8.8- Dicyano-5,6,7,8-tetrahydroquinolines

The relative ease of the reaction with the (l,l-dicyanopentynyl)pyrimi-dines induced studies of the reaction of 5-R-2-(l,l-dicyanohex-5-yn-1-yl)pyrimidine (R = H, NO2, Ar). They react with more difficulty, due to the longer tether between diene ad dienophile leading to a decreased entropic assistance. At a considerable higher temperature than observed for the (l,l-dicyanopentynyl)pyrimidines (210°C instead of 130°C and 8,8-dicyano-5,6,7,8-tetrahydroquinolines are formed (89T5151) (Scheme 33). 

Hydroxy-l,2,3,4-tetrahydroquinolines 304 were obtained by cyclization of oxime 2,4-dinitrophenyl ethers 303 in the presence of system NaBH3CN/NaH/l,4-dioxane (equation 131) . If the reductive cyclization was followed by oxidation with DDQ (2,3-dichloro-4,5-dicyano-p-benzoquinone) the corresponding 8-hydroxyquinolines 305 were obtained . ... 

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). 

Quinoline 82a could be prepared as a sole product by the following one-pot procedure after quenching the reaction with acetic acid, oxidation of the mixture with a 0.5 molar amount of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) gave quinoline 82a in 80% yield from 80a without forming tetrahydroquinoline 83a (Scheme 36). 

A similar reaction is found in the formation of 5-amino-l-cyano-2,4-dimethyl-3.106-dihydrobenzo[c][2,7]naphthyridine by reduction of 3,5-dicyano-2,6-dimethyl-4-(2-nitro-phenyl)-l,4-dihydropyridine [87] and tetrahydroquino[2,l-c]benzodiazepines from 2-cyano-l-(2-nitrobenzyl)-l,2,3,4-tetrahydroquinoline [88], When the hydroxylamino group has a choice between a nitrile or an ester group, as in 3-ethoxycarbonyl-5-cyano-2,6-dimethyl-4-(2-nitrophenyl)-1,4-dihydropyridine, it condenses with the nitrile in acidic solution and with the ester in basic media [89]. 

The obtained 1,2,3,4-tetrahydroquinolines V are rather stable, but the 1,2-dihydroquinolines VI are highly reactive and usually oxidized to the corresponding quinolines Vn (Scheme 13.110) on exposure to air [200, 202, 211, 212] or by a hydrogen transfer reaction [207b] in a domino one-pot process with the elimination [202, 212] or the Povarov reaction itself [202, 207a, b, 209a, 211], Also direct oxidation of the 1,2,3,4-tetrahydroquinolines can be accomplished stoi-chiometrically with 2,3-dichloro-5,6-dicyano-l,4-benzoqui-none (DDQ) [201] or catalytically with palladium combined with air [212],... 

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