DISSYMMETRIC BAMBUSURILS: SYNTHESIS AND SUPRAMOLECULAR PROPERTIES

Bambusurils (BUs) are unique macrocycles assembled from N,N-dimethyl-glycoluril units that are connected by methylene bridges. The convex faces of the glycoluril units form the interior of the cavitand with the two methine hydrogen atoms of each glycoluril unit pointing inward. This orientation is the key for their strong binding interactions with anions. While BU[4] is too small to accommodate even the smallest anion, F^‒, BU[6] is an excellent anion receptor that exhibits high selectivity for I^‒ over various inorganic anions.

Up to date, only N,N-disubstituted glycolurils with identical substituents have been developed for the synthesis of symmetric R₁₂BU[6]s and R₈BU[4]s, respectively, and their binding properties were studied. Recently, we have demonstrated that sulfur replacement in bambusurils could significantly alter their binding properties.¹ Similarly, BUs can have intriguing stereochemistry in terms of configurational isomers based on geometric isomers or optical isomers leading to unique binding properties of amino acids.

Here we describe for the first time the synthesis of chiral glycolurils as well as asymmetric N,N`-disubstituted glycolurils and their use as starting materials for the synthesis of dissymmetric BUs. In a preliminary study, we were able to isolate four different diastereomers of Bn₄Me₄BU[4] based on the standard reaction conditions that are commonly used for the preparation of BU[4]. In contrast, a single diastereomer of the hexameric bambusuril was obtained under anion-template reaction conditions, i.e., Pr₆Me₆BU[6]*TBAX exhibits as a single stereoisomer according to NMR. We anticipate that these unique dissymmetric BUs could be used in chiral recognition and asymmetric organocatalysis.

1. Singh, M.; Solel, E.; Keinan, E. and Reany O. Chem. Eur. J. 2015, 21, 536-540.