Reactions of 1,3‐Dipoles with Heterocycles, 8. — Synthesis of 1,8a‐Dihydro[1,2,4]triazolo[4,3‐a]pyridines and Benzologues
The C,N double bond of pyridine, quinoline and isoquinoline as heterodipolarophile react with diarylnitrilimines 2, generated in situ by dehydrohalogenation of N‐phenylbenzhydrazonoyl chlorides 1, in a cycloaddition with complete regioselectivity. A facile route to hitherto unreported 1,3‐diaryl‐1,8a‐dihydro[1,2,4]triazolo[4,3‐a]pyridines 3, 1,3‐diaryl‐3,3a‐dihydro[1,2,4]triazolo[4,3‐a]quinolines 4, and 1,3‐diaryl‐1,10b‐dihydro[1,2,4]triazolo[3,4‐a]isoquinolines 5 has been developed. In a similar way, cycloadditions are carried out with C‐ethoxy‐carbonyl‐ and C‐acetyl‐N‐phenylnitrilimines. The ring cleavage of 3 in acidic medium yields the corresponding [(arylhydrazono)methyl]pyridinium chlorides 7. The conversion of the open‐chain products back to 3 has been carried out in pyridine containing triethylamine. Anodic oxidation of 3–5 in aprotic medium affords the [1,2,4]triazolohetarenium perchlorates 9–11. The yields in such reactions are either similar or better than with chemical oxidants. The thermally initiated cycloreversion of 3 and 5 is discussed, judging from the facts that the thermolysis afford 2:1 cycloadducts.