Conformational insights into furo- and thieno[2,3-b]indolines derived from coupling constants and molecular modeling

Abstract

The extent to which conformational preferences of fused heterocyclic five-membered rings change with the nature of the heteroatom (O and S) was investigated in furo- (1, 2) and thieno[2,3-b]indolines (3, 4) by the combined use of 1H NMR spectroscopy and density functional theory (DFT) calculations. In contrast to the behavior observed for pyrroloindolines, the furo- and thienoindolines exist in solution in only one conformer, with structures in the 2E-2T3 (1,2) and 2T3-E (3,4) North/West region of the pseudorotational wheel, and with pseudorotation phase angles (P) of 315.8, 311.6, 337.2 and 331.6 degrees, respectively.