2 -Pyridinone, 1-phenacyl-, cyclization

The 1 phenacyl-2( 1 //)-pyridinone (66) is cyclized to the oxazolo[3,2-a]pyridinium salt (67) by sulfuric acid (67JHC66). Sometimes the loss of a proton can give a neutral product as in the conversion of (68) into the ring-fused system (69) (71JOC222). For further examples see Section 3.3.3.4.1 CHEC 4.29 CHEC-II, 8.10.3 and 8.10.5. F... 

Azine approach. Oxazolo[3,2-a]pyridinium salts (210) were first obtained from the cyclodehydration reaction of l-phenacyl-2(lii)-pyridinone in sulfuric acid (67JHC66). These salts can also be prepared from 2-halo-l-phenacylpyridinium derivatives (211) by treatment with a base which causes ylide formation and hence cyclization by intramolecular substitution (69JOC2129, 76CB3646). It is recommended that a bulky tertiary amine is used as base in order to avoid opening of the ring or substitution of the 2-halo substituent in the starting material (211). Isoquinoline and quinoline analogues have also been prepared by these methods. 

Diaza-compounds. Phenacyl bromide reacts with 2-aminopyridine 1-oxide to yield 2-phenylimidazo[l,2-a]pyridin-3-ol (769) another derivative of this ring system, compound (771), is produced by the action of diphenylketen on the sulphilimide (770), dimethyl sulphide being eliminated.(6-Methyl-2-pyri-dyl)acetyl azide (772) forms the dihydroimidazo[3,4-a]pyridinone (773) on heating." Whereas l-methylpyrrole-2-aldehyde and other heterocyclic aldehydes react with tosylmethyl isocyanide to give oxazoles, e.g. (774), pyrrole-2-aldehyde affords the 6-azaindolizine (775). The action of potassium amide on the derivative (776) of 3-bromopyridine furnishes the pyrrolo[3,4-c]pyridine (777) by cyclization of an intermediate pyridyne." ... 

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See also in sourсe #XX -- [ ]

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