Identification of a Foldaxane Kinetic Byproduct during Guest-Induced Single to Double Helix Conversion

An aromatic oligoamide sequence was designed and synthesized
to
fold in a single helix having a large cavity and to behave as a host
for a dumbbell-shaped guest derived from tartaric acid. NMR, molecular
modeling, and circular dichroism (CD) evidence demonstrated the rapid
formation of this 1:1 host–guest complex and induction of the
helix handedness of the host by the guest. This complex was found
to be a long-lived kinetic supramolecular byproduct, as it slowly
transformed into a 2:2 host–guest complex with two guest molecules
bound at the extremities of a double helix formed by the host, as
shown by NMR and CD spectroscopy and a solid-state structure. The
guest also induced the handedness of the double helical host, but
with an opposite bias. The chiroptical properties of the system were
thus found to revert with time as the 1:1 complex formed first, followed
by the 2:2 complex.