Stereoselective synthesis of new conformationally restricted analogues of a potent CGRP receptor antagonist

Article abstract of DOI:10.1021/ol0510062

(Chemical Equation Presented) A stereocontrolled racemic synthesis of conformationally restricted analogues 2a and 2b of a potent CGRP receptor antagonist 1 by novel functionalization of 2-substituted octahydropyrido[1,2-a] pyrazin-6-ones is described. Th

Full text of DOI:10.1021/ol0510062

ORGANIC
LETTERS
2005
Vol. 7, No. 12
2465-2468
Stereoselective Synthesis of New
Conformationally Restricted Analogues
of a Potent CGRP Receptor Antagonist
Dmitry Zuev,* Jodi A. Michne,† Hong Huang, Brett R. Beno, Dedong Wu, Qi Gao,
John R. Torrente, Cen Xu, Charles M. Conway, John E. Macor, and
Gene M. Dubowchik
Bristol-Myers Squibb Pharmaceutical Research Institute, P.O. Box 5100,
Wallingford, Connecticut 06492
dmitry.zueV@bms.com
Received May 3, 2005
ABSTRACT
A stereocontrolled racemic synthesis of conformationally restricted analogues 2a and 2b of a potent CGRP receptor antagonist 1 by novel
functionalization of 2-substituted octahydropyrido[1,2-a]pyrazin-6-ones is described. The new diastereoselective LDA-promoted
intermediate lactams established the required trans-configuration in the desired products.
r-nitration of
Conformational restriction of biologically active molecules
has significant importance in medicinal chemistry.1 When
the pharmacophore of the molecule of interest is locked into
its binding conformation, the result is greater affinity to the
complementary receptor site, resulting in increased binding
potency. Among the other anticipated benefits are increased
receptor selectivity and metabolic stability of the new
molecule. Moreover, innovative chemistry efforts may
culminate in the identification of original compounds, which
reward a research chemist with a novel chemotype of interest.
It is for these reasons that the strategy of conformational
restriction has attracted widespread attention.2
nists as potential drug candidates for treatment of migraine.3
In the course of our exploration program we proposed struc-
tures 2a and 2b as constrained analogues of a potent CGRP
receptor antagonist 1 (Compound 1) (Figure 1), reported by
Merck scientists.4
Initial molecular modeling results (not shown) suggested
that compound 2a should be a reasonable mimic of com-
pound 1. A subsequent, more sophisticated analysis, which
is described below, supports the original conclusion.5 Ten
conformers of compound 1 predicted to be low in energy
were superimposed with conformers of compound 2a that
For a few years, we have been interested in the discovery of
new calcitonin gene-related peptide (CGRP) receptor antago-
(3) Calcitonin gene-related peptide (CGRP), a 37-amino acid peptide
belonging to the calcitonine family of peptides, is believed to play an
important role in the pathophysiology of migraine.17,18 CGRP binds to a
CGRP receptor, which is a G-protein coupled receptor, consisting of a
classical 7-transmembrane component calcitonine receptor-like receptor
(CRLR) and a receptor activity modifying protein (RAMP).19,20
(4) Edvinsson, L.; Sams, A.; Jansen-Olesen, I.; Tajti, J.; Kane, S. A.;
Rutledge, R. Z.; Koblan, K. S.; Hill, R. G.; Longmore, J. Eur. J. Pharmacol.
2001, 415, 39.
† Present address: AstraZeneca R&D Boston, Infection Discovery, 35
Gatehouse Park, Waltham, MA, 02451.
(1) Mann, A. In Practice of Medicinal Chemistry, 2nd ed.; Wermuth, C.
G., Ed.; Academic Press: San Diego, 2003; Chapter 15, p 233.
(2) Wermuth, C. G. In Practice of Medicinal Chemistry, 2nd ed.;
Wermuth, C. G., Ed.; Academic Press: San Diego, 2003; Chapter 14, p 215.
10.1021/ol0510062 CCC: $30.25
© 2005 American Chemical Society
Published on Web 05/17/2005

Figure 3. Conformationally restricted analogues 4a and 4b of
lidoflazine 5.
to several different low energy conformations of 1 suggested
that 2a should be a reasonable mimic of 1. Encouraging mol-
ecular modeling results prompted us to accomplish the stereo-
selective synthesis of structurally complex molecule 2a and
its analogue 2b, containing the previously unknown 2,6,6-tri-
substituted octahydropyrido[1,2-a]pyrazin-6-one ring system.
A decade ago, Compernolle et al. reported a series of
bicyclic lactams as useful scaffolds for the synthesis of
conformationally restricted analogues of several piperazine-
containing drugs.6,7 As an example, lactam 2-benzyloctahy-
dropyrido[1,2-a]pyrazin-6-one 3b was used in the synthesis
of analogues 4a and 4b of the coronary vasodilator lidofla-
zine 5 (Figure 3).7 In the present publication we wish to
report a novel stereoselective functionalization of bicyclic
lactams 3a and 3b in the synthesis of proposed constrained
analogues 2a and 2b of the CGRP receptor antagonist 1.
Figure 1. Conformationally restricted analogues 2a and 2b of
potent CGRP receptor antagonist 1.
were predicted to be within 5.0 kcal/mol of the lowest energy
conformer identified. An in-house program that superimposes
structures by matching classes of atoms in a reference
structure to similar classes in a query structure and scores
the alignment on the basis of the proximity of similar
functional groups in the query and reference was used. The
overlay of the best-fitting conformer of 2a with a low energy
conformer of 1 is shown in Figure 2. The energies of the
conformers of compounds 1 and 2a are 0.5 and 1.3 kcal/mol
above those of the lowest energy conformers identified for
1 and 2a, respectively. Although none of the conformers of
1 used as references necessarily represent its bioactive con-
formation, the ability of 2a to adopt conformations similar
Scheme 1. Synthesis of 2-Substituted
Octahydropyrido[1,2-a]pyrazin-6-ones 3a and 3b
Figure 2. Overlay of predicted low energy conformations of potent
CGRP antagonist 1 (stick rendering) and compound 2a (ball-and-
stick rendering).
Synthesis of 2-phenyloctahydropyrido[1,2-a]pyrazin-6-one
3a and 2-benzyloctahydropyrido[1,2-a]pyrazin-6-one 3b was
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Org. Lett., Vol. 7, No. 12, 2005

performed by analogy with the previously described protocol
(Scheme 1). With one of the amino functionalities blocked by
an appropriately chosen protecting group, 6a8 and 6b were al-
kylated regioselectively with methyl 6-chloro-5-oxohex-
anoate to give ketoesters 7a and 7b. Subsequent deprotection,
followed by internal reductive amination with sodium
cyanoborohydride in methanol at pH 5, yielded pipera-
zinebutanoic esters 8a and 8b. Potassium carbonate induced
cyclization of these compounds afforded the desired 2-sub-
stituted octahydropyrido[1,2-a]pyrazin-6-ones 3a and 3b in
respective 45% and 85% yields for three steps.
Scheme 3. Further Novel Elaboration of Lactams 3a and 3b
Further novel elaboration of 3a and 3b involved treatment
with LDA and 1,3-dibromo-5-iodomethyl-2-methoxybenzene
10 (prepared in 92% yield from 1,3-dibromo-4-methoxy-
benzaldehyde 9,9 according to Scheme 2) in THF to give
Scheme 2. Synthesis of
1,3-Dibromo-5-iodomethyl-2-methoxybenzene
mixtures of diastereomeric lactams 11a and 11b in corre-
sponding 67% and 71% combined yields (Scheme 3).10
Although the LDA-promoted nitration of amide R-methylene
groups has been previously established,11 no examples of
analogous R-methyne nitrations have been reported to date.
Without separation, pairs of diastereomers 11a and 11b were
deprotonated with LDA and quenched with n-propyl nitrate
to afford a mixture of trans- and cis-nitro-derivatives 12a
and 12b in 5:1 and 8:1 ratios. Despite the moderate yields
(41% and 37%), these clean reactions allowed quantitative
recovery of unreacted starting materials.
obtained in 55% and 73% yields, respectively, by separation
from the minor isomers 14a and 14b using reverse phase
preparative HPLC.12
X-ray crystallographic analysis of the monohydrate of the
double TFA salt of major diastereomer 13b (Figure 4)
Mixtures of nitro-compounds 12a and 12b were carefully
reduced to the corresponding amines with zinc in concen-
trated hydrochloric acid at room temperature to avoid
potential problems of debenzylation and debromination,
commonly observed with transition metal catalyzed reduc-
tions. The resulting major diastereomers 13a and 13b were
(5) For a detailed description of the molecular modeling procedure, see
Supporting Information.
(6) Van den Branden, S.; Compernolle, F.; Hoornaert, G. J. Chem. Soc.,
Perkin Trans. 1 1992, 1035.
(7) Saleh, M. A.; Compernolle, F.; Van den Branden, S.; De Buysser,
W.; Hoornaert, G. J. Org. Chem. 1993, 58, 690.
(8) Nate, H. et al. Chem. Pharm. Bull. 1987, 35, 2825.
(9) Boukouvalas, J.; Lachance, N.; Ouellet, M.; Trudeau, M. Tetrahedron
Lett. 1998, 39, 7665.
(10) Trost, B. M.; Kunz, R. A. J. Org. Chem. 1974, 39, 2475.
(11) Feuer H.; Panda, C. S.; Hou, L.; Bevinakatti, H. S. Synthesis 1983,
187.
Figure 4. X-ray structure of amine 13b.
(12) Conditions for a reverse phase preparative HPLC separation of 13a
and 13b included: column, XTERRA 30 mm × 150 mm S5; injection
volume, 2000 µL; solvent A, CH3OH (10%)/H2O (90%)/TFA (0.1%);
solvent B, CH3OH (90%)/H2O (10%)/TFA (0.1%); starting % B, 20; final
% B, 80; gradient time, 20 min; end time, 25 min.
revealed that the molecule has the trans-configuration at C2
and C5.13 In the solid state, it exists in a double chair
conformation with the 3,5-dibromo-4-methoxybenzyl group
Org. Lett., Vol. 7, No. 12, 2005
2467

Both conformationally restricted analogues were tested in
a [125I]CGRP binding assay,16 where 2a (Ki ) 3.6 µM) was
found to be only weakly active and activity for 2b (Ki > 5
µM) could not be measured. The loss in binding affinity of
2a compared to that of 1 (reported Ki ) 16 nM)4 clearly
indicated a significant difference between the calculated low
energy and binding conformations of the latter compound.
Nonetheless, our chemistry efforts resulted in the preparation
of structurally novel CGRP receptor binding agent 2a and
its binding affinity is the subject of further optimization.
In summary, we developed a racemic synthesis of con-
formationally restricted analogues 2a and 2b of potent CGRP
receptor antagonist 1 by appropriate functionalization of
2-substituted octahydropyrido[1,2-a]pyrazin-6-ones 3a and
3b. The new stereocontrolled LDA-promoted nitration of
lactams 13a and 13b established the required trans-config-
uration in the desired products.
Scheme 4. Completion of the Synthesis of 2a and 2b
Supporting Information Available: Full experimental
procedures and spectral data for new compounds. Complete
details of computational methods for 1 and 2a. SK-N-MC
cell membrane preparation and CGRP binding assay descrip-
tions. This material is available free of charge via the Internet
at http://pubs.acs.org.
positioned axially and the amino group equatorially at C2.
Based on the determined trans configuration of amine 13b,
we concluded that the attack of n-propyl nitrate on the enolate
derived from mixtures 11a and 11b occurred predominantly
from the equatorial direction.
OL0510062
(13) Full crystallographic data for 13b have been deposited to the
Cambridge Crystallographic Data Center (CCDC reference number 238932).
(14) Takeda, K.; Ogura, H. Synth. Commun. 1980, 10, 373.
(15) McOmie, J. F. W.; Watts, M. L.; West, D. E. Tetrahedron 1968,
24, 2289.
Coupling of amines 13a and 13b to commercially available
1-piperidin-4-yl-1,3-dihydro-benzoimidazol-2-one 16 in the
presence of disuccinimidyl carbonate formed ureas 15a and
15b (Scheme 4).14 Low-temperature O-demethylation pro-
moted by boron tribromide15 in dichloromethane cleanly
afforded target molecules 2a and 2b in 87% yields.
(16) For a detailed description of the CGRP binding assay, see Supporting
Information.
(17) Brain, S. D.; Cambridge, H. Gen. Pharmacol. 1996, 27, 607.
(18) Edvinsson, L. CNS Drugs 2001, 15, 745.
(19) Juaneda, C.; Dumont, Y.; Quirion, R. Trends Pharmacol. Sci. 2000,
21, 432.
(20) McLatchie, L. M.; Fraser, N. J.; Main, M. J.; Wise, A.; Brown, J.;
Thompson, N.; Solari, R.; Lee, M. G.; Foord, S. M. Nature 1998, 393, 333.
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