supplementary materials


is5298 scheme

Acta Cryst. (2013). E69, o1504-o1505    [ doi:10.1107/S1600536813023817 ]

(2R*,4R*,7S*,10R*,12R*)-3,11,13,15-Tetraoxapentacyclo[5.5.3.01,7.02,4.010,12]pentadeca-5,8-dien-14-one

G. Mehta, S. Sen and C. S. A. Kumar

Abstract top

The title compound, C11H8O5, features a `skipped' diene, an anti-bis(epoxide) and a cyclic carbonate, all embedded in a densely functionalized [4.4.3]propellane scaffold. The crystal packing of this diepoxide is effected primarily by C-H...O hydrogen bonds, which link the molecules into tapes along the b axis. Inter-tape connectivity is brought about by centrosymmetrically disposed pairs of C...O contacts [3.183 (4) Å] between the C[delta]+=O[delta]- dipoles of neighbouring carbonate moieties.

Comment top

In a recent communication, we had introduced the C—H···O hydrogen bonded self-assembly of the crystalline tetraene 1 – the synthetic precursor of the title compound 2 (Fig. 1) – as a foil to highlight the rather singular crystal packing (lacking in any obvious hydrogen bonds) observed in two structurally related [4.4.3]propellanes (Mehta & Sen, 2011). During the course of this study, we recognized that the tricyclic carbonate 1 features a novel structural attribute – namely, two abutting 1,3-cyclohexadiene (CHD) units embedded in a rigid 11,13-dioxa[4.4.3]propellane framework (Ashkenazi et al., 1978; Paquette et al., 1990). In view of our on-going activity in delineating the patterns of self-assembly in oxygenated CHDs (Mehta & Sen, 2010), it was of considerable interest to investigate the diversely oxy-functionalized CHD moieties that might be accessed from the tetraene 1.

m-Chloroperbenzoic acid (mCPBA) mediated epoxidation of 1 was carried out and though sluggish, led to the formation of the title compound 2 in presence of excess peracid (Fig. 1). Routine characterization of 2 by NMR spectroscopy revealed it to be the product of an unsymmetrical bis-epoxidation of the C2v-symmetric 1. This was decidedly an uncanny result that was quite unpredictable and could not be readily reconciled with stereo-electronic preferences. In order to settle its stereo-structure by single-crystal XRD analysis, the diepoxide 2 was crystallized by slow evaporation of its saturated solution in 1:2 EtOAc-hexanes.

The crystal structure of 2 was solved and refined in the centrosymmetric monoclinic space group P21/c (Z = 4). Interestingly, the two epoxy functionalities in 2 were found to have a 1,4-anti relationship to one another (Fig. 2). As suggested by their conformational analysis [Ring C1–C6: q2 = 0.225 (3) Å, q3 = 0.086 (3) Å, φ2 = 322.5 (9)°, QT = 0.241 (3) Å, θ2 = 69.2 (7)°; Ring C1/C6–C10: q2 = 0.201 (4) Å, q3 = -0.080 (4) Å, φ2 = 200.6 (10)°, QT = 0.215 (3) Å, θ2 = 111.8 (11)°], the two cyclohexene rings in 2 adopt a puckered form, somewhat intermediate between a pure twist-boat (TB) and a pure half-chair (HC) conformation (Cremer & Pople, 1975). Puckering parameters of the central five-membered ring [q2 = 0.238 (3) Å, φ2 = 51.9 (7)°] are close to the values expected for an ideal C2-symmetric 'twist' (T) conformation (Cremer & Pople, 1975).

Molecular packing in 2 was brought about primarily via the agency of C—H···O hydrogen bonds which linked the molecules into zigzag tapes essentially along the b axis (Fig. 3). Within each of these tapes, a pair of hydrogen bonds (C2—H2···O4 and C10—H10···O5), defining an R22(11) motif (Etter et al., 1990; Bernstein et al., 1995) connected the diepoxide molecules related by the 21 symmetry, while another H-bond (C3—H3···O5) consolidated the architecture by linking the molecules, translated along the b axis. While no inter-tape hydrogen bonds were observed, a closer analysis of the crystal packing in 2 revealed C···O short contacts [C11···O5, d = 3.183 (4) Å, symmetry code: –x + 1, –y, –z + 1] between the molecular tapes along the longest c axis. These C···O contacts involved centrosymmetrically disposed pairs of neighbouring Cδ+Oδ- dipoles (Fig. 3) and described an interesting cyclic dipolar interaction motif, much akin to an R22(4) O—H···O hydrogen bonding loop. In fact, a Cambridge Structural Database search [CSD version 5.33 (November 2011); Allen, 2002; Bruno et al., 2002] with such a C···O interaction quadrilateral involving the carbonyl group of carbonate moieties (Fig. 4), generated only 10 hits (CSD codes: ARUJEC, CHPCBO, JOKSUX, MHIQXI, QENJUP, SECPUL, SEGCAI, VECQUP, WIZQOL and XEXVEB). Even within this coterie, the observed C···O interaction motif was, in most cases, adventitious/supportive in nature and resulted merely on account of centrosymmetrically related pairs of hydrogen bonds forcing the carbonyls to approach closer to one another. It is worth mentioning at this point that the centrosymmetric C···O interaction motif in 2, though unsupported by hydrogen bonds, might owe its existence to a congenial synergy between the shape and charge distribution in the molecule. As illustrated in Figure 5, each of the two carbonyl groups, involved in the C···O short contacts, fits in a complementary lock-and-key fashion within the single accessible 'groove' (defined by the central five-membered ring and the cyclohexene, bearing the endo epoxy moiety) that the diepoxide 2 presents. Coincidentally, this groove also bears that 'face' of the carbonyl functionality in 2 which features a well defined Cδ+Oδ- dipolar charge separation (Fig. 6).

To summarize, we have provided herein the first report of an attempted oxyfunctionalization of the tetraene 1 and the complete structural elucidation of the diepoxide 2, obtained in the endeavour. The supramolecular structure of 2, as obtained from the analysis of its single-crystal XRD data, was found to be quite noteworthy, particularly because the carbonyl group of the carbonate moiety in 2 not only functioned as the donor in two C—H···O hydrogen bonds, but participated in a scarcely encountered cyclic dipolar interaction motif as well.

Related literature top

For our reference related to the crystal structure of (1s,6s)-11,13-dioxatricyclo[4.4.3.01,6]trideca-2,4,7,9-tetraen-12-one, the synthetic precursor of the title compound, see: Mehta & Sen (2011). For salient references related to the chemistry of molecules, featuring two abutting 1,3-cyclohexadiene (CHD) units embedded in a rigid 11,13-dioxa[4.4.3]propellane framework, see: Ashkenazi et al. (1978); Paquette et al. (1990). For references representing our own previous studies on the modes of self-assembly in oxygenated CHDs, see: Mehta et al. (2010) and citations therein. For a reference related to VEGA ZZ 3.0.0 the program used to generate the MEP surface diagram of the title compound, see: Pedretti et al. (2004). For hydrogen-bond motifs, see: Bernstein et al. (1995); Etter et al. (1990). For details of ring conformations, see: Cremer & Pople (1975). For details of the Cambridge Structural Database, see: Allen (2002); Bruno et al. (2002).

Experimental top

As delineated in Figure 1, the title compound was obtained by mCPBA mediated epoxidation of the tetraene 1. Thus, 1 (0.188 g, 1.00 mmol) was dissolved in dichloromethane (8 ml) and solid m-chloroperbenzoic acid (70% purity, 0.740 g, 3.00 mmol) was added portion wise to the stirred solution, cooled on an ice-bath. Thereafter, the reaction mixture was allowed to stir while gradually warming to room temperature on its own. The progress of the reaction was monitored by thin layer chromatography. Even after allowing the reaction to proceed for three days at room temperature, the TLC profile showed no apparent change beyond the disappearance of the starting material and the formation of 2 as the predominant product. The excess peracid was therefore decomposed with saturated Na2SO3 solution and the reaction mixture extracted thrice with dichloromethane. The combined extracts were washed with saturated NaHCO3 solution and then dried over anhydrous Na2SO4. Evaporation of the solvent and purification of the residue by column chromatography over silica gel with 40% EtOAc-hexanes furnished the pure diepoxide 2 (0.170 g, 77%) as a colourless solid. M.p. 158 – 159 °C.

Single crystals of the diepoxide 2, suitable for X-ray diffraction, were obtained by slow evaporation of its saturated solution in 1:2 EtOAc-hexanes.

Refinement top

H atoms were placed in geometrically idealized positions with C—H distances 0.93 or 0.98 Å and allowed to ride on their parent atoms with Uiso(H) = 1.2Ueq(C).

Computing details top

Data collection: SMART (Bruker, 1998); cell refinement: SAINT (Bruker, 1998); data reduction: SAINT (Bruker, 1998); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009) and CAMERON (Watkin et al., 1993); software used to prepare material for publication: PLATON (Spek, 2009).

Figures top
[Figure 1] Fig. 1. Preparation of the diepoxide 2 from the tetraene 1.
[Figure 2] Fig. 2. View of the diepoxide 2, with the atom numbering scheme. Displacement ellipsoids for non-H atoms are drawn at the 30% probability level. H atoms are shown as small spheres of arbitrary radii.
[Figure 3] Fig. 3. The molecular packing of the diepoxide 2. Non-interacting hydrogen atoms have been omitted for the sake of clarity. Dotted lines indicate the C—H···O hydrogen bonds and C···O short contacts. Symmetry codes: (i) –x + 1, y + 1/2, –z + 1/2; (ii) x, y + 1, z; (iii) –x + 1, –y, –z + 1.
[Figure 4] Fig. 4. The centrosymmetric C···O interaction quadrilateral, employed in the CSD search.
[Figure 5] Fig. 5. Space filling representation of two molecules of the diepoxide 2, involved in the cyclic centrosymmetric C···O interaction motif.
[Figure 6] Fig. 6. Molecular electrostatic potential (MEP) surface diagram of the diepoxide 2, as generated by VEGA ZZ 3.0.0 (Pedretti et al., 2004). The MEP calculation was performed after the semi-empirical charges were assigned by a single point MOPAC calculation, employing the MNDO method.
(2R*,4R*,7S*,10R*,12R*)-3,11,13,15-Tetraoxapentacyclo[5.5.3.01,7.02,4.010,12]pentadeca-5,8-dien-14-one top
Crystal data top
C11H8O5F(000) = 456
Mr = 220.17Dx = 1.599 Mg m3
Monoclinic, P21/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybcCell parameters from 2221 reflections
a = 7.170 (3) Åθ = 2.9–25.2°
b = 7.809 (3) ŵ = 0.13 mm1
c = 16.357 (6) ÅT = 291 K
β = 93.301 (6)°Block, colorless
V = 914.3 (6) Å30.23 × 0.17 × 0.13 mm
Z = 4
Data collection top
Bruker SMART APEX CCD area-detector
diffractometer
1691 independent reflections
Radiation source: fine-focus sealed tube1319 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.040
φ and ω scansθmax = 25.4°, θmin = 2.5°
Absorption correction: multi-scan
(SADABS; Sheldrick, 2003)
h = 88
Tmin = 0.971, Tmax = 0.984k = 99
6504 measured reflectionsl = 1919
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.072Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.162H-atom parameters constrained
S = 1.18 w = 1/[σ2(Fo2) + (0.0604P)2 + 0.6586P]
where P = (Fo2 + 2Fc2)/3
1691 reflections(Δ/σ)max < 0.001
145 parametersΔρmax = 0.33 e Å3
0 restraintsΔρmin = 0.22 e Å3
Crystal data top
C11H8O5V = 914.3 (6) Å3
Mr = 220.17Z = 4
Monoclinic, P21/cMo Kα radiation
a = 7.170 (3) ŵ = 0.13 mm1
b = 7.809 (3) ÅT = 291 K
c = 16.357 (6) Å0.23 × 0.17 × 0.13 mm
β = 93.301 (6)°
Data collection top
Bruker SMART APEX CCD area-detector
diffractometer
1691 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 2003)
1319 reflections with I > 2σ(I)
Tmin = 0.971, Tmax = 0.984Rint = 0.040
6504 measured reflectionsθmax = 25.4°
Refinement top
R[F2 > 2σ(F2)] = 0.072H-atom parameters constrained
wR(F2) = 0.162Δρmax = 0.33 e Å3
S = 1.18Δρmin = 0.22 e Å3
1691 reflectionsAbsolute structure: ?
145 parametersAbsolute structure parameter: ?
0 restraintsRogers parameter: ?
Special details top

Experimental. IR (KBr, ν cm-1) 3057, 2203, 1805, 1524, 1204, 1263, 1032, 943, 854, 758, 725, 665, 619; 1H NMR (400 MHz, CDCl3, δ, p.p.m.) 6.46 (dd, J = 8, 3 Hz, 1H), 6.34 (dd, J = 8, 3 Hz, 1H), 5.91 (d, J = 8 Hz, 1H), 5.84 (dd, J = 8, 1 Hz, 1H), 4.00 (d, J = 3 Hz, 1H), 3.85 (d, J = 3 Hz, 1H), 3.73 (ddd appearing as a dt, J = 3, 1 Hz, 1H), 3.57 (ddd appearing as a dt, J = 3, 1 Hz, 1H); 13C NMR (100 MHz, CDCl3, δ, p.p.m.) 151.7, 130.9, 129.8, 128.1, 125.4, 78.3, 76.2, 56.8, 51.8, 49.5, 46.6.

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
O10.5301 (3)0.1663 (3)0.36989 (14)0.0500 (7)
O20.7967 (3)0.5656 (3)0.37864 (14)0.0514 (7)
O30.7693 (3)0.0174 (3)0.42208 (14)0.0432 (6)
O40.7672 (4)0.0902 (3)0.23628 (15)0.0688 (8)
O50.4863 (4)0.1009 (3)0.41171 (16)0.0638 (8)
C10.6909 (4)0.2708 (4)0.35416 (19)0.0373 (8)
C20.6407 (5)0.4528 (4)0.3733 (2)0.0447 (8)
C30.6966 (5)0.5281 (4)0.4515 (2)0.0468 (9)
C40.7949 (5)0.4205 (4)0.5129 (2)0.0445 (8)
C50.8586 (4)0.2693 (4)0.49495 (19)0.0400 (8)
C60.8441 (4)0.1914 (4)0.41165 (18)0.0340 (7)
C71.0317 (5)0.1766 (4)0.3782 (2)0.0430 (8)
C81.0625 (5)0.1947 (5)0.3007 (2)0.0538 (10)
C90.9122 (6)0.2221 (5)0.2393 (2)0.0593 (11)
C100.7260 (5)0.2568 (4)0.2652 (2)0.0511 (9)
C110.5881 (5)0.0159 (4)0.4026 (2)0.0442 (8)
H100.64660.32790.22810.061*
H20.52580.49740.34580.054*
H30.61670.61890.47160.056*
H40.81270.46070.56630.053*
H50.91710.20570.53710.048*
H71.13310.15310.41450.052*
H81.18470.18990.28460.065*
H90.94550.27140.18700.071*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O10.0374 (13)0.0479 (14)0.0635 (15)0.0075 (11)0.0084 (11)0.0113 (12)
O20.0623 (17)0.0365 (13)0.0565 (15)0.0047 (11)0.0147 (12)0.0086 (11)
O30.0452 (14)0.0288 (12)0.0544 (14)0.0040 (10)0.0083 (10)0.0050 (10)
O40.090 (2)0.0617 (17)0.0536 (16)0.0063 (16)0.0078 (14)0.0158 (13)
O50.0650 (18)0.0488 (15)0.0770 (19)0.0260 (14)0.0016 (14)0.0047 (13)
C10.0344 (18)0.0367 (18)0.0401 (18)0.0036 (14)0.0049 (13)0.0074 (14)
C20.0398 (19)0.0388 (19)0.056 (2)0.0083 (15)0.0026 (15)0.0143 (16)
C30.050 (2)0.0364 (18)0.056 (2)0.0000 (15)0.0219 (17)0.0005 (16)
C40.055 (2)0.0409 (19)0.0387 (18)0.0115 (16)0.0104 (15)0.0048 (15)
C50.0436 (19)0.0398 (19)0.0358 (17)0.0093 (15)0.0051 (14)0.0055 (14)
C60.0356 (17)0.0268 (15)0.0388 (17)0.0038 (13)0.0050 (13)0.0055 (13)
C70.0393 (19)0.0357 (18)0.053 (2)0.0008 (15)0.0044 (15)0.0055 (15)
C80.051 (2)0.050 (2)0.061 (3)0.0007 (18)0.0114 (19)0.0129 (18)
C90.083 (3)0.056 (2)0.039 (2)0.006 (2)0.014 (2)0.0057 (17)
C100.071 (3)0.042 (2)0.0390 (19)0.0004 (18)0.0117 (17)0.0021 (16)
C110.051 (2)0.0364 (19)0.045 (2)0.0097 (17)0.0016 (16)0.0011 (15)
O10.0374 (13)0.0479 (14)0.0635 (15)0.0075 (11)0.0084 (11)0.0113 (12)
O20.0623 (17)0.0365 (13)0.0565 (15)0.0047 (11)0.0147 (12)0.0086 (11)
O30.0452 (14)0.0288 (12)0.0544 (14)0.0040 (10)0.0083 (10)0.0050 (10)
O40.090 (2)0.0617 (17)0.0536 (16)0.0063 (16)0.0078 (14)0.0158 (13)
O50.0650 (18)0.0488 (15)0.0770 (19)0.0260 (14)0.0016 (14)0.0047 (13)
Geometric parameters (Å, º) top
O1—C11.448 (4)C4—C31.460 (5)
O1—C111.346 (4)C4—H40.9300
O2—C21.422 (4)C5—C41.305 (5)
O2—C31.456 (4)C5—C61.490 (4)
O3—C111.320 (4)C5—H50.9300
O3—C61.474 (3)C6—C11.535 (4)
O4—C101.421 (4)C6—C71.486 (4)
O4—C91.462 (5)C7—C81.307 (5)
O5—C111.183 (4)C7—H70.9300
C1—C101.495 (5)C8—C91.446 (6)
C2—C11.504 (4)C8—H80.9300
C2—C31.443 (5)C9—H90.9800
C2—H20.9800C10—C91.449 (5)
C3—H30.9800C10—H100.9800
O1—C1—C10108.2 (3)C4—C3—H3117.0
O1—C1—C2107.1 (3)C4—C5—C6124.7 (3)
O1—C1—C6102.3 (2)C4—C5—H5117.7
O2—C2—C1113.7 (3)C5—C4—C3121.8 (3)
O2—C2—C361.1 (2)C5—C4—H4119.1
O2—C2—H2116.4C5—C6—C1113.9 (3)
O2—C3—C4116.0 (3)C6—C5—H5117.7
O2—C3—H3117.0C6—C7—H7118.1
O3—C11—O1111.4 (3)C7—C6—C1115.9 (3)
O3—C6—C1101.0 (2)C7—C6—C5110.6 (3)
O3—C6—C5106.1 (2)C7—C8—C9122.0 (4)
O3—C6—C7108.3 (2)C7—C8—H8119.0
O4—C10—C1116.1 (3)C8—C7—C6123.8 (3)
O4—C10—C961.3 (2)C8—C7—H7118.1
O4—C10—H10115.9C8—C9—C10119.1 (3)
O4—C9—H9117.1C8—C9—H9117.1
O5—C11—O1122.9 (3)C8—C9—O4114.8 (3)
O5—C11—O3125.6 (3)C9—C10—C1120.5 (3)
C1—C10—H10115.9C9—C10—H10115.9
C1—C2—H2116.4C9—C8—H8119.0
C2—C1—C6115.2 (3)C10—C1—C2109.1 (3)
C2—C3—C4118.3 (3)C10—C1—C6114.3 (3)
C2—C3—H3117.0C10—C9—H9117.1
C2—C3—O258.8 (2)C10—C9—O458.4 (2)
C2—O2—C360.2 (2)C10—O4—C960.3 (2)
C3—C2—C1120.8 (3)C11—O1—C1109.4 (2)
C3—C2—H2116.4C11—O3—C6109.9 (2)
C3—C4—H4119.1
O1—C1—C10—C9131.7 (3)C4—C5—C6—O3130.7 (3)
O1—C1—C10—O461.1 (4)C5—C4—C3—C211.0 (5)
O2—C2—C1—C1076.4 (3)C5—C4—C3—O255.9 (4)
O2—C2—C1—C653.7 (4)C5—C6—C1—C10153.6 (3)
O2—C2—C1—O1166.7 (2)C5—C6—C1—C226.0 (4)
O2—C2—C3—C4104.8 (3)C5—C6—C1—O189.7 (3)
O3—C6—C1—C1093.2 (3)C5—C6—C7—C8145.4 (3)
O3—C6—C1—C2139.2 (3)C6—C1—C10—C918.4 (4)
O3—C6—C1—O123.5 (3)C6—C1—C10—O452.1 (4)
O3—C6—C7—C898.8 (4)C6—C5—C4—C31.6 (5)
O4—C10—C9—C8102.8 (4)C6—C7—C8—C93.4 (5)
C1—C10—C9—C82.3 (5)C6—O3—C11—O17.5 (4)
C1—C10—C9—O4105.0 (3)C6—O3—C11—O5172.8 (3)
C1—C2—C3—C43.0 (5)C7—C6—C1—C1023.6 (4)
C1—C2—C3—O2101.8 (3)C7—C6—C1—C2104.0 (3)
C1—C6—C7—C813.8 (4)C7—C6—C1—O1140.3 (3)
C1—O1—C11—O39.5 (4)C7—C8—C9—C109.7 (5)
C1—O1—C11—O5170.2 (3)C7—C8—C9—O456.5 (5)
C2—C1—C10—C9112.2 (4)C9—O4—C10—C1112.1 (4)
C2—C1—C10—O4177.3 (3)C10—O4—C9—C8110.2 (3)
C2—O2—C3—C4108.7 (3)C11—O1—C1—C1099.9 (3)
C3—C2—C1—C10145.7 (3)C11—O1—C1—C2142.6 (3)
C3—C2—C1—C615.6 (4)C11—O1—C1—C621.1 (3)
C3—C2—C1—O197.4 (3)C11—O3—C6—C119.8 (3)
C3—O2—C2—C1113.4 (3)C11—O3—C6—C599.3 (3)
C4—C5—C6—C120.5 (4)C11—O3—C6—C7142.0 (3)
C4—C5—C6—C7112.0 (4)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C2—H2···O4i0.982.533.509 (5)174
C3—H3···O5ii0.982.553.313 (4)134
C10—H10···O5i0.982.493.379 (4)151
Symmetry codes: (i) x+1, y+1/2, z+1/2; (ii) x, y+1, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
C2—H2···O4i0.982.533.509 (5)174
C3—H3···O5ii0.982.553.313 (4)134
C10—H10···O5i0.982.493.379 (4)151
Symmetry codes: (i) x+1, y+1/2, z+1/2; (ii) x, y+1, z.
Acknowledgements top

GM wishes to thank the Government of India for the award of National Research Professorship. SS and CSA thank the University Grants Commission, India, for the award of Dr D. S. Kothari postdoctoral fellowships. GM acknowledges the research support from Eli Lilly and Jubilant–Bhartia Foundations, and the facilities extended by the University of Hyderabad.

references
References top

Allen, F. H. (2002). Acta Cryst. B58, 380–388.

Altomare, A., Cascarano, G., Giacovazzo, C., Guagliardi, A., Burla, M. C., Polidori, G. & Camalli, M. (1994). J. Appl. Cryst. 27, 435.

Ashkenazi, P., Kalo, J., Rüttimann, A. & Ginsburg, D. (1978). Tetrahedron, 34, 2161–2165.

Bernstein, J., Davis, R. E., Shimoni, L. & Chang, N.-L. (1995). Angew. Chem. Int. Ed. Engl. 34, 1555–1573.

Bruker (1998). SMART and SAINT. Bruker AXS Inc. Madison. Wisconsin, USA.

Bruno, I. J., Cole, J. C., Edgington, P. R., Kessler, M., Macrae, C. F., McCabe, P., Pearson, J. & Taylor, R. (2002). Acta Cryst. B58, 389–397.

Cremer, D. & Pople, J. A. (1975). J. Am. Chem. Soc. 97, 1354–1358.

Etter, M. C., MacDonald, J. C. & Bernstein, J. (1990). Acta Cryst. B46, 256–262.

Mehta, G. & Sen, S. (2010). Tetrahedron Lett. 51, 503–507.

Mehta, G. & Sen, S. (2011). Cryst. Growth Des. 11, 3721–3724.

Paquette, L. A., Liang, S., Waykole, L., DeLucca, G., Jendralla, H., Rogers, R. D., Kratz, D. & Gleiter, R. (1990). J. Org. Chem. 55, 1598–1611.

Pedretti, A., Villa, L. & Vistoli, G. (2004). J. Comput. Aided Mol. Des. 18, 167–173.

Sheldrick, G. M. (2003). SADABS. University of Göttingen, Germany.

Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122.

Spek, A. L. (2009). Acta Cryst. D65, 148–155.

Watkin, D. M., Pearce, L. & Prout, C. K. (1993). CAMERON. Chemical Crystallography Laboratory, University of Oxford, England.