Application of the copper catalysed N-arylation of amidines in the synthesis of analogues of the chemical tool, blebbistatin

Article abstract of DOI:10.1039/c0cc03624b

A robust protocol for the CuI-catalysed arylation of amidines is presented. Whilst the initially identified conditions were useful for benzamidine-derived substrates, difficulties were encountered with more complex substrates. This problem was overcome following a change in ligand type, enabling the synthesis of analogues of the chemical tool, blebbistatin.

Full text of DOI:10.1039/c0cc03624b

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Application of the copper catalysed N-arylation of amidines
in the synthesis of analogues of the chemical tool, blebbistatinw
Christopher P. A. T. Lawson, Alexandra M. Z. Slawin and Nicholas J. Westwood*
Received 2nd September 2010, Accepted 26th October 2010
DOI: 10.1039/c0cc03624b
A robust protocol for the CuI-catalysed arylation of amidines is
presented. Whilst the initially identified conditions were useful
for benzamidine-derived substrates, difficulties were encountered
with more complex substrates. This problem was overcome
following a change in ligand type, enabling the synthesis of
analogues of the chemical tool, blebbistatin.
function.6 We have previously reported the synthesis of 3
and its analogues in a sequence starting from anthranilates
such as 4 and lactam 5a.7
Considerable advances have been made in the copper-catalysed
Ullmann N-arylation reaction due to the large demand for
N-arylated heterocycles in natural product synthesis, chemical
biology and drug discovery. Whilst examples of the Ullmann
N-arylation of amines, anilines, amides, carbamates and N-
containing heterocycles abound,1,2 there are relatively few
reports of the use of simple amidines. Exceptions include
elegant work on the copper-catalysed arylation of amidines en
route to various heterocyclic systems3 and the functionalisation
of several amidine-containing heterocycles.4
A recent report5 described the use of amidine hydrochlorides
in the copper-catalysed synthesis of arylamines with the
amidine salt serving as an ammonia surrogate (Scheme 1).
This useful reaction was proposed to occur via initial N-
arylation of the amidine hydrochloride (1 or 2a) in the
presence of CuI, L-proline and Cs2CO3. Hydrolysis of the
resulting N-arylamidine by water, produced in the initial
reaction of the amidine salt with Cs2CO3, delivers the desired
aniline. Based on this mechanistic proposal, we reasoned that if
water was removed, either by use of the free base of the
amidine or by addition of molecular sieves, it should be
possible to isolate the N-arylated amidine. Here we report
the results of our attempts to optimise the conditions required
to N-arylate amidine-containing substrates using Ullmann-
type procedures. Our studies culminate in the synthesis of
novel analogues of blebbistatin (3) (Scheme 2), an
increasingly popular chemical tool used to study myosin
Scheme 2 Previous approach to (S)-(À) blebbistatin (3).7
Initial studies focused on the reaction of acetamidine
hydrochloride (1) with p-iodoanisole (6a). The only product
isolated was the corresponding aniline, anisidine 7a
(Scheme 1), albeit in lower yield than previously reported
(48% vs. 78%,5 Table 1, entry 1). Repeating the reaction
with the sole change of the addition of predried 4 A
molecular sieves only resulted in a decrease in the yield of 7a
(entry 2) with no additional products being isolated.
In line with the literature5 we found that benzamidine
hydrochloride (2a) was a less efficient substrate for the
synthesis of aniline 7a than 1 (entry 3). However, trace
amounts of the desired amidine 8a were also isolated from this
1
reaction. Small molecule X-ray crystallographic and H NMR
analysis of 8a was consistent with the tautomer shown in
Table 1.w When predried 4 A molecular sieves were added to
this reaction, a 48% isolated yield of 8a was achieved (entry 4).
8a could also be prepared when commercially available
benzamidine 2b was used in place of its hydrochloride salt
(entry 5). The presence of sieves improved the yield further to
45% (entry 6) and these conditions also enabled the synthesis of
8b in 66% yield (entry 7). No reaction was observed in the
absence of copper iodide, although a 24% yield of 8a was
obtained in the absence of ligand (entries 8 and 9). Changing
the solvent used from DMF to toluene leads to a significant
improvement in the reaction yields with 8a and 8b being
prepared in 73% and 79%, respectively (entries 10 and 11).
Attempts to use DMSO as the reaction solvent led to a decrease
in yield of 8a (entry 12). Finally, an attempt was made to reduce
the catalyst loading. Whilst a reduction in the yield of 8a from
73% (entry 10) to 60% (entry 13) was observed when 5 mol%
copper iodide and 10 mol% L-proline were used, it was decided
to explore the scope of the reaction by varying the aryl halide
using this lower catalyst loading (entries 14–22). Under the
chosen conditions, a preference for electron-donating groups
at the 4-position of the iodide was observed. Strong electron-
withdrawing groups such as 4-nitro were not well tolerated at
this position,3a but substrates containing the relatively weakly
electron-withdrawing chloro-group at either the 4- or 3-position
Scheme 1 Reported5 conversion of amidine salts to anilines.
School of Chemistry and the Biomedical Sciences Research Complex,
University of St Andrews, North Haugh, St Andrews, Fife,
Scotland KY16 9ST, UK. E-mail: njw3@st-andrews.ac.uk;
Fax: +44 (0)1334 462595; Tel: +44 (0)1334 463816
w Electronic supplementary information (ESI) available: Experimental
procedures and spectroscopic characterisation for all new compounds
are provided. CCDC 791559–791561 (8a, 8b, 9b). For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc03624b
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 1057–1059 1057

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Table 1 Amidine N-arylation
Entry
Amidine
Halide 6e
CuI/mol%
L-prolinef/mol%
Solvent
Yield of 8 (and 7a)
1
2
3
4
5
6
7
8
1
6a
6a
6a
6a
6a
6a
6b
6a
6a
6a
6b
6a
6a
6b
6c
6d
10
10
10
10
10
10
10
0
10
10
10
10
5
5
5
5
5
5
5
5
5
20
20
20
20
20
20
20
20
0
20
20
20
10
10
10
10
10
10
10
10
10
10
10
10
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Toluene
Toluene
DMSO
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
0 (48d)
1 + Sieves
2a
2a + Sieves
2b
0 (28d)
Trace (29d)
48
37
45
66
0
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
2b + Sieves
9
24
73
79
49
60
69
41
17
50
53
36
37
52
21
46
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
6e
6f
6g
6h
6i
6j
5
5
5
p-Bromo-toluene
p-Chloro-toluene
a
b
c
d
2-Iodothiophene. 4-Iodopyridine. 4-Phenyl-iodobenzene. Isolated yield of 7a. 1.2 eq. with respect to amidine. Difficulties in dissolving all
e
f
the proline and base were experienced under the reaction conditions.
led to the desired amidines in moderate yields. Heteroaromatic
iodides were also shown to work (entries 20 and 21). No attempt
was made to optimise further the yields of these
transformations. During the course of these experiments, a
small amount of an additional product was frequently
observed. In one case, this product was isolated and shown to
be the diarylated amidine 9b (Table 1) from the reaction with 6b.
The structure of 9b was confirmed by X-ray crystallographic
analysis.3a w The synthesis of 8b using p-bromo-toluene was also
successful although p-chloro-toluene did not work under these
conditions (entries 23 and 24).
therefore appealing. The required substrate for these studies,
amidine 10 (Scheme 3), was prepared in racemic form in 6 steps
from methyl 4-methyl anthranilate (4). Key steps in this
sequence included the C3a-chlorination9 of quinolone 11 to
Having demonstrated that it was possible to prepare N-
arylated benzamidines using this approach, our focus turned to
the blebbistatin system (Schemes 2 and 3). Detailed inspection
of the recently reported X-ray crystal structure of (S)-
(À)-blebbistatin (3) bound to non-muscle myosin II from
D. discoideum showed that whilst some 75% of 3 is enclosed
within a glove like binding pocket, the N1-phenyl ring in 3
protrudes out of the main binding site.8 The synthesis of
alternative N1-arylated analogues of 3 may therefore lead to
an increase in inhibitor potency or induce selectivity within the
myosin family. In our previous approach to 3 and its analogues
(Scheme 2), the N1-substitutent was incorporated via lactam 5
early in the synthesis. The potential for the late stage
incorporation of a range of N1-aryl or heteroaryl groups was
Scheme 3 Reagents and conditions: (a) 5b (1.2 eq.), POCl3 (1.2 eq.),
DCM, RT, 3 h then 4 (1.0 eq.), reflux 18 h, 91%; (b) LiHMDS (2.5 eq.),
THF, À78 1C to 0 1C, 3 h, 89%; (c) dichloroisocyanuric acid (0.5 eq.),
THF : H2O (1 : 1), RT, 4 h, 91%; (d) aq. NaOH (1.85 eq.), THF : H2O
(1 : 1), 18 h, 52%; (e) TIPSOTf (3.0 eq.), iPr2NEt, (4.0 eq.), reflux, 6 h,
80%; (f) CAN (4.5 eq.), MeCN : H2O (1 : 1), 8 h, 50%.
c
1058 Chem. Commun., 2011, 47, 1057–1059
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Table 2 N1-arylation of 10: ligand screen
to good yields. Again, a drop off in the yields at lower catalyst
loading and a preference for the use of electron-rich iodides
were observed. To date no evidence for the formation of the
N9-isomer of 3a,b,e,g–j has been observed in these reactions.
When this protocol was applied to the synthesis of a number of
the benzamidines described in Table 1 (see Table S1, ESIw for
details) analogous reactivity was observed although the
reaction yields were lower.
In summary, a novel approach to analogues of the small
molecule tool blebbistatin
approach enables the facile introduction of N1-aryl
substituents providing flexible and rapid route to
3 has been developed. This
a
modifying a key region of this bioactive molecule. Whilst
initial studies focused on the modification of a literature
protocol,5 these conditions proved applicable only to
reactions involving simple amidine synthesis. A change in the
catalyst ligand was required to enable successful reaction of the
more complex substrate 10. Whilst it is difficult to provide a
clear rationalisation as to why the two systems behaved so
differently it is possible that the steric environment around the
amide functional group in 10 plays a role. Assessment of the
biological activity of the new analogues of 3 is ongoing and will
be reported in the near future.
Ligand
Entry Solvent type
CuI/
mol% mol%
Ligand/ Temperature/ Yield
(%)
1C
1
2
3
4
5
6
7
8
Toluene L-Proline 10
DMSO L-Proline 10
20
20
20
20
20
20
20
10
110
80
—
—
—
—
7
25
80
47
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
A
B
C
D
E
E
10
10
10
10
10
5
110
110
110
110
110
110
give 12 and the CAN-mediated PMP deprotection of 13 to give
10, both of which proceeded in reasonable yields.
Unfortunately, all our attempts to N1-arylate 10 using the
conditions optimised for synthesis of, for example, 8b (Table 1,
entry 11), failed to give 3 or its N9-arylated isomer. In the light
of this result, it was decided to resume our search for a catalyst
system that would deliver successful N-arylation of both
benzamidine 2b and 10. Table 2 summarises the results of a
limited ligand screen carried out on the reaction of 10 with
iodobenzene (6k). The ligands were selected based on their use
in previous Ullmann-type procedures.10–12
We would like to acknowledge Dr Tomas Lebl for advice
regarding the analysis of NMR spectra.
Notes and references
1 (a) D. Ma and Q. Cai, Acc. Chem. Res., 2008, 41, 1450–1460;
(b) F. Monnier and M. Taillefer, Angew. Chem., Int. Ed., 2008, 47,
3096–3099; (c) L. Jiang, G. E. Job, A. Klapars and S. L. Buchwald,
Org. Lett., 2003, 5, 3667–3669; (d) S. V. Ley and A. W. Thomas,
Angew. Chem. Int. Ed., 2003, 42, 5400–5449; (e) A. Klapars,
J. C. Antilla, X. Huang and S. L. Buchwald, J. Am. Chem. Soc.,
2001, 123, 7727–7729.
2 For review of related protocols involving palladium see
(a) J. F. Hartwig, Acc. Chem. Res., 1998, 31, 852–860;
(b) J. P. Wolfe, S. Wagaw, J.-F. Marcoux and S. L. Buchwald,
Acc. Chem. Res., 1998, 31, 805–818.
3 (a) X. Deng and N. S. Mani, Eur. J. Org. Chem., 2010, 680–686;
(b) X. Deng, H. McAllister and N. S. Mani, J. Org. Chem., 2009,
74, 5742–5745; (c) X. Liu, H. Fu, Y. Jiang and Y. Zhao, Angew.
Chem., Int. Ed., 2009, 48, 348–351; (d) C. Huang, Y. Fu, H. Fu,
Y. Jiang and Y. Zhao, Chem. Commun., 2008, 6333–6335;
(e) J. Kim, S. Y. Lee, J. Lee, Y. Do and S. Chang, J. Org. Chem.,
2008, 73, 9454–9457; (f) R. D. Viirre, G. Evindar and R. A. Batey,
J. Org. Chem., 2008, 73, 3452–3459; (g) B. G. Szczepankiewicz,
J. J. Rohde and R. Kurukulasuriya, Org. Lett., 2005, 7, 1833–1835.
4 (a) Y. Liu, Y. Bai, J. Zhang, Y. Li, J. Jiao and X. Qi, Eur. J. Org.
Chem., 2007, 6084–6088; (b) C. Ran, Q. Dai and R. G. Harvey,
J. Org. Chem., 2005, 70, 3724–3726.
Whilst none of the desired product 14 was observed with
amino acid-based ligands10 (Table 2, entries 1–3) or the b-
diketone ligand B11 (entry 4), limited success was observed
with the diamine ligand C12 (entry 5). Use of trans-
diaminocyclohexyl-based ligands12 resulted in further
improvements in yield (entries 6 and 7) enabling the synthesis
of 14 as a single regioisomer in 80% yield with a catalyst loading
of 10 mol%. A reduction in yield was observed when a lower
catalyst loading was used (entry 8). The protocol using ligand E
followed by desilylation of the resulting crude products was then
applied to the synthesis of blebbistatin analogues 3a,b,e,g–j by
reaction of 10 with a range of aryliodides (Table 3).
5 X. Gao, H. Fu, R. Qiao, Y. Jiang and Y. Zhao, J. Org. Chem.,
2008, 73, 6864–6866.
The newly developed protocol proved reasonably robust
with a series of novel analogues being prepared in moderate
6 A. F. Straight, A. Cheung, J. Limouze, I. Chen, N. J. Westwood,
J. R. Sellers and T. J. Mitchison, Science, 2003, 299, 1743–1747.
7 (a) C. Lucas-Lopez, J. S. Allingham, T. Lebl, C. P. A. T. Lawson,
R. Brenk, J. R. Sellers, I. Rayment and N. J. Westwood, Org.
Biomol. Chem., 2008, 6, 2076–2084; (b) C. Lucas-Lopez,
S. Patterson, T. Blum, A. F. Straight, J. Toth, A. M. Z. Slawin,
T. J. Mitchison, J. R. Sellers and N. J. Westwood, Eur. J. Org.
Chem., 2005, 1736–1740.
Table 3 N1-arylation of 10: halide screen
Entry Halidea CuI/mol% Ligand E/mol% Product Yield (%)
1
2
3
4
5
6
7
6a
6b
6e
6g
6h
6i
10
10
10
5
10
10
5
20
20
20
10
20
20
10
3a
3b
3e
3g
3h
3i
66
63
66
19
53
71
34
8 J. S. Allingham, R. Smith and I. Rayment, Nat. Struct. Mol. Biol.,
2005, 12, 378–379.
9 J. L. C. Marais, W. Pickl and B. Staskun, J. Org. Chem., 1990, 70, 1969.
10 H. Zhang, Q. Cai and D. Ma, J. Org. Chem., 2005, 70, 5164–5173.
11 A. Shafir and S. L. Buchwald, J. Am. Chem. Soc., 2006, 128, 8742–8743.
12 J. C. Antilla, A. Klapars and S. L. Buchwald, J. Am. Chem. Soc.,
2002, 124, 11684–11688.
6j
3j
a
See Table 1 for structures of aryliodides.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 1057–1059 1059