Traceless solid-phase synthesis of mappicine ketone library via multiple chemoselective palladium-catalyzed reactions on benzenesulfonate linker

Article abstract of DOI:10.1055/s-2008-1077964

We report here a solid-phase synthesis of a library of mappicine ketone, a leading antiviral compound with activity against herpes viruses and human cytomegalovirus. The synthesis is based on multiple chemoselective palladium-catalyzed reactions involving

Full text of DOI:10.1055/s-2008-1077964

LETTER
2005
Traceless Solid-Phase Synthesis of Mappicine Ketone Library via Multiple
Chemoselective Palladium-Catalyzed Reactions on Benzenesulfonate Linker
Solid-Phase
Synthiesis of
r
o
e
Ketone
L
i
k
brary azu Tsukamoto,* Risako Suzuki, Yoshinori Kondo
Graduate School of Pharmaceutical Sciences, Tohoku University, Aramaki-aza aoba 6-3, Aoba-ku, Sendai 980-8578, Japan
Fax +81(22)7953906; E-mail: hirokazu@mail.pharm.tohoku.ac.jp
Received 16 April 2008
Abstract: We report here a solid-phase synthesis of a library of
mappicine ketone, a leading antiviral compound with activity
against herpes viruses and human cytomegalovirus. The synthesis is
based on multiple chemoselective palladium-catalyzed reactions in-
volving a regioselective intramolecular Heck reaction at C-2, sub-
stitution of Cl at C-7 by Suzuki–Miyaura cross-coupling reaction
and Buchwald–Hartwig amination, and reductive cleavage of the
benzenesulfonate linker at C-10.
Key words: mappicine ketone, traceless solid-phase synthesis,
benzenesulfonate linker, palladium, chemoselective
Mappicine ketone (1) is an antiviral lead compound with
activity against herpes viruses and human cytomegalovi-
rus in the low micromolecular range (Scheme 1).1 Al-
though there are plenty of solution-phase syntheses of 1
and structurally related compounds, that is, mappicine (2)
and camptothecin (3),1–4 solid-phase synthesis has never
Scheme 1 Synthetic strategy for library of 1–3
been reported. Development of solid-phase synthesis
based on combinatorial chemistry is important for the
drug-discovery process because it facilitates rapid prepa-
ration of many analogues through a split-pool method us-
O
O
H
a
ing tag technologies. Curran’s group synthesized the
library of 1, 2, and their analogues in solution with a fluo-
rous-tagging strategy, but there are still problems intrinsic
to solution-phase synthesis such as intermediate purifica-
tion.3 To disclose the structure–activity relationship and
improve activity, we developed the traceless solid-phase
synthesis of mappicine ketone and a mappicine library
based on multiple chemoselective Pd-catalyzed reactions
on the benzenesulfonate linker (Scheme 1).5,6
S
O
R
R
6
7
R = electron-donating or -withdrawing group
PPh2
P(t-Bu)2
PPF-t-Bu =
Fe
Traceless linkers represent an exciting aspect of solid-
phase organic synthesis due to the potential to make mol-
ecules lacking any extraneous functionality.7 We recently
revisited the benzenesulfonate linker 6 to transform phe-
nols into arenes 7 under Pd catalysis and developed new
reaction conditions applicable to traceless cleavage of
phenols bearing both electron-donating and electron-
withdrawing groups (Scheme 2).6 We also showed the
feasibility of Pd-catalyzed transformations of halogens
and trifluoromethanesulfonate on the benzenesulfonate
linker. Our strategy for mappicine ketone synthesis makes
good use of the traceless linker. It consists of the follow-
ing reactions: 1) N-alkylation of solid-supported 2,4-
Scheme 2 Pd-catalyzed traceless cleavage of benzenesulfonate lin-
ker 6. Reagents and conditions: (a) HCO2H, Et3N, cat. Pd(OAc)2–
PPF–t-Bu, t-BuOH, MW, 120 °C.
dichloroquinoline 4 with 2-hydroxypyridine derivatives
5; 2) Pd-catalyzed regioselective intramolecular Heck re-
action at C-2; 3) Pd-catalyzed substitution of Cl at C-7;
and 4) Pd-catalyzed reductive cleavage of the linker
(Scheme 1). The chain of reactions allows introduction of
diverse substituents at C-7, C-15, and C-16 of the tetracy-
clic core.
Our synthetic study started with the development of the
reaction conditions in the solution-phase synthesis of
SYNLETT 2008, No. 13, pp 2005–2010
x
x
.x
x
.2
0
0
8
Advanced online publication: 15.07.2008
DOI: 10.1055/s-2008-1077964; Art ID: U03608ST
© Georg Thieme Verlag Stuttgart · New York

2006
H. Tsukamoto et al.
LETTER
mappicine analogues 14–17a (Scheme 4). Preparation of
2,4-dichloroquinoline 4a–c is outlined in Scheme 3. Com-
mercially available 5-hydroxyanthranilic acid (8) was
converted into its methyl ester8 and the hydroxyl group
was selectively tosylated with p-toluenesulfonyl chloride
and triethylamine in the presence of an amino group.9 The
acylation of the amino group in 9a with tert-butyl (chloro-
formyl)acetate10 and subsequent Dieckman condensation
gave 2,4-dihydroxyquinoline 10a.11 The NaBH4 reduction
of the acid chloride formed by treatment of tert-butyl ester
10a with POCl3 gave 2,4-dichloro-3-hydroxymethylquin-
oline (4a),12 which was transformed into the correspond-
ing bromide 4c with PBr3 in DMF.
X
HO
CO2H
NH2
O
O
CO2Me
NH2
a, b
S
O
8
9a X = Me
9b X = I
X
OH
c, d
O
CO2t-Bu
10a X = Me
S
10b X = I
O
O
N
H
O
X
Cl
e, f
4a X = Me, Y = OH
4b X = I, Y = OH
O
S
Y
g
Reaction conditions for N-selective alkylation of 2-hy-
droxypyridine (5a) with alcohol 4a and bromide 4c are
summarized in Table 1. While Mitsunobu reaction with
4a and alkylation with 4c and KOt-Bu in DME (Comins’s
methods13,14) gave a mixture of N- and O-alkylation prod-
ucts 11a and 12a, alkylation with 4c in the presence of
NaH and LiBr in DME–DMF (4:1, Curran’s method15) af-
forded N-alkylation product 11a exclusively (entry 3).
Further investigation led to discovery of simpler condi-
tions [LiOt-Bu, DME–DMF (4:1) or Cs2CO3, 1,4-diox-
ane,16 entries 4, 5].
O
O
4c X = Me, Y = Br
N
Cl
Scheme 3 Preparation of 7-benzenesulfonyloxy-2,4-dichloroquino-
lines 4a–c. Reagents and conditions: (a) MeOH, H2SO4, reflux; (b) p-
X-C6H4SO2Cl (X = Me or I), Et3N, CH2Cl2, 0 °C (9a: 89%, 9b: 99%
for two steps); (c) ClOCCH2CO2t-Bu, pyridine, CH2Cl2 (0.1 M), 0 °C;
(d) NaOEt, THF–EtOH, r.t.; (e) POCl3, 100 °C; (f) NaBH4, THF–
EtOH, 0 °C (4a: 69%, 4b: 83% for four steps); (g) Br3P, DMF, 0 °C
(4c: 90%).
10-OTs groups. The tosylate at C-10, as well as chlorine
at C-7, turned out to be susceptible to reductive cleavage
using the Pd(OAc)2–1,3-bis(diphenylphosphino)propane
(dppp) catalytic system.20 However, the chlorine atom in
13a could be selectively substituted by the Suzuki–
Miyaura cross-coupling reaction with methyl- and aryl-
boronic acids21 and Buchwald–Hartwig amination with a
primary aliphatic amine22 to provide 15–17a, without af-
fecting the tosylate group.
The regioselective intramolecular Heck reaction at C-2 in
11a proved to proceed effectively under Comins’s condi-
tions [a catalytic amount of Pd(OAc)2(PPh3)2, KOAc,
MeCN, 80 °C]14b after all our efforts to screen phosphine
ligands, ratios of the phosphine to Pd, bases, and solvents
(Scheme 4). The yield seems still to be insufficient, but
much better than that observed in the regioselective Heck
reaction for the synthesis of homocamptothecin (only
3%).17 The higher reactivity of halogen at C-2 of quino-
line compared to C-4 towards Pd-catalyzed reactions was
also observed in other cases.17,18 The regioselectivity was
determined by transformation of 13a into 14a, a known
tetracyclic compound,19 by reduction of both the 7-Cl and
On the basis of solution-phase reaction conditions, the
solid-phase synthesis of mappicine ketone and the mappi-
cine library was carried out (Scheme 5).23 2-Hydroxypyri-
dine building blocks 5b–g were prepared from 2-
fluoropyridine by Comins’s method (Scheme 6).24 For
improvement of poor reactivity on solid phase and simpli-
Table 1 Reaction Conditions for N-Alkylation of 4a and 4c with 2-Hydroxypyridine 5a
O
HN
Cl
N
O
Cl
N
4a
or
TsO
TsO
5a
N
O
N
+
r.t.
Cl
Cl
4c
11a
12a
Entry
Substrate
Conditions
Yield (%)
70
11a/12aa
5.5:1
1
2
3
4
5
4a
4c
4c
4c
4c
Ph3P, DIAD, THF
KOt-Bu, DME
92
19:1
NaH, LiBr, DME–DMF (4:1)
LiOt-Bu, DME–DMF (4: 1)
Cs2CO3, 1,4-dioxane
quant
quant
quant
11a only
11a only
11a only
a The ratio was determined by 1H NMR.
Synlett 2008, No. 13, 2005–2010 © Thieme Stuttgart · New York

LETTER
Solid-Phase Synthesis of Mappicine Ketone Library
2007
Y
N
Y
X
TsO
c, d,
or e
a
O
O
11a
N
N
N
13a X = OTs, Y = Cl
14a X = Y = H
15a Y = Me
16a Y = 4-AcC6H4
17a Y = NHBn
b
Scheme 4 Regioselective Heck reaction and substitution of 7-Cl. Reagents and conditions: (a) cat. Pd(OAc)2(PPh3)2, KOAc, MeCN, 80 °C,
65%; (b) HCO2H, Et3N, cat. Pd(OAc)2–dppp, 1,4-dioxane, 100 °C (14a: quant.); (c) MeB(OH)2, K3PO4, cat. Pd(OAc)2–S-Phos, toluene, 100
°C (15a: 67%); (d) 4-AcC6H4B(OH)2, K3PO4, cat. Pd(OAc)2–S-Phos, toluene, 100 °C (16a: 80%); (e) BnNH2, K2CO3, cat. Pd(OAc)2–
XANTPHOS, 1,4-dioxane, 100 °C (17a: 76%). Ligands: dppp = 1,3 bis(diphenylphosphino)propane, S-Phos = 2-dicyclohexylphosphino-2′,6′-
dimethoxy-1,1′-biphenyl, XANTPHOS = 9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene.
a
b
OH
O
JandaJelTM-OH
O
19
Cl
Cl
N
JJ-TsO
JJ-TsO
O
Y
N
f, g
d, e
R1
N
Cl
or h
20 Y = OH
21 Y = Br
22 X = O
23 X = H, OH
R2
c
i
X
j
R3
R3
JJ-TsO
H
O
O
N
N
j
R1
R1
R2
N
N
R2
25 X = O
26 X = H, OH
24 X = O
X
X
O
HN
O
O
R1
R2
O
O
S
O
JJ-TsO
O
5b–g
Scheme 5 Solid-phase synthesis of mappicine ketone and mappicine library 25 and 26. Reagents and conditions: (a) 6-heptynoic acid, 2,6-
dichlorobenzyoyl chloride, pyridine, DMF, r.t., 24 h; (b) 4b, Et3N, cat. PdCl2(PPh3)2–CuI, DMF, r.t., 8 h; (c) Br3P, DMF, r.t., 8 h; (d) 5b–g,
LiOt-Bu, DME–DMF (4:1), r.t., 24 h; (e) cat. Pd(OAc)2–Ph3P, KOAc, PhCN, 80 °C, 12 h; (f) MeB(OH)2, K3PO4, cat. Pd(OAc)2–S-Phos, to-
luene, 100 °C, 12 h; (g) 4-AcC6H4B(OH)2, K3PO4, cat. Pd(OAc)2–S-Phos, toluene, 100 °C, 12 h; (h) BnNH2, K2CO3, cat. Pd(OAc)2–
XANTPHOS, 1,4-dioxane, 100 °C, 12 h; (i) NaBH4, THF–EtOH (3:1), r.t., 3 h; (j) HCO2H, Et3N, cat. Pd(OAc)2–dppp, 1,4-dioxane, 100 °C,
12 h.
fication of both loading and preparation of 2,4-dichloro- reaction solvent should be changed from acetonitrile to
quinoline, the Sonogashira reaction was employed as the benzonitrile because the resin does not swell in acetoni-
fourth Pd-catalyzed reaction to attach p-iodobenzene- trile. The chlorine in 22 could be substituted in the same
sulfonate 4b – prepared according to the procedures for way as in the solution-phase synthesis, to give solid-sup-
the synthesis of tosylate 4a (Scheme 2) – to terminal ported mappicine ketone analogues 24. The NaBH4 reduc-
alkyne-containing JandaJelTM resin 19 (Scheme 5).25 The tion of the ketone functionality in 22 could provide further
reaction conditions using LiOt-Bu (Table 1, entry 4) diversity, leading to the formation of solid-supported
turned out to be better for N-alkylation reaction of 2-hy- mappicine analogues 23. Final traceless cleavage of the
droxypyridines with the solid-supported bromide 21 – benzenesulfonate linker furnished library 25 and 26,
prepared by bromination of alcohol 20 – than those using shown in Table 2.
Cs2CO3 (Table 1, entry 5), due to poor solubility of cesi-
The present study offers the first solid-phase synthesis of
um salt of 2-hydroxypyridines in 1,4-dioxane. The subse-
mappicine ketone and mappicine library. The synthesis
quent Heck reaction on solid phase required that the
consists of multiple Pd-catalyzed reactions on the ben-
Synlett 2008, No. 13, 2005–2010 © Thieme Stuttgart · New York

2008
H. Tsukamoto et al.
LETTER
F
F
F
Table 2 Mappicine Ketone and Mappicine Analogues Obtained by
Solid-Phase Synthesis
N
N
N
c, d
e
a, b
R1
R1
R2
5b–g
Entry Product
R1
R2
R3
Yield
(%)a
I
O
1
2
25bA
25bB
25bC
25bD
25cA
25cB
25cC
25cD
25dA
25dB
25dC
25dD
25eA
25eB
25eC
25eD
25fA
25fB
25fD
25gA
25gB
25gC
25gD
26cA
26dA
26eA
26fA
26gA
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Bn
Et
H
60
52
38
40
46
40
22
27
41
49
29
33
49
45
24
50
36
22
33
39
36
25
33
52
42
38
31
34
Scheme 6 Preparation of 2-hydroxypyridines 5b–g. Reagents and
conditions: (a) LDA, THF then I2, 61%; (b) LDA, THF, then R1X (ha-
logen dance); (c) i-PrMgCl, THF then R2CHO; (d) Swern oxidation;
(e) 3 M HCl, reflux, 12 h. Yields for four steps are 82% for 5b (R1 =
Me, R2 = Et), 77% for 5c (R1 = Me, R2 = i-Bu), 64% for 5d (R1 = Me,
R2 = c-C6H11), 76% for 5e (R1 = Me, R2 = 4-MeC6H4), 73% for 5f (R1 =
Me, R2 = 2-thienyl), and 31% for 5g (R1 = Bn, R2 = Et).
Et
Me
3
Et
4-AcC6H4
4
Et
NHBn
5
i-Bu
H
zenesulfonate linker, which is compatible with halogen
transformations and can be readily cleaved by the special
conditions. The linker has a possibility of functionalized
cleavage as well as reductive cleavage to give more di-
verse compounds. The solid-phase synthesis can be ap-
plied to a split-pool method using tag technologies.
Further study on the polished library synthesis applicable
to a wider variety of transformations26 and building blocks
and an assay of its biological activity are under way in our
laboratory.
6
i-Bu
Me
7
i-Bu
4-AcC6H4
8
i-Bu
NHBn
9
c-C6H11
c-C6H11
c-C6H11
c-C6H11
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-MeC6H4
2-thienyl
2-thienyl
2-thienyl
Et
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
Me
4-AcC6H4
NHBn
H
Acknowledgment
Me
This work was partly supported by a Grant-in-Aid from the Japan
Society for Promotion of Sciences (No.16790004) and Japan Com-
binatorial Chemistry Focus Group Award in Synthetic Organic
Chemistry, Japan.
4-AcC6H4
NHBn
H
References and Notes
Me
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Et
Me
Bn
Et
4-AcC6H4
Bn
Et
NHBn
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Me
Me
Bn
i-Bu
H
H
H
H
H
c-C6H11
4-MeC6H4
2-thienyl
Et
a Yields refer to the purified compounds by preparative TLC and were
determined based on the loading of 21, which was calculated by in-
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Synlett 2008, No. 13, 2005–2010 © Thieme Stuttgart · New York

LETTER
Solid-Phase Synthesis of Mappicine Ketone Library
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Simmons, C. A.; Shearin, J. J. Med. Chem. 2005, 48, 2243.
(e) Pal, M.; Khanna, I.; Subramanian, V.; Padakanti, S.;
Pillarisetti, S. WO 2006058201, 2006; Chem. Abstr. 2006,
145, 8041.
(19) (a) Kametani, T.; Nemoto, H.; Takeda, H.; Takano, S.
Tetrahedron 1970, 26, 5753. (b) Bowman, W. R.; Bridge,
C. F.; Brookes, P.; Cloonan, M. O.; Leach, D. C. J. Chem.
Soc., Perkin Trans. 1 2002, 58. (c) Nguyen, T.; Wicki, M.
A.; Snieckus, V. J. Org. Chem. 2004, 69, 7816. (d)Dai,W.;
Petersen, J. L.; Wang, K. K. Org. Lett. 2006, 8, 4665.
(e) Zhou, H.-B.; Liu, G.-S.; Yao, Z.-J. Org. Lett. 2007, 9,
2003.
To a suspension of 19 (2.0 mmol), 4b (2.55 g, 5.0 mmol),
PdCl2(PPh3)2 (140 mg, 0.20 mmol), and CuI (76.2 mg, 0.40
mmol) in anhyd DMF (16 mL) was added Et3N (5.3 mL, 38
mmol) at r.t. under argon. The mixture was agitated at r.t. for
12 h. The resin was washed alternately with DMF and H2O
(3×), CHCl3 and MeOH (3×), and Et2O, and then dried under
vacuum to yield 20 (2.68 g).
Preparation of 21 (Step c)
To a suspension of 20 (2.0 mmol) in anhyd DMF (22 mL)
was added Br3P (0.95 mL, 10 mmol) at r.t. under argon. The
mixture was agitated at r.t. for 6 h. The resin was washed
alternately with DMF and H2O (3×), CHCl3 and MeOH (3×),
and Et2O, and then dried under vacuum to yield 21 (2.66 g,
0.46 mmol/g calculated by increased resin weight).
General Procedure for N-Alkylation (Step d)
To a solution of 5b–g (3.9 mmol) in DME (5.2 mL) and
DMF (1.3 mL) was added t-BuOLi (3.6 mmol) under argon.
The mixture was agitated at r.t. for 30 min, after which 21
(0.80 g, 0.37 mmol) was added. The mixture was then
agitated at r.t. for 24 h. The resin was washed alternately
with DMF and H2O (3×), CHCl3 and MeOH (3×), and Et2O,
and then dried under vacuum to yield N-alkylation products.
General Procedure for Heck Reaction (Step e)
To a test tube containing the N-alkylation product (0.045
mmol), Pd(OAc)2 (0.032 mmol), Ph3P (0.064 mmol), and
KOAc (0.64 mmol) was added benzonitrile (0.8 mL) under
argon. The mixture was agitated at 80 °C for 12 h. The resin
was washed alternately with DMF and H2O (3×), CHCl3 and
MeOH (3×), and Et2O, and then dried under vacuum to yield
22.
General Procedure for Methylation of 22 (Step f)
To a test tube containing 22 (0.041 mmol), MeB(OH)2 (0.45
mmol), Pd(OAc)2 (0.027 mmol), S-Phos (0.054 mmol), and
K3PO4⋅nH2O (0.45 mmol) was added toluene (0.8 mL) under
argon. The mixture was agitated at 100 °C for 12 h. The resin
was washed alternately with DMF and H2O (3×), CHCl3 and
MeOH (3×), and Et2O, and then dried under vacuum.
General Procedure for Arylation of 22 (Step g)
To a test tube containing 22 (0.041 mmol), p-Ac-
C6H4B(OH)2 (0.45 mmol), Pd(OAc)2 (0.027 mmol), S-Phos
(0.054 mmol), and K3PO4 (0.45 mmol) was added toluene
(0.8 mL) under argon. The mixture was agitated at 100 °C
for 12 h. The resin was washed alternately with DMF and
H2O (3×), CHCl3 and MeOH (3×), and Et2O, and then dried
under vacuum.
General Procedure for Amination of 22 (Step h)
To a test tube containing 22 (0.041 mmol), BnNH2 (0.46
mmol), Pd(OAc)2 (0.066 mmol), XANTPHOS (0.073
mmol), and K2CO3 (0.46 mmol) was added 1,4-dioxane (0.8
mL) under argon. The mixture was agitated at 100 °C for 12
h. The resin was washed alternately with DMF and H2O
(3×), CHCl3 and MeOH (3×), and Et2O, and then dried under
vacuum.
(20) (a) Cabri, W.; Candiani, I.; Zarini, F.; Penco, S.; Bedeschi,
A. Tetrahedron Lett. 1995, 36, 9197. (b) Bedeschi, A.;
Zarini, F.; Cabri, W.; Candiani, I.; Penco, S.; Capolongo, L.;
Ciomei, M.; Farao, M.; Grandi, M. Bioorg. Med. Chem. Lett.
1996, 6, 671. (c) Fu, Q.; Chen, Z. Synthesis 2006, 1940.
(21) (a) Walker, S. D.; Barder, T. E.; Martinelli, J. R.; Buchwald,
S. L. Angew. Chem. Int. Ed. 2004, 43, 1871. (b) Barder, T.
E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L. J. Am.
Chem. Soc. 2005, 127, 4685.
Synlett 2008, No. 13, 2005–2010 © Thieme Stuttgart · New York

2010
H. Tsukamoto et al.
LETTER
General Procedure for NaBH4 Reduction of 22 (Step i)
To a suspension of 22 (0.021 mmol) in THF–EtOH (3:1, 0.5
mL) was added NaBH4 (0.11 mmol) under argon. The
mixture was agitated at r.t. for 3 h. The resin was washed
alternately with DMF and H2O (3×), CHCl3 and MeOH (3×),
and Et2O, and then dried under vacuum to yield 23.
General Procedure for Reductive Cleavage (Step j)
To a test tube containing 23 or 24 (0.021 mmol), Pd(OAc)2
(0.0084 mmol), and dppp (0.0092 mmol) were added anhyd
1,4-dioxane (0.5 mL), Et3N (0.63 mmol), and formic acid
(0.63 mmol) under argon. The mixture was agitated at
100 °C for 12 h. The mixture was filtered and thoroughly
washed with CHCl3. The filtrate was concentrated in vacuo
and the residue was purified by PLC eluting with 6%
MeOH–CHCl3 to yield 25 or 26. Spectral data of 25bA
agreed with the reported ones.2b,d
(24) (a) Comins, D. L.; Saha, J. K. Tetrahedron Lett. 1995, 36,
7995. (b) Rocca, P.; Cochennec, C.; Marsais, F.;
Thomas-dit-Dumont, L.; Mallet, M.; Godard, A.;
Quéguiner, G. J. Org. Chem. 1993, 58, 7832. (c) Saitton,
S.; Kihjberg, J.; Luthman, K. Tetrahedron 2004, 60, 6113.
(25) Pd-catalyzed attachment of p-iodobenzenesulfonate of
aliphatic alcohols to solid support has been reported:
(a) Takahashi, T.; Inoue, H.; Yamamura, Y.; Doi, T. Angew.
Chem. Int. Ed. 2001, 40, 3230. (b) Doi, T.; Kinbara, A.;
Inoue, H.; Takahashi, T. Chem. Asian J. 2007, 2, 188.
(26) (a) Najiba, D.; Carpentier, J.-F.; Castanet, Y.; Biot, C.;
Brocard, J.; Mortreux, A. Tetrahedron Lett. 1999, 40, 3719.
(b) Wolf, C.; Lerebours, R. J. Org. Chem. 2003, 68, 7077.
(c) Wolf, C.; Lerebours, R. Org. Biomol. Chem. 2004, 2,
2161. (d) Luo, Y.; Gao, H.; Li, Y.; Huang, W.; Lu, W.;
Zhang, Z. Tetrahedron 2006, 62, 2465.
Synlett 2008, No. 13, 2005–2010 © Thieme Stuttgart · New York