tert-Butyl 6-bromo-1,4-dimethyl-9H-carbazole-9-carboxyl­ate

Jean-François Lohier; Anna Caruso; Jana Sopková-de Oliveira Santos; Jean-Charles Lancelot; Sylvain Rault

doi:10.1107/S1600536810026528

. 2010 Jul 10;66(Pt 8):o1971–o1972. doi: 10.1107/S1600536810026528

tert-Butyl 6-bromo-1,4-dimethyl-9H-carbazole-9-carboxylate

Jean-François Lohier ^a,^*, Anna Caruso ^b, Jana Sopková-de Oliveira Santos ^b, Jean-Charles Lancelot ^b, Sylvain Rault ^b

PMCID: PMC3007274 PMID: 21588291

Abstract

The title compound, C₁₉H₂₀BrNO₂, consists of a carbazole skeleton with methyl groups at positions 1 and 4, a protecting group located at the N atom and a Br atom at position 6. The pyrrole ring is oriented at dihedral angles of 1.27 (7) and 4.86 (7)° with respect to the adjacent benzene rings. The dihedral angle between the benzene rings is 5.11 (7). The crystal structure is determined mainly by intramolecular C—H⋯O and intermolecular π–π interactions. π-stacking between adjacent molecules forms columns with a parallel arrangement of the carbazole ring systems. The presence of the tert-butoxycarbonyl group on the carbazole N atom and the intramolecular hydrogen bond induce a particular conformation of the exocyclic N—C bond within the molecule.

Related literature

For the pharmaceutical properties of carbazole derivatives, see: Itoigawa et al. (2000 ▶); Laronze et al. (2005 ▶); Thevissen et al. (2009 ▶). For their electroactivity and luminescent properties, see: Grazulevicius et al. (2003 ▶) and for their their applications in the light-emitting field, see: Zhang et al. (2006 ▶). For the synthesis of carbazoles and ellipticine derivatives, see: Ergün et al. (1998 ▶); Knölker et al. (2002 ▶); Liu et al. (2007 ▶). For related structures, see: Caruso et al. (2007 ▶); Sopková-de Oliveira Santos et al. (2008 ▶). For bond-length data, see: Allen et al. (1987 ▶). The title compound constitutes a cheap and reactive intermediate for the preparation of new analogs of the anticancer agent 9-methoxyellipticine, see: Le Pecq et al. (1974 ▶). A lengthening of N—C bond lengths due to the presence of a protecting group has been observed in similar compounds, see: Back et al. (2001 ▶); Chakkaravarthi et al. (2009 ▶); Terpin et al. (1998 ▶) For N-sulfonyl carbazole derivatives with similar conformations, see: Chakkaravarthi et al. (2008 ▶). For non N-atom-substituted analogs, see: Viossat et al. (1988 ▶).

Experimental

Crystal data

C₁₉H₂₀BrNO₂
M _r = 374.27
Triclinic,
a = 7.521 (4) Å
b = 9.715 (5) Å
c = 11.930 (6) Å
α = 91.10 (4)°
β = 96.40 (4)°
γ = 90.96 (4)°
V = 865.9 (8) Å³
Z = 2
Mo Kα radiation
μ = 2.38 mm⁻¹
T = 291 K
0.46 × 0.37 × 0.34 mm

Data collection

Bruker–Nonius APEXII KappaCCD diffractometer
Absorption correction: numerical (SAINT; Bruker, 2007 ▶) T _min = 0.378, T _max = 0.429
37091 measured reflections
5718 independent reflections
4268 reflections with I > 2σ(I)
R _int = 0.025

Refinement

R[F ² > 2σ(F ²)] = 0.034
wR(F ²) = 0.092
S = 1.02
5718 reflections
213 parameters
H-atom parameters constrained
Δρ_max = 0.63 e Å⁻³
Δρ_min = −0.52 e Å⁻³

Data collection: APEX2 (Bruker, 2007 ▶); cell refinement: SAINT (Bruker, 2007 ▶); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: Mercury (Macrae et al., 2008 ▶); software used to prepare material for publication: SHELXL97.

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536810026528/om2338sup1.cif

e-66-o1971-sup1.cif^{(19KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536810026528/om2338Isup2.hkl

e-66-o1971-Isup2.hkl^{(279.9KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C8—H8⋯O2	0.93	2.33	2.863 (3)	116

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Table 2. π–π interactions (Å, °).

Cg1, Cg2 and Cg3 are the centroids of the N9–C9A–C4A–C5A–C8A, C9A–C1–C2–C3–C4–C4A and C5A–C5–C6–C7–C8–C8A rings, respectively, ccd is the distance between ring centroids, sa is the mean slippage angle (angle subtended by the intercentroid vector to the plane normal) and ipd is the mean interplanar distance (distance from one plane to the neighbouring centroid). For details, see Janiak (2000 ▶).

Group 1/group 2	ccd	sa	ipd
Cg2/Cg3ⁱ	3.755 (2)	24	3.532 (1)
Cg3/Cg2ⁱ	3.755 (2)	20	3.433 (1)
Cg1/Cg1ⁱ	3.927 (2)	22	3.638 (1)
Cg2/Cg3ⁱⁱ	3.811 (2)	18	3.654 (1)
Cg3/Cg2ⁱⁱ	3.811 (2)	16	3.626 (1)
Cg1/Cg1ⁱⁱ	4.199 (2)	32	3.578 (1)

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Symmetry codes: (i) −x + 1, −y, −z + 1; (ii) −x, −y, −z + 1.

supplementary crystallographic information

Comment

Over the past few years, large interest has been observed in chemistry of carbazole derivatives since they can be widely used as organic materials due to their electroactivity and luminescent properties (Grazulevicius et al., 2003) or their applications in the light-emitting field (Zhang et al., 2006). This class of compounds also displays various pharmacological activities such as, among others, anticancer (Itoigawa et al., 2000; Laronze et al., 2005), antibacterial and antifungal activities (Thevissen et al., 2009).

Many elegant methods for the synthesis of ellipticine and related carbazole alkaloids have been reported (Ergün et al., 1998; Knölker et al., 2002; Liu et al., 2007). In our laboratory, the quest to discover new potential bioactive compounds possessing a carbazole core has attracted all our attention and recently, we have synthesized and characterized a series of carbazole derivatives (Caruso et al., 2007; Sopková-de Oliveira Santos et al., 2008). In this paper, we present the results of structural investigation of a new intermediate (Scheme 1): 6-bromo-9-tert-butoxycarbonyl-1,4-dimethyl-9H- carbazole (Fig. 1) which constitutes a very interesting, cheap and reactive intermediate for the preparation of new analogs of the anticancer agent 9-methoxyellipticine (Le Pecq et al., 1974).

The carbazole ring system (C1—C9A/N9) is nearly planar and the maximum deviation from the least-squares planes does not exceed 0.0662 (14) Å. The pyrrole ring is oriented with respect to the adjacent benzene rings at dihedral angles of 1.27 (7) and 4.86 (7)°.

The N—C bond lengths, namely N9—C8A and N9—C9A [1.408 (2)Å and 1.417 (2) Å] deviate slightly from the normal mean value reported in the literature (Allen et al., 1987). This indicates that the presence of protecting group at atom N9, probably through its electron-withdrawing character, causes the lengthening of N—C bond lengths which has been already observed with similar compounds (Back et al., 2001; Terpin et al., 1998; Chakkaravarthi et al., 2009). Methyl substituent C9 is coplanar with the aromatic rings, methyl substituent C10 closed to N-protecting group displays slight deviation from the carbazole plane with torsion angle values C4A—C9A—C1—C10 of -172.72 (15). This is probably due to minimize the steric hinderance induced by the carbamate group. No particular increase in the widening angle, namely C9A—C1—C10, has been observed compared to non substituted nitrogen atom analogs (Viossat et al., 1988). Weak intramolecular C—H···O interaction is present in the molecule. In fact, atom C8 acts, throught H8, as hydrogen-bond donor to O2, distance between H8 and O2 being 2.33 Å (Table 1). Thus, in order to optimize previous H-bond and minimize steric hinderance of N-protecting group, carbamate is forced to adopt a particular conformation, specially a very twisted torsion angle which have been also seen with N-sulfonyl carbazole derivatives displaying intramolecular H-bonds (Chakkaravarthi et al., 2008). Thus, the torsion angle C1—C9A—N9—C11 is as high as 30.8 (2)°.

In the crystal packing, π–π interactions may be effective in the stabilization of the structure. Stacking interactions occur between aromatic rings leading to columns along a axis. The arrangement of carbazole ring systems within column is parallel but non equally spaced and molecules rotate of 180° alternatively. More precisely, π–π contacts are present with Cg2···Cg3 distance = 3.755 (2)Å [symmetry code: 1 - x,-y, 1 - z] and 3.811 (2)Å [symmetry code:-x,-y, 1 - z]. Cg1···Cg1 distance is 3.927 (2)Å [symmetry code: 1 - x,-y, 1 - z] and 4.199 (2)Å [symmetry code:-x,-y, 1 - z] with a center-to-edge arrangement (Table 2). Cg1, Cg2 and Cg3 are the centroids of N9—C9A—C4A—C5A—C8A, C5A—C5—C6—C7—C8—C8A and C9A—C1—C2—C3—C4—C4A rings, respectively. The carbazole systems are inclined at an angle of about 13.4° to [100] plan.

In conclusion, the crystal structure of an interesting carbazole intermediate has been elucidated. A strong displacement of the N-protecting group out of the plane has been observed. Nevertheless, presence of the tert-Butyloxycarbonyl group does not prevent parallel arrangement of carbazole systems by π stacking. Thus, flat similar compounds could be used as anticancer agents through their intercalation effect like ellipticine.

Experimental

6-Bromo-9-tert-butoxycarbonyl-1,4-dimethyl-9H-carbazole was prepared by reaction of 6-bromo-1,4-dimethyl-9H-carbazole (5.0 g, 18.2 mmol) with di-tert-butyl dicarbonate (8.0 g, 36.5 mmol) in the presence of DMAP (4.46 g, 36.5 mmol) and triethylamine (5.1 ml, 36.5 mmol) in acetonitrile (70 ml). The mixture was stirred for 1 h at 0°C, then left at room temperature for 3 h. The residue obtained after removal of the solvent was diluted with EtOAc (100 ml) and shaken with water (2 x 100 ml). The residue obtained after an usual work-up was purified by silica gel column chromatography using cyclohexane/ether (7:3) as eluent to give the compound as a yellow solid (63% yield). Transparent crystals suitable for X-ray analysis were grown from an acetonitrile solution at room temperature.