3- Amino-5-phenyl-1,2,4-triazine 1 -oxide formation

Phenyl-l,2,4-triazine (5) leads (via transition state 11) to the 3,4-dihydropyridine 6, which (after oxidation and Cope elimination) affords the 2-phenylcyclopenta[c]pyridine 7 6-phenyl-l,2,4-triazine (8) leads to the 3,4-dihydropyridine 9 (via transition state 12) and (after amine elimination) to 5-phenylcyclopenta[c]pyridine 10. These results indicate that product formation is decisively influenced by secondary orbital interactions between the amino and phenyl groups in the transition states preceding cycloaddition. 

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