Tetra­hydro­berberine, a pharmacologically active naturally occurring alkaloid

The crystal structure of racemic tetra­hydro­berberine, a pharmacologically active alkaloid, shows inter­molecular 1,3-benzodioxole –CH₂⋯OCH₃ and –OCH₃⋯OCH₃ inter­actions.

Abstract

Tetra­hydro­berberine (systematic name: 9,10-dimeth­oxy-5,8,13,13a-tetra­hydro-6H-benzo[g][1,3]benzodioxolo[5,6-a]quinolizine), C₂₀H₂₁NO₄, a widely distributed naturally occurring alkaloid, has been crystallized as a racemic mixture about an inversion center. A bent conformation of the mol­ecule is observed, with an angle of 24.72 (5)° between the arene rings at the two ends of the reduced quinolizinium core. The inter­molecular hydrogen bonds that play an apparent role in crystal packing are 1,3-benzodioxole –CH₂⋯OCH₃ and –OCH₃⋯OCH₃ inter­actions between neighboring mol­ecules.

Introduction  

Tetra­hydro­berberine, also known as canadine and xanthopuccine, is a naturally occurring alkaloid that occurs in widely distributed shrubs, both deciduous and evergreen, of the genus Berberis. Berberine (see Scheme 1), the fully oxidized form of the compound, has long been known to exert a broad variety of potentially useful pharmacological and therapeutic properties, ranging from anti­malarial (Ho, 2013 ▸; Bansal & Silakari, 2014 ▸) to anti­cancer agents (Kaboli et al., 2014 ▸; Anwarul Bashar et al., 2014 ▸; Ortiz et al., 2014 ▸). The understanding of these properties and their application to human medicine continue to be active areas of research. Only more recently has recognition emerged that the fully reduced form of berberine, i.e. tetra­hydro­berberine, has significant pharmacological activity that differs from the parent berberine. For example, in contrast to the cytotoxic effects of berberine, tetra­hydro­berberine has been reported to show little cytotoxicity toward several lines of cells, but instead to be effective as an anti­oxidant (Correché et al., 2008 ▸). Consequently, it holds promise as an anti-inflammatory agent. Other studies have reported biological effects of tetra­hydro­berberine and its derivatives as a Ca²⁺ channel blocker (Kubota et al., 1994 ▸; Li et al., 1995 ▸, 2002 ▸; Dai et al., 1996 ▸), which enables the induction of vascular muscle relaxation and its use as anti­hypertension and anti-arrhythmia agents. Tetra­hydro­berberine has also been observed to block ATP-sensitive K⁺ ion channels that are associated with the pathogenesis of Parkinson’s disease (Wu et al., 2010 ▸), indicating an important neuroprotective role. Still other research has found an inhibitory effect upon platelet aggregation, suggesting an important role in protecting against thrombosis (Bo et al., 1994 ▸).

In earlier work (Pingali et al., 2014 ▸), we described the structure of di­hydro­berberine in the crystalline state and noted that it shows a capacity to engage in CH₂⋯X (X = O, N) hydrogen bonding that could have pertinence to the nature of inter­actions with in vivo systems. As noted in that report, di­hydro­berberine and derivatives of it show manifold pharmacological effects that may have use in the treatment of human disease. In a continuing effort to more fully characterize small organic mol­ecules that exert important therapeutic properties, we report the crystal structure of (±)-tetra­hydro­berberine, which has heretofore not been described. Earlier works by others have reported the structures of protonated (Sakai et al., 1987 ▸) and N-methyl­ated (Kamigauchi et al., 2003 ▸) forms of tetra­hydro­berberine and of tetra­hydro­berberine in a 2:1 complex with (+)-2,3-di-p-tol­uyl­tartaric acid (Gao et al., 2008 ▸).

Experimental  

Synthesis and crystallization  

To a stirred refluxing solution of berberine chloride (3.71 g, 10 mmol) and K₂CO₃ (3.6 g, 26 mmol) in MeOH (125 ml), solid NaBH₄ (0.4 g, 10 mmol) was added portionwise. The reaction mixture was allowed to reflux for an additional 20 min, during which time it became a homogeneous solution. Stirring was continued for an additional 4 h at ambient temperature, and the precipitated product was then collected by filtration and recrystallized from absolute EtOH (400 ml) to afford 2.6 g (77%) of material as pale-yellow–brown needles (m.p. 447–449 K).

Spectroscopic data  

MS (m/z) 339; ¹H NMR (DMSO-d ₆): δ 6.90 (s, 1H), 6.87 (d, 1H, J = 8.4 Hz), 6.84 (s, 1H, J = 8.4 Hz), 6.65 (s, 1H), 5.93 (d, 2H, J = 2.4 Hz), 4.04 (d, 1H, J = 16.0 Hz), 3.76 (s, 3H), 3.71 (s, 3H), 3.35 (d, 1H, J = 16.0 Hz), 3.27–3.36 (m, 2H), 3.08 (q, 1H, J = 3.6 Hz), 2.85–2.93 (m, 1H), 2.61 (m, 1H), 2.55 (q, 1H, J = 4.0 Hz), 2.43 (q, 1H, J = 3.2 Hz).

Refinement  

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. H atoms were identified in the later difference maps, and their positions were refined, as were their isotropic displacement parameters. The latter were approximately 1.2–1.5 times those of the C atoms to which they were attached.

Table 1. Experimental details.

Crystal data
Chemical formula	C20H21NO4
M r	339.38
Crystal system, space group	Monoclinic, P21/n
Temperature (K)	150
a, b, c (Å)	10.516 (3), 14.796 (5), 10.620 (3)
β (°)	101.986 (4)
V (Å3)	1616.3 (9)
Z	4
Radiation type	Mo Kα
μ (mm−1)	0.10
Crystal size (mm)	0.39 × 0.17 × 0.03
Data collection
Diffractometer	Bruker SMART APEX CCD diffractometer
Absorption correction	Multi-scan (SADABS; Sheldrick, 2009 ▸)
T min, T max	0.650, 0.997
No. of measured, independent and observed [I > 2σ(I)] reflections	23937, 3306, 2468
R int	0.069
(sin θ/λ)max (Å−1)	0.625
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.050, 0.132, 1.09
No. of reflections	3306
No. of parameters	310
H-atom treatment	All H-atom parameters refined
Δρmax, Δρmin (e Å−3)	0.25, −0.28

Computer programs: APEX2 (Bruker, 2010 ▸), SAINT (Bruker, 2009 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELXL2014 (Sheldrick, 2015 ▸) and SHELXTL (Bruker, 2008 ▸).

Results and discussion  

The structure of tetra­hydro­berberine, (I), is presented in part (a) of Scheme 1 and rendered three dimensionally in part (b) such that its nonplanarity is emphasized. The compound may be described as a saturated quinolizine system fused at the 1,2-positions to a 1,3-benzodi­oxole system and to a di­meth­oxy­benzene fragment at the 7,8-positions. The saturation of bonds within the central quinolizine system induces pseudo-chair-type conformations to its two rings and necessitates chirality at the tertiary N atom and at atom C13A. The H atom bound to C13A has an anti disposition relative to the electron lone pair of N7 [see part (b) of Scheme 1, and Fig. 1 ▸]. In the displacement ellipsoid plot shown in Fig. 1 ▸, atoms N7 and C13A display R and S configurations, respectively. Because of the centrosymmetry of the space group, the S (N7) and R (C13A) enanti­omer must occur in the cell as well. Pairs of enanti­omers are arranged about the inversion centers at the mid-points of the a edges of the cell (Fig. 2 ▸). The dihedral angle between the C₆ arene groups at the two ends of the mol­ecule is 24.72 (5)°, which is the simplest way to qu­antify its departure from planarity. The corresponding value in di­hydro­berberine was observed to be modestly larger at 27.94 (5)° (Pingali et al., 2014 ▸).

Figure 1.

Displacement ellipsoid plot drawn at the 50% probability level, showing the complete atom labeling for tetra­hydro­berberine.

Figure 2.

Pairs of tetra­hydro­berberine enanti­omers arranged about the inversion centers at the mid-points of the a edges of the unit cell.

A dihedral angle of 31.5 (1)° between the mean planes of the C₆ arene rings at the ends of the tetra­hydro­berberine mol­ecule in the 2:1 tetra­hydro­berberine (+)-2,3-di-p-toluyltartaric acid complex has been reported (Gao et al., 2008 ▸). For a rigid mol­ecule such as tetra­hydro­berberine, this appreciably larger value is greater than would be plausibly attributed to packing effects or to statistical variance. Our independent assessment of the structure reported by Gao and co-workers finds values of 25 (1) and 22 (1)° for the the two independent mol­ecules in the unit cell. Rather than being a single optical isomer as reported, the two tetra­hydro­berberine mol­ecules appear to be enanti­omers, and the occurrence of noncentric P1 as space group appears to be due to the presence of the optically pure (+)-2,3-di-p-toluyltartaric acid cocrystallite.

As was found in the crystal structure of di­hydro­berberine, weak inter­molecular hydrogen bonding appears to play a role in governing the packing of mol­ecules in the crystalline state of tetra­hydro­berberine. A notable difference between the packing arrangements of the two mol­ecules is that tetra­hydro­berberine mol­ecules are disposed relative to one another in such a way that their tertiary N atoms are unable to engage potential hydrogen-bond donors. Whereas the acetal-type –CH₂– group of 1,3-benzodioxole of di­hydro­berberine participates in both CH₂⋯N and CH₂⋯OMe hydrogen bonds with neighboring mol­ecules, only the latter type of hydrogen bond is found in the arrangement of mol­ecules for tetra­hydro­berberine. As seen in Fig. 3 ▸, mol­ecules of tetra­hydro­berberine related by translation along the b axis of the unit cell present the –CH₂– group of the 1,3-benzodioxole fragment near the meth­oxy O atom of the next mol­ecule such that it can serve as a hydrogen-bond acceptor (Table 2 ▸). Weak hydrogen bonding may also be operative between a meth­oxy H atom of one mol­ecule and the meth­oxy O atom of an adjoining mol­ecule further along in the direction of the c axis of the unit cell (Fig. 4 ▸ and Table 2 ▸). These inter­molecular hydrogen bonds are undoubtedly weaker than those involving the acetal-type –CH₂– hydrogens, as the latter are more activated by the proximity of two electronegative O atoms rather than one.

Figure 3.

Tetra­hydro­berberine mol­ecules related by translation along the b axis of the unit cell. Dashed lines show the inter­molecular C—H⋯OCH₃ hydrogen bonds.

Table 2. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C2—H2B⋯O10i	0.94 (2)	2.58 (2)	3.212 (3)	124.5 (17)
C16—H16A⋯O9ii	0.95 (2)	2.53 (2)	3.455 (3)	162.9 (18)

Symmetry codes: (i) ; (ii) .

Figure 4.

Cell packing diagram for tetra­hydro­berberine, with inter­molecular OCH₃⋯OCH₃ hydrogen bonds shown as dashed lines.

Much remains to be learned about the physical and chemical underpinnings of the pharmacological activity of tetra­hydro­berberine. The capacity for tetra­hydro­berberine to act as both hydrogen-bond donor and acceptor suggest that this property be considered as a key basis for some of its selective activity in vivo.

Supplementary Material

CCDC reference: 1051389