Synthesis of 7-substituted benzolactam-V8s and their selectivity for protein kinase C isozymes.

Article abstract of DOI:10.1021/ol026125l

[reaction: see text] Condensation of L-valine benzyl ester toluenesulfonic acid salt with a substituted cyclohexadione followed by aromatization with the assistance of NBS provides an N-aryl L-valine benzyl ester. This intermediate is converted into 7-sub

Full text of DOI:10.1021/ol026125l

ORGANIC
LETTERS
2002
Vol. 4, No. 14
2377-2380
Synthesis of 7-Substituted
Benzolactam-V8s and Their Selectivity
for Protein Kinase C Isozymes
,†
,‡
Dawei Ma,* Guozhi Tang,† and Alan P. Kozikowski*
State Key Laboratory of Bioorganic and Natural Products Chemistry,
Shanghai Institute of Organic Chemistry, 354 Fenglin Lu, Shanghai 200032, China,
Drug DiscoVery Laboratory, Institute for CognitiVe and Computational Sciences,
Georgetown UniVersity Medical Center, 3970 ReserVoir Road, N.W.,
Washington, DC 20007-2197
madw@pub.sioc.ac.cn
Received May 2, 2002
ABSTRACT
Condensation of L-valine benzyl ester toluenesulfonic acid salt with a substituted cyclohexadione followed by aromatization with the assistance
of NBS provides an N-aryl L-valine benzyl ester. This intermediate is converted into 7-substituted benzolactam-V8s using an asymmetric
Strecker reaction as the key step. The target molecules show a different pattern of isozyme selectivity relative to the 8-substituted benzolactam-
V8s.
Protein kinase C (PKC), comprised of at least eleven
isozymes, plays an important role in signal transduction
pathways, mediating signals that orginate from the induction
of lipid hydrolysis.1 These isozymes exhibit a distinct pattern
of tissue-specific expression and play diverse roles in both
physiological and pathophysiological processes.1,2 To clarify
the biological roles of the individual isozymes, isozyme-
specific modulators of PKC are required.1,2 In the past two
decades, both academic and industrial research has been
directed to the discovery of PKC modulators possessing such
selectivity. To date, several isozyme-selective inhibitors of
PKC have been discovered.3 However, few isozyme-selective
activators have been reported.4 During studies aimed at
elucidating the structure-activity relationships (SARs) of the
teleocidins, a group of natural products that are potent but
relatively nonselective PKC activators, the simplified ana-
logue benzolactam-V8 1a5,6 was found to be a potent PKC
activator.6b In previous studies we have demonstrated that
compound 2a, a benzolactam-V8 analogue with an acetylene
(3) Bradshaw, D.; Hill, C. H.; Nixon, J. S.; Wilkinson, S. E. Agents
Actions 1993, 38, 135. Jirousek, M. R.; Gillig, J. R.; Gonzalez, C. M.; Heath,
W. F.; McDonald, J. H., III; Neel, D. A.; Rito, C. J.; Singh, U.; Stramm,
L. E.; Melikian-Badalian, A.; Baevsky, M.; Ballas, L. M.; Hall, S. E.;
Winneroski, L. L.; Faul, M. M. J. Med. Chem. 1996, 39, 2664. Wilkinson,
S. E.; Parker, P. J.; Nixon, J. S. Biochem. J. 1993, 294, 335. Martiny-
Baron, G.; Kazanietz, M. G.; Mischak, H.; Blumberg, P. M.; Kochs, G.;
Hug, H.; Marme, D.; Schachele, C. J. Biol. Chem. 1993, 268, 9194.
(4) Szallasi, Z.; Denning, M. F.; Smith, C. B.; Dlugosz, A. A.; Yuspa,
S. H.; Pettit, G. R.; Blumberg, P. M. Mol. Pharmacol. 1994, 46, 840 and
references therein.
† Shanghai Institute of Organic Chemistry.
‡ Georgetown University Medical Center.
(1) For review, see: (a) Nishizuka, Y. Nature 1988, 334, 661. (b) Basu,
A. Pharmacol. Ther. 1993, 59, 257. (c) Stabel, S.; Parker, P. J. Pharmacol.
Ther. 1991, 51, 71.
(5) For reviews, see: (a) Ma, D. Curr. Med. Chem. 2001, 8, 191. (b)
Kishi, Y.; Rando, R. R. Acc. Chem. Res. 1998, 31, 163.
(2) Dekker, L. V.; Parker, P. J. Trends Biochem. Sci. 1994, 19, 73.
10.1021/ol026125l CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/08/2002

Scheme 1
Figure 1. Structures of the teleocidin family and substituted
benzolactam-V8s.
side chain at the 8-position, shows some selectivity for
classical isozymes, while its analogue 2b with a saturated
side chain is less potent.7 On the basis of this finding and
results from molecular modeling, we believed that it would
be valuable to examine the isozyme selectivity of the
7-substituted benzolactam-V8s 3. Herein we report a novel
method for the synthesis of these compounds together with
their PKC binding profile. As outlined in Scheme 1, our
synthesis started with construction of the N-aryl L-valine
moiety using an unprecedented condensation/oxidative aro-
matization strategy. Accordingly, alkylation of 1,3-cyclo-
hexanedione with ICH2CH2OBn mediated by KOH in a
mixture of ethanol, water, and dioxane provided 4 in 64%
yield. These reaction conditions were found to be essential
for minimizing the generation of products of O-alkylation.
The diketone 4 was condensed with L-valine benzyl ester
toluenesulfonic acid salt under the action of pyridine in
refluxing toluene to afford enamine 5.10 Aromatization of 5
was required next in order to generate the N-aryl L-valine
ester intermediates. After a number of failed attempts that
included MnO2 oxidation and dehydrogenation by Pd/C, we
soon found that bromination of 5 with 2 equiv of NBS was
effective. This bromination reaction may involve the forma-
tion of compounds 6 and 7 as intermediates, which undergo
elimination of HBr assisted by NaHCO3 to produce aryl
bromide 8 in 74% yield together with a small amount of 9.
When only 1 equiv of NBS was used, this reaction gave 9
as the major product, but the overall yield was lower. Next,
the mixture of 8 and 9 was subjected to a reductive amination
reaction with HCHO/NaBH3CN to introduce a N-methyl
group. Removal of the extraneous bromine atom by Pd/C-
catalyzed hydrogenolysis yielded the N-aryl amino acid 10
in 91% yield. The overall yield from 4 to 10 was 65%. Next,
the hydroxyl and carboxyl group of 10 were protected by
treatment with BnBr under the action of potassium carbonate
to provide 11. Chiral HPLC analysis of 11 indicated that its
enantiomeric purity was ∼98%, which implies that little if
any racemization had occurred during the condensation or
aromatization steps. The present chemistry thus provides an
efficient method for gaining access to optically pure N-aryl
R-amino acids from optically pure R-amino acids, com-
pounds which represent the core structures of a number of
biologically important molecules.11
(6) (a) Kozikowski, A. P.; Ma, D.; Pang, Y.-P.; Shum, P.; Likic, V.;
Mishra, P. K.; Macura, S.; Basu, A.; Lazo, J. S.; Ball, R. G. J. Am. Chem.
Soc. 1993, 115, 3957. (b) Endo, Y.; Ohno, M.; Hirano, M.; Itai, A.; Shudo,
K. J. Am. Chem. Soc. 1996, 118, 1841. (c) Endo, Y.; Takehana, S.; Ohno,
M.; Driedger, P. E.; Stabel, S.; Mizutani, M. Y.; Tomioka, N.; Itai, A.;
Shudo, K. J. Med. Chem. 1998, 41, 1476.
(7) Kozikowski, A. P.; Wang, S.; Ma, D.; Yao, J.; Ahmad, S.; Glazer,
R. I.; Bogi, K.; Acs, P.; Modarres, S.; Lewin, N. E.; Blumberg, P. M. J.
Med. Chem. 1997, 40, 1316.
(8) Ma, D.; Tang, W.; Kozikowski, A. P.; Lewin, N. E.; Blumberg, P.
M. J. Org. Chem. 1999, 64, 6366.
(9) Ma, D.; Zhang, T.; Wang, G.; Kozikowski, A. P.; Lewin, N. E.;
Blumberg, P. M. Bioorg. Med. Chem. Lett. 2001, 11, 99.
(10) (a) Baraldi, P. G.; Simoni, D.; Manfredini, S. Synthesis 1983, 902.
(b) Backer, H. G. O. J. Prakt. Chem. 1961, 12, 294.
Next, Dess-Martin oxidation of 11 produced the aldehyde
12, which was subjected to an asymmetric Strecker reaction
with (R)-phenylglycinol and trimethylsilyl cyanide in metha-
nol to deliver a separable mixture of diastereomers 13 and
(11) Ma, D.; Zhang, Y.; Yao, J.; Wu, S.; Tao, F. J. Am. Chem. Soc.
1998, 120, 12459.
2378
Org. Lett., Vol. 4, No. 14, 2002

14 (Scheme 2).12 The undesired isomer 14 was converted to
thermodynamically stable 13 by heating it in methanol at
80 °C.
Scheme 4
Scheme 2
Treatment of 13 with HCl-saturated methanol provided
the ester 15, which was hydrogenated to bring about removal
of the benzyl protecting groups to give the corresponding
amino acid. Occasionally, we found that the resulting amino
acid was cyclized to give the desired lactam 16 under the
action of di-tert-butyl dicarbonate.13 However, the lactam-
ization yield (47%) was not so satisfactory (Scheme 3).
protecting groups, the acid was transformed to the lactam
18 using the activated ester method.8,14 Although this route
required more steps, the overall yield (65%) from 15 to the
lactam was higher.
The target molecule 3a was obtained from 18 in 76% yield
by the following steps: (1) treatment of 18 with trifluoro-
methylsulfonic anhydride to give the triflate; (2) Pd/CuI-
catalyzed coupling of the triflate with 1-decyne;8,15 and (3)
reduction of the ester to the alcohol with lithium borohydride.
Finally, hydrogenation of 3a provided 3b in 98% yield.
Compounds 3a and 3b have been evaluated for their ability
to displace phorbol 12,13-dibutyrate (PDBU) binding from
recombinant PKCR, PKCâ, PKCγ, PKCδ, and PKCꢀ.7 The
Ki values for 3a and 3b are summarized in Table 1. For
Scheme 3
Table 1. Ki Values for the Inhibition of [3H]PDBu Binding by
PKC Activators (nM)
compd
3a
3b
2a a
2ba
PKCR
PKCâ
PKCγ
PKCδ
PKCꢀ
4533
513
309
409
169
60
14.7
17.4
40.7
122
142
46.6
58.2
140
185
187
1691
>10 000
2098
To improve the overall yield of the lactam, an alternative
route was explored (Scheme 4). The chiral auxiliary of 13
was subjected to oxidative cleavage,12a and the resulting free
amine was protected with di-tert-butyl dicarbonate to produce
17. After Pd/C-catalyzed hydrogenation to remove the benzyl
2562
a Data taken from ref 7.
comparison, The Ki values of the 8-substituted benzolactam-
V8s 2a and 2b are also provided. As is apparent, the
(12) (a) Chakraborty, T. K.; Hussain, K. A.; Reddy, G. V. Tetrahedron,
1995, 51, 9179. (b) Sakamuri, S.; Kozikowski, A. P. Chem. Commun. 2001,
475.
(13) A report regarding the formation of dipeptides promoted by di-tert-
butyl dicarbonate has appeared; see: Mohapatra, D. K.; Datta, A. J. Org.
Chem. 1999, 64, 6879.
(14) Endo, Y.; Hasegawa, M.; Itai, A.; Shudo, K. Tetrahedron 1987,
43, 3695.
(15) Sonogashira, K. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Ed.; Pergamon Press: Oxford; 1999, Vol. 3, p 521.
Org. Lett., Vol. 4, No. 14, 2002
2379

7-substituted benzolactam-V8s display a different isozyme
selectivity pattern in comparison to the 8-substituted benzo-
lactam-V8s. Compound 3b possessing a saturated side chain
is more potent than its acetylene-bearing counterpart 3a, and
it shows an 8-fold selectivity for PKCꢀ over PKCR. On the
other hand, in the 8-substituted series, 2a with an acetylenic
side chain is about 10-fold more selective for PKCR than
PKCꢀ. A detailed explanation for these subtle differences
using computer modeling, as well as the design of more
selective compounds based upon this information, is in
progress.
of China (Grant 20132030 to D.M.), the Qiu Shi Science
and Technologies Foundation (to D.M.), and the National
Institutes of Health (CA79601 to A.P.K.) for their financial
support. We also thank the NIMH Psychoactive Drug
Screening Program (KO2MH01366) for the PKC data.
Supporting Information Available: Experimental pro-
cedures and characterizations for compounds 4-18, 3a, and
3b. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. The authors are grateful to the Chinese
Academy of Sciences, National Natural Science Foundation
OL026125L
2380
Org. Lett., Vol. 4, No. 14, 2002