Beiträge zur Chemie der 4‐Hydroxyindol‐Verbindungen 10. Mitteilung über synthetische indolverbindungen [1]

Abstract

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 POCl 3 /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.