A useful synthesis of the Phe-Arg phosphinic acid dipeptide isostere

Article abstract of DOI:10.1016/S0040-4039(02)00957-7

A modular method for construction of polypeptides containing the Phe-Arg phosphinic acid isostere is described.

Full text of DOI:10.1016/S0040-4039(02)00957-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 4973–4976
A useful synthesis of the Phe-Arg phosphinic acid dipeptide
isostere
Andrew S. Kende,a,* Han-Qing Dong,a Xuewei Liub and Frank H. Ebetinob
aChemistry Department, University of Rochester, Rochester, NY 14627-0216, USA
bProcter & Gamble Pharmaceuticals, Health Care Research Center, Mason, OH 45040-8006, USA
Received 27 March 2002; accepted 14 May 2002
Abstract—A modular method for construction of polypeptides containing the Phe-Arg phosphinic acid isostere is described.
© 2002 Elsevier Science Ltd. All rights reserved.
Peptidomimetic analogs have found wide application as
bioavailable and potent mimetics of naturally occurring
biologically active peptides.1 Work continues in the
field to develop diverse non-peptidic scaffolds and
amide isosteres to increase metabolic stability, to
restrict the conformational properties of short peptides
and to provide three-dimensional mimics of peptide
motifs such as b-turns and a-helices.2,3
sis of Phe-Arg carba analog Boc-
D
-Phe-C[CH2CH2]-L-
Arg(NO2]-OH.6 Rich et al.7 reported the protected
hydroxyethylene dipeptide isostere of Phe-Arg and
tripeptide derivatives as components of potential pepti-
dase inhibitors. Recently, we also reported a novel
asymmetric synthesis of C[PheP-Phe] analogs.8
We now report the successful extension of this work to
the ready preparation of phosphinic acid dipeptide
During our design and screening of peptidomimetic
receptor agonists, we became interested in synthesizing
isosteres of Phe-Arg. Arginine bears a basic guanidino
group which is positively charged at neutral pH and is
involved in many important physiological and patho-
physiological processes. Many enzymes and receptors
display a preference for the arginine residue that is
found in natural substrates and in synthetic
inhibitors.4,5 Previously, we reported a practical synthe-
isosteric
PheP-Arg
analogs,
since
unnatural
C[P(O)OHCH2] peptide analogs also provide the pepti-
domimetic chemist additional metabolically stable
isosteres/scaffolds. We expect these and related analogs
will allow further opportunities to scan the conforma-
tional requirements within receptor ligand pharma-
cophores. For example, several of the analogs reported
herein include potential simplified pharmacophoric
mimics of the melanocortin receptor agonist tetra-
Scheme 1.
Keywords: phosphinic acid dipeptide; PheP-Arg isostere; methylenemalonate ester; phosphinate conjugate addition.
* Corresponding author. Fax: 716-473-6889; e-mail: kende@chem.rochester.edu
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00957-7

4974
A. S. Kende et al. / Tetrahedron Letters 43 (2002) 4973–4976
peptide recognition sequence His-Phe-Arg-Trp, based
on related attempts by Hruby, Benoit, Bednarek, Wik-
berg and others cited by them.9,10
Phosphinic acid dipeptides have been synthesized by
conjugate addition of a mono-substituted phosphinate
to certain a-substituted acrylates in which the a-sub-
stituent comprises the C-terminus side chain, however
this route may fail depending on the nature of the side
chain.11 We now describe an alternative modular strat-
egy in which di-tert-butyl methylenemalonate serves as
the Michael acceptor,12 permitting subsequent alkyla-
tion next to the C-terminus from a common phos-
phinylmethylenemalonate intermediate to introduce
side chain substituents at will. Scheme 1 illustrates this
strategy for the construction of dipeptides containing
the Phe-Arg phosphinic acid core.
Scheme 2. Reagents and conditions: (a) iPr2NEt, TMS–Cl,
CH2Cl2; (b) CH2N2 for 3a (68% from 1a), EDCI, PrOH,
CH2Cl2 for 3a% (72% from 1a); (c) t-BuOK, I(CH2)3N3, DME,
(44% for 4a, 88% for 4a%).
i
Scheme 3. Reagents and conditions: (a) TFA, CH2Cl2; (b) toluene, reflux; (c) CH2N2, 61% (from 4a).
Scheme 4. Reagents and conditions: (a) TFA, CH2Cl2; (b) toluene, reflux; (c) 1,1%-carbonyldiimidazole (CDI), 2-Nap-NHMe, THF;
(d) CH2N2, 46% (from 4b); (e) Lindlar catalyst, H2, N,N%-bis(tert-butoxycarbonyl)-1H-pyrazole-1-carboxamidine, EtOH, 63%; (f)
10% Pd–C, HCO2NH4, MeOH; (g) AcNH-Tyr(t-Bu)-OH, HOBT, EDCI, N-methylmorpholine, THF, 79% (from 9); (h) TMS–Br,
CH2Cl2; (i) TFA, CH2Cl2, 100%.

A. S. Kende et al. / Tetrahedron Letters 43 (2002) 4973–4976
4975
In the specific example of Scheme 2, the racemic phos-
phinic acid 1a13 was activated by silylation,14 then
reacted with di-tert-butyl methylene malonate followed
by esterification with diazomethane to afford adduct 3a
in 68% yield. Alkylation of 3a with 1-azido-3-
iodopropane15 using potassium tert-butoxide resulted
in partial demethylation to give azide 4a in only 44%
yield. However, when 2a was converted to the bulkier
isopropyl ester 3a%, subsequent alkylation gave azide 4a%
in 88% yield.
5. Hoekstra, W. J.; Poulter, B. L. Curr. Med. Chem. 1998, 5,
195.
6. Kende, A. S.; Dong, H.-Q.; Mazur, A. W.; Ebetino, F.
H. Tetrahedron Lett. 2001, 42, 6015.
7. Brewer, M.; Rich, D. H. Org. Lett. 2001, 3, 945.
8. Liu, X.; Hu, X.; Tian, X.; Mazur, A. W.; Ebetino, F. H.
J. Organomet. Chem. 2002, 646, 212.
9. Haskell-Luevano, C.; Hentrata, S.; North, C.; Sawyer, T.
K.; Hadley, M. E.; Hruby, V. J.; Dickinson, C.; Gantz, I.
J. Med. Chem. 1997, 40, 2133.
10. (a) Benoit, S. C.; Schwartz, M. W.; Lachey, J. L.; Hagan,
M. M.; Rushing, P. A.; Blake, K. A.; Yagaloff, K. A.;
Kurylko, G.; Franco, L.; Danhoo, W.; Seeley, R. J. J.
Neurosci. 2000, 20, 3442; (b) Bednarek, M. A.; MacNeil,
T.; Kalyani, R. N.; Tang, R.; Van der Ploeg, L. H. T.;
Weinberg, D. H. Biochem. Biophys. Res. Commun. 2000,
272, 23; (c) Prusis, P.; Muceniece, R.; Mutule, I.; Mutulis,
F.; Wikberg, J. E. S. Eur. J. Med. Chem. 2001, 36, 137.
11. (a) Vassilious, S.; Mucha, A.; Cuniasse, P.; Georgiadis,
D.; Lucet-Levannier, K.; Beau, F.; Kannan, R.; Murphy,
G.; Knauper, V.; Rio, M.-C.; Basset, P.; Yiotakis, A.;
Dive, V. J. Med. Chem. 1999, 42, 2610; (b) Georgiadis,
D.; Matziari, M.; Vassiliou, S.; Dive, V.; Yiotakis, A.
Tetrahedron 1999, 55, 14635; (c) Matziari, M.; Geor-
giadis, D.; Dive, V.; Yiotakis, A. Org. Lett. 2001, 3, 659.
12. Ballesteros, P.; Roberts, B. W.; Wong, J. J. Org. Chem.
1983, 48, 3603.
13. (a) Baylis, E. K.; Campbell, C. D.; Dingwall, J. G. J.
Chem. Soc., Perkin Trans. 1 1984, 2845; (b) Optical
rotation for 1b: [h]D=+62.8° (c=0.72, EtOH), see:
McKittrick, B. A.; Stamford, A. W.; Weng, X.; Ma, K.;
Chackalamannil, S. Bioorg. Med. Chem. Lett. 1996, 6,
1629. [h]D=+63° (c=1.0, EtOH).
14. (a) Boyd, E. A.; Corless, M.; James, K.; Regan, A. C.
Tetrahedron Lett. 1990, 31, 2933; (b) Boyd, E. A.; Boyd,
M. E. K.; Loh, V. M., Jr. Tetrahedron Lett. 1996, 37,
1651.
As shown in Scheme 3, treatment of 4a or 4a% with
TFA–CH2Cl2 (1:1, v/v) converted either ester to triacid
5, presumably through intermediate A.16 When 5 was
refluxed in toluene for 4 h, the desired diacid 6 was
obtained, which was characterized through the corre-
sponding dimethyl ester 7 (mixture of diastereomers).17
To illustrate the convenience of this approach to a
tetrapeptide containing the PheP-Arg core, we con-
ducted the same initial sequence starting with the
known optically pure phosphinic acid 1b.13b The
diastereomeric diacids 6b were coupled with optically
pure (S)-2-naphthylalanine-N-methylamide18 to give 8
in 46% overall yield from 4b. Conversion of the azide to
guanidine was most conveniently carried out by a one-
pot sequence of chemoselective Lindlar reduction19 and
in-situ guanidination with N,N%-bis(tert-butoxycar-
bonyl)-1H-pyrazole-1-carboxamidine, as summarized in
Scheme 4.
Debenzylation of 9 at the N-terminus using ammonium
formate and 10% Pd–C, followed by coupling with
AcNH-Tyr(t-Bu)-OH,20 afforded tetrapeptides 10 in
79% yield. Compounds 8, 9, and 10 are all mixtures of
four diastereomers, clearly reflected in their H-decou-
pled 31P NMR spectra, showing four singlet peaks.21
Demethylation of 10 with bromotrimethylsilane,22 fol-
lowed by treatment with trifluoroacetic acid, produced
free tetrapeptides 11 which could be separated by
HPLC to give approximately equal amounts of both
stereochemically pure tetrapeptides 11a and 11b,23 in
which phosphorus is no longer a stereogenic center due
to prototropic equilibration.
15. Lebreton, L.; Jost, E.; Carboni, B.; Annat, J.; Vaultier,
M.; Dutartre, P.; Renaut, P. J. Med. Chem. 1999, 42,
4749.
16. Georgiadis, D.; Dive, V.; Yiotakis, A. J. Org. Chem.
2001, 66, 6604.
17. Compound 7 is a mixture of eight isomers (i.e. four pairs
of enantiomers): 1H NMR (CDCl3, 400 MHz): l 1.51–
1.72 (m, 5H), 2.27 (m, 1H), 2.84 (m, 2H), 3.24–3.30 (m,
3H), 3.65–3.76 (m, 6H, 2×Me), 4.36 (m, 1H), 4.92–5.14
(m, 3H, NH+Cbz), 7.21–7.35 (m, 10H) ppm; 31P NMR
(CDCl3, 162 MHz): l 52.9, 53.0, 53.1, 54.2 ppm; IR
(CH2Cl2): 3227 (NH), 3031, 2952, 2098 (N3), 1716 (CꢀO),
1260, 1209, 1040 cm−1; MS (APCI, Pos.): 503 (M+H).
18. Hirayama, R.; Yamamoto, M.; Tsukida, T.; Matsuo, K.;
Obata, Y.; Sakamoto, F.; Ikeda, S. Bioorg. Med. Chem.
1997, 5, 765.
The scope of this strategy toward diverse PheP-Arg
polypeptides is under investigation. Unfortunately, the
series exemplified above did not exhibit biological activ-
ity similar to cyclic peptide mimics of the His-Phe-Arg-
Trp pharmacophore, suggesting this isosteric approach
did not provide conformations necessary for the
melanocortin receptors.
19. Corey, E. J.; Nicolaou, K. C.; Balanson, R. D.; Machida,
References
Y. Synthesis 1975, 590.
20. AcNH-Tyr(t-Bu)-OH was commercially available from
BACHEM. Plummer, M. S.; Lunney, E. A.; Para, K. S.;
Shahripour, A.; Stankovic, C. J.; Humblet, C.; Fergus, J.
H.; Marks, J. S.; Herrera, R.; Hubbell, S.; Saltiel, A.;
Sawyer, T. K. Bioorg. Med. Chem. 1997, 5, 41.
21. Analytical and spectral data for selected compounds:
Compound 3b: a white solid, mp: 85–86°C, 1H NMR
(CDCl3, 400 MHz): l 1.30 (d, J=6.1 Hz, 3H), 1.38 (d,
1. Gante, J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1699.
2. Obrecht, D.; Altorfer, M.; Robinson, J. A. Adv. Med.
Chem. 1999, 4, 1.
3. Etzkorn, F. A.; Travins, J. M.; Hart, S. A. Advances in
Amino Acid Mimetics and Peptdomimetics 1999, 2, 125.
4. Hauptmann, J.; Sturzebecher, J. Thromb. Res. 1999, 93,
203.

4976
A. S. Kende et al. / Tetrahedron Letters 43 (2002) 4973–4976
J=6.1 Hz, 3H), 1.48 (s, 18H), 2.37 (m, 2H), 2.85 (m, 1H),
2.35–2.55 (m, 3H), 2.55–2.80 (m, 5H), 2.80–3.30 (m, 3H),
3.49–3.78 (m, 5H), 4.42–4.75 (m, 3H), 6.85–7.10 (m, 4H),
7.15–7.35 (m, 5H), 7.35–7.50 (m, 3H), 7.70–7.85 (m, 4H)
ppm; 31P NMR (CD3OD, 162 MHz): l 53.9, 55.0, 56.3,
56.8 ppm; IR (CH2Cl2): 3280 (NH), 2977, 1721 (CꢀO),
1651 (CꢀO), 1644 (CꢀO), 1161, 1134, 1048 cm−1; MS
(API-ES, Pos.): 1042 (M+H), 1064 (M+Na); HRMS (M+
H): calcd 1042.5419, found: 1042.5400. Anal. calcd for
C55H76N7O11P: C, 63.38; H, 7.35; N, 9.41; found: C,
63.29; H, 7.25; N, 9.15%.
3.26 (m, 1H), 3.61 (dt, J=11.6 Hz, 6.9 Hz, 1H), 4.37 (m,
1H), 4.78 (m, 1H), 4.94, 5.02 (AB, JAB=12.4 Hz, 2H),
5.31 (br d, J=10.3 Hz, NH), 7.18–7.32 (m, 10H) ppm;
31P NMR (CDCl3, 162 MHz): l 50.2 ppm; IR (CH2Cl2):
3222 (NH), 2979, 1728 (CꢀO), 1258, 1137 cm−1; MS
(APCI, Pos.): 590 (M+H). Anal. calcd for C31H44NO8P:
C, 63.14; H, 7.52; N, 2.38; found: C, 63.12; H, 7.70; N,
2.38%. Compound 4b: a white gum, 1H NMR (CDCl3,
400 MHz): l 1.28 (d, J=6.1 Hz, 3H), 1.39 (d, J=6.1 Hz,
3H), 1.47 (s, 9H), 1.48 (s, 9H), 1.45–1.53 (m, 2H), 2.17
(m, 2H), 2.38 (m, 2H), 2.79 (m, 1H), 3.23–3.34 (m, 3H),
4.31 (m, 1H), 4.79 (m, 1H), 4.93–5.00 (m, 3H, NH+Cbz),
7.17–7.37 (m, 10H) ppm; 31P NMR (CDCl3, 162 MHz): l
49.8 ppm; IR (CH2Cl2): 3225 (NH), 2978, 2934, 2097
(N3), 1728 (CꢀO), 1258, 1141 cm−1. Anal. calcd for
C34H49N4O8P: C, 60.70; H, 7.34; N, 8.33; Found: C,
22. McKenna, C. E.; Higa, M. T.; Cheung, N. H.; McKenna,
M.-C. Tetrahedron Lett. 1977, 2, 155.
23. Preparative HPLC separation was performed on a Polaris
C18-A 10 m column (50 mm×25 cm), operated at room
temperature and eluted at 60 mL/min flow rate, using a
linear gradient of water containing 0.1% TFA (from 95 to
0%) and acetonitrile (from 5 to 100%) over 60 min, with
UV detection at 214 nm. Compound 11a (with retention
1
60.44; H, 7.52; N, 8.09%. Compound 8: a white solid, H
1
time=8.61 min): H NMR (CD3OD, 300 MHz): l 1.30,
NMR (CD3OD, 400 MHz): l 1.10–1.40 (m, 3H), 1.70–
1.95 (m, 1H), 2.18–2.24 (m, 2H), 2.70, 2.71, 2.77, 2.78
(4×s, 3H, N-Me), 2.40–2.80 (m, 2H), 2.88–3.00 (m, 2H),
3.10–3.50 (m, 3H), 3.51, 3.64, 3.70, 3.79 (4×d, J=10.4
Hz, 3H, O-Me), 4.20 (m, 1H), 4.63–4.90 (m, 1H), 4.94–
5.03 (m, 2H, Cbz), 7.18–7.31 (m, 10H), 7.40–7.50 (m,
3H), 7.70–7.85 (m, 4H) ppm; 31P NMR (CD3OD, 162
MHz): l 55.1, 55.7, 56.7, 56.8 ppm; MS (API-ES, Pos.):
699 (M+H), 721 (M+Na); HRMS (M+H): calcd
699.3060, found: 699.3081. Compound 9: a white solid,
1H NMR (CD3OD, 400 MHz): l 1.08–1.31 (m, 3H), 1.48,
1.49, 1.50, 1.53, 1.54, 1.55, 1.57 (s, 18H, 2×Boc), 1.75 (m,
1H), 2.25 (m, 1H), 2.50 (m, 1H), 2.68, 2.69, 2.76, 2.78
(4×s, 3H, N-Me), 2.65–3.50 (m, 7H), 3.53, 3.66, 3.70, 3.77
(4×d, J=10.4 Hz, 3H, OMe), 4.21 (m, 1H), 4.60 (m, 1H),
4.92–5.00 (m, 2H, Cbz), 7.17–7.29 (m, 10H), 7.40–7.45
(m, 3H), 7.74–7.82 (m, 4H) ppm; 31P NMR (CD3OD, 162
MHz): l 55.2, 55.9, 56.80, 56.84 ppm; IR (CH2Cl2): 3284
(NH), 2977, 1721 (CꢀO), 1644 (CꢀO), 1538, 1134, 1048
cm−1; MS (API-ES, Pos.): 915 (M+H), 937 (M+Na);
HRMS (M+H): calcd 915.4422, found: 915.4429. Anal.
calcd for C48H63N6O10P: C, 63.01; H, 6.94; N, 9.18;
found: C, 62.96; H, 7.01; N, 8.92%. Compound 10: a
1.51 (m, 2H), 1.82–1.88 (m, 5H), 2.25 (m, 1H), 2.37–2.55
(m, 3H), 2.55–2.76 (m, 5H), 2.80–3.30 (m, 3H), 3.49–3.78
(m, 5H), 4.20–4.40 (m, 3H), 6.24 (m, 4H), 6.80–6.65 (m,
5H), 6.91–7.00 (m, 3H), 7.20–7.31 (m, 4H) ppm; 13C
NMR (CD3OD, 75 MHz): l 174.6, 171.5, 171.4, 170.4,
155.7, 154.5, 136.6, 136.5, 134.2, 132.2, 131.1, 128.4,
127.6, 126.8, 126.5, 126.4, 126.0, 125.9, 125.6, 125.0,
124.6, 124.1, 113.5, 53.8, 52.1 49.7, 48.4, 39.1, 38.5, 36.0,
34.9, 31.6, 29.8, 29.6, 28.4, 24.1, 23.9, 19.8; 31P NMR
(CD3OD, 121 MHz): l 47.4; MS (ESI): m/z 772.6 (M+1).
Compound 11b (with retention time=8.96 min): 1H
NMR (CD3OD, 300 MHz): 1H NMR (CD3OD, 300
MHz): l 1.32, 1.52 (m, 2H), 1.83–1.87 (m, 5H), 2.26 (m,
1H), 2.37–2.55 (m, 3H), 2.55–2.78 (m, 5H), 2.80–3.31 (m,
3H), 3.49–3.78 (m, 5H), 4.20–4.40 (m, 3H), 6.24 (m, 4H),
6.80–6.45 (m, 5H), 6.85–7.00 (m, 3H), 7.22–7.34 (m, 4H)
ppm; 13C NMR (CD3OD, 75 MHz): l 174.4, 171.6,
170.8, 170.4, 155.2, 154.5, 136.2, 136.1, 134.2, 132.1,
131.0, 128.5, 127.8, 126.8, 126.4, 126.0, 125.9, 125.6,
125.0, 124.6, 124.1, 113.5, 53.8, 52.1 49.7, 48.4, 39.1, 38.5,
36.4, 34.9, 31.6, 29.3, 29.6, 28.4, 24.2, 23.8, 19.8; 31P
NMR (CD3OD, 121 MHz): l 49.0; MS (ESI): m/z 772.6
(M+1). Because we have been unable to obtain an X-ray
crystal of either 11a or 11b, their stereochemical assign-
ments are undetermined.
1
white solid, H NMR (CD3OD, 400 MHz): l 1.30, 1.31
(s, 9H, t-Bu), 1.48, 1.49, 1.50, 1.55, 1.56 (s, 18H, 2×Boc),
1.14–1.50 (m, 2H), 1.82–1.88 (m, 5H), 2.25 (m, 1H),