1,3-dipolar cycloaddition heterocyclic adducts formation

Hydroxypyridine (201) itself possesses latent 1,3-dipolar character because of tautomerism involving 1-protiopyridinium 3-oxide (202). Aprotic diazotization of anthranilic acid in the presence of 201 gives two heterocyclic products [196 (20%) and 203 (23%)] which were isolated in separate experiments run under almost identical conditions.103,105 Formation of the bis-adduct 196 must involve cycloaddition of benzyne to 202 and N-phenylation and there is some evidence from related additions to 2//-phthalazin-1 -one (208) that the steps occur in this order.3 7b Formation of the isocoumarin structure 203 apparently involves electrophilic substitution of 201 by the benzyne precursor 5, followed by lactonization. From 3-hydroxy-6-methylpyridine compounds analogous to 196 and 203 were also obtained (10 and 29%, respectively). 3-Hydroxyquinoline afforded only the corresponding isocoumarin 204 (20%) whereas 4-hydroxyisoquinoline gave 4-phenoxyisoquinoline (12%) and the bis-adduct 205 (12%) with benzyne.103,105... 

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