Preparation and host-guest interactions of novel cage-type cyclophanes bearing chiral binding sites provided by dipeptide residues

Abstract

Novel cage-type cyclophanes, which are constructed with two rigid macrocyclic skeletons, tetraaza[3.3.3.3]paracyclophanes, and four bridging segments composed of either β- l-aspartyl- l-aspartyl or β- d-aspartyl- d-aspartyl residues were prepared [(−)- 1 and (+)- 1, respectively]. Structural and asymmetric properties of these cage-type cyclophanes were characterized by 1H NMR and circular dichroism (CD) spectroscopy. The guest-binding behavior of the cage-type hosts toward fluorescent guests, such as 8-anilinonaphthalene-1-sulfonate and 6- p-toluidinonaphthalene-2-sulfonate, was examined in comparison with those demonstrated by the corresponding non-cage hosts in aqueous media. The microenvironmental polarity around the fluorescent guests and their fluorescence polarization values in the presence of the cage-type hosts revealed that these guest molecules were incorporated into apolar host cavities, and their rotational motion was largely restricted. Furthermore, chiral host-guest interactions between the hosts and a hydrophobic guest, pamoic acid (PA), were examined by CD spectroscopy. PA was subjected to stereochemical changes to assume P- and M-helix configurations in the presence of (−)- 1 and (+)- 1, respectively.