Multi-gram synthesis of precursors of bibrachial diaza-paracyclophanes. Complexes with Zn 2+ , Cu 2+ and Co 2+ ions

A multi-step synthetic strategy to obtain N -4-nitrophenyl-pyruvoyl-amino esters at a multi-gram scale has been developed. The subsequent domino reaction promoted by catalytic hydrogenation of the nitro group gave diaza-paracyclophane 3 , 13 and 14 . Complexation with Zn 2+ , Cu 2+ and Co 2+ ions has been studied by UV-vis and 1 H NMR spectroscopic methods


Introduction
2][3] This property is mainly linked to "face-to-face" transannular interactions of the aromatic rings and to hydrogen bonding with functional groups of the aliphatic chains.Azacyclophanes, in which one or more methylene units are substituted by nitrogen atoms, have a particular behaviour since the nitrogen bridges play an important role in their excited states, 4 and they may offer additional lone pairs from nitrogen atoms for complexation with metal ions. 5In particular Wurster's cyclophanes, which are composed of pphenylenediamine units, contain redox-active moieties and their coordinating ability can be altered by physical or chemical means. 6Different azacyclophanes containing p-phenylenedi(or tri)methylene subunits have been synthesized to study their coordination capabilities. 7The most common synthetic strategies to get these ligands are the alkylation of terminal N,N'ditosyldiamines with p-phenylenedimethylene dihalides, 8 the alkylation of terminal p-phenylene-polymethylenediamine with a polymethylenedibromide, 9 or the condensation of polymethylenediamine or polyamine derivatives with an arenedialdehyde.In this respect we found that m and p-diazacyclophanes may be obtained through the catalytic hydrogenation of the nitro group in methyl N- [3-(3 or 4-nitrophenyl)-pyruvoyl]-amino esters because this reaction promotes a [1+1]-condensation to give a macrocycle and a final hydrogenation of the two imine functions (Scheme 1).

Scheme 1
The synthesis of p-diazacyclophane 3 (Scheme 2) started with the acid-promoted methanolysis of the 2,5-piperazinedione derivative 1 to give 2, followed by catalytic reduction. 10The final hydrogenation of the two imine functions, that could give mixtures of diastereomers with a syn or anti stereochemistry, was diastereoselective.The anti isomer was ruled out because the 1 H and 13 C-NMR spectra showed the symmetry of the molecule, which is only compatible with the syn isomer, that must be a mixture of (2S,6S)-and (2R,6R)-enantiomers although only the last one is shown.The syn configuration and the boat-like conformation of 3 were in accordance to ab initio calculations using the 3-21G(d) basis set at the Hartree-Fock (HF) density functional level in the gas phase. 13This study also showed that benzene rings are parallel and nearly superposed and the two arms are equatorially disposed.The boat-like conformation has been proposed for other diazacyclophanes. 14cheme 2. Reported approach to paracyclophane 3: i) HCl/MeOH (10%), microwave irradiation time 5 min, 130 ºC.ii) H2, 10% Pd-C, EtOAc, 2 x 10 -2 M.
These bibrachial macrocycles 11 contain two amino groups incorporated into the cyclophane ring 12 and could be ligands for different ions, but the synthetic approach depicted in Scheme 2 is not convenient to be extended to enantiomerically pure dipeptide anhydrides such as cyclo(Gly-L-Ala) or cyclo(Gly-L-Phe), because of the lability of stereocenters to the basic media required in the aldol-type condensation with p-nitrobenzaldehyde to obtain compounds of type 1.Furthermore, the methanolysis of these compounds to obtain nitrophenylpyruvoyl amino esters requires to be improved for a multi-gram scale synthesis of diazacyclophane precursors because, by solubility reasons, it had to be performed at high dilutions (up to 0.003 M).Unfortunately, all attempts to find out other experimental conditions were unsuccessful, which moved us to study a multi-step approach to obtain these amino esters at a multi-gram scale.Here we describe this approach and the ability of diazacyclophanes for complexation with metal ions.

Synthesis of azacyclophanes (13) and (14)
We first obtained compound 5 by acid hydrolysis of azalactone 4 15 following well known procedures. 16The subsequent coupling of 5 with methyl L-alaninate in the presence of EDC and Et3N gave 6 in very low yield, indicating that protection of the ketone functional group was necessary, but the apparently simple transformation of compound 5 15 or its methyl ester 7 17 into the corresponding ketals was not a straight reaction.Treatment of 5 with ethylenediol, under BF3. .OEt2 18 or pTsOH 19 catalysis gave the 2-hydroxyethoxy ester 8, and compound 7 was the only product obtained in the attempted synthesis of the dimethoxy ketal by treatment of 5 with trimethyl orthoformate. 20Fortunately, we could obtain the 1,3-dithiane 9, and this compound was condensed under standard conditions with methyl L-alaninate or L-phenylalaninate to afford compounds 10 or 11 with excellent yields (Scheme 3).
Deprotection of the -ketoamide function in compounds 10 and 11 was performed by oxidation with HgO and BF3.OEt2 21 to give 6 and 12, respectively (Scheme 4, Ar = 4nitrophenyl).After a very careful purification to eliminate traces of any sulfur-containing derivative to avoid the poison of the catalyst, these compounds were submitted to catalyzed hydrogenation, that promoted a domino process 22 implying reduction of the nitro group, intermolecular condensation, and diastereoselective reduction of the two imine functions, to give 13 and 14.In spite of the X-ray data absence, the symmetry showed in the NMR spectra of both compounds was only compatible with a 2,6 syn configuration.Intracyclic benzene protons in the 1 H NMR spectra of 3, 13 and 14 were located in the normal aromatic region. 1 H and 13 C NMR spectra of compound 3 at room temperature were very simple, while 1 H NMR spectrum of 13, and specially of 14, showed less resolved signals.We think that the observed double signals are due to the presence of different rotamers, due to the expected higher energy barriers in the dynamic equilibrium between different conformations because of the presence of the Me or Bn substituents in the side-chain.The possible contamination with the (S,2S,6S,S)-diastereomer seams less probable, since chemical shift differences in double signals are very small and attempts to isolate both possible diastereomers by column chromatography were unsucsessful.It appears that the chiral side-chains induce the diastereoselective hydrogenation of the imine functions, giving the (S,2R,6R,S)-diastereomer.

Complexation studies
Cyclophanes 3 and 14 were tested for their complexation behaviour with Cu(II), Zn(II) and Co(II) through UV-vis and 1 H NMR spectroscopic methods, since we have been unable up to now to get proper crystals for an X-ray study.

UV-vis spectra studies
Compounds 3 and 14 solved in MeOH showed UV-vis spectra with max at 257, 247, 205, and 334 nm for 3 and 293, 240 and 209 nm for 14 (see Figure 1 and Supplementary Material).These data point out that the benzene rings in both ligands are rather unstrained, since the absorption maxima are similar to that of p-xylene (max 268 nm). 23Upon addition of CuCl2, CoCl2 or ZnCl2, the intensity of the bands increased without no significant shifts in max values.In the case of the complex of 14 with Co 2+ the absorption band at 240 nm disappeared.The stoichiometry of the complexes with azacyclophane 3 was investigated by using the UV/Vis spectrum titration method and, according to Job plot experiments, 24 this compound forms a 1:1 complex with Zn 2+ (Figure 1).The same stoichiometry was observed for the complex with Co 2+ , while three complexes with 1:1, 1:2, and 1:3 stoichiometries appeared for Cu 2+ (see Supplementary Material).

NMR spectra studies
We chosed the Zn 2+ ion to shed further light on the complexation modes in solution because it forms diamagnetic complexes.Upon addition of this ion to diazacyclophanes 3 and 14, the main differences were found in the chemical shifts of the intracyclic aromatic protons at the orthoposition respect to the amine group (o-NH-H(Ar)), which were higher in the complexes than in the free ligands.Lower field shifts were also observed in the CH-CH2 intracyclic protons of the complexes while changes in the chemical shifts of the side-chain protons were not observed (Table 1).We show in Figure 2a the 1 H NMR spectrum of 3 in CD3OD and in Figure 2b the spectrum of a 0.5 nM solution of Zn 2+ in CD3OD after addition to a 1 nM solution of 3 in the same solvent and 2 hours of reflux.The observed set of resonance signals of the same intensity in this solution, where the 3/Zn 2+ ratio is 1:0.5, indicates the presence of an equimolar amount of the free ligand and of the complex.Figure 2c shows the spectrum of a similarly treated solution where the ligand/metal ratio was 1:1.In this case, all proton signals corresponding to the free ligand has disappeared and only the proton signals of the complex are observed, indicating the formation of a stable 1:1 complex in solution.In conclusion, although to be sure of the true structure of the new complexes X-ray data should be necessary, it is rather clear that the trans-annular amine groups are involved in the complexation, because the main 1 H NMR differences between ligand and complexes have been found in the aromatic protons at the ortho-position respect to these amino groups and in the H-2 and H-6 protons These interactions would be compatible with 1:1 endo-complexes or with 2:2 exo-complexes, where the metal ions would be inside or outside of the pi-cavity, respectively.

Experimental Section
General.All reagents were of commercial quality and were used as received.Solvents were dried and purified using standard techniques.Reactions were monitored by thin layer chromatography, on aluminium plates coated with silica gel with fluorescent indicator.Separations by flash chromatography were performed on silica gel with 40-63 m particle size.Melting points were measured in a hot stage microscope, and are uncorrected.Infrared spectra were recorded on a FT-IR spectrophotometer, with solid compounds compressed into KBr pellets and liquid compounds examined as films on NaCl disks.NMR spectra were obtained at 250 MHz for 1 H and 63 MHz for 13 C, with CDCl3, DMSO-d6 and CD3OD as solvents (Servicio de Resonancia Magnetica Nuclear, Universidad Complutense).Elemental analysis were determined by the Servicio de Microanalisis Elemental, Universidad Complutense.

General procedure for the deprotection reaction of (10) and (11)
A solution of 10 or 11 (5 mmol) in THF/H2O (85:15, 100 mL) was added to a suspension of HgO (15.5 mmol) and BF3.OEt2 (15.5 mmol) in THF/H2O (85:15) at 0 ºC.The reaction mixture was stirred for 20 h at 60 ºC, then CH2Cl2 (100 mL) was added and the precipitate filtered.The organic layer was washed with brine, dried over Na2SO4 and the solvent evaporated in vacuo.The crude product was purified by flash column chromatography.

Synthesis of diazacyclophanes
A solution of compound 6 or 12 (2.4 mmol) in EtOAc (240 mL) with a 20% of Pd/C was hydrogenated at room temperature for 16 h.After filtration over Celite and evaporation of the solvent under reduced pressure, the crude product was purified by flash column chromatography.

Figure 2 .
Figure 2. (a) 1 H NMR spectrum of 3 in CD3OD.(b)1 H NMR spectrum of a CD3OD solution of 3/Zn 2+ in 1:0.5 ratio after 2 hours of reflux.(c)1 H NMR spectrum of a similarly treated solution where the ligand/metal ratio was 1:1.In conclusion, although to be sure of the true structure of the new complexes X-ray data should be necessary, it is rather clear that the trans-annular amine groups are involved in the complexation, because the main 1 H NMR differences between ligand and complexes have been found in the aromatic protons at the ortho-position respect to these amino groups and in the H-2 and H-6 protons These interactions would be compatible with 1:1 endo-complexes or with 2:2 exo-complexes, where the metal ions would be inside or outside of the pi-cavity, respectively.