Cyclizative Condensations. I. 2-Methylindole with Acetone and Methyl Ethyl Ketone

Evidence, both chemical and spectroscopic, has been obtained to indicate that the monobasic 2:2 condensation product from 2-methylindole and acetone, first assigned structure 1a by Scholtz, has structure Va or Via, the position of the olefinic double bond being uncertain. The same compound was also obtained from reactions of 2-methylindole with: (a) the 1:1 adduct of 2-methylindole and mesityl oxide (IIIa), (b) phorone, and (c) mesityl oxide. Likewise, action of acetic anhydride on the bisindole IIe produced the monobase Va as its monoacetyl derivative, which was also obtained directly by acetylation of the monobase. It has been shown that the acetyl, propionyl, and n-butyryl derivatives, previously assigned structures IIa, IIb, and IIe by Scholtz, are, in fact, simple acyl derivatives of the monobase. The presence of both a basic and an acidic NH group in the monobase has been established by stepwise monomethylation (to Vb and Vd), in which the sequence can be reversed, but which leads to the same dimethyl derivative (Vc) in either case. The presence of an olefinic double bond in the monobase is established by hydrogenation to the dihydro derivative VII. A mechanism for formation of the monobase is proposed, involving the intermediates VIII-XI. That indole monobases will also form, however, from cyclic keto monobases is shown by the formation of XII in 70% yield from condensation of 2-methylindole with IVa. An analogy for the proposed mechanism of monobase formation is presented involving formation of the dimer XIVa from the vinylindole XIII. An attempt to prepare a similar product (XIVb) by addition-condensation of 2-methylindole with benzalacetophenone gave 3,3′-benzylidene-2,2′-dimethylbisindole. Condensation of 1,2-dimethyilndole with acetone gave only the bisindole IIf, and none of the monobase Vc. 1,2-Dimethylindole and mesityl oxide gave the 1:1 adduct IIIb. Condensation of 2-methylindole with methyl ethyl ketone gave two isomeric monobases (XV).