Readily available amino acid building blocks for the synthesis of phosphole-containing peptides

Article abstract of DOI:10.1016/j.tetlet.2007.02.102

Nucleophilic substitution of a phospholide anion onto protected 3-iodoalanine leads to the formation of an amino acid with an appended phosphole in excellent yield. Manipulation of the protecting groups, leads to building blocks suitable for the synthesis of phosphole-containing polypeptides.

Full text of DOI:10.1016/j.tetlet.2007.02.102

Tetrahedron Letters 48 (2007) 2857–2859
Readily available amino acid building blocks for the synthesis
of phosphole-containing peptides
Steven van Zutphen,* Vicente J. Margarit, Guilhem Mora and Pascal Le Floch
Laboratoire ‘He´te´roe´le´ments et Coordination’, Ecole Polytechnique, CNRS, 91128 Palaiseau Cedex, France
Received 13 December 2006; revised 12 February 2007; accepted 22 February 2007
Available online 25 February 2007
Abstract—Nucleophilic substitution of a phospholide anion onto protected 3-iodoalanine leads to the formation of an amino acid
with an appended phosphole in excellent yield. Manipulation of the protecting groups, leads to building blocks suitable for the syn-
thesis of phosphole-containing polypeptides.
Ó 2007 Elsevier Ltd. All rights reserved.
Phospholes, unsaturated phosphorus-containing five-
membered heterocycles, exhibit a wide range of chemis-
try, making them attractive building blocks for a wide
variety of purposes including ligands in catalysis, pre-
cursors for other phosphorus heterocycles and new
materials.1–3 In particular, complexes of phospholes
with late transition metals, such as palladium and rho-
dium, have been used successfully in various important
catalytic transformations.4–6 In order to develop novel
biocompatible transition metal catalysts, we set out to
synthesise amino acids with a phosphole in the side
chain. When suitably protected, these may serve as
building blocks to be integrated into polypeptides, thus
forming the basis for biocompatible transition metal
catalysts quite different to native enzymes.
to be promising glutamate receptor agonists, racemic
mixtures are of limited interest as ligands for asymmetric
catalysis. The easiest way to obtain enantiopure nonnat-
ural amino acids is by using a natural amino acid as the
starting material. Such an approach was adopted by
Gilbertson and co-workers in a general synthesis of
phosphine-containing amino acids derived from ser-
ine.12 By coupling aryl and alkyl phosphine chlorides
to an amino acid zinc/copper reagent, the desired phos-
phine-containing amino acids were obtained in moder-
ate to good yields. Unfortunately, when applying this
methodology to phospholes, the zinc/copper reagent
reduces the electrophilic halogenophosphole to the cor-
responding bis-phosphole and no phosphole amino acid
is formed. Furthermore, this approach is limited by the
fact that only
a few 1-halogenophospholes are
To the best of our knowledge, no phosphole-containing
amino acids have been previously described, although
phospholes with appended carboxylic acids,7 esters8
and amines9,10 have been reported. Typically, the rela-
tive low stability of poorly substituted phospholes limits
the scope of available reagents and transformations that
can be carried out successfully, thus complicating the
synthesis of a molecule containing a phosphole linked
to other functional groups. On the other hand, several
phosphine amino acids have been reported. An amino
acid with an appended phospholane was synthesised as
a racemic mixture.11 Although these compounds appear
available.13,14
Based on these initial results, we decided to investigate
the substitution of nucleophilic phospholide species on
an electrophilic amino acid derivative. The synthesis is
depicted in Scheme 1. Starting with N-Boc protected
serine 1, the acid was protected selectively using MeI.
Subsequently, using Ph3P and I2 in the presence of imid-
azole, the fully protected 3-iodioalanine methyl ester 2
was formed.15 This compound was found to be relatively
unreactive towards lithium 2,5-diphenylphospholide,13
even in the presence of Pd(dba)2 as catalyst. This failure
probably results from the extensive delocalisation of the
negative charge within the ring and the concomitant de-
crease of the nucleophilicity of the phosphorus atom.
More satisfying results were obtained with the 3,4-di-
methyl derivative which displays much higher nucleo-
philicity. Reaction of 1 equiv of lithium 3,4-
Keywords: Phosphole; Amino acid; Fmoc protected; Phosphorus;
Catalysis; Peptide synthesis.
*
Corresponding author. Tel.: +33 169 33 47 93; fax: +33 169 33 39
90; e-mail: steven.zutphen@polytechnique.edu
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.02.102

2858
S. van Zutphen et al. / Tetrahedron Letters 48 (2007) 2857–2859
cleanly resulted in phosphole amino acid 5.22 To take
S
full advantage of the extensively developed Fmoc-based
peptide chemistry,23 5 was subsequently treated with
FmocOSu to give the desired Fmoc protected phosphole
amino acid 6.24 We are currently investigating the appli-
cation of this building block in the formation of phosp-
hole-containing polypeptides.
OH
OH
I
P
iii
i, ii
O
O
BocHN
BocHN
BocHN
92%
76%
O
O
O
1
2
3
iv
97%
Acknowledgements
S
S
P
S
P
P
v
vi
The authors would like to thank the European Commu-
nity (Adventure-STREP Project No. 15471) as well as
the Ecole Polytechnique for financial support of this
work.
OH
OH
.TFA
OH
FmocHN
H2N
100%
81%
BocHN
O
O
O
6
5
4
Scheme 1. Synthesis of phosphole amino acid 6. Reagents and
conditions: (i) MeI, KHCO3 in DMF; (ii) PPh3, I2, imidazole in
CH2Cl2 for 3.5 h at 0 °C; (iii) Li-3,4-dimethylphosphole, then S8 in
THF; (iv) 1 M NaOH, overnight; (v) 50% TFA in DCM for 3 h; (vi)
FmocOSu, KHCO3 in H2O/dioxane.
References and notes
1. Carmichael, D.; Mathey, F. New Trends in Phosphame-
tallocene Chemistry, New Aspects in Phosphorus Chemistry
I; Springer: Berlin, 2002.
2. Mathey, F.; Mercier, F.; Robin, F.; Ricard, L. J.
Organomet. Chem. 1998, 557, 117–120.
dimethylphospholide salt16 with 2 at 0 °C in THF gave a
new compound which exhibited a signal at À13 ppm in
31P NMR spectroscopy.
3. Baumgartner, T.; Reau, R. Chem. Rev. 2006, 106, 4681–
4727.
4. Mora, G.; van Zutphen, S.; Thoumazet, C.; Le Goff, X.
F.; Ricard, L.; Grutzmacher, H.; Le Floch, P. Organo-
metallics 2006, 25, 5528–5532.
5. Piechaczyk, O.; Thoumazet, C.; Jean, Y.; le Floch, P. J.
Am. Chem. Soc. 2006, 128, 14306–14317.
6. Le Floch, P. Phosphorus–Carbon Heterocyclic Chemistry:
The Rise of a New Domain; Pergamon: Amsterdam, 2001.
7. Melaimi, M.; Ricard, L.; Mathey, F.; Le Floch, P. Org.
Lett. 2002, 4, 1245–1247.
8. Ferao, A. E.; Deschamps, B.; Mathey, F. Bull. Soc. Chim.
Fr. 1993, 130, 695–699.
9. Huy, N. H. T.; Mathey, F. Phosphorus Sulfur Silicon
Relat. Elem. 1990, 47, 477–481.
10. Tan, W.; Foucaud, A. Tetrahedron Lett. 1988, 29, 4581–
4582.
11. Conway, S. J.; Miller, J. C.; Bond, A. D.; Clark, B. P.;
Jane, D. E. J. Chem. Soc., Perkin Trans. 1 2002, 1625–
1627.
12. Greenfield, S. J.; Gilbertson, S. R. Synthesis 2001, 2337–
2340.
This compound could be isolated, but was highly prone
to oxidation, as expected for a 1-alkyl-3,4-dimethyl-
phosphole derivative. We therefore chose to protect
the phosphorus lone pair with sulfur. Addition of
1.1 equiv of S8 smoothly protected the phosphole within
1 h at room temperature. A colour change from yellow
to deep red was observed and the 31P NMR confirmed
the total conversion to the S-protected phosphole prod-
uct 3 with a chemical shift at 46.7 ppm.17 The sulfur not
only protected the compound against oxidation, but
also greatly enhanced its acid and base stability, two
properties which are essential for further use of this
compound as a building block in peptide synthesis.
The sulfur could be removed cleanly by reaction with
P(CH2CH2CN)3 at 70 °C in 4 h,18 but this must be done
only after the amino acid analogue is integrated into the
desired polypeptide. Although 3 is relatively stable, it
could not be purified by flash chromatography without
considerable product degradation, probably due to rear-
rangement reactions that occur on the silica gel.19,20 The
phosphole amino acid was therefore purified through
precipitation from a toluene solution with petroleum
ether, and obtained in 92% yield. The purity was con-
firmed by NMR analysis. When performing the substi-
tution reaction at À78 °C instead of 0 °C, the yield
was reduced to approximately 50%. Apparently, at low
temperature, the phospholide anion also reacts as a base
to deprotonate the a-proton of 2. The N-Boc amido-
acrylic acid formed in this side reaction, can be recov-
ered from the petroleum ether fraction.
13. Charrier, C.; Bonnard, H.; Mathey, F.; Neibecker, D. J.
Organomet. Chem. 1982, 231, 361–367.
14. Visseaux, M.; Nief, F.; Ricard, L. J. Organomet. Chem.
2002, 647, 139–144.
15. Trost, B. M.; Rudd, M. T. Org. Lett. 2003, 5, 4599–4602;
To a solution of triphenylphosphine (3.28 g, 12.5 mmol)
and imidazole (850 mg, 12.5 mmol) in CH2Cl2 (30 mL)
was added I2 (3.17 g, 12.5 mmol) at 0 °C. On complete
dissolution of the I2 the temperature was raised to room
temperature and after 10 min, re-cooled to 0 °C. N-(tert-
Butoxycarbonyl)-L-serine methyl ester (2.19 g, 10 mmol)
was added and the mixture was stirred for 3.5 h at 0 °C.
The reaction was quenched with aqueous sodium thiosul-
fate, extracted using EtOAc, washed with brine, dried
(MgSO4) and evaporated to dryness. The crude product
was purified on silica gel (10–30% EtOAc in petroleum
ether) to yield 2 in 76% yield. 1H NMR (300 MHz,
CDCl3): d = 1.27 (s, 9H, CCH3), 3.40 (m, 2H, CH2I), 3.62
(s, 3H, OCH3), 4.36 (m, 1H, aCH), 5.49 (d, J = 8 Hz, 1H,
NH). 13C NMR (75.5 MHz, CDCl3): d = 169.8 (COOMe),
154.7 (COO-t-Bu), 80.0 (t-Bu), 53.8 (Me), 52.9 (aC), 29 (t-
Bu), 7.79 (bC).
The phosphole sulfide moiety proved to be remarkably
stable during the manipulation of the amino acid protec-
tion groups. The desired Fmoc protected amino acid
was thus obtained in near quantitative yield with respect
to phosphole 3. Overnight treatment of phosphole 3
with aqueous NaOH yielded the Boc protected phosp-
hole amino acid 4.21 Further deprotection using TFA

S. van Zutphen et al. / Tetrahedron Letters 48 (2007) 2857–2859
2859
16. Breque, A.; Mathey, F.; Savignac, P. Synthesis 1981, 983–
985.
bC), 28.3 (t-Bu), 17.4 (d, J = 17.8 Hz) and 17.3 (d,
J = 17.7 Hz, Me-phosphole). 31P NMR (121.5 MHz,
CDCl3): 46.7 ppm. HRMS EI+: 331.0867 (calcd
331.1007, C14H22NO4PS). Mp: >250 °C.
17. To a stirred solution of 3,4-dimethyl-1-phenylphosphole
(1.43 mL, 7.60 mmol) in dry THF (15 mL) was added Li
wire (4 cm, ꢀ26 mmol). When the reaction was complete
as indicated by 31P NMR, the excess Li was removed and
AlCl3 (169 mg, 1.27 mmol) was added. After 30 min, 2
(2.50 g, 7.60 mmol) in THF (15 mL) was added at 0 °C.
When the addition was complete, the cold bath was
removed. After 10 min, S8 (269 mg, 8.40 mmol) was added
and the solution was stirred for another 30 min. The
reaction was quenched with sodium thiosulfate, extracted
with EtOAc, washed with brine, dried (MgSO4) and
evaporated to dryness. The crude compound was then
redissolved in toluene and precipitated with petroleum
ether to yield 3 in 92% yield. 1H NMR (300 MHz CDCl3):
d = 1.45 (s, 9H, CCH3), 2.05 (s, 6H, 2CH3), 2.65 (m, 2H,
CH2P), 3.78 (s, 3H, OCH3), 4.60 (m, 1H, aCH), 5.61 (d,
J = 7.8 Hz, 1H, NH), 6.00 (d, J = 30 Hz, 2H, PCH). 13C
NMR (75.5 MHz, CDCl3): d = 171.4 (COOMe), 154.9
(COO-t-Bu), 153.5 (d, J = 18.0 Hz, C-3 phosphole), 123.6
(d, J = 80.3 Hz, C-2 phosphole), 80.4 (t-Bu), 52.8 (Me),
49.9 (aC), 32.8 (d, J = 53.2 Hz, bC), 28.3 (t-Bu), 17.5 (d,
J = 17.7 Hz) and 17.42 (d, J = 17.6 Hz, Me-phosphole).
31P NMR (121.5 MHz, CDCl3): 46.9 ppm. HRMS EI+:
345.0358 (calcd 345.1164, C15H24NO4PS). Mp: >250 °C.
18. Deschamps, E.; Mathey, F. Bull. Soc. Chim. Fr. 1992, 129,
486–489.
22. To a solution of 4 (1.95 g, 5.90 mmol) in CH2Cl2 (10 mL)
was added TFA (9 mL) and water (1 mL). The mixture
was stirred for 3 h, diluted with toluene and dried under
reduced pressure to yield 5 as a yellow solid in quantitative
yield. 1H NMR (300 MHz, MeOD): d = 2.13 (s, 6H,
2CH3), 2.54 (m, 2H, CH2P), 4.50 (m, 1H, aCH), 6.16 (m,
2H, PCH). 13C NMR (75.5 MHz, MeOD): d = 170.6
(CO), 156.3 (d, J = 19.0 Hz) and 155.8 (d, J = 19.0 Hz, C-
3 phosphole), 124.9 (d, J = 22.6 Hz) and 123.8 (d, J =
23.0 Hz, C-2 phosphole), 50.2 (aC), 31.9 (d, J = 50.0 Hz,
bC), 17.4 (d, J = 18.2 Hz) and 17.3 (d, J = 18.2 Hz,
Me-phosphole). 31P NMR (121.5 MHz, MeOD): 47.3
ppm. 19F NMR (282.4 MHz, MeOD): 78.7 ppm. Mp:
120–121 °C.
23. Chan, W. C.; White, P. D. Fmoc Solid Phase Peptide
Synthesis; Oxford University Press: Oxford, 2000.
24. To a solution of FmocOSu (1.99 g, 5.90 mmol) in dioxane
(25 mL) was added 5 (1.94, 5.90 mmol) and KHCO3
(1.48 g, 14.75 mmol) in water (25 mL) at 0 °C. After
30 min, the cold bath was removed and the solution was
stirred for another 4 h. The reaction mixture was diluted
with EtOAc and extracted with NaHCO3. The organic
layer was washed with brine, dried (MgSO4) and evapo-
rated to dryness. The crude product was redissolved in hot
toluene and precipitated with petroleum ether to give 6 as
a light yellow crystalline solid in 81% yield. 1H NMR
(300 MHz, CDCl3): d = 1.93 (s, 6H, 2CH3), 2.65 (m, 2H,
CH2P), 4.06 (m, 1H, CHCH2), 4.35 (m, 2H, CH2O), 4.73
(m, 1H, aCH), 5.97 (d, J = 31.6 Hz) and 5.94 (d,
J = 31.6 Hz, 2H, PCH), 6.22 (d, J = 6.7 Hz, 1H, NH),
7.31–7.77 (m, 8H, aromatic Fmoc), 8.43 (br s, 1H, OH).
13C NMR (75.5 MHz, CDCl3): d = 174.6 (COOH), 156.0
(NCO), 154.1 (d, J = 18.0 Hz) and 154.0 (d, J = 17.8 Hz,
C-3 phosphole), 143.7, 143.5, 141.2, 127.8, 127.1, 124.8,
120.1 (aromatic Fmoc), 123.1 (d, J = 80.3 Hz, C-2 phosp-
hole), 67.5 (CH2O), 53.6 (aC), 46.9 (CHCH2), 33.0 (d,
J = 51.6 Hz, bC), 17.3 (d, J = 17.4 Hz, Me-phosphole).
31P NMR (121.5 MHz, CDCl3): 46.6 ppm. HRMS EI+:
453.0475 (calcd 353.1164, C24H24NO4PS). Mp: 134–
135 °C.
19. Kashman, Y.; Wagenstein, I.; Rudi, A. Tetrahedron 1976,
32, 2427–2431.
20. Mathey, F.; Muller, G. Tetrahedron 1972, 28, 5645.
21. To a solution of 3 (2.1 g, 6.08 mmol) in THF (12 mL) was
added 1 M NaOH (12 mL) and the solution was stirred
overnight. The reaction was acidified with dilute HCl,
extracted with CH2Cl2, washed with brine, dried (MgSO4)
and evaporated to dryness to yield 4 as a yellow crystalline
1
solid in 97% yield. H NMR (300 MHz, CDCl3): d = 1.43
(s, 9H, CCH3), 2.03 (s, 6H, 2CH3), 2.63 (m, 2H, CH2P),
4.60 (m, 1H, aCH), 5.70 (d, J = 7.4 Hz, 1H, NH), 6.02 (d,
J = 31.3 Hz, 2H, PCH). 13C NMR (75.5 MHz, CDCl3):
d = 174.9 (COOH), 155.2 (COO-t-Bu), 153.8 (d,
J = 18.4 Hz) and 153.6 (d, J = 18.5 Hz, C-3 phosphole),
124.0 (d, J = 80.4 Hz) and 123.4 (d, J = 80.8 Hz, C-2
phosphole), 80.6 (t-Bu), 49.8 (aC), 33.0 (d, J = 51.6 Hz,