New Cucurbituril Homologues:  Syntheses, Isolation, Characterization, and X-ray Crystal Structures of Cucurbit[n]uril (n = 5, 7, and 8)

Abstract

Cucurbituril (CB[6]) is a hexameric macropolycyclic compound self-assembled from an acid-catalyzed condensation reaction of glycoluril and formaldehyde. 1,2 Its easy synthesis and rigid structure with a hollow core make CB[6] one of the attractive synthetic receptors. Its inclusion properties have been investigated extensively by Mock and co-workers. 3 Enzyme-like behavior of CB[6] effecting a large rate acceleration in a cycloaddition reaction has also been observed. 4 Its ability to bind alkali metal ions at the portals of the cavity allowed us to study molecular container systems capable of reversible encapsulation and release of guest molecules. 5 We also have demonstrated that CB [6] can be used as a molecular "bead" in self-assembly of interlocked structures such as polyrotaxanes and molecular necklaces. 6 CB- [6] is also known to be effective for removal of dyes from waste waters. 7 Although several homologues are available in calixarenes, which are obtained from a base-catalyzed condensation of phenol and formaldehyde, cucurbituril homologues with greater or fewer glycoluril units have not been reported. 8 Here we report the new family of cucurbituril, cucurbit[n]uril (CB[n]) (n ) 5, 7, and 8) containing five, seven, and eight glycoluril units, respectively.Reaction of glycoluril with formaldehyde in 9 M sulfuric acid at ∼75°C for 24 h and then at ∼100°C for 12 h yielded a mixture of CB[n] (n ) 5-11) as evidenced by electrospray ionization mass spectrometry. 9 Although its contents vary from batch to batch, the mixture typically contains ∼60% of CB[6], ∼10% of CB [5], ∼20% of CB [7], and ∼10% of other higher CB homologues. Apparently, the reaction of glycoluril and formaldehyde first generates linear oligomeric products which then cyclize to produce a library of cucurbituril. The cyclization at the lower temperature compared to that employed in the conventional synthesis 2 of CB[6] allows formation of significant amounts of other CB homologues besides CB [6]. The major product CB [6] was separated from the mixture by fractional dissolution of other CB homologues with acetone/water. From the acetone/water soluble portion, CB[5] and CB[7] were isolated by fractional crystallization/precipitation. 9 The homologue CB[8] was isolated from a CB[n] mixture that had been prepared by a modified procedure. Reaction of glycoluril and formaldehyde in the presence of HCl in a high-pressure reactor at 115°C for 24 h produced a white powder which, when treated with dilute sulfuric acid in a way similar to the one described above, yielded a mixture of CB[n] having a similar composition with slightly higher CB[7] and CB[8] contents. Upon standing, crystals of CB[8] were produced from a solution of the CB[n] mixture in 6 M sulfuric acid. 9 The new CB homologues CB[5], CB[7], and CB [8] have been characterized by various spectroscopic methods and X-ray crystallography. In the 1 H NMR spectra, CB homologues show the same peak pattern as that of CB[6], but their chemical shift values are slightly and yet distinctivel...

Halloysite nanotubes loaded with peppermint essential oil as filler for functional biopolymer film

“…Cucurbit[6]uril was synthesized through the procedure reported by Kim et al (2000) Peppermint essential oil was purchased from AromaZone.com.…”

Antimicrobial cationic polymers: from structural design to functional control

“…From another perspective, they can be also regarded as the guest motifs for host-guest complexations. By using a water-soluble macrocyclic host, such as cucurbit[n]urils (CB[n]s) 79 and cyclodextrin, 80 the hydrophobic group can be encapsulated into the cavity of the host. Besides, the cationic group can further enhance the host-guest complexation by introducing electrostatic interactions.…”

The Delivery of Triamterene by Cucurbit[7]uril: Synthesis, Structures and Pharmacokinetics Study

“…26−30 Cucurbituril (Scheme 1) contains a hydrophobic cavity with two uniform hydrophilic portals composed of carbonyl functional groups. 27,31 Among the CB [n] homologues, the size and shape of CB [6], CB [7] and CB [8] are close to those of α-, β-, and γ-cyclodextrin, respectively. Unlike the poor solubility of CB [6] and CB [8] (below 10 −5 M), CB [7] is aqueous soluble (0.02− 0.03 M).…”

“…Cucurbit [7]uril was synthesized by the reported methods, 27,32,50 which was further purified by recrystallization from distilled water, and characterized by single X-ray analysis, 1 H NMR spectra were recorded on a Varian Mercury 300 MHz spectrometer. 13 C NMR spectra were recorded on Bruker UltraShield 400 MHz spectrometer.…”