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Unsubstituted Bambusurils: Post-Macrocyclization Modification of Versatile Intermediates

A new bambusuril derivative, (H)BU[6], lacking substituents on the ureidic nitrogen atoms, has been isolated and characterized. This macrocycle was prepared by the deprotection of bambusuril (PMB)BU[6]. (H)BU[6] is attractive for use as a starting compound for the preparation of other bambusuril derivatives, which was demonstrated via propargylation and the copper-catalyzed click reaction performed on the macrocycle.


■ INTRODUCTION
Bambusurils are a family of macrocyclic compounds consisting of n glycoluril units connected by n methylene bridges (where n = 4 and 6). 1,2 Four-membered bambusurils (BU [4]) are not able to include any guests inside of their small cavity. On the contrary, six-membered bambusurils (BU [6]) are prized for their excellent affinity for inorganic anions in water and many organic solvents. 3−6 The high affinity of six-membered bambusurils toward anions is further shared among other hemicucurbituril derivatives, 7−9 including biotinurils. 10,11 Recently, bambusuril derivatives, semithio-and semiaza-bambusurils, have been prepared by replacing oxygen atoms on the portals of the bambusurils by sulfur and nitrogen, respectively. 12,13 The bambusuril macrocycle has been used as parts of anion recognition systems, 14 electromembrane anion extraction, 15 photosensitive materials, 16 and systems for anion transmembrane transport. 17 Both the solubility and binding properties of bambusurils can be influenced by the type of substituents attached to the nitrogen atoms on the opposed macrocycle portals. Until now, there has been only one synthetic approach enabling the synthesis of bambusurils and thus also only one synthetic approach enabling the modification of substituents on their framework. This approach is based on the preparation of desired derivatives of glycoluril (Scheme 1) and its subsequent acid-catalyzed macrocyclization reaction with formaldehyde yielding a desired bambusuril. However, some glycolurils bearing certain types of substituents have proved to be unstable under these conditions, and no macrocycle could be isolated. Herein, we present a new synthetic protocol which allows for the preparation of bambusurils which are not achievable by the traditional synthetic approach.

■ RESULTS AND DISCUSSION
Our novel synthetic protocol is based on the preparation of a bambusuril bearing protecting groups on the nitrogen atoms of the glycoluril units (Scheme 1). The protecting group would subsequently be removed yielding bambusuril (H)BU [6]/ (H)BU [4], with nitrogen atoms on the macrocycle portals available for further reactions. Several protecting groups were tested with this approach. Our first choice, a p-methoxyphenyl substituent (Figure 1), which is removable by oxidizing agents, 18 proved to be unsuitable because of the low solubility of the corresponding glycoluril. In addition, steric hindrance by the rigid substituent prevented the glycoluril building blocks from assembling into a macrocyclic structure. We also investigated deprotection of the benzyl substituent on the already available macrocycles (Bn)BU [6] or (Bn)BU [4] (see Figure 1 for the structure). Unfortunately, deprotection of the macrocycles via hydrogenation or their treatment with a solution of sodium metal in liquid ammonia resulted either in only partial deprotection ( Figure S28) or decomposition of the macrocycle. Finally, we prepared bambusurils (PMB)BU [4] and (PMB)BU [6] bearing a p-methoxybenzyl protecting group (PMB, Scheme 2), which we successfully converted to the bambusurils (H)BU [6] and (H)BU [4]. The synthetic procedure for PMB derivatives started with the quantitative synthesis of the corresponding urea, which can then undergo an acid-catalyzed reaction with 4,5-dihydroxyimidazolidin-2-one to give the glycoluril, with a yield of 81%. The macrocyclization reaction was complicated by the unwanted reaction between formaldehyde and the PMB protecting groups on the glycolurils, which takes place under the acidic conditions required for the reaction.
Thus, moderate formation of the desired macrocycles was only observed when we limited the amount of formaldehyde to an amount equivalent with regard to the monomer unit. Even a slight excess of formaldehyde leads to a complete degradation of the macrocycles formed, and no macrocycle could then be isolated.
The main focus was on the six-membered bambusuril homologue because this macrocycle, unlike its four-membered homologue, acts as an excellent receptor for a number of anions. The best conversion of the glycoluril to (PMB)BU [6] was achieved when 1,4-dioxane was used as the solvent, and the reaction was carried out with only a catalytic amount of H 2 SO 4 under a longer reaction time (48 h) and at an elevated temperature (Scheme 2). Given the acid lability of the PMB group, we were unable to access an anion-free derivative by the use of a HSO 4 − template as recently published; 5 instead, an iodide template was employed. (PMB)BU [6] was isolated in the pure form by column chromatography as the TBAI complex, with a 19% yield. The removal of the iodide template by oxidation 4,19 was not possible because of the instability of PMB groups toward the oxidizing reagents. The (PMB)BU [6] complex with TBAI was used for the deprotection reaction.
The first attempts were made with cerium(IV) ammonium nitrate (CAN) in either a CH 3 CN/H 2 O mixture 20 or DMSO/ H 2 O. Unfortunately, because of the low solubility of the partially deprotected macrocycle (in CH 3 CN/H 2 O) and difficult separation (in DMSO/H 2 O), these attempts did not lead to the desired compound. Strong acids, such as CF 3 COOH, have also been reported in the literature as an alternative for the removal of this protecting group at elevated temperatures. 21,22 Neat CF 3 COOH, or its 50% solution in CHCl 3 , proved to function both as a deprotecting agent and also as a solvent. Complete deprotection was achieved after 1 h at 100°C in a 50% CF 3 COOH/CHCl 3 mixture or 15 min in neat CF 3 COOH under microwave irradiation. If a chromatographically purified sample of (PMB)BU [6] was used for the deprotection, it was possible to obtain pure (H)BU [6], with a yield of 58%.
We also investigated if the deprotection of a crude mixture containing (PMB)BU [6] without its chromatography purification could be used to obtain (H)BU [6]. Given the poor solubility of this unprotected macrocycle, we expected that the majority of linear oligomers, and other side products present in the crude material, could be separated because of their better solubilities. Indeed, after deprotection, most of the unwanted oligomers were washed out by MeOH and CH 3 COOH, and (H)BU [6] was isolated with a purity sufficient for further reactions. Unfortunately, the atom efficiency of the deprotection reactions is low, as the removal of 12 protecting groups radically decreases the molecular weight of the compound. Furthermore, the isolated (PMB)BU [6]·TBAI complex is, in the presence of strong acid, transformed into the (H)BU [6]·HI complex, and the molecular weight of the starting material decreases from 2736 to 1053 g mol −1 in the product. In addition, (PMB)BU [4] was prepared in 1,4-dioxane with ptoluenesulfonic acid (PTSA) used as a catalyst, and in the absence of a template, with the low yield of 6%. Deprotection of (PMB)BU [4] was successfully achieved by its reaction with CAN in a DMSO/MeOH/CH 3 CN mixture. 20 Given the low amount of the isolated macrocycle as well as its inability to bind anions, its modification was not studied further and (H)BU [4] was only characterized by the 1 H NMR spectrum in DMSO-d 6 Figure 1. Compounds unsuccessfully tested for bambusuril deprotection.

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Article ( Figure S3). The 1 H and 13 C NMR spectra of (H)BU [6] retain the high symmetry exhibited by all bambusuril derivatives ( Figure 2). The presence of ureidic NH groups on the macrocycle was supported by the measured 1 H− 15 N heteronuclear single quantum coherence (HSQC) spectrum ( Figure  S11). The inclusion complexes between six-membered bambusurils and anions are usually observed by 1 H NMR spectroscopy.
A chemical shift of anion-free bambusuril protons (particularly methine proton c) differs significantly from that of anionfree macrocycles. We tried to follow the chemical shift of an anion-free (H)BU [6], generated inside a NMR tube by the addition of an excess of anion-free (Bn)BU [6] receptor into the solution of the (H)BU [6] complex with HI. New signals corresponding to an iodide complex with (Bn)BU [6] indicated that the ion was transferred from (H)BU [6] to (Bn)BU [6]. Interestingly, we only observed a negligible change in the chemical shift of the NH and CH signals of (H)BU [6], which would correspond to the removal of I − from the complex ( Figure S11). This unexpected feature is currently under investigation in our laboratory.
We decided to illustrate the use of (H)BU [6] as a precursor for other bambusuril derivatives. Free NH positions on a bambusuril scaffold can conveniently be alkylated in DMSO in the presence of KOH. As reported, 23 powdered KOH in DMSO acts as a strong base which is just strong enough to deprotonate the ureidic NH groups. (H)BU [6] prepared from a crude (PMB)BU [6] mixture was propargylated with an excess of propargyl bromide (Scheme 2). A small amount of insoluble oligomeric impurities were removed after the alkylation. According to MS analysis ( Figure S32), the majority of the NH groups on bambusuril reacted and only traces of a macrocycle with 11 substituents were present.
Alkynes readily react with organic azides under copper(I) catalysis (azide−alkyne cycloaddition), and we used (propargyl)BU [6] (Scheme 2) as an alkyne component in this reaction. After the reaction, we observed the disappearance of the NMR signal of the starting alkyne compound; however, the characteristic signals of product 1 were only observed after the addition of an excess of TBAI.
The alkylation reaction was also attempted on 2,4dimethylglycoluril, which is structurally similar to the (H)BU [6] macrocycle. The propargyl derivative of glycoluril as well as compound 2 (Figure 3) was used to further prove the structures of the novel bambusurils ((propargyl)BU [6] and 1). Unfortunately, no crystals suitable for X-ray analysis were obtained from any of the novel macrocycles ((H)BU [6], (propargyl)BU [6], or 1). To validate the structure of 1 and to observe the spatial arrangement of its bulky substituents on the glycoluril monomer, we attempted to crystallize compound 2.
This compound was prepared prior the modification of the macrocycle as a test compound (see the Supporting Information). Slow diffusion of diethyl ether vapors into CH 2 Cl 2 solution of 1 produced monocrystals suitable for the Xray diffraction analysis. The crystal structure ( Figure 4) showed that the bulky substituents are oriented in such a way that the aromatic ring can possibly engage in π−π stacking interactions.

■ CONCLUSIONS
In conclusion, for the first time, four-and six-membered bambusurils lacking substituents on the nitrogen atoms of their portals were synthesized and characterized. This was achieved by careful selection of the protecting group as well as the conditions of macrocyclization and the deprotection reaction. A deprotected bambusuril represents a versatile bambusuril intermediate, which will allow for the preparation of derivatives that are not achievable by classical acid-catalyzed macrocyclization. This post-macrocyclization derivatization of bambusurils was demonstrated by the preparation of the propargyl derivative, which was later used in azide−alkyne cycloaddition.

■ EXPERIMENTAL SECTION
General Methods. NMR spectra were recorded on a Bruker AVANCE III 300 spectrometer with working frequencies of 300.15 MHz for 1 H and 75.47 MHz for 13 C or

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Article on a Bruker AVANCE III 500 spectrometer with working frequencies of 500.11 MHz for 1 H and 125.75 for 13 C. Both spectrometers were equipped with a BBFO probe. All experiments were recorded at 303.15 K and were processed using a MestReNova v. 9.1.0 program. NMR chemical shifts (δ) are reported in parts per million (ppm) using the residual solvent signal as a reference for the measured spectra CDCl 3 ( 1 H = 7.26, 13 C = 77.16) and DMSO-d 6 ( 1 H = 2.50, 13 C = 39.52). The proton resonances are annotated in text as: chemical shift, multiplicity (s, singlet; d, doublet; t, triplet; q, quartet; and m, multiplet), coupling constant [J (Hz)], and integration. In several instances, the position of carbon signal for reported compounds was deduced from HSQC and heteronuclear multiple bond correlation (HMBC) experiments as 13 C NMR produced only weak signals for quaternary groups. HRMS analysis was recorded on an Agilent 6224 Accurate-Mass TOF LC−MS. Samples were ionized by electrospray ionization (ESI). The MALDI-TOF mass spectrum was recorded on a MALDI-TOF Axima CFR spectrometer. The sample was ionized with the aid of a nitrogen laser (a wavelength of 337 nm and a maximum power of 6 MW); αcyano-4-hydroxycinnamic acid or 2,5-dihydroxybenzoic acid was used as a matrix. To improve the accuracy of the measurements, a calibration experiment was performed by mixing the sample with synthetic peptides. The mixture was analyzed, and the obtained calibration function was applied as a correction for the analysis of a pure sample of macrocycle. All column chromatography procedures were performed on columns packed with a silica gel (40−60 μm). Thin-layer chromatography (TLC) was performed using silica gel plates Silica Gel 60 F254 (0.2 mm thickness, Merck) and visualized under a UV lamp (254 and 366 nm) or with CAM stain. All solvents and chemicals were used as purchased or purified by standard procedures when necessary.