Five-membered heterocyclic ureas suitable for the donor–donor–acceptor hydrogen-bonding modules

Five-membered heterocyclic ureas are capable of forming the unfolded conformer without preorganization by using the intramolecular hydrogen bond, and are suitable for the DDA hydrogen-bonding modules. In contrast, six-membered heterocyclic ureas are destabilized by an effect of steric repulsion due to the closer distance of CHc⋯O and their conformational equilibriums are biased toward the stable folded conformer.