Configuration, stereodynamics and crystal structure of 3-substituted-2-oxofuro[2,3-b]indoles

The molecular structure of 3a-alkyl-3-cyano-2-oxofuro[2,3-b]indoles 1a-d an d 3a-alkyl-3-methoxycarbonyl-2-oxofuro[2,3-b]indoles 2a-c has been determin ed by H-1 NMR spectroscopy, X-ray diffraction and semiempirical (AM1) and a b initio (HF/3-21G* level) quantum chemical calculations. Considering detai led NOE measurements and the anisotropic effects of the C(3) carbonyl ester group, the preferred configurations were determined in solution. In these compounds the thermodynamic preference of the C(3) electron-attracting subs tituent for the endo or exo face appears to be controlled by the steric dem and of the C(3a) alkyl substituent. The structurally folded furoindoles end o-2a-c evidence a repulsive interaction between the sterically crowding CO2 Me group and the pi system of the benzene ring. An increase in the solvent polarity significantly alters the endo:exo ratio in favor of the endo isome r. The solid-state structures of 1d and 2a demonstrate that these compounds assume configurations which differ in the spacial orientation of the C(3) substituent, with the CN group in 1d oriented to the endo face and the CO2M e group in 2a to the exo face of the molecule, in agreement with those conf igurations determined in CD2Cl2 solution and from theoretical calculations.