Synthesis of a potent and selective inhibitor of p90 Rsk

Article abstract of DOI:10.1021/ol0500463

(Chemical Equation Presented) The synthesis of the naturally occurring kaempferol glycoside SL0101 has been accomplished, as has its biochemical evaluation. SL0101 exhibits selective and potent p90 Rsk inhibitory activity at nanomolar concentrations without inhibiting the function of upstream kinases such as MEK, Raf, or PKC. The synthesis verified the structural assignment of the natural product and has provided access to material sufficient for detailed biological evaluation.

Full text of DOI:10.1021/ol0500463

ORGANIC
LETTERS
2005
Vol. 7, No. 6
1097-1099
Synthesis of a Potent and Selective
Inhibitor of p90 Rsk
David J. Maloney and Sidney M. Hecht*
Departments of Chemistry and Biology, UniVersity of Virginia,
CharlottesVille, Virginia 22901
sidhecht@Virginia.edu
Received January 10, 2005
ABSTRACT
The synthesis of the naturally occurring kaempferol glycoside SL0101 has been accomplished, as has its biochemical evaluation. SL0101
exhibits selective and potent p90 Rsk inhibitory activity at nanomolar concentrations without inhibiting the function of upstream kinases such
as MEK, Raf, or PKC. The synthesis verified the structural assignment of the natural product and has provided access to material sufficient
for detailed biological evaluation.
The overexpression of proteins in the mitogen-activated
protein kinase (MAPK) pathway has been noted for a number
of human malignancies,1 suggesting the possible utility of
inhibitors of these proteins in cancer therapy. However,
MAPK is involved in many fundamental cellular processes
such as apoptosis, survival, differentiation, and proliferation.2
Consequently, the inhibition of protein mediators involved
in multiple processes may result in effects on both normal
and cancer cells in a nonselective manner. Logically, the
identification of downstream mediators in the MAPK
pathway essential for tumor growth might lead to inhibitors
capable of selectively targeting cancer cells.
have been identified.3 Recent studies have demonstrated the
role of Rsk in cell survival signaling via the phosphorylation
and inactivation of the proapoptotic protein BAD. Rsk has
also been shown to directly promote cell survival by
regulating the expression and activation of prosurvival
proteins such as CREB (cyclic adenosine monophosphate
response element binding protein).4 The combination of
promoting cell survival and prevention of apoptosis causes
excessive cell survival, eventually leading to diseases such
as cancer and autoimmune disorders.4b Additionally, it has
been found that Rsk2 is overexpressed in more than 50% of
human breast cancers, validating the Rsk family as a potential
target for drug design.
Rsks are a family of 90 kDa ribosomal S6 kinases, which
are downstream effectors of MAPK; to date, four isoforms
Recently, we described three kaempferol glycosides,
isolated from a methanol extract of Forsteronia refracta, that
selectively inhibit p90 Rsk (Figure 1).5 Interestingly, SL-
(1) (a) Bang, Y.-J.; Kwon, J.-H.; Kang, S.-H.; Kim, J.-W.; Yang, Y.-C.
Biochem. Biophys. Res. Commun. 1998, 250, 43. (b) Schmidt, C. M.;
McKillop, I. H.; Cahil, P. A.; Sitzmann, J. V. 1997, 236, 54. (c) Janes, P.
W.; Daly, R. J.; deFazio, A.; Sutherland, R. L. Oncogene 1994, 9, 3601.
(d) Kurokawa, H.; Lenferink, A. E.; Simpson, J. F.; Pisacane, P. L.;
Sliwkowski, M. X.; Forbes, J. T.; Arteaga, C. L. Cancer Res. 2000, 60,
5887.
(2) (a) Lewis, T. S.; Shapiro, P. S.; Ahn, N. G. AdV. Cancer. Res. 1998,
74, 49. (b) Cobb, M. H. Prog. Biophys. Mol. Biol. 1999, 71, 479. (c) Kolch,
W. Biochem. J. 2000, 351, 289.
(3) Yntema, H. G.; van den Helm, B.; Kissing, J.; van Dujinhove, G.;
Poppelaars, F.; Chelly, J.; Moraine, C.; Fryns, J. P.; Hamel, B. C.;
Helbronner, H.; Pander, H. J.; Brunner, H. G.; Ropers, H. K.; Cremers, F.
P.; van Bokhoven, H. Genomics 1999, 62, 332.
(4) (a) Nebreda, A. R.; Gavin, A. C. Science 1999, 286, 1309. (b) Ballif,
B. A.; Blenis, J. Cell Growth. Diff. 2001, 12, 397.
(5) Smith, J. A.; Poteet-Smith, C. E.; Xu, Y.; Errington, T. M.; Hecht,
S. M.; Lannigan, D. A. Cancer Res. In press.
10.1021/ol0500463 CCC: $30.25
© 2005 American Chemical Society
Published on Web 02/19/2005

Preparation of the carbohydrate moiety is outlined in
Scheme 2. Compound 5 was synthesized from L-rhamnose
Scheme 2
Figure 1. Structures of SL0101 (1), 4′′-O-acetyl SL0101, and 2′,4′′-
O-diacetyl SL0101 and their Rsk2 inhibitory activities.
0101 (1) inhibited Rsk1 and Rsk2 to a greater extent than
Rsk3, although Rsk2 and Rsk3 are 80% homologous at the
level of primary sequence. Further, in comparison to its
potent inhibition of Rsk2 (IC50 ∼89 nM), 1 was found not
to inhibit upstream kinases such as MEK, Raf, and PKC.5
The interesting biological activities of 1,5 and its limited
availability from natural sources, prompted an investigation
into the synthesis of 1.
by known methods.9 Since O-3 and O-4 of 1 are both
acetylated, while O-2 is unprotected, we sought an appropri-
ate orthogonal protecting group for O-2. Accordingly, using
a procedure reported by Crich and co-workers,10 regioselec-
tive protection of the O-3 and O-4 hydroxyl groups was
achieved using 2,3-butanedione and trimethylorthoformate
to give 6 in 89% yield. Benzyl protection of O-2 proceeded
in 85% yield via the agency of NaH and benzyl bromide in
THF to give the fully protected rhamnose. Benzylation of
this OH group was chosen to allow for a mild and efficient
global deprotection at the end of the synthesis. Removal of
the O-3,4 protecting group was accomplished using TFA-
H2O in CH2Cl2 to afford 7 in 93% yield. Bis-acetylation of
7 with Ac2O, NEt3, and catalytic 4-(dimethylamino)pyridine
(DMAP) gave 2-O-benzyl-3,4-di-O-acetylrhamnose deriva-
tive 8 in yields exceeding 90%. Conversion to the rhamnosyl
bromide 9 was accomplished in 84% yield by treatment with
Br2 in CH2Cl2 at 0 °C. Condensation of 4 and 9 in the
presence of Ag2O provided perbenzylated SL0101 (10),
exclusively as the R-anomer, in 60% yield (Scheme 3). Other
commonly used glycosylation methods11 such as benzyltri-
ethylamine bromide and dilute aqueous KOH failed. Global
debenzylation using Pearlman’s catalyst (Pd(OH)2/C) in the
The short and convergent synthetic approach to 1 started
with the preparation of flavonol 4, as outlined in Scheme 1.
Scheme 1
Naringenin (4′,5,7-trihydroxyflavanone) was treated with
benzyl bromide and excess K2CO3, resulting in concomitant
â-elimination and benzyl protection to give the chalcone 2
in 81% yield. Formation of the desired flavone 3 was
accomplished in good yields (70-80%) using catalytic I2 in
DMSO at 140 °C.6 Introduction of a 3-OH group was
achieved using dimethyldioxirane (DMDO),7 followed by
opening of the formed epoxide with catalytic p-toluene-
sulfonic acid to afford flavonol 4 in 78% yield.8
(7) (a) Adam, W.; Hadijiarapoglou, L.; Smerz, A. Chem. Ber. 1991, 124,
227. (b) Adam, W.; Chan, Y.-Y.; Cremer, D.; Scheutzow, D.; Schindler,
M. J. Org. Chem. 1987, 52, 2800. (c) Murray, R. W.; Jeyaraman, R. J.
Org. Chem. 1987, 50, 3890.
(8) (a) Adam, W.; Golsch, D.; Hadjiarapoglou, L. J. Org. Chem. 1991,
56, 7292. (b) Lee, Y.-J.; Wu, T.-D. J. Chin. Chem. Soc. 2001, 48, 201.
(9) (a) Groneberg, R. D.; Miyazaki, T.; Stylianides, N. A.; Schulze, T.
J.; Stahl, W.; Schreiner, E. P.; Suzuki, T.; Iwabuchi, Y.; Smtih, A. L.;
Nicolaou, K. C. J. Am. Chem. Soc. 1993, 115, 7593. (b) Pozsgay, V.
Carbohydr. Res. 1992, 235, 1992. (c) Bashir, N. B.; Phythian, S. J.; Reason,
A. J.; Roberts, S. M. J. Chem. Soc., Perkin Trans. 1 1995, 2203.
(10) Crich, D.; Vinod, A. U.; Picione, J. J. Org. Chem. 2003, 68, 8453.
(11) Demetzos, C.; Skaltsounis, A. L.; Tillequin, F.; Koch, M. Carbohydr.
Res. 1990, 207, 131.
(6) (a) Kitagawa, M.; Yamamoto, K.; Katakura, S.; Kanno, H.; Yamada,
K. Chem. Pharm. Bull. 1991, 39, 2681. (b) Khan, M. S. Y.; Sharma, P.
Indian J. Chem. Sect. B. 1993, 32, 817.
1098
Org. Lett., Vol. 7, No. 6, 2005

In conclusion, a short and efficient approach to the
naturally occurring p90 Rsk inhibitor 1 has been completed.
This convergent approach has permitted access to 1 on a
preparative scale and should provide facile access to structur-
ally related analogues. Further investigations into the biologi-
cal activity of 1 are currently underway.
Scheme 3
Acknowledgment. We thank Drs. Deborah Lannigan and
Jeffrey Smith for biological assay data. This work was
supported by NIH Research Grant CA50771, awarded by
the National Cancer Institute.
Supporting Information Available: Experimental pro-
cedures and characterization data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
presence of H2 gave SL0101 (1) in 94% yield. The synthetic
compound had spectral data in full agreement with the
authentic, naturally derived sample.
OL0500463
Org. Lett., Vol. 7, No. 6, 2005
1099