Quinolines 1.2- dihydro-, disproportionation

Friedlander synthesis, 2, 445 Quinoline, alkenyl-1,2-dihydro-disproportionation, 2, 369 Quinoline, 2-alkoxy-reactions, 2, 351 Quinoline, 4-alkoxy-reactions, 2, 351 Quinoline, 2-alkoxy-4-hydroxy-synthesis... 

In the case of 8-methylquinoline, the intermediate dihydro derivative yields nearly equal amounts of the two disproportionation products, while with quinoline itself only a trace of the tetrahydro compound was formed. ° In the latter case some other oxidizing agent, perhaps traces of oxygen, must have been involved. 

Oximes are good precursors to reductive cyclization. A reductive base-induced cyclization of O-aryl oxime 4 will yield the tetrahydroquinoline <1998CL437>. The reduction prevented the normal dihydro-cyclization product from disproportionating to the quinoline and tetrahydroquinoline <1998BCJ2945>. By adding 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) after cyclization, the quinoline is the sole product (Scheme 9). 

Lithium in liquid ammonia conditions can produce l,4-dihydroquinoline" and 3,4-dihydroisoquinoline." Conversely, lithium aluminium hydride reduces generating l,2-dihydroquinoline" and 1,2-dihydroisoquinoline. These dihydro-heterocycles can be easily oxidised back to the fully aromatic systems, or disproportionate, especially in acid solution, to give a mixture of tetrahydro and re-aromatised compounds. Stable dihydro-derivatives (see also 9.13) can be obtained by trapping following reduction, as a urethane, by reaction with a chloroformate. Quaternary salts of quinoline and isoquinoline are particularly easily reduced, either catalyticaUy or with a borohydride in protic solution, giving... 

---