| Abstract: | Catalytic hydrogenation of 4-benzyloxyindoles does not stop at the hydroxyindole stage, but slowly leads to the 4,5,6,7-tetrahydro-4-ox-indoles 3 . Some procedures for the selective preparation of 4-hydroxyindoles 2 are described. When 4-benzyloxy-3-(1-hydroxyimino-ethyl)-indole ( 4c ) is warmed with trifluoroacetic acid, cleavage of the ether results as well as partial benzylation of the free hydroxyindole in the position 5 ( 5a, 5b ); no Beckmann rearrangement is observed. Esters of 4-benzyloxy-indole-2-carboxylic acid are formylated with POCl3/dimethylformamide in the 7-position to give 7a ; in the corresponding dimethylamide, on the other hand, the formyl group enters the 3-position to give 8 . Both 4- and 7-hydroxyindole are oxidized with Frémy's salt to the 4, 7-quinone 13 ; on reduction this yields 4, 7-dihydroxyindole 14 , which is tautomerized by base-catalysis to 5, 6-dihydro-4, 7-dioxo-indole 15 . The course of the etherification of 4-hydroxyindoles with epichlorohydrin and related compounds is described, and the resulting side-chains are characterized by their NMR. spectra. |
