research communications
Synthesis and S,4aR,8aR)-6-oxo-2,4a,6,8a-tetrahydropyrano[3,2-b]pyran-2-carboxamide
of (2aDepartment of Chemistry, Villanova University, 800 E Lancaster Avenue, Villanova, PA, USA, and bDepartment of Chemistry and Biochemistry, College of Charleston, 66 George Street, Charleston, SC, USA
*Correspondence e-mail: robert.giuliano@villanova.edu
The pyranopyran amide (2S,4aR,8aR)-6-oxo-2,4a,6,8a-tetrahydropyrano[3,2-b]pyran-2-carboxamide, C9H9NO4, 3, was prepared by a chemoselective hydration of the corresponding nitrile, 2, using a heterogeneous catalytic method based on copper(II) supported on molecular sieves, in the presence of acetaldoxime. Compound 3 belongs to a new class of pyranopyrans that possess antibacterial and phytotoxic activity. Crystallographic analysis of 3 shows a bent structure for the cis-fused bicyclic pyranopyran, similar to nitrile 2. Evidence of an intramolecular hydrogen bond involving the amide group and ring oxygen was not observed; however, two separate intermolecular hydrogen-bonding interactions were observed between the amide hydrogen atoms and adjacent carbonyl oxygen atoms along the b- and a-axis directions. The latter interaction may also be supported by an intermolecular C—H⋯O hydrogen bond. The lattice is filled out by close-packed layers of this hydrogen-bonded network along the c-axis direction, related from one to the next by a 21 screw axis.
Keywords: diplopyrone; pyranopyran; crystal structure.
CCDC reference: 1980773
1. Chemical context
The phytotoxin diplopyrone 1 was isolated from the fungus Diplodia mutila and reported in 2003 (Evidente et al., 2003). This fungus is considered a causative agent of cork oak decline and diplopyrone is implicated as the main phytotoxin responsible for this disease, the economic and environmental impacts of which are well known (Giorgio et al., 2005). The proposed structure of diplopyrone contains a cis-fused pyranopyran core and four centers, originally assigned as 9S,6R,8aS,4aS, but revised recently to 9R,6S,8aS,4aS (Fusè et al., 2019). In 2019, our laboratory published the synthesis and biological evaluation of pyranopyran analogs based on the structure of diplopyrone (Lazzara et al., 2019). These enantiomeric analogs showed antibacterial and phytotoxic activity, in one case exceeding the activity of a commercially used antibiotic that is used to treat bacterial diseases in pond-raised catfish, which is the largest segment of aquaculture in the United States. Pyranopyran nitrile 2 was approximately 100 times more potent in bioassay than florfenicol against Edwardsiella ictaluri, which causes enteric septicemia (ESC), a disease that can result in losses of tens of millions of dollars to the industry annually. Compound 2 was also phytotoxic in an assay using the aquatic plant Lemna paucicostata (L.) Hegelm. (duckweed). As part of our ongoing efforts to synthesize additional analogs of 1 for testing as new antibacterials and herbicides, we have recently prepared amide 3, by a heterogeneous catalytic method that uses copper(II) supported on molecular sieves, in the presence of acetaldoxime to carry out chemoselective hydration of 2 (Kiss & Hell, 2011).
2. Structural commentary
Pyranopyrans in which the two rings are cis-fused are relatively rare compared to trans-fused pyranopyrans (Giuliano, 2014). A consequence of the cis ring fusion is that the molecule has more of a bent shape than it would if trans-fused, which is demonstrated by the O1—C8A—C4A—O5 torsion angle of 72.95 (15)° versus 177° for a comparable trans-fused pyranopyran (Yu et al., 2017). Both rings adopt half-chair conformations, placing the amide group in a near 1,3-diaxial interaction with H4A. These features are consistent with the results in the computational study reported (Evidente et al., 2003). The study suggests the hydroxyethyl side chain is involved in an intramolecular hydrogen bond between the hydroxyl group and the O5 ring oxygen. By contrast, the amide side chain in 3 does not exhibit a similar intramolecular hydrogen bond with its amino group in the solid state, as shown in Fig. 1. The overall structure of 3 is nearly identical to that of the pyranopyran nitrile 2 with obvious deviation at the side chain.
The NMR spectra of the pyranopyran amide 3 are similar to those of pyranopyran nitrile 2. The most obvious difference in the 13C spectra is the presence of the additional (amide) carbonyl carbon in 3 at δ 174.1 ppm and the absence of the nitrile carbon that occurs at δ 114.9 in 2. The 1H spectrum of 3 shows slight changes in the chemical shifts of most protons, for example there is a downfield shift of H4A from δ 4.45 ppm in the nitrile to δ 4.61 ppm in the amide. The vinyl hydrogen H4 is also further downfield in the amide (δ 7.10 ppm vs 6.91 ppm). The torsion angle of 45.8° for H4A—C4A—C8A—H8A in 3 is consistent with the observed vicinal coupling constant of 4.5 Hz for H4A—H8A in the associated 1H NMR spectrum.
3. Supramolecular features
The amino hydrogen atoms of 3 are involved in intermolecular hydrogen bonding with adjacent carbonyl oxygen atoms: H1A with O2i and H1B with O3ii (Fig. 3, Table 1, Symmetry codes: (i) x, y + 1, z; (ii) x + 1, y, z.). A packing diagram of 3 (Fig. 2a) shows the N—H⋯O hydrogen-bonding interactions forming molecular planes defined by the crystallographic a- and b-axes; packing of these hydrogen-bonded layers appears to be a function of solvent exclusion and van der Waals contact alone, lacking any hydrogen bonding.
The hydrogen-bonded network in the ab plane also presents an arrangement of C—H⋯O and C—H⋯π interactions that suggests two potential additional forces at play within the lattice of 3. Fig. 2b and Table 1 depict distances between H6 and O3ii and C6 and O3ii of adjacent copies of 3. These distances fall within parameters for C—H⋯O hydrogen bonding as has been described in well-characterized membrane proteins and peptidomimetics (Senes et al., 2001; Giuliano et al., 2009); the α-protons implicated in these systems are structurally analogous to the C6—H6 bond of 3. While we will not speculate on the energetic significance of this interaction, which can arise as a coincidence of crystal packing (Dunitz & Gavezzotti, 2005), we note that such interactions have been spectroscopically measured within the core of the dimeric membrane peptide glycophorin A (Arbely & Arkin, 2004). Further, solid-state NMR studies have observed that 1H and 13C NMR shifts change for anomeric C—H bonds in crystalline maltose samples, suggesting that such interactions as described in this study (the C6—H6 bond in 3 is pseudo-anomeric) are not consequences of an energetically dominant lattice arrangement and N—H⋯O hydrogen bonding, but rather have some measurable, albeit weak, energetic contribution to intermolecular association (Yates et al., 2005).
Within the ab plane, H4A of one copy of 3 comes into close approach with its closest neighbor along the a axis. Fig. 3c depicts these distances, which place the centroid of the C7=C8 double bond within distance parameters similar to those calculated for aliphatic C—H⋯π interactions (Karthikeyan et al., 2013). We investigated this further using a semi-empirical protocol to generate partial charges for the atoms of 3. This only allows for qualitative comparison, and, as the color coding in Fig. 2c reveals, the C7=C8 bond (pink, negative) is electrostatically matched with H4A (light blue, positive). Proper exploration of this would require more advanced QM/MM methods, however, the crystal packing of 3 is at least suggestive of a favorable geometry and electrostatic environment for C—H⋯π interactions.
4. Database survey
A search of the Cambridge Structural Database (CSD Version 5.41, November 2019; Groom et al., 2016) using the core fused ring lactone in the search query revealed only three similar structures (Somarathne et al., 2019; Lazzara et al., 2019) in which the pyranopyran ring system is cis-fused and the two double bonds are in the same location as they are in 3. Among the total 40 structures that were found in the search, the pyranopyran core of several were trans-fused, for example, the bergenins and also truncated ladder related to brevitoxin. Some compounds possessed aryl rings fused to the pyranopyran system while others had double bonds at alternate positions including the ring junction.
5. Synthesis and crystallization
(2S,4aR,8aR)-6-Oxo-2,4a,6,8a-tetrahydropyrano[3,2-b]pyran-2-carboxamide 3:
Compound 3 was prepared by the procedure of Kiss & Hell (2011) with a change of solvent from methanol to tert-butanol. A mixture of (4aR,6S,8aR)-6-cyano-6,8a-dihydropyrano-[3,2-b]pyran-2-(4aH)-one 2 (0.040 g, 0.226 mmol), CuII-4 Å catalyst (0.022 g), acetaldoxime (0.040 g, 0.678 mmol) and tert-butanol (2 mL) was stirred at 343 K for 4 h. The mixture was filtered through a pad of Celite and concentrated to a yellow–brown solid that was purified by cartridge on a Waters vacuum manifold system using 5% methanol/chloroform as eluant (flash was also successful using 10% methanol/chloroform). Concentration of fractions left a white solid; yield, 0.0227 g (51.5%). Single crystals were obtained from a solution of 3 in 10% methanol/chloroform at 253 K. Rf = 0.2 (10% methanol/ chloroform); mp 433-437 K; [α]D20 −268 (c, 0.8, methanol); IR (ATR) ν 3425, 3325, 3219, 1710, 1670, 1618 cm−1; 1H NMR (300 MHz, CDCl3) δ 7.10 (dd, 1H, J3,4 = 10.1, J4,4a = 5.4 Hz, H-4), 6.40 (ddd, 1H, J7,8 = 10.2, J6,7 = 3.6, J7,8a = 1.2 Hz, H-7), 6.16 (d, 1H, J3,4 = 10.5, H-3), 6.10 (m, 1H, H-8), 4.86 (bs, 2H, NH2), 4.80 (m, 2H, H-6, H-8a), 4.61 (ddd, 1H, J4a,4 = J4a,8a = 4.5, J4a,8 = 1.2 Hz, H-4a); 13C{1H} NMR (CDCl3) δ 174.1, 164.8, 143.7, 131.8, 124.5, 123.3, 74.0, 70.2, 64.4. HRMS (ESI–TOF) m/z calculated for C9H10NO4 196.0610, found 196.0607.
6. Refinement
Crystal data, data collection, and structure . The was known from the synthetic route and assigned accordingly. The amino hydrogen atoms were found in the electron difference map and refined isotropically, while all other hydrogen atoms were treated as idealized contributions with C—H = 0.95–1.00 Å and Uiso(H) = 1.2Ueq(C).
details are summarized in Table 2
|
Supporting information
CCDC reference: 1980773
https://doi.org/10.1107/S2056989020001292/zl4038sup1.cif
contains datablock I. DOI:Structure factors: contains datablock I. DOI: https://doi.org/10.1107/S2056989020001292/zl4038Isup2.hkl
Supporting information file. DOI: https://doi.org/10.1107/S2056989020001292/zl4038Isup3.cml
Data collection: APEX3 (Bruker, 2016); cell
SAINT (Bruker, 2015); data reduction: SAINT (Bruker, 2015); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).C9H9NO4 | Dx = 1.558 Mg m−3 |
Mr = 195.17 | Mo Kα radiation, λ = 0.71073 Å |
Orthorhombic, P212121 | Cell parameters from 8408 reflections |
a = 4.9279 (1) Å | θ = 2.3–30.2° |
b = 10.6350 (3) Å | µ = 0.12 mm−1 |
c = 15.8788 (4) Å | T = 100 K |
V = 832.18 (4) Å3 | Prism, colourless |
Z = 4 | 0.4 × 0.3 × 0.18 mm |
F(000) = 408 |
Bruker SMART APEXII area detector diffractometer | 2461 independent reflections |
Radiation source: sealed tube | 2366 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.031 |
Detector resolution: 8 pixels mm-1 | θmax = 30.2°, θmin = 2.3° |
ω and φ scans | h = −5→6 |
Absorption correction: multi-scan (SADABS; Krause et al., 2015) | k = −15→14 |
Tmin = 0.654, Tmax = 0.746 | l = −22→22 |
14252 measured reflections |
Refinement on F2 | Hydrogen site location: mixed |
Least-squares matrix: full | H atoms treated by a mixture of independent and constrained refinement |
R[F2 > 2σ(F2)] = 0.032 | w = 1/[σ2(Fo2) + (0.0446P)2 + 0.205P] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.080 | (Δ/σ)max < 0.001 |
S = 1.04 | Δρmax = 0.35 e Å−3 |
2461 reflections | Δρmin = −0.20 e Å−3 |
135 parameters | Absolute structure: Flack x determined using 928 quotients [(I+)-(I-)]/[(I+)+(I-)] (Parsons et al., 2013) |
0 restraints | Absolute structure parameter: −0.1 (3) |
Primary atom site location: structure-invariant direct methods |
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. |
Refinement. 1. Fixed Uiso At 1.2 times of: All C(H) groups 2.a Ternary CH refined with riding coordinates: C4A(H4A), C6(H6), C8A(H8A) 2.b Aromatic/amide H refined with riding coordinates: C3(H3), C4(H4), C7(H7), C8(H8) 3. N1(H1A) and N1(H1B) located from the difference map with refined Uiso |
x | y | z | Uiso*/Ueq | ||
O1 | 0.7129 (2) | 0.30730 (10) | 0.31022 (7) | 0.0145 (2) | |
C2 | 0.5987 (3) | 0.22091 (15) | 0.36070 (9) | 0.0141 (3) | |
O2 | 0.7069 (3) | 0.11890 (11) | 0.36663 (8) | 0.0198 (3) | |
C3 | 0.3563 (3) | 0.25659 (15) | 0.40997 (9) | 0.0155 (3) | |
H3 | 0.2482 | 0.1928 | 0.4350 | 0.019* | |
C4 | 0.2869 (3) | 0.37655 (14) | 0.41980 (10) | 0.0154 (3) | |
H4 | 0.1413 | 0.3982 | 0.4562 | 0.019* | |
C4A | 0.4377 (3) | 0.47756 (14) | 0.37369 (9) | 0.0121 (3) | |
H4A | 0.3071 | 0.5453 | 0.3574 | 0.015* | |
O5 | 0.6340 (2) | 0.52880 (10) | 0.43085 (6) | 0.0118 (2) | |
C6 | 0.7796 (3) | 0.63226 (13) | 0.39613 (9) | 0.0107 (3) | |
H6 | 0.9519 | 0.6405 | 0.4289 | 0.013* | |
C7 | 0.8579 (3) | 0.61248 (14) | 0.30510 (9) | 0.0126 (3) | |
H7 | 0.9754 | 0.6718 | 0.2792 | 0.015* | |
C8 | 0.7700 (3) | 0.51613 (14) | 0.25988 (9) | 0.0140 (3) | |
H8 | 0.8362 | 0.5046 | 0.2042 | 0.017* | |
C8A | 0.5687 (3) | 0.42536 (14) | 0.29495 (9) | 0.0124 (3) | |
H8A | 0.4244 | 0.4103 | 0.2518 | 0.015* | |
C9 | 0.6280 (3) | 0.75809 (14) | 0.40406 (8) | 0.0112 (3) | |
O3 | 0.3793 (2) | 0.76516 (11) | 0.40436 (7) | 0.0163 (2) | |
N1 | 0.7940 (3) | 0.85729 (13) | 0.40610 (9) | 0.0151 (3) | |
H1A | 0.735 (5) | 0.933 (2) | 0.4068 (15) | 0.033 (6)* | |
H1B | 0.967 (5) | 0.845 (2) | 0.4125 (13) | 0.017 (5)* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0173 (5) | 0.0092 (5) | 0.0170 (5) | 0.0032 (4) | 0.0028 (4) | −0.0007 (4) |
C2 | 0.0161 (7) | 0.0111 (7) | 0.0151 (6) | −0.0009 (5) | −0.0026 (5) | −0.0014 (5) |
O2 | 0.0235 (6) | 0.0102 (5) | 0.0255 (6) | 0.0034 (4) | 0.0003 (5) | 0.0013 (4) |
C3 | 0.0137 (6) | 0.0138 (7) | 0.0189 (7) | −0.0026 (6) | 0.0003 (5) | 0.0016 (6) |
C4 | 0.0121 (6) | 0.0140 (7) | 0.0202 (7) | −0.0014 (5) | 0.0027 (5) | −0.0012 (5) |
C4A | 0.0104 (6) | 0.0093 (6) | 0.0166 (6) | 0.0006 (5) | 0.0004 (5) | −0.0011 (5) |
O5 | 0.0140 (5) | 0.0093 (5) | 0.0121 (4) | −0.0021 (4) | 0.0006 (4) | 0.0006 (4) |
C6 | 0.0103 (6) | 0.0087 (6) | 0.0130 (6) | −0.0001 (5) | −0.0001 (5) | 0.0005 (5) |
C7 | 0.0116 (6) | 0.0119 (6) | 0.0145 (6) | 0.0021 (5) | 0.0030 (5) | 0.0029 (5) |
C8 | 0.0170 (7) | 0.0133 (7) | 0.0117 (6) | 0.0035 (6) | 0.0023 (5) | 0.0018 (5) |
C8A | 0.0153 (7) | 0.0088 (6) | 0.0130 (6) | 0.0021 (5) | −0.0017 (5) | −0.0005 (5) |
C9 | 0.0144 (6) | 0.0096 (6) | 0.0097 (5) | 0.0009 (5) | 0.0001 (5) | −0.0001 (5) |
O3 | 0.0121 (5) | 0.0128 (5) | 0.0240 (5) | 0.0019 (4) | 0.0016 (4) | −0.0016 (4) |
N1 | 0.0155 (6) | 0.0082 (6) | 0.0215 (6) | 0.0000 (5) | −0.0023 (5) | 0.0015 (5) |
O1—C2 | 1.3428 (18) | C6—H6 | 1.0000 |
O1—C8A | 1.4629 (18) | C6—C7 | 1.511 (2) |
C2—O2 | 1.2126 (19) | C6—C9 | 1.538 (2) |
C2—C3 | 1.478 (2) | C7—H7 | 0.9500 |
C3—H3 | 0.9500 | C7—C8 | 1.324 (2) |
C3—C4 | 1.330 (2) | C8—H8 | 0.9500 |
C4—H4 | 0.9500 | C8—C8A | 1.492 (2) |
C4—C4A | 1.497 (2) | C8A—H8A | 1.0000 |
C4A—H4A | 1.0000 | C9—O3 | 1.2280 (19) |
C4A—O5 | 1.4343 (18) | C9—N1 | 1.335 (2) |
C4A—C8A | 1.513 (2) | N1—H1A | 0.86 (3) |
O5—C6 | 1.4243 (17) | N1—H1B | 0.87 (2) |
C2—O1—C8A | 118.85 (12) | C7—C6—C9 | 108.88 (11) |
O1—C2—C3 | 118.63 (14) | C9—C6—H6 | 107.1 |
O2—C2—O1 | 118.31 (14) | C6—C7—H7 | 118.6 |
O2—C2—C3 | 122.97 (14) | C8—C7—C6 | 122.88 (13) |
C2—C3—H3 | 119.5 | C8—C7—H7 | 118.6 |
C4—C3—C2 | 121.08 (14) | C7—C8—H8 | 119.5 |
C4—C3—H3 | 119.5 | C7—C8—C8A | 121.06 (13) |
C3—C4—H4 | 119.9 | C8A—C8—H8 | 119.5 |
C3—C4—C4A | 120.23 (14) | O1—C8A—C4A | 112.66 (12) |
C4A—C4—H4 | 119.9 | O1—C8A—C8 | 107.10 (12) |
C4—C4A—H4A | 108.9 | O1—C8A—H8A | 108.7 |
C4—C4A—C8A | 110.65 (12) | C4A—C8A—H8A | 108.7 |
O5—C4A—C4 | 107.33 (12) | C8—C8A—C4A | 110.78 (12) |
O5—C4A—H4A | 108.9 | C8—C8A—H8A | 108.7 |
O5—C4A—C8A | 112.00 (12) | O3—C9—C6 | 122.57 (14) |
C8A—C4A—H4A | 108.9 | O3—C9—N1 | 124.26 (15) |
C6—O5—C4A | 112.85 (11) | N1—C9—C6 | 113.09 (13) |
O5—C6—H6 | 107.1 | C9—N1—H1A | 122.5 (17) |
O5—C6—C7 | 113.04 (12) | C9—N1—H1B | 118.8 (14) |
O5—C6—C9 | 113.33 (12) | H1A—N1—H1B | 118 (2) |
C7—C6—H6 | 107.1 | ||
O1—C2—C3—C4 | −15.0 (2) | O5—C4A—C8A—C8 | −47.00 (16) |
C2—O1—C8A—C4A | 41.32 (17) | O5—C6—C7—C8 | 8.0 (2) |
C2—O1—C8A—C8 | 163.37 (12) | O5—C6—C9—O3 | −30.03 (19) |
C2—C3—C4—C4A | 6.2 (2) | O5—C6—C9—N1 | 153.21 (13) |
O2—C2—C3—C4 | 161.46 (16) | C6—C7—C8—C8A | 4.7 (2) |
C3—C4—C4A—O5 | −98.05 (16) | C7—C6—C9—O3 | 96.70 (16) |
C3—C4—C4A—C8A | 24.4 (2) | C7—C6—C9—N1 | −80.06 (15) |
C4—C4A—O5—C6 | −176.04 (11) | C7—C8—C8A—O1 | −108.78 (16) |
C4—C4A—C8A—O1 | −46.75 (16) | C7—C8—C8A—C4A | 14.4 (2) |
C4—C4A—C8A—C8 | −166.70 (12) | C8A—O1—C2—O2 | 173.12 (13) |
C4A—O5—C6—C7 | −41.21 (16) | C8A—O1—C2—C3 | −10.21 (19) |
C4A—O5—C6—C9 | 83.29 (14) | C8A—C4A—O5—C6 | 62.33 (15) |
O5—C4A—C8A—O1 | 72.95 (15) | C9—C6—C7—C8 | −118.85 (16) |
D—H···A | D—H | H···A | D···A | D—H···A |
N1—H1A···O2i | 0.86 (3) | 2.08 (3) | 2.8840 (18) | 156 (2) |
N1—H1B···O3ii | 0.87 (2) | 2.21 (2) | 3.0462 (18) | 163.1 (19) |
C6—H6···O3ii | 1.00 | 2.52 | 3.2787 (19) | 133 |
Symmetry codes: (i) x, y+1, z; (ii) x+1, y, z. |
Funding information
The authors thank Villanova University for financial support of this work.
References
Altomare, A., Cascarano, G., Giacovazzo, G., Guagliardi, A., Burla, M. C., Polidori, G. & Camalli, M. J. (1994). Appl. Cryst. 27, 435. Google Scholar
Arbely, E. & Arkin, I. T. (2004). J. Am. Chem. Soc. 126, 5362–5363. PubMed CAS Google Scholar
Bruker (2015). SAINT. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Bruker (2016). APEX3. Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Dolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K. & Puschmann, H. (2009). J. Appl. Cryst. 42, 339–341. Web of Science CrossRef CAS IUCr Journals Google Scholar
Dunitz, J. D. & Gavezzotti, A. (2005). Angew. Chem. Int. Ed. 44, 1766–1787. Web of Science CrossRef CAS Google Scholar
Evidente, A., Maddau, L., Spanu, E., Franceschini, A., Lazzaroni, S. & Motta, A. (2003). J. Nat. Prod. 66, 313–315. PubMed CAS Google Scholar
Fusè, M., Mazzeo, G., Longhi, G., Abbate, S., Masi, M., Evidente, A., Puzzarini, C. & Barone, V. (2019). J. Phys. Chem. B, 123, 9230–9237. PubMed Google Scholar
Giorgio, E., Maddau, L., Spanu, E., Evidente, A. & Rosini, C. (2005). J. Org. Chem. 70, 7–13. PubMed CAS Google Scholar
Giuliano, M. W., Horne, W. S. & Gellman, S. H. (2009). J. Am. Chem. Soc. 131, 9860–9861. PubMed CAS Google Scholar
Giuliano, R. M. (2014). Curr. Org. Chem. 18, 1686–1700. CAS Google Scholar
Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171–179. Web of Science CrossRef IUCr Journals Google Scholar
Karthikeyan, S., Ramanathan, V. & Mishra, B. K. (2013). J. Phys. Chem. A, 117, 6687–6694. Web of Science CrossRef CAS PubMed Google Scholar
Kiss, A. & Hell, Z. (2011). Tetrahedron Lett. 52, 6021–6023. CAS Google Scholar
Krause, L., Herbst-Irmer, R., Sheldrick, G. M. & Stalke, D. (2015). J. Appl. Cryst. 48, 3–10. Web of Science CSD CrossRef ICSD CAS IUCr Journals Google Scholar
Lazzara, N. C., Rosano, R. J., Vagadia, P. P., Giovine, M. T., Bezpalko, M. W., Piro, N. A., Kassel, Wm. S., Boyko, W. J., Zubris, D. L., Schrader, K. K., Wedge, D. E., Duke, S. O. & Giuliano, R. M. (2019). J. Org. Chem. 84, 666–678. CAS PubMed Google Scholar
Parsons, S., Flack, H. D. & Wagner, T. (2013). Acta Cryst. B69, 249–259. Web of Science CrossRef CAS IUCr Journals Google Scholar
Pettersen, E. F., Goddard, T. D., Huang, C. C., Couch, G. S., Greenblatt, D. M., Meng, E. C. & Ferrin, T. E. (2004). J. Comput. Chem. 25, 1605–1612. Web of Science CrossRef PubMed CAS Google Scholar
Senes, A., Ubarretxena-Belandia, I. & Engelman, D. M. (2001). Proc. Natl Acad. Sci. USA, 98, 9056–9061. Web of Science CrossRef PubMed CAS Google Scholar
Sheldrick, G. M. (2015). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Somarathne, K. K., McCone, J. A. J., Brackovic, A., Rivera, J. L. P., Fulton, J., Russell, E., Field, J. J., Orme, C. L., Stirrat, H. L., Riesterer, J., Teesdale–Spittle, P. H., Miller, J. H. & Harvey, J. E. (2019). Chem. Asian J. 14, 1230–1237. CAS PubMed Google Scholar
Wang, J., Wang, W., Kollman, P. A. & Case, D. A. (2006). J. Mol. Graphics Modell. 25, 247–260. Web of Science CrossRef Google Scholar
Yates, J. R., Pham, T. N., Pickard, C. J., Mauri, F., Amado, A. M., Gil, A. M. & Brown, S. P. (2005). J. Am. Chem. Soc. 127, 10216–10220. Web of Science CrossRef PubMed CAS Google Scholar
Yu, K., Wu, W., Li, S., Dou, L., Liu, L., Li, P. & Liu, E. (2017). Nat. Prod. Res. 31, 2581–2586. CAS PubMed Google Scholar
This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.