First total syntheses of chrestifoline-B and (±)-chrestifoline-C, and improved synthetic routes to bismurrayafoline-A, bismurrayafolinol and chrestifoline-D

Article abstract of DOI:10.1039/c4ob00609g

We describe an efficient synthesis of the methylene-bridged biscarbazole alkaloids bismurrayafoline-A, bismurrayafolinol and chrestifoline B-D using an Ullmann-type coupling at the benzylic position. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00609g

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First total syntheses of chrestifoline-B and
( )-chrestifoline-C, and improved synthetic routes
to bismurrayafoline-A, bismurrayafolinol and
chrestifoline-D†‡
Cite this: Org. Biomol. Chem., 2014,
12, 3831
Received 21st March 2014,
Accepted 17th April 2014
DOI: 10.1039/c4ob00609g
Carsten Börger, Arndt W. Schmidt and Hans-Joachim Knölker*
www.rsc.org/obc
We describe an efficient synthesis of the methylene-bridged bis-
carbazole alkaloids bismurrayafoline-A, bismurrayafolinol and
chrestifoline B–D using an Ullmann-type coupling at the benzylic
position.
A wide range of carbazole alkaloids has been isolated and
investigated towards their biological activity.1–3 Much less is
known about the pharmaceutical potential of biscarbazoles
and only a few synthetic approaches have been described.1,4
We have developed several methods for the synthesis of carba-
zoles.1,2 Using our palladium-catalysed construction of the car-
bazole framework,5 we recently described efficient synthetic
routes to biscarbazole alkaloids,6–8 for example the oxygen-
bridged biscarbazole oxydimurrayafoline and the N-aryl linked
murrastifoline-A.7,8 The biscarbazole linkage was constructed
by etherification,7 the Buchwald–Hartwig amination,9 or the
Ullmann coupling.10 Herein, we report the synthesis of methy-
lene-bridged biscarbazole alkaloids by palladium(0)- and
copper(I)-catalysed coupling reactions at the benzylic position
at C-3 of the carbazole framework.
The first methylene-bridged biscarbazole alkaloid obtained
from natural sources was bismurrayafoline-A (1), isolated in
1983 by Furukawa et al. from the root bark of Murraya euchres-
tifolia Hayata (Fig. 1).11 Bismurrayafoline-A (1) showed a weak
activity against some cancer cell lines.12 In 2001, Bringmann
et al. reported the formation of bismurrayafoline-A (1) in up to
19% yield as a by-product of the reduction of mukonine (6d)
(7 steps, 9% overall yield of 1).13 We have described a total syn-
thesis of bismurrayafoline-A (1) via an unprecedented
rearrangement during an Ullmann coupling (6 steps, 28%
overall yield of 1).8 In 1987, Furukawa et al. isolated bismur-
rayafolinol (2) from the root bark of Murraya euchrestifolia
Hayata.14 Bringmann et al. reported the formation of bismur-
Fig. 1 Bismurrayafoline-A (1), bismurrayafolinol (2), chrestifoline-B (4),
chrestifoline-C (5), chrestifoline-D (3), the 1-methoxycarbazole alkaloids
6a–d, girinimbine (7a) and mahanimbine (7b).
rayafolinol (2) as a by-product in up to 6% yield (7 steps, 3%
overall yield of 2) during their synthesis of murrayafoline-A
(6a).13 Chrestifoline-D (3) was isolated in 1992 by Furukawa
and Wu et al. from the root bark of M. euchrestifolia.15 The first
synthetic approach by partial synthesis starting from bismur-
rayafolinol (2) was reported along with the isolation. Chrestifo-
line-B (4) and chrestifoline-C (5) were isolated in 1990 by
Furukawa et al. from the same natural source.16 Chrestifoline-
C (5) has been obtained in an optically active form ([α]D
=
−5.6, CHCl3),16 but its absolute configuration is not known.
The biscarbazoles 1–5 have an identical benzylic subunit
which corresponds to murrayafoline-A (6a), but they differ in
the benzylic substituent (Fig. 1). For the biscarbazoles 1–3, the
benzylic linkage leads to the nitrogen atom of a 1-oxygenated
tricyclic carbazole corresponding to murrayafoline-A (6a),
koenoline (6b) or murrayanine (6c), respectively. For chrestifo-
Department Chemie, Technische Universität Dresden, Bergstrasse 66, 01069 Dresden,
Germany. E-mail: hans-joachim.knoelker@tu-dresden.de
†Part 117 of Transition Metals in Organic Synthesis; for Part 116, see ref. 6b.
‡Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
of the biscarbazole alkaloids 1–5. See DOI: 10.1039/c4ob00609g
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line-B (4) and chrestifoline-C (5), the pyrano[3,2-a]carbazole
alkaloids girinimbine (7a) and mahanimbine (7b) serve as
benzylic substituents. The mono-carbazole building blocks of
the biscarbazoles 1–5 are found in nature as well. The 1-meth-
oxycarbazole alkaloids, murrayafoline-A (6a), koenoline (6b),
murrayanine (6c) and mukonine (6d), have been obtained
from diverse plants of the Rutaceae family.1 The pyrano[3,2-a]-
carbazole alkaloids girinimbine (7a) and mahanimbine (7b)
were isolated first by Chakraborty et al. from Murraya koenigii
Spreng.17,18 Our group previously described synthetic routes to
all individual carbazole fragments present in the biscarbazoles
1–5. An efficient iron-mediated synthesis of the 1-methoxycar-
bazole alkaloids 6a–d was reported early on.7,19 More recently,
an optimised palladium-catalysed synthesis of mukonine (6d)
was described.7 Several synthetic routes to girinimbine (7a)
and an efficient access to mahanimbine (7b) were also develo-
ped by our group.20
For the synthesis of the biscarbazoles 1–5 we envisaged a
retrosynthetic cleavage of the C–N linkage between both carba-
zole units which leads to the carbazoles 8 and 9 as precursors
(Scheme 1). In our approach the nitrogen atom of carbazole 8
serves as a nucleophile. Thus, carbazole 8 is represented by
the unprotected naturally occurring carbazoles 6a–c, 7a and
7b. The fragment 9 should have a leaving group at the benzylic
position and would be represented by an appropriate koeno-
line derivative. Compound 9 can be prepared from mukonine
(6d) which is readily available using the arylamine 10 as the
starting material.7
Scheme 2 Synthesis of bismurrayafolinol (2) and chrestifoline-D (3).
Reagents and conditions: (a) 1.2 equiv. PhBr, 6 mol% Pd(OAc)2, 12 mol%
SPhos, 1.4 equiv. Cs2CO3, toluene, reflux, 40 h, 100%; (b) 10 mol%
Pd(OAc)2, 10 mol% K2CO3, PivOH, 115 °C, air, 14 h, 91%; (c) 2 equiv. Boc2O,
1 equiv. DMAP, MeCN, rt, 17 h, 97%; (d) 3.2 equiv. DIBAL-H, Et2O, −78 °C,
3.5 h, 100%; (e) 1.5 equiv. p-(NO2)C6H4COCl, 1.5 equiv. DMAP, CH2Cl2, rt,
1.5 h, 100%; (f) 1.3 equiv. mukonine (6d), 0.2 equiv. CuBr, 0.4 equiv.
pyrrole-2-carboxylic acid, 4 equiv. K3PO4, DMSO, 110 °C, 40 h, 63% 13b;
(g) 3.2 equiv. DIBAL-H, Et2O, −78 °C, 4 h, 96%; (h) 5 equiv. MnO2, CH2Cl2,
rt, 24 h, 78%.
The Buchwald–Hartwig coupling of bromobenzene and aryl-
amine 10 in the presence of SPhos followed by palladium(II)-
catalysed oxidative cyclisation afforded mukonine (6d) in 91%
yield over both steps (Scheme 2).7 Boc-protection of 6d and
subsequent reduction of the ester group led to the protected the benzylic position of the koenoline derivative 11,7 the
koenoline 11. Next, the hydroxy group had to be transformed resulting compound should have sufficient stability in order to
into a leaving group. However, in light of the high reactivity of serve as the central intermediate en route to the methylene-
bridged biscarbazole alkaloids. Treatment of 11 with p-nitro-
benzoyl chloride in the presence of stoichiometric amounts of
DMAP afforded quantitatively the p-nitrobenzoate 12. In con-
trast to the corresponding mesylate,7 the p-nitrobenzoate 12 is
much more stable towards benzyl cation formation. Thus, the
koenoline derivative 12 with an activated benzylic position is
available in five steps and 88% overall yield and was sub-
sequently used as a relay compound for the synthesis of the
methylene-bridged biscarbazole alkaloids 1–5.
First, we envisaged a Buchwald–Hartwig coupling reaction
of the p-nitrobenzoate 12 with mukonine (6d). Only a few
methods have been described so far using transition metal-
catalysed coupling reactions at the benzylic position.21 Reac-
tion of 12 and 6d in the presence of catalytic amounts of palla-
dium(II) acetate, rac-BINAP and stoichiometric amounts of
caesium carbonate in toluene at reflux provided the desired
biscarbazole 13a in 27% yield along with the deprotected bis-
carbazole 13b in 35% yield (Scheme 2, Table 1). The concomi-
tant partial removal of the Boc group could result either from
a pericyclic reaction under thermal conditions22 or from a
Scheme 1 Retrosynthetic analysis of the methylene-bridged biscarba-
zoles 1–5.
base-promoted saponification. Coupling of 12 and 6d under
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the methylene-bridged biscarbazoles 1, 4 and 5 became
directly available (Scheme 3).
Table 1 Coupling of the p-nitrobenzoate 12 with mukonine (6d)
Products,
yield (%)
Thus, reaction of murrayafoline-A (6a)8 with the p-nitro-
benzoate 12 provided bismurrayafoline-A (1) in 46% yield.§
The Ullmann-type coupling of the nitrobenzoate 12 with giri-
nimbine (7a)20c afforded chrestifoline-B (4) and the reaction of
12 with mahanimbine (7b)20c provided ( )-chrestifoline-C (5).§
A comparison of the four Ullmann-type couplings described
herein reveals that mukonine (6d), which obviously is more
stable under the reaction conditions, provided the best yield
(63%) for the copper-induced C–N bond formation with com-
pound 12.
Reaction conditions
1.3 equiv. 6d, 10 mol% Pd(OAc)2, 10 mol%
rac-BINAP, 1.4 equiv. Cs2CO3, toluene, reflux, 4 d.
13a, 27%;
13b, 35%
1.3 equiv. 6d, 0.2 equiv. CuBr, 0.4 equiv. pyrrole-
13b, 63%
2-carboxylic acid, 4 equiv. K3PO4, DMSO, 110 °C, 40 h.
Ullmann conditions, using substoichiometric amounts of
copper(I) bromide in the presence of pyrrole-2-carboxylic
acid,23 led only to the deprotected biscarbazole 13b in 63%
yield (see the Experimental procedure). Therefore, the
Ullmann coupling reaction is superior for the present syn-
thesis as it provides directly the biscarbazole 13b and thus
avoids an additional deprotection step. Reduction of biscarba-
zole 13b with diisobutylaluminium hydride provided bismur-
rayafolinol (2) in 96% yield. The spectroscopic data of 2 are in
full agreement with those reported for the natural product.§
Reduction of 13b using lithium aluminium hydride afforded
bismurrayafolinol (2) in 90% yield but did not lead to bismur-
rayafoline-A (1). Apparently, N-substituted carbazoles do not
form the quinone imine methide intermediate required for
the complete reduction to a 3-methylcarbazole,5i,13,24 and
thus, further reduction of the 3-(hydroxymethyl)carbazole does
not occur. Oxidation of bismurrayafolinol (2) with manganese(IV)
oxide25,26 led to chrestifoline-D (3).§
In conclusion, five naturally occurring methylene-bridged
biscarbazole alkaloids were synthesised using an Ullmann-
type coupling at the benzylic carbon atom of the relay com-
pound 12 as the key reaction. Using the present route, the bis-
carbazole alkaloids 1–5 become available via short and
efficient synthetic routes, bismurrayafoline-A (1): 6 steps, 41%
overall yield; bismurrayafolinol (2): 7 steps, 53% overall yield;
chrestifoline-D (3): 8 steps, 42% overall yield; chrestifoline-B
(4): 6 steps, 36% overall yield; and ( )-chrestifoline-C (5): 6
steps, 28% overall yield based on the arylamine 10. Chresti-
foline-B (4) and ( )-chrestifoline-C (5) have been obtained by
total synthesis for the first time. Studies concerning the bio-
activity of the biscarbazole alkaloids 1–5 are in progress.
Using the Ullmann-type coupling of the p-nitrobenzoate 12
with the appropriate carbazole building blocks 6a, 7a and 7b,
Experimental procedure
Experimental procedure for the Ullmann-type coupling to the
biscarbazole 13b:
A solution of the p-nitrobenzoate 12 (66.6 mg, 0.140 mmol),
freshly dried potassium phosphate (120 mg, 0.566 mmol),
mukonine (6d) (46.4 mg, 0.182 mmol), copper(I) bromide
(4.0 mg, 28 µmol), and pyrrole-2-carboxylic acid (6.2 mg,
56 µmol) in DMSO (1 mL) was heated at 110 °C for 40 h. Water
(1.5 mL) and a saturated aqueous solution of ammonium
chloride (3 mL) were added, the layers were separated and the
aqueous layer was extracted three times with ethyl acetate. The
combined organic layers were dried over magnesium sulfate
and the solvents were removed under reduced pressure. Purifi-
cation of the residue by flash chromatography on silica gel
(gradient elution with petroleum ether–ethyl acetate, 19 : 1 to
7 : 3) afforded the biscarbazole 13b (40.9 mg, 63%) as a colour-
less solid; mp 152.5–153 °C. UV (MeOH): λ = 240 (sh), 252 (sh),
270, 292 (sh), 325 nm; IR (ATR): ν = 3360, 3055, 2992, 2949,
2933, 2836, 1771, 1734, 1716, 1692, 1653, 1625, 1582, 1542,
1503, 1489, 1448, 1401, 1358, 1338, 1306, 1266, 1251, 1200,
1167, 1134, 1103, 1037, 1011, 991, 947, 906, 861, 837, 764, 729,
1
694, 665, 613 cm−1; H NMR (500 MHz, CDCl3): δ = 3.81 (s, 3
Scheme 3 Synthesis of bismurrayafoline-A (1), chrestifoline-B (4), and H), 3.99 (s, 3 H), 4.02 (s, 3 H), 6.05 (s, 2 H), 6.76 (d, J = 0.8 Hz,
chrestifoline-C (5). Reagents and conditions: (a) 1.5 equiv. 6a, 0.4 equiv.
CuBr, 0.8 equiv. pyrrole-2-carboxylic acid, 4 equiv. K3PO4, DMSO,
110 °C, 2 d, 46%; (b) 1.5 equiv. 7a, 0.2 equiv. CuBr, 0.4 equiv. pyrrole-2-
carboxylic acid, 4 equiv. K3PO4, DMSO, 125 °C, 40 h, 41%; (c) 1.5 equiv.
1 H), 7.17 (ddd, J = 7.9 Hz, 6.9 Hz, 1.1 Hz, 1 H), 7.27 (ddd, J =
7.8 Hz, 7.0 Hz, 0.8 Hz, 1 H), 7.37 (ddd, J = 8.1 Hz, 7.0 Hz,
1.1 Hz, 1 H), 7.40–7.44 (m, 2 H), 7.497 (d, J = 8.1 Hz, 1 H),
7.500 (s, 1 H), 7.66 (d, J = 1.3 Hz, 1 H), 7.93 (d, J = 7.8 Hz, 1 H),
8.14 (d, J = 7.8 Hz, 1 H), 8.20 (br s, 1 H), 8.54 (d, J = 1.3 Hz,
7b, 0.2 equiv. CuBr, 0.4 equiv. pyrrole-2-carboxylic acid, 4 equiv. K3PO4,
DMSO, 120 °C, 40 h, 32%.
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1 H); 13C NMR and DEPT (125 MHz, CDCl3): δ = 49.76 (CH2),
52.18 (CH3), 55.51 (CH3), 56.08 (CH3), 104.89 (CH), 108.20
(CH), 110.38 (CH), 110.99 (CH), 111.09 (CH), 116.38 (CH),
119.46 (CH), 120.23 (CH), 120.56 (CH), 120.66 (CH), 121.50 (C)
123.51 (C), 123.65 (C), 124.17 (C), 124.55 (C), 125.88 (CH),
126.48 (CH), 129.13 (C), 130.65 (C), 133.19 (C), 139.49 (C),
141.76 (C), 145.87 (C), 146.57 (C), 168.03 (C); ESI-MS (+50 V):
m/z = 487 [M + Na]+, (−100 V): m/z = 463 [M − H]−; MS (EI):
m/z (%) = 464 (26, M+), 255 (32), 240 (8), 224 (8), 210 (100), 167
(11); HRMS: m/z calcd for C29H24N2O4 (M+): 464.1736; found:
464.1741.
O. Kataeva, I. Bauer and H.-J. Knölker, Chem. – Eur. J.,
2012, 18, 770; (h) L. Huet, R. Forke, A. Jäger and
H.-J. Knölker, Synlett, 2012, 1230; (i) C. Börger and
H.-J. Knölker, Tetrahedron, 2012, 68, 6727.
6 (a) V. P. Kumar, K. K. Gruner, O. Kataeva and H.-J. Knölker,
Angew. Chem., 2013, 125, 11279, (Angew. Chem. Int. Ed.,
2013, 52, 11073); (b) C. Börger, A. W. Schmidt and
H.-J. Knölker, Synlett, 2014, DOI: 10.1055/s-0033-1338621.
7 C. Börger, M. P. Krahl, M. Gruner, O. Kataeva and
H.-J. Knölker, Org. Biomol. Chem., 2012, 10, 5189.
8 C. Börger, O. Kataeva and H.-J. Knölker, Org. Biomol.
Chem., 2012, 10, 7269.
9 (a) A. S. Guram, R. A. Rennels and S. L. Buchwald, Angew.
Chem., 1995, 107, 1456, (Angew. Chem. Int. Ed. Engl., 1995,
34, 1348); (b) J. Louie and J. F. Hartwig, Tetrahedron Lett.,
1995, 36, 3609; (c) S. Shekhar, P. Ryberg, J. F. Hartwig,
J. S. Mathew, D. G. Blackmond, E. R. Strieter and
S. L. Buchwald, J. Am. Chem. Soc., 2006, 128, 3584;
(d) B. Schlummer and U. Scholz, Adv. Synth. Catal., 2004,
346, 1599.
10 (a) F. Ullmann, Ber. Dtsch. Chem. Ges., 1903, 36, 2382;
(b) F. Ullmann, Ber. Dtsch. Chem. Ges., 1904, 37, 853;
(c) I. Goldberg, Ber. Dtsch. Chem. Ges., 1906, 39,
1691; (d) S. V. Ley and A. W. Thomas, Angew. Chem.,
2003, 115, 5558, (Angew. Chem. Int. Ed., 2003, 42,
5400).
Notes and references
§Bismurrayafoline-A (1): Light brownish solid, mp 209–210 °C; UV (MeOH): λ =
224, 243, 251, 262 (sh), 282, 292, 323 (sh), 338, 352 (sh) nm; MS (EI): m/z (%) =
420 (M+, 40), 210 (100).
Bismurrayafolinol (2): Colourless solid, mp 120–121.5 °C; UV (MeOH): λ = 225,
243, 252, 261 (sh), 281, 292, 331, 343 (sh) nm; MS (EI): m/z (%) = 436 (M+, 12),
210 (100).
Chrestifoline-D (3): Colourless solid, mp 163.5–164 °C; UV (MeOH): λ = 222,
242, 251, 275, 291, 338, 346 (sh) nm; MS (EI): m/z (%) = 434 (M+, 22), 210 (100).
Chrestifoline-B (4): Light brownish solid, mp 106–107 °C; UV (MeOH): λ = 227
(sh), 239, 251 (sh), 258 (sh), 290, 325, 338, 358 (sh) nm; ESI-MS (−10 V): m/z =
471 [M − H]−.
( )-Chrestifoline-C (5): Light brownish solid, mp 90 °C; UV (MeOH): λ = 228
(sh), 242, 251 (sh), 282 (sh), 290, 327, 340, 360 nm; ESI-MS (+10 V): m/z = 541
[M + H]+.
For the 1H and 13C NMR spectra of the biscarbazoles 1–5, see: ESI.‡
11 H. Furukawa, T.-S. Wu and T. Ohta, Chem. Pharm. Bull.,
1983, 31, 4202.
1 (a) H.-J. Knölker and K. R. Reddy, Chem. Rev., 2002, 102, 12 M. Itoigawa, Y. Kashiwada, C. Ito, H. Furukawa,
4303; (b) H.-J. Knölker and K. R. Reddy, in The Alkaloids,
ed. G. A. Cordell, Academic Press, Amsterdam, 2008, vol.
Y. Tachibana, K. F. Bastow and K.-H. Lee, J. Nat. Prod.,
2000, 63, 893.
65, p. 1; (c) A. W. Schmidt, K. R. Reddy and H.-J. Knölker, 13 G. Bringmann and S. Tasler, Tetrahedron, 2001, 57,
Chem. Rev., 2012, 112, 3193. 2337.
2 (a) H.-J. Knölker, Top. Curr. Chem., 2005, 244, 115; 14 C. Ito, T.-S. Wu and H. Furukawa, Chem. Pharm. Bull., 1987,
(b) I. Bauer and H.-J. Knölker, Top. Curr. Chem., 2012, 309,
203.
3 (a) D. P. Chakraborty and S. Roy, in Progress in the Chem-
istry of Organic Natural Products, ed. W. Herz, H. Grisebach,
35, 450.
15 C. Ito, N. Okahana, T.-S. Wu, M.-L. Wang, J.-S. Lai,
C.-S. Kuoh and H. Furukawa, Chem. Pharm. Bull., 1992, 40,
230.
G. W. Kirby, W. Steglich and C. Tamm, Springer-Verlag, 16 C. Ito, T.-S. Wu and H. Furukawa, Chem. Pharm. Bull., 1990,
Wien, 1991, vol. 57, p. 71; (b) D. P. Chakraborty, in The 38, 1143.
Alkaloids, ed. G. A. Cordell, Academic Press, New York, 17 D. P. Chakraborty, B. K. Barman and P. K. Bose, Sci. Cult.,
1993, vol. 44, p. 257; (c) C. J. Moody, Synlett, 1994, 681; 1964, 30, 445.
(d) D. P. Chakraborty and S. Roy, in Progress in the Chem- 18 D. P. Chakraborty, K. C. Das and P. K. Bose, Sci. Cult., 1966,
istry of Organic Natural Products, ed. W. Herz, H. Grisebach, 32, 83.
G. W. Kirby, W. Steglich and C. Tamm, Springer-Verlag, 19 (a) H.-J. Knölker and M. Bauermeister, J. Chem. Soc., Chem.
Wien, 2003, vol. 85, p. 125.
Commun.,
1990,
664;
(b)
H.-J.
Knölker
and
4 (a) H. Furukawa, Trends Heterocycl. Chem., 1993, 3, 185;
(b) S. Tasler and G. Bringmann, Chem. Rec., 2002, 2, 113.
5 Reviews: (a) H.-J. Knölker, Curr. Org. Synth., 2004, 1, 309;
(b) H.-J. Knölker, Chem. Lett., 2009, 38, 8; applications:
M. Bauermeister, Tetrahedron, 1993, 49, 11221;
(c) H.-J. Knölker and M. Wolpert, Tetrahedron Lett., 1997,
38, 533; (d) H.-J. Knölker and M. Wolpert, Tetrahedron,
2003, 59, 5317.
(c) H.-J. Knölker and N. O’Sullivan, Tetrahedron, 1994, 50, 20 (a) K. K. Gruner and H.-J. Knölker, Org. Biomol. Chem.,
10893; (d) H.-J. Knölker and K. R. Reddy, Heterocycles, 2003,
60, 1049; (e) R. Forke, A. Jäger and H.-J. Knölker, Org.
Biomol. Chem., 2008, 6, 2481; (f) R. Forke, M. P. Krahl,
F. Däbritz, A. Jäger and H.-J. Knölker, Synlett, 2008, 1870;
(g) T. Gensch, M. Rönnefahrt, R. Czerwonka, A. Jäger,
2008, 6, 3902; (b) K. K. Gruner, T. Hopfmann,
K. Matsumoto, A. Jäger, T. Katsuki and H.-J. Knölker, Org.
Biomol. Chem., 2011, 9, 2057; (c) R. Hesse, K. K. Gruner,
O. Kataeva, A. W. Schmidt and H.-J. Knölker, Chem. – Eur.
J., 2013, 19, 14098.
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21 See for example: (a) R. A. Earley and M. J. Gallagher, 23 R. A. Altman, K. W. Anderson and S. L. Buchwald, J. Org.
J. Chem. Soc. C, 1970, 158; (b) U. Nettekoven and Chem., 2008, 73, 5167.
J. F. Hartwig, J. Am. Chem. Soc., 2002, 124, 1166; 24 D. P. Chakraborty, S. Roy and A. K. Dutta, J. Indian Chem.
(c) S. L. Marquard, D. C. Rosenfeld and J. F. Hartwig, Angew. Soc., 1987, 64, 215.
Chem., 2010, 122, 805, (Angew. Chem. Int. Ed., 2010, 49, 793). 25 (a) A. J. Fatiadi, Synthesis, 1976, 65; (b) H.-J. Knölker,
22 (a) V. H. Rawal and M. P. Cava, Tetrahedron Lett., 1985, 26, J. Prakt. Chem., 1995, 337, 75.
6141; (b) K. E. Knott, S. Auschill, A. Jäger and H.-J. Knölker, 26 Manganese dioxide (precipitated, active) from Merck (art.
Chem. Commun., 2009, 1467.
805958).
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