New bisphosphine ligands for palladium-catalyzed desymmetrization of meso-cyclopent-2-en-1,4-diol biscarbamate

Article abstract of DOI:10.1055/s-2005-871963

A new class of bisphosphine ligands (5, 6) with a cyclobutane backbone have been designed and synthesized on the basis of a privileged C₂ scaffold of head-to-head coumarin dimer, among which ligand 5a was found to show excellent activity and en

Full text of DOI:10.1055/s-2005-871963

LETTER
2067
New Bisphosphine Ligands for Palladium-Catalyzed Desymmetrization of
meso-Cyclopent-2-en-1,4-diol Biscarbamate
New
C
hiral Bispho
o
sphines for
De
symmetriz
g
ationof
B
isc
b
arbamate o Zhao, Zheng Wang, Kuiling Ding*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Road, Shanghai 200032, P. R. China
Fax +86(21)64166128; E-mail: kding@mail.sioc.ac.cn
Received 8 June 2005
on the synthesis of a new class of bisphosphine ligands (5,
6) having cyclobutane backbone, which can be considered
as the structural analogues of Trost’s ligand 4.5 Among
the new ligands developed in this work, ligand 5a was
Abstract: A new class of bisphosphine ligands (5, 6) with a cyclo-
butane backbone have been designed and synthesized on the basis
of a privileged C2 scaffold of head-to-head coumarin dimer, among
which ligand 5a was found to show excellent activity and enantiose-
lectivity in Pd-catalyzed desymmetrization of the biscarbamate of found to show excellent activity and enantioselectivity in
meso-cyclopent-2-en-1,4-diol, affording oxazolidin-2-one in up to
90% yield with 97% ee.
Pd-catalyzed desymmetrization of the biscarbamate of
meso-cyclopent-2-en-1,4-diol, affording oxazolidin-2-
Key words: C2-symmetric, bisphosphine, palladium, desymmetri-
zation, meso-diols
one in up to 90% yield with 97% ee.
The key intermediates for the syntheses of bisphosphine
ligands 5a and 5b, chiral diamines 11a and 11b, were
prepared with an overall yield of 70% and 68%, respec-
tively, by following the reaction sequences as shown in
Scheme 1. As we described previously, the enantiopure
anti-head-to-head coumarin dimer (–)-1 could be easily
obtained in large scale by optical resolution of the corre-
sponding racemic coumarin dimer through molecular
complexation with TADDOL.3,4 One of the advantages of
this coumarin dimer was its high functionality. Lactone-
opening reaction of (–)-1 with Me2SO4 afforded the corre-
sponding diesters (7) in excellent yields.6 Hydrolysis of 7
followed by treatment with thionyl chloride7 and
subsequent condensation with anhydrous ammonia gas in
chloroform gave the diamide 9, which was converted
through 10 to the target diamine 11a by Hoffmann
rearrangement using phenyl iodosylacetate [PhI(OAc)2]
The enantiopure anti head-to-head coumarin dimer (–)-11
is a highly strained but readily accessible compound
which, upon lactone-opening reactions, can be easily
transformed into various C2 symmetric cyclobutane-con-
taining bisphosphine ligands, such as 22 and 3.3,4 Despite
the facts of excellent asymmetric induction of ligand 3
and its PEG-supported analogues in Pd-catalyzed asym-
metric allylic substitutions of acyclic substrate,4 ligand 3
exhibited poor enantioselective control of palladium-cat-
alyzed desymmetrization of meso-cyclopent-2-ene-1,4-
diol derivative (25% ee). As an extension of our research
to the development of new bisphosphine ligands for asym-
metric catalysis on the basis of privileged C2 scaffold of
1,3,4 in the present letter, we report our preliminary results
O
O
ROOC
Ar2P
COOR
O
O
OMe
Ph2P
OMe
PPh2
PAr2
1
3
2
PPh2 Ph2P
PPh2
Ph2P
H
O
O
NH
HN
H
N
NH HN
N
n
n
O
O
O
O
PPh2 Ph2P
4
MeO
RO
OMe
OR
5a: n = 0
5b: n = 1
6a: R = Me
6b: R = Bn
Figure 1
SYNLETT 2005, No. 13, pp 2067–2071
1
9
.0
8
.2
0
0
5
Advanced online publication: 12.07.2005
DOI: 10.1055/s-2005-871963; Art ID: U17505ST
© Georg Thieme Verlag Stuttgart · New York

2068
D. Zhao et al.
LETTER
O
O
O
XOC
COX
MeO2C
MeO
CO2Me
O
a
b
OMe
MeO
OMe
8a: X = OH
8b: X = Cl
c
(–)-1
7
H2N
NH2
H2NOC
CONH2
MeO2CHN
NHCO2Me
d
e
f
MeO
MeO
R'O
MeO
OMe
OMe
OMe
11a
10
9
OR'
N3
N3
H2N
NH2
j
g
i
7
MeO
12a: R' = H
OMe
MeO
OMe
MeO
11b
OMe
h
13
12b: R' = Ms
Scheme 1 Synthesis of chiral 1,2-diamine 11a and chiral 1,4-diamine 11b. Reagents and conditions: a) Me2SO4, aq KOH, acetone, >95%; b)
MeOH, aq KOH, reflux, then 2 N HCl, >95%; c) SOCl2, benzene, reflux; d) CHCl3, NH3 (g), r.t., >99%; e) KOH, 2 equiv PhI(OAc)2, MeOH,
0 °C, reflux, 83.3%; f) aq KOH, MeOH, reflux, 48h, >95%; g) 8 equiv LiAlH4, Et2O, 95%; h) 4 equiv MsCl, Et3N, CH2Cl2, 90%; i) 10 equiv
NaN3, DMF, reflux, 98%; j) Pd/C, H2 (1 atm), 86%.
in alkaline methanol. On the other hand, synthesis of
PPh2
NH
Ph2P
chiral 1,4-diamine 11b, a more flexible counterpart of
11a, was accomplished via a four-step reaction procedure
starting from 7 (Scheme 1). Reduction of the ester func-
tionalities of 7 with lithium aluminum hydride gave diol
12a, which was mesylated on the hydroxyl groups to yield
the corresponding dimesylate 12b. Nucleophilic substitu-
tion of 12b with sodium azide, followed by Pd/C catalytic
hydrogenation at ambient pressure, afforded the desired
chiral 1,4-diamine 11b.
NH2
NH2
n
n
NH
n
n
O
O
a
MeO
OMe
MeO
OMe
11a: n = 0
11b: n = 1
5a: n = 0; 50%
5b: n = 1; 78%
PPh2 Ph2P
With the chiral diamines 11a,b in hand, the transforma-
tion to the target chiral biphosphine ligands 5a,b was quite
straightforward. Condensation of 11a,b with 2-(diphe-
nylphosphino)benzoic acid (DPPBA)8 in the presence of
DCC and a catalytic amount of DMAP proceeded smooth-
ly affording bisphosphines 5a9 and 5b10 in 50% and 78%
yields, respectively (Scheme 2).
NH
HN
COCl
ClOC
b
O
O
R'O
OR'
RO
OR
8b: R' = Me
8c: R' = Bn
6a: R' = Me; 95%
6b: R' = Bn; 80%
To investigate the possible influences of different amide
linkage on the asymmetric induction capability of this
type of bisphosphine ligands, two other chiral bisphos-
phine ligands 6a,b, which have similar trans-cyclobutane
backbones as that of 5a or 5b but with inversed amide
linkages, were subsequently prepared. By following
Trost’s procedure,11 condensation of the corresponding
dichlorides 8b or 8c (prepared by following the similar
procedure for the preparation of 8b) with 2-(diphe-
nylphosphino)aniline (DPPA)12 (Scheme 2) afforded 6a13
and 6b14 as white solids in 95% and 80% yield,
respectively.
Scheme 2 Synthesis of bisphosphine ligands 5a,b and 6a,b.
Reagents and conditions: a) 2.4 equiv DPPBA, 3 equiv DCC, DMAP,
CH2Cl2, r.t.; b) 3 equiv DPPA, CH2Cl2, r.t.
The reaction is formally an intramolecular allylic substi-
tution reaction of a meso-substrate with two enantiotopic
leaving groups.5a Owing to its mechanically well-defined
nature, the reaction was also often adopted as a standard
test reaction for chiral ligands designed for use in asym-
metric allylic substitution reactions.5,16 Given these facts,
we moved on to examine the enantiodiscrimination abili-
ties of our chiral bisphosphines 5a,b and 6a,b in palladi-
um-catalyzed desymmetrization of meso-cyclopent-2-en-
1,4-diol bis(carbamate) 15,17 and the results obtained
were summarized in Table 1.
Palladium-catalyzed desymmetrization of meso-alkene-
diols or their derivatives (Scheme 3) was developed by
Trost et al.5,11 and has been successfully used in the syn-
thesis of a number of biologically significant products.15
Synlett 2005, No. 13, 2067–2071 © Thieme Stuttgart · New York

LETTER
New Chiral Bisphosphines for Desymmetrization of Biscarbamate
2069
OCONHTs
conformations) competing with each other,5a thus leading
to the significantly poorer enantiocontrol performance. It
is interesting to note that for the ‘invertomer’ ligands
6a,b, the reaction proceeded in high yields and moderate
ee values, but the sense of product configuration was op-
posite to that obtained by the ‘normal’ ligand 5a (entries
4, 5 vs. entries 1, 2). This phenomenon has also been re-
ported for ligand 4 and its analogue in the literature,18 in-
dicating that the asymmetric induction capabilities of
[Pd(η3-C3H5)Cl]2
OH
O
TsN=C=O
THF
(2.5 mol%)
O
N
ligand (5 or 6 mol%)
1 equiv Et3N, THF
Ts
OCONHTs
OH
14
15
16
Scheme 3 Palladium-catalyzed desymmetrization of meso-alkene-
diols or their derivatives.
The reaction was initially conducted in a one-pot fashion, these ligands are highly sensitive to the subtle changes in
i.e., using the in situ generated bis(carbamate) 15 (from the ligand structures. Finally, the temperature effect on the
equal molar amounts of the corresponding meso-diol and enantioselectivity of this reaction was examined using
tosyl isocyanate) directly in the desymmetrization reac- ligands 5a and 6a (entries 6–11). In both series, the reac-
tion with the palladium catalyst of ligand 5a (entry 1). It tion rate decreases with the lowering of the temperature,
was found that ligand 5a showed remarkably high activity accompanied by an enhancement of the enantioselectivity
and very good enantioselectivity in the titled reaction, af- (entries 2 vs. 6–8 or entries 4, 9–11). When the reaction
fording the oxazolidinone 16 in 61% isolated yield and was conducted at –60 °C, high enantioselectivity up to
89% ee within two hours at 0 °C. Encouraged by this re- 97% ee was achieved with ligand 5a (entry 8), whereas
sult, we turned to studying the reactions with isolated bis- the ‘invertomer’ ligand 6a gave the allylic product 16 in
carbamate 15 as substrate. Under the otherwise identical 78% ee with an opposite configuration (entry 11).
conditions, the isolated yield of the reaction using pure
biscarbamate 15 was much higher (quantitative) than that
In summary, new C2-symmetric chiral bisphosphine
ligands with different skeleton flexibilities (5a vs. 5b) or
of the one-pot reaction using in situ prepared 15, albeit the
different amide linkages (6a,b vs. 5a,b) were synthesized
enantioselectivity of the product 16 remained at the same
starting from enantiopure anti-head-to-head coumarin
dimer (–)-1. These Trost-type ligands were tested as chiral
inducers in Pd-catalyzed desymmetrization of meso-cy-
clopent-2-en-1,4-diol biscarbamate (15), affording the al-
level (entry 2 vs. entry 1). Remarkably, the flexible ligand
5b exhibited a drastically lowered enantiodiscriminating
capability for this reaction, produced almost racemic
product in 81% yield under the otherwise identical condi-
lylic substitution product oxazolidinone 16 in excellent
tions (entry 3). This can be rationalized by the fact that
yields and various degrees of enantioselectivities (up to
ligand 5b is considerably more flexible than 5a owing to
97% ee with ligand 5a). The enantiodiscrimination capa-
the presence of extra methylene moieties, which should in
bilities of this type of ligands were found to be highly sen-
principle give rise to more transition state structures (or
sitive to the subtle changes in the ligand structures, with
Table 1 Pd-Catalyzed Enantioselective Intramolecular Cyclization of meso-Biscarbamate 15 Using Trost-Type Bisphosphine Ligands 5 and 6a
Entry
1b
2
Ligand
5a
Temp (°C)
Time (h)
Yield (%)c
61
ee (%)d
89
Confige
3R,4S
3R,4S
3S,4R
3S,4R
3S,4R
3R,4S
3R,4S
3R,4S
3S,4R
3S,4R
3S,4R
0
0
2
2
5a
>99
81
88
3
5b
0
2
8
4
6a
0
2
>99
>99
>99
98
63
5
6b
0
2
35
6
5a
–15
–30
–60
–15
–30
–60
12
12
18
12
12
18
93
7
5a
94
8
5a
90
97
9
6a
92
78
10
11
6a
>99
89
75
6a
78
a Molar ratio of 15:Et3N:[p-allylPdCl]2:ligand = 1:1:0.025:0.06; ligands 5, 6 used were all of S,S,S,S configuration.
b One-pot reaction.
c Isolated yield.
d The ee values were determined by HPLC on a Chiralcel AD column.
e The absolute configurations of 16 were assigned to be 3R,4S and 3S,4R, respectively, based on their optical rotations.
Synlett 2005, No. 13, 2067–2071 © Thieme Stuttgart · New York

2070
D. Zhao et al.
LETTER
(10) Synthesis of (S,S,S,S)-5b.
the rigid 5a giving considerably superior performance to
its flexible counterpart 5b. This fact suggests that a broad
space of structural modifications is still possible for fur-
ther ligand optimizations. Extension of the substrate scope
in this reaction and utilization of the present catalyst sys-
tem in other types of asymmetric reactions are currently in
progress in this laboratory.
Following the same procedure for the preparation of
(S,S,S,S)-5a, reaction of (S,S,S,S)-11b with 2-
(diphenylphosphino)benzoic acid afforded (S,S,S,S)-5b as a
white solid (yield 78%). [a]D20 –32.2 (c 1.04, CHCl3). IR
(KBr): n = 2929, 1627, 1584, 1531, 1492, 1461, 1435, 1245,
1026, 747, 696, 545 cm–1. 1H NMR (300 MHz, CDCl3): d =
2.69–2.73 (m, 2 H), 2.98–3.07 (m, 2 H), 3.25–3.33 (m, 2 H),
3.83 (s, 6 H), 4.30 (d, J = 6.6 Hz, 2 H), 6.29 (br, 2 H), 6.78–
7.44 (m, 36 H). 31P NMR (121.46 MHz, CDCl3): d = –8.86.
13C NMR (75 MHz, CDCl3): d = 169.50, 157.65, 141.95,
141.61, 137.10, 137.00, 136.30, 136.04, 134.29, 134.23,
134.18, 134.01, 133.90, 130.34, 129.21, 129.05, 128.95,
128.84, 128.79, 128.34, 127.96, 127.92, 127.68, 120.90,
109.82, 55.49, 41.82, 39.52, 36.73. MS (ESI): m/z (%) =
903.45 (100) [M+ + 1]. HRMS (FT): m/z calcd for
Acknowledgment
Financial support from the National Natural Science Foundation of
China, the Chinese Academy of Sciences, the Major Basic Research
Development Program of China (Grant no. G2000077506), and the
Ministry of Science and Technology of Commission of Shanghai
Municipality is gratefully acknowledged.
C58H52O4N2NaP2 [M+ + Na]: 925.3295; found: 925.3277.
(11) For early examples, see: (a) Trost, B. M.; Van Vranken, D.
L.; Bingel, C. J. Am. Chem. Soc. 1992, 114, 9327. (b)Mori,
M.; Nukui, S.; Shibasaki, M. Chem. Lett. 1991, 1797.
(c) Voshizaki, H.; Satoh, H.; Sato, Y.; Nukui, S.; Shibasaki,
M. J. Org. Chem. 1995, 60, 2016. (d) Yoshizaki, H.;
Yoshioka, K.; Sato, Y.; Mori, M. Tetrahedron 1997, 53,
5433.
References
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(5) For a comprehensive review, see: (a) Trost, B. M.; van
Vranken, D. L. Chem. Rev. 1996, 96, 395. (b) For recent
example, see: Trost, B. M.; Patterson, D. E. J. Org. Chem.
1998, 63, 1339.
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Synth. 1982, 21, 178.
(9) Synthesis of (S,S,S,S)-5a.
(12) Copper, M. K.; Downes, J. M.; Duckworth, P. A.; Kerby, M.
C.; Powell, R. J.; Soucek, M. D. Inorg. Synth. 1988, 27, 129.
(13) Synthesis of (S,S,S,S)-6a.
Anhyd CHCl3 (5 mL) was added to the freshly prepared
dichloride 8b (0.2 mmol) and 2-(diphenylphosphino)aniline
(0.17 g, 0.6 mmol). The resultant yellow mixture was stirred
at r.t. until TLC indicated complete reaction. The mixture
was concentrated in vacuo and chromatographed on silica
gel with EtOAc–hexane (1:3) as eluent to afford diamide 6a
as an amorphous white solid (0.16 g, 95%). [a]D20 –36.9 (c
1.09, CHCl3). IR (KBr): n = 3302, 1671, 1574, 1506, 1494,
1434, 1289, 1248, 1028, 746, 696 cm–1. 1H NMR (300 MHz,
CDCl3): d = 3.66–3.81 (m, 8 H), 4.79–4.89 (m, 2 H), 6.61–
7.54 (m, 36 H), 7.90–8.10 (m, 2 H). 31P NMR (121.46 MHz,
CDCl3): d = –21.27. 13C NMR (75 MHz, CDCl3): d = 170.59,
157.44, 141.73, 141.47, 135.97, 135.87, 135.19, 135.10,
134.15, 134.10, 133.90, 133.84, 133.65, 130.23, 129.44,
129.18, 129.06, 128.98, 128.15, 127.62, 127.53, 126.97,
126.83, 124.88, 122.68, 120.83, 110.06, 55.76, 55.72, 45.73,
39.53. MS (ESI): m/z (%) = 875.5 (100) [M+ + 1]. HRMS
(FT): m/z calcd for C56H49O4N2P2 [M+ + 1]: 875.3162;
found: 875.3163.
Anhyd CH2Cl2 (4 mL) was added to the diamine 11a (0.06
g, 0.2 mmol), 2-(diphenylphosphino)benzoic acid (0.15 g,
0.48 mmol), DCC (0.12 g, 0.6 mmol) and 5 mol% DMAP.
The resultant yellow, chalky mixture was stirred at r.t. until
TLC indicated complete reaction. The mixture was filtered
through Celite to remove dicyclohexylurea, and the filter
cake was washed with CH2Cl2 (2 ꢀ 10 mL). The filtrate was
concentrated in vacuo and chromatographed on silica gel
with EtOAc–hexane (1:2) as eluent to afford diamide 5a as
an amorphous white solid (0.08 g, 50%). [a]D20 –26.0 (c
1.02, CHCl3). IR (KBr): n = 2930, 2852, 1654, 1628, 1584,
1493, 1460, 1435, 1328, 1245, 1027, 745, 696 cm–1. 1H
NMR (300 MHz, CDCl3): d = 3.75 (s, 6 H), 4.42 (d, J = 8.4
Hz, 2 H), 5.13 (br, 2 H), 6.15 (d, J = 8.4 Hz, 2 H), 6.87–7.51
(14) Following the same procedure for the preparation of 6a,
reaction of 8c with 2-diphenylphosphino)aniline afforded
(S,S,S,S)-6b as a white solid (yield 80%). [a]D20 –28.7 (c
1.00, CHCl3). IR (KBr): n = 1682, 1575, 1508, 1450, 1434,
1288, 1244, 745, 695 cm–1. 1H NMR (300 MHz, CDCl3):
d = 3.70–3.74 (m, 2 H), 4.96–5.01 (m, 4 H), 5.17 (d, J = 12.3
Hz, 2 H), 6.68–7.54 (m, 46 H), 7.90–8.10 (m, 2 H). 31P NMR
(121.46 MHz, CDCl3): d = –21.32. 13C NMR (75 MHz,
CDCl3): d = 170.46, 156.41, 154.96, 141.77, 141.51, 137.54,
136.13, 136.03, 135.10, 135.00, 134.13, 133.87, 133.80,
133.55, 130.44, 130.27, 129.37, 129.04, 128.97, 128.94,
128.90, 128.84, 128.09, 127.92, 127.84, 127.71, 127.21,
127.14, 124.88, 122.81, 121.27, 111.76, 70.24, 70.19, 45.95,
39.70. MS (ESI): m/z (%) = 1027.35 (30) [M+ + 1]. HRMS
(FT): m/z calcd for C68H56O4N2P2Na [M+ + Na]: 1049.3608;
found: 1049.3616.
(m, 36 H). 31P NMR (121.46 MHz, CDCl3): d = –10.04. 13
C
NMR (75 MHz, CDCl3): d = 167.78, 158.13, 141.06, 140.74,
138.52, 138.35, 137.98, 137.68, 134.97, 134.29, 134.01,
133.74, 130.54, 129.05, 128.81, 128.71, 128.57, 127.17,
126.69, 121.14, 110.66, 55.72, 52.75, 40.29. MS (EI): m/z
(%) = 305 (100), 285 (45), 306 (29), 56 (28), 91 (25), 57
(24), 147 (24), 277 (20). MS (ESI): m/z = 875.1 (100) [M+ +
1]. HRMS (MALDI): m/z calcd for C56H49O4N2P2 [M+ + 1]:
875.3162; found: 875.3144.
(15) (a) Trost, B. M.; van Vranken, D. L. J. Am. Chem. Soc. 1993,
115, 444. (b) Trost, B. M.; Patterson, D. E. Chem.–Eur. J.
1999, 5, 3279. (c) Buschmann, N.; Rückert, A.; Blechert, S.
J. Org. Chem. 2002, 67, 4325.
Synlett 2005, No. 13, 2067–2071 © Thieme Stuttgart · New York

LETTER
New Chiral Bisphosphines for Desymmetrization of Biscarbamate
2071
(16) (a) Lee, S.; Lim, C. W.; Song, C. E.; Kim, K. M.; Jun, C. H.
J. Org. Chem. 1999, 64, 4445. (b) Lim, C. W.; Lee, S.
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solvent under reduced pressure, the residue was submitted to
flash chromatography on silica gel with hexane–EtOAc
(3:1) as eluent to product (3R,4S)-16 as a colorless oil in
90% yield. [a]D20 –136.6 (c 0.90, CHCl3); 97% ee. IR (KBr):
n = 1771, 1355, 1191, 1172, 1146, 1090, 707, 663, 609, 562,
542 cm–1. 1H NMR (300 MHz, CDCl3): d = 2.45 (s, 3 H),
2.67–2.69 (m, 1 H), 2.82–2.84 (m, 1 H), 5.11 (ddd, J = 8.3,
5.8, 1.8 Hz, 1 H), 5.29 (dd, J = 7.4, 1.3 Hz, 1 H), 6.02–6.04
(m, 2 H), 7.36 (d, J = 8.1 Hz, 2 H), 7.95 (d, J = 8.4 Hz, 2 H).
MS (ESI): m/z (%) = 279.95 (5) [M+ + 1]. HRMS (FT): m/z
calcd for C13H14NO4S [M+ + 1]: 280.0638; found: 280.0636.
The ee was determined with HPLC on a Chiralcel AD
column, flow rate: 1.0 mL/min, n-hexane–i-PrOH = 85:15,
30.6 min (3S,4R), 36.6 min (3R,4S); l = 254 nm.
(17) A General Procedure for Pd-Catalyzed Intramolecular
Cyclization of meso-Biscarbamate 15.
To a Schlenk tube containing [Pd(C3H5)Cl]2 (1.3 mg, 0.0036
mmol, 2.5 mol%) and chiral ligand 5a (0.009 mmol, 6.0
mol%) was added dried THF (2 mL), and the mixture was
stirred at r.t. for 30 min. Then, addition of biscarbamate 15
(74.8 mg, 0.15 mmol) was followed by Et3N (0.024 mL, 0.15
mmol). The resulting clear solution stirred at the indicated
temperature for the stated times, and then was quenched with
a sat. aq NH4Cl solution (5 mL). The aqueous phase was
extracted with Et2O, the combined organic phase was
separated and dried over Na2SO4. After removal of the
(18) (a) Trost, B. M.; Breit, B.; Peukert, S.; Zambrano, J.; Ziller,
J. W. Angew. Chem., Int. Ed. Engl. 1995, 34, 2386.
(b) Trost, B. M.; Zambrano, J. L.; Richter, W. Synlett 2001,
907.
Synlett 2005, No. 13, 2067–2071 © Thieme Stuttgart · New York