Confonnation of cis-4-Hydroxy-Praziquantel

Conformation of cis-4-Hydroxy-Praziquantel and its Crystal andMolecular Structure

Janina Karolak-Wojciechowska", Witold Kwiatkowski", Katarzyna Kiec-Kononowicz" b), and Gottfried Blaschkec)

a) Instituteof GeneralChemistry,TechnicalUniversity.90-924Lodz,Zwirki36, Polandb) Instituteof Pharmaceutical Chemistry,Medicinal Academy 31-065 Krakow, Skateczna 10,Polandc) Instituteof Pharmaceutical Chemistry,University, Hittorf Sir. 58-62, 0-4400 MUnster, Gennany

ReceivedMay 28,1990

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The crystal structureof racemiccis-4-0H-PZQ - the main metabolite of theanthelmintic drug Praziquantel - has been determined by X-rayanalysisandrefined to R = 0.064. In the unit cell there are two independent molecules.but with a similar confonnation. The molecules of the same enantiomer(joined by H-bonds) fonn in the crystal a column along a-axis direction.Molecular mechanics calculations indicate that the conformation in the solidstate does not correspond to the energy minimum and there are two lowerenergy confonnations. Thus, in solutions these two confonners can mostprobably exist.Theirconcentration in solutions dependson the solventpola­rity and the value of the dipole moment for the respective fonn of themolecule.

Praziquantel (PZQ), an anthelmintic drug showing broadtrematocidal and cestocidal activityl,2), is a pyrazino[2,1-a]isoquinolin-4-one derivative possessing an asymmetric cen­ter. The anthelmintic activity is mainly concerned with the(-)-R-enantiomer3-5). In our previous work6) we have de­scribed the spectral properties of PZQ and some relatedderivatives of pyrazino[2,I-a]isoquinolin-4-0ne. We haveobtained acyl (aromatic and alicy.clic) derivatives of pyrazi­no[2,I-a]isoquinolin-4-one. In IH_NMR spectra of arylderivatives the signals were very broad and the 13C_NMRsignals were difficult to assign. In 13C_NMR spectra of ali­cyclic derivatives measured in MeOD two sets of signalsexist with nearly equal intensities for nearly all carbons

8 7

Konformatlon des tls-4-0H·Prazlquantel und seIne Krlstall- und Mole­kularstruktur

Es wurde die Kristallstruktur des razemischen cis-4-0H-P'.lQ - des Haupt­metaboliten des Anthelmintikums Praziquantel - rllntgenographisch mit ei­nem GUtefaktor R = 0.064 bestimmt, In dem asymmetrischen Teil der Ele­mentarzelle befinden sich zwei MolekUle dieser Verbindung. Die MolekUledesselben Enantiomers sind durch eine H-BrUcke miteinander verbundenund bildeneine Kette der a-Achseentlang. Aus MolekUlmechanikrechnun­gen folgt, daB die Konformation des MolekUls im festen Zustand nichl demEnergieminimum entspricht und es zwei Konrormationen niedrigerer Ener­gie gibt. In Lllsunggibt es daher zweistabile Konformationen. Ihre Konzen­tration in Lllsungen hlIngt von der Polaritat des Losungsmtuels und demDipolmomera der entsprecbenden MolekUlfonn abo

whereas in CDCh one pair of signals is more intensive. Wehave presumed that these results were connected with dy­namic properties of the pyrazin-4-one ring or with the rota­tion around the N-2 - C-12 bond.

Taking into account the above results it seemed interes­ting to continue investigations on the preferred conforma­tions of these compounds on the basis of x-ray data. In thepharmacological studies was stated that 4-0H-PZQ is themain metabolite of pzQI,7), so as a representative compoundracemic 2-(4-cis-hydroxy-cyclohexylcarbonyl)-1,2,3,6,7,II b­hexahydr0-4H-pyrazino-[2,I-cx]-isoquinolin-4-0ne (cis-4-0H­PZQ) was chosen. The stereoselective synthesis of cis-4­OH-PZQ was described8

).

9

ill

Scheme 1 FIg. 1: Molecularstructureof cis-4-0H-Praziquantel.

Arch. Pharm. (Weinheim) 324.479482 (/99/) ClVCH Verlagsgesellschaft mbH. 0-6940 Weinheim.1991 036S-6233/9I/0808-0479 S3.50+ .25/0

480

,,

Fig. 2: Unitcellof cis-4-0H·Praziquantel.

Selected bond lengths and bond angles data of the solvedstructures of 4-0H-PZQ are given in Tables 2 and 3 for twoindependent molecules [with descriptions (1) or (2), respec­tively). From the PLUTO drawing (Fig. 1) it is visible thatboth molecules of the same enantiomers, are joined by aH-bond 016-H16, ....04 1 of 2.907 (3) A [with a respectivebond angle equal to 174.2 (2) 0]. In the crystal the moleculesof the same enantiomer are forming two independent col­umns along [100] direction (Fig. 2). The link in the columnsare two H-bonds, the one mentioned above and the secondone between 016 1 - HI6, I and 04 (i) with symmetry cod (i)2+x,y,z and equal 2.916 (2) A[angle equal to 172.9 (2) 0]. Itis necessary to point out that atom 012 is not used in H­bond formation. Both molecules in the independent part ofthe unit cell show only very small differences.

The remarkably feature in the molecules under discussionis the conformation of all six-membered rings. [The sub­scription of the rings are given in Scheme I.] Phenyl rings(A-rings) are nearly planar. As indicated by the torsionangles values, the B-rings (piperidine rings) are in the halfchair form with a plane of symmetry going through theatoms C6 and Clla. Duax asymmetry parameters9) (calcu­lated from the torsion angles) for these planes of symmetryequal ~Cm (1) =13.05° and ~Cm (2) =12.26°, respectively,for molecule 1 and 2. The piperazinone rings (C rings) havea screw conformation with 2-fold axis bisecting CI-N2 andC4-N5 bonds and asymmetry parameters equal aC2 (1) =5.85° and ~C2 (2) =3.52°, respectively, for both molecules.The H-atom at Cllb atoms (being the chiral center) are axialin both fused rings. In the cyclohexyl rings, having the chairform (all t.Cm calculated from the torsion angle values aresmaller than I), the OR groups at Cl6 atoms occupy axialpositions, while the C13-C12 bonds, connecting this ringswith the main skeletons of the molecule, are equatorial. Allbond distances and valence angles have expected values.

Kiec-Kononowicz and coworkers

Fig. 3: Strain energydistribution forcis-4-0H-Praziquanlel.

Both independent molecules of cis-4-0H-PZQ in the crys­tal have all cyclic systems sufficiently close to the planartransoidal arrangement (torsional angles C3-N2-CI2-CI3equal 7.9 (6) 0 and 9.5 (3) 0, respectively, for both mole­cules]. This conformation permit 1t overlap between N2atom and012=C12 group. In this context the bond lengthsand angles of the fragment N2-CI2-012 indicate a partialdouble bond (conjugated) system. To understand betterwhich conformational isomer might be preferred in the solu­tion, we have calculated the energies of all isomers uponclockwise rotation about the C3-N2-CI2-CI3 angle for themolecule 1. This calculation was performed by molecularmechanics programme MMX IO) taking 1t overlap into ac­count. The energy was minimized after each 10° from 0° to3600. The results are depicted in Fig. 3. The calculationsindicate that in the minimum energy state the conformationof the molecule is 120° twisted from coplanarity. So, in theplanar form with the unfavorable steric interaction, the fa­vorable resonance interaction exists. The maxima energy atabout 0° and 180° are about 6 kcal/mol higher than minimaenergy at 120° and 250°. The X-ray conformation of cis-4­OH-PZQ is calculated to be nearly at the maximum energywith the torsional angle being 0°. In the dissolution processeach molecule is reconstructed by rotation around the N2­CI2 partially double bond to a minimum energy conforma­tion. In solution the ratio of isomers in the two energy mini­ma depends on the nature of the solvent. In polar solvents(e.g. MeOD) both isomers with cI> = 1200 and 250° arepresent in similar concentration. At the same time the dif­ference in energy between them was found to be very small,but the energy barrier, even at room temp., was found to besufficiently high to be a real barrier. However, in non-polarsolvents (e.g. CDCl) one isomer should be favored in orderto balance between steric and coulombic factors. Normallythe preferred conformation can be the form with the mini­mum dipole moment. Semiempirical molecular orbital cal­culation (MNDO-2) indicates that the isomer with cI> =2500

has a lower dipole moment (2.68 Debye) than this with cI> =

Arch.PluJrm. (Weinheim) 324. 479-482 (/99/)

Confonnation ofcis-4-HYdroxy-Praziquantel

1200 (4.69 Debye). On the basis of this argumentation wehave assigned the conformation isomer with «1l = 2500 to bethe major isomer in non-polar solvents. This discussionpresumably can be applied as an interpretation for the 13C_NMR spectra of cis-4-0H-PZQ.

This research was supported by P.R.II project of the Polish Ministry ofEducation.

Experimental Part

X-RayDataCollection

Crystals were recrystallized from acetonein very thin long needles. One

crystal wasselectedwith 0.06 x 0.1 x 0.2 mm size.The crystallographic sys­

tem and unit cell dimensions were first detennined from the de Joung-Bou­man photographs. than refined from the angular selling of 25 reflectionsmeasured on aEnraf-Nonius CAD4 single-crystaldiffractometer, employing

Cuka (A=1.54178A) radiation with a graphitemonochromator. The intensi­

ty dataof6944reflections (5255 independent)weremeasured in therangeh:-6 to 6; k: -18 to 0; 1:·26 to 26, using the ro-29-scan mode.The intensity ofthe primary beamwascheckedthroughout thedatacollection by monitoring

threereferencereflections every 70 reflections. The variation in the inten­sities over the courseof the datacollection was not observed.After Lorentzandpolarization corrections. thedatawereconsideredas"observed"with Fa

> 3a (Fa) andthenumber of thedatawerereducedto 3264reflections.

Table 1: Non-hydrogen fractional atomic coordinates (10-4). equivalenttemperature factors (A2) (x 10-3)with e.s.d.'s in parentheses.

Cl 5648(7) 10389(3) 4229(2) 44(1)N2 4500(6) 9401(2) 4055(1) 44(1)C3 1938(7) 9225(3) 3705(2) 54( 1)C4 1651(8) 9808(3) 3190(2) 47(1)04 -334(5) 9609(2) 2812(1) 64(1)N5 3539(6) 10510(2) 3167(1) 46(1)C6 3295(8) 11060(3) 2659(2) 59(1)C7 3128(7) 12024(3) 2964(2) 56(1)C7A 5360(7) 12424(3) 3489(2) 41(1)C8 6162(8) 13382(3) 3663(2) 59(1)C9 8264(9) 13769(3) 4110(2) 61(1)Cl0 9614(8) 13200(3) 4384(2) 66(1)Cll 8833(8) 12243(3) 4227(2) 54(1)CllA 6688(7) 11846(3) 3774(2) 39(1)CllB 5913(7) 10789(3) 3620(2) 41(1)C12 5670(8) 8748(3) 4267(2) 41(1)012 7737(5) 8973(2) 4603(1) 48(1)C13 4342(7) 7734(3) 4055(2) 39(1)C14 4685(7) 7405(3) 3364(2) 46(1)CIS 3359(7) 6388(3) 3140(2) 53(1)016 6839(5) 5721(2) 3541(1) 70(1)C16 4285(8) 5758(3) 3577(2) 57(1)C17 4096(7) 6115(3) 4272(2) 54(1)C18 5416(7) 7131(3) 4492(2) 49(1)Cl 1 11025(7) 4612(3) 772(2) 48(1)N2 1 11053(6) 5597(2) 948(1) 44(1)C3 1 9001(7) 5766(3) 1286(2) 51(1)C4 1 8659(7) 5194(3) 1808(2) 46(1)04 1 7242(5) 5387(2) 2188(1) 62(1)N5 1 9863(6) 4493(2) 1835(1) 47(1)C6 1 9571(8) 3936(3) 2343(2) 59(1)C7 1 8153(8) 2975(3) 2032(2) 55(1)C7Al 9447(7) 2571(3) 1510(2) 44(1)C8 1 9104(8) 1617(3) 1336(2) 53(1)C9 1 10389(9) 1224(3) 889(2) 64(1)CI01 12037(9) 1802(3) 615(2) 65(1)Cl11 12372(8) 2763(3) 774(2) 56(1)CI1Al 11066(7) 3147(3) 1224(2) 40(1)Cl1Bl 11499(7) 4205(3) 1379(2) 41(1)C12 1 12658(7) 6249(3) 734(2) 40(1)012 1 14162(5) 6027(2) 399(1) 48(1)C13 1 12561(7) 7264(3) 945(2) 40(1)C14 1 13919(7) 7607(3) 1638(2) 46(1)C15 1 13833(7) 8616(3) 1862(2) 51(1)C16 1 14938(8) 9237(3) 1422(2) 53(1)016 1 17561(5) 9274(2) 1456(1) 71(1)C17 1 13724(8) 8892(3) 726(2) 56(1)C18 1 13791(7) 7872(3) 509(2) 47(1)

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Table 2: Selected bond lengths (A) with e.s.d.ts in parentheses.

Molecule 1 Molecule 2

Cl N2 1. 464(5) Cl 1 N2 1 1. 458(5)Cl CllB 1.523(6) Cl 1 Cl1Bl 1. 525(£,)N2 C3 1. 470(5) N2 1 C3 1 1. 460(5)N2 C12 1. 363(6) N2 1 C121 1. 363(51C3 C4 1. 510(7) C3 1 C4 1 1.511(7)C4 04 1.234(5) C4 1 04 1 1. 238(5)C4 N5 1.345(5) C4 1 N5 1 1.342(6)N5 C6 1. 461(5) N5 1 C6 1 1. 469(5)N5 CllB 1.477(5) N5 1 Cl1Bl 1. 482(5)C6 C7 1.513(6) C6 1 C7 1 1.510(5)C7 C7A 1.520(5) C7 1 C7Al 1. 512(6)CllA CllB 1.529(6) CIIAI CllBl 1. 530(6)C12 012 1. 228(5) C121 0121 1.223(5)C12 C13 1. 527(6) C121 C131 1.527(6)016 C16 1. 432(5) C161 0161 1. 431(5)

Table 3: Selected bond angles(deg) with e.s.d.'s in parentheses.

Molecule 1 Molecule 2

N2 Cl CllB 109.0(3) N2 1 Cl 1 Cl1Bl 109.2(3)Cl N2 C3 113.0(3) Cl 1 N2 1 C3 1 112.5(3)Cl N2 C12 121.1(3) Cl 1 N2 1 C121 121.2(3)C3 N2 C12 125.6(3) C3 1 N2 1 C121 125.8(3)N2 C3 C4 113.3(3) N2 1 C3 1 C4 1 113.1(3)C3 C4 04 117.4(4) C3 1 C4 1 04 1 118.2(4)C3 C4 N5 119.0(4) C3 1 C4 1 N5 1 119.0(4)04 C4 N5 123.7(4) 04 1 C4 1 N5 1 122.8(4)C4 N5 C6 119.2(3) C4 1 N5 1 C6 1 119.7(3)C4 N5 CllB 125.5(3) C4 1 N5 1 CllBl 125.3(3)C6 N5 C11B 115.4(3) C6 1 N5 1 Cl1Bl 115.0(3)N5 C6 C7 108.0(3) N5 1 C6 1 C7 1 107.7(3)C6 C7 C7A 109.7(3) C6 1 C7 1 C7Al 110.6(4)Cl CllB N5 110.3(3) Cl 1 Cl1Bl N5 1 109.9(3)Cl CllB CllA 110.7(3) Cl 1 Cl1Bl CllAl 111.0(3)N5 CllB CllA 109.9(3) N5 1 CllBl CllAI 110.4(3)N2 C12 012 120.7(4) N2 1 C121 0121 121.0(4)N2 C12 C13 117.4(4) N2 1 C121 C131 117.5(3)012 C12 C13 121.8(4) 0121 C121 C131 121.5(4)C15 C16 016 111.0(3) C151 C161 0161 111.0(3)016 C16 C17 108.2(3) 0161 C161 C171 107.7(3)

Crystalclata: C19H24N20l: M = 328.4; lriclinic; pT: a = 5.5193 (5). b =14.9075(II), c = 21.3846 (14) A: a = 98.4587 (59). ~ = 97.4001 (63). y=100.6323 (63) 0; V = 1688.35 A-3: d.,= 1.292 g . cm·3; J.l = 6.69 em":F (000) = 704; T =293 K.

Structure determination andrefinement

The space group PI wasdecidedon the bases of E-values distribution. Inthis space group the phaseproblem was solved by mean of SHELXS-86program with R (E) = 0.198. E-map hasshown the positions of all non-H­

atoms in two molecules. This model was refined by full-matrix least­squares in two blocks (218 parameters in eachblock) on F's values(SHEL­

X-76). Anisotropic temp. factors were applied to all non-H-atoms. H-atomswere locatedon Ap map and their positional parameters as well as isotropic

temp. factors (equal 1.5 of the respective values for parent C-atorns) were

not refined. At the end of refinement the agreement factors were equal toR = 0.064 and Rw = 0.046 with w = 2.3S43/a2 (Fa) for 3264 reflections.

The maximum Aa =0.06 in the last cycle of refinement; maximum andminimum electron densities in final Ap map were 0.05 e.A-3 and -0.085

e.A-3•The empirical correctionof the secondary extinction with g =0.00457

wasapplied.All calculation wereperfonned on PC Amstrad 1512computer. TIle final

atomic parameters are listed in Table III).

References

P. Andrews. H. Thomas, R. Pohlke. and J. Seubert, Med. Res. Rev. 3.147(1983): C.A. 99. 47364f(1983).

2 H. Frohberg and M. Schulze Schencking. Arzneim. Forsch. 3J. 555(1981).

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3 P. Andrews. Phannacol. TIler. 29. 129 (1985); C.A. /04. 199314z(1986).

4 Y. Liu, M. Qian, X. Wang. J. Jia. Q. Wang. Y. Jiang. R. Wang. S.Van. and B. Chen. Chin. Med. J. 99. 935 (1986); C.A. /06. 207232e(1987).

5 G. Blaschke and B. Walther. Chern. Ber. 1/8.4620 (1985).6 K. Kiec-Kononowicz. Arch. Pharm. (Weinheim) in press.7 K.U. Buhring, H.W. Diekman. H. Muller. A. Garbe. and H. Nowak.

Eur.1. Drug Metab. Phannacok. 3.179 (1978).

Kiec-Kononowicz and coworkers

8 K. Kiec-Kononowicz, Z.S. Farghaly, and G. Blaschke. in preparation.9 J.F. Griffin. W.L. Duax, Ch.M. Weeks. Atlas of Steroid Structures

Vol. 2.IFIJPlenurn.IV New York-Washington-London (1984).10 N.L. Allinger and Y.A. Yuh (1973). MMX86 is a version of MM2P

supplied by Gwen Nelson and C. Scott Owen adapted to MicrosoftFortran 77 v 3.20 by JJ. Gajewski and K.E. Gilbert.

II Further information can be obtained from the Fachinformationszen­trum Energie. Physik. Mathematik. 0·7514 Eggenstein-Leopoldshafen2. citing the deposit no...... the authors and the journal. [Ph836]

Arch. Pharm. (WeinheimJ 324. 479-482 (/99/)