DOI: 10.17344/acsi.2017.3566 Acta Chim. Slov. 2017, 64, 737-746 Scientific paper A 26-Membered Macrocycle Obtained by a Double Diels-Alder Cycloaddition Between Two 2H-Pyran-2-one Rings and Two 1,1'-(Hexane-1,6-diyl)bis (1H-pyrrole-2,5-dione)s Bor Lucijan Turek, Marijan Kočevar, Krištof Kranjc* and Franc Perdih* Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, SI-1000 Ljubljana, Slovenia * Corresponding author: E-mail: franc.perdih@fkkt.uni-lj.si; kristof.kranjc@fkkt.uni-lj.si Received: 19-05-2017 Dedicated to Professor Emeritus Miha Tišler, University of Ljubljana, on the occasion of his 90th birthday. Abstract With the application of a double dienophile 1,1'-(hexane-1,6-diyl)bis(1ff-pyrrole-2,5-dione) for a [4+2] cycloaddition with a substituted 2H-pyran-2-one a novel 26-membered tetraaza heteromacrocyclic system 3 was prepared via a direct method under solvent-free conditions with microwave irradiation. The macrocycle prepared is composed of two units of the dienophile and two of the diene. The structure of the macrocycle was characterized on the basis of IR, 1H and 13C NMR and mass spectroscopy, as well as by the elemental analysis and melting point determination. With X-ray diffraction of a single crystal of the macrocycle we have determined that the two acetyl groups (attached to the bridging double bond of the bicyclo[2.2.2]octene fragments) are oriented towards each other (and also towards the inside of the cavity of the macrocycle), therefore, mostly filling it completely. Keywords: Macrocycles, 2H-Pyran-2-ones, [4+2] Cycloaddition, Crystal structure, Hydrogen bonds, Polymorphs 1. Introduction Macrocycles are privileged molecule structures that are of paramount importance in many areas of chemistry, including drug development,1 formation of coordination compounds and metal-organic frameworks.2 Generally they possess properties (structural, chemical, physical and biological) that set them apart from their linear or small-ring analogues, the reason being that they can often provide sufficient flexibility for interactions with other molecules (e.g. for binding to an enzyme's active site or for a coordination to a guest ion during phase catalysis) combined with the advantages brought by the fact that they often contain more than one binding motif. This means that all of the interactions between the host (or enzyme) and the macrocycle are taking place between two molecules only and consequently the enthropy of the interaction is not so unfavourable as would be in the case where more (smaller) ligands interact simultaneously with the host. Even though the synthesis of macrocycles has achieved some remarkable successes, there is still a lack of general approach towards them.3 There were many successful attempts towards the preparation of macrocycles, one of the most-often used being dilution techniques triggering the macrocyclization via lactonization, lactamiza-tion, metathesis reaction etc. (that were recently used for the first asymmetric total synthesis of aspergillide D4 or for the total synthesis of mandelalide A).5 Other options include the template-induced cyclization (around the host ion)6 and cyclization on a solid support (like Merri-field-based synthesis of cyclic peptides or such inspired by non-ribosomal peptide aldehydes).7 More contemporary approaches are based on multi multicomponent macrocy-clizations (MiBs)8 that include various bifunctional building blocks. However, neither of the above mentioned ap- proaches can be applied universally. So there is still place for new routes. Recently, a lot of effort was devoted to mul-ticomponent reactions that efficiently offer access to various macrocycles, including the possibility to incorporate points of diversity, which are, nevertheless, generally introduced before or after the key cyclization step.9 However, even this approach is generally applied just to obtain the requisite linear precursors that are latter assembled via a suitable ring-closing reaction into the final macrocyclic target.10-14 Of interest are also preparations of calix[4]arene systems linked with 1,2,4-triazole and 1,3,4-oxadiazole derivatives,15 as well as other tetraaza macrocycles applied as ligands in various coordination compounds.16 Herein we present another approach, where two double Diels-Alder cycloadditions between two molecules of the substituted 2H-pyran-2-ones (each acting as a "double" diene)17 and two molecules of the double dieno-phile provide a 26-membered tetraaza macrocyclic system. This strategy can be termed a multicomponent reaction (as four molecules react to form the macrocycle) with four individual [4+2] pericyclic reactions representing the crucial ring-closing steps. 2. Experimental 2. 1. Materials and Measurements Melting points were determined on a micro hot stage apparatus and are uncorrected. !H NMR spectra were recorded at 29 °C with a Bruker Avance III 500 spectrometer at 500 MHz using Me4Si as an internal standard. 13C NMR spectra were recorded at 29 °C with a Bruker Avance III 500 spectrometer at 125 MHz and were referenced against the central line of the solvent signal (CDCl3 triplet at 77.0 ppm or DMSO-d6 septet at 39.5 ppm). The coupling constants (J) are given in Hertz. IR spectra were obtained with a Bruker Alpha Platinum ATR FT-IR spectrometer on a solid support as microcrystalline powder. MS spectra were recorded with an Agilent 6624 Accurate Mass TOF LC/MS instrument (ESI ionization). Elemental analyses (C, H, N) were performed with a Perkin Elmer 2400 Series II CHNS/O Analyzer. TLC was carried out on Fluka silica-gel TLC-cards. The starting 2H-pyran-2-one 1 was prepared by the method devised by Kepe, Kocevar et al.18 as follows: from acetylacetone, N,N-dimethylformamide dimethyl acetal (DMFDMA) and hippuric acid by heating in acetic anhydride according to the published procedure 5-acetyl-3-ben-zoylamino-6-methyl-2H-pyran-2-one was obtained; followed by the removal of the benzoyl group (in concentrated H2SO4 upon heating) analogously as previously described19,20 and subsequent derivatization of the free 3-amino group with acetyl chloride the 2H-pyran-2-one 1 was obtained.21 Dienophile 2 was prepared by a modification of the procedures published by Cava et al.22 All other reagents and solvents were used as received from commercial suppliers. Microwave reactions were performed in air using a focused microwave unit (Discover by CEM Corporation, Matthews, NC, USA). The machine consists of a continuous, focused microwave power-delivery system with an operator-selectable power output ranging from 0 to 300 W. Reactions were conducted in darkness in glass vessels (capacity 10 mL) sealed with rubber septum. The pressure was controlled by a load cell connected to the vessel via the septum. The temperature of the reaction mixtures was monitored using a calibrated infrared temperature controller mounted below the reaction vessel and measuring the temperature of the outer surface of the reaction vessel. The mixtures were stirred with a Teflon-coated magnetic stirring bar in the vessel. Temperature, pressure, and power profiles were recorded using commercially available software provided by the manufacturer of the microwave unit. Synthesis of 1,1'-(Hexane-1,6-diyl)bis(1H-pyrrole-2,5-dione) (2)22 To a clear solution of maleic anhydride (2.03 g, 20 mmol) in diethyl ether (30 mL) a separately prepared mixture of hexane-1,6-diamine (2.07 g, 10 mmol) in diethyl ether (10 mL) is added dropwise at room temperature. The viscous suspension is further stirred at room temperature for 1 h and thereafter cooled on ice. Precipitated product is isolated by vacuum filtration and used in the next step without drying or additional purification. The entire obtained solid is slowly added to a mixture of sodium acetate (0.66 g, 8 mmol) and acetic anhydride (8 mL) in an Erlenmayer flask while vigorously stirring at room temperature. After the completion of the addition, the reaction mixture is heated on water bath (ap-prox. 100 °C) for 1 h, cooled to room temperature and poured onto ice-water mixture (30 g). The precipitated product is isolated by vacuum filtration, rinsed 3 times with distilled water and once with a few mL of petroleum ether yielding crude 2 (0.56 g, 20%) that is further crystallized from ethanol. M.p. 139-141 °C (EtOH), m.p. (lit.)23 139-141 °C (EtOH). IR (ATR) 3104, 3087, 2936, 2856, 1686, 1453, 1418, 1372, 1327, 1240, 1129 cm-1. 1H NMR (500 MHz, CDCl3): 5 1.29 (m, 4H, 2 x NCH2CH2CH2), 1.59 (m, 4H, 2 x NCH2CH2CH2), 3.51 (t, 4H, 2 x NCH2CH2CH2), 6.69 (s, 4H, 4 x CH). 13C NMR (125 MHz, CDCl3): 5 25.6, 27.8, 36.9, 124.4, 171.1. MS (ESI+) m/z 277 (MH+). HRMS (ESI+) calcd. for C14H17N2O4 (MH+): 277.1183. Found: 277.1181. Anal. calcd. for C14H16N2O4-0.1 H2O: C, 60.47; H, 5.87; N, 10.07. Found: C, 60.43; H, 5.89; N, 9.95. Synthesis of the Macrocycle 3 A 10 mL quartz microwave vessel is loaded with 2H-pyran-2-one 1 (105 mg, 0.5 mmol), dienophile 2 (152 mg, 0.55 mmol) and «-butanol (100 mg). A stirring bar is added and the vessel closed with the rubber septum. The reaction mixture is irradiated with microwaves (150 W) at 150 °C for 45 min. Thereafter, the reaction mixture is cooled to room temperature and diisopropyl ether is added (0.5 mL). The precipitated product is collected by vacuum filtration providing crude macrocycle 3 (150 mg, 34%) that is further crystallized from DMF. M.p. 255-257 °C (DMF). IR (ATR) 3368, 2940, 2860, 1766, 1698, 1548, 1437, 1399, 1367 cm-1. 1H NMR (500 MHz, DMSO-d6): 5 1.03 (m, 8H, 2 x NCH2CH2 CH2CH2CH2CH2N), 1.23 (m, 8H, 2 x NCH2CH2CH,, CH2CH2CH2N), 1.85 (s, 6H, 2 x Me), 1.95 (s, 6H, 2 x NHCOCH3), 2.02 (s, 6H, 2 x COMe), 3.00 (d, J = 7.5 Hz, 4H, 2 x 3a-H, 4a-H), 3.18 (m, 8H, 2 x NCH2CH2CH2CH2 CH2CH2N), 4.11 (d, J = 7.5 Hz, 4H, 2 x 7a-H, 8a-H), 6.82 (s, 2H, 2 x CH), 8.43 (s, 2H, 2 x NH). 13C NMR (125 MHz, DMSO-d6): S 18.2, 23.5, 25.8, 27.0, 27.4, 37.7, 41.2, 42.9, 48.8, 57.2, 138.2, 142.7, 170.3, 174.1, 175.3, 195.8. MS (ESI+) m/z 884 (MH+). HRMS (ESI+) calcd. for C^HNA, (MH+): 883.3872. Found: 883.3844. Anal. cal- 46 55 6 12 x ' cd. for C H NO,-0.8 H,O: C, 61.57; H, 6.24; N, 9.37. 46 54 6 12 2 Found: C, 61.57; H, 6.36; N, 9.27. 2. 2. Crystallography Single-crystal X-ray diffraction data were collected at room temperature on a Nonius Kappa CCD diffractom-eter using graphite monochromated Mo-Ka radiation (A = 0.71073 A). The data were processed using DENZO.24 Structures were solved by direct methods implemented in SIR9725 and refined by a full-matrix least-squares procedure based on F2 with SHELXL-2014.26 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were readily located in a difference Fourier maps and were subsequently treated as riding atoms in geometrically idealized positions, with C-H = 0.93 (aromatic), 0.98 (methine), 0.97 (methylene) or 0.96 A (CH3), N-H = 0.86 A and with Uiso(H) = kUeq(C or N), where k = 1.5 for methyl groups, which were permitted to rotate but not to tilt, and 1.2 for all other H atoms. To improve the refinement results, two reflections in the case of 2a, eleven reflections in the case of 2b and twenty eight reflections in the case of 3-2DMF with too high values of 5(F2)/e.s.d. and with Fo2 < Fc2 were deleted from the refinement. In 2b a proposed twin law has been applied according to Platon analysis and the R1 factor has improved from 8.13% to 7.75%, however, instead of estimated BASF 0.19 the refined BASF was found to be 0.00939. In the crystal structure of 3-2DMF a solvate DMF molecule is disorder over two positions with refined ratio 0.82:0.18 and ISOR instruction was used for the refinement of C25B atom in DMF. Crystallographic data are listed in Table 1. X-Ray powder diffraction data were collected at room temperature using a PANalytical X'Pert PRO MPD diffractometer with 9-29 reflection geometry, Table 1. Crystal data and refinement parameters for the compounds 2a, 2b and 3-2DMF. Compound 2a 2b 3-2DMF CCDC 1547701 1547702 1547703 Molecular formula C14H16N2O4 276.29 C14H16N2O4 276.29 C52H68N8O14 1029.14 Molecular weight Crystal system Triclinic Monoclinic Orthorhombic Space group P-1 P 21/a P c a n a (À) 4.5975(2) 8.4999(3) 10.4260(10) b (À) 5.5190(3) 6.6347(2) 17.5217(2) c (À) 14.1680(10) 12.6120(5) 27.8937(4) « (°) 93.956(3) 90 90 P (°) 97.222(4) 98.295(2) 90 y (°) 97.692(4) 90 90 V (À3) 352.08(4) 703.80(4) 5095.7(5) Z 1 2 4 Dcalc (g cm-3) 1.303 1.304 1.341 p (mm1) 0. 097 0.097 0.098 F(000) 146 292 2192 Crystal dimensions (mm) 0.60 x 0.35 x 0.05 0.60 x 0.50 x 0.05 0.28 x 0.13 x 0.08 Reflections collected 2464 3070 11000 Data / restraints / parameters 1540 / 0/91 1566 / 0 / 93 5819 / 6 / 369 Rint 0.0227 0.0204 0.0320 R1, wR2 [I > 2a(I)]a 0.0444, 0.1231 0.0775, 0.2475 0.0486, 0.1157 R1, wR2 (all data)b 0.0580, 0.1343 0.0854, 0.2540 0.0855, 0.1335 Goodness of fit on F2, Sc 1.039 1.097 1.010 Extinction coefficient - 0.62(15) - Ap ,Ap . (e À-3) ' max ' min v ' 0.128, -0.150 0.226, -0.229 0.263, -0.306 0 R = X||FJ - |FJ|/X|FJ.b wR2 = (Z[w(Fo2 - Fc2)2]/X[w(Fo2)2]}1/2. c S = (Z[(Fo2 - Fc2)2]/(n/p)}1/2 where n is the number of reflections and p is the total number of parameters refined. primary side Johansson type monochromator and Cu-Ka1 radiation (A = 1.54059 â). 3. Results and Discussion 3. 1. Synthesis The strategy for the synthesis of the macro cycle 3 was based on our previous experiences with the [4+2] cycloadditions of variously substituted 2H-pyran-2-ones and appropriate dienophiles, including N-substituted maleim-ides.27-31 Namely, it was already observed that 2H-pyran-2-ones can act as "double" dienes, reacting in two consecutive Diels-Alder reactions with two distinctive molecules of the dienophiles, yielding bicyclo [2.2.2] octenes.30 The initial cycloaddition step leads to the formation of CO2-bridged oxabicyclo[2.2.2]octenes that in the next step eliminate a molecule of CO2 (via a retro-hetero-Diels-Al-der reaction) providing cyclohexadiene systems that act as new dienes for another molecule of dienophile finally providing the double cycloadducts. On the other hand, if the two molecules of the dienophile would be connected by a suitable tether, it would be possible to expect that the second cycloaddition step would take place intramolecularly. At least in theory, the smallest possible cyclic product would consist of just one bicyclo [2.2.2] octene fragment (formed out of one 2H-pyran-2-one ring) and one molecule of the double dienophile. Related examples were already described by the application of cycloocta-1,4-di-ene.32 Of course, it could be also possible that larger cycles would be obtained, for example such that contain two bi-cyclo[2.2.2]octene moieties and two molecules of the double dienophile. Here, we have focused our attention to a 3-acetyl-amino-6-methyl-2H-pyran-2-one (1) and 1,1'-(hexane- n-buoh NHCOMe 150'C