3,8-Diazabicyclo[3.2.1]octan-2-one peptide mimetics: synthesis of a conformationally restricted inhibitor of farnesyltransferase.

Article abstract of DOI:10.1021/ol015504w

A new synthesis of the 3,8-diazabicyclo[3.2.1]octan-2-one framework is described. Transannular enolate alkylation of piperazinone derivatives provides a flexible route to highly constrained bicyclic peptidomimetic synthons with substitution at the Calpha

Full text of DOI:10.1021/ol015504w

ORGANIC
LETTERS
2001
Vol. 3, No. 6
865-868
3,8-Diazabicyclo[3.2.1]octan-2-one
Peptide Mimetics: Synthesis of a
Conformationally Restricted Inhibitor of
Farnesyltransferase†
,‡
Christopher J. Dinsmore,* Jeffrey M. Bergman,‡ Michael J. Bogusky,‡
J. Christopher Culberson,§ Kelly A. Hamilton,| and Samuel L. Graham‡
Departments of Medicinal Chemistry, Molecular Systems, and Cancer Research,
Merck Research Laboratories, P.O. Box 4, West Point, PennsylVania 19486
christopher•dinsmore@merck.com
Received January 2, 2001
ABSTRACT
A new synthesis of the 3,8-diazabicyclo[3.2.1]octan-2-one framework is described. Transannular enolate alkylation of piperazinone derivatives
provides a flexible route to highly constrained bicyclic peptidomimetic synthons with substitution at the Cr position. The chemistry was used
to produce a conformationally constrained farnesyltransferase inhibitor, which aided the elucidation of enzyme-bound conformation.
The manipulation of amino acid conformation is an important
strategy in the chemical investigation of biological systems.
Conformationally constrained amino acid segments have
been important tools for drug design in support of under-
standing and optimizing interactions between ligands and
macromolecules.1 Methods of affixing desired dihedral angles
within the backbone of a peptide segment are numerous,1a-c
and many employ the Freidinger strategy of incorporating
ring-induced constraints.2
in restriction of the φi, ψi, and ωi torsion angles.3 Amino
group acylation induces an A(1,3)-strain-enforced pseudo-
axial position of the CR side chain (R2) substituent, further
restricting the conformation.4 A more rigid variant of 2 with
better defined structure is achieved by the incorporation of
a transannular bridge. Thus, the 3,8-diazabicyclo[3.2.1]octan-
2-one compound 3 is a piperazinone-proline hybrid in which
the side chain (R2) substituent is forced to occupy a
pseudoequatorial position with respect to the piperazinone
ring by virtue of the diaxially disposed ethano bridge. Since
chair-chair interconversion is prevented, the peptide back-
bone torsion angles are severely constrained. While methods
(1) Recent reviews: (a) Hanessian, S.; McNaughton-Smith, G.; Lombart,
H.-G.; Lubell, W. D. Tetrahedron 1997, 53, 12789. (b) Liskamp, R. M. J.
Recl. TraV. Chim. Pays-Bas 1994, 113, 1. (c) Gante, J. Angew. Chem., Int.
Ed. Engl. 1994, 33, 1699. (d) Adang, A. E. P.; Hermkens, P. H. H.; Linders,
J. T. M.; Ottenheijm, H. C. J.; van Staveren, C. J. Recl. TraV. Chim. Pays-
Bas 1994, 113, 63. (e) Giannis, A.; Kolter, T. Angew. Chem., Int. Ed. Engl.
1993, 32, 1244.
(2) Freidinger, R. M.; Veber, D. F.; Perlow, D. S.; Brooke, J. R.;
Saperstein, R. Science 1980, 210, 656.
(3) (a) DiMaio, J.; Belleau, B. J. Chem. Soc., Perkin Trans. 1 1989,
1687. (b) Kolter, T.; Dahl, C.; Giannis, A. Liebigs Ann. 1995, 625.
(4) (a) Hoffman, R. W. Angew. Chem., Int. Ed. Engl. 1992, 31, 1124
and references therein. (b) N-Sugg, E. E.; Griffin, J. F.; Portoghese, P. S.
J. Org. Chem. 1985, 50, 5032.
Piperazinones 2 function as peptidomimetics wherein the
Ni and Ni-1 positions of an amino acid backbone fragment
1 are linked by an ethylene unit. This ring constraint results
† Dedicated to Professor David A. Evans on the occasion of his 60th
birthday.
‡ Department of Medicinal Chemistry.
§ Department of Molecular Systems.
| Department of Cancer Research.
10.1021/ol015504w CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/17/2001

exist for the construction of the unsubstituted scaffold (3,
R2 ) H) from pyroglutamate derivatives,5 most of these
syntheses have provided racemic material,5b-f and none
features the incorporation of a CR side chain substituent. In
this report we describe a versatile synthetic approach to 3
that enables facile substitution at the CR position. We also
discuss the synthesis and biological characterization of a
novel 3,8-diazabicyclo-[3.2.1]octan-2-one farnesyltransferase
inhibitor that provides insight into the enzyme-bound con-
formation of related flexible inhibitors (vide infra).
An amino acid CR side chain substituent was easily
introduced into the bicyclic framework from the piperazinone
intermediate 7, providing a method for the preparation of a
constrained phenylalanine isostere.11 Thus, alkylation of 7
with benzyl bromide (Scheme 2) provided the trans-
Scheme 2a
The synthesis is based on the preparation of an ap-
propriately substituted piperazinone3,6 followed by a pivotal
transannular alkylation reaction7 to install the [3.2.1] bicyclic
ring system (Scheme 1). Boc-(S)-allylglycine 4 was con-
Scheme 1a
a Reagents and conditions: (a) LiHMDS, THF, -78 °C; BnBr,
-78 °C, 3.5 h, 74% borsm. (b) OsO4, NMO, t-BuOH, THF, H2O,
2.5 h; NaIO4, NaHCO3, 2 h. (c) NaBH4, EtOH, 0 °C, 20 min, 70%
two steps. (d) PhSO2Cl, Et3N, CH2Cl2, 0 °C, 1 h, 90%. (e) LDA,
THF, -78 f 0 °C, 2 h, 63%.
disubstituted compound 10 (diastereoselectivity g 95:5).
Smooth conversion as before to the benzenesulfonate 11,
followed by cyclization with strong base (LDA, THF, -78
to 0 °C),12 afforded the constrained bicyclic analogue 9b.
The intermediate 9a, prepared above, was used to syn-
thesize a conformationally restricted inhibitor of farnesyl-
transferase (FTase). FTase is an important posttranslational
processing enzyme that prenylates proteins and enables the
participation of some in signal transduction during cell
proliferation.13 Inhibitors of this enzyme (FTI) are promising
antitumor agents, and several are currently being evaluated
in human clinical trials.14 During our investigations of
peptidomimetic 1-aryl-2-piperazinone FTIs (e.g., 12, Figure
a Reagents and conditions: (Dmb ) 2,4-dimethoxybenzyl) (a)
MeO(Me)NH‚HCl, EDC, HOBt, DMF, 0 °C, 24 h, 67%. (b)
LiAlH4, Et2O, -50 f 5 °C, 3 h, 85%. (c) 2,4-(MeO)BnNH2,
Na(AcO)3BH, 4 Å MS, ClCH2CH2Cl, 16 h, 90%. (d) ClCH2COCl,
NaHCO3, EtOAc-H2O, 0 °C, 30 min, 93%. (e) Cs2CO3, DMF, 65
°C, 16 h, 75%. (f) OsO4, NMO, t-BuOH, THF, H2O, 7 h; NaIO4,
NaHCO3, 1.5 h, 96% (g) NaBH4, EtOH, 0 °C f rt, 1 h, 92%. (h)
PhSO2Cl, Et3N, CH2Cl2, 0 °C f rt, 1 h, 83%. (i) LiHMDS, THF,
-78 f 0 °C, 40 min, 86%.
verted to the corresponding aldehyde 58 and then used to
reductively alkylate 2,4-dimethoxybenzylamine to give 6.
Chloroacetylation and base-promoted cyclization provided
the 5-allyl-2-piperazinone 7. Refunctionalization of the olefin
in 7 was accomplished by oxidative cleavage, aldehyde
reduction, and conversion to the benzenesulfonate 8. Finally,
after treatment of 8 with LiHMDS in THF at -78 °C and
warming the reaction to 0 °C, an intramolecular enolate
alkylation occurred to afford the bicyclic framework 9a in
86% yield.9,10
(9) Compound 9a was subjected to Boc-deprotection (HCl, EtOAc, 0
°C), conversion to the (+)- and (-)-10-camphorsulfonamides (10-cam-
phorsulfonyl chloride, Et3N, DMF), and HPLC analysis to confirm g 97%
enantiomer purity.
(10) Transannular alkylation of the methanesulfonate i under the same
conditions provided an 8:1 mixture of 9a and the alcohol derived from
cleavage of the methanesulfonyl group. Attempts to cyclize the sulfonium
salt ii derived from L-methionine were unsuccessful.
(5) (a) Jain, S.; Sujatha, K.; Krishna, K. V. R.; Roy, R.; Singh, J.; Anand,
N. Tetrahedron 1992, 48, 4985. (b) Sturm, P. A.; Henry, D. W. J. Med.
Chem. 1974, 17, 481. (c) Cignarella, G.; Testa, E. Farmaco. Ed. Sci. 1969,
24, 419. (d) Blackman, S. W.; Baltzly, R. J. Org. Chem. 1961, 26, 2750.
(e) Cignarella, G.; Nathansohn, G. J. Org. Chem. 1961, 26, 2747. (f)
Cignarella, G.; Nathansohn, G. J. Org. Chem. 1961, 26, 1500.
(6) (a) Schanen, V.; Riche, C.; Chiaroni, A.; Quiron, J.-C.; Husson, H.-
P. Tetrahedron Lett. 1994, 35, 2533. (b) Pohlmann, A.; Schanen, V.;
Guillaume, D.; Quirion, J.-C.; Husson, H.-P. J. Org. Chem. 1997, 62, 1016.
(c) Bhatt, U.; Mohamed, N.; Just, G.; Roberts, E. Tetrahedron Lett. 1997,
38, 3679. (d) Weissman, S. A.; Lewis (nee Scull), S.; Askin, D.; Volante,
R. P.; Reider, P. J. Tetrahedron Lett. 1998, 39, 7459. (e) Dinsmore, C. J.;
Zartman, C. B. Tetrahedron Lett. 2000, 41, 6309.
(11) Gibson (nee Thomas), S. E.; Guillo, N.; Tozer, M. J. Tetrahedron
1999, 55, 585.
(12) Attempted cyclization using LiHMDS (THF, -78 to 0 °C, 5 h) gave
only recovered 11. The use of KHMDS under the same conditions resulted
in gradual conversion to the dihydrooxazinone iii.
(7) Hart, B. P.; Rapoport, H. J. Org. Chem. 1999, 64, 2050.
(8) Goel, O. P.; Krolls, U.; Stier, M.; Kesten, S. Org. Synth. 1988, 67,
69.
(13) (a) Kato, K.; Cox, A. D.; Hisaka, M. M.; Graham, S. M.; Buss, J.
E. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 6403. (b) Rowinsky, E. K.;
Windle, J. L.; Von Hoff, D. D. J. Clin. Oncol. 1999, 17, 3631.
866
Org. Lett., Vol. 3, No. 6, 2001

1),15 we discovered that they adopt a folded conformation
when bound to the enzyme and that a macrocyclic constraint
(e.g., 13) enhances potency by stabilizing the bound con-
exposure to triflic acid and triethylsilane in a solubilizing
dichloromethane-acetonitrile mixture to effect deprotection.
The robustness of this protecting group is likely due to
protonation of the piperazinone amino group in 15, which
impedes protonation of the nearby amide. Alkylation of the
bridged piperazinone with bromide 17 gave 18, which was
subjected to a tandem base-promoted methanesulfonate group
deprotection and SNAr cyclization to provide the cyclophane
19.19
The calculated lowest-energy structure of 19 reveals the
anticipated conformation with the ethylene bridge projecting
to the exterior of the macrocyclic ring (Figure 2, gray).20
Figure 1. Piperazinone farnesyltransferase inhibitors 12 and 13.
formation.16 The importance of the carbonyl group position
for FTase inhibitory activity was also established.17 We
sought to further elucidate enzyme-bound conformations of
these compounds with respect to the position of the carbonyl
group. In principle, this could be accomplished by constrain-
ing the conformational flexibility of the piperazinone ring
and inhibiting the positional mobility of the carbonyl group.
On the basis of these considerations, a 3,8-diazabicyclo-
[3.2.1]octan-2-one analogue of the macrocycle 13 was
selected as a target for synthesis.
The intermediate 9a was deprotected and reductively
alkylated with 1-(3-fluoro-4-cyanobenzyl)imidazole-5-car-
boxaldehyde (14)16 to provide amine 15 (Scheme 3). The
Figure 2. Superposition of the calculated lowest energy conforma-
tions of 19 (gray) and 13 (pink), and a conformation of 13 with
the carbonyl group projecting back (yellow).
Scheme 3a
1
Variable temperature H NMR studies revealed differential
line broadening below 0 °C, with two sets of resonances at
-50 °C (∼5:1 ratio), indicating slow conformational ex-
change. NOE data obtained at room temperature are con-
sistent with a mixture of available species, most likely
(15) (a) Williams, T. M.; Dinsmore, C. J.; Bergman, J. M.; Hutchinson,
J. H.; MacTough, S. C.; Stump, C. S.; Wei, D. D.; Zartman, C. B.; Bogusky,
M. J.; Chen, I.-W.; Culberson, J. C.; Koblan, K. S.; Kohl, N. E.; Lobell, R.
B.; Motzel, S. L.; Salata, J. J.; Gibbs, J. B.; Graham, S. L.; Hartman, G.
D.; Oliff, A. I.; Huff, J. R. 216th National Meeting of the American
Chemical Society, Boston, MA, 1998; American Chemical Society:
Washington, DC, 1998; MEDI 309. (b) manuscript in preparation.
(16) Dinsmore, C. J.; Bogusky, M. J.; Culberson, J. C.; Bergman, J. M.;
Homnick, C. F.; Zartman, C. B.; Mosser, S. D.; Schaber, M. D.; Robinson,
R. G.; Koblan, K. S.; Huber, H. E.; Graham, S. L.; Hartman, G. D.; Huff,
J. R.; Williams, T. M. J. Am. Chem. Soc. 2001, 123, in press.
(17) Dinsmore, C. J.; Bergman, J. M.; Wei, D. D.; Zartman, C. B.;
Davide, J. P.; Greenberg, I. B.; Liu, D.; O’Neill, T. J.; Gibbs, J. B.; Koblan,
K. S.; Kohl, N. E.; Lobell, R. B.; Chen, I.-W.; McLoughlin, D. A.; Olah,
T. V.; Graham, S. L.; Hartman, G. D.; Williams, T. M. Bioorg. Med. Chem.
Lett. 2001, 11, 537.
a Reagents and conditions: (a) HCl, EtOAc, 0 °C, 0.5 h, 100%.
(b) 1-(3-fluoro-4-cyanobenzyl)imidazole-5-carboxaldehyde (14),
Na(AcO)3BH, 4 Å MS, DCE, 24 h, 56%. (c) MsCl, Et3N, CH2Cl2,
30 min. (d) NBS, AIBN, CCl4, 80 °C, 14 h, 70%, two steps. (e)
TfOH, Et3SiH, CH2Cl2-CH3CN, 14 d, 69% based on 75% conv.
(f) NaH, DMF, 17, 0 °C, 1 h, 55%. (g) Cs2CO3, DMSO, 80 °C, 5
h, 34%.
(18) (a) Schlessinger, R. H.; Bebernitz, G. R.; Lin, P.; Poss, A. Y. J.
Am. Chem. Soc. 1985, 107, 1777. (b) DeShong, P.; Ramesh, S.; Elango,
V.; Perez, J. J. J. Am. Chem. Soc. 1985, 107, 5219.
(19) Dinsmore, C. J.; Zartman, C. B. Tetrahedron Lett. 1999, 40, 3989.
(20) (a) Molecular modeling: Conformations were generated using metric
matrix distance geometry algorithm JG (S. Kearsley, Merck & Co., Inc.,
unpublished). The structures were subjected to energy minimization within
Macromodel (ref 20b) using the MMFF force field. (b) Mohamadi, F.;
Richards, N. G. J.; Guida, W. C.; Liskamp, R.; Caufield, C.; Chang, G.;
Hendrickson, T.; Still, W. C. J. Comput. Chem. 1990, 11, 440.
2,4-dimethoxybenzyl amide protecting group, normally
sensitive to treatment with TFA,18 required prolonged
(14) (a) Oliff, A. Biochim. Biophys. Acta 1999, 1423, C19. (b) End, D.
W. InVest. New Drugs 1999, 17, 241. (c) Gibbs, J. B. J. Clin. InVest. 2000,
105, 9.
Org. Lett., Vol. 3, No. 6, 2001
867

derived from mobility of the imidazole, cyanophenyl, and
chlorophenyl rings relative to the 3,8-diazabicyclo[3.2.1]-
octan-2-one ring.21 Compound 13, in its lowest energy state
(Figure 2, pink),20 adopts the same conformation as does 19.
The corresponding orientation of 13 with the carbonyl group
projected in the opposite direction22 is only 2.63 kcal/mol
higher in energy (Figure 2, yellow). In comparison, a
conformation of 19 with the alternative rearward orientation
of the carbonyl group (not shown) is 39.2 kcal/mol higher
in energy than the ground state, since it requires that the
ethylene bridge reside in the congested interior of the
macrocycle.
than the parent unbridged compound 13 (IC50 19 ) 0.13 nM,
IC50 13 ) 0.7 nM). Taken with the conformational analyses
described above, this result suggests that the FTase-bound
conformation of 13 is one in which the carbonyl group is
projected in a forward orientation (S-configuration of the
piperazinone plane), as in the constrained compound 19. This,
in conjunction with earlier results,16 implies that 1-aryl-2-
piperazinone FTIs (e.g., 12) adopt a similar enzyme-bound
conformation as well.
In summary, a new method for the synthesis of the 3,8-
diazabicyclo[3.2.1]octan-2-one framework enables access to
CR-substituted derivatives as constrained peptide mimetics.
A bicyclic-constrained analogue of a piperazinone farnesyl-
transferase inhibitor was prepared and was useful for the
elucidation of enzyme-bound conformation.
Compound 19 was assayed for inhibition of FTase-
catalyzed incorporation of [3H]-farnesylpyrophosphate into
recombinant Ras-CVIM23 and was found to be more potent
(21) This interpretation is supported by computational analyses (ref 20),
which revealed only moderate energy increases for the following confor-
mational perturbations relative to lowest energy 19 (cf. Figure 2): front-
to-back rotations of the imidazole, cyanophenyl rings and back-to-front
rotation of the chlorophenyl ring (1.66 kcal/mol); axial-equatorial isomer-
ization (2.94 kcal/mol).
Acknowledgment. We are grateful to C. W. Ross III, A.
B. Coddington, and H. G. Ramjit for analytical support and
M. A. Guttman for manuscript assistance.
(22) An alternative conformation of 13 with the carbonyl projected in
the opposite direction relative to 19 is the enantiomer of lowest energy 13.
(23) For assay conditions, see: Graham, S. L.; deSolms, S. J.; Giuliani,
E. A.; Kohl, N. E.; Mosser, S. D.; Oliff, A. I.; Pompliano, D. L.; Rands,
E.; Breslin, M. J.; Deanna, A. A.; Garsky, V. M.; Scholz, T. H.; Gibbs, J.
B.; Smith, R. L. J. Med. Chem. 1994, 37, 725.
Supporting Information Available: Experimental pro-
cedures and characterization for compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL015504W
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