Configurations and conformations of sanguinarine and chelerythrine free bases stereoisomers

Journal of Molecular Structure 613 (2002) 103–113 www.elsevier.com/locate/molstruc

Configurations and conformations of sanguinarine and chelerythrine free bases stereoisomers Jaromı´r Tousˇeka,*, Roger Dommisseb, Jirˇ´ı Dosta´lc, Zdirad Zˇa´kd, Luc Pieterse, Radek Marekf a

Department of Theoretical and Physical Chemistry, Faculty of Science, Masaryk University, Kotla´rˇska´ 2, CZ-61137 Brno, Czech Republic b Department of Chemistry, University of Antwerp RUCA, Groenenborgerlaan 171, B-2020 Antwerpen, Belgium c Department of Biochemistry, Faculty of Medicine, Masaryk University, Komenske´ho na´m 2, CZ-66243 Brno, Czech Republic d Department of Inorganic Chemistry, Masaryk University, Kotla´rˇska´ 2, CZ-61137 Brno, Czech Republic e Department of Pharmaceutical Sciences, University of Antwerp UIA, Universiteitsplein 1, B-2610 Antwerpen, Belgium f National Center for Biomolecular Research, Faculty of Science, Masaryk University, Kotla´rˇska´ 2, CZ-61137 Brno, Czech Republic Received 15 February 2002; accepted 12 March 2002

Abstract The configurations and conformations of bimolecular aminoacetals of benzo[c ]phenanthridine alkaloids sanguinarine and chelerythrine were investigated by NMR spectroscopy, quantum chemical calculations, and X-ray analysis. The results of the complete computational conformational analysis, the calculation of the chemical shielding of the considerably populated conformers, the determination of averaged chemical shifts and comparison with experimental NMR chemical shifts observed in solution are reported. Based on these results, the relative configurations at the stereogenic centers of two diastereomers of bis(dihydrosanguinarinyl) ether were determined. The structure of the major diastereomer of bis(dihydrosanguinarinyl) ether was confirmed by X-ray analysis and specified for the solid state. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Sanguinarine; Chelerythrine; NMR spectroscopy; X-ray crystallography; Quantum chemical calculations

1. Introduction Sanguinarine (1a) and chelerythrine (1b) (Fig. 1) are two members of the benzo[c ]phenanthridine alkaloids and are known since 1827 and 1839, respectively. They occur in many plant species of the Papaveraceae family (Sanguinaria canadensis, Chelidonium majus, Dicranostigma lactucoides, Macleaya microcarpa, and some other) [1]. They * Corresponding author. Tel.: þ420-05-4112-9321; fax: þ 42005-4121-1214. E-mail address: [email protected] (J. Tousˇek).

are quaternary iminium salts, brightly colored, responsible for a typical coloring of tissue and latex of the above-mentioned plants. Red sanguinarine (1a) and yellow chelerythrine (1b) are considerably susceptible to nucleophilic attack at the carbon C-6 and yield colorless adducts in such reactions [2]. In alkaline environment, like most other alkaloids [3], the salts of sanguinarine and chelerythrine [4] are converted to free bases. Depending on the environment the free bases can adopt either the pseudobase structure [5,6] or the structure of bimolecular aminoacetal [7 –9] or both. Their constitutions have been determined by spectral methods including 2D

0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 8 6 0 ( 0 2 ) 0 0 1 3 8 - 2

104

J. Tousˇek et al. / Journal of Molecular Structure 613 (2002) 103–113

Fig. 1. Structural formulae with atom numbering.

Atom

1 4 6 9 10 11 12 13-Ha 13-Hb 14-Ha/14 14-Hb/21 20 19 a

c

e

Chelerythrine

C (2aa)

C (3a)

C (4a)

C (5ab)

C (6ac)

D (3a)

B (2a)

B (3a)

7.11 7.65 5.83 6.93 7.42 7.76 7.49 6.04 6.04 6.12 6.07 2.74 2.15

7.13 7.96 6.33 6.71 7.23 7.65 7.43 6.08 6.11 5.17 5.66 2.98 –

7.08 7.17 5.86 6.87 7.37 7.73 7.41 6.04 6.04 5.69 5.90 2.59 –

7.11 7.99 5.75 6.68 7.20 7.63 7.43 6.07 6.10 5.31 5.69 2.86

7.09 7.48 5.23 6.84 7.31 7.71 7.42 6.02 6.02 5.87 5.93 2.49

7.04 7.76 5.71 6.75 7.21 7.66 7.32 5.34 5.35 5.33 5.34 2.36

e

e

7.30 7.87 6.23 6.87 7.38 7.74 7.51 6.20 6.23 5.22 5.80 2.91 –

7.09 8.18 6.49 6.51 7.08 7.62 7.31 5.35 5.40 4.69 5.13 2.73 –

e

Ref. [5]. N NMR (d, ppm) 39.2 (N5) [28]. 15 N NMR (d, ppm) 42.9 (N5); 13C NMR (d, ppm) 40.97 (NMe), 66.12 (C6). Ref. [6]. Not observed.

b 15

d

Sanguinarine B (4a)

C (2b)

C (3b)

C (5b)

B (2bd)

B (5b)

D (2b)

D (3b)

D (5b)

e

2.51

7.13 7.67 6.01 7.07 7.65 7.79 7.49 6.05 6.05 4.01 3.95 2.72

e

e

7.16 7.92 6.60 6.85 7.49 7.69 7.45 6.11 6.12 2.42 3.73 3.05 –

7.15 7.99 5.99 6.80 7.43 7.65 7.43 6.11 6.11 2.57 3.73 2.91 1.43

7.05 7.79 5.98 6.70 7.52 7.78 7.37 5.36 5.36 3.96 3.39 2.40 1.64

7.10 8.23 6.19 6.58 7.39 7.68 7.32 5.35 5.41 2.80 3.24 2.70 1.40

7.31 7.54 5.58 7.16 7.70 7.88 7.54 6.12 6.15 3.87 3.84 2.57 5.77

7.39 7.86 6.41 7.02 7.61 7.80 7.55 6.21 6.24 2.33 3.67 2.98 –

7.36 7.88 5.85 6.96 7.54 7.75 7.52 6.19 6.23 2.55 3.67 2.82 1.20

7.46 6.20 e e e e e e e e

J. Tousˇek et al. / Journal of Molecular Structure 613 (2002) 103–113

Table 1 1 H NMR chemical shifts (ppm) of sanguinarine (a) and chelerythrine (b) derivatives 2–6 at 30 8C (C: chloroform-d; D: DMSO-d6; B: C6D6)

105

J. Tousˇek et al. / Journal of Molecular Structure 613 (2002) 103–113

106

Table 2 13 C NMR chemical shifts (ppm)a of sanguinarine (a) and chelerythrine (b) derivatives 2–5 in CDCl3, DMSO-d6, and C6D6 at 30 8C (C: chloroform-d; D: DMSO-d6; B: C6D6) Atom

1 2 3 4 4a 4b 6 6a 7 8 9 10 10a 10b 11 12 12a 13 14 21 20 a b c

Sanguinarine

Chelerythrine

C (3a)

C (4a)

D (3a)

B (3a)

C (2b)

C (3b)

C (5b)

D (5b)

B (2b)

B (3bb)

B (5b)

104.28 147.10 147.23 102.25 127.30 138.96 79.33 112.98 145.08 147.02 108.34 115.71 126.03 123.03 120.45 123.47 130.94 100.92 101.14 – 41.92

104.35 147.43 147.58 101.65 126.37 139.33 81.56 113.92 145.08 147.18 108.56 115.80 125.91 122.93 120.08 123.02 131.00 100.96 101.53 – 41.72

103.86 147.02 147.18 101.42 126.35 138.49 78.92 112.22 144.24 146.43 108.38 115.64 125.26 121.89 119.63 123.24 130.47 100.15 100.88 – 41.20

104.79 147.67 148.02 102.32

104.58 147.47 148.08 100.77 126.90 138.16 79.01

104.51 147.53 148.08 101.11 126.96 138.45 77.51 126.18 146.38 152.13 112.36 118.69 125.56 123.04 119.84 123.26 131.23 100.87 60.41 55.69 40.87

104.34 147.39 147.94 101.02 127.74 139.42 64.01 128.32 145.83 151.94 111.49 118.60 125.22 123.08 119.81 123.15 131.16 101.00 60.32 55.59 41.00

104.17 147.02 147.56 100.22 126.94 138.69 63.58 127.17 145.05 151.51 112.28 118.52 124.39 122.45 119.67 123.01 130.72 101.11 59.39 55.51 40.77

104.95 148.07 148.67 101.38 127.84 139.22 79.31

104.93 148.02 148.64 101.41 127.85 139.32 78.18 127.46 147.79 152.86 113.27 118.41 126.19 123.80 120.26 123.59 131.79 101.09 60.34 55.54 40.73

104.74 148.02 148.60 101.64 128.58 140.38 64.74 129.25 147.23 152.76 112.63 118.61 126.12 123.94 120.32 123.50 131.82 101.01 60.28 55.52 40.91

c

139.39 79.55 113.72 145.61 147.67 108.52 116.06

c

146.78 152.20 c c

c

c

123.13 120.91 123.92 131.57 100.95 101.29 – 41.43

c c c c

101.15 61.88 56.05 40.33

c

147.82 152.80 113.43 118.95 c c

120.29 124.23 131.73 100.94 61.59 55.59 40.14

Commercially available solvents were used without further purification. Ref. [6]. Not observed, not determined.

NMR (GHMBC [10] and GSQMBC [11]). In this communication, we report detailed treatment of both bis(dihydrosanguinarinyl) ether diastereomers (3a/ 3b). For this study, a combined NMR, X-ray, and theoretical approach were used. The results of the NMR measurements (the values of the chemical shifts) and the X-ray analysis (the geometry of the most stable conformer) are compared with the theoretically predicted data.

2. Experimental 2.1. Preparation of compounds Bis(dihydrosanguinarinyl) ether (3a), bis(dihydrochelerythrinyl) ether (3b), bis(dihydrosanguinarinyl)amine (5a), and bis(dihydrochelerythrinyl)amine (5b) were obtained by alkalization of sanguinarine

chloride (1a) and chelerythrine chloride (1b) as described in Refs. [7,8], respectively. 2.2. NMR measurements Two-dimensional NMR spectra were recorded using a Bruker Avance DRX-400 and Bruker Avance DRX500 spectrometers operating at frequencies of 400.13 MHz (1H), 100.62 MHz (13C) and 500.13 MHz (1H), 125.76 MHz (13C), and 50.68 MHz (15N), respectively. 1H and 13C spectra were also measured on Varian UNITY-400 spectrometer. The temperature of the measurement was 303 K. Samples were prepared by dissolving the alkaloid free base (1–25 mg) in 500– 700 ml of CDCl3, DMSO-d6 or C6D6. TMS was used as an internal standard for 1H and 13C spectra. NOESY [12], GHSQC [13,14] and GSQMBC [15] spectra were measured in the phase-sensitive mode and with magnetic field gradients. For detailed experimental setup see e.g. Refs. [4,6].

J. Tousˇek et al. / Journal of Molecular Structure 613 (2002) 103–113

107

Fig. 2. Portion of GSQMBC spectrum (optimized for coupling of 7.5 Hz) of the mixture of compounds 2a, 3a, 4a, 5a, and 6a with one-bond H6–C6 and three-bond H6–C60 interactions.

2.3. NMR spectra of oxosanguinarine (7) 1

H NMR (CDCl3), 3.90 (20-H), 6.09 (13-H), 6.26 (14-H), 7.15 (1-H), 7.23 (9-H), 7.52 (12-H), 7.57 (4H), 7.75 (10-H), 7.96 (11-H). 13C NMR (CDCl3), 40.83 (C-20), 101.56 (C-13), 102.55 (C-4), 102.90 (C14), 104.74 (C-1), 113.18 (C-9), 115.47 (C-10), 118.73 (C-11), 123.53 (C-12), 131.91 (C-12a), 135.63 (C-4b), 147.26 (C-2), 147.54 (C-3). Carbon atoms C-4a, C-6, C-6a, C-7, C-8, C-10a, C-10b not detected or determined due to very low concentration of oxosanguinarine in the mixture of 2a, 3a, and 4a. 2.4. Crystal structure determination of bis(dihydrosanguinarinyl) ether (3a)

T ¼ 150(2) K, crystal size ¼ 0.25 £ 0.18 £ 0.10 mm3, colorless, 19708 reflections collected, 5508 unique (Rint ¼ 0.0402), R1 ¼ 0.0679, wR2 ¼ 0.0868 (all data). 2.5. Quantum chemical calculations Quantum chemical calculations were carried out with the Gaussian 94 software [17] installed on a Power Challenge SGI computer with an R10000 processors.

3. Results and discussion 3.1. NMR studies

Diffraction data were collected on a KUMA KM-4 four-circle single crystal diffractometer using the v – 2u scan mode. The structure was solved by the direct method using a SHELXS86 program [16]. Atomic coordinates and geometric parameters of compound 3a have been deposited at the Cambridge Crystallographic Data Centre (CCDC), Cambridge, UK (Deposition No. CCDC 168391). Crystal data. C40H28N2O9, M ¼ 680.64, monocli˚ , b ¼ 10.2420(10) A ˚, nic, P21/c, a ¼ 14.5630(10) A ˚ c ¼ 21.784(2) A , a ¼ 908, b ¼ 105.290(10)8, ˚ 3, Z ¼ 4, calculated g ¼ 908, V ¼ 3134.2(5) A 23 density ¼ 1.442 g cm , m(Mo Ka) ¼ 0.103 mm21,

1

H and 13C NMR chemical shifts of the bimolecular compounds 3–6 as well as the 6-hydroxy-5,6-dihydro derivatives 2 of sanguinarine and chelerythrine are summarized in Tables 1 and 2, respectively. Only the previously unreported chemical shifts are presented in this contribution. The other data are in excellent agreement with previously published values. The changes between the chemical shielding of the hydrogen atoms of the pseudobase (2) and the bimolecular aminoacetal (3) have been discussed in our previous paper [6]. 1H NMR chemical shifts were assigned by two-dimensional NOE experiments [12] following

108

J. Tousˇek et al. / Journal of Molecular Structure 613 (2002) 103–113

Fig. 3. 1H NMR spectrum of bis(dihydrochelerythrinyl)amine (5b) in DMSO-d6. The signals of NH and H-6 hydrogen atoms with characteristic splitting due to the NH –CH scalar coupling are shown.

closely the assignment strategy reported previously [4, 6]. The sample of sanguinarine free base dissolved in CDCl3 contains a mixture of compounds 3a, 2a, and 4a. A NOESY spectrum with a mixing time of 700 ms indicates relatively fast chemical exchange between the signals of 6-hydroxy-5,6-dihydrosanguinarine (2a) [6] and the minor diastereomer 4a. This clearly detects the exchange between pairs: 2.74(2a) – 2.59(4a) NCH3 (H-20), 6.07(2a)– 5.88(4a) H-14b,

6.11(2a) –5.68(4a) H-14a, and 7.65(2a)– 7.19(4a) H4. The NMR spectrum of sanguinarine free base showed one more set of minor signals, which were identified as oxosanguinarine (7). Compound 7 is the product of pseudobase 2a oxidation or disproportionation. This result agrees with our previous finding of oxosanguinarine [8]. 13C NMR chemical shifts were assigned based on the gradient-selected single-bond (HSQC) [13,14] and multiple-bond (GSQMBC) [15] heteronuclear chemical shift correlation experiments. The sample of bimolecular NH compound (mixture of 5a and 6a), when dissolved in CDCl3, undergoes hydrolysis to the pseudobase 2a with subsequent transformation to 3a and 4a. Fig. 2 shows a part of the GSQMBC spectrum of the mixture of compounds 2a, 3a, 4a, 5a, and 6a with the H6 –C6 scalar interactions. Compounds 2a, 3a, 4a, 5a, and 6a were easily detected and recognized in a single experiment [10, 11]. The hydrogen atom of the NH group of amino derivatives 5 is significantly broadened when 5 is dissolved in CDCl3 and remarkably sharpened for DMSO-d6 solutions. The scalar coupling between H-6 atoms and the hydrogen atom of the NH group was clearly detected in DMSO-d6 solution of compound 5a and 5b and the corresponding coupling constant J , 6.5 Hz was observed. Part of the 1H NMR spectrum of bis(dihydrochelerythrinyl)amine (5b) is shown in Fig. 3. 3.2. X-Ray diffraction analysis of 3a

Fig. 4. The side view of two molecules of bis(dihydrosanguinarinyl) ether (3a).

Bis(dihydrosanguinarinyl) ether (3a) consists of two benzo[c ]phenanthridine units which are connected by the oxygen atom O-19. The bond angle around the sp3 oxygen atom C6 –O19 –C60 (112.258) is somewhat enlarged. All the six aromatic rings in both parts of the bimolecular compound are reasonably planar. The conformations of the two partially saturated nitrogen heterocycles resemble distorted half-chairs with the atoms C-6 (C-60 ) and N-5 (N-50 ) significantly deviated from the best mean planes of the isoquinoline moieties. The dihedral angles along the oxygen bridge H6 – C6 – O19 – C60 and H60 – C60 – O19 –C6 are 37.38 and 37.28, respectively. Consequently, both tetracyclic benzophenanthridine systems are almost perpendicular; their best planes make an angle of 74.178. The shape of the molecule

J. Tousˇek et al. / Journal of Molecular Structure 613 (2002) 103–113

109

Table 3 Selected geometric parameters, AM1 energies and Boltzmann distribution of the conformers found by the conformational analysis Diastereomer

Conformera

E (kJ mol21)

Boltzmann distributionb (%)

Boltzmann distributionc (%)

3a (S,S ) 3a (S,S ) 3a (S,S ) 3a (S,S ) 4a (R,S ) 4a (R,S ) 4a (R,S ) 4a (R,S ) 4a (R,S )

a,a; 69,69 a,e; 52,43 e,e; 41,41 All conformers e,a; 13,176 a,a; 55,7 a,a; 21,175 e,a; 42, 2 23 All conformers

2369.0 2366.4 2362.3 – 2359.6 2358.4 2357.6 2356.2 –

70.3 25.1 4.6 – 43.4 26.3 19.2 11.1 –

67.7 24.2 4.5 96.4 1.6 0.9 0.7 0.4 3.6

a,a: the position of the O-19 atom with respect to the non-aromatic rings (a: axial; e: equatorial); the value of the H6–C6 –O19–C60 , H60 – C6 –O19–C6 dihedral angles. b Boltzmann distribution for each diastereomer separately. c Boltzmann distribution for the mixture of diastereomers. a

0

resembles the ‘L’ letter (Fig. 4). There are no Hbonds; the molecules are linked together by van der Waals contacts. The compound bears two stereogenic centers, the atoms C-6 and C-60 . According to the centrosymmetric space group, the crystal examined was a racemate. The same conclusion was made for a similar derivative of sanguilutine, bis(dihydrosanguilutinyl) ether [18]. 3.3. Quantum chemical calculations The aim of the computational part of this work is the calculation of the NMR chemical shifts of the sanguinarine and chelerythrine aminoacetals 3 and 4 and comparison of the calculated values with the experimental data for predicting the relative configurations. The geometry of the most stable conformer of sanguinarine is compared with the structure obtained by the X-ray analysis. The calculation of shielding constants of the flexible molecules [19,20] includes several steps, namely conformational analysis [21], geometry optimization, calculation of the chemical shifts of the particular conformers and calculation of the average chemical shifts according to the Boltzmann distributions. In order to find the conformers of both diastereomers of bis(dihydrosanguinarinyl) ether (3a and 4a), a systematic conformer search was performed. Sanguinarine aminoacetal contains mostly planar

aromatic rings and displays limited flexibility. The only non-planar ring in the molecule is partially saturated nitrogen containing heterocycle. Depending on the conformation of this ring, oxygen O-19 can be found in the axial (a ) or equatorial (e ) position. Moreover, rotation around the C6 – O19 and C60 – O19 bonds can occur. A number of input structures were generated that mutually differed in the geometric parameters mentioned above. The geometries of these input structures were optimized using quantum chemical semiempirical method AM1 [22]. Optimized structures represent the particular conformers. Conformers with a Boltzmann distribution of at least 1% (Table 3) were included in the chemical shifts calculations. Chemical shielding constants were computed by DFT approach, with B3LYP functional at the 631Gp p level, GIAO method [23]. DFT approach was selected because it provides better results than conventional SCF-based methods with comparable computational efforts [24]. Chemical shifts were computed using the following equation:

di ¼ sst 2 si where di is the chemical shift of the given nucleus, sst the shielding constant of the standard [25], and si the shielding constant of the given nucleus. The chemical shifts were computed for the conformers summarized in Table 3. Averaged chemical shifts were then calculated according to the

J. Tousˇek et al. / Journal of Molecular Structure 613 (2002) 103–113

110

Table 4 Calculated NMR chemical shifts (ppm) for the 1H nucleus (d(calc1): geometry optimized using AM1 method; d(calc2): geometry from the Xray analysis; d(calc3): geometry optimized using RHF/6-31Gp p method) Atom

3a [d(calc1)]

3a [d(calc2)]

4a [d(calc1)]

3b [d(calc1)]

3b [d(calc3)]

1 4 6 9 10 11 12 13-Ha 13-Hb 14-Ha/14 14-Hb/21 20 Rmsd

6.59 7.50 6.42 6.37 6.99 7.45 6.97 6.36 6.36 5.99 6.00 3.39 0.41

6.91 8.03 6.50 6.61 7.41 8.03 7.71 5.87 6.24 5.61 6.13 1.37 0.53

6.44 7.06 6.04 6.48 6.99 7.30 6.76 6.30 6.29 6.52 6.39 3.27 0.49

6.63 7.69 6.67 6.59 7.23 7.49 6.97 6.35 6.36 3.32 3.76 3.53 0.40

6.98 8.10 6.32 6.86 7.49 7.76 7.49 6.25 6.26 2.81 3.55 2.72 0.20

Boltzmann distribution for each particular diastereomer. An empirical constant was added to the calculated chemical shifts in order to compensate for possible systematic errors. This constant was found by minimizing the root-mean-square-deviation (rmsd) between the theoretical and experimental

values. The calculated values of the chemical shifts are referred to as d (calc) and are summarized in Table 4 (1H nucleus) and Table 5 (13C nucleus). Tables 4 and 5 also contain chemical shifts calculated for X-ray geometry. The rmsd expresses the overall agreement between

Table 5 Calculated NMR chemical shifts (ppm) for the 13C nucleus (d(calc1): geometry optimized using AM1 method; d(calc2): geometry from the Xray analysis; d(calc3): geometry optimized using RHF/6-31Gp p method) Atom

3a [d(calc1)]

3a [d(calc2)]

4a [d(calc1)]

3b [d(calc1)]

3b [d(calc3)]

1 2 3 4 4a 4b 6 6a 7 8 9 10 10a 10b 11 12 12a 13 14 21 20 Rmsd

104.15 146.11 147.29 99.47 123.22 138.07 84.90 112.97 146.14 147.44 105.22 113.36 124.82 123.53 118.63 122.16 126.92 106.94 107.39 – 45.07 3.02

101.61 148.24 148.89 102.00 131.66 145.21 85.32 116.67 146.69 148.38 104.09 112.58 131.18 128.74 117.23 118.67 132.90 97.54 97.03 – 28.85 4.73

103.69 146.07 146.58 100.50 122.23 139.10 89.47 113.27 145.69 147.17 105.53 113.42 124.67 121.77 118.23 121.18 127.04 106.83 107.62 – 45.64 3.26

103.85 145.76 146.69 99.43 122.33 138.31 84.29 126.17 146.80 149.78 114.73 115.52 125.25 122.62 117.96 121.19 126.68 106.58 61.22 59.54 45.95 3.09

103.14 143.75 144.78 99.50 127.09 138.70 81.01 128.71 147.15 149.96 115.04 116.68 127.99 126.07 119.22 122.76 129.48 100.93 59.08 57.23 42.39 2.07

J. Tousˇek et al. / Journal of Molecular Structure 613 (2002) 103–113

111

Fig. 5. The most stable conformer of bis(dihydrosanguinarinyl) ether (3a) found by the systematic search method (a) and from X-ray data (b). The hydrogen atoms are omitted for clarity except for H-6 and H-60 .

the calculated and experimental values. The experimental values obtained for CDCl3 solutions (Tables 1 and 2) were used for calculating the rmsd referred in Tables 4 and 5. In the case of the 1H nuclei, rmsd ¼ 0.41 for 3a and rmsd ¼ 0.49 for 4a were obtained. These values represent approximately 5% of the 1H chemical shifts scale. For the 13C nuclei: rmsd ¼ 3.02 (3a) and rmsd ¼ 3.26 (4a); roughly 2% of the 13C chemical shifts scale. The results obtained with the AM1 optimized structures are significantly better than the results obtained with the X-ray-derived geometry. The rmsd between the theoretical data and experimental values obtained for the major diastereomer in benzene and DMSO solutions were also calculated. The values are 3.14 ppm (13C) and 0.65 ppm (1H) for benzene and 3.05 ppm (13C) and 0.44 ppm (1H) for DMSO. The larger differences between the calculated and experimental values for benzene solution, especially for 1H nuclei, is supposed to be on account of the ring current effects of this aromatic solvent. Noticeable differences in the experimental chemical shifts between the hydrogen atoms H-4, H-6, H14a, H-20 and carbon atoms C-6, C-6a, C-4, and C-4a for two diastereomers were found. These differences are reproduced with the quantum chemical calculations (Tables 4 and 5). The chemical shifts of the chelerythrine amino-

acetal 3b were also calculated. In this case, the calculations of the only most stable conformer, derived from the analogous structure of sanguinarine aminoacetal 3a, were performed. The 7,8-OCH2O – substituent was replaced by the two – OCH3 substituents and the structure was optimized by the AM1 and RHF/6-31Gp p methods. Comparison (Tables 4 and 5) of the two optimization procedures indicates a significant improvement of the results when a RHF/631Gp p method is applied. However, the CPU time required for this optimization (approximately 50 days) is unacceptably high. Based on the energy of particular conformers, the relative stability of the two diastereomers of sanguinarine aminoacetal 3a, 4a can be determined and compared with the experimental values. Based on this comparison the NMR spectra can be assigned to the particular diastereomers. According to the calculations, compound 3a (6S,60 S and 6R,60 R ) is more stable than 4a (6R,60 S ; 6S,60 R ) with Boltzmann populations of 96.4 and 3.6%, respectively. Experimentally determined population of the major diastereomer is 93 – 97%. The geometry of the most stable conformer determined by quantum chemical calculation is compared with the X-ray-derived structure (Fig. 5) and agreement between these two structures is remarkable. The oxygen atom O-19 is in the axial position with respect to both non-aromatic rings. The

112

J. Tousˇek et al. / Journal of Molecular Structure 613 (2002) 103–113

values of the H6 – C6 – O19 – C60 dihedral angles differ; its value is 698 for conformer found by the conformational analysis and 378 for the X-ray-derived structure. This torsion can also be extracted from the proton – carbon long-range coupling constant 3J(H6– C6 – O19 –C60 ). The value of the corresponding threebond H – C –O – C proton– carbon coupling constant for compound 3a was determined to be 3 JH,C ¼ 3.5 Hz (see Ref. 11). The value of the dihedral angle obtained by using a parameterized Karplus equation [26]: 3

Acknowledgments This work was supported by grants from the Ministry of Education of the Czech Republic (LN00A016), the Concerted Research Action of the Flemish Government (multidisciplinary spectroscopic research of organic materials and biomolecules), and the visiting scientist grant from the University of Antwerp (RUCA) to R.M. We also express our gratitude to the staff of supercomputing centers in Brno and Prague for CPU time.

JH;C ¼ 5:7 cos2 F 2 0:6 cos F þ 0:5 References

published by Tvarosˇka et al. [27] is 378. The value is accidentally identical (considering accuracy of the NMR measurement) to that obtained by X-ray analysis. However, the agreement of both the methods is excellent.

4. Conclusions The methodology of the chemical shifts calculations for the molecule with the limited flexibility described in this article (systematic conformer search, AM1 optimization of the geometry of particular conformers, B3LYP/6-31Gp p calculations of the chemical shifts) provides results that fit well with the experimental data. The error between the calculated and experimental chemical shifts was approximately 5% for the 1H nuclei and 2% for the 13 C nuclei, although no solvent model was used for the calculations. The differences between the experimental chemical shifts of two diastereomers of sanguinarine free base are reproduced as well. Results of the calculations support the suggested relative configurations at the stereogenic centers for the major diastereomer in solution and provide a complete view of its conformational behavior. The geometry of the most stable conformer agrees well with the X-ray-derived structure. The relative configuration determined in the crystal is identical with that of the major diastereomer determined in the solution and the agreement among the conformation parameters determined by different approaches is excellent.

[1] B.D. Krane, M.O. Fagbule, M. Shamma, B. Go¨zler, J. Nat. Prod. 47 (1984) 1. [2] J. Dosta´l, M. Pota´cˇek, Collect. Czech. Chem. Commun. 55 (1990) 2840. [3] J. Dosta´l, J. Chem. Educ. 77 (2000) 993. [4] R. Marek, J. Tousˇek, J. Dosta´l, J. Slavı´k, R. Dommisse, V. Sklena´rˇ, Magn. Reson. Chem. 37 (1999) 781. [5] J. Dosta´l, R. Marek, J. Slavı´k, E. Ta´borska´, M. Pota´cˇek, V. Sklena´rˇ, Magn. Reson. Chem. 36 (1998) 869. [6] P. Secˇka´rˇova´, R. Marek, J. Dosta´l, R. Dommisse, E.L. Esmans, Magn. Reson. Chem. 40 (2002) 147. [7] J. Dosta´l, E. Ta´borska´, J. Slavı´k, M. Pota´cˇek, E. de Hoffmann, J. Nat. Prod. 58 (1995) 723. [8] J. Dosta´l, H. Bochorˇa´kova´, E. Ta´borska´, J. Slavı´k, M. Pota´cˇek, M. Budeˇsˇ´ınsky´, E. de Hoffmann, J. Nat. Prod. 59 (1996) 599. [9] J. Dosta´l, J. Slavı´k, M. Pota´cˇek, R. Marek, O. Humpa, V. Sklena´rˇ, J. Tousˇek, E. de Hoffmann, R. Rozenberg, Collect. Czech. Chem. Commun. 63 (1998) 1045. [10] R. Marek, V. Sklena´rˇ, J. Dosta´l, J. Slavı´k, Tetrahedron Lett. 37 (1996) 1655. [11] R. Marek, J. Tousˇek, L. Kra´lı´k, J. Dosta´l, V. Sklena´rˇ, Chem. Lett. (1997) 369. [12] J. Jeener, B.H. Meier, P. Bachmann, R.R. Ernst, J. Chem. Phys. 71 (1979) 4546. [13] G. Bodenhausen, D.J. Ruben, Chem. Phys. Lett. 69 (1980) 185. [14] W. Willker, D. Leibfritz, R. Kerssebaum, W. Bermel, Magn. Reson. Chem. 31 (1993) 287. [15] R. Marek, L. Kra´lı´k, V. Sklena´rˇ, Tetrahedron Lett. 38 (1997) 665. [16] G.M. Sheldrick, Acta Crystallogr. A46 (1990) 467. [17] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. AlLaham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox,

J. Tousˇek et al. / Journal of Molecular Structure 613 (2002) 103–113

[18]

[19] [20] [21]

J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. HeadGordon, C. Gonzalez, J.A. Pople, Gaussian 94, Revision D.4, Gaussian Inc., Pittsburg, PA, 1995. J. Dosta´l, J. Slavı´k, M. Pota´cˇek, R. Marek, V. Sklena´rˇ, E. de Hoffmann, R. Rozenberg, B. Tinant, J.P. Declercq, Phytochemistry 47 (1998) 879. M. Stahl, U. Schopfer, G. Frenking, R.W. Hoffmann, Mol. Phys. 92 (1997) 569. M. Stahl, U. Schopfer, G. Frenking, R.W. Hoffmann, J. Org. Chem. 62 (1997) 3702. M. Saunders, K.N. Houk, Y.-D. Wu, W.C. Still, M. Lipton, G. Chang, W.C. Guida, J. Am. Chem. Soc. 112 (1990) 1419.

113

[22] M.J.S. Dewar, E.G. Zoebish, E.F. Healy, J.J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902. [23] R. Ditchfield, Mol. Phys. 27 (1974) 789. [24] J.R. Cheeseman, G.W. Trucks, T.A. Keith, M.J. Frisch, J. Chem. Phys. 104 (1996) 5497. [25] A.K. Jameson, C.J. Jameson, Chem. Phys. Lett. 134 (1987) 461. [26] M. Karplus, J. Chem. Phys. 30 (1959) 11. [27] I. Tvarosˇka, M. Hricovini, E. Petra´kova´, Carbohydr. Res. 189 (1989) 359. [28] R. Marek, O. Humpa, J. Dosta´l, J. Slavı´k, V. Sklena´rˇ, Magn. Reson. Chem. 37 (1999) 195.