Synthesis of macrocyclic β-strand templates by ring closing metathesis

Article abstract of DOI:10.1021/jo802723w

We report the synthesis of macrocycles 1-6 via ring closing metathesis of dienes 7-12. Addition of chlorodicyclohexylborane to thermal and microwave assisted RCM of dienes 8 and 9 markedly improves the yield. The geometric isomers of macrocycles 1-3 and 5

Full text of DOI:10.1021/jo802723w

reported for microwave assisted RCM in the solid phase
synthesis of R-helices, with an optimum temperature of 120 °C
using Grubbs second generation catalyst.6 Despite such reports
of improvements, some structural features within the substrate
still limit the efficiency of RCM. For example, co-ordination
of a ruthenium intermediate with a proximate polar functional
group has been reported7 to be detrimental to RCM because of
the formation of a stable intramolecular ruthenium carbene
chelate. In these cases, the addition of a Lewis acid can improve
yields by suppressing the formation of this chelate.8 Chlorodi-
cyclohexylborane has been reported to be particularly good at
facilitating such metathesis reactions.9 In this paper we report
efforts to improve the efficiency of the key RCM macrocycli-
sation of an extended set of peptide based dienes (7-12, Scheme
1), using chlorodicyclohexylborane as an additive under thermal
and microwave-assisted conditions.
Synthesis of Macrocyclic ꢀ-Strand Templates by
Ring Closing Metathesis
Andrew D. Abell,*,† Nathan A. Alexander,
Steven G. Aitken, Hongyuan Chen, James M. Coxon,
Matthew A. Jones, Stephen B. McNabb, and
Andrew Muscroft-Taylor
Department of Chemistry, UniVersity of Canterbury,
PriVate Bag 4800, Christchurch, New Zealand
andrew.abell@adelaide.edu.au
ReceiVed December 23, 2008
The starting dienes 7-12 used in the study were prepared
by standard peptide coupling of either O-allyl or O-homoallyl
N-Boc-Tyr-LeuOH with an appropriate alkene containing amino
acid, Scheme 2. All dienes were then cyclized by RCM (Scheme
1) using the optimum thermal conditions developed in our earlier
study to maintain active catalyst2 (i.e., 3 separate additions of
Grubbs second generation catalyst and 18 h reaction, conditions
A in Table 1). This gave the required macrocycles 1-6 as
mixtures of geometric isomers in modest yields of 49, 22, 51,
34, 22, and 41%, respectively, after purification by column
chromatography. Microwave-assisted RCM of 7-12 under
Conditions B, again using 3 separate additions of Grubbs second
generation catalyst, resulted in slightly improved yields of 1-6
isolated as the same mixtures of alkene isomers in 58, 37, 58,
38, 24, and 42% respectively, Table 1.
We report the synthesis of macrocycles 1-6 via ring closing
metathesis of dienes 7-12. Addition of chlorodicyclohexy-
lborane to thermal and microwave assisted RCM of dienes
8 and 9 markedly improves the yield. The geometric isomers
of macrocycles 1-3 and 5 have been assigned using X-ray
crystallography and NMR.
Importantly, the addition of chlorodicyclohexylborane to the
thermal RCM of dienes 8 and 9 (Conditions C, Table 1) resulted
in a significantly improved yield of 2 and 3. These were isolated
in 82 and 74%, respectively, as the same mixtures of isomers
after purification by column chromatography. The addition of
chlorodicyclohexylborane under microwave-assisted conditions
(D) gave still further improvement, with 91% and quantitative
yield of 2 and 3 respectively (Table 1). Here chlorodicyclo-
hexylborane is considered to disrupt the stable and nonproduc-
tive six-membered ruthenium carbene chelate that would form
on reaction of the catalyst with the alkene proximal to the ester
of the respective dienes (see Figure 1).
Some comparative studies were carried out using titanium(IV)
isoproxide as an alternative Lewis acid, given reports8 of its
use in disrupting such chelates. In particular, RCM of 8 in the
presence of titanium(IV) isoproxide under thermal conditions
(analogous to conditions C) gave a much reduced yield of 2
(34%). In addition, RCM of 8 with a single addition of catalyst
(10 mol %) with either titanium(IV) isoproxide or chlorodicy-
clohexylborane as additive, gave alkene along with significant
Proteases are known to bind their substrates and inhibitors
in a ꢀ-strand geometry.1 On the basis of this observation, we
recently reported2 the design and synthesis of macrocyclic
ꢀ-strand templates (1-3 and 6) for use in the development of
cysteine protease inhibitors. A key step in their synthesis
required thermal ring-closing metathesis (RCM) of an appropri-
ate peptide-based diene (7-9 and 12), however, yields were
generally moderate.
Factors that influence the efficiency of RCM, such as the
nature of the solvent and reaction temperature, have been well
studied.3,4 In addition, the effect of microwave irradiation has
been reported to increase the yield and the rate of RCM.5 For
example, improved yields and short reaction times have been
† Current address, School of Chemistry and Physics, University of Adelaide,
South Australia.
(1) Tyndall, J. D. A.; Nall, T.; Fairlie, D. P. Chem. ReV. 2005, 105, 973–
999.
(2) Abell, A. D.; Jones, M. A.; Coxon, J. M.; Morton, J. D.; Aitken, S. G.;
McNabb, S. B.; Lee, H. Y.-Y.; Mehrtens, J. M.; Alexander, N. A.; Stuart, B. G.;
Neffe, A. T.; Bickerstaffe, R. Angew. Chem., Int. Ed. 2009, 48, 1455–1458.
(3) Champagne, T. M.; Hong, S. H.; Lee, C. W.; Ung, T. A.; Stoianova,
D. S.; Pederson, R. L.; Kuhn, K. M.; Virgil, S. C.; Grubbs, R. H. Abstracts of
Papers, 236th ACS National Meeting, Philadelphia, PA, United States, August
17-21, 2008, ORGN-077.
(4) Tsantrizos, Y. S.; Ferland, J.-M.; McClory, A.; Poirier, Farina, V.; Nathan,
K.; Yee, Wang, X.-J.; Haddad, N.; Wei, X.; Xu, J.; Zhang, L. J. Organomet.
Chem. 2006, 691, 5163–5171.
(5) (a) Yang, C.; Murray, W. V.; Wilson, L. J. Tetrahedron Lett. 2003, 44,
1783–1786. (b) Mayo, K. G.; Nearhoof, E. H.; Kiddle, J. J. Org. Lett. 2002, 4,
1567–1570.
(6) Chapman, R. N.; Arora, P. S. Org. Lett. 2006, 8, 5825–5828.
(7) Furstner, A.; Langemann, K. J. Am. Chem. Soc. 1997, 119, 9130–9136.
(8) Yang, Q.; Xiao, W.-J.; Yu, K. Org. Lett. 2005, 7, 871–874.
(9) Vedrenne, E.; Dupont, H.; Oualef, S.; Elkaim, L.; Grimaud, L. Synlett
2005, 4, 670–672.
4354 J. Org. Chem. 2009, 74, 4354–4356
10.1021/jo802723w CCC: $40.75 2009 American Chemical Society
Published on Web 04/27/2009

SCHEME 1. RCM of Dienes 7-12a
TABLE 1. RCM of Dienes 7-12
diene RCM conditiona productb ring size
ratioc
conversion (%)d
7
A
B
C
D
A
B
C
D
A
B
C
D
A
B
C
D
A
B
C
D
A
B
C
D
1
16
17
18
18
19
19
19(E):1(Z)
20(E):1(Z)
19(E):1(Z)
20(E):1(Z)
10(E):1(Z)
9(E):1(Z)
9(E):1(Z)
9(E):1(Z)
1.3(E):1(Z)
1.7(E):1(Z)
1.9(E):1(Z)
1.8(E):1(Z)
-
49
58
50
48
22
37
82
91
51
58
74
quant
34
38
41
39
22
24
30
33
41
42
45
40
8
2
9
3
10
11
12
4e
5
-
-
-
8(E):1(Z)
8(E):1(Z)
9(E):1(Z)
10(E):1(Z)
10:1
9:1
9:1
11:1
6f
a All reactions performed in 1,1,2 TCE with 3 separate additions of
10 mol % Grubbs 2nd generation catalyst. Condition A: Thermal reflux
18 h. Condition B: Microwave reflux 1 h. Condition C: Thermal reflux
with Lewis acid 18 h. Conditions D: Microwave reflux with Lewis acid
1 h. b RCM metathesis result in mixtures of geometric isomers. c Ratios
determined by 1H NMR before purification (conditions D). d Isolated
yields following column chromatography. e Multiple macrocyclic prod-
ucts were formed due to alkene migration. f Configuration of alkene
unable to be assigned.
SCHEME 2. Preparation of Dienes 7-12 from O-Allyl or
O-Homoallyl N-Boc-Tyr-LeuOHa
a Reagents and conditions: (i) Grubbs 2nd generation catalyst, see Table 1.
returned starting material. Thus the optimum conditions for
RCM would appear to be the use of chlorodicyclohexylborane
as Lewis acid and 3 sequential additions of Grubbs second
generation catalyst, with microwave irradiation showing some
advantage.
a Reagents and conditions: (i) (S)-homoallyl-Gly-OMe, DMF, DIPEA,
EDC, HOAT (76%); (ii) (S)-O-allyl-Ser-OMe.HCl, DMF, DIPEA, HATU
(77%).
Next we investigated the influence of chlorodicyclohexylbo-
rane on the RCM of dienes 7 and 10-12. Here the addition of
chlorodicyclohexylborane under both thermal and microwave
conditions did not give a significant improvement in the
efficiency of macrocyclisation, Table 1. The less stable seven-
and eight-membered chelates that might form in reactions of
10-12 (Figure 1) would unlikely interfere with cyclization. Thus
we expect these reactions to be unaffected by the addition of a
Lewis acid, as was observed. Reaction of the vinyl group of
diene 7 with Grubbs catalyst would give a particularly stable
five-membered ring chelate that would likely be unaffected by
the addition of chlorodicyclohexylborane, again as was the case.
The near quantitative formation of 2 and 3 using chlorodicy-
clohexylborane and microwave RCM conditions thus provides
the optimum route for scale up synthesis of these important
macrocycles.2
FIGURE 1. Structure of ruthenium chelates.
as such isomer ratios are not reported for this case. Macrocycle
2 was recrystallized from methanol to give the major alkene as
fine-plate like needles. Single X-ray analysis of this material
revealed the (E)-configuration (Figure 2).10 This is consistent11
with the coupling constant observed for the alkenic protons
(J ) 15.5 Hz) in the major alkene of the mixture. The crystal
structure also revealed a ꢀ-strand geometry for the peptide
backbone as defined by P2(Leu) Φ and Ψ torsion angles within
The ratio of isomers of alkenes was similar in samples of
1-6 formed under each of the four reaction conditions, with
Table 1 showing results for Conditions D. RCM of 10 resulted
in some ring contraction to give a mixture of 16-, 17- and 18-
membered macrocycles as revealed by mass spectrometry and
(10) Structures were solved by standard methods and atomic coordinates have
been deposited with the Cambridge Structural Database.
(11) Coupling constants of J ) 13.0 to 15.0 Hz are associated with E-alkenes
(Kessler, H.; Seip, S. NMR of Peptide. In Two-dimensional NMR Spectroscopy:
Application for Chemists and Biochemists, 2nd ed.; Croasmun, W. R., Carlson,
R. M. K., Eds.; VCH Publishers: New York, 1994; pp 619-650).
J. Org. Chem. Vol. 74, No. 11, 2009 4355

To a solution of diene in anhydrous 1,1,2-trichloroethane (0.01 M)
under an inert atmosphere was added Grubb’s second generation
catalyst (10 mol %), and the reaction mixture was heated for 20
min in a microwave (1200 W, 110-115 °C). Two further portions
of catalyst (2 × 10 mol %) were added with 20 min heating between
each addition. The reaction mixture was cooled, stirred overnight
with activated charcoal, filtered, and the solvent removed in Vacuo.
Condition C. Thermal reflux with Lewis acid. To a solution of
diene in anhydrous 1,1,2-trichloroethane (0.01 M) under an inert
atmosphere was added Grubb’s second generation catalyst (10 mol
%) and chlorodicyclohexyl borane (1 M solution in hexane, 10 mol
%). The reaction mixture was heated under reflux for 1 h and then
treated as for A with two further equivalents of catalyst. Condition
D. Microwave reflux with Lewis acid. To a solution of diene in
anhydrous 1,1,2-trichloroethane (0.01 M) under an inert atmosphere
was added Grubb’s second generation catalyst (10 mol %) and
chlorodicyclohexyl borane (1 M solution in hexane, 10 mol %).
The reaction mixture was then heated for 20 min in a microwave
(1200 W, 110-115 °C) and then treated as for B with two further
equivalents of catalyst.
(E)/(Z)-(7S,10S,13S)-13-tert-Butoxycarbonylamino-10-isobutyl-
9,12-dioxo-2-oxa-8,11-diaza-bicyclo[13.2.2]nonadeca-1(18),4,15(19),16-
tetraene-7-carboxylic acid Methyl Ester (2). Diene 8 (2.00 g, 3.67
mmol) was subjected to RCM under optimum conditions D. The crude
material was purified by flash chromatography on silica using a gradient
of ethyl acetate and (50/70) petrol ether to give 2, a white solid (1.73
g, 91%) as a 9:1 mixture of isomers. mp 241-243 °C; [R]D20 +32.5
(c 1.0, CHCl3); 1H NMR for major isomer from mixture (500 MHz in
CDCl3): δ 7.01 (2H, app d, J ) 5.5 Hz, HArO), 6.78 (2H, app d, J )
5.5 Hz, HArO), 5.87 (1H, d, J ) 8.5 Hz, NH), 5.81 (1H, d, J ) 7.0
Hz, NH), 5.54 (1H, app dt, J ) 16.0, 3.8, and 3.8 Hz,
OCH2CHCHCH2), 5.44 (1H, ddd, J ) 16.0, 6.5, and 1.5 Hz,
OCH2CHCHCH2), 5.34 (1H, d, J ) 8.5 Hz, NH), 4.73 (1H, ddd, J )
9.0, 8.5, and 3.0 Hz, CHCO2CH3), 4.64 (1H, J ) 15.5 Hz OCHH-
CHCHCH2) 4.59 (1H, J ) 15.5 and J ) 4.2 Hz, OCHHCHCHCH2),
4.09-4.20 (2H, m, CHCH2ArO and CHCH2CH(CH3)2), 3.74 (3H, s,
OCH3), 3.08 (1H, dd, J ) 12.7 and 5.0 Hz, CHCHHAr), 2.73 (1H, J
) 12.7 and 12.1 Hz, CHCHHAr), 2.67 (1H, J ) 14.6 Hz,
CHHCHCO2CH3), 2.34 (1H, m, CHHCHCO2CH3), 1.52-1.58 (3H,
m, CH(CH3)2 and CHCH2CH(CH3)2), 1.44 (9H, s, (CH3)3), 0.84-0.88
(6H, m, CHCH2CH(CH3)2); 13C NMR for major isomer from mixture
(75 MHz, CDCl3): δ 171.9, 171.0, 170.7, 156.1, 155.0, 129.7, 128.4,
128.1, 127.5, 115.8, 79.8, 66.4, 57.0, 52.5, 51.8, 51.6, 42.8, 38.8, 34.7,
28.3, 24.5, 24.3, 22.7, 22.5; HRMS (ES) 518.2869 (MH+). C27H40N3O7
requires 518.2866.
FIGURE 2. ORTEP diagram of the X-ray crystal structure of (E)-2
showing a ꢀ-strand peptide backbone with Φ ) -147.3° and Ψ )
119.7° with respect to the P2(Leu) Φ and Ψ angles.
the ranges -160° < Φ < -100° and 90° < Ψ < 160° (Figure
2). This conformation is central to the ability of these derivatives
to bind to proteases and is thus a key finding.1
The configuration of the alkenes in macrocycles 1, 3, and 5
1
were established by H NMR analysis of the major isomer
evident in each of the crude reaction mixtures. Alkene coupling
constants (J ) 15.5, 15.0, and 16.0 Hz, respectively) were
observed in each case which is consistent with an (E) config-
uration.11 The H NMR spectrum of the metathesis product
1
mixture from reaction of 10 was complicated due to the before
mentioned formation of multiple macrocyclic products resulting
from alkene migration. The alkene vicinal coupling constant of
6 was not measurable due to the chemical shift of the vinyl
protons of the major isomer having identical chemical shifts.
The major geometric alkene isomer from the RCM of 10 and
12 could, therefore, not be definitively assigned.
In conclusion, addition of chlorodicyclohexylborane to ther-
mal and microwave promoted RCM of dienes 8 and 9 affords
near quantitative formation of macrocycles 2 and 3. This is of
particular significance in that the macrocycle 2 is a key
intermediate in the synthesis of a potent inhibitor of calpain 2,
which shows much promise in the treatment of cataract.2 These
results may also be applicable to other studies where long
reactions times for RCM have been reported.12 The yield of
macrocycle reflects inversely on the stability of a reuthenium
carbene chelate and directly on the ability of chlorodicyclohexyl
borane to disrupt chelation. The crystal structure of (E)-2 defined
its absolute configuration and revealed a ꢀ-strand geometry for
its peptide backbone as is required for binding to a protease.
Diene 8 (50 mg, 0.1 mmol) was also subjected to RCM under
conditions C using titanium(IV) isoproxide (10 mol %) in place of
chlorodicyclohexyl borane to give (E)-2 as a white solid after
purification by chromatography as described above (16 mg, 34%).
Data as recorded above.
Experimental Section
Acknowledgment. We acknowledge Dr M. Polson (X-ray
crystallography) and financial support from New Zealand Public
Good Science and Technology Fund, Foundation for Research
Science and Technology, Australian Research Council, and
Douglas Pharmaceuticals Limited.
Ring Closing Metathesis: Condition A. Thermal reflux. To a
solution of diene in anhydrous 1,1,2-trichloroethane (0.01M) under
an inert atmosphere was added Grubb’s second generation catalyst
(10 mol %) and the mixture heated under reflux. After 1 h a second
portion of catalyst (10 mol %) was added, and the mixture
washeated for 1 h before the final portion (10 mol %) was added.
The reaction mixture was heated under reflux for a further 16 h,
cooled, stirred overnight with activated charcoal, filtered, and the
solvent was removed in Vacuo. Condition B. Microwave reflux.
Supporting Information Available: 1H NMR spectra for all
compounds, X-ray structural analysis of (E)-2, and the prepara-
tion of diene 11 and compounds 1 and 3-6 under Conditions
D for RCM. This material is available free of charge via the
Internet at http://pubs.acs.org.
(12) Boyle, T. P.; Bremner, J. B.; Jonathan Coates, J.; Deadman, J.; Keller,
P. A.; Pyne, S. G.; Rhodes, D. I. Tetrahedron 2008, 64, 11270–11290.
JO802723W
4356 J. Org. Chem. Vol. 74, No. 11, 2009