Zur Kenntnis der Indolreaktion nach Fischer. II. Thermische und säurekatalysierte Indolisierung von 1′-Alkenyl-2′-methyl-2′-phenylacetohydraziden

On the Fischer-Indole Reaction. II. Thermal and Acid Catalysed Indolization of 1′-Alkenyl-2′-methyl-2′-phenylacetohydrazides Seven different 1′-alkenyl-2′-methyl-2′-phenylacetohydrazides, 6a-g , have been prepared by treatment of the methylphenylhydrazones 7 of appropriate ketones and aldehydes with acetyl chloride in pyridine. At 170° 6a-g are transformed into the N-methylindoles 3a-g and acetamide in moderate yield. N-Methylaniline is the other major reaction product indicating that homolytic cleavage of the weak N, N-bond in 6 is a major primary reaction step. It is likely but not proven that the N-methylindoles 3 are formed in a reaction sequence initiated by an uncatalysed concerted 3, 3]-sigmatropic rearrangement. Upon treatment of 6 with 0.5N dichloroacetic acid in anhydrous acetonitrile at room temperature a quantitative conversion to 3 is observed, interpreted as proceeding by a charge induced 3, 3]-sigmatropic rearrangement of protonated 6 in the rate determining step. The ketone derivatives 6a-e (R¹ = alkyl) react 40-1000 times faster with acid than the aldehyde derivatives 6f and 6g (R¹ = H). This is rationalized as a consequence of the increased basicity of 6a-e relative to 6f and 6g caused by a steric effect.