Efficient solid-phase synthesis of compounds containing phenylalanine and its derivatives via side-chain attachment to the polymer support

Full text of DOI:10.1021/ja991679d

J. Am. Chem. Soc. 1999, 121, 8407-8408
8407
desirable for the rapid preparation of a multitude of compounds
with important biological activities.
Efficient Solid-Phase Synthesis of Compounds
Containing Phenylalanine and Its Derivatives via
Side-Chain Attachment to the Polymer Support
The most efficient way to design a library of phenylalanine-
containing molecules would be to attach the phenylalanine side
chain to the polymer support so that the residues at both the N-
and C-termini could be varied, leading to libraries with high
diversity. Recently, several novel strategies utilizing resin-bound
arylsilane as a “traceless linker” have been developed for solid-
phase synthesis of aromatics or heteroaromatic compounds.10-14
This method allows the attachment of substrates to the support
at an inert site within the molecule. Upon cleavage from the resin
by protiodesilylation, no trace or memory of attachment to the
polymer support is left. Also, silicone-directed ipso-substitution
of arylsilanes is frequently used for regiospecific introduction of
functional groups such as bromine and iodine into the aromatic
ring, thereby permitting an even higher degree of diversity in a
desired chemical library.15,16 Libraries of compounds constructed
on a solid support using silyl linkages include 1,4-benzodiaz-
epines,10 biaryls,12 benzofurans,12 and tricyclics.13 We reasoned
that an appropriate arylsilyl linkage could be used for tethering
phenylalanine to the polymer via its phenyl group, and then further
elongation of the amino acid backbone would allow rapid
synthesis of phenylalanine-containing molecules. Here we report
the design of polymer-bound phenylalanine precursors as new
building blocks for solid-phase synthesis and their application in
dipeptide synthesis on a solid support.
Younghee Lee and Richard B. Silverman*
Department of Chemistry, Northwestern UniVersity
EVanston, Illinois 60208-3113
ReceiVed May 20, 1999
Combinatorial chemistry, combined with recent advances in
robotic screening, has become an important tool in drug discov-
ery.1 Solid-phase synthesis allows the use of excess reagents to
drive the reaction to completion and easy removal of the reagents
and side products by simple washing with solvent. Since the
introduction of the Merrifield method for peptide synthesis,2 large
libraries of linear biopolymers such as peptides and oligonucleo-
tides as well as organic compounds have been generated.3
Generally, the synthesis of peptides on a solid support is carried
out from the C-terminus to the N-terminus for a variety of
reasons.4 A more efficient approach to the synthesis of peptides
involves the anchoring of the side chain to the polymer, an
approach that has been well utilized, especially for head-to-tail
cyclization of cyclic peptides. This methodology provides minimal
risks for side reactions, such as dimerization and oligomerization,
even under reaction conditions of high concentration. Furthermore,
this side-chain anchoring strategy allows the generation of more
diverse libraries of compounds than the conventional unidirec-
tional way (C- to N- or N- to C-elongation). Unfortunately, this
useful technology has been applicable only to a few amino acids
with polar side chains, such as Asp/Glu (COOH), Cys (SH), Ser/
Tyr (OH), Lys (NH2), and His (imidazole).5
Because of the nonpolar nature and steric bulkiness of its side
chain, phenylalanine is one of the key pharmacophores in
peptidomimetics for favorable hydrophobic interactions with
biological targets.6 Almost every aspartyl protease, such as HIV
protease, renin, and cathepsins D and E, has a preference for
hydrophobic amino acid side chains at the P1 position;7 conse-
quently, peptidomimetics designed to inhibit these enzymes
generally contain phenylalanine mimics or other bulky hydro-
phobic groups at the P1 position. A large number of naturally
occurring linear peptides with antineoplastic activity, such as
Dolastatin 10 and 15,8 and numerous families of cyclic peptides9
that are hydrophobic also contain at least one phenylalanine
residue. Therefore, a new methodology utilizing phenylalanine
or masked phenylalanine as the building block would be highly
Scheme 1 illustrates the synthesis of a phenylalanine derivative
having a silyl linker attached to the phenyl ring and the loading
of this compound to brominated polystyrene resin. A carbon-
carbon bond-forming reaction between 2 and the lithitiated
bislactim ether17 3 in THF at -78 °C provided a mixture of
(6) (a) Garrett, G. S.; McPhail, S. J.; Tornheim, K.; Correa, P. E.; McIver,
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* Corresponding author. Phone: (847) 491-5653. Fax: (847) 491-7713.
E-mail:Agman@chem.nwu.edu.
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M. A. J. Med. Chem. 1994, 37, 1385-1401. (b) Baum, R. Chem. Eng. News
1996, 74 (12 Feb), 28.
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Brenk, A. L.; Watters, D. J. J. Med. Chem. 1990, 33, 1634-1638. (c) Degnan,
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amino acids are commercially available. Stewart, J. M.; Young, J, D. Solid-
Phase Peptide Synthesis; Pierce Chemical Co.; Rockford, IL, 1984.
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10.1021/ja991679d CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/31/1999

8408 J. Am. Chem. Soc., Vol. 121, No. 36, 1999
Communications to the Editor
Scheme 1a
Scheme 2
h. C-Terminal amide coupling was performed on this carboxylic
acid with glycine methyl ester under the conditions described
above. After completion of the synthesis, cleavage of the desired
dipeptide analogue from the resin was carried out with 50% TFA
in dichloromethane for 24 h, giving protected dipeptide 9a in 93%
yield. The purity of the crude sample was determined to be about
a Conditions: (a) n-BuLi, THF, -78 °C, then allylchlorodimethylsilane;
(b) TsOH (catalytic), MeOH, 16 h; (c) PPh3, CBr4, CH2Cl2; (d) n-BuLi,
THF, -78 °C; (e) 9-BBN, THR, 5 h, then bromopolystyrene, DMF,
Pd(PPh3)4, K2CO3, 60 °C, 24 h; (f) 8:1 THF/1 N HCl, 3 h; (g) 1:1 MeOH/1
N HCl, 30 min, then FmocCl or (Boc)2O, NaHCO3, dioxane/H2O; (h)
LiOH (5 equiv), 5:1 THF/H2O; (i) 20% piperidine in DMF.
1
95% by H NMR spectroscopy. Alternatively, cleavage of the
dipeptides from the resin by ipso-substitution of the silyl group
with electrophiles (Br2 or ICl in dichloromethane for 5 min)
provides the corresponding halogen-substituted compounds (9b,
97%; 9c, 94%) with purities higher than 95% as determined by
1H NMR.
diastereomers in a ratio of 94:6. The major diastereomer (4),
isolated in 71% yield after silica gel column chromatography,
was attached to the polymer in a two-step sequence. Hydrobo-
ration of the allylsilane (9-BBN in THF), followed by in situ
Suzuki coupling18 of the borane complex with bromopolystyrene
resin19 [Pd(0), K2CO3, in THF], provided the resin-bound phe-
nylalanine precursor 5. Treatment of 5 with a solution of THF/1
N HCl (8:1) for 3 h at room temperature afforded polymer-bound
phenylalanine ethyl ester 6, which is ready for further derivati-
zation. Alternatively, 4 can be hydrolyzed under mild acidic
conditions (1:1 MeOH/1 N HCl), and the resulting amine can be
protected as either an N-Fmoc carbamate (FmocCl, NaHCO3) (7a)
or an N-Boc carbamate ((Boc)2O, NaHCO3) (7b); saponification
of 7b with LiOH in THF/H2O gives 7c. Hydroboration and Suzuki
coupling of carbamates 7a and 7c with bromopolystyrene resin
under the conditions described above provided the resin-bound
phenylalanine derivatives 8a and 8c, respectively. The loading
level of the amino acid derivatives bound to the polymer was
determined by the Fmoc release UV/vis assay20 or by mass
balance of the corresponding products released from the resin
after treatment of the resin with TFA or Br2 in dichloromethane.21
The utility of the polymer-bound phenylalanine precursors was
demonstrated by the preparation of dipeptide analogues from both
the N- and C-termini. As shown in Scheme 2, coupling (EDC,
HOBT, TEA in DMF) of 6 with benzoic acid formed the
N-terminal amide. Hydrolysis of the ester did not proceed at all
under the standard conditions (LiOH [5 equiv] in THF/H2O
(8:1), 16 h at room temperature), possibly because of poor
swelling properties of the bromopolystyrene resin under the
reaction conditions. However, the desired hydrolysis proceeded
to completion when the same solution was heated to reflux for 1
Similarly, but in a reversed reaction sequence, the same
compound (9a) was synthesized on the solid support starting from
the N-Boc-protected phenylalanine precursor 8c. C-Terminal
amide coupling with glycine methyl ester, followed by depro-
tection of the Boc group with 50% TFA in dichloromethane for
10 min, and subsequent coupling of the resulting amine with
benzoic acid, afforded 9a in 91% yield after cleavage from the
resin (50% TFA in CH2Cl2 for 24 h). These two operations
demonstrate that small phenylalanine-containing peptides can be
prepared on a solid support in any order of reaction sequence
desired. The polymer-bound phenylalanine can be modified in
either the N- or C-terminal direction with readily available
reagents, such as amines and carboxylic acids, and diversity can
be further increased by cleavage of the peptide from the resin
with halogens to give para-substituted halophenylalanines. These
halogenated analogues, then, can be even further elaborated by a
variety of alkyl22 or aryl18 substitution reactions at the halide.
The arylsilyl linkage was found to be resistant to moderate
acidic [1 N HCl/THF (1:8); 50% TFA/CH2Cl2 for 10 min] and
basic conditions [LiOH, THF/H2O (8:1), heat] as well as to the
general amide coupling reactions. Both N-Boc and N-Fmoc
protection strategies employed for the preparation of dipeptides
9 indicate that this versatility may be of great value for the solid-
phase synthesis of complex molecules requiring various orthogo-
nal protection of amine intermediates. Furthermore, this meth-
odology may be applied to cyclic peptide synthesis by head-to-
tail cyclization of a peptide on a solid support.
Acknowledgment. The authors are grateful for the financial support
of this research by the National Institutes of Health (GM 49725).
(18) Miyarura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457.
(19) Farrall, M. J.; Frechet, J. M. J. Org. Chem. 1976, 41, 3877.
(20) Bunin, B. A. The Combinatorial Index; Academic Press: San Diego,
1998.
Supporting Information Available: Complete experimental details
for synthesis of the compounds discussed herein (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(21) The cleavage conditions and loading levels for several of the polymer-
bound compounds are as follows: 5, Br2 in CH2Cl2, 5 min, 0.40 mequiv/g; 6,
Br2 in CH2Cl2, 5 min, 0.38 mequiv/g; 7a, UV/vis assay at 290 nm, 0.57
mequiv/g; 7c, 50% TFA in CH2Cl2, 24 h, 0.37 mequiv/g.
JA991679D
(22) Ishiyama, T.; Abe, S.; Miyaura, N.; Suzuki, A. Chem. Lett. 1992, 691.