Mixed Indole Dimers, Trimers, and Their Acyl Derivatives

The synthesis of the first mixed indole and indole:pyrrole dimers is described. They are derived from an A component (an indole having an open 2-position) and a B component (an indole or pyrrole more nucleophilic than the A component). Included are the dimers from indole with 2-methylindole (Ib), 1,2-dimethylindole (Ic), 2-phenylindole (Id), or 2,5-dimethylpyrrole (XIIa), and the dimer from skatole with 2-methylindole (IIIa). Structures are assigned on the basis of spectral data and by analogy with the proven structure of diindole (Ia). The competitive dimerization reaction of indole and 1-methylindole permitted isolation, after acylation with maleic anhydride, of only maleyldiindole (XVa). The competitive reaction of indole and skatole proceeded in rather clean-cut stepwise fashion with formation first of diindole hydrochloride and then of diskatole hydrochloride. A new homodimer, 1,3-dimethylindole dimer (VIb), has been prepared. Evidence is presented concerning the structure of diskatole (VIa). As byproducts in the preparation of the mixed dimers, the first mixed trimers have been prepared. Included are diindole:2-methylindole trimer (IIc); diindole: 1,2-dimethylindole trimer (IId); and indole :di-2,o-dimethylpyrrole trimer (XIII). Diindole: 1,2-dimethylindole trimer (IId) is different from the dimethyltriindole obtained by methylation of triindole (IIa). Evidence is presented that, contrary to a previous report, the latter dimethyltriindole is in reality an N,N-dimethylaniline derivative (IIb) resulting from dimethylation of the primary amino group of triindole. Acyl derivatives of the mixed dimers and trimers have been prepared from maleic, succinic, itaconic, and citraconic anhydrides. Whereas acylation of diindole, indole: 2-phenylindole dimer, and indole: 2,5-dimethylpyrrole dimer with maleic anhydride gave only the maleyl derivatives, the corresponding reactions with indole: 2-methylindole dimer and indole: 1,2-dimethylindole dimer gave not only the maleyl derivatives (XVb-c) but also lesser amounts of the fumaryl derivatives (XVI b-c). In the two cases where both maleyl and fumaryl derivatives were obtained, hydrogenation of these stereoisomers gave the succinyl derivatives (XVIIb-c), which were also prepared independently by acylation of the dimers with succinic anhydride. Skatole: 2-methylindole dimer with maleic anhydride gave the maleyl derivative (IIIb), but in more polar solvents an isomer was obtained to which the spiro indolenine structure XXI is assigned.