T. Shono, Y. Matsumura, Kenji Inoue
Oct 1, 1982
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Journal of the American Chemical Society
Abstract
A versatile synthetic method of tetrahydroquinolines and julolidines has been developed. The method involves the anodic oxidation of N,N-dimethylaniline in methanol to afford a-methoxylated or a,&-dimethoxylated compounds and subsequent treatment of the products with Lewis acids in the presence of nucleophiles. Simple and electron-rich olefins such as alkenes, styrene, enol ethers, silyl enol ethers, enamines, and enol esters are usable as the nucleophiles. The intermediary formation of iminium ions from the methoxylated compounds is proposed as one of the key steps. The nucleophilic reaction of Grignard reagents with the methoxylated compounds in the presence of Lewis acid is also described. Although anodic oxidation of amines or their derivatives (1) (Scheme I ) has been known to be a versatile tool in generating iminium ion intermediates (2),l the trapping of 2 with carbanion or the like under conditions of anodic oxidation (path a ) is almost impossible, since except cyanide ion,2 the nucleophiles mentioned above are generally unstable under the reaction conditions. W e have recently found, however, that 2 (R' = C 0 2 C H , ) can be trapped efficiently with a variety of nucleophiles when 2 is first trapped with methanol to give a-methoxylated carbamates (4, R' = C 0 2 C H 3 ) followed by regeneration of 2 from 4 with Lewis acid catalysts in the presence of nucleophiles (path b).3 This concept of trapping and regeneration of iminium ion may be applicable to amines other than carbamates of aliphatic amines. According to the above concept, we describe herein a versatile perparation of tetrahydroquinolines (9) and julolidines (34 and 35) starting from N-methyl-N-alkylanilines (5). Scheme I1 il(1) For examples, see (a) Barnes, K. K.; Mann, C. K. J. Org. Chem. 1967, 32, 1474. (b) Barry, J. E.; Finkelstein, M.; Mayeda, E. A,; Ross, S . D. Ibid. 1974,39,2695. (c) Shono, T.; Hamaguchi, H.; Matsumara, Y . J. Am. Chem. SOC. 1975, 97, 4264. (2) Chiba, T.; Takaya, Y . J . Org. Chem. 1977, 42, 2973. (3) (a) Shono, T.; Matsumura, Y.; Tsubata, K. J . Am. Chem. SOC. 1981, 103, 1172. (b) Shono, T.; Matsumura, Y.; Tsubata, K.; Takata, J. Chem. Lett. 1981, 1121. (c) Shono, T.; Matsumura, Y. ; Tsubata, K. Tetrahedron Let?. 1981, 22, 2411. (d) Ibid. 1981, 22, 3249. Scheme 1