Potent thrombin inhibitors via a 20-membered ring olefin metathesis macrocyclization

Article abstract of DOI:10.1016/S0040-4039(03)00216-8

Twenty-membered ring pyrazinone derived macrocycles were prepared as a means to enhance the potency of existing thrombin inhibitors. Macrocyclization was accomplished via Grubbs olefin metathesis of a highly functionalized allyl-alloc scaffold, thus furth

Full text of DOI:10.1016/S0040-4039(03)00216-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2401–2404
Potent thrombin inhibitors via a 20-membered ring olefin
metathesis macrocyclization
Philippe G. Nantermet* and Harold G. Selnick
Merck Research Laboratories, PO Box 4, West Point, PA 19486, USA
Received 13 January 2003; revised 20 January 2003; accepted 20 January 2003
Abstract—Twenty-membered ring pyrazinone derived macrocycles were prepared as a means to enhance the potency of existing
thrombin inhibitors. Macrocyclization was accomplished via Grubbs olefin metathesis of a highly functionalized allyl-alloc
scaffold, thus further confirming the power of such methodology. © 2003 Elsevier Science Ltd. All rights reserved.
The search for orally active, direct thrombin inhibitors1
has lead our laboratories to the discovery of
pyrazinone2 based small molecules which inhibit
thrombin with a high degree of potency and selectivity.
Studies with proline3 and pyrazinone4 (1, Fig. 1) based
thrombin inhibitors have shown that access to the S4
affinity pocket of the enzyme both from a P1 phenoxy
group or by branching off the P3 group results in
potency enhancement. Examination of X-ray data3
revealed the close spatial proximity of both P4 groups
derived from either P1 or P3. The lure of enhanced
potency by conformational preorganization5 and poten-
tial added metabolic stability6 directed us toward the
design of macrocycles of type 2 which would tether the
P3-branched aminomethyl group and the P4 phenoxy
substituent. We anticipated that the Grubbs olefin
metathesis7 might be an efficient method for the macro-
cyclization of such densely functionalized molecules,
and herein we report on the superiority of such
methodology versus standard macrolactamization
protocols.
The application of the proposed strategy is outlined in
Scheme 1. Starting from commercially available (S)-
Boc-phenylalaninol 3, we could prepare the corre-
sponding azide under Mitsunobu conditions.8 Removal
of the Boc protecting group with anhydrous HCl pro-
vided aminoazide 4. Reaction between bromopyrazi-
none 52 and amine 4 in EtOH at 100°C in a sealed tube
lead to the formation of aminopyrazinone 6 in 73%
isolated yield. Chlorination with N-chlorosuccinimde
and reduction of the azide to the corresponding amine
7, followed by installation of an allyl carbamate and
hydrolysis of the ethyl ester with 1N LiOH in THF lead
to the alloc-carboxylic acid 8. A P1 substituent bearing
an allyl substituent was then installed via the EDC/
HOAt coupling of benzyl amine 9.9 Exposure of
10 to a catalytic amount of Grubbs catalyst10 in
Figure 1.
* Corresponding author.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00216-8

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P. G. Nantermet, H. G. Selnick / Tetrahedron Letters 44 (2003) 2401–2404
Scheme 1.
dichloromethane (0.002 M) at room temperature,
overnight, resulted in the formation of macrocycles 11
and 12 in a 3.5:1 E:Z ratio and in 76% isolated yield.11
This amounts to a fairly impressive result considering
the functional complexity of the system and the pres-
ence on the substrate of various chelating and basic
groups which could immobilize the metal–substrate
complex in a non-productive conformation. The ease of
cyclization could, however, be in part due to substrate
preorganization via internal H-bonding. Catalytic
hydrogenation with Pd/C under 1 atm hydrogen
allowed for the preparation of the saturated macrocy-
cles 13 and 14.12
similar system, attachment of the sarcosine containing
P1–P4 group 249 to 7 followed by deprotection and
macrolactamization under similar conditions as for
compounds 21–23, provided macrocycle 25 in 40% iso-
lated yield. Assuming that both the Boc removal and
ester hydrolysis steps were nearly quantitative in each
of these examples, macrolactamizations occurred in 29–
56% yield suggesting that the more traditional macro-
lactamization strategy is less efficient than the olefin
metathesis approach, at least in this case.
Macrocycles 11–14, 21–23 and 25 were found to be
potent and selective thrombin inhibitors with Ki against
the enzyme in the single digit nanomolar to sub-
nanomolar range.14
As a comparison to the metathesis strategy, Scheme 2
illustrates a more conventional mode of macrocycliza-
tion via the formation of an amide bond. Amine 7 was
coupled to bromides 15–179 to provide intermediates
18–20 which display tethers of 4–6 carbons length. Boc
removal, ester hydrolysis and macrolactamization using
EDC/HOAt conditions in DMF (0.0025 M) at 50°C,
overnight, lead to the formation of macrocycles 21–23
in 38, 56 and 29%, respective isolated yields.13 In a
The results shown in this letter suggest that the Grubbs
olefin metathesis ring closure strategy is more efficient
than the conventional macrolactamization approach.
The facility by which this 20-membered ring cyclization
of a rather densely functionalized molecule occurs is yet
another tribute to the synthetic utility of the olefin
metathesis reaction.

P. G. Nantermet, H. G. Selnick / Tetrahedron Letters 44 (2003) 2401–2404
2403
Scheme 2.
Acknowledgements
Cooper, C. M.; Vacca, J. P. J. Med. Chem. 1998, 41,
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A. M.; Chen, Z.; Naylor-Olsen, A. M.; Lewis, S. D.;
Lucas, B. J.; Freidinger, R. M.; Kuo, L. C. J. Med.
Chem. 1997, 40, 830–832.
We thank Professor M. T. Crimmins for helpful discus-
sions and J. Murphy, S. Pitzenberger, S. Varga, A.
Coddington, C. Ross and H. Ramjit for analytical
support.
4. Unpublished results from our laboratories. Also see: US
Patent, US 6,180,627 B1, issued Jan. 30, 2001.
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6. Bell, I. M.; Gallicchio, S. N.; Abrams, M.; Beese, L. S.;
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1H). HRMS ES calculated for C27H27Cl2N5O5: 572.1462,
found 572.1469.
9. Obtained from the alkylation of the corresponding phe-
nol. US Patent, US 5,792,779, issued Jun. 8, 1998.
12. Compounds 13 and 14 were isolated in a 1:1 ratio. It
should, however, be possible to alter this ratio and per-
haps eliminate the formation of 14 by modifying the
reaction parameters such as the nature of the catalyst
(rhodium for example) and reaction time.
10. Bis(tricyclohexylphosphine)benzylidine
ruthenium(IV)
dichloride, 20 mol%, 0.002 M.
1
11. Compound 11: H NMR (CD3OD+CDCl3, 400 MHz): l
7.83 (bt, J=5.2 Hz, 1H); 7.36–7.10 (m, 7H); 6.83 (s, 1H);
6.82 (d, J=9.2 Hz, 1H); 5.89 (A of ABX, dt, J=16, 4 Hz,
1H); 5.80 (B of ABX, dt, J=16, 4.5 Hz, 1H); 5.16 (A of
AB, d, J=16 Hz, 1H); 4.56–4.29 (m, 6H); 4.29 (B of AB,
d, J=16 Hz, 1H); 3.60 (A of ABX, dd, J=14, 3.6 Hz,
1H); 3.11 (B of ABX, dd, J=14, 8.4 Hz, 1H); 2.99–2.82
(m, 2H). HRMS ES calculated for C27H27Cl2N5O5:
572.1462, found 572.1471. Compound 12: 1H NMR
(CD3OD+CDCl3, 400 MHz): l 7.32–7.12 (m, 7H); 6.89
(d, J=7.8 Hz, 1H); 6.85 (s, 1H); 5.80 (dt, J=11, 5.2 Hz,
1H); 5.64 (dt, J=11, 6.7 Hz, 1H); 4.96 (A of AB, d,
J=16.3 Hz, 1H); 4.69–4.52 (m, 3H); 4.46 (B of AB, d,
J=16.3 Hz, 1H); 4.29–4.20 (m, 1H); 4.16–4.07 (m, 1H);
3.50–3.41 (m, 1H); 3.21–3.12 (m, 1H); 2.96 (A of ABX,
dd, J=14, 7.4 Hz, 1H); 2.79 (B of ABX, dd, J=14, 7 Hz,
13. Compound 21 (dihydrochloride): 1H NMR (d6-DMSO,
400 MHz): l 8.78 (bs, 2H); 7.82 (bs, 1H); 7.68 (d, J=8.5
Hz, 1H); 7.40–7.15 (m, 7H); 7.00 (d, J=7.9 Hz, 1H); 6.92
(s, 1H); 4.85 (bd, J=15 Hz, 1H); 4.60 (bs, 2H); 4.40 (bd,
J=15 Hz, 1H); 4.05–3.95 (m, 1H); 3.95–3.60 (m, H count
obliterated by residual water); 3.42–3.28 (m, 1H); 3.05–
3.15 (m, 1H); 3.15–2.80 (m, 3H); 2.80–2.65 (m, 1H);
1.80–1.55 (m, 2H); 1.55–1.45 (m, 1H); 1.45–1.30 (m, 1H).
HRMS ES calculated for C26H29Cl2N5O3: 530.1720,
found: 530.1737. CHN calculated for C26H29Cl2N5O3+
0.4H2O+2HCl: C, 51.14; H, 5.25; N, 11.47; found: C,
51.13; H, 5.34; N, 11.13%. Rotation: [h]d=−15.4° (c 0.21,
MeOH).
14. Full details to be published elsewhere in due time.