J. Am. Chem. Soc. 1998, 120, 6629-6630
6629
bination of molecular modeling and site-directed mutagenesis
studies.6 To both simplify and to rigidify the ILV structure,
conceptually we considered linking C-9 and N-13, as these atoms
are close to one another in ILV’s twist conformation (Figure 1a),
to arrive at the pyrrolidone derivatives 6 (Figure 2). In order for
this type of compound to interact efficiently with PKC, modeling
studies revealed that the isopropyl and phenyl groups must be
cis oriented and trans to the hydroxymethyl group. With this
stereochemistry, the pyrrolidone is capable of engaging in the
same hydrogen-bond network to PKC as identified for ILV
(Figure 1). Its isopropyl group interacts with the side chain of
Leu 24, thereby mimicking the isopropyl group of ILV. Also,
its phenyl group is parallel to Pro 11, thus allowing for strong
hydrophobic interactions. However, the important interaction
of the N-methyl group of ILV with Pro 11 and Leu 20 is ab-
sent (the absence of this group in ILV results in a 100-fold
reduction in potency11). In addition, the optimal water solubility
values (log WS)14 can be adjusted by introduction of an
appropriate substituent. This side chain generally enhances a
ligand’s binding affinity through interaction with the lipid
membrane.
The phenyl-bearing pyrrolidone 6a was synthesized starting
from L-glutamic acid15 via 1 (Scheme 1). Copper-catalyzed con-
jugate addition of PhMgBr to 1 furnished 2a. Subsequent aldol
condensation with acetone gave rise to the corresponding tertiary
alcohol 3a which was dehydrated by the Burgess reagent to afford
a mixture of two olefins. Isomerization of the nonconjugated
olefin with DBU provided conjugated lactam 4a as a single
compound. A hydroxyl-directed hydrogenation16 over 10% Pd/C
was carried out after removal of the silyl group. Last, deprotection
of Boc group with TFA yielded 6a. By using p-BrPhMgBr in
the conjugate addition step, and then at the stage of 4 performing
a palladium-catalyzed coupling reaction with 1-nonyne,6 we
acquired access to the pyrrolidone 6b possessing a hydrophobic
alkyl residue. The results obtained with 6a and 6b (Table 1)
suggested that our design concepts were correct. However,
additional modifications were clearly needed to obtain compounds
of nanomolar affinity.
Molecular modeling suggested that replacement of phenyl by
R-naphthyl can partially compensate for the absence of ILV’s
N-methyl group in our γ-lactams. Attempts to synthesize these
naphthylpyrrolidone derivatives by the conjugate addition strategy
of Scheme 1 failed. Instead, as shown in Scheme 2, D-serine
methyl ester hydrochloride was acylated with R-bromoisovaleryl
chloride, and the hydroxyl and amido groups were protected by
reaction with 2,2-dimethoxypropane to provide 8. The ester was
then reduced to aldehyde by DIBAL-H treatment, followed by
addition of R-naphthylmagnesium bromide. Perruthenate-
catalyzed oxidation of the resulting secondary alcohol furnished
the ketone 9a. Its SmI2-mediated ring closure gave only a single
diastereoisomer 10a, with the stereochemistry being confirmed
by X-ray diffraction. The Barton deoxygenation served to remove
the tertiary alcohol and to invert the C-4′ stereocenter, thereby
delivering the desired cis-C-3′/C-4′ stereochemistry as evidenced
by the shielding of the methine proton belonging to the isopro-
pyl group in 12 to the extent of 0.8 ppm compared to that in
10. After deprotection by transketalization with ethanedithiol in
the presence of BF3‚Et2O, the resulting product 13a was tested
and found to exhibit a 23-fold improvement in activity compared
to 6a.
Structure-Based Design of a New Class of Protein
Kinase C Modulators
Lixin Qiao,† Shaomeng Wang,† Clifford George,‡
Nancy E. Lewin,§ Peter M. Blumberg,§ and
Alan P. Kozikowski*,†
Drug DiscoVery Program, Georgetown UniVersity
Medical Center, Washington, D.C. 20007
NaVal Research Laboratory, Washington, D.C. 20375
Molecular Mechanisms of Tumor Promotion Section
Laboratory of Cellular Carcinogenesis and
Tumor Promotion, National Cancer
Institute, Bethesda, Maryland 20892
ReceiVed February 17, 1998
Protein kinase C (PKC) is a ubiquitous diacylglycerol (DAG)-
activated signal transducing enzyme system that is coupled to
diverse biological events including regulation of ion channels,
neurotransmitter release, growth and differentiation, apoptosis,
and neuronal plasticity.1 Apart from diacylglycerol,2 the endog-
enous activator of PKC, several complex natural products and
their analogues such as the phorbol esters, bryostatin, teleocidin,
and indolactam V (ILV) can mimic DAG to activate PKC at low
concentrations.3-5 Synthesis,6-8 molecular modeling,6,9-11 and
structure-activity relationships12,13 of ILV and its analogues have
been reported. The available X-ray structure of PKCδ in com-
plex with phorbol 13-acetate10 provides information invaluable
to the design of new classes of PKC modulators. This paper
presents the first example of the structure-based design, synthesis,
and biological activities of certain γ-lactams as novel mimics of
ILV.
The search for a simpler structural template that retains PKC-
activating properties was driven largely by our desire to work
with a compound that was readily amenable to modification so
that ultimately we can discover isozyme selective modulators for
the DAG superfamily. On the basis of the X-ray structure of
PKCδ CRD2 (cysteine-rich domain) in complex with phorbol 13-
acetate, we have determined how the high-affinity ligands ILV
and the eight-membered ring benzolactam bind through a com-
† Georgetown University Medical Center.
‡ Naval Research Laboratory.
§ National Cancer Institute.
(1) For a review, see: Protein Kinase C. Current Concepts and Future
PerspectiVes; Lester, D. S., Epand, R. M., Eds.; Ellis Horwood: New York,
1992.
(2) Wender, P. A.; Gribbs, C. M. In AdVances in Medicinal Chemistry;
Maryanoff, B. E., Maryanoff, C. A., Eds.; JAI Press: Greenwich, CT, 1992;
Vol. 1, pp 1-53.
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(5) Jeffrey, A. M.; Liskamp, R. M. J. Proc. Natl. Acad. Sci. U.S.A. 1986,
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Chem. 1997, 40, 1316 and references therein.
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(8) Moreno, O. A.; Kishi, Y. J. Am. Chem. Soc. 1996, 118, 8180.
(9) Endo, Y.; Hasegawa, M.; Itai, A.; Shudo, K.; Tori, M.; Asakawa, Y.;
Sakai, S. Tetrahedron Lett. 1985, 26, 1069.
(10) Zhang, G.; Kazanietz, M. G.; Blumberg, P. M.; Hurley, J. H. Cell
1995, 81, 917.
(14) Marquez, V.; Lee, J.; Sharma, R.; Teng, K.; Wang, S.; Lewin, N. E.;
Bahador, A.; Kazanietz, M. G.; Blumberg, P. M. Bioorg. Med. Chem. Lett.
1994, 4, 355.
(11) Irie, I.; Okuno, S.; Kajiyama, S.; Koshimizu, K.; Nishino, H.;
Iwashima, A. Carcinogenesis 1991, 12, 1883.
(12) Krauter, G.; Von Der Lieth, C. W.; Schmidt, R.; Hecker, E. Eur. J.
Biochem. 1996, 242, 417.
(15) Ackermann, J.; Matthes, M.; Tamm, C. HelV. Chim. Acta 1990, 73,
122.
(13) Endo, Y.; Ohno, M.; Takehana, S.; Driedger, P. E.; Stabel, S.; Shudo,
K. Chem. Pharm. Bull. 1997, 45, 424.
(16) Baldwin, J. E.; Fryer, A. M.; Spyvee, M. R.; Whitehead, R. C.; Wood,
W. E. Tetrahedron Lett. 1996, 37, 6923.
S0002-7863(98)00513-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/19/1998