A new approach for modification of phenylalanine peptides by Suzuki-Miyaura coupling reaction.

Article abstract of DOI:10.1016/S0960-894X(01)00582-0

For the first time, we have modified phenylalanine peptides by the Suzuki-Miyaura coupling reaction which may be useful in developing combinatorial libraries of peptidomimetics.

Full text of DOI:10.1016/S0960-894X(01)00582-0

Bioorganic & Medicinal Chemistry Letters 11 (2001) 2887–2890
A New Approach for Modification of Phenylalanine Peptides by
Suzuki–Miyaura Coupling Reaction
Sambasivarao Kotha* and Kakali Lahiriy
Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai 400 076, India
Received 19 July 2001; accepted 29 August 2001
Abstract—For the first time, we have modified phenylalanine peptides by the Suzuki–Miyaura couplingreaction which may be
useful in developingcombinatorial libraries of peptidomimetics. # 2001 Elsevier Science Ltd. All rights reserved.
Peptides are natural messenger molecules of the human
body and hence ideal lead compounds for initiation of
drugdiscovery research. Examples of disorders for
which peptide-based therapy exists include osteoporosis,
diabetes, hypertension and diuresis. However, unfa-
vourable pharmacological properties, such as non-
selectivity, poor oral availablity and rapid proteolysis,
limit severely the clinical usage of unmodified native
peptides.1 It is a general goal of peptide research to aim
for structure modifications that may confer oral
absorptivity, metabolic stability or function specificity,
thus improvingthe pharmacological profile of a native
peptide.2 Molecular modifications are either undertaken
rationally to alter the stucture or conformational fea-
tures of the peptide pharmacophore, or combinatorially
followed by screeningto look for variant that will dis-
play desired modification in the activity profiles. A
common theme of both peptidomimetics and combina-
torial chemistry research is that stereochemical
restraints are introduced into the native pharmacophore
and, chemistries to introduce such restraints directly in
the peptide architecture evoke interest in the rapidly
even provide multitude of peptides, which could not
possibly be made from component amino acids because
the individual amino acid may require special coupling
methods and/or require special protectinggroups.
As a part of research program,6 we were interested in
usingthe Suzuki–Miyaura couplingreaction 7as a key
step in the modification of the peptides. Aromatic
AAAs like phenylalanine and tyrosine are important
structural elements in several peptide pharmacophores.
To test this idea initially, the Suzuki couplingreaction
on simple AAA derivative was attempted. In this
regard, the known compound 2,8 obtained from com-
mercially available 4-iodo-l-phenylalanine was reacted
with furanboronic acid in presence of tetrakis(triphenyl-
phosphine)palladium(0) catalyst to give modified AAA
3 in 85% yield. Alongsimilar lines, various arylboronic
acid derivatives were reacted with 2 to give the corre-
spondingcouplingproducts
shown in Scheme 1. The structure of the couplingpro-
3–8 and the results are
1
ducts were confirmed by 300 MHz H NMR spectral
data.
3
developingfield of combinatorial chemistry.
In connection with our interest to prepare unusual a-
amino acid derivatives and peptide modifications we
4,5
have conceived a ‘buildingblock approach’
for the
generation of large number of compounds starting from
a common precursor. By usingour ‘buildingblock
approach’ one will be in a position to synthesise a series
of oligopeptides without having to repeat the whole
sequence of peptide synthesis. Such a procedure might
*Correspondingauthor. Tel.:+91-22-576-7160; fax:+91-22-572-3480;
e-mail: srk@chem.iitb.ac.in
yNee Chakraborty.
Scheme 1. Reagents and conditions (i) (Boc)2O, CHCl3, Et3N (ii)
ArB(OH)2, Pd(PPh3)4, Na2CO3–H2O, THF/toluene (1:1).
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00582-0

2888
S. Kotha, K. Lahiri / Bioorg. Med. Chem. Lett. 11 (2001) 2887–2890
Table 1. List of various modified phenylalanine peptide derivatives
prepared by the Suzuki–Miyaura couplingreaction
Startingpeptide
Couplingproduct Ar (Yield%)
Scheme 2. Reagents and Conditions. (i) BocHNXxxCO2H, HOBt,
EDC1, Dry THF, Et3N; (ii) MeOH, NaOH; (iii) BocNHYyyCO2Me,
CF3CO2H, HOBt, EDC1, Dry THF, NMM.
Havingestablished the Suzuki–Miyaura couplingreac-
tion with compound 2, attention was turned towards
the modification of phenylalanine peptides. In this
regard, the tripeptides 9 and 10 were prepared by the
condensation of the ester protected amino acid building
block 1 with various tert-butoxycarbonyl (Boc)-pro-
tected amino acid derivatives by standard DCC medi-
8
ated peptide couplingstrategy. However, in each case,
the tripeptide was found to be contaminated with the
byproduct, dicyclohexyl urea (DCU). To avoid this
problem, water-soluble DCC, 1-[3-(dimethylamino)pro-
pyl]3-ethylcarbodiimide hydrochloride (EDCl) was used
for the couplingreaction. The peptides were purified by
silica gel chromatography. The purity of the tripeptides
was confirmed by TLC and high field (300 MHz) 1H
NMR spectral data. Suzuki–Miyaura couplingon these
tripeptides was performed usingthe conditions devel-
oped in the case of phenylalanine AAA derivatives and
the yields of the reactions were found to be good. Simi-
larly, pentapeptide 11 was prepared from the tripeptide
9 usingthe same couplingprotocol. The purity of the
compounds was judged by TLC and 300 MHz 1H NMR
modification of unusual phenylalanine peptides using
Suzuki–Miyaura couplingreaction as a key step. This
strategy which gave a good overall yield may be useful
to prepare various biologically active peptides in a short
period of time. Moreover, the strategy developed here
may find useful application in developingcombinatorial
synthesis of peptidomimetics and therefore our results
may find useful applications in bioorganic and medic-
inal chemistry.
1
spectral data. The 300 MHz H NMR spectrum of the
peptide 11 in CDCl3 showed three peaks instead of five
amide peaks as expected. However, when the spectrum
was recorded in DMSO-d6, all the amide protons were
accounted. Temperature dependence of NH shift
experiment was carried on the pentapeptide 11 in
DMSO-d6.9 It is noteworthy to mention that one of the
amide proton at d=8 ppm (at rooms temperature) was
having[ ꢀd/T] value 2Â10À3 ppm/ꢀC. This indicates the
amide proton is solvent shielded which may be due to
intramolecular H-bondingwith C ¼O moiety present in
the peptide residue.
Selected data for the compounds 3–20
26
3. ½ꢀꢁD 57.29 (c 1, CHCl3).7e
26
4. ½ꢀꢁD À28.57 (c 0.7, CHCl3).
5. ½ꢀꢁD 52.92 (c 1, CHCl3).7e
26
26
6. ½ꢀꢁD 57.99 (c 1, CHCl3).7e
26
Then, attention was directed towards the modification
of the pentapeptide 11. In this regard, compound 11 was
reacted with furan, thiophene and phenylboronic acids
successively to generate various modified analogues and
the results are summarized in Table 1. A typical experi-
mental procedure for the Suzuki–Miyaura coupling
reaction involves treatment of the iodo compound (1
equiv) with boronic acid (2 equiv) at 80 ꢀC in presence
of tetrakis(triphenylphosphine)palladium(0) (5 mol%)
catalyst and aqueous sodium carbonate (2 equiv) in
THF/toluene solvent. At the conclusion of the reaction
(TLC), the two layers were separated and the aqueous
layer was extracted with dichloromethane. The product
was purified on a silica gel column by eluting with pet
ether/ethyl acetate (Scheme 2 ).
7. ½ꢀꢁD 32.99 (c 1, CHCl3).
26
8. ½ꢀꢁD 24.44 (c 0.9, CHCl3).
1
9. H NMR 300 MHz CDCl3. d 0.87–0.92 (m, 6H), 1.32
(d, J=6.9 Hz, 3H), 1.44 (s, 9H), 1.50–1.64 (m, 3H), 3.00
(dd, J=5.9, 13.9 Hz, 1H), 3.11 (dd, J=5.9, 13.9 Hz,
1H), 3.72 (s, 3H), 4.07–4.13 (m, 1H), 4.33–4.40 (m, 1H),
4.76–4.83 (m, 1H), 4.90 (bs, 1H), 6.61 (bs, 2H), 6.86 (d,
26
J=8.1 Hz, 2H), 7.61 (d, J=8.4 Hz, 2H); ½ꢀꢁD À5.55 (c
0.9, CHCl3); mp 164 ꢀC.
10. 1H NMR 300 MHz CDCl3. d 0.87–0.92 (m, 9H),
0.97 (d, J=7.0 Hz, 3H), 1.44 (s, 9H), 1.49–1.65 (m, 3H),
2.11–2.21 (m, 1H), 3.00 (dd, J=6.6, 13.9 Hz, 1H), 3.10
(dd, J=5.5, 13.9 Hz, 1H), 3.71 (s, 3H), 3.87 (dd, J=5.9,
8.1 Hz, 1H), 4.38–4.45 (m, 1H), 4.78 (dd, J=6.4 Hz,
In conclusion, for the first time we have demonstrated
an exceptionally simple and versatile method for the

S. Kotha, K. Lahiri / Bioorg. Med. Chem. Lett. 11 (2001) 2887–2890
2889
13.7, 1H), 4.97 (bs, 1H), 6.31 (bs, 1H), 6.67 (bs, 1H),
6.88 (d, J=8.4 Hz, 2H), 7.62 (d, J=8.4 Hz, 2H);
2.12–2.19 (m, 1H), 3.12 (dd, J=6.2, 13.9 Hz, 1H), 3.19
(dd, J=5.9, 13.9 Hz, 1H), 3.72 (s, 3H), 3.89 (dd, J=5.9,
7.9 Hz, 1H), 4.42–4.49 (m, 1H), 4.84 (dd, J=6.1,
13.6 Hz, 1H), 4.99 (d, J=7.1 Hz, 1H), 6.38 (d,
J=7.3 Hz, 1H), 6.68 (bs, 1H), 7.20 (d, J=8.1 Hz, 2H),
7.32–7.36 (m, 1H), 7.41–7.50 (m, 2H), 7.54 (d,
26
½ꢀꢁD 22.99 (c=1, CHCl3); mp. 153–154 ꢀC.
11. 1H NMR 300 MHz DMSO-d6. d 0.73–0.88 (m,
18H), 1.12 (d, J=7.3 Hz, 3H), 1.36 (s, 9H), 1.46–1.51
(m, 3H), 1.91–2.07 (m, 2H), 2.69–2.84 (m, 1H), 2.88–
2.94 (m, 1H), 3.63 (s, 3H), 3.88–3.94 (m, 1H), 4.16–4.24
(m, 2H), 4.31–4.36 (m, 1H), 4.63–4.70 (m, 1H), 6.90 (d,
J=12.0 Hz, 1H), 7.05 (d, J=8.1 Hz, 2H), 7.57 (d,
J=8.1 Hz, 2H), 7.68 (d, J=8.4 Hz, 1H), 7.96–8.03 (m,
26
J=8.2 Hz, 2H), 7.57–7.60 (m, 2H); ½ꢀꢁD 11.99 (c 0.5,
CHCl3); mp. 137–139 ꢀC.
18. 1H NMR 300 MHz DMSO-d6. d 0.72–0.88 (m,
18H), 1.09 (d, J=7.0 Hz, 3H), 1.35 (s, 9H), 1.23–1.51
(m, 3H), 1.89–2.07 (m, 2H), 2.72–2.84 (m, 1H), 2.95–
3.01 (m, 1H), 3.63 (s, 3H), 3.91–3.96 (m, 1H), 4.17–4.26
(m, 2H), 4.33–4.38 (m, 1H), 4.64–4.69 (m, 1H), 6.56 (dd,
J=1.8, 3.3 Hz, 1H), 6.86 (d, J=3.3 Hz, 1H), 6.91 (d,
J=6.6 Hz, 1H), 7.28 (d, J=8.4 Hz, 2H), 7.56 (d,
J=8.4 Hz, 2H), 7.67 (s, 1H), 7.70–7.71 (m, 1H), 8.00 (t,
26
2H), 8.24 (d, J=8.1 Hz, 1H); ½ꢀꢁD À25.71 (c 0.7,
CHCl3); mp 245 ꢀC.
12. 1H NMR 300 MHz CDCl3. d 0.87–0.91 (m, 6H),
1.29 (d, J=7.0 Hz, 3H), 1.43 (s, 9H), 1.50–1.65 (m, 3H),
3.07 (dd, J=6.3, 13.9 Hz, 1H), 3.15 (dd, J=6.0, 13.7 Hz,
1H), 3.72 (s, 3H), 4.07–4.11 (m, 1H), 4.35–4.40 (m, 1H),
4.79–4.86 (m, 1H), 4.89 (bs, 1H), 6.44–6.46 (m, 1H),
6.55 (bs, 2H), 6.62 (d, J=3.3 Hz, 1H), 7.12 (d,
J=8.0 Hz, 2H), 7.44 (s, 1H), 7.59 (d, J=8.0 Hz, 2H);
26
J=6.6 Hz, 2H), 8.25 (d, J=8.1 Hz, 1H); ½ꢀꢁD À24.28 (c
0.7, CHCl3); mp. 241 ꢀC.
19. 1H NMR 300 MHz DMSO-d6. d 0.67–0.88 (m,
18H), 1.10 (d, J=6.9 Hz, 3H), 1.35 (s, 9H), 1.14–1.56
(m, 3H), 1.89–2.10 (m, 2H), 2.76–2.84 (m, 1H), 2.94–
3.11 (m, 1H), 3.63 (s, 3H), 3.91–3.96 (m, 1H), 4.17–4.26
(m, 2H), 4.33–4.38 (m, 1H), 4.64–4.72 (m, 1H), 6.92 (d,
J=7.7 Hz, 1H), 7.11 (dd, J=3.7, 4.8 Hz, 1H), 7.27 (d,
J=8.1 Hz, 2H), 7.43 (d, J=2.9 Hz, 1H), 7.49–7.52 (m,
3H), 7.69 (d, J=8.1 Hz, 1H), 7.99–8.02 (m, 2H), 8.25 (d,
26
½ꢀꢁD À23.33 (c 0.6, CHCl3); mp. 148–149 ꢀC.
13. 1H NMR 300 MHz CDCl3. d 0.82–0.91 (m, 6H),
1.29 (d, J=7.0 Hz, 3H), 1.43 (s, 9H), 1.48–1.69 (m, 3H),
3.08 (dd, J=6.1, 13.8 Hz, 1H), 3.16 (dd, J=5.9, 13.7 Hz,
1H), 3.72 (s, 3H), 4.12 (q, J=7.1 Hz, 1H), 4.37–4.44 (m,
1H), 4.84 (dd, J=6.0, 13.7 Hz, 1H), 4.96 (d, J=7.0,
1H), 6.62 (bs, 2H), 7.07 (dd, J=3.6 Hz, 5.0 Hz, 1H),
7.11 (d, J=8.2 Hz, 2H), 7.25–7.29 (m, 2H), 7.53 (d,
26
J=8.4 Hz, 1H); ½ꢀꢁD À57.50 (c 0.4, CHCl3); mp. 250–
251 ꢀC.
26
J=8.1 Hz, 2H); ½ꢀꢁD 5.99 (c 1, CHCl3); mp. 163–164 ꢀC.
20. 1H NMR 300 MHz DMSO-d6. d 0.67–0.88 (m,
18H), 1.10 (d, J=7.3 Hz, 3H), 1.35 (s, 9H), 1.14–1.55
(m, 3H), 1.89–2.07 (m, 2H), 2.81–2.89 (m, 1H), 2.96–
3.08 (m, 1H), 3.63 (s, 3H), 3.92–3.96 (m, 1H), 4.19 (dd,
J=6.6, 8.4 Hz, 1H), 4.26–4.37 (m, 2H), 4.66–4.73 (m,
1H), 6.93 (d, J=7.7 Hz, 1H), 7.31–7.62 (m, 9H) 7.69 (d,
J=8.1 Hz, 1H), 7.98–8.04 (m, 2H), 8.23 (d, J=8.4 Hz,
14. 1H NMR 300 MHz CDCl3. d 0.89 (t, J=6.2 Hz,
6H), 1.29 (d, J=7.0 Hz, 3H), 1.43 (s, 9H), 1.49–1.70 (m,
3H), 3.11 (dd, J=6.0, 13.9 Hz, 1H), 3.20 (dd, J=6.0,
14.0 Hz, 1H), 3.74 (s, 3H), 4.08–4.13 (m, 1H), 4.38–4.45
(m, 1H), 4.86 (dd, J=6.0, 13.6 Hz, 1H), 4.95 (d,
J=7.1 Hz, 1H), 6.61 (bs, 2H), 7.16–7.59 (m, 9H);
26
26
½ꢀꢁD 11.99 (c 1, CHCl3); mp. 155–157 ꢀC.
1H); ½ꢀꢁD À11.66 (c 0.6, CHCl3); mp 242 ꢀC.
15. 1H NMR 300 MHz CDCl3. d 0.84–0.90 (m, 9H),
0.94 (d, J=6.6 Hz, 3H), 1.44 (s, 9H), 1.49–1.67 (m, 3H),
2.12–2.18 (m, 1H), 3.07 (dd, J=6.9, 13.8 Hz, 1H), 3.14
(dd, J=5.7, 13.8 Hz, 1H), 3.71 (s, 3H), 3.88 (dd, J=5.8,
8.1 Hz, 1H), 4.40–4.46 (m, 1H), 4.81 (dd, J=6.0 Hz,
13.7, 1H), 4.97 (bs, 1H), 6.32 (d, J=7.7 Hz, 1H), 6.46 (dd,
J=1.8, 3.3 Hz, 1H), 6.64 (d, J=2.9 Hz, 1H), 6.66 (bs, 1H),
7.14 (d, J=8.1 Hz, 2H), 7.45 (d, J=1.5 Hz, 1H), 7.60 (d,
Acknowledgements
We would like to acknowledge the DST for the financial
support, RSIC-Mumbai and TIFR for recordingspec-
tral data. K.L. thanks IIT-Bombay for the award of the
fellowship.
26
J=8.4 Hz, 2H); ½ꢀꢁD 55.00 (c 0.8, CHCl3); mp. 134–136ꢀC.
1
References and Notes
16. H NMR 300 MHz CDCl3. 0.84–0.91 (m, 9H), 0.94
(d, J=6.6 Hz, 3H), 1.43 (s, 9H), 1.50–1.69 (m, 3H),
2.12–2.19 (m, 1H), 3.07 (dd, J=6.6, 13.9 Hz, 1H), 3.15
(dd, J=6.0, 13.9 Hz, 1H), 3.71 (s, 3H), 3.89 (dd, J=5.7,
7.9 Hz, 1H), 4.41–4.45 (m, 1H), 4.82 (dd, J=6.2,
13.9 Hz, 1H), 4.99 (d, J=6.6 Hz, 1H), 6.36 (bs, 1H),
6.68 (bs, 1H), 7.07 (dd, J=3.7, 5.1 Hz, 1H), 7.14 (d,
J=8.1 Hz, 2H), 7.27 (d, J=3.3 Hz, 1H), 7.30 (d,
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26
J=3.7 Hz, 1H), 7.54 (d, J=8.1 Hz, 2H); ½ꢀꢁD À12.50 (c
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17. 1H NMR 300 MHz CDCl3. d 0.07–0.91 (m, 9H),
0.94 (d, J=6.8 Hz, 3H), 1.43 (s, 9H), 1.50–1.67 (m, 3H),

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