SmI₂-Promoted Radical Addition Reactions with N-(2-Indolylacyl)oxazolidinones:  Synthesis of Bisindole Compounds

The treatment of N-acyl oxazolidinones of N-benzyl 2-indolecarboxylic acids varying in the substitution pattern of the indole ring with samarium diiodide at −78 °C led to the formation of two indole dimer products. The major product isolated in yields from 55 to 59% represents an unsymmetrical dimer arising from 1,4-addition to the 2-indolecarboxylic acid derivative of a possible ketyl-type radical anion intermediate originating from the reduction of the exocyclic carbonyl group of the N-acyl oxazolidinone. The minor dimer, represented by a symmetrical diketone, was produced in yields ranging from 11 to 23%. Even in the presence of an α,β-unsaturated amide, dimerization was the preferred pathway rather than the formation of a γ-keto amide. Upon treatment with acid, the unsymmetrical indole dimer cyclized to form a diindolequinone. Finally, the N-acyl oxazolidinones of pyrrole-2-carboxylic acid and 3-indolecarboxylic acid preferred in both cases to undergo C−C bond formation with an acrylamide in the presence of SmI₂ rather than dimerization.