acetonyl-2-pyridone, which can be cyclized in sulfuric acid or perchloric acid to oxazolo[3,2-fl]pyridinium salts (RJ. 1115), which have been isolated as their perchlorates (XII-7SS). ... 

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