Hydrogen-bonded sheets in 4-amino-8,8-dimethyl-2-(methyl­sulfan­yl)-8,9-dihydro­pyrimidino[4,5-b]quinolin-6(7H)-one

Cruz, S.; Cobo, J.; Low, J.N.; Glidewell, C.

doi:10.1107/S1600536806040633

organic compounds

CRYSTALLOGRAPHIC
COMMUNICATIONS

ISSN: 2056-9890

Volume 62| Part 11| November 2006| Pages o4933-o4935

https://doi.org/10.1107/S1600536806040633

Hydrogen-bonded sheets in 4-amino-8,8-dimethyl-2-(methylsulfanyl)-8,9-dihydropyrimidino[4,5-b]quinolin-6(7H)-one

Silvia Cruz,^a Justo Cobo,^b John N. Low ^c and Christopher Glidewell ^d ^*

^aDepartamento de Química, Universidad de Nariño, Ciudad Universitaria, Torobajo, AA 1175, Pasto, Colombia, ^bDepartamento de Química Inorgánica y Orgánica, Universidad de Jaén, 23071 Jaén, Spain, ^cDepartment of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB24 3UE, Scotland, and ^dSchool of Chemistry, University of St Andrews, Fife KY16 9ST, Scotland
^*Correspondence e-mail: cg@st-andrews.ac.uk

(Received 2 October 2006; accepted 2 October 2006; online 13 October 2006)

The title molecule, C₁₄H₁₆N₄OS, shows strong bond fixation within the fused heterocyclic rings. In the crystal structure, molecules are linked into sheets by a combination of N—H⋯N and N—H⋯O hydrogen bonds.

Comment

We report here the structure of the title compound, (I) (Fig. 1), which was prepared by microwave irradiation of a two-component mixture of a 6-aminopyrimidine and the condensation product formed from dimedone and formaldehyde. By contrast a similar reaction between 6-aminopyrimidines, 5,5-dimethylcyclohexane-1,3-dione and a large excess of formaldehyde yielded spiranopyridopyrimidines (Quiroga et al., 2006 ).

The bond distances within the fused heterocyclic system (Table 1) provide evidence for significant bond fixation of the naphthalene type. Thus, for example, the bonds N1—C2, N3—C4 and C9A—N10 are all significantly shorter than the bonds C2—N3, N10—C10A and C10A—N1, while C5—C5A is the shortest of the C—C bonds. The carbocyclic ring adopts a conformation best described as intermediate between an envelope form, with the fold across the vector C7⋯C9, and a half-chair form. The ring-puckering parameters (Cremer & Pople, 1975 ) for the atom sequence C5A—C6—C7—C8—C9—C9A are θ = 129.2 (2)° and φ = 342.4 (4)°; the idealized values for the envelope and half-chair forms, respectively, are θ = 126.3 and 129.8°, and φ = (60k) and (60k + 30)°, where k is zero or an integer. The two S—C distances are clearly different, and atom C21 lies almost in the plane of the adjacent pyrimidine ring.

The molecules are linked into sheets by two hydrogen bonds (Table 2), and the formation of the sheets is readily analysed in terms of two simple substructures, each formed by just one hydrogen bond. In the first substructure, amino atom N4 in the molecule at (x, y, z) acts as a hydrogen-bond donor, via H4A, to the pyrimidine ring atom N3 in the molecule at (1 − x, 1 − y, 2 − z), so generating by inversion an R₂²(8) (Bernstein et al., 1995 ) ring centred at ( $[{1\over 2}]$ , $[{1\over 2}]$ , 1) (Fig. 2). In the second substructure, amino atom N4 at (x, y, z) acts as a hydrogen-bond donor, via H4B, to atom O6 in the molecule at (x, $[{3\over 2}]$ − y, $[{1\over 2}]$ + z), so forming a simple C(8) chain running parallel to the [001] direction and generated by the c-glide plane at y = 0.75 (Fig. 3). The combination of these two substructures generates a sheet parallel to (100) (Fig. 4), but there are no direction-specific interactions between adjacent sheets; in particular C—H⋯π hydrogen bonds and π–π stacking interactions are absent.

Figure 1
The molecular structure of compound (I)

, showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 2
Part of the crystal structure of compound (I)

, showing the formation of the R₂²(8) substructure. Hydrogen bonds are shown as dashed lines and for the sake of clarity the H atoms bonded to C atoms have been omitted. The atoms marked with an asterisk (*) are at the symmetry position (1 − x, 1 − y, 2 − z).

Figure 3
Part of the crystal structure of compound (I)

, showing the formation of the C(8) substructure. The hydrogen bonds are shown as dashed lines and for the sake of clarity the H atoms bonded to C atoms have been omitted. The atoms marked with an asterisk (*) or a hash (#) are at the symmetry positions (x, $[{3\over 2}]$ − y, $[{1\over 2}]$ + z) and (x, $[{3\over 2}]$ − y, − $[{1\over 2}]$ + z), respectively.

Figure 4
A stereoscopic view of part of the crystal structure of compound (I)

, showing the formation of a sheet parallel to (100). The hydrogen bonds are shown as dashed lines and for the sake of clarity the H atoms bonded to C atoms have been omitted.

Experimental

A mixture of 4,6-diamino-2-methylsulfanylpyrimidine (1.0 mmol), 2,2-methylenebis(3-hydroxy-5,5-dimethylcyclohex-2-en-1-one) (1.0 mmol) and triethylamine (0.5 mmol) was placed in an open Pyrex-glass vessel and irradiated in a domestic microwave oven for 80 s at 600 W. The resulting solid product was collected by filtration, washed with cold ethanol, dried and then recrystallized from ethanol to provide crystals of compound (I) suitable for single-crystal X-ray diffraction; yield 60%, m.p. 580 K.

Crystal data

C₁₄H₁₆N₄OS
M_r = 288.37
Monoclinic, P 2₁ /c
a = 10.7138 (11) Å
b = 15.1368 (15) Å
c = 8.9112 (6) Å
β = 101.208 (6)°
V = 1417.6 (2) Å³
Z = 4
D_x = 1.351 Mg m⁻³
Mo Kα radiation
μ = 0.23 mm⁻¹
T = 120 (2) K
Block, colourless
0.40 × 0.24 × 0.18 mm

Data collection

Bruker–Nonius KappaCCD diffractometer
φ and ω scans
Absorption correction: multi-scan (SADABS; Sheldrick, 2003 ) T_min = 0.926, T_max = 0.960
20540 measured reflections
3245 independent reflections
2115 reflections with I > 2σ(I)
R_int = 0.072
θ_max = 27.5°

Refinement

Refinement on F²
R[F² > 2σ(F²)] = 0.054
wR(F²) = 0.143
S = 1.02
3245 reflections
184 parameters
H-atom parameters constrained
w = 1/[σ²(F_o²) + (0.0693P)² + 0.6375P] where P = (F_o² + 2F_c²)/3
(Δ/σ)_max < 0.001
Δρ_max = 0.35 e Å⁻³
Δρ_min = −0.33 e Å⁻³

Table 1
Selected geometric parameters (Å, °)

N1—C2	1.314 (3)
C2—N3	1.364 (3)
N3—C4	1.338 (3)
C4—C4A	1.445 (3)
C4A—C5	1.401 (3)
C5—C5A	1.384 (3)
C5A—C9A	1.413 (3)
C9A—N10	1.328 (3)
N10—C10A	1.356 (3)
C10A—N1	1.371 (3)
C4A—C10A	1.408 (3)
C2—S2	1.753 (2)
S2—C21	1.794 (3)
C4—N4	1.332 (3)

C2—S2—C21	101.78 (12)

N1—C2—S2—C21	2.1 (2)
N3—C2—S2—C21	−177.21 (17)

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N4—H4A⋯N3ⁱ	0.90	2.17	3.067 (3)	173
N4—H4B⋯O6ⁱⁱ	0.90	2.15	2.946 (3)	147
Symmetry codes: (i) -x+1, -y+1, -z+2; (ii) $[x, -y+{\script{3\over 2}}, z+{\script{1\over 2}}]$ .

All H atoms were located in difference maps and then treated as riding atoms with distances C—H = 0.95 Å (aromatic), 0.98 Å (CH₃) or 0.99 Å (CH₂) and N—H = 0.90 Å, and with U_iso(H) = kU_eq(C,N), where k = 1.5 for methyl H atoms and 1.2 for all other H atoms.

Data collection: COLLECT (Hooft, 1999 ); cell refinement: DENZO (Otwinowski & Minor, 1997 ) and COLLECT; data reduction: DENZO and COLLECT; program(s) used to solve structure: OSCAIL (McArdle, 2003 ) and SHELXS97 (Sheldrick, 1997 ); program(s) used to refine structure: OSCAIL and SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003 ); software used to prepare material for publication: SHELXL97 and PRPKAPPA (Ferguson, 1999 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: https://doi.org/10.1107/S1600536806040633/lh2204sup1.cif

Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S1600536806040633/lh2204Isup2.hkl

Computing details top

Data collection: COLLECT (Hooft, 1999); cell refinement: DENZO (Otwinowski & Minor, 1997) and COLLECT; data reduction: DENZO and COLLECT; program(s) used to solve structure: OSCAIL (McArdle, 2003) and SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: OSCAIL and SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97 and PRPKAPPA (Ferguson, 1999).

4-Amino-8,8-dimethyl-2- (methanesulfanyl)-8,9-dihydropyrimidino[4,5-b]quinolin-6(7H)-one top

Crystal data top

C₁₄H₁₆N₄OS	F(000) = 608
M_r = 288.37	D_x = 1.351 Mg m⁻³
Monoclinic, P2₁/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 3245 reflections
a = 10.7138 (11) Å	θ = 2.4–27.5°
b = 15.1368 (15) Å	µ = 0.23 mm⁻¹
c = 8.9112 (6) Å	T = 120 K
β = 101.208 (6)°	Block, colourless
V = 1417.6 (2) Å³	0.40 × 0.24 × 0.18 mm
Z = 4

Data collection top

Bruker–Nonius KappaCCD diffractometer	3245 independent reflections
Radiation source: Bruker-Nonius FR591 rotating anode	2115 reflections with I > 2σ(I)
Graphite monochromator	R_int = 0.072
Detector resolution: 9.091 pixels mm^-1	θ_max = 27.5°, θ_min = 2.4°
φ and ω scans	h = −13→13
Absorption correction: multi-scan (SADABS; Sheldrick, 2003)	k = −19→19
T_min = 0.926, T_max = 0.960	l = −11→11
20540 measured reflections

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)] = 0.054	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.143	H-atom parameters constrained
S = 1.02	w = 1/[σ²(F_o²) + (0.0693P)² + 0.6375P] where P = (F_o² + 2F_c²)/3
3245 reflections	(Δ/σ)_max < 0.001
184 parameters	Δρ_max = 0.35 e Å⁻³
0 restraints	Δρ_min = −0.33 e Å⁻³

Special details top

Experimental. MS (70 eV) m/z (%) 290 (100, M⁺), 289 (62), 275 (34), 259 (5), 245 (9), 220 (17)

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

	x	y	z	U_iso*/U_eq
N1	0.74462 (19)	0.39484 (12)	0.7310 (2)	0.0288 (5)
C2	0.6764 (2)	0.39091 (15)	0.8384 (3)	0.0290 (5)
S2	0.69070 (6)	0.29872 (4)	0.95954 (7)	0.0350 (2)
C21	0.8107 (3)	0.23543 (17)	0.8931 (3)	0.0355 (6)
N3	0.59209 (19)	0.45107 (12)	0.8743 (2)	0.0283 (5)
C4	0.5710 (2)	0.52293 (15)	0.7856 (2)	0.0250 (5)
N4	0.48914 (19)	0.58258 (13)	0.8191 (2)	0.0301 (5)
C4A	0.6340 (2)	0.53342 (14)	0.6572 (2)	0.0250 (5)
C5	0.6103 (2)	0.60132 (15)	0.5486 (2)	0.0260 (5)
C5A	0.6734 (2)	0.60094 (14)	0.4270 (2)	0.0247 (5)
C6	0.6463 (2)	0.66958 (15)	0.3058 (2)	0.0274 (5)
O6	0.58072 (17)	0.73385 (11)	0.32040 (18)	0.0338 (4)
C7	0.7028 (2)	0.65422 (16)	0.1666 (3)	0.0311 (6)
C8	0.8376 (2)	0.61671 (15)	0.2029 (2)	0.0274 (5)
C81	0.8854 (3)	0.59856 (16)	0.0546 (3)	0.0334 (6)
C82	0.9273 (3)	0.68183 (16)	0.3016 (3)	0.0358 (6)
C9	0.8344 (2)	0.52914 (15)	0.2897 (3)	0.0312 (6)
C9A	0.7636 (2)	0.53374 (14)	0.4198 (2)	0.0252 (5)
N10	0.78922 (18)	0.46986 (12)	0.5234 (2)	0.0279 (5)
C10A	0.7220 (2)	0.46742 (15)	0.6375 (2)	0.0263 (5)
H21A	0.7841	0.2243	0.7832	0.053*
H21B	0.8226	0.1790	0.9479	0.053*
H21C	0.8910	0.2684	0.9119	0.053*
H4A	0.4592	0.5712	0.9047	0.036*
H4B	0.4941	0.6379	0.7837	0.036*
H5	0.5517	0.6471	0.5583	0.031*
H7A	0.7041	0.7109	0.1115	0.037*
H7B	0.6476	0.6128	0.0975	0.037*
H81A	0.8307	0.5543	−0.0060	0.050*
H81B	0.9729	0.5764	0.0793	0.050*
H81C	0.8831	0.6534	−0.0044	0.050*
H82A	0.9243	0.7388	0.2489	0.054*
H82B	1.0144	0.6587	0.3190	0.054*
H82C	0.9008	0.6897	0.4000	0.054*
H9A	0.7943	0.4834	0.2165	0.037*
H9B	0.9230	0.5103	0.3305	0.037*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
N1	0.0383 (12)	0.0191 (10)	0.0309 (11)	0.0019 (9)	0.0112 (9)	0.0049 (8)
C2	0.0341 (14)	0.0219 (12)	0.0303 (12)	−0.0026 (10)	0.0048 (10)	0.0034 (9)
S2	0.0459 (4)	0.0266 (3)	0.0350 (4)	0.0031 (3)	0.0143 (3)	0.0098 (3)
C21	0.0468 (16)	0.0229 (12)	0.0380 (14)	0.0050 (12)	0.0111 (12)	0.0083 (10)
N3	0.0351 (11)	0.0213 (10)	0.0298 (11)	0.0004 (9)	0.0094 (8)	0.0029 (8)
C4	0.0286 (12)	0.0194 (12)	0.0266 (11)	−0.0033 (10)	0.0047 (9)	−0.0015 (9)
N4	0.0429 (12)	0.0191 (10)	0.0309 (11)	−0.0006 (9)	0.0137 (9)	0.0007 (8)
C4A	0.0317 (13)	0.0169 (11)	0.0267 (12)	−0.0025 (10)	0.0064 (9)	−0.0022 (9)
C5	0.0308 (13)	0.0197 (12)	0.0279 (12)	−0.0015 (10)	0.0068 (10)	−0.0022 (9)
C5A	0.0306 (13)	0.0156 (11)	0.0280 (12)	−0.0026 (9)	0.0058 (10)	−0.0012 (9)
C6	0.0338 (13)	0.0219 (12)	0.0258 (12)	−0.0026 (11)	0.0042 (10)	0.0007 (9)
O6	0.0452 (11)	0.0210 (9)	0.0362 (9)	0.0063 (8)	0.0104 (8)	0.0039 (7)
C7	0.0376 (14)	0.0277 (13)	0.0284 (12)	0.0010 (11)	0.0078 (10)	0.0048 (10)
C8	0.0372 (14)	0.0200 (12)	0.0265 (12)	−0.0008 (10)	0.0102 (10)	−0.0008 (9)
C81	0.0450 (16)	0.0258 (14)	0.0321 (13)	−0.0013 (11)	0.0138 (11)	0.0000 (10)
C82	0.0400 (15)	0.0307 (14)	0.0391 (14)	−0.0059 (12)	0.0137 (11)	−0.0054 (11)
C9	0.0448 (15)	0.0218 (12)	0.0300 (13)	0.0031 (11)	0.0152 (11)	0.0018 (10)
C9A	0.0329 (13)	0.0171 (11)	0.0264 (12)	−0.0009 (10)	0.0081 (10)	0.0001 (9)
N10	0.0381 (12)	0.0182 (10)	0.0294 (10)	0.0025 (9)	0.0117 (9)	0.0028 (8)
C10A	0.0336 (13)	0.0198 (12)	0.0257 (11)	−0.0018 (10)	0.0062 (9)	0.0001 (9)

Geometric parameters (Å, º) top

N1—C2	1.314 (3)	C5—H5	0.95
C2—N3	1.364 (3)	C5A—C6	1.486 (3)
N3—C4	1.338 (3)	C6—O6	1.221 (3)
C4—C4A	1.445 (3)	C6—C7	1.501 (3)
C4A—C5	1.401 (3)	C7—C8	1.526 (3)
C5—C5A	1.384 (3)	C7—H7A	0.99
C5A—C9A	1.413 (3)	C7—H7B	0.99
C9A—N10	1.328 (3)	C8—C82	1.530 (3)
N10—C10A	1.356 (3)	C8—C81	1.532 (3)
C10A—N1	1.371 (3)	C8—C9	1.538 (3)
C4A—C10A	1.408 (3)	C81—H81A	0.98
C2—S2	1.753 (2)	C81—H81B	0.98
S2—C21	1.794 (3)	C81—H81C	0.98
C21—H21A	0.98	C82—H82A	0.98
C21—H21B	0.98	C82—H82B	0.98
C21—H21C	0.98	C82—H82C	0.98
C4—N4	1.332 (3)	C9—C9A	1.506 (3)
N4—H4A	0.90	C9—H9A	0.99
N4—H4B	0.90	C9—H9B	0.99

C2—N1—C10A	114.9 (2)	C8—C7—H7B	108.8
N1—C2—N3	128.9 (2)	H7A—C7—H7B	107.7
N1—C2—S2	119.38 (18)	C7—C8—C82	110.2 (2)
N3—C2—S2	111.68 (17)	C7—C8—C81	110.20 (19)
C2—S2—C21	101.78 (12)	C82—C8—C81	109.5 (2)
S2—C21—H21A	109.5	C7—C8—C9	108.33 (19)
S2—C21—H21B	109.5	C82—C8—C9	109.8 (2)
H21A—C21—H21B	109.5	C81—C8—C9	108.84 (18)
S2—C21—H21C	109.5	C8—C81—H81A	109.5
H21A—C21—H21C	109.5	C8—C81—H81B	109.5
H21B—C21—H21C	109.5	H81A—C81—H81B	109.5
C4—N3—C2	116.59 (19)	C8—C81—H81C	109.5
N4—C4—N3	117.5 (2)	H81A—C81—H81C	109.5
N4—C4—C4A	122.1 (2)	H81B—C81—H81C	109.5
N3—C4—C4A	120.4 (2)	C8—C82—H82A	109.5
C4—N4—H4A	114.8	C8—C82—H82B	109.5
C4—N4—H4B	117.8	H82A—C82—H82B	109.5
H4A—N4—H4B	122.0	C8—C82—H82C	109.5
C5—C4A—C10A	118.0 (2)	H82A—C82—H82C	109.5
C5—C4A—C4	125.3 (2)	H82B—C82—H82C	109.5
C10A—C4A—C4	116.6 (2)	C9A—C9—C8	114.27 (19)
C5A—C5—C4A	119.2 (2)	C9A—C9—H9A	108.7
C5A—C5—H5	120.4	C8—C9—H9A	108.7
C4A—C5—H5	120.4	C9A—C9—H9B	108.7
C5—C5A—C9A	118.7 (2)	C8—C9—H9B	108.7
C5—C5A—C6	120.7 (2)	H9A—C9—H9B	107.6
C9A—C5A—C6	120.6 (2)	N10—C9A—C5A	122.9 (2)
O6—C6—C5A	121.0 (2)	N10—C9A—C9	115.76 (19)
O6—C6—C7	123.0 (2)	C5A—C9A—C9	121.32 (19)
C5A—C6—C7	116.0 (2)	C9A—N10—C10A	118.2 (2)
C6—C7—C8	113.59 (19)	N10—C10A—N1	114.8 (2)
C6—C7—H7A	108.8	N10—C10A—C4A	122.7 (2)
C8—C7—H7A	108.8	N1—C10A—C4A	122.4 (2)
C6—C7—H7B	108.8

C10A—N1—C2—N3	−3.9 (4)	C6—C7—C8—C82	−62.0 (3)
C10A—N1—C2—S2	176.91 (17)	C6—C7—C8—C81	177.1 (2)
N1—C2—S2—C21	2.1 (2)	C6—C7—C8—C9	58.1 (3)
N3—C2—S2—C21	−177.21 (17)	C7—C8—C9—C9A	−47.8 (3)
N1—C2—N3—C4	2.8 (4)	C82—C8—C9—C9A	72.6 (3)
S2—C2—N3—C4	−177.99 (16)	C81—C8—C9—C9A	−167.7 (2)
C2—N3—C4—N4	−179.9 (2)	C5—C5A—C9A—N10	1.1 (3)
C2—N3—C4—C4A	1.2 (3)	C6—C5A—C9A—N10	−178.5 (2)
N4—C4—C4A—C5	−5.2 (4)	C5—C5A—C9A—C9	179.4 (2)
N3—C4—C4A—C5	173.6 (2)	C6—C5A—C9A—C9	−0.3 (3)
N4—C4—C4A—C10A	177.7 (2)	C8—C9—C9A—N10	−161.0 (2)
N3—C4—C4A—C10A	−3.5 (3)	C8—C9—C9A—C5A	20.6 (3)
C10A—C4A—C5—C5A	0.2 (3)	C5A—C9A—N10—C10A	2.6 (3)
C4—C4A—C5—C5A	−176.9 (2)	C9—C9A—N10—C10A	−175.7 (2)
C4A—C5—C5A—C9A	−2.5 (3)	C9A—N10—C10A—N1	173.8 (2)
C4A—C5—C5A—C6	177.2 (2)	C9A—N10—C10A—C4A	−5.2 (3)
C5—C5A—C6—O6	9.5 (3)	C2—N1—C10A—N10	−177.8 (2)
C9A—C5A—C6—O6	−170.8 (2)	C2—N1—C10A—C4A	1.1 (3)
C5—C5A—C6—C7	−169.9 (2)	C5—C4A—C10A—N10	3.8 (3)
C9A—C5A—C6—C7	9.7 (3)	C4—C4A—C10A—N10	−178.9 (2)
O6—C6—C7—C8	140.8 (2)	C5—C4A—C10A—N1	−175.0 (2)
C5A—C6—C7—C8	−39.8 (3)	C4—C4A—C10A—N1	2.3 (3)

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
N4—H4A···N3ⁱ	0.90	2.17	3.067 (3)	173
N4—H4B···O6ⁱⁱ	0.90	2.15	2.946 (3)	147

Symmetry codes: (i) −x+1, −y+1, −z+2; (ii) x, −y+3/2, z+1/2.

Acknowledgements

X-ray data were collected at the EPSRC X-ray Crystallographic Service, University of Southampton, England. JC and JT thank the Consejería de Innovación, Ciencia y Empresa (Junta de Andalucía, Spain) and the Universidad de Jaén for financial support. SC thanks COLCIENCIAS and UDENAR (Universidad de Nariño, Colombia) for financial support.

References

Bernstein, J., Davis, R., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555–1573. CrossRef CAS Web of Science Google Scholar
Cremer, D. & Pople, J. A. (1975). J. Am. Chem. Soc. 97, 1354–1358. CrossRef CAS Web of Science Google Scholar
Ferguson, G. (1999). PRPKAPPA. University of Guelph, Canada. Google Scholar
Hooft, R. W. W. (1999). COLLECT. Nonius BV, Delft, The Netherlands. Google Scholar
McArdle, P. (2003). OSCAIL for Windows. Version 10. Crystallography Centre, Chemistry Department, NUI Galway, Ireland. Google Scholar
Otwinowski, Z. & Minor, W. (1997). Methods in Enzymology, Vol. 276, Macromolecular Crystallography, Part A, edited by C. W. Carter Jr & R. M. Sweet, pp. 307–326. New York: Academic Press. Google Scholar
Quiroga, J., Cruz, S., Insuasty, B., Abonía, R., Nogueras, M. & Cobo, J. (2006). Tetrahedron Lett. 47, 27–30. Web of Science CrossRef CAS Google Scholar
Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of Göttingen, Germany. Google Scholar
Sheldrick, G. M. (2003). SADABS. Version 2.10. University of Göttingen, Germany. Google Scholar
Spek, A. L. (2003). J. Appl. Cryst. 36, 7–13. Web of Science CrossRef CAS IUCr Journals Google Scholar

© International Union of Crystallography. Prior permission is not required to reproduce short quotations, tables and figures from this article, provided the original authors and source are cited. For more information, click here.