Synthesis of new enantiopure dimethyl-and diisobutyl -substituted pyridino-18-crown-6 ethers containing a halogen atom or a methoxy group at position 4 of the pyridine ring for enantiomeric recognition studies

New enantiomerically pure dimethyl-and diisobutyl-substituted pyridino-18-crown-6 ethers containing a halogen atom or a methoxy group at position 4 of the pyridine ring [( S,S )- 1 , ( S,S )- 2 , ( S,S )- 3, ( S,S )- 4 ] have been synthesized. A new synthetic route and the solid state structure of the reported enantiopure dimethyl-substituted pyridino-18-crown-6 ether [( S,S )- 5 ] containing a chlorine atom at position 4 of the pyridine ring are also described. These ligands are good candidates for enantiomeric recognition studies of protonated primary amines, amino acids and their derivatives


Introduction
There are numerous examples of molecular recognition in nature, such as the immunological response, storage and retrieval of genetic information by DNA, enzyme-substrate interactions, selective complexation and transport of metal ions across cell membranes by ionophore antibiotics and the metabolism of single enantiomeric forms of amino acids and sugars in biochemical pathways can be mentioned.The last example refers to enantiomeric recognition.
Enantiomeric recognition, as a special case of molecular recognition, involves the discrimination between the enantiomers of a chiral guest molecule by a chiral host molecule.Since Cram and his co-workers published their seminal work on the use of chiral macrocyclic ligands in enantiomeric recognition, 1 a great number of chiral macrocycles have been synthesized and studied. 2,3][12][13][14] In continuation of our studies in this area of research, our attention turned to the preparation of new enantiopure dimethyl-and diisobutyl-substituted pyridino-18-crown-6 ether type macrocycles containing a halogen atom or a methoxy group at position 4 of the pyridine ring [(S,S)-1, (S,S)-2, (S,S)-3, (S,S)-4, (S,S)-5, see Figure 1].These macrocycles can be transformed to derivatives which are able to function as effective sensor and selector molecule. 9Regarding the former derivatives up to now, only the dimethyl-substituted pyridino-18-crown-6 ether containing a chlorine atom at position 4 of the pyridine ring [(S,S)-5] has been synthesized from the appropriate enantiopure dimethyl-substituted pyridino-crown ether. 9In this paper we describe a new route for the synthesis of the reported ligand (S,S)-5 and also the preparation of new enantiopure dimethyl-and diisobutyl-substituted pyridino-18-crown-6 ethers containing a chlorine or a bromine atom or a methoxy group at position 4 of the pyridine ring [(S,S)-1─(S,S)-4, see Figure 1].

Synthesis
For the synthesis of enantiopure dimethyl-substituted 4-bromo-pyridino-crown ether [(S,S)-2], two synthetic pathways were investigated.In both cases, a solution of the enantiopure dimethylsubstituted tetraethylene glycol [(S,S)-6] 15 in THF was converted into the corresponding dialkoxide using NaH as a strong base and then this dialkoxide was reacted with the pyridine precursor 7 or 8 performing a Williamson-type macrocyclic ether formation reaction as shown in Scheme 1. Scheme 1. Synthesis of pyridino-crown-ether derivatives (S,S)-1-(S,S)-5.
Pyridino-crown ether (S,S)-1 substituted with a chlorine atom at position 4 of the pyridine ring was also synthesized from pyridono-crown ether (S,S)-12 reacting the latter with thionyl chloride in CHCl3 in the presence of a catalytic amount of N,N-dimethylformamide (DMF) (see Scheme 2).Pyridino-crown ether (S,S)-2 substituted with a bromine atom at position 4 of the pyridine ring was also prepared from pyridono-crown ether (S,S)-13.We used phosphorus pentabromide and CHCl3 as a solvent in this case.Scheme 2. Synthesis of pyridino-crown ether derivatives (S,S)-1 and (S,S)-2 from enantiopure pyridono-crown ethers (S,S)-12 and (S,S)-13.
Tetraethylene glycols (S,S)-6 and (S,S)-10 (see Scheme1) were obtained as described in the literature. 8,15pon reduction of diesters 14, 15 and 16 with NaBH4, 4-substituted pyridine-2,6-dimethanols 17, 18 and 19 were obtained (see Scheme3) in a similar manner as described in the literature for analogous compounds. 18In the latter literature, this type of diols were not isolated.A modified procedure of Lüning et al 19 was used to synthesize chloro-diol 18.Instead of the long time (2 days) continuous extracting, we isolated 18 by recrystallization from water.All of these diols 17, 18 and 19 were converted to ditosylates 7, 9 and 11 with tosyl chloride in a mixture of CH2Cl2 and 40 % aqueous KOH.Bis-bromomethyl derivative 8 was prepared from bromo-diol 17 using phosphorus tribromide in ether 20 (Scheme3).Scheme 3. Synthesis of precursors 7, 8, 9 and 11 for pyridino-crown ether derivatives.
Dihydro-4-oxo-2,6-pyridinedicarboxylic acid 20 was prepared as reported 18 from commercially available and cheap starting materials like acetone, sodium, EtOH, diethyl oxalate and ammonia.Pyridone derivative 20 was treated with bromine and phosphorus tribromide and then with EtOH to yield diethyl 4-bromopyridine-2,6-dicarboxylate 14, applying a modified procedure of the one reported by Takalo and Kankare 21 (Scheme 4).Dimethyl chelidamate 21 was prepared from chelidamic acid as reported. 6iester 21 was treated with thionyl chloride and a catalytic amount of DMF for 2 days at reflux temperature in CHCl3 to obtain dimethyl 4-chloro-pyridine-2,6-dicarboxylate 15 (see Scheme 4).This method gave 15 with a higher yield (91%) than applying the reported 22 one (69%), where Markees and Kidder used the more expensive and more dangerous PCl5 for this transformation.Dimethyl 4-methoxypyridine-2,6-dicarboxylate 16 was synthesized according to the literature 18 .Scheme 4. Synthesis of pyridine diester derivatives 14, 15 and 16 containing a bromine or a chlorine atom or a methoxy group at position 4 of the pyridine ring

X-ray analysis
In order to obtain more information about the structure of (S,S)-5 in the solid state, a single crystal was grown using a mixture of heptane and CH2Cl2.Several dimethyl-substituted pyridino-crown ethers prepared from (S,S)-5 form complexes with one molecule of water. 9X-ray analysis proved that the water molecule is not complexed by the crystalline form of macrocycle (S,S)-5.Figure 2 shows the molecular diagram of crystalline (S,S)-5 (where only the major disorder component is displayed).This paper reports only the synthesis of the new ligands, their precursors and the solid state structure of ligand (S,S)-5.Their transformation to enantioselective sensor and selector molecules and the applications of the latter compounds will be published in due course.

Experimental Section
General.Infrared spectra were recorded on a Zeiss Specord IR 75 spectrometer.Optical rotations were taken on a Perkin-Elmer 241 polarimeter that was calibrated by measuring the optical rotations of both enantiomers of menthol.NMR spectra were recorded in CDCl3 either on a Bruker DRX-500 Avance spectrometer (at 500 MHz for 1 H and at 125 MHz for 13 C spectra) or on a Bruker 300 Avance spectrometer (at 300 MHz for 1 H and at 75 MHz for 13 C spectra) and it is indicated in each individual case.Mass spectra were recorded on a ZQ 2000 MS instrument (Waters Corp.) using ESI method.Elemental analyses were performed in the Microanalytical Laboratory of the Department of Organic Chemistry, Institute for Chemistry, L. Eötvös University, Budapest, Hungary.Melting points were taken on a Boetius micro-melting point apparatus and were uncorrected.Starting materials were purchased from Aldrich Chemical Company unless otherwise noted.Silica gel 60 F254 (Merck) and aluminium oxide 60 F254 neutral type E (Merck) plates were used for TLC.Aluminium oxide (neutral, activated, Brockman I) and silica gel 60 (70-230 mesh, Merck) were used for column chromatography.Ratios of solvents for the eluents are given in volumes (mL/mL).Solvents were dried and purified according to well established methods 23 .Evaporations were carried out under reduced pressure unless otherwise stated.
X-ray measurements.Intensity data were collected on an RAXIS-RAPID diffractometer (graphite monochromator Cu-Kα radiation, λ = 1.54187Ǻ) at 293(2) K in the range 7.15 ≤θ≤ 70.05°.A multi-scan absorption correction was applied to the data (the minimum and maximum transmission factors were 0.666 and 1.000) 24 .The structure was solved by direct methods (SIR2004) 25 .Anisotropic full-matrix least-squares refinement 26 on F 2 for all non-hydrogen atoms yielded R1 = 0.0578 and wR2 = 0.1542 for 1332 [I>2(I)] and R1 = 0.0652 and wR2 = 0.1668 for all (3376) intensity data, (number of parameters = 248, S = 1.126, absolute structure parameter x = -0.03(3), the maximum and mean shift/esd is 0.003 and 0.000).The maximum and minimum residual electron density in the final difference map was 0.436 and -0.285 e.Å -3 .Hydrogen atomic positions were calculated from assumed geometries.Hydrogen atoms were included in structure factor calculations but they were not refined.The isotropic displacement parameters of the hydrogen atoms were approximated from the U(eq) value of the atom they were bonded to.

General procedure for the preparation of the chiral pyridino-crown ethers (S,S)-1─(S,S)-5 (see Scheme 1
).In a dry three-necked round-bottom flask equipped with a reflux condenser, argon inlet and a dropping funnel, a suspension of NaH (1.53 g, 38.4 mmol, 60% dispersion in mineral oil) in pure and dry THF (30 mL) was stirred vigorously at 0 °C for 2 min.
To this suspension was added slowly the appropriate optically active tetraethylene glycol (S,S)-6 or (S,S)-10 (13.7 mmol) dissolved in pure and dry THF (170 mL) under Ar at 0 °C.The reaction mixture was stirred at 0 °C for 10 min, at room temperature for 30 min and at reflux temperature for 4 h.The mixture was cooled to -10°C and 2,6-bis(tosyloxymethyl)pyridine derivative 7 or 8 or 9 or 11 (14.5 mmol) dissolved in pure and dry THF (200 mL) was added in 0.5 h.After addition of the appropriate ditosylate the reaction mixture was allowed to warm up slowly to room temperature and it was stirred under these conditions until the TLC analysis (Al2O3 TLC; EtOHtoluene 1:30) showed the total consumption of the starting materials (2 days).The solvent was evaporated, and the residue was dissolved in a mixture of CH2Cl2 (100 mL) and ice-water (100 mL).The phases were shaken thoroughly and separated.The aqueous phase was extracted with CH2Cl2 (3×200 mL).The combined organic phase was dried over anhydrous MgSO4, filtered and evaporated.The crude product was purified as described below for each compound to result in the optically active pyridino-crown ethers [(S,S)-1, (S,S)-2, (S,S)-3, (S,S)- A. From (S,S)-10 and 9 (see Scheme 1).Crown ether (S,S)-1 was prepared as described above in the General procedure starting from diisobutyl-substituted tetraethylene glycol (S,S)-10 (3.78 g, 12.3 mmol) and ditosylate 9 (5.50 g, 13.1 mmol).The crude product was purified by column chromatography first on neutral aluminium oxide using EtOH-toluene (1:140) mixture as an eluent followed by aluminium oxide preparative thin layer chromatography using isopropylalcohol-toluene (1:40) mixture to yield (S,S)-1 (710 mg, 13%) as a pale yellow oil.Rf: 0.87 (aluminium oxide TLC, EtOH-toluene 1:20 B. From (S,S)-12 and thionyl chloride (see Scheme 2).To a solution of diisobutyl-substituted pyridono-crown ether (S,S)-12 (937 mg, 2.16 mmol) in pure and dry CHCl3 (20 mL) was added firstly a catalytic amount of pure and dry DMF (three drops) followed by thionyl chloride (3.9 mL, 6.26 g, 53.5 mmol), and the resulting mixture was stirred at reflux temperature under Ar for 1.5 h.The volatile components were removed and the residue was dissolved in a mixture of CH2Cl2 (100 mL) and 12.5% aqueous tetramethylammonium hydroxide (30 mL).The aqueous layer was extracted with CH2Cl2 (3X30 mL).The combined organic phase was dried over MgSO4, filtered and the solvent evaporated.The crude product was purified by column chromatography on neutral aluminium oxide using EtOH-toluene (1:160) mixture as an eluent to gain (S,S)-1 (297 mg, 31 %) as a pale yellow oil.Macrocycle (S,S)-1 obtained this way was identical in every aspect to that prepared by the previous procedure (A).

4-Chloro-2,6-bis[(p-tolylsulfonyl
)oxymethyl]pyridine (9) (see Scheme 3).Chloro diol 18 (1.0 g, 5.76 mmol) was vigorously stirred in a mixture of CH2Cl2 and 40 % aqueous KOH solution (60 mL of each) at 0 °C and tosyl chloride (2.31 g, 12.1 mmol) was added to it in one portion.The mixture was stirred at 0 °C for one hour then at room temperature until the TLC analysis (SiO2 TLC; MeOH-toluene 1:4) showed the total consumption of the starting material and only one main spot for the product.The mixture was washed into a separatory funnel with water and CH2Cl2 (30 mL of each).The resulting mixture was shaken well and separated.The aqueous phase was shaken with CH2Cl2 (3X30 mL).The combined organic phase was dried over anhydrous MgSO4, filtered and the solvent was evaporated.(2.43 g, 14.4 mmol) was stirred in a mixture of CH2Cl2 and 40 % aqueous KOH solution (120 mL of each) at 0 °C and tosyl chloride (7.39 g, 38.99 mmol) was added to it in one portion.The mixture was stirred at 0 °C for one hour, then at room temperature until the TLC analysis (silica gel TLC, MeOH-toluene 1:4) showed the complete conversion of the starting materials and only one main spot for the product.The mixture was washed into a separatory funnel with water and CH2Cl2 (60 mL of each).The resulting mixture was shaken well and separated.The aqueous phase was shaken with CH2Cl2 (3X60 mL).The combined organic phase was dried over anhydrous MgSO4, filtered and the solvent was removed.The residue was recrystallyzed from CH2Cl2-MeOH to give ditosylate 11 (7.06 g, 78%) as white crystals.Mp 77-78 °C (CH2Cl2-MeOH) (lit.mp: 77-78 °C18 (ClCH2CH2Cl-MeOH)).Rf: 0.37 (silica gel TLC, acetone-hexane 1:2).Ditosylate 11 obtained this way had the same spectroscopic data than those of reported 18 .Diethyl 4-bromo-2,6-pyridinedicarboxylate (14) (see Scheme 4).Bromo diester 14 was prepared with a modification of a reported procedure 21 .In a dry three-necked round-bottom flask equipped with a reflux condenser, argon inlet and a dropping funnel was stirred vigorously a solution of bromine (3.8 mL, 74.5 mmol) in pure and dry CHCl3 (38 mL).To this solution was added phosphorus tribromide (8.4 mL, 89.5 mmol) drop by drop at 0 °C.Stirring was continued at 0 °C for 5 minutes to obtain phosphorus pentabromide.Dry dihydro-4-oxo-2,6pyridinedicarboxylic acid 20 (5.0 g, 25 mmol) was added to the reaction mixture and after stirring it at room temperature for 5 minutes, the temperature was raised to 90 °C and maintained at this temperature overnight.After cooling down the mixture to 0 °C, EtOH (300 mL) was added slowly to it and after half an hour the solution was concentrated under reduced pressure.
The residue was triturated with a mixture of ice and water (50 g of each), to obtain white crystals.The TLC analysis (SiO2 TLC, acetone-hexane 1:2) showed the total consumption of the starting material and only one main spot for the product.This crude product was recrystallized from diisopropyl ether, to yield bromo diester 14 (11.4 g, 50 %) as white crystals.Mp:95-96 °C (diisopropyl ether) (lit.mp: 95-96 °C21 (hexane)).Rf: 0.39 (silica gel TLC, acetone-hexane 1:2).Bromodiester 14 obtained this way had the same spectroscopic data than those of reported 21 .Dimethyl 4-chloro-2,6-pyridinedicarboxylate (15) (see Scheme 4).In a dry three-necked round-bottom flask equipped with a reflux condenser, argon inlet and a dropping funnel was stirred vigorously a solution of dimethyl 4-hydroxy-pyridine-2,6-dicarboxylate 21 (3.0 g, 14.2 mmol) in pure and dry CHCl3 (30 mL).Thionyl chloride (10.4 ml, 142 mmol) was added slowly at 0 °C to the mixture followed a catalytic amount of DMF (2 drops) was added.The reaction mixture was stirred at 0 °C for 10 min, then at reflux temperature for 2 days.The volatile components were evaporated under reduced pressure.The residue was dissolved in a mixture of CH2Cl2 and 10% aqueous NaHCO3 solution (30 mL of each).The phases were shaken well and separated.The organic phase was shaken with water (2×30 mL), dried over MgSO4, filtered and the solvent was evaporated.The residue was recrystallized from MeOH to give chloro diester 15

Figure 1 .
Figure 1.Enantiopure pyridino-18-crown-6 ethers containing a halogen atom or a methoxy group at position 4 of the pyridine ring.

Figure 2 .
Figure 2. Molecular diagram of (S,S)-5 with the numbering of atoms.

Table 1 .
Crystal data and details of the structure determination for (S,S)-5