Multiple threading of a triple-calix[6]arene host

The synthesis of the triple-calix[6]arene derivative 6 in which three calix[6]arene macrocycles are linked to a central 1,3,5-trimethylbenzene moiety is reported. Derivative 6 is able to give multiple-threading processes in the presence of dialkylammonium axles. The formation of pseudo[2]rotaxane, pseudo[3]rotaxane, and pseudo[4]rotaxane by threading one, two, and three, respectively, calix-wheels of 6 has been studied by 1D and 2D NMR, DOSY, and ESI-FT-ICR MS/MS experiments. The use of a directional alkylbenzylammonium axle led to the stereoselective formation of endo-alkyl pseudo[n]rotaxane stereoisomers.

With the aim to increase the complexity of our system we have designed the triple-calix [6]arene host 6 ( Figure 2) bearing three calix [6]arene wheels symmetrically-linked to a central benzene unit. Now the question arises as to whether the triplecalix [6]arene system 6 is also capable to form pseudo[n]rotaxanes by multiple-threading with dialkylammonium axles.

Results and Discussion
The synthesis of triple-calix [6]arene derivative 6 is outlined in Scheme 1.
Analogous results were obtained when 6 was titrated with dipentylammonium axle 4 + , as TFPBsalt (Scheme 3). An  collisionally dissociated to give 4 + 6, by de-threading of one dipentylammonium axle. When an excess of 4 + ·TFPB − salt was added to the CDCl 3 solution of 6 then evidences for the formation of the (4 + ) 3 6 pseudo [4]rotaxanes was obtained by a 1 H NMR study (Figure 8d). A careful analysis of the ArCH 2 Ar region of these spectra evidenced again a syn orientation of the aromatic rings of calix [6]-wheels corresponding to a cone conformation, which was also confirmed by the minimum-energy structure of (4 + ) 3 6 obtained by molecular mechanics calculations ( Figure 8).

Conclusion
In this study we described the synthesis of a triple-calix [6]arene host (6) in which three pentamethoxy-mono-hydroxy units are linked to a central 1,3,5-trimethylbenzene moiety. We have shown that 6 is able to give multiple-threading processes in the presence of dipentylammonium or dibenzylammonium axles. The formation of pseudo [2]rotaxanes, pseudo [3]rotaxanes, and pseudo [4]rotaxanes in CDCl 3 solution was ascertained by 1D and 2D NMR, DOSY, and ESI-FT-ICR MS/MS experiments. In addition, in the presence of a directional butylbenzylammonium axle, the stereoselective formation of endo-alkyl pseudorotaxane stereoisomers was observed.

Experimental
HR mass spectra were acquired on a FT-ICR mass spectrometer equipped with a 7T magnet. The mass spectra were calibrated externally, and a linear calibration was applied. All chemicals were reagent grade and were used without further purification. Tetrahydrofuran was dried by heating under reflux over sodium wire in the presence of benzophenone as indicator while dimethylformamide was dried by activated 3 Å molecular sieves. When necessary the compounds were dried in vacuum over CaCl 2 . Reactions were monitored by TLC silica gel plates (0.25 mm) and visualized by 254 nm UV light, or by spraying with H 2 SO 4 -Ce(SO 4 ) 2 . The derivative 9 has been synthesized according to literature procedures [27]. NMR spectra were recorded on a 600 [600 ( 1 H) and 150 MHz ( 13 C)] spectrometer. Chemical shifts are reported relative to the residual solvent peak. COSY spectra were taken using a relaxation delay of 2 s with 30 scans and 170 increments of 2048 points each. HSQC spectra were performed with gradient selection, sensitivity enhancement, and phasesensitive mode using the Echo/Antiecho-TPPI procedure.
Synthesis of derivative 6. In a dry round flask, under N 2 , derivative 8 (3.11 g, 2.98 mmol) was dissolved in dry THF/DMF (180 mL, 7:3 v/v). Subsequently, NaH (1.05 g, 43.86 mmol) was added at 0 °C. After 15 minutes, 1,3,5-tris(bromomethyl)benzene (0.36 g, 1.00 mmol) was added to the reaction mixture at room temperature. The reaction was stirred at reflux for 12 h under a nitrogen atmosphere. Afterwards the reaction was stopped by addition of 1 M HCl and the solution was extracted with chloroform. The organic phase was dried over anhydrous Na 2 SO 4 , filtered and evaporated of the solvent. The raw was purified through chromatography column on silica gel and using solvent mixture dichloromethane/diethyl ether 96:4 as  Preparation of pseudo[n]rotaxane. Derivative 6 (5.00 mg, 1.5 × 10 −3 mmol) and dialkylammonium axle 4 + , 7 + or 8 + [n × (1.5 × 10 −3 mmol), n = 1-6] were dissolved in 0.5 mL of CDCl 3 . Then, the solution was sonicated for 5 min and transferred in a NMR tube for 1D and 2D NMR spectra acquisition.

Determination of K app values by quantitative 1 H NMR analysis.
The samples were prepared by dissolving 6 (1.5 × 10 −3 mmol) and the appropriate alkylammonium guest 4 + , 7 + or 8 + as TFPB − salt (1.5 × 10 −3 mmol) in CDCl 3 (0.5 mL) containing 1 μL of 1,1,2,2-tetrachloroethane (d = 1.586 g/mL, 0.019 M) as internal standard (IS). The complex concentration [complex] was evaluated by integration of the 1 H NMR signals of 1,1,2,2-tetrachloroethane vs the shielded signals of the guest molecules. The following equation was used to obtain the moles of the complex: Where: Ga = grams of IS; Gb = grams of complex, Fa and Fb = areas of the signals of 1,1,2,2-tetrachloroethane and signal of the guest, Na and Nb = numbers of nuclei which cause the signals (Na for IS = 2; Nb for guest) and Ma and Mb = molecular masses of IS (a) and complex (b)

Supporting Information
Supporting Information File 1 1 H and 13 C NMR spectra, 1 H qNMR spectra, 2D COSY and HSQC spectra of pseudorotaxanes.