A new efficient enantioselective synthesis of malonylphenylalanyl and malonylmethylphenylalanyl derivatives suitable for solid phase peptide synthesis

Article abstract of DOI:10.1016/j.tetlet.2005.03.105

A new synthesis of enantiomerically pure malonylphenylalanyl and malonylmethylphenylalanyl derivatives was developed in which the corresponding prochiral enamides were treated by asymmetric hydrogenation using the Rh(I)-(S,S)-Me-DuPHOS system. These unnat

Full text of DOI:10.1016/j.tetlet.2005.03.105

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 3319–3322
A new efficient enantioselective synthesis of
malonylphenylalanyl and malonylmethylphenylalanyl
derivatives suitable for solid phase peptide synthesis
Huixiong Chen, Jean-Philippe Luzy and Christiane Garbay*
´ ´ ´
Laboratoire de pharmacochimie moleculaire et cellulaire, INSERM, U648, CNRS FRE 2718, Faculte de Medecine des
` ´ ´
Saints-Peres, Universite Rene Descartes, 75270 Paris Cedex 06, France
Received 9 February 2005; revised 15 March 2005; accepted 16 March 2005
Available online 5 April 2005
Abstract—A new synthesis of enantiomerically pure malonylphenylalanyl and malonylmethylphenylalanyl derivatives was devel-
oped in which the corresponding prochiral enamides were treated by asymmetric hydrogenation using the Rh(I)-(S,S)-Me-DuPHOS
system. These unnatural amino acids were suitably protected and can be used in solid phase peptide synthesis.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Many of the signalling pathways and regulatory systems
in eukaryotic cells are controlled by phosphorylation
and dephosphorylation of tyrosine residues in proteins.
Phosphotyrosyl residue (pY) and surrounding amino
acids serve as high affinity and specific binding sites
for intracellular proteins interacting via Src-homology
2 (SH2) or phosphotyrosyl-binding (PTB) domains.
Due to the critical function of signalling transduction
in normal cell growth, differentiation and activation,
dysfunction of these signal pathways can participate in
the etiology of human neoplastic diseases.1 The impor-
tant role of pY residues in protein tyrosine kinase
(PTK)-dependent signal transduction has generated
great interest in synthesising stable non-hydrolysable
phosphotyrosine mimics in the design of peptide-based
signalling transduction inhibitors. As part of our pro-
gram to develop inhibitors of SH2 domains of Grb fam-
ily, we required enzymatically stable, non-hydrolysable
analogues of phosphotyrosine. In particular we were
interested in para-malonylphenylalanine (Pmf), which
is the most potent non-phosphorus-containing pY mi-
metic reported against Grb2 SH2 domains.2 Pmf-con-
dent hepatocyte growth factor-induced cell migration
at 3 nM concentration in cell-based assays.3
The synthesis of Fmoc-protected Pmf was previously re-
ported by Gao and Burke,4 using the method of Wil-
liam.5 This compound has also been synthesised in our
laboratory by another pathway, using a camphor sultam
as chiral auxiliary (not published). However, such a syn-
thetic pathway is rather lengthy and has a low yield. Re-
cently, it was shown that Rh complexes bearing
DuPHOS ligand are extremely effective catalysts in
enantioselective hydrogenation of a variety of prochiral
unsaturated substrates including enamide esters with an
ee up to 99%,6 providing a new enantioselective route to
a-aminoacids. In this letter we report a new short enan-
tioselective synthesis of para-malonylphenylalanine
(Pmf), para-malonylmethylphenylalanine (Pmmf) in
which the amino group was protected with a widely used
protecting group such as Fmoc, Boc or Cbz (Schemes 1
and 2). Such derivatives can be incorporated into pep-
tides by solid phase synthesis.
taining
compounds
show
potent
inhibition
As shown in Scheme 1, the differently protected Pmfs
were conveniently achieved in a four-step synthesis
starting from 4-bromobenzaldehyde diethyl acetal by
using the strategy of asymmetric catalytic hydrogena-
tion of the corresponding unsaturated derivatives.
(IC50 = 8 nM) of Grb2 SH2 domain binding in ELISA
assays and almost completely inhibit Met PTK-depen-
Keywords: Enantioselective synthesis; Phosphotyrosine mimetics; SH2
domain.
The palladium catalysed cross-coupling reaction7 of 4-
bromobenzaldehyde diethyl acetal with di-tert-butyl
malonate was performed in anhydrous dioxane at
*
Corresponding author. Tel.: +33 1428 64080; fax: +33 1428 64082;
e-mail addresses: Huixiong.chen@univ-paris5.fr; christiane.garbay@
univ-paris5.fr
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.03.105

3320
H. Chen et al. / Tetrahedron Letters 46 (2005) 3319–3322
t-BuO
t-BuO
O
t-BuO
t-BuO
O
O
O
Ot-Bu
Ot-Bu
O
O
O
H
ii
i
iii
Br
O
O
R
O
R
O
N
N
H
H
O
O
1
2
3
2a R = Boc
2b R = Cbz
3a R = Boc
3b R = Cbz
t-BuO
O
t-BuO
iv
O
R
OH
N
H
O
4
4a R = Boc
4b R = Cbz
4c R = Fmoc
v
Scheme 1. Asymmetric synthesis of N-protected p-malonylphenylalanine. Reagents and conditions: (i) tbutyl malonate/Pd(dba)2/P(tBu)3/KOtBu;
citric acid 10%; (ii) methyl 2-(N-Boc-amino)-2-dimethylphosphonylacetate or methyl 2-(N-Cbz-amino)-2-dimethylphosphonylacetate/TMG; (iii) H2
10 atm/Rh(COD)2OTf/(S,S) Me-DuPhOS; (iv) Na2CO3/MeOH; (v) H2/Pd–C; FmocOSu/Na2CO3.
O
O
O
O
O
H
O
t-BuO
t-BuO
t-BuO
t-BuO
t-BuO
iii
i
ii
H
O
t-BuO
O
O
R1
OR2
R1
OR2
O
N
H
N
H
O
O
5
6
7
7a R1 = Boc, R2 = Me
7b R1 = Cbz, R2 = Me
7c R2 = Cbz, R2 = H
6a R1 = Boc, R2 = Me
6b R1 = Cbz, R2 = Me
v
ii
i
H
iv
O
O
O
t-BuO
t-BuO
v or vi
OH
t-BuO
t-BuO
R1
O
N
H
O
O
O
R1
R1
OR2
N
H
N
H
10
O
9
O
8
10a R1 = Boc
10b R1 = Cbz
9a R1 = Boc
9c R1 = Fmoc
8a R1 = Boc, R2 = Me
8c R1 = H, R2 = H
Scheme 2. Asymmetric synthesis of N-protected p-malonylmethylphenylalaninyl derivative. Reagents and conditions: (i) tbutyl malonate/TiCl4/
pyridine; (ii) methyl 2-(N-Boc-amino)-2-dimethylphosphonylacetate or methyl 2-(N-Cbz-amino)-2-dimethylphosphonylacetate/TMG; (iii) H2
10 atm/Rh(COD)2OTf/(S,S) Me-DuPhOS; (iv) H2/Pd–C; (v) Na2CO3/MeOH; (vi) FmocOSu/Na2CO3.
80 ꢁC for 6 h, using P(tBu)3 as ligand and KOtBu as
base.
proceeded smoothly with methyl 2-(N-Boc-amino)-
2-dimethylphosphonylacetate or methyl 2-(N-Cbz-ami-
no)-2-dimethylphosphonylacetate using tetramethyl-
guanidine (TMG) as base at ꢀ78 ꢁC, followed by
warming up to ambient temperature overnight. The
(Z)-enamido esters 2a–b were formed in yields of 76%
Aldehyde 1 was obtained in 75% yield, after treatment
with 10% citric acid. A Horner–Emmons (HE) type olef-
ination of the resulting aldehyde 1 in anhydrous THF

H. Chen et al. / Tetrahedron Letters 46 (2005) 3319–3322
3321
Figure 1. Reverse phase HPLC chromatograms of crude dipeptides 13 (left) and 14 (right), using a linear gradient: 10–25% B/15 min (A: 0.1% TFA
in water; B: 70% acetonitrile in aqueous solution with 0.09% TFA) on a Vydac C-18 column (5 lm, 4.6 · 250 mm) at the flow 1 mL/min.
and 79%, respectively. The configuration was based on
the fact that didehydroamino acid derivatives with an
E-configuration have been reported to display NOE ef-
fects between the olefinic CH and NH protons.8 No such
effects were observed for 2a–b. Asymmetric hydrogena-
tion of 2a–b using BurkÕs catalytic Rh(I)-(S,S)-Me-Du-
PHOS system6 (0.1 mol %) in deoxygenated MeOH
under reaction conditions of 10 atm H2 at 25 ꢁC for
24 h afforded the fully protected L-Pmf derivatives 3a–
b in 94% and 96% yields, respectively. The absolute con-
figurations were assigned as S based on the selectivity of
the (S,S)-Me-DuPHOS ligand.6,9 The synthesis of 4a
was initially attempted by standard saponification pro-
cedures, but was unsuccessful due to the concomitant
partial deprotection of the malonyl tert-butyl ester.
Thus in order to avoid such a hydrolysis, compounds
3a–b were treated in mild conditions (Na2CO3/MeOH,
25 ꢁC, 16 h) to provide the desired acids 4a–b10,11 in
yields of 86% and 89%. Finally, Pd-catalysed hydrogeno-
lytic removal of Cbz in 4b in MeOH followed by
treatment with N-Fmoc succinimide (Fmoc-OSu) in
dioxane–water (1:1) using Na2CO3 as base afforded the
N-Fmoc form 4c,12 in yield of 93% by two steps.
afforded 7b, which was subsequently saponified in
MeOH, using Na2CO3 as base to generate the acid
7c14 with a yield of 83% by two steps. The latter was
finally hydrogenated on palladium and protected using
Fmoc-OSu to give the Fmoc derivative 9c15 in yield of
92%.
In order to verify the enantiomeric purity of Pmf and
Pmmf, dipeptides H-(D,L)-Pmf-(L)-Ala-OH 11 and H-
(L)-Pmf-(L)-Ala-OH 12 or H-(D,L)-Pmmf-(L)-Ala-OH
13 and H-(L)-Pmmf-(L)-Ala-OH 14 were prepared by
DCC, HOBt, NMP coupling and TFA deprotection,
using tert-butyl alaninate with Boc-(L)-Pmf 4a or Boc-
(L)-Pmmf 9a or their racemic mixtures. The latter
compounds Boc-(D,L)-Pmf and Boc-(D,L)-Pmmf were
obtained by treatment of 2a or 6a in MeOH using Pd-
catalysed hydrogenation followed by mild hydrolysis.
Comparison of HPLC profiles of 13 and 14 (Fig. 1)
showed a very high enantiomeric purity (>96%). This
was also observed by 1H NMR spectroscopy (DMSO-
d6). Thus, in 13, the NH (Ala) gave two doublets at
8.82 and 8.65 ppm, and the CH3b (Ala) group two dou-
blets at 1.33 ppm and 1.13 ppm. In 14, the NH (Ala) ap-
peared as a single peak at 8.82 ppm and the CH3b (Ala)
group led to a doublet at 1.33 ppm. Finally, the similar
results were also observed for the peptides 11 and 12.
The new Pmmf derivatives were prepared following a
similar synthetic pathway which is outlined in Scheme 2.
The commercially available 4-(diethoxymethyl)-benzal-
dehyde was converted into a,b-unsaturated di-tert-butyl
malonate 5 by a mild Knoevenagel condensation with
1 equiv of di-tert-butyl malonate in the presence of
1 equiv of TiCl4 and 1 equiv of pyridine in anhydrous
THFat 0 ꢁC, followed by warming up to ambient tem-
perature for 4 h in yield of 82%. A HE olefination of
aldehyde group in 5 with methyl 2-(N-Boc-amino)-2-
dimethylphosphonylacetate or methyl 2-(N-Cbz-ami-
no)-2-dimethylphosphonylacetate in anhydrous THF,
using TMG as base gave (Z)-enamido esters 6a–b in
yields of 80% and 75%, respectively. Alternatively, these
compounds could be obtained by a HE olefination of
4-(diethoxymethyl)benzaldehyde, followed by Knoevena-
gel condensation. Subsequent generation of 8a was
easily achieved by hydrogenation of 6a in the presence of
Rh(I)-(S,S)-Me-DuPHOS catalyst (0.1 mol %) and by
Pd-catalysed hydrogenation in yield of 93%. The car-
boxylic methyl ester 8a was saponified in mild condi-
tions (Na2CO3/MeOH) to give the corresponding acid
9a13 in yield of 87%. Hydrogenation of 6b in the pres-
ence of Rh(I)-(S,S)-Me-DuPHOS catalyst (0.1 mol %)
In conclusion, we have developed a very rapid synthesis
of optically pure malonylphenylalanyl and malonyl-
methylphenylalanyl derivatives. The applicability of
these unnatural amino acids to solid phase peptide syn-
thesis is now in progress in our laboratory.
Acknowledgements
´
We thank the Ligue Nationale contre le Cancer (Comite
de Paris) for financial support.
References and notes
1. Edwarss, D. R. Trends Pharmacol. Sci. 1994, 15, 239–244.
2. Gao, Y.; Luo, J.; Yao, Z. J.; Zou, H.; Kelly, J.; Voigt, J.
H.; Yang, D.; Burke, T. R., Jr. J. Med. Chem. 2000, 43,
911–920.
3. Atabey, N.; Yao, Z. J.; Breckenridge, D.; Soon, L.;
Soriano, J. V.; Burke, T. R.; Bottaro, D. P., Jr. J. Biol.
Chem. 2001, 276, 14308–14314.

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H. Chen et al. / Tetrahedron Letters 46 (2005) 3319–3322
4. Gao, Y.; Burke, T. R., Jr. Synlett 2000, 1, 134–136.
5. William, R. M.; Im, M. N. J. Am. Chem. Soc. 1991, 113,
9276–9286.
6. Burk, M. J.; Feaster, J. E.; Nugent, W. A.; Harlow, R. L.
J. Am. Chem. Soc. 1993, 115, 10125–10138.
7. Beare, N. A.; Hartwig, J. F. J. Org. Chem. 2002, 67, 541–
555.
12. Compound 4c: 1H NMR (250 MHz, CDCl3) d 1.49 (s,
18H, 2 · tBu), 3.20 (d, 2H, CH2b), 4.10–4.55 (m, 4H,
CH2–CH, CH(CO2tBu)2), 4.72 (t, 1H, CHa), 5.25 (s, 1H,
NH), 7.16–7.81 (m, 12H, aromatic). HRMS(ESI): calcd
for C35H39NO8Na: 624.2573 (M+Na)+. Found: 624.2581.
20
½aꢁD +24.2 (c 1, CHCl3).
13. Compound 9a: 1H NMR (250 MHz, CDCl3) d 1.42 (s,
27H, 3 · tBu), 3.0–3.3 (m, 4H, CH2b, CH2), 3.52 (t, 1H,
CH(CO2tBu)2), 4.42 (m, 1H, CHa), 5.06 (s, 1H, NH), 7.13
(s, 4H, aromatic). MS(ESI): 516.6 (M+Na)+, 532.6
8. Shimohigashi, Y.; Nitz, T. J.; Stammer, C. H.; Inubushi,
T. Tetrahedron Lett. 1982, 23, 3235–3323.
9. RajanBabu, T. V.; Radetich, B.; You, K. K.; Ayers, T. A.;
Casalnuovo, A. L. J. Org. Chem. 1999, 64, 3429–
3447.
20
(M+K)+. ½aꢁD 11.9 (c 1, CHCl3).
14. Compound 7c: 1H NMR (250 MHz, CDCl3) d 1.54 (s,
18H, 2 · tBu), 3.22 (m, 2H, CH2b), 4.70 (m, 1H, CHa);
5.12 (s, 2H, CH2Ph), 5.25 (s, 1H, NH), 7.1–7.4 (m, 9H,
aromatic), 7.53 (s, 1H, @CH). MS(ESI): 548.6 (M+Na)+,
10. Compound 4a: 1H NMR (250 MHz, CDCl3) d 1.41 (s,
27H, 3 · tBu), 2.78–3.10 (m, 2H, CH2b), 4.41 (s, 1H,
CH(CO2tBu)2), 4.52 (t, 1H, CHa), 5.34 (s, 1H, NH), 7.13
(s, 4H, aromatic). MS(ESI): 502.4 (M+Na)+, 518.5
20
564.5 (M+K)+. ½aꢁD +26.5 (c 1, CHCl3).
20
(M+K)+. ½aꢁD +13.6 (c 1, CHCl3).
15. Compound 9c: 1H NMR (250 MHz, CDCl3) d 1.42 (s,
18H, 2 · tBu), 3.14–3.44 (m, 4H, CH2b, CH2), 3.50 (s, 1H,
CH(CO2tBu)2), 4.21–4.46 (m, 3H, CH2–CH), 4.68 (t, 1H,
CHa), 5.20 (s, 1H, NH), 7.16–7.81 (m, 12H, aromatic).
HRMS(ESI): calcd for C36H41NO8Na: 638.2730
11. Compound 4b: 1H NMR (250 MHz, CDCl3) d 1.49 (s,
18H, 2 · tBu), 3.18 (d, 2H, CH2b), 4.47 (s, 1H,
CH(CO2tBu)2), 4.72 (t, 1H, CHa), 5.15 (s, 2H, CH2Ph),
5.44 (s, 1H, NH), 7.18–7.57 (m, 9H, aromatic). MS(ESI):
20
20
536.6 (M+Na)+, 552.5 (M+K)+. ½aꢁD +17.7 (c 1, CHCl3).
(M+Na)+. Found: 638.2929. ½aꢁD +15.2 (c 1, CHCl3).