Design and synthesis of a β-amino phosphotyrosyl mimetic suitably protected for peptide synthesis

Article abstract of DOI:10.1016/S0960-894X(02)00783-7

Mimetics of phosphotyrosine (pTyr) such as phosphonomethylphenylalanine (Pmp) have traditionally retained α-amino functionality. However, β-amino acids represent isomeric variants, which may exhibit properties that are distinct from the parent. Reported h

Full text of DOI:10.1016/S0960-894X(02)00783-7

Bioorganic & Medicinal Chemistry Letters 12 (2002) 3399–3401
Design and Synthesis of a ꢀ-Amino Phosphotyrosyl Mimetic
Suitably Protected for Peptide Synthesis
Kyeong Lee,a Manchao Zhang,b Dajun Yangb and Terrence R. Burke, Jr.a,*
aLaboratory of Medicinal Chemistry, Center for Cancer Research, NCI-Frederick, National Institutes of Health,
Frederick, MD 21702, USA
bDepartment of Hematology/Oncology, University of Michigan, Ann Arbor, MI 48109, USA
Received 9 July 2002; accepted 28 August 2002
Abstract—Mimetics of phosphotyrosine (pTyr) such as phosphonomethylphenylalanine (Pmp) have traditionally retained a-amino
functionality. However, b-amino acids represent isomeric variants, which may exhibit properties that are distinct from the parent.
Reported herein is the first b-amino pTyr mimetic (Pmpb) bearing protection suitable for peptide synthesis. Preparation of Pmpb
was accomplished enantioselectively in 43% overall yield from commercially available 4-vinylbenzyl chloride.
# 2002 Elsevier Science Ltd. All rights reserved.
Physiochemical properties of peptides can be highly
dependent on solution conformations. For this reason
amino acid analogues are often employed in order to
induce desired bioactive backbone or side-chain geome-
tries.1,2 Beta-amino acids are isomeric variants of
a-amino acids which can serve as useful building blocks
due to their ability to induce turn structures either
when used exclusively,3,4 or when used in combination
with a-amino acids.5 Phosphotyrosyl residues (pTyr, 1)
provide critical recognition motifs in a variety of signal
transduction pathways, including those associated with
diseases such as cancers.6 Phosphotyrosyl-dependent
binding phenomena mediated by Src homology 2(SH2)
and phosphotyrosyl-binding (PTB) domains are key to
several of these processes,7,8 and disruption of these
interactions may afford new therapeutic approaches.9
Accordingly, a number of phosphatase-stable pTyr
mimetics have been developed for such purposes.10,11
(Pmp, 2, Fig. 1). Since in a variety of contexts, b-amino
variants may provide useful alternatives to traditional
a-amino pTyr mimetics, b-amino phosphonomethyl-
phenylalanine (Pmpb, 3) was designed as the first exam-
ple of a b-amino pTyr mimetic (Fig. 1). Presented herein
is the synthesis of 3 bearing amino protection suitable
for Boc-based peptide synthesis, as well as its
incorporation into a model tripeptide.
Asymmetric syntheses of b-amino acids have been
achieved by several methods,17 including addition of
nitrogen nucleophiles to a,b unsaturated esters, enolate
additions to imines and hydrogenation of b-enamino
esters. Our initial attempts at the asymmetric synthesis
of b-amino-containing 7 utilizing conjugate addition
onto Evan’s auxillary-containing 4 of either protected
amines or azide, failed (Fig. 2). Subsequently, Mannich-
type reaction of chiral enolate 6 with imine 5 derived
from a variety of amines, including trimethylsilylamine
and p-methoxybenzylamine, also failed due to instability
of the imine species. Recently, tert-butanesulfinamide
In spite of the fact that critical binding functionality can
originate from the amino-proximal region of the pTyr
residue12À14 and that bend conformations can be pre-
ferred by both SH2domains and PTB domains, 15,16 most
pTyr mimetics have retained the natural a-amino func-
tionality of parent pTyr. An example of such a pTyr
mimetic is provided by phosphonomethylphenylalanine
*Corresponding author. Tel.: +1-301-846-5906; fax: +1-301-846-
6033; e-mail: tburke@helix.nih.gov
Figure 1. Structures of pTyr (1), representative a-amino pTyr mimetic
Pmp (2) and a new b-amino pTyr mimetic Pmpb (3).
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00783-7

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K. Lee et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3399–3401
(11, Scheme 1)18 has been employed in a variety of
contexts, including the synthesis of imines, which can be
converted to b-amino acids.19,20 Sulfinylimines derived
from 11 are chemically stable and contain chirality
appropriate for induction of asymmetric addition by
achiral nucleophiles. In addition, after creation of a new
stereogenic center, the tert-butanesulfinyl auxiliary can
be removed easily by acid treatment or utilized as an
acid-labile amine-protecting group. Accordingly, (R)-
(À)-tert-butanesulfinylamide 11 was employed for the
efficient synthesis of protected b-amino pTyr mimetic 14
(Scheme 1). Preparation of requisite arylaldehyde 10
began by reaction of commercially available 4-vinyl-
benzyl chloride (8) with lithiated di-benzylphosphite in
the presence of tetra-n-butylammonium iodide as a cat-
alyst. Resulting phosphonate 9 provided 10, following
oxidative cleavage of the vinyl group (66% yield from 8).
according to the method of Tang et al.19 gave 13 in 83%
yield without b-lactam formation. Diastereoselectivity
of the reaction was determined by the method of
Mosher.26 In summary, amino-deprotection of 13 by
treatment with 4 N HCl in dioxane followed by acylation
using either (S)-(À)- or (R)-(+)-a-methoxy-a-trifluoro-
methylphenyl acetyl chloride, yielded diastereomeric
Mosher-amides 15a and 15b, respectively. Characteriza-
tion of these derivatives by both proton and fluorine NMR
indicated high diastereoselectivity (d.e. of >90% for the
(3S) isomer). Finally, saponification of 13 provided title
compound 14 in quantitative yield.
In order to validate the suitability of analogue 14 for the
synthesis of b-amino pTyr mimetic-containing peptides,
compound 2027 (Scheme 2) was chosen as a model sys-
tem, since it had previously been shown to function as a
Grb2SH2domain-binding ligand (IC
value of 2 mM).
50
Of note, although aldehydes of type 10 have previously
been prepared as versatile intermediates for a variety of
pTyr mimetics,21À23 the current synthetic approach is
significantly improved relative to these previous meth-
ods.20,22 Transformation of 10 to chiral aldimine 12 was
achieved by refluxing with sulfinylamide 11 in the pre-
sence of Ti(O-Pri)2 (79% yield from 10).24 Treatment of
12 with the sodium enolate of methyl acetate, which was
generated from NaHMDS in ether at À78ꢀ C, failed to
give the desired product 12.25 Alternatively, using the
titanium enolate (formed by transmetallation of lithium
methylacetate using 2.3 equiv of ClTi(O-Pri)3 in THF),
Using a functionalized variant of 20, earlier modeling
studies had highlighted the hydrophobic binding inter-
actions of the carboxy-terminal naphthylpropyl group
with a hydrophobic region of the Grb2protein formed
by residues Lys109 and Leu111.28
As shown in Figure 3 (Panel A), hydrophobic inter-
actions could also reasonably be expected in the binding
of 20. Accordingly, tripeptide 19 was designed utilizing
b-amino p-Tyr mimetic 3, wherein a similar type of
hydrophobic interaction was envisioned by translocat-
ing the naphthyl group from the carboxy-terminal to
the amino-terminal position (Fig. 3, Panel B). A close
correspondence of naphthyl-functionality binding could
potentially be achieved, as shown in Panel C.
Synthesis of 19 commenced with the preparation of
dipeptide 16, which was obtained in 90% yield by cou-
pling the HOBt-active ester of N-Fmoc-1-aminocyclo-
hexanecarboxylic acid with Asn-amideꢁ HCl in the
presence of iPr2NEt, followed by standard N-Fmoc
deprotection using piperidine (Scheme 2). Introduction
of N-ButS(O)-protected 14 was then accomplished using
HOBt active ester coupling (HOBt/diisopropyl-carbo-
diimide), to provide 17 in 57% yield. Attempted
removal of N-ButS(O)-protection using TFA failed,
however, treatment with 4N HCl/dioxane in MeOH
Figure 2. Unsuccessful approaches toward asymmetric introduction of
b-amino functionality.
Scheme 1. (a) HP(O)(OBn)2, n-BuLi, TBAI, À78ÀC to rt; (b) (i) cat.
OsO4, NMO, THF, water, (ii) NaIO4, THF/H2O, 1:1; (c) Ti(OiPr)4,
reflux; (d) methyl acetate, n-BuLi, THF, then ClTi(OiPr)3; (e) 1 M
LiOH, THF/MeOH, 1:1, rt; (f) 4 N HCl/dioxane, MeOH, rt; (g) (S)-
(À)-a-methoxy-a-(trifluoromethyl)phenylacetiyl chloride for 15a, (R)-
(À)-a-methoxy-a-(trifluoro-methyl)phenylacetiyl chloride for 15b.
Scheme 2. (a) HOBt, DIPCDI, DMF; (b) (i) 4 N HCl/dioxane,
MeOH, (ii) 2-naphthyloxyacetic acid, HOBt, DIPCDI, DMF; (c) H2,
10% Pd-C, MeOH, 40 psi.

K. Lee et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3399–3401
3401
Figure 3. Simulated binding of 20 (panel A) and 19 (Panel B) to the Grb2SH2domain comparing potential interaction of naphthyl units with a
10. Burke, T. R., Jr.; Yao, Z.-J.; Smyth, M. S.; Ye, B. Curr.
gave the b-amino analogue quantitatively, which was
then acylated using commercially available 2-naphthyl-
Pharm. Des. 1997, 3, 291.
11. Burke, T. R., Jr.; Yao, Z.-J.; Liu, D.-G.; Voigt, J.; Gao,
oxyacetic of 18, using 10% palladium on activated
Y. Biopolymers 2001, 60, 32 .
carbon in MeOH at 40 psi, 2h, and purification by
reverse phase HPLC, led to b-amino pTyr-containing 19
(81% yield). Although in an ELISA-based Grb2SH2
domain assay,29 19 exhibited significantly less affinity
than parent 20 (data not reported), the original intent of
demonstrating the suitability of 14 for peptide synthesis
was successfully achieved.
12. Furet, P.; Gay, B.; GarciaEcheverria, C.; Rahuel, J.;
Fretz, H.; Schoepfer, J.; Caravatti, G. J. Med. Chem. 1997, 40,
3551.
13. Rahuel, J.; GarciaEcheverria, C.; Furet, P.; Strauss, A.;
Caravatti, G.; Fretz, H.; Schoepfer, J.; Gay, B. J. Mol. Biol.
1998, 279, 1013.
14. Lee, T. R.; Lawrence, D. S. J. Med. Chem. 1999, 42, 784.
15. Trub, T.; Choi, W. E.; Wolf, G.; Ottinger, E.; Chen, Y.;
Weiss, M.; Shoelson, S. E. J. Biol. Chem. 1995, 270, 18205.
16. Ettmayer, P.; France, D.; Gounarides, J.; Jarosinski, M.;
Martin, M. S.; Rondeau, J. M.; Sabio, M.; Topiol, S.; Weid-
mann, B.; Zurini, M.; Bair, K. W. J. Med. Chem. 1999, 42,
971.
17. (a) Cole, D. C. Tetrahedron 1994, 50, 9517. (b) Sewald, N.
Amino Acids 1996, 11, 397. (c) Enantioselective Synthesis of ꢀ-
Amino Acids, Juaristi, E., Ed. Wiley-VCH: New York, 1997.
(d) Abdel-magid, A. F.; Cohen, J. H.; Maryanoff, C. A. Curr.
Med. Chem. 1999, 6, 955. (e) Juaristi, E.; Lopez-Ruiz, H. Curr.
Med. Chem. 1999, 6, 983.
In conclusion, reported herein is the design and efficient
synthesis of the first reported b-amino pTyr mimetic,
bearing protection suitable for peptide synthesis. This
new amino acid analogue may expand the range of
peptide structures available for investigators studying
signal transduction processes.
Acknowledgements
18. Cogan, D. A.; Liu, G.; Kim, K.; Backes, B. J.; Ellman,
J. A. J. Am. Chem. Soc. 1998, 120, 8011.
19. Tang, T. P.; Ellman, J. A. J. Org. Chem. 1999, 64, 12 .
20. Lee, Y.; Silverman, R. B. Org. Lett. 2000, 2, 303.
21. Burke, T. R.; Li, Z.-H.; Bolon, J. B.; Marquez, V. E.
J. Med. Chem. 1991, 34, 1577.
Appreciation is expressed to Dr. James Kelley of the
LMC for mass spectral analysis.
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