Cyclization of 2-acetaldehyde Diethyl Acetals to Indoles. Evidence for Stereoelectronic Effects in Intramolecular Electrophilic Aromatic Substitution

Article abstract of DOI:10.1021/jo00176a007

Methanesulfonamides of N-(2,2-diethoxyethyl)anilines can be cyclized to indoles in aromatic solvents by reaction with titanium tetrachloride.The temperature of the cyclization is substituent dependent, occurring at 0 deg C for the m-methoxy derivative but requiring 130 deg C for the p-bromo compound.Yields are good for various alkoxy-, alkyl-, and haloindoles, ranging from 60percent to 90percent.Meta-substituted reactants give rise to mixtures of 4- and 6-substituted indoles in which the 6-substituted product dominates by 2-4:1.The cyclization fails for ortho-substituted reactants.The major reaction process is N-dealkylation in the case of ortho-substituted compounds.An analogous cyclization occurs with the methanesulfonamides of N-(3,3-diethoxypropyl)anilines to give 1-(methylsulfonyl)-4-chloro-1,2,3,4-tetrahydroquinolines.This cyclization is much more rapid than for the five-membered ring closure leading to indoles and indicates a substantial rate retardation due to stereoelectronic effects in the indole cyclization.Ortho substitution also prevents cyclization in the six-membered-ring case.