L-Prolinol as a chiral auxiliary in the photochemical synthesis of a new aryltetraline lignan analogue

Article abstract of DOI:10.1016/j.tetasy.2011.06.007

Prochiral (E,E)-fulgide 8 was reacted with l-prolinol to afford cyclic (M)-fulgamide 10 in a two step reaction sequence. The UV-light-driven photocyclization of 10 proceeded stereospecifically, giving lignan analogue 1 in 40% yield. The structure of 1 was

Full text of DOI:10.1016/j.tetasy.2011.06.007

Tetrahedron: Asymmetry 22 (2011) 1103–1107
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
L-Prolinol as a chiral auxiliary in the photochemical synthesis
of a new aryltetraline lignan analogue
Krzysztof K. Krawczyk a, Daria Madej a, Jan K. Maurin b,c, Zbigniew Czarnocki a,
⇑
a Faculty of Chemistry, University of Warsaw, Pasteura 1, Warsaw 02-093, Poland
b National Medicines Institute, Chełmska 30/34, Warsaw 00-725, Poland
c
´
Institute of Atomic Energy, Otwock-Swierk 05-400, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Prochiral (E,E)-fulgide 8 was reacted with
L
-prolinol to afford cyclic (M)-fulgamide 10 in a two step reac-
Received 27 April 2011
Accepted 6 June 2011
Available online 19 July 2011
tion sequence. The UV-light-driven photocyclization of 10 proceeded stereospecifically, giving lignan
analogue 1 in 40% yield. The structure of 1 was confirmed by X-ray crystallography.
Ó 2011 Elsevier Ltd. All rights reserved.
OH
1. Introduction
2
9
7
1
6
8
O
A
D
Within the group of aryltetraline lignans, many show a broad
scope of biological activity, of which the ability to intercalate
DNA and inhibit topoisomerase II is by far the most important.1,2
In particular, the biological activity of podophyllotoxin 2 has been
the subject of in-depth studies and numerous publications.3
In an excellent review, Castro et al.1 concluded that the most
crucial feature of the podophyllotoxin-type lignans to show antitu-
mour activity is the relative rigidity of the C ring (the structure and
customary numeration of the rings and atoms of podophyllotoxin
is given in Fig. 1). Also, the relative cis-stereochemistry of the sub-
stituents at C-70 and C-80 are of high importance for the bioactivity.
Podophyllotoxin and its derivatives can undergo irreversible aro-
matization of the C ring, to produce arylnaphthalene derivatives,
which do not exhibit significant activity. The bulkiness of the sub-
stituent at C-7 seems to not play an important role in metabolic
transformations and a number of differently C-7 substituted podo-
phyllotoxin-derived lignans show extensive biological activity. We
expected that the introduction of additional methyl groups onto
the C ring of podophyllotoxin-like lignans could be of interest, pri-
marily by the inhibition of an undesired aromatisation. Also, the
rigidity of the C ring would be enhanced, which could enhance
the bioactivity.1
B
C
O
9'
O
8'
4
1'
O
2'
6'
E
O
O
4'
O
Figure 1. Structure of podophyllotoxin 2.
skeleton.4–7 Other aryltetraline lignan syntheses utilising the
described approach yielded a racemate, until Charlton et al. used
(ꢀ)-ephedrine for the asymmetric synthesis of (+)-lyoniresinol di-
methyl ether.8
Inspired by this work, we chose another bidentate chiral adju-
vant L-prolinol, which is inexpensive and easily available from nat-
ural proline in enantiomerically pure form. The presence of the
secondary amine should be beneficial, since we found that for pri-
mary fulgamides, the formation of a five-membered fulgimide is
always more favoured than the formation of amide esters.
Herein, we report on the successful photocyclization of com-
pound 10 derived from a bisbenzylidenesuccinic acid and
ol to yield (1S,2S)-dihydro-1-arylnaphtalene 1 (Fig. 2).
L-prolin-
The ability of 2,3-dibenzylidene derivatives to undergo a con-
certed electrocyclic reaction followed by a 1,5-hydrogen shift to
yield 1,2-dehydro-4-aryltetralines, makes them interesting candi-
dates for the synthesis of cyclolignans. Some naphthalene, dihydro-
naphthalene, and tetrahydronaphthalene lignans were synthesized
using this approach, mainly those with an aromatic naphthalene
2. Results and discussion
Fulgide 8 was prepared according to an analogous procedure
found in the literature.9,10 The Stobbe condensation of diethylsuc-
cinate and 4-methoxyacetophenone yielded both the (E)- and the
(Z)-isomers of 3, which was in accordance with the literature re-
port.9 The mixture of (E)- and (Z)-3 was esterified with ethanol
in the presence of sulfuric acid and was used for the second Stobbe
⇑
Corresponding author. Tel.: +48 22 822 02 11; fax: +48 22 822 59 96.
E-mail address: czarnoz@chem.uw.edu.pl (Z. Czarnocki).
0957-4166/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2011.06.007

1104
K. K. Krawczyk et al. / Tetrahedron: Asymmetry 22 (2011) 1103–1107
O
4
11 O
1"
3
2
2"
3"
12 N
7
O
1
1'
4"
O
5"
3'
4'
O
Figure 2. Structure of compound 1.
condensation without separation of the isomers. The partial isom-
erization of the undesired (E,Z)-isomer in the presence of a strong
base appears to be the reason as to why (E,E)-5a was produced in a
quantity exceeding the expected yield for only (E)-3. After esterifi-
cation of the resulting acid ester, the diastereomeric succinates
could be easily separated by column chromatography on silica
gel to yield 60% of pure (E,E)-isomer 6a (Scheme 1). Saponification
of this product with lithium hydroxide yielded diacid 7, which can
be easily converted to the stable anhydride (fulgide) 8. For this
product we managed to grow a crystal suitable for X-ray analysis,
which confirmed the (E,E)-configuration of the benzylidene groups
Figure 3. The molecule of (E,E)-8. All non-hydrogen atoms are shown as 30%
probability ellipsoids.
stable atropoisomers (M)-10 and (P)-10, which were produced in
an approximately 19:1 ratio (Scheme 1). These isomers could be
separated by column chromatography and fully characterized.
Compound (M)-10 underwent a facile photocyclization when
irradiated with a medium pressure mercury lamp (kmax ꢁ365 nm).
During the reaction, the formation of a deep-red colored intermedi-
ate product 11 could be observed (Scheme 2). After column chroma-
tography of the reaction mixture, compound 1 crystallized while the
solvent was being evaporated. We recorded a very low value for the
(Fig. 3). Fulgide 8 reacted quantitatively with L-prolinol in the pres-
ence of triethylamine. The resulting acid amide 9 was converted
into 10 with a good yield (78%) by treatment with BOP in THF at
35 °C. All other attempts to close 9 to form the eight-membered
cyclic amide ester 10 were unsuccessful or gave an unsatisfactory
yield due to polymerization or decomposition (e.g., the use of DCC/
DMAP and CDI). The amide ester 10 can exist in the form of two
O
O
O
O
OR
O
O
O
OR
O
O
O
O
O
3a, 4a
EtO2C
O
5a, 5b R=H
tBuOK,
tBuOK, 4
toluene, rt
5a, 6a
+
toluene, rt
K2CO3, EtI,
O
CO2Et
O
DMSO/acetone,
O
reflux
OR
6a, 6b
R=Et
O
3b, 4b
O
O
O
OR
O
5b, 6b
O
3a, 3b
R=H
EtOH,
H2SO4
4a, 4b R=Et
O
O
O
OH
OH
O
O
AcCl
MeOH, LiOH,
reflux
O
6a
O
O
O
8
7
O
O
O
O
OH
HO
O
O
O
O
N
HO
HN
O
O
CH2Cl2
BOP, TEA,
THF, 35oC
O
+
O
N
O
O
9
8
(M)-10 + (P)-10
(~19:1)
Scheme 1. Synthesis of atropoisomers (M)- and (P)-10.

K. K. Krawczyk et al. / Tetrahedron: Asymmetry 22 (2011) 1103–1107
1105
O
O
O
O
O
N
O
N
[1,5]-H shift
hν
1
CHCl3 (anhydrous, Ar)
O
O
H
O
(M)-10
O
11
Scheme 2. Photocyclization of (M)-10.
specific rotation for compound 1 ð½a D20
¼ þ16Þ. In contrast, the value
ꢂ
measured for the (+)-lyoniresinol dimethyl ether synthesized by the
Charlton’s group was much higher ð½a D20
ꢂ
¼ þ476Þ.8
In the NMR spectrum of (M)-10 we observed an interesting phe-
nomenon associated with the restricted rotation of the aryl substi-
tuent at C-1 at 25 °C. Accidentally, room temperature was the
temperature of coalescence for the signals of the protons at C-20,
C-60, C-3,0 and C-50. The NMR spectra recorded at ꢀ55 °C showed
the presence of four different protons attached to the aryl group.
When the spectrum was measured at +50 °C, it showed almost a
free rotation. The signals of the protons at C-20 and C-60 started
to appear in the spectrum, while the protons at C-30 and C-50
formed a broad doublet.
The signalsof the methylgroups at C-1 and C-4 had different mul-
tiplicities; while one of them appeared as a singlet, the other formed
a doublet with J = 3 Hz. We first concluded that the cyclization must
have been followed by a 1,3-H shift to give 10, as such reactivity was
observed in our previous experiments with fulgides, fulgimides, and
fulgenates as already reported in the literature.8,11–13
To prove as to whether the obtained product had the structure 1
or 10, HSQC, HMBC, and ROESY measurements were employed.
From the HSQC and ROESY spectra it was concluded that the
methyl protons at C-4 appeared as a doublet, whereas the methyl
protons at C-1 formed a singlet peak. In HMBC, the methyl group at
C-1 showed coupling with the aromatic proton at C-9 and C-10 as
well as with C-1 and C-2, while the methyl protons at C-4 were
coupled only with aryl and vinyl protons, proving that the product
obtained possesses structure 1. We thus report on a rather rare
example of an NMR coupling of methyl protons attached to C-4
with a proton at C-2 through five bonds, of which only one is a dou-
ble bond.
Based on the analysis of the NMR data, we were unable to deter-
mine whether the cyclization occurred at the aryl ring attached to
the amide moiety or at the ester group. Charlton et al. reported that
the cyclization occurred regioselectively only at the aryl ring of the
amide. In our studies (ROESY), an NOE between the aryl proton at
C-20 and proton at C-200a was observed, while the methyl protons at
C-4 were coupled with one of the methylene protons at C-100a. This
suggested that the regioselectivity in photocyclization had been
opposite to that stated by Charlton et al.8
Figure 4. The molecule of 1. All non-hydrogen atoms are shown as 30% probability
ellipsoids.
3. Conclusion
In conclusion, we described the synthesis of compound 1 which
was obtained with the use of the photocyclization of a chiral atrop-
oisomer containing L-prolinol as a bidental chiral auxiliary. The
photocyclization has occurred exclusively at the aryl ring adjacent
to the ester functionality, resulting in the creation of two new
stereogenic centers of which one is a quaternary carbon. The sub-
stituents at C-3 and C-4 are in a relative cis position, as expected.
4. Experimental
4.1. General procedures
The NMR spectra were recorded on a Varian Unity Plus spec-
trometer operating at 500 MHz for 1H NMR and at 125 MHz for
13C NMR. The spectra were measured in CDCl3 and are given as d
values (in ppm) relative to TMS. Mass spectra were collected on
Quattro LC Micromass and LCT Micromass TOF HiRes apparatus.
Optical rotations were measured on Perkin–Elmer 241 polarimeter.
TLC analyses were performed on silica gel plates (Merck Silica Gel
60 F254) and visualized using UV-light. Column chromatography
was carried out at atmospheric pressure using Silica Gel 60
(230–400 mesh, Merck). Melting points were determined by a Boe-
tius hot-plate microscope and are uncorrected. Solvents used in
reactions were anhydrous. Toluene was dried by boiling with so-
dium metal, distilled, and kept over molecular sieves 3 Å. THF
The final proof for the structure of 1 was obtained by X-ray crys-
tallography which showed that, the cyclization had occurred
exclusively at the aryl group adjacent to the ester functionality
(Fig. 4) which is opposite to that obtained by Charlton et al. for
the (ꢀ)-ephedrine amide ester.8
The described results seem to be interesting, and while the pho-
tochemical cyclization of fulgenates can be found in the litera-
ture,6,11,12,14,15a,b there has been no report on the cyclization of
fulgenamides to date.
The presence of a double bond in compound 1 between C-3 and
C-4 carbon atoms, allows further diastereoselective transforma-
tions and work on this subject is currently underway.

1106
K. K. Krawczyk et al. / Tetrahedron: Asymmetry 22 (2011) 1103–1107
was dried with CaH2, and distilled under argon directly into the
reaction vessel. The single-crystal X-ray measurements were car-
4.5. (E,E)-2,3-Bis-(4-methoxy-a-phenylethylidene)succinic
anhydride 8
ried out on Oxford Diffraction Xcalibur R
j-axis diffractometer
with CCD Ruby detector. In all cases, Cu K characteristic radiation
a
Product 6a (7.24 g, 16.54 mmol) was dissolved in ethanol
(200 mL) and an excess of LiOH (3.97 g, 165.4 mmol, 10 equiv)
was added. The mixture was refluxed overnight, EtOH was evapo-
rated and the residue was dissolved in distilled water and ex-
tracted with ethyl acetate. The organic extracts were discarded
and the aqueous phase was acidified with concd HCl. Diacid 7
was extracted with ethyl acetate, dried over MgSO4, filtered, and
evaporated to give a dense residue, which was dissolved in acetyl
chloride and refluxed for 8 h. Volatile compounds were evaporated
in vacuo and the yellow residue was subjected to chromatography
on a short silica column to give (E,E)-anhydride 8 (4.09 g, 68%),
which crystallized from diethyl ether; mp 199–201 °C; 1H NMR d
6.73 (br d, J = 8 Hz, 4H), 6.55 (dd, J = 7.8, 1 Hz, 4H), 3.78 (s, 3H),
2.44 (s, 3H); 13C NMR d 164.19, 159.86, 154.96, 134.20, 128.70,
was applied. After initial corrections and data reduction, intensities
of reflections were used to solve and consecutively refine struc-
tures using SHELXS9716 and SHELXL9717 programs. Further
absorption corrections were applied in final steps of refinement.
BOP and all other reactants were purchased from Aldrich.
4.2. Ethyl (E)-(4-methoxy-a-phenylethylidene)succinate acid 3
A mixture of potassium tert-butoxide (7.66 g, 68 mmol) in
freshly distilled toluene was placed in a two necked round bot-
tomed flask. To this suspension, a mixture of diethyl succinate
(10 g, 57 mmol, 1 equiv) and 4-methoxyacetophenone (8.55 g,
57 mmol, 1 equiv) in anhydrous toluene was added dropwise
with vigorous stirring. The reaction proceeded for 1 h at room
temperature. Then most of the solvent was evaporated and the
residue was dissolved in distilled water and extracted with ethyl
acetate. The organic extracts were discarded, the water solution
was acidified with concd HCl, and extracted several times with
ethyl acetate. The organic phase was then washed with water
and brine and dried over anhydrous MgSO4. The solvent was
evaporated to give a mixture of acid esters 3a and 3b with a yield
of 94% (14.89 g).
118.47, 113.70, 55.35, 22.71; HRMS calcd for
C22H20O5Na
(M+Na)+ 387.1208. Found: 387.1203.
A monocrystal suitable for X-ray analysis was grown by slow
evaporation of the diethyl ether solution. The detailed structural
parameters have been deposited with the Cambridge Crystallo-
graphic Data Centre under the number CCDC 818576.
4.6. (E,E)-2,3-Bis-(4-methoxy-a-phenylethylidene)succinate
L-prolinol cyclic amide ester 10
4.3. Diethyl (E)-(4-methoxy-
a
-phenylethylidene)succinate 4
Fulgide 8 (103.74 mg, 0.285 mmol) was dissolved in 1 mL of
CH2Cl2 and -prolinol (30.22 mg, 0.299 mmol, 1.05 equiv) was
L
A mixture of 3a and 3b (14.89 g, 53.56 mmol) was dissolved in
anhydrous ethanol (125 mL) and after the addition of a catalytic
amount of H2SO4, it was refluxed for 10 h under argon. The mixture
was then cooled to room temperature and approximately 80% of
the solvent was evaporated. Ethyl acetate was added and the or-
ganic layer was extracted with 5% NaHCO3. The combined aqueous
extracts were washed again with ethyl acetate. The organic ex-
tracts were combined and dried over MgSO4. The residue after
evaporation of the solvent (13.60 g, 44 mmol) was used for the
next reaction without further purification.
added. The mixture immediately turned dark red. After the addi-
tion of TEA (43.18 mg, 0.428 mmol, 1.5 equiv), the solution lost
its color within minutes. Ethyl acetate (10 mL) was added and
the mixture was washed with 10% aqueous solution of citric acid.
The organic extracts were combined and dried over Na2SO4. The
product was filtered off and the solvent was evaporated in vacuo.
Compound 9 (132.5 mg, 0.285 mmol) was dissolved in freshly dis-
tilled THF and BOP (132.3 mg, 0.299 mmol, 1.05 equiv) was added.
The stirred mixture was cooled in an ice bath and TEA (43.18 mg,
0.428 mmol, 1.5 equiv) was added dropwise. The solution was al-
lowed to warm to room temperature and was slowly heated to
35 °C. After 1 h, the solvent was evaporated, the residue was dis-
solved in ethyl acetate and washed with 10% citric acid, 5% NaH-
CO3, water, and brine. The solvent was then evaporated and the
residue was purified by column chromatography using diethyl
ether–hexane (1:1 v/v)—diethyl ether to give products (P)-10 and
(M)-10. Compound (P)-10 eluted first and was collected in 3.9%
yield (4 mg), whereas compound (M)-10 was isolated in 83% yield
(85 mg).
4.4. Diethyl (E,E)-2,3-bis-(4-methoxy-a-phenylethylidene)
succinate 6
Potassium tert-butoxide (5.91 g, 52.8 mmol, 1.2 equiv) was
placed in a two necked round bottomed flask together with
200 mL of dry toluene. The reaction vessel was purged with argon
and the mixture of 4a and 4b together with 4-methoxyacetophe-
none (7.26 g, 48.4 mmol, 1.1 equiv) dissolved in 100 mL of dry
toluene were added through a dropping funnel over a period of
30 min. After 30 min the solvent was evaporated, the residue dis-
solved in distilled water, and extracted with ethyl acetate. The or-
ganic extracts were discarded and the aqueous phase was
acidified with concd HCl. After extraction with ethyl acetate, dry-
ing, and evaporation of the solvent, the resulting gum of 5a and
5b was dissolved in a 4:1 (v/v) mixture of acetone and DMSO.
An excess of K2CO3 (22.77 g, 0.165 mmol, 5 equiv) and EtI
(25.74 g, 0.165 mmol, 5 equiv) were added and the mixture was
refluxed for 15 h. Acetone and unreacted EtI were evaporated,
and the residue dissolved in distilled water and extracted with
ethyl acetate. The mixture was purified by flash column chroma-
tography with hexane–ethyl acetate (82:18) to give 7.24 g
(16.54 mmol) of (E,E)-bisbenzylidenesuccinate 6a (29% overall
yield from diethylsuccinate); 1H NMR d 7.21 (d, J = 8.6 Hz, 4H),
6.88 (d, J = 8.6 Hz, 4H), 3.96 (q, J = 7 Hz, 4H), 3.82 (s, 6H), 2.15
4.6.1. (M)-10
½
a 2D0
ꢂ
¼ þ42 (c 0.30, CHCl3); 1H NMR d 7.40–7.37 (m, 2H), 7.07–
7.04 (m, 2H), 6.87–6.84 (m, 2H), 6.80–6.77 (m, 2H), 4.36 (dd, J = 12,
8.5 Hz, 1H), 4.09 (dd, J = 12, 2 Hz, 2H), 3.82–3.3.78 (m, 1H), 3.77 (s,
3H), 3.75 (s, 3H), 3.49–3.44 (m, 1H), 2.22–2.15 (m, 1H), 2.13 (s, 3H),
1.92 (s, 3H), 1.91–1.87 (m, 2H), 1.75–1.69 (m, 1H); 13C NMR d
170.12, 169.71, 159.82, 159.37, 147.81, 139.26, 133.89, 133.19,
130.48, 129.10, 128.54, 128.28, 113.94, 113.72, 73.63, 60.76,
55.42, 55.37, 46.51, 29.92, 23.50, 22.42, 21.68; HRMS calcd for
C
27H29NO5Na (M+Na)+ 470.1943. Found: 470.1942.
4.6.2. (P)-10
½
a 2D0
ꢂ
¼ þ270 (c 0.40, CHCl3); 1H NMR d 7.262–7.243 (m, 2H),
7.237–7.215 (m, 2H), 6.883–6.859 (m, 2H), 6.859–6.835 (m, 2H),
4.37 (dd, J = 12.5, 6 Hz, 1H), 4.18 (dd, J = 12.5, 1.5 Hz, 1H), 3.96–
3.92 (m, 1H), 3.805 (s, 3H), 3.804 (s, 3H), 3.40–3.34 (m, 1H),
3.42–3.32 (m, 1H), 2.33 (s, 3H), 2.15 (s, 3H), 2.11–2.07 (m, 1H),
(s, 6H), 0.97 (t, J = 7 Hz, 6H); HRMS calcd for
C26H30O6Na
(M+Na)+ 461.2042. Found: 461.2039.

K. K. Krawczyk et al. / Tetrahedron: Asymmetry 22 (2011) 1103–1107
1107
1.79–1.72 (m, 1H), 1.69–1.58 (m, 2H); 13C NMR d 170.25, 168.75,
Acknowledgments
159.98, 159.40, 148.16, 142.52, 133.80, 133.46, 130.03, 128.49,
127.31, 114.15, 113.84, 74.26, 60.12, 55.50, 55.45, 46.44, 31.73,
23.61, 21.93, 21.16; HRMS calcd for
470.1943. Found: 470.1939.
The authors thank the Polish Ministry of Science and Higher
Education for financial support (Grant No N N204 030636). We
are also grateful to Dr Marcin Wilczek from the Faculty of Chemis-
try, University of Warsaw, for valuable consultation concerning
NMR peak assignment.
C
27H29NO5Na (M+Na)+
4.7. Aryltetraline lignan analogue 1
The product (M)-10 (65 mg, 0,145 mmol) was dissolved in
50 mL of dry and freshly distilled CHCl3 which was additionally
purged with argon for 30 min. The solution was placed in a quartz
cuvette and irradiated with a mercury lamp for 40 min. During this
time the temperature of the solution rose to 50 °C. The solution
turned deep red on irradiation due to the formation of the interme-
diate product 11, and faded immediately. The solvent was then
evaporated and the residue was subjected to column chromatogra-
phy using diethyl ether as an eluent. The product crystallized while
the solvent was being evaporated. Compound 1 was obtained in a
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yield of 40%; mp 207–212 °C; ½a D20
ꢂ
¼ þ16 (c 1.00, CHCl3); 1H NMR
(ꢀ55 °C) d 7.44 (dd, J = 8.5, 2.5 Hz, 1H {20}), 7.42 (d, J = 9 Hz, 1H
{5}), 6.88 (dd, J = 9, 2.5 Hz, 1H {6}), 6.84 (dd, J = 8.5, 2.5 Hz, 1H
{30}), 6.77 (d, J = 2.5 Hz, 1H {8}), 6.62 (dd, J = 8.5, 2.5 Hz, 1H {50}),
6.25 (dd, J = 8.5, 2.5 Hz, 1H {60}), 4.16 (dd, J = 12.5, 7.5 Hz, 1H
{13a}), 4.13 (q, J = 2.5 Hz, 1H {2}), 3.82 (s, 3H {7-OMe}), 3.79 (br
m, 1H {14a}), 3.77 (s, 3H {40-OMe}), 3.52–3.45 (m, 1H {13b}),
3.47–3.42 (m, 1H {17b}), 3.13–3.07 (m, 1H {17a}), 2.20 (s, 3H {1-
Me}), 2.17 (d, J = 2.5 Hz, 3H {4-Me}), 1.73–1.68 (m, 1H {16a}),
1.65–1.59 (br m, 1H {16b}), 1.12 (dd, J = 12.5, 6 Hz, 1H {15b}),
0.58–0.52 (m, 1H {15a}); HRMS calcd for C27H29NO5Na (M+Na)+
470.1943. Found: 470.1945
Crystals suitable for X-ray analysis were grown by slow evapo-
ration of diethyl ether solution. The detailed structural parameters
have been deposited with the Cambridge Crystallographic Data
Centre under the number CCDC 819367.
17. Sheldrick, G. M. SHELXL97. Program for crystal structure refinement; University
of Goettingen: Germany, 1997.