Isomerization of Bicyclic Hydrazine Monoesters to N-Amino-2-pyridone Esters, Precursors to Bicyclic "Isosydnones"

Selective aqueous alkaline hydrolysis of the diazetidine diesters 4 gives the basic monoesters 1 which isomerize quantitatively in trifluoroacetic acid or by refluxing with sodium methoxide in dioxane to the N-aminopyridone esters 2 . The bridged pyridazine ester 5 a can also be selectively hydrolyzed to the presumed intermediate 10 , hydrolysis being accompanied by immediate isomerization to 2 a. A common mechanistic pattern is proposed for these isomerizations.Heating of 2 causes loss of alcohol to give the mesoionic "isosydnone" 3 , a fairly stable ring system slowly opened by ethanol to 2 b and by concentrated alkali to the N-aminopyridone 16 a. Oxidation of 16 a with lead tetraacetate gives the pyridone 18 a.