All products, except quinolyl derivative 4, were additionally characterized by single crystal X-ray crystallography (Supplementary section 3). The solid-state structures are (as expected) of the energetically most stable Z isomers. The structural similarity between these Z isomers is high, i.e., the torsional angles between 3-oxindole and N-heterocycle moieties are all below 3° and the N(H) … N bond distances are found between 2.72-2.86 Å, which is within hydrogen bond range (Supplementary Table ). The presence of a stabilizing intramolecular hydrogen bond is thus supported in all cases. Important to note is that for (Z)-1 this hydrogen bond distance (2.82 Å) is similar to the one in the DFT calculated structure (2.81 Å).