organic compounds
(1aR*,2R*,7S*,7aS*)-rel-3,6-Dimethoxy-2-methyl-1a,2,7,7a-tetrahydro-2,7-epoxy-1H-cyclopropa[b]naphthalene
aDepartment of Chemistry, University of Toronto, Toronto, Ontario, M5S 3H6, Canada, and bDepartment of Chemistry, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
*Correspondence e-mail: alough@chem.utoronto.ca
In the racemic title compound, C14H16O3, the dihedral angle formed by the mean planes of the cyclopropane and benzene rings is 5.0 (2)°. In the crystal, a pair of weak C—H⋯O hydrogen bonds connect two molecules related by a twofold rotation axis, thus forming a dimer with an R22(10) motif.
Keywords: crystal structure; racemic; cyclopropane; twofold rotation axis; naphthalene; tetracycle.
CCDC reference: 1456342
Structure description
We have recently investigated the palladium-catalysed cyclopropanation reactions of [2.2.1] heterobicyclic compounds (Carlson et al., 2016). Substituted 7-oxabenzonorbornadiene (I) reacts with diazomethane in the presence of catalytic Pd(OAc)2 in THF to give the cyclopropane (II) as a single stereoisomer (see Fig. 1). The stereochemistry of (II) was determined by this single-crystal X-ray analysis. Of the exo or endo isomers which could be formed, the reaction was found to give solely the exo stereoisomer.
The molecular structure of the title compound is shown in Fig. 2. The dihedral angle formed by the mean planes of the cyclopropane and benzene rings C3/C4/C5 and C1/C7/C8/C9/C10/C11, respectively is 5.0 (2)°. In the crystal, a pair of weak C—H⋯O hydrogen bonds (Table 1 and Fig. 3) between two molecules related by a twofold rotation axis forms a dimer with an (10) motif.
Synthesis and crystallization
HAZARD ALERT! Diazomethane can be fatal if inhaled and capable of detonation if appreciably concentrated. See Carlson et al. (2016) for a detailed figure of the experimental apparatus. Alkene (I) (9.2 mmol), Pd(OAc)2 (0.092 mmol) and THF (40 ml) were stirred in a sealed reaction flask at 273 K, connected to a 1:1 glacial acetic acid: water trap and vented to the back of the fumehood. Diazomethane was generated in a separate flask by dropwise addition (2 ml min−1) of 12.5 M aqueous NaOH (1.3 mol) to Diazald (23.8 mmol) stirred in 95% EtOH (50 ml), and directed under a steady stream of argon to the reaction flask. Formation of the light-yellow CH2N2 was observed with the dissolution of Diazald. Upon completion of the reaction (monitored by TLC and dissipation of any yellow colour), the crude reaction mixture was filtered through Celite, rinsed with Et2O (3 × 10 ml), concentrated and purified by (EtOAc:hexanes = 1:9) followed by recrystallization from pentane solution to give the exo cyclopropane (II) in the form of colourless plates in 82% yield.
Refinement
Crystal data, data collection and structure .
details are summarized in Table 2Structural data
CCDC reference: 1456342
10.1107/S2414314616003412/hb4021sup1.cif
contains datablock I. DOI:Structure factors: contains datablock I. DOI: 10.1107/S2414314616003412/hb4021Isup2.hkl
Supporting information file. DOI: 10.1107/S2414314616003412/hb4021Isup3.cml
HAZARD ALERT! Diazomethane can be fatal if inhaled and capable of detonation if appreciably concentrated. See Carlson et al. (2016) for a detailed figure of the experimental apparatus. Alkene (I) (9.2 mmol), Pd(OAc)2 (0.092 mmol) and THF (40 ml) were stirred in a sealed reaction flask at 273 K, connected to a 1:1 glacial acetic acid: water trap and vented to the back of the fumehood. Diazomethane was generated in a separate flask by dropwise addition (2 ml min−1) of 12.5 M aqueous NaOH (1.3 mol) to Diazald (23.8 mmol) stirred in 95% EtOH (50 ml), and directed under a steady stream of argon to the reaction flask. Formation of the light-yellow CH2N2 was observed with the dissolution of Diazald. Upon completion of the reaction (monitored by TLC and dissipation of any yellow colour), the crude reaction mixture was filtered through Celite, rinsed with Et2O (3 × 10 ml), concentrated and purified by
(EtOAc:hexanes = 1:9) followed by recrystallization from pentane solution to give the exo cyclopropane (II) in the form of colourless plates in 82% yield.We have recently investigated the palladium-catalysed cyclopropanation reactions of [2.2.1] heterobicyclic compounds (Carlson et al., 2016). Substituted 7-oxabenzonorbornadiene (I) reacts with diazomethane in the presence of catalytic Pd(OAc)2 in THF to give the cyclopropane (II) as a single stereoisomer (see Fig. 1). The stereochemistry of (II) was determined by this single-crystal X-ray analysis. Of the exo or endo isomers which could be formed, the reaction was found to give solely the exo stereoisomer.
The molecular structure of the title compound is shown in Fig. 2. The dihedral angle formed by the mean planes of the cyclopropane and benzene rings C3/C4/C5 and C1/C7/C8/C9/C10/C11, respectively is 5.0 (2)°. In the crystal, a pair of weak C—H···O hydrogen bonds (Table 1) between two molecules related by a twofold rotation axis forms a dimer with an R22(10) motif.
Data collection: APEX2 (Bruker, 2014); cell
APEX2 (Bruker, 2014); data reduction: SAINT (Bruker, 2014); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: publCIF (Westrip, 2010).Fig. 1. The reaction scheme. | |
Fig. 2. The molecular structure of the title compound showing 30% probability ellipsoids. | |
Fig. 3. A pair of molecules connected by weak hydrogen bonds shown as dashed lines. |
C14H16O3 | F(000) = 992 |
Mr = 232.27 | Dx = 1.322 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
a = 16.401 (2) Å | Cell parameters from 4973 reflections |
b = 6.9386 (9) Å | θ = 2.5–27.4° |
c = 20.666 (2) Å | µ = 0.09 mm−1 |
β = 97.089 (3)° | T = 147 K |
V = 2333.8 (5) Å3 | Plate, colourless |
Z = 8 | 0.33 × 0.23 × 0.06 mm |
Bruker Kappa APEX DUO CCD diffractometer | 1972 reflections with I > 2σ(I) |
Radiation source: sealed tube with Bruker Triumph monochromator | Rint = 0.037 |
φ and ω scans | θmax = 27.5°, θmin = 2.0° |
Absorption correction: multi-scan (SADABS; Bruker, 2014) | h = −21→21 |
Tmin = 0.703, Tmax = 0.746 | k = −8→9 |
17093 measured reflections | l = −17→26 |
2681 independent reflections |
Refinement on F2 | 0 restraints |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.041 | H-atom parameters constrained |
wR(F2) = 0.104 | w = 1/[σ2(Fo2) + (0.0431P)2 + 2.2112P] where P = (Fo2 + 2Fc2)/3 |
S = 1.02 | (Δ/σ)max < 0.001 |
2681 reflections | Δρmax = 0.27 e Å−3 |
157 parameters | Δρmin = −0.20 e Å−3 |
C14H16O3 | V = 2333.8 (5) Å3 |
Mr = 232.27 | Z = 8 |
Monoclinic, C2/c | Mo Kα radiation |
a = 16.401 (2) Å | µ = 0.09 mm−1 |
b = 6.9386 (9) Å | T = 147 K |
c = 20.666 (2) Å | 0.33 × 0.23 × 0.06 mm |
β = 97.089 (3)° |
Bruker Kappa APEX DUO CCD diffractometer | 2681 independent reflections |
Absorption correction: multi-scan (SADABS; Bruker, 2014) | 1972 reflections with I > 2σ(I) |
Tmin = 0.703, Tmax = 0.746 | Rint = 0.037 |
17093 measured reflections |
R[F2 > 2σ(F2)] = 0.041 | 0 restraints |
wR(F2) = 0.104 | H-atom parameters constrained |
S = 1.02 | Δρmax = 0.27 e Å−3 |
2681 reflections | Δρmin = −0.20 e Å−3 |
157 parameters |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
x | y | z | Uiso*/Ueq | ||
O1 | 0.02986 (6) | 0.82417 (15) | 0.09283 (5) | 0.0194 (2) | |
O2 | 0.09941 (7) | 1.03830 (15) | 0.28403 (5) | 0.0253 (3) | |
O3 | 0.26711 (6) | 1.04938 (15) | 0.06547 (5) | 0.0248 (3) | |
C1 | 0.11743 (8) | 0.9435 (2) | 0.17734 (7) | 0.0167 (3) | |
C2 | 0.04984 (9) | 0.7949 (2) | 0.16232 (7) | 0.0190 (3) | |
H2A | 0.0027 | 0.8064 | 0.1885 | 0.023* | |
C3 | 0.09260 (9) | 0.5976 (2) | 0.16523 (7) | 0.0207 (3) | |
H3A | 0.1212 | 0.5476 | 0.2074 | 0.025* | |
C4 | 0.06766 (10) | 0.4617 (2) | 0.10921 (8) | 0.0249 (3) | |
H4A | 0.0822 | 0.3239 | 0.1154 | 0.030* | |
H4B | 0.0148 | 0.4864 | 0.0818 | 0.030* | |
C5 | 0.13667 (9) | 0.6032 (2) | 0.10507 (7) | 0.0191 (3) | |
H5A | 0.1946 | 0.5565 | 0.1073 | 0.023* | |
C6 | 0.11289 (9) | 0.8013 (2) | 0.07536 (7) | 0.0185 (3) | |
C7 | 0.15865 (9) | 0.9463 (2) | 0.12220 (7) | 0.0167 (3) | |
C8 | 0.22780 (9) | 1.0607 (2) | 0.12042 (7) | 0.0177 (3) | |
C9 | 0.25244 (9) | 1.1778 (2) | 0.17404 (7) | 0.0189 (3) | |
H9A | 0.2988 | 1.2596 | 0.1735 | 0.023* | |
C10 | 0.20992 (9) | 1.1766 (2) | 0.22880 (7) | 0.0187 (3) | |
H10A | 0.2273 | 1.2587 | 0.2646 | 0.022* | |
C11 | 0.14249 (9) | 1.0565 (2) | 0.23120 (7) | 0.0173 (3) | |
C12 | 0.11176 (10) | 0.8263 (2) | 0.00304 (7) | 0.0260 (4) | |
H12A | 0.1676 | 0.8113 | −0.0085 | 0.039* | |
H12B | 0.0911 | 0.9551 | −0.0097 | 0.039* | |
H12C | 0.0758 | 0.7287 | −0.0198 | 0.039* | |
C13 | 0.10349 (12) | 1.1913 (3) | 0.32903 (8) | 0.0383 (5) | |
H13A | 0.0652 | 1.1671 | 0.3609 | 0.057* | |
H13B | 0.0886 | 1.3120 | 0.3059 | 0.057* | |
H13C | 0.1595 | 1.2013 | 0.3516 | 0.057* | |
C14 | 0.33542 (10) | 1.1738 (3) | 0.06247 (8) | 0.0320 (4) | |
H14A | 0.3577 | 1.1538 | 0.0211 | 0.048* | |
H14B | 0.3779 | 1.1452 | 0.0988 | 0.048* | |
H14C | 0.3177 | 1.3081 | 0.0654 | 0.048* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0152 (5) | 0.0205 (5) | 0.0219 (5) | 0.0000 (4) | −0.0003 (4) | −0.0001 (4) |
O2 | 0.0300 (6) | 0.0246 (6) | 0.0233 (5) | −0.0053 (5) | 0.0113 (5) | −0.0050 (5) |
O3 | 0.0249 (6) | 0.0280 (6) | 0.0232 (5) | −0.0074 (5) | 0.0095 (4) | −0.0026 (5) |
C1 | 0.0156 (7) | 0.0139 (7) | 0.0203 (7) | 0.0002 (5) | 0.0009 (6) | 0.0015 (6) |
C2 | 0.0181 (7) | 0.0195 (8) | 0.0194 (7) | −0.0016 (6) | 0.0029 (6) | −0.0003 (6) |
C3 | 0.0221 (8) | 0.0168 (7) | 0.0233 (7) | −0.0014 (6) | 0.0026 (6) | 0.0009 (6) |
C4 | 0.0266 (8) | 0.0165 (7) | 0.0311 (8) | −0.0032 (6) | 0.0024 (7) | −0.0021 (6) |
C5 | 0.0177 (7) | 0.0157 (7) | 0.0239 (7) | 0.0002 (6) | 0.0030 (6) | −0.0027 (6) |
C6 | 0.0178 (7) | 0.0171 (7) | 0.0206 (7) | −0.0008 (6) | 0.0028 (6) | −0.0009 (6) |
C7 | 0.0184 (7) | 0.0131 (7) | 0.0181 (7) | 0.0015 (5) | 0.0000 (6) | 0.0004 (5) |
C8 | 0.0179 (7) | 0.0165 (7) | 0.0189 (7) | 0.0012 (6) | 0.0034 (6) | 0.0024 (6) |
C9 | 0.0163 (7) | 0.0159 (7) | 0.0242 (7) | −0.0030 (6) | 0.0010 (6) | 0.0008 (6) |
C10 | 0.0203 (7) | 0.0161 (7) | 0.0192 (7) | −0.0001 (6) | −0.0004 (6) | −0.0025 (6) |
C11 | 0.0186 (7) | 0.0156 (7) | 0.0179 (7) | 0.0023 (6) | 0.0029 (6) | 0.0009 (6) |
C12 | 0.0314 (9) | 0.0264 (8) | 0.0198 (7) | −0.0061 (7) | 0.0016 (7) | −0.0013 (6) |
C13 | 0.0530 (12) | 0.0355 (10) | 0.0302 (9) | −0.0071 (9) | 0.0204 (9) | −0.0129 (8) |
C14 | 0.0301 (9) | 0.0336 (9) | 0.0350 (9) | −0.0122 (8) | 0.0153 (7) | −0.0030 (8) |
O1—C2 | 1.4474 (17) | C5—H5A | 1.0000 |
O1—C6 | 1.4598 (16) | C6—C12 | 1.503 (2) |
O2—C11 | 1.3776 (16) | C6—C7 | 1.527 (2) |
O2—C13 | 1.4074 (19) | C7—C8 | 1.3886 (19) |
O3—C8 | 1.3755 (16) | C8—C9 | 1.393 (2) |
O3—C14 | 1.4218 (18) | C9—C10 | 1.4006 (19) |
C1—C11 | 1.382 (2) | C9—H9A | 0.9500 |
C1—C7 | 1.3951 (19) | C10—C11 | 1.391 (2) |
C1—C2 | 1.5177 (19) | C10—H10A | 0.9500 |
C2—C3 | 1.536 (2) | C12—H12A | 0.9800 |
C2—H2A | 1.0000 | C12—H12B | 0.9800 |
C3—C4 | 1.510 (2) | C12—H12C | 0.9800 |
C3—C5 | 1.5137 (19) | C13—H13A | 0.9800 |
C3—H3A | 1.0000 | C13—H13B | 0.9800 |
C4—C5 | 1.509 (2) | C13—H13C | 0.9800 |
C4—H4A | 0.9900 | C14—H14A | 0.9800 |
C4—H4B | 0.9900 | C14—H14B | 0.9800 |
C5—C6 | 1.536 (2) | C14—H14C | 0.9800 |
C2—O1—C6 | 97.26 (10) | C7—C6—C5 | 104.78 (11) |
C11—O2—C13 | 117.86 (12) | C8—C7—C1 | 120.60 (13) |
C8—O3—C14 | 116.97 (12) | C8—C7—C6 | 134.34 (12) |
C11—C1—C7 | 121.62 (13) | C1—C7—C6 | 105.01 (12) |
C11—C1—C2 | 133.42 (12) | O3—C8—C7 | 117.17 (12) |
C7—C1—C2 | 104.93 (12) | O3—C8—C9 | 124.78 (13) |
O1—C2—C1 | 100.24 (10) | C7—C8—C9 | 118.05 (12) |
O1—C2—C3 | 102.13 (11) | C8—C9—C10 | 120.98 (13) |
C1—C2—C3 | 106.21 (11) | C8—C9—H9A | 119.5 |
O1—C2—H2A | 115.4 | C10—C9—H9A | 119.5 |
C1—C2—H2A | 115.4 | C11—C10—C9 | 120.64 (13) |
C3—C2—H2A | 115.4 | C11—C10—H10A | 119.7 |
C4—C3—C5 | 59.87 (9) | C9—C10—H10A | 119.7 |
C4—C3—C2 | 116.57 (13) | O2—C11—C1 | 117.11 (12) |
C5—C3—C2 | 101.93 (12) | O2—C11—C10 | 124.83 (13) |
C4—C3—H3A | 120.4 | C1—C11—C10 | 118.04 (12) |
C5—C3—H3A | 120.4 | C6—C12—H12A | 109.5 |
C2—C3—H3A | 120.4 | C6—C12—H12B | 109.5 |
C5—C4—C3 | 60.20 (9) | H12A—C12—H12B | 109.5 |
C5—C4—H4A | 117.8 | C6—C12—H12C | 109.5 |
C3—C4—H4A | 117.8 | H12A—C12—H12C | 109.5 |
C5—C4—H4B | 117.8 | H12B—C12—H12C | 109.5 |
C3—C4—H4B | 117.8 | O2—C13—H13A | 109.5 |
H4A—C4—H4B | 114.9 | O2—C13—H13B | 109.5 |
C4—C5—C3 | 59.93 (10) | H13A—C13—H13B | 109.5 |
C4—C5—C6 | 116.73 (12) | O2—C13—H13C | 109.5 |
C3—C5—C6 | 103.18 (11) | H13A—C13—H13C | 109.5 |
C4—C5—H5A | 120.1 | H13B—C13—H13C | 109.5 |
C3—C5—H5A | 120.1 | O3—C14—H14A | 109.5 |
C6—C5—H5A | 120.1 | O3—C14—H14B | 109.5 |
O1—C6—C12 | 109.65 (12) | H14A—C14—H14B | 109.5 |
O1—C6—C7 | 99.92 (10) | O3—C14—H14C | 109.5 |
C12—C6—C7 | 119.96 (12) | H14A—C14—H14C | 109.5 |
O1—C6—C5 | 101.35 (11) | H14B—C14—H14C | 109.5 |
C12—C6—C5 | 118.17 (12) | ||
C6—O1—C2—C1 | 53.68 (11) | C11—C1—C7—C6 | 179.16 (13) |
C6—O1—C2—C3 | −55.53 (11) | C2—C1—C7—C6 | 1.12 (14) |
C11—C1—C2—O1 | 147.94 (15) | O1—C6—C7—C8 | −150.61 (15) |
C7—C1—C2—O1 | −34.35 (13) | C12—C6—C7—C8 | −30.9 (2) |
C11—C1—C2—C3 | −106.10 (17) | C5—C6—C7—C8 | 104.75 (17) |
C7—C1—C2—C3 | 71.61 (14) | O1—C6—C7—C1 | 32.10 (13) |
O1—C2—C3—C4 | −27.71 (15) | C12—C6—C7—C1 | 151.76 (13) |
C1—C2—C3—C4 | −132.31 (13) | C5—C6—C7—C1 | −72.55 (13) |
O1—C2—C3—C5 | 34.38 (13) | C14—O3—C8—C7 | 176.96 (13) |
C1—C2—C3—C5 | −70.21 (13) | C14—O3—C8—C9 | −3.2 (2) |
C2—C3—C4—C5 | 88.68 (14) | C1—C7—C8—O3 | 177.28 (13) |
C3—C4—C5—C6 | −90.21 (14) | C6—C7—C8—O3 | 0.3 (2) |
C2—C3—C5—C4 | −113.95 (13) | C1—C7—C8—C9 | −2.6 (2) |
C4—C3—C5—C6 | 113.46 (13) | C6—C7—C8—C9 | −179.56 (14) |
C2—C3—C5—C6 | −0.49 (14) | O3—C8—C9—C10 | −178.34 (13) |
C2—O1—C6—C12 | −179.66 (12) | C7—C8—C9—C10 | 1.5 (2) |
C2—O1—C6—C7 | −52.73 (11) | C8—C9—C10—C11 | 0.8 (2) |
C2—O1—C6—C5 | 54.68 (12) | C13—O2—C11—C1 | −158.53 (14) |
C4—C5—C6—O1 | 29.64 (15) | C13—O2—C11—C10 | 22.9 (2) |
C3—C5—C6—O1 | −33.09 (13) | C7—C1—C11—O2 | −177.80 (13) |
C4—C5—C6—C12 | −90.13 (16) | C2—C1—C11—O2 | −0.4 (2) |
C3—C5—C6—C12 | −152.87 (13) | C7—C1—C11—C10 | 0.9 (2) |
C4—C5—C6—C7 | 133.21 (13) | C2—C1—C11—C10 | 178.31 (14) |
C3—C5—C6—C7 | 70.48 (13) | C9—C10—C11—O2 | 176.62 (13) |
C11—C1—C7—C8 | 1.4 (2) | C9—C10—C11—C1 | −2.0 (2) |
C2—C1—C7—C8 | −176.63 (12) |
D—H···A | D—H | H···A | D···A | D—H···A |
C2—H2A···O2i | 1.00 | 2.44 | 3.2777 (17) | 141 |
Symmetry code: (i) −x, y, −z+1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
C2—H2A···O2i | 1.00 | 2.44 | 3.2777 (17) | 141 |
Symmetry code: (i) −x, y, −z+1/2. |
Experimental details
Crystal data | |
Chemical formula | C14H16O3 |
Mr | 232.27 |
Crystal system, space group | Monoclinic, C2/c |
Temperature (K) | 147 |
a, b, c (Å) | 16.401 (2), 6.9386 (9), 20.666 (2) |
β (°) | 97.089 (3) |
V (Å3) | 2333.8 (5) |
Z | 8 |
Radiation type | Mo Kα |
µ (mm−1) | 0.09 |
Crystal size (mm) | 0.33 × 0.23 × 0.06 |
Data collection | |
Diffractometer | Bruker Kappa APEX DUO CCD |
Absorption correction | Multi-scan (SADABS; Bruker, 2014) |
Tmin, Tmax | 0.703, 0.746 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 17093, 2681, 1972 |
Rint | 0.037 |
(sin θ/λ)max (Å−1) | 0.650 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.041, 0.104, 1.02 |
No. of reflections | 2681 |
No. of parameters | 157 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.27, −0.20 |
Computer programs: APEX2 (Bruker, 2014), SAINT (Bruker, 2014), SHELXT (Sheldrick, 2015a), SHELXL2014 (Sheldrick, 2015b), PLATON (Spek, 2009), publCIF (Westrip, 2010).
References
Bruker (2014). APEX2, SAINT and SADABS, Bruker AXS Inc., Madison, Wisconsin, USA. Google Scholar
Carlson, E., Duret, G., Blanchard, N. & Tam, W. (2016). Synth. Commun. 46, 55–62. Web of Science CrossRef CAS Google Scholar
Sheldrick, G. M. (2015a). Acta Cryst. A71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Sheldrick, G. M. (2015b). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Spek, A. L. (2009). Acta Cryst. D65, 148–155. Web of Science CrossRef CAS IUCr Journals Google Scholar
Westrip, S. P. (2010). J. Appl. Cryst. 43, 920–925. Web of Science CrossRef CAS IUCr Journals 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.