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ISSN: 2056-9890

Crystal structure of (R)-6-fluoro-2-[(S)-oxiran-2-yl]chroman

aUniversite de Bourgogne, ICMUB–UMR6302, 9 avenue Alain Savary, 21000 Dijon, France, and bCordenPharma-Synkem, 47 rue de Longvic, 21301 Chenove, France
*Correspondence e-mail: yoann.rousselin@u-bourgogne.fr

Edited by M. Gdaniec, Adam Mickiewicz University, Poland (Received 20 June 2015; accepted 30 June 2015; online 8 July 2015)

The title compound, C11H11FO2, is a building block in the synthesis of the active pharmaceutical ingredient DL-nebivolol. The synthesis starting from the enanti­omerically pure (R)-6-fluoro-4-oxo-3,4-di­hydro-2H-chromene-2-carb­oxy­lic acid resulted in a mixture of two stereoisomers, namely (R)-6-fluoro-2-[(S)-oxiran-2-yl]chroman and (R)-6-fluoro-2-[(R)-oxiran-2-yl]chroman. The mixture was separated by column chromatography but only one stereoisomer crystallized. The X-ray structure analysis revealed that the solid consisted of the R,S isomer. A similar procedure was repeated for (S)-6-fluoro-4-oxo-3,4-di­hydro-2H-chromene-2-carb­oxy­lic acid and, in this case, the S,R isomer was produced as a crystalline solid. Thus, all four stereoisomers of the title epoxide were obtained and their absolute configuration was assigned. The crystal studied was refined as an inversion twin.

1. Related literature

For the synthesis of the enanti­opure title product, see: Jas et al. (2011[Jas, G., Freifeld, I. & Kesseler, K. (2011). Patent WO 2011091968 (Corden PharmaChem GmbH).]). For pharmacological properties of nebivolol, see: Van Lommen et al. (1990[Van Lommen, G. R. E., De Bruyn, M. F. L. & Schroven, M. F. J. (1990). J. Pharm. Belg. 45, 355-360.]). For a study of related isomers, see: Horiguchi et al. (1997[Horiguchi, A., Kuge, Y. & Mochida, K. (1997). J. Mol. Catal. B Enzym. 3, 285-292.]). For the determination of absolute structure, see: Flack (2003[Flack, H. D. (2003). Helv. Chim. Acta, 86, 905-921.]).

[Scheme 1]

2. Experimental

2.1. Crystal data

  • C11H11FO2

  • Mr = 194.20

  • Monoclinic, P 21

  • a = 9.3742 (3) Å

  • b = 4.76845 (12) Å

  • c = 11.0212 (3) Å

  • β = 114.202 (4)°

  • V = 449.35 (3) Å3

  • Z = 2

  • Cu Kα radiation

  • μ = 0.94 mm−1

  • T = 100 K

  • 0.05 × 0.05 × 0.02 mm

2.2. Data collection

  • Agilent SuperNova Dual Source diffractometer with an Atlas detector

  • Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2012[Agilent (2012). CrysAlis PRO. Agilent Technologies Ltd, Yarnton, Oxfordshire, England.]) Tmin = 0.64, Tmax = 1

  • 14207 measured reflections

  • 1847 independent reflections

  • 1820 reflections with I > 2σ(I)

  • Rint = 0.050

2.3. Refinement

  • R[F2 > 2σ(F2)] = 0.032

  • wR(F2) = 0.092

  • S = 1.10

  • 1847 reflections

  • 128 parameters

  • 1 restraint

  • H-atom parameters constrained

  • Δρmax = 0.20 e Å−3

  • Δρmin = −0.16 e Å−3

  • Absolute structure: crystal refined as an inversion twin (Flack, 2003[Flack, H. D. (2003). Helv. Chim. Acta, 86, 905-921.])

  • Absolute structure parameter: 0.0 (2)

Data collection: CrysAlis PRO (Agilent, 2012[Agilent (2012). CrysAlis PRO. Agilent Technologies Ltd, Yarnton, Oxfordshire, England.]); cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015[Sheldrick, G. M. (2015). Acta Cryst. C71, 3-8.]); molecular graphics: OLEX2 (Dolomanov et al., 2009[Dolomanov, O. V., Bourhis, L. J., Gildea, R. J., Howard, J. A. K. & Puschmann, H. (2009). J. Appl. Cryst. 42, 339-341.]); software used to prepare material for publication: OLEX2.

Supporting information


Structural commentary top

6-Fluoro-2-(oxiran-2-yl)chroman (Fig. 1) is a building block in the synthesis of dl-nebivolol. This active pharmaceutical ingredient is a highly cardioselective vasodilatory β-receptor blocker used in treatment of hypertension.

The synthesis starts from enanti­opure 2-chloro-1-(6-fluoro-chroman-2-yl)-1-ethanol (Jas et al., 2011) that is transformed into enanti­opure 6-fluoro-4-oxo-3,4-di­hydro-2H-chromene-2-carb­oxy­lic acid. Formation of the epoxide from the carb­oxy­lic acid results in a new stereogenic center and a mixture of two stereoisomers is formed from each enanti­opure form: R,S and R,R from the R enanti­omer of the acid and S,R and S,S from the S enanti­omer. The mixtures of stereoisomers can be separated by column chromatography and all four stereoisomers in the reactions can be isolated in a pure form (Fig. 2). These epoxide inter­mediates can be further used in a ring-opening reaction with benzyl­amine to yield, after catalytic hydrogenation, nebivolol isomer with four chiral center.

Of the four stereisomers of the title compound only two form solids, and the remaining two are liquids under normal conditions. X-ray structural analysis from a single crystal of a solid stereoisomer obtained from (R)-6-fluoro-4-oxo-3,4-di­hydro-2H-chromene-2-carb­oxy­lic acid revealed that it has R,S configuration at the stereogenic centers (Fig. 1). Crystal data for this stereoisomer are reported in this paper. As expected, X-ray structural analysis of the solid epoxide obtained from the S enanti­omer of the acid revealed the S,R configuration at the stereogenic centers.

Synthesis and crystallization top

(R)-6-Fluoro-2-[(S)-oxiran-2-yl]chroman was prepared from enanti­opure 2-chloro-1-(6-fluoro­chroman-2-yl)-1-ethanol according to the procedure reported by Jas et al. (2011).

Crystals suitable for X-ray analysis were obtained in a fraction from column chromatography with heptane/ethyl acetate as eluent.

Refinement top

Crystal data, data collection and structure refinement details are summarized in Table 1. All C-bound H atoms were placed at calculated positions and refined as riding on their carriers with C—H = 1.00 Å (methine), 0.99 Å (methyl­ene) or 0.95 Å (aromatic) and with Uiso(H) = 1.2Ueq(C). TWIN/BASF refinement type in SHELXL-2014 (Sheldrick, 2015) was used to determine absolute configuration from anomalous scattering using the Flack method (Flack, 2003).

Related literature top

For the synthesis of the enantiopure title product, see: Jas et al. (2011). For pharmacological properties of nebivolol, see: Van Lommen et al. (1990). For a study of related isomers, see: Horiguchi et al. (1997). For the determination of absolute structure, see: Flack (2003).

Structure description top

6-Fluoro-2-(oxiran-2-yl)chroman (Fig. 1) is a building block in the synthesis of dl-nebivolol. This active pharmaceutical ingredient is a highly cardioselective vasodilatory β-receptor blocker used in treatment of hypertension.

The synthesis starts from enanti­opure 2-chloro-1-(6-fluoro-chroman-2-yl)-1-ethanol (Jas et al., 2011) that is transformed into enanti­opure 6-fluoro-4-oxo-3,4-di­hydro-2H-chromene-2-carb­oxy­lic acid. Formation of the epoxide from the carb­oxy­lic acid results in a new stereogenic center and a mixture of two stereoisomers is formed from each enanti­opure form: R,S and R,R from the R enanti­omer of the acid and S,R and S,S from the S enanti­omer. The mixtures of stereoisomers can be separated by column chromatography and all four stereoisomers in the reactions can be isolated in a pure form (Fig. 2). These epoxide inter­mediates can be further used in a ring-opening reaction with benzyl­amine to yield, after catalytic hydrogenation, nebivolol isomer with four chiral center.

Of the four stereisomers of the title compound only two form solids, and the remaining two are liquids under normal conditions. X-ray structural analysis from a single crystal of a solid stereoisomer obtained from (R)-6-fluoro-4-oxo-3,4-di­hydro-2H-chromene-2-carb­oxy­lic acid revealed that it has R,S configuration at the stereogenic centers (Fig. 1). Crystal data for this stereoisomer are reported in this paper. As expected, X-ray structural analysis of the solid epoxide obtained from the S enanti­omer of the acid revealed the S,R configuration at the stereogenic centers.

For the synthesis of the enantiopure title product, see: Jas et al. (2011). For pharmacological properties of nebivolol, see: Van Lommen et al. (1990). For a study of related isomers, see: Horiguchi et al. (1997). For the determination of absolute structure, see: Flack (2003).

Synthesis and crystallization top

(R)-6-Fluoro-2-[(S)-oxiran-2-yl]chroman was prepared from enanti­opure 2-chloro-1-(6-fluoro­chroman-2-yl)-1-ethanol according to the procedure reported by Jas et al. (2011).

Crystals suitable for X-ray analysis were obtained in a fraction from column chromatography with heptane/ethyl acetate as eluent.

Refinement details top

Crystal data, data collection and structure refinement details are summarized in Table 1. All C-bound H atoms were placed at calculated positions and refined as riding on their carriers with C—H = 1.00 Å (methine), 0.99 Å (methyl­ene) or 0.95 Å (aromatic) and with Uiso(H) = 1.2Ueq(C). TWIN/BASF refinement type in SHELXL-2014 (Sheldrick, 2015) was used to determine absolute configuration from anomalous scattering using the Flack method (Flack, 2003).

Computing details top

Data collection: CrysAlis PRO (Agilent, 2012); cell refinement: CrysAlis PRO (Agilent, 2012); data reduction: CrysAlis PRO (Agilent, 2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); 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).

Figures top
[Figure 1] Fig. 1. View of the molecular structure of the title compound with 50% probability displacement ellipsoids for the non-hydrogen atoms.
[Figure 2] Fig. 2. Reaction scheme for the synthesis of nebivolol
(R)-6-Fluoro-2-[(S)-oxiran-2-yl]chroman top
Crystal data top
C11H11FO2F(000) = 204
Mr = 194.20Dx = 1.435 Mg m3
Monoclinic, P21Cu Kα radiation, λ = 1.54184 Å
a = 9.3742 (3) ÅCell parameters from 7897 reflections
b = 4.76845 (12) Åθ = 4.4–76.3°
c = 11.0212 (3) ŵ = 0.94 mm1
β = 114.202 (4)°T = 100 K
V = 449.35 (3) Å3Needle, colourless
Z = 20.05 × 0.05 × 0.02 mm
Data collection top
Agilent SuperNova Dual Source
diffractometer with an Atlas detector
1847 independent reflections
Radiation source: SuperNova (Cu) X-ray Source1820 reflections with I > 2σ(I)
Mirror monochromatorRint = 0.050
Detector resolution: 5.2474 pixels mm-1θmax = 76.5°, θmin = 4.4°
CCD rotation images, thick slices scansh = 1111
Absorption correction: multi-scan
(CrysAlis PRO; Agilent, 2012)
k = 55
Tmin = 0.64, Tmax = 1l = 1313
14207 measured reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.032H-atom parameters constrained
wR(F2) = 0.092 w = 1/[σ2(Fo2) + (0.0529P)2 + 0.0952P]
where P = (Fo2 + 2Fc2)/3
S = 1.10(Δ/σ)max < 0.001
1847 reflectionsΔρmax = 0.20 e Å3
128 parametersΔρmin = 0.16 e Å3
1 restraintAbsolute structure: crystal refined as an inversion twin (Flack, 2003)
0 constraintsAbsolute structure parameter: 0.0 (2)
Crystal data top
C11H11FO2V = 449.35 (3) Å3
Mr = 194.20Z = 2
Monoclinic, P21Cu Kα radiation
a = 9.3742 (3) ŵ = 0.94 mm1
b = 4.76845 (12) ÅT = 100 K
c = 11.0212 (3) Å0.05 × 0.05 × 0.02 mm
β = 114.202 (4)°
Data collection top
Agilent SuperNova Dual Source
diffractometer with an Atlas detector
1847 independent reflections
Absorption correction: multi-scan
(CrysAlis PRO; Agilent, 2012)
1820 reflections with I > 2σ(I)
Tmin = 0.64, Tmax = 1Rint = 0.050
14207 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.032H-atom parameters constrained
wR(F2) = 0.092Δρmax = 0.20 e Å3
S = 1.10Δρmin = 0.16 e Å3
1847 reflectionsAbsolute structure: crystal refined as an inversion twin (Flack, 2003)
128 parametersAbsolute structure parameter: 0.0 (2)
1 restraint
Special details top

Experimental. Analyzes were recorded in the "Pole Chimie Moleculaire", the technological platform for chemical analysis and molecular synthesis (http://www.wpcm.fr) which relies on the Institute of the Molecular Chemistry of University of Burgundy and Welience"TM", a Burgundy University private subsidiary.

1H and 13C NMR measurements were performed in deuterated methanol on Bruker Avance III, recorded at 500 MHz and 125 MHz, respectively. Chemical shifts (δ) and coupling constants are reported respectively in p.p.m. and hertz (Hz). The optical rotation was measured using a UV Visible Perkin Elmer Lambda 12, polarimeter at 589 nm. High-resolution mass spectrometry (HRMS) was performed in ESI a positive mode. The infrared spectrum (IR) was generated by ATR using a Spectrometer Infrared Avatar 370. A scan range of 4000 - 400 cm-1 was used.

(2S,2R)-(6-fluoro-2-chromanyl)oxirane:

δ(1H, DMSO-d6, 300 MHz, ppm): 1.72 (1H, m); 2.00 (1H, m); 2.74 (2H, m); 2.82 (2H, m); 3.17 (1H, ddd); 3.94 (1H, ddd); 6.77 (1H, dd); 6.92 (2H, m).

δ(13C DMSO-d6, 75.47MHz, ppm): 23.4, 23.7, 44.5, 52.3, 75.3, 113.7 (d, 23.2 Hz), 115.2 (d, 22.3 Hz), 117.2 (d, 8.2 Hz), 123.5(d, 7.5 Hz), 150.1 (d, 1.5 Hz), 156.0(d, 234.8 Hz).

[α]29D +67.5o (c =1.0 in CHCl3)

HRMS (ESI) for C18H21FNO2[M+H]+ m/z = 195.08158, found m/z = 195.08120.

IR (cm-1) 3050, 3001, 2951, 1492, 1220

Data CCDC-1407326 contains the enantiomer structure of (S)-6-Fluoro-2-[(R)-oxiran-2-yl]chroman. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif

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.

Refinement. Refined as a 2-component inversion twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
C10.7376 (2)0.5622 (5)0.50701 (19)0.0255 (4)
C60.8296 (2)0.4711 (5)0.4448 (2)0.0254 (4)
H60.90950.33660.48700.030*
C50.8061 (2)0.5754 (4)0.32005 (19)0.0226 (4)
C40.6872 (2)0.7732 (4)0.26151 (18)0.0214 (4)
C30.5962 (2)0.8655 (5)0.3267 (2)0.0255 (4)
H30.51651.00090.28570.031*
C20.6216 (2)0.7604 (5)0.4512 (2)0.0261 (4)
H20.56080.82310.49680.031*
C70.9061 (2)0.4801 (4)0.24929 (19)0.0253 (4)
H7A0.87590.28680.21570.030*
H7B1.01720.47700.31320.030*
C80.8876 (2)0.6721 (5)0.13380 (19)0.0240 (4)
H8A0.93060.58030.07550.029*
H8B0.94580.84880.16780.029*
C90.7148 (2)0.7350 (5)0.05528 (19)0.0235 (4)
H90.65680.55390.02670.028*
C100.6765 (2)0.9185 (5)0.0645 (2)0.0271 (4)
H100.56510.98140.10820.033*
C110.7573 (2)0.8967 (5)0.1530 (2)0.0286 (5)
H11A0.83770.74870.13400.034*
H11B0.69680.93840.24850.034*
O10.65326 (15)0.8871 (3)0.13795 (13)0.0246 (3)
O20.79031 (19)1.1254 (4)0.06003 (15)0.0329 (4)
F10.76157 (14)0.4549 (3)0.62882 (12)0.0333 (3)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
C10.0249 (9)0.0259 (10)0.0236 (9)0.0037 (8)0.0079 (7)0.0017 (8)
C60.0206 (8)0.0216 (10)0.0299 (10)0.0010 (7)0.0064 (7)0.0003 (8)
C50.0191 (8)0.0202 (10)0.0261 (9)0.0036 (8)0.0070 (7)0.0049 (8)
C40.0185 (8)0.0201 (10)0.0238 (9)0.0035 (8)0.0070 (7)0.0029 (8)
C30.0214 (9)0.0265 (11)0.0276 (9)0.0035 (7)0.0090 (7)0.0011 (8)
C20.0245 (9)0.0274 (11)0.0282 (10)0.0013 (8)0.0128 (8)0.0051 (8)
C70.0224 (8)0.0220 (10)0.0303 (9)0.0040 (8)0.0097 (8)0.0013 (8)
C80.0180 (8)0.0252 (10)0.0288 (9)0.0010 (7)0.0096 (7)0.0042 (8)
C90.0201 (8)0.0252 (11)0.0266 (9)0.0009 (7)0.0109 (7)0.0042 (8)
C100.0225 (9)0.0309 (11)0.0273 (9)0.0016 (8)0.0096 (7)0.0015 (9)
C110.0272 (9)0.0315 (11)0.0286 (9)0.0033 (9)0.0130 (8)0.0049 (9)
O10.0224 (6)0.0275 (8)0.0247 (6)0.0066 (6)0.0106 (5)0.0019 (6)
O20.0442 (9)0.0260 (9)0.0323 (7)0.0047 (7)0.0195 (6)0.0041 (6)
F10.0343 (6)0.0384 (8)0.0268 (6)0.0030 (6)0.0121 (5)0.0062 (6)
Geometric parameters (Å, º) top
C1—C61.374 (3)C7—C81.519 (3)
C1—C21.381 (3)C8—H8A0.9900
C1—F11.366 (2)C8—H8B0.9900
C6—H60.9500C8—C91.521 (2)
C6—C51.392 (3)C9—H91.0000
C5—C41.399 (3)C9—C101.500 (3)
C5—C71.514 (2)C9—O11.456 (2)
C4—C31.393 (3)C10—H101.0000
C4—O11.377 (2)C10—C111.463 (3)
C3—H30.9500C10—O21.439 (3)
C3—C21.387 (3)C11—H11A0.9900
C2—H20.9500C11—H11B0.9900
C7—H7A0.9900C11—O21.440 (3)
C7—H7B0.9900
C6—C1—C2122.35 (18)C7—C8—H8B109.9
F1—C1—C6119.21 (18)C7—C8—C9108.92 (16)
F1—C1—C2118.43 (17)H8A—C8—H8B108.3
C1—C6—H6119.9C9—C8—H8A109.9
C1—C6—C5120.12 (19)C9—C8—H8B109.9
C5—C6—H6119.9C8—C9—H9108.9
C6—C5—C4118.06 (17)C10—C9—C8115.48 (16)
C6—C5—C7121.35 (18)C10—C9—H9108.9
C4—C5—C7120.59 (17)O1—C9—C8110.18 (15)
C3—C4—C5121.04 (18)O1—C9—H9108.9
O1—C4—C5122.66 (16)O1—C9—C10104.31 (16)
O1—C4—C3116.30 (17)C9—C10—H10115.1
C4—C3—H3119.9C11—C10—C9122.88 (19)
C2—C3—C4120.19 (19)C11—C10—H10115.1
C2—C3—H3119.9O2—C10—C9117.62 (16)
C1—C2—C3118.22 (17)O2—C10—H10115.1
C1—C2—H2120.9O2—C10—C1159.51 (13)
C3—C2—H2120.9C10—C11—H11A117.8
C5—C7—H7A109.3C10—C11—H11B117.8
C5—C7—H7B109.3H11A—C11—H11B115.0
C5—C7—C8111.62 (16)O2—C11—C1059.42 (13)
H7A—C7—H7B108.0O2—C11—H11A117.8
C8—C7—H7A109.3O2—C11—H11B117.8
C8—C7—H7B109.3C4—O1—C9115.60 (15)
C7—C8—H8A109.9C10—O2—C1161.08 (13)
C1—C6—C5—C40.0 (3)C7—C5—C4—O11.0 (3)
C1—C6—C5—C7179.71 (17)C7—C8—C9—C10178.77 (17)
C6—C1—C2—C31.3 (3)C7—C8—C9—O163.4 (2)
C6—C5—C4—C30.7 (3)C8—C9—C10—C1138.9 (3)
C6—C5—C4—O1179.29 (18)C8—C9—C10—O231.0 (3)
C6—C5—C7—C8165.51 (18)C8—C9—O1—C449.5 (2)
C5—C4—C3—C20.4 (3)C9—C10—C11—O2105.1 (2)
C5—C4—O1—C917.2 (2)C9—C10—O2—C11113.8 (2)
C5—C7—C8—C944.7 (2)C10—C9—O1—C4174.00 (14)
C4—C5—C7—C814.2 (2)O1—C4—C3—C2179.56 (17)
C4—C3—C2—C10.5 (3)O1—C9—C10—C11159.93 (19)
C3—C4—O1—C9162.78 (17)O1—C9—C10—O290.0 (2)
C2—C1—C6—C51.0 (3)F1—C1—C6—C5179.30 (18)
C7—C5—C4—C3178.99 (18)F1—C1—C2—C3179.04 (19)

Experimental details

Crystal data
Chemical formulaC11H11FO2
Mr194.20
Crystal system, space groupMonoclinic, P21
Temperature (K)100
a, b, c (Å)9.3742 (3), 4.76845 (12), 11.0212 (3)
β (°) 114.202 (4)
V3)449.35 (3)
Z2
Radiation typeCu Kα
µ (mm1)0.94
Crystal size (mm)0.05 × 0.05 × 0.02
Data collection
DiffractometerAgilent SuperNova Dual Source
diffractometer with an Atlas detector
Absorption correctionMulti-scan
(CrysAlis PRO; Agilent, 2012)
Tmin, Tmax0.64, 1
No. of measured, independent and
observed [I > 2σ(I)] reflections
14207, 1847, 1820
Rint0.050
(sin θ/λ)max1)0.631
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.032, 0.092, 1.10
No. of reflections1847
No. of parameters128
No. of restraints1
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.20, 0.16
Absolute structureCrystal refined as an inversion twin (Flack, 2003)
Absolute structure parameter0.0 (2)

Computer programs: CrysAlis PRO (Agilent, 2012), SHELXS97 (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015), OLEX2 (Dolomanov et al., 2009).

 

Acknowledgements

We thank Ms Marie-Jose Penouilh for the NMR spectra and the ESI mass spectra.

References

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