4-Benzyl-6-bromo-2-phenyl-4H-imidazo[4,5-b]pyridine

Ouzidan, Y.; Obbade, S.; Capet, F.; Essassi, E.M.; Ng, S.W.

doi:10.1107/S160053681001038X

organic compounds

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ISSN: 2056-9890

Volume 66| Part 4| April 2010| Page o946

doi:10.1107/S160053681001038X

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4-Benzyl-6-bromo-2-phenyl-4H-imidazo[4,5-b]pyridine

Y. Ouzidan,^a S. Obbade,^b F. Capet,^c El Mokhtar Essassi ^d and Seik Weng Ng ^e ^*

^aLaboratoire de Chimie Organique Appliquée, Faculté des Sciences et Techniques, Université Sidi Mohamed Ben Abdallah, Fés, Morocco, ^bLaboratoire d'Electrochimie et de Physicochimie des Matériaux et des Interfaces, Domaine Universitaire, 38402 St Martin d'Hères Cedex, Grenoble, France, ^cUnité de Catalyse et de Chimie du Solide, Ecole Nationale Supérieure de Chimie de Lille, Lille, France, ^dLaboratoire de Chimie Organique Hétérocyclique, Pôle de Compétences Pharmacochimie, Université Mohammed V-Agdal, BP 1014 Avenue Ibn Batout, Rabat, Morocco, and ^eDepartment of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
^*Correspondence e-mail: seikweng@um.edu.my

(Received 11 March 2010; accepted 19 March 2010; online 27 March 2010)

The imidazopyridine fused ring in the title compound, C₁₉H₁₄BrN₃, is almost coplanar with the phenyl ring at the 2-position of the five-membered ring [dihedral angle = 2.4 (1). The crystal structure features short Br⋯Br contacts [3.562 (1) Å].

Related literature

For the synthesis of imidazo[4,5-b]pyridines, see: Aridoss et al. (2006 ); Benham et al. (1995 ); Cundy et al. (1997 ); Kale et al. (2009 ); Walsh et al. (1994 ); Zaki & Proença (2007 ).

Experimental

Crystal data

C₁₉H₁₄BrN₃
M_r = 364.24
Monoclinic, P 2₁ /c
a = 8.6613 (6) Å
b = 19.7631 (13) Å
c = 9.3683 (6) Å
β = 99.647 (3)°
V = 1580.93 (18) Å³
Z = 4
Mo Kα radiation
μ = 2.60 mm⁻¹
T = 293 K
0.28 × 0.24 × 0.20 mm

Data collection

Bruker X8 APEXII diffractometer
Absorption correction: multi-scan (SADABS; Sheldrick, 1996 ) T_min = 0.529, T_max = 0.624
57936 measured reflections
4613 independent reflections
3492 reflections with I > 2σ(I)
R_int = 0.035

Refinement

R[F² > 2σ(F²)] = 0.032
wR(F²) = 0.098
S = 1.00
4613 reflections
208 parameters
H-atom parameters constrained
Δρ_max = 0.63 e Å⁻³
Δρ_min = −0.51 e Å⁻³

Data collection: APEX2 (Bruker, 2008 ); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: X-SEED (Barbour, 2001 ); software used to prepare material for publication: publCIF (Westrip, 2010 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S160053681001038X/pk2232sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S160053681001038X/pk2232Isup2.hkl

checkCIF report

Comment top

Imidazo[4,5-b]pyridines are a class of sedative drugs exemplified by Zolpidem, Alpidem, Saripidem and Necopidem. There is intense interest in designing new synthetic routes; for example, an eco-friendly synthesis by oxidation in aqueous medium has been claimed (Kale et al., 2009). Other methods require more than one step (Aridoss et al., 2006; Benham et al., 1995; Cundy et al., 1997; Walsh et al., 1994; Zaki & Proença, 2007).

We have been able to react 6-bromo-2-phenyl-1H-imidazo[4,5-b]pyridine with benzyl chloride in the presence of a catalytic quantity of tetra-n-butylammonium bromide under mild conditions to furnish the title compound (Scheme I, Fig. 1). The imidazopyridine fused-ring in C₁₉H₁₄BrN₃ is co-planar with the phenyl ring at the 2-position [dihedral angle 2.4 (1) °]. In the five-membered imidazo portion, the carbon–nitrogen bond whose carbon atom is also connected to the pyridine nitrogen atom is predominantly a double bond [1.329 (2) Å], whereas the carbon–nitrogen bond whose atom is connected to the pyridine carbon atom is predominantly a single bond [1.372 (2) Å].

Related literature top

For the synthesis of imidazo[4,5-b]pyridines, see: Aridoss et al. (2006); Benham et al. (1995); Cundy et al. (1997); Kale et al. (2009); Walsh et al. (1994); Zaki & Proença (2007).

Experimental top

To a solution of the 6-bromo-2-phenyl-1H-imidazo[4,5-b]pyridine (0.30 g, 1.09 mmol), potassium carbonate (0.20 g, 1.42 mmol) and tetra-n-butylammonium bromide (0.04 g (0,1 mmol) in DMF (15 ml) was added benzyl chloride (0.15 ml, 1.31 mmol). Stirring was continued at room temperature for 12 hours. The salt was removed by filtration and the filtrate concentrated under reduced pressure. The residue was separated by chromatography on a column of silica gel with ethyl acetate/hexane (1/1) as eluent. Brown crystals were isolated when the solvent was allowed to evaporate.

Computing details top

Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: X-SEED (Barbour, 2001); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures top

Fig. 1. Thermal ellipsoid plot (Barbour, 2001) of C₁₉H₁₄BrN₃ at the 50% probability level. Hydrogen atoms are drawn as spheres of arbitrary radius.

4-Benzyl-6-bromo-2-phenyl-4H-imidazo[4,5-b]pyridine top

Crystal data top

C₁₉H₁₄BrN₃	F(000) = 736
M_r = 364.24	D_x = 1.530 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 9876 reflections
a = 8.6613 (6) Å	θ = 2.4–27.2°
b = 19.7631 (13) Å	µ = 2.60 mm⁻¹
c = 9.3683 (6) Å	T = 293 K
β = 99.647 (3)°	Prism, brown
V = 1580.93 (18) Å³	0.28 × 0.24 × 0.20 mm
Z = 4

Data collection top

Bruker X8 APEXII diffractometer	4613 independent reflections
Radiation source: fine-focus sealed tube	3492 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.035
ϕ and ω scans	θ_max = 30.1°, θ_min = 2.4°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −12→11
T_min = 0.529, T_max = 0.624	k = −27→27
57936 measured reflections	l = −13→13

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.032	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.098	H-atom parameters constrained
S = 1.00	w = 1/[σ²(F_o²) + (0.0518P)² + 0.5269P] where P = (F_o² + 2F_c²)/3
4613 reflections	(Δ/σ)_max = 0.001
208 parameters	Δρ_max = 0.63 e Å⁻³
0 restraints	Δρ_min = −0.51 e Å⁻³

Crystal data top

C₁₉H₁₄BrN₃	V = 1580.93 (18) Å³
M_r = 364.24	Z = 4
Monoclinic, P2₁/c	Mo Kα radiation
a = 8.6613 (6) Å	µ = 2.60 mm⁻¹
b = 19.7631 (13) Å	T = 293 K
c = 9.3683 (6) Å	0.28 × 0.24 × 0.20 mm
β = 99.647 (3)°

Data collection top

Bruker X8 APEXII diffractometer	4613 independent reflections
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	3492 reflections with I > 2σ(I)
T_min = 0.529, T_max = 0.624	R_int = 0.035
57936 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.032	0 restraints
wR(F²) = 0.098	H-atom parameters constrained
S = 1.00	Δρ_max = 0.63 e Å⁻³
4613 reflections	Δρ_min = −0.51 e Å⁻³
208 parameters

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
Br1	0.10158 (3)	0.475089 (12)	0.85824 (2)	0.06483 (10)
N1	0.29485 (16)	0.43405 (7)	0.49901 (15)	0.0385 (3)
N2	0.26154 (17)	0.60722 (7)	0.40338 (16)	0.0424 (3)
N3	0.35446 (17)	0.50867 (7)	0.31224 (15)	0.0387 (3)
C1	0.1792 (2)	0.48667 (9)	0.68312 (19)	0.0454 (4)
C2	0.1807 (2)	0.55060 (9)	0.62007 (19)	0.0458 (4)
H2	0.1429	0.5886	0.6616	0.055*
C3	0.2408 (2)	0.55460 (8)	0.49376 (18)	0.0392 (3)
C4	0.29944 (19)	0.49489 (8)	0.43321 (17)	0.0367 (3)
C5	0.2352 (2)	0.43003 (9)	0.62362 (18)	0.0437 (4)
H5	0.2322	0.3884	0.6694	0.052*
C6	0.32810 (19)	0.57716 (8)	0.30027 (17)	0.0379 (3)
C7	0.37246 (19)	0.61435 (8)	0.17772 (18)	0.0390 (3)
C8	0.3423 (2)	0.68335 (9)	0.1595 (2)	0.0459 (4)
H8	0.2938	0.7067	0.2261	0.055*
C9	0.3845 (2)	0.71728 (10)	0.0425 (2)	0.0547 (5)
H9	0.3644	0.7634	0.0310	0.066*
C10	0.4557 (3)	0.68322 (11)	−0.0569 (2)	0.0574 (5)
H10	0.4829	0.7062	−0.1357	0.069*
C11	0.4869 (3)	0.61517 (11)	−0.0399 (2)	0.0624 (5)
H11	0.5353	0.5922	−0.1069	0.075*
C12	0.4459 (3)	0.58097 (10)	0.0773 (2)	0.0538 (5)
H12	0.4679	0.5350	0.0888	0.065*
C13	0.3544 (2)	0.37271 (8)	0.43491 (19)	0.0427 (3)
H13A	0.4041	0.3433	0.5121	0.051*
H13B	0.4331	0.3859	0.3778	0.051*
C14	0.22599 (19)	0.33413 (8)	0.34017 (17)	0.0380 (3)
C15	0.1392 (2)	0.36407 (9)	0.21824 (19)	0.0481 (4)
H15	0.1605	0.4084	0.1945	0.058*
C16	0.0214 (3)	0.32854 (11)	0.1321 (2)	0.0574 (5)
H16	−0.0379	0.3494	0.0522	0.069*
C17	−0.0082 (3)	0.26217 (11)	0.1643 (2)	0.0585 (5)
H17	−0.0859	0.2380	0.1050	0.070*
C18	0.0769 (3)	0.23206 (10)	0.2833 (3)	0.0598 (5)
H18	0.0570	0.1873	0.3047	0.072*
C19	0.1930 (2)	0.26789 (9)	0.3728 (2)	0.0517 (4)
H19	0.2487	0.2473	0.4548	0.062*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
Br1	0.08695 (19)	0.06436 (15)	0.05128 (13)	0.00593 (10)	0.03507 (11)	0.00630 (9)
N1	0.0419 (7)	0.0355 (6)	0.0382 (7)	0.0001 (5)	0.0074 (5)	0.0003 (5)
N2	0.0515 (8)	0.0338 (6)	0.0437 (7)	−0.0007 (6)	0.0131 (6)	−0.0028 (5)
N3	0.0443 (7)	0.0337 (6)	0.0394 (7)	−0.0009 (5)	0.0104 (5)	−0.0012 (5)
C1	0.0501 (10)	0.0498 (10)	0.0386 (8)	−0.0024 (7)	0.0140 (7)	0.0004 (7)
C2	0.0535 (10)	0.0417 (9)	0.0444 (9)	−0.0007 (7)	0.0147 (7)	−0.0056 (7)
C3	0.0431 (8)	0.0349 (7)	0.0402 (8)	−0.0015 (6)	0.0088 (6)	−0.0046 (6)
C4	0.0390 (8)	0.0345 (7)	0.0366 (7)	−0.0020 (6)	0.0061 (6)	−0.0035 (6)
C5	0.0485 (9)	0.0430 (8)	0.0403 (8)	−0.0019 (7)	0.0094 (7)	0.0036 (7)
C6	0.0407 (8)	0.0345 (7)	0.0385 (7)	−0.0028 (6)	0.0068 (6)	−0.0023 (6)
C7	0.0420 (8)	0.0353 (7)	0.0395 (7)	−0.0036 (6)	0.0064 (6)	−0.0004 (6)
C8	0.0471 (9)	0.0360 (8)	0.0553 (10)	−0.0020 (7)	0.0105 (8)	−0.0007 (7)
C9	0.0567 (11)	0.0391 (9)	0.0678 (12)	−0.0031 (8)	0.0092 (9)	0.0118 (8)
C10	0.0647 (12)	0.0560 (11)	0.0530 (10)	−0.0070 (9)	0.0141 (9)	0.0153 (9)
C11	0.0851 (15)	0.0567 (12)	0.0516 (10)	0.0037 (11)	0.0298 (10)	0.0062 (9)
C12	0.0782 (13)	0.0393 (9)	0.0486 (10)	0.0062 (9)	0.0240 (9)	0.0040 (7)
C13	0.0430 (9)	0.0371 (8)	0.0483 (9)	0.0064 (7)	0.0091 (7)	0.0015 (7)
C14	0.0419 (8)	0.0331 (7)	0.0416 (8)	0.0029 (6)	0.0147 (6)	−0.0002 (6)
C15	0.0608 (11)	0.0390 (8)	0.0446 (9)	−0.0013 (8)	0.0092 (8)	0.0021 (7)
C16	0.0660 (12)	0.0574 (11)	0.0462 (10)	−0.0037 (9)	0.0015 (9)	−0.0030 (8)
C17	0.0603 (12)	0.0584 (12)	0.0587 (11)	−0.0140 (9)	0.0152 (9)	−0.0156 (9)
C18	0.0683 (13)	0.0396 (9)	0.0754 (14)	−0.0103 (9)	0.0233 (11)	−0.0022 (9)
C19	0.0583 (11)	0.0388 (9)	0.0593 (11)	0.0013 (8)	0.0139 (9)	0.0088 (8)

Geometric parameters (Å, º) top

Br1—C1	1.8882 (18)	C9—H9	0.9300
N1—C4	1.355 (2)	C10—C11	1.376 (3)
N1—C5	1.356 (2)	C10—H10	0.9300
N1—C13	1.483 (2)	C11—C12	1.385 (3)
N2—C6	1.344 (2)	C11—H11	0.9300
N2—C3	1.372 (2)	C12—H12	0.9300
N3—C4	1.329 (2)	C13—C14	1.508 (2)
N3—C6	1.374 (2)	C13—H13A	0.9700
C1—C5	1.375 (3)	C13—H13B	0.9700
C1—C2	1.396 (3)	C14—C19	1.385 (2)
C2—C3	1.373 (2)	C14—C15	1.390 (2)
C2—H2	0.9300	C15—C16	1.382 (3)
C3—C4	1.438 (2)	C15—H15	0.9300
C5—H5	0.9300	C16—C17	1.379 (3)
C6—C7	1.468 (2)	C16—H16	0.9300
C7—C12	1.388 (3)	C17—C18	1.366 (3)
C7—C8	1.394 (2)	C17—H17	0.9300
C8—C9	1.385 (3)	C18—C19	1.390 (3)
C8—H8	0.9300	C18—H18	0.9300
C9—C10	1.376 (3)	C19—H19	0.9300

C4—N1—C5	119.22 (14)	C11—C10—H10	120.0
C4—N1—C13	120.17 (14)	C9—C10—H10	120.0
C5—N1—C13	120.61 (14)	C10—C11—C12	119.8 (2)
C6—N2—C3	102.99 (13)	C10—C11—H11	120.1
C4—N3—C6	101.13 (13)	C12—C11—H11	120.1
C5—C1—C2	122.44 (16)	C11—C12—C7	120.86 (18)
C5—C1—Br1	117.06 (13)	C11—C12—H12	119.6
C2—C1—Br1	120.49 (14)	C7—C12—H12	119.6
C3—C2—C1	116.66 (16)	N1—C13—C14	112.30 (13)
C3—C2—H2	121.7	N1—C13—H13A	109.1
C1—C2—H2	121.7	C14—C13—H13A	109.1
N2—C3—C2	133.11 (16)	N1—C13—H13B	109.1
N2—C3—C4	106.70 (14)	C14—C13—H13B	109.1
C2—C3—C4	120.18 (16)	H13A—C13—H13B	107.9
N3—C4—N1	127.72 (15)	C19—C14—C15	118.70 (17)
N3—C4—C3	111.64 (14)	C19—C14—C13	120.47 (16)
N1—C4—C3	120.64 (15)	C15—C14—C13	120.82 (15)
N1—C5—C1	120.85 (16)	C16—C15—C14	120.56 (17)
N1—C5—H5	119.6	C16—C15—H15	119.7
C1—C5—H5	119.6	C14—C15—H15	119.7
N2—C6—N3	117.54 (14)	C17—C16—C15	120.1 (2)
N2—C6—C7	122.76 (14)	C17—C16—H16	119.9
N3—C6—C7	119.70 (14)	C15—C16—H16	119.9
C12—C7—C8	118.69 (17)	C18—C17—C16	119.88 (19)
C12—C7—C6	120.15 (15)	C18—C17—H17	120.1
C8—C7—C6	121.17 (16)	C16—C17—H17	120.1
C9—C8—C7	120.11 (18)	C17—C18—C19	120.47 (18)
C9—C8—H8	119.9	C17—C18—H18	119.8
C7—C8—H8	119.9	C19—C18—H18	119.8
C10—C9—C8	120.42 (18)	C14—C19—C18	120.25 (18)
C10—C9—H9	119.8	C14—C19—H19	119.9
C8—C9—H9	119.8	C18—C19—H19	119.9
C11—C10—C9	120.08 (18)

C5—C1—C2—C3	0.3 (3)	N2—C6—C7—C12	177.72 (18)
Br1—C1—C2—C3	179.25 (13)	N3—C6—C7—C12	−2.4 (2)
C6—N2—C3—C2	179.20 (19)	N2—C6—C7—C8	−2.2 (3)
C6—N2—C3—C4	−0.16 (17)	N3—C6—C7—C8	177.69 (16)
C1—C2—C3—N2	−179.77 (18)	C12—C7—C8—C9	0.4 (3)
C1—C2—C3—C4	−0.5 (3)	C6—C7—C8—C9	−179.61 (16)
C6—N3—C4—N1	−179.97 (16)	C7—C8—C9—C10	0.2 (3)
C6—N3—C4—C3	−0.15 (18)	C8—C9—C10—C11	−0.6 (3)
C5—N1—C4—N3	179.35 (16)	C9—C10—C11—C12	0.2 (4)
C13—N1—C4—N3	−0.7 (3)	C10—C11—C12—C7	0.5 (4)
C5—N1—C4—C3	−0.5 (2)	C8—C7—C12—C11	−0.8 (3)
C13—N1—C4—C3	179.47 (15)	C6—C7—C12—C11	179.25 (19)
N2—C3—C4—N3	0.21 (19)	C4—N1—C13—C14	−94.67 (18)
C2—C3—C4—N3	−179.25 (16)	C5—N1—C13—C14	85.26 (19)
N2—C3—C4—N1	−179.95 (15)	N1—C13—C14—C19	−119.47 (17)
C2—C3—C4—N1	0.6 (2)	N1—C13—C14—C15	60.9 (2)
C4—N1—C5—C1	0.3 (2)	C19—C14—C15—C16	0.5 (3)
C13—N1—C5—C1	−179.68 (16)	C13—C14—C15—C16	−179.83 (17)
C2—C1—C5—N1	−0.2 (3)	C14—C15—C16—C17	−1.8 (3)
Br1—C1—C5—N1	−179.17 (13)	C15—C16—C17—C18	1.4 (3)
C3—N2—C6—N3	0.1 (2)	C16—C17—C18—C19	0.2 (3)
C3—N2—C6—C7	180.00 (15)	C15—C14—C19—C18	1.1 (3)
C4—N3—C6—N2	0.05 (19)	C13—C14—C19—C18	−178.56 (17)
C4—N3—C6—C7	−179.88 (14)	C17—C18—C19—C14	−1.5 (3)

Experimental details

Crystal data
Chemical formula	C₁₉H₁₄BrN₃
M_r	364.24
Crystal system, space group	Monoclinic, P2₁/c
Temperature (K)	293
a, b, c (Å)	8.6613 (6), 19.7631 (13), 9.3683 (6)
β (°)	99.647 (3)
V (Å³)	1580.93 (18)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	2.60
Crystal size (mm)	0.28 × 0.24 × 0.20

Data collection
Diffractometer	Bruker X8 APEXII diffractometer
Absorption correction	Multi-scan (SADABS; Sheldrick, 1996)
T_min, T_max	0.529, 0.624
No. of measured, independent and observed [I > 2σ(I)] reflections	57936, 4613, 3492
R_int	0.035
(sin θ/λ)_max (Å⁻¹)	0.705

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.032, 0.098, 1.00
No. of reflections	4613
No. of parameters	208
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.63, −0.51

Computer programs: APEX2 (Bruker, 2008), SAINT (Bruker, 2008), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), X-SEED (Barbour, 2001), publCIF (Westrip, 2010).

Acknowledgements

We thank Université Sidi Mohammed Ben Abdallah, Université Mohammed V-Agdal and the University of Malaya for supporting this study.

References

Aridoss, G., Balasubramanian, S., Parthiban, P. & Kabilan, S. (2006). Eur. J. Med. Chem. 41, 268–275. Web of Science CrossRef PubMed CAS Google Scholar
Barbour, L. J. (2001). J. Supramol. Chem. 1, 189–191. CrossRef CAS Google Scholar
Benham, C. D., Blackburn, T. P., Johns, A., Kotecha, N. R., Martin, R. T., Thomas, D. R., Thompson, M. & Ward, R. W. (1995). Bioorg. Med. Chem. Lett. 5, 2455–2460. CrossRef CAS Web of Science Google Scholar
Bruker (2008). APEX2 and SAINT. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Cundy, D. J., Holan, G., Otaegui, M. & Simpson, G. W. (1997). Bioorg. Med. Chem. Lett. 7, 669–674. CrossRef CAS Web of Science Google Scholar
Kale, R. P., Shaikh, M. U., Jadhav, G. R. & Gill, C. H. (2009). Tetrahedron Lett. 50, 1780–1782. Web of Science CrossRef CAS Google Scholar
Sheldrick, G. M. (1996). SADABS. University of Göttingen, Germany. Google Scholar
Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122. Web of Science CrossRef CAS IUCr Journals Google Scholar
Walsh, T. F., Fitch, K. J., MacCoss, M., Chang, R. S. L., Kivlighn, S. D., Lotti, V. J., Siegl, P. K. S., Patchett, A. & Greenlee, W. J. (1994). Bioorg. Med. Chem. Lett. 4, 219–222. CrossRef CAS Web of Science Google Scholar
Westrip, S. P. (2010). publCIF. In preparation. Google Scholar
Zaki, M. E. A. & Proença, M. F. (2007). Tetrahedron, 63, 3745–3753. Web of Science CrossRef CAS Google Scholar