Cycloaddition or Metathesis: Selectivity in a Series of Indole-2,3-Quinodimethane Derivatives

The efficiency of combining experimental methods and quantum chemical calculations (DFT and ab initio) for optimizing conditions and predicting mechanisms for the interaction of acyclic and cyclic derivatives of indole-2,3-quinodimethane with heterodienophiles is demonstrated. For the reactions of acyclic derivatives of indole-2,3-quinodimethane with nitriles and carbon disulfide, indoles, [b]-fused with nitrogen- and sulfur-containing six-membered rings, were synthesized. Quantum chemistry methods have shown that the addition of nitriles and carbon disulfide to indole-2,3-quinodimethane derivatives occurs through cyclic transition states involving a lithium atom. Calculations have substantiated the choice of a non-polar aprotic solvent to increase the yield of γ-carbolines. It has been shown for the first time that the replacement of the oxygen atom in cyclic derivatives of quinodimethane—pyrano[4,3-b]indol-3(5H)-ones by sulfur leads to a complete change in the direction and mechanism of their reaction with dimethyl acetylenedicarboxylate: instead of the usual [4+2] cycloaddition in the case of an oxygen-containing substrate, cascade processes are realized, including alkyne-thiocarbonyl metathesis and [3+2] cycloaddition.