Peptide bond formation using polymer-bound BOP

Article abstract of DOI:10.1002/ejoc.200300798

A new polymer-supported BOP (P-BOP) has been prepared starting from the commercially available polystyrene-bound 1-hydroxybenzotriazole (P-HOBt) and successfully used as a solid-supported reagent for peptide-coupling reactions. Compared to BOP, less epime

Full text of DOI:10.1002/ejoc.200300798

FULL PAPER
Peptide Bond Formation Using Polymer-Bound BOP
[a]
[a]
[b]
[a]
´
Sorin V. Filip,* Valerie Lejeune, Jean-Pierre Vors, Jean Martinez, and
Florine Cavelier*[a]
Keywords: Peptide synthesis / Polymer-supported reagents / BOP
A new polymer-supported BOP (P-BOP) has been prepared
starting from the commercially available polystyrene-bound
1-hydroxybenzotriazole (P-HOBt) and successfully used as a
is also a suitable activating reagent for difficult peptide coup-
ling reactions involving α,α-dialkyl amino acids.
solid-supported reagent for peptide-coupling reactions. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Compared to BOP, less epimerization was observed. P-BOP
Germany, 2004)
Introduction
(e.g. Aib).[8] In connection with our research directed
towards the development of new methodologies for auto-
mated synthesis of peptides, we wish to report the synthesis
of a new polymer-supported peptide coupling reagent P-
BOP (2)[9] and its use in model peptide couplings.
In recent years, the benefits of performing organic syn-
thesis using polymer-supported reagents have been fully
recognized. One major advantage lies in the fact that
workup procedures are simplified. Moreover, combination
of benefits of polymer-supported reactions with those of
solution-phase chemistry (e.g., easy monitoring of the reac-
tion progress by simply applying LC-MS or TLC tech-
niques) made polymer-supported reagents a widespread ap-
Results and Discussion
P-BOP (2) was prepared by the reaction of commercially
preciated alternative. Peptide synthesis also takes the advan- available P-HOBt (1) (1 mmol/g resin loading) with 5 equiv.
tage of the emerging supported reagents technology, es- of bromotris(dimethylamino)phosphonium hexafluoro-
pecially in those aspects regarding the formation of amide phosphate (Brop), at room temperature for 18 h, in the
bonds. Carbodiimide (P-EDC),[1] 1-hydroxybenzotriazole presence of triethylamine (TEA) as base and in acetonitrile
(P-HOBt)[2,3] and 4-(dimethylamino)pyridine (P-DMAP)[4] as solvent (Scheme 1).
supported on normal cross-linked polystyrene beads or on
macroporous polystyrene were used as activating agents, P-
HOBt and P-DMAP requiring the presence of an activating
reagent like PyBrop (bromotrispyrrolidinophosphonium
hexafluorophosphate) or DCC (dicyclohexylcarbodiimide).
Recently, the synthesis of polymeric O-(benzotriazol-1-yl)-
N,N,NЈ,NЈ-tetramethyluronium
tetrafluoroborate
(P-
TBTU)[5] was reported. P-TBTU allows direct formation
of the amide bond and recovery of immobilized HOBt by
filtration once the reaction is complete.
(Benzotriazol-1-yloxy)tris(dimethylamino)phosphonium
hexafluorophosphate (BOP) has been described as an acti-
vation reagent ideally suited for solid phase peptide syn-
thesis and the rate of coupling compares favorably to other
methods of activation.[6,7] BOP has also proved to be very
efficient for the difficult coupling of α,α-dialkyl amino acids
Scheme 1. Synthesis of P-BOP (2)
The reaction progress between P-HOBt (1) and Brop was
monitored by IR spectroscopy and was considered to be
complete after the disappearance of the broad OϪH
stretching band of P-HOBt at ν˜ ϭ 3314 cmϪ1. The resulting
polymer 2 was filtered, washed successively with aceto-
nitrile, methanol, dichloromethane and diethyl ether, and
[a]
University of Montpellier II, LAPP, UMR-CNRS 5810, CC19,
34095 Montpellier, France
[b]
Bayer CropScience, La Dargoire Research Centre,
14Ϫ20 rue Pierre Baizet, 69009 Lyon, France
Fax: (internat.) ϩ 33-4-67144866
E-mail: florine@univ-montp2.fr
1936
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejoc.200300798
Eur. J. Org. Chem. 2004, 1936Ϫ1939

Peptide Bond Formation Using Polymer-Bound BOP
FULL PAPER
dried at 50 °C in vacuo (6 mbar). IR spectra of the resulting
resin displayed three new strong bands at ν˜ ϭ 843 (assigned
to PϪF) and 1064 and 1320 cmϪ1 (both corresponding to
PϪNMe2) (Figure 1).
Figure 2. The HPLC chromatogram of the crude reaction mixture
resulting after the coupling between 3 and 4, in dichloromethane
and using pyridine as base, at λ ϭ 214 (A), 254 (B) and 263 nm
(C), respectively
area of Fmoc-βAla-NHiPr (9.12 min) at 263 nm; A2 ϭ peak
area of unchanged Fmoc-βAla-OH (8.75 min) at 263 nm.
Reaction of Fmoc-βAla-OH (3) with isopropylamine (4)
showed to be solvent-dependent and slightly influenced by
the nature of the base, i.e. pyridine or TEA (Table 1). HPLC
analysis of the crude reaction mixture showed that the
Fmoc protecting group was stable under the reaction con-
ditions, since no side-product absorbing at λ ϭ 254 nm was
detected by HPLC (Figure 2, B). The effective resin loading
was found to be around 0.6Ϫ0.7 mmol/g, depending on the
nature (and the dryness) of the solvent, which suggested
that not all polymer-bound BOP (2) was available for coup-
ling or that a part of it was destroyed by the water of the
solvent (Table 1).
Figure 1. The IR spectra of P-BOP (2)
1
Gel-phase H NMR of the resin in CD2Cl2 as solvent,
showed the appearance of a broad single peak at δ ϭ
2.90 ppm, which was attributed to the NϪMe groups. El-
emental analysis of both P-HOBt (1) and P-BOP (2)
showed a molecular ratio N/S of 3.98:1 for resin 1, while
for 2 the molecular ratio N/S was 6.78:1. These values are
in agreement with the structural formula of both polymer-
supported reagents and clearly show an increase in nitrogen
content for P-BOP (2) compared to the starting P-HOBt
(1). Based on these values, the loading of the resin can be
calculated at 0.91 mmol/g. However, the effective resin load-
ing was determined based on the synthesis of Fmoc-βAla-
NHiPr (5) since part of the functionality might not be ac-
cessible. (Scheme 2).
Table 1. Variation of the effective resin loading of P-BOP (2) ac-
cording to the nature of the solvent and base
Entry
Solvent
Base [0.033 ]
Loading [mmol/g]
1
2
3
4
MeCN
MeCN
DCM
DCM
pyridine
TEA
pyridine
TEA
0.61
0.60
0.76
0.72
Scheme 2. Reaction used for determination of the effective resin
loading of P-BOP (2)
Starting from N-protected amino acids and amino acid
For the coupling reaction between 3 and 4, a theoretical ester hydrochlorides, a series of dipeptides was prepared
value of 1 mmol/g resin loading was used for P-BOP (2), by using P-BOP (2) as solid-supported coupling reagent
virtually considering the total conversion of the P-HOBt (Table 2) in the presence of an organic base. Acetonitrile
(1). In a standard procedure, to a suspension of 1 equiv. of and dichloromethane were tested as solvents and a better
P-BOP (2) (50 mg) in 3 mL of the corresponding solvent, 2 resin swelling was observed for DCM. Pyridine and TEA
equiv. of Fmoc-βAla-OH (3) was added, followed by 2 were tested as bases (Table 2, Entries 1Ϫ4), the latter af-
equiv. of organic base, namely pyridine or TEA. The mix- fording higher yields. In a standard procedure, as detailed
ture was shaken at room temperature for 15 min, then iso- in the Exp. Sect., to a suspension of P-BOP in the corre-
propylamine (4) was added and the shaking continued for sponding solvent, the N-protected amino acid was added,
16 h. After filtration, the resin was subsequently washed followed by the amino ester hydrochloride and the organic
with acetonitrile and dichloromethane (DCM), and the sol- base, namely pyridine, TEA or DIPEA.
vents were removed in vacuo. The resulting crude reaction
The yields of isolated peptides were good and the levels
mixture was dissolved in acetonitrile and methanol, and of epimerization in Young’s and Anteunis’s tests (Table 2,
submitted to the HPLC analysis. The chromatogram was Entries 5 and 6) were similar to those reported in the litera-
recorded at 263 nm (Figure 2, C) and the effective resin ture for other polymer-supported peptide coupling re-
loading was calculated using the following formula: Load- agents.[5] A more hindered base such as N,NЈ-diisopropyle-
ing [mmol/g] ϭ (A1 ϫ n)/[(A1 ϩ A2) ϫ q]; n ϭ mmol of thylamine (DIPEA) was used in place of pyridine or TEA
Fmoc-βAla-OH; q ϭ mg of P-BOP (2) resin; A1 ϭ peak to minimize racemization. Chiral HPLC analysis of the di-
Eur. J. Org. Chem. 2004, 1936Ϫ1939
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1937

S. V. Filip, V. Lejeune, J.-P. Vors, J. Martinez, F. Cavelier
FULL PAPER
Table 2. Peptides prepared using P-BOP (2) as coupling reagent
Entry
Peptide[a]
Solvent
Base
Yield [%][b]
1
2
3
4
5
6
7
8
BocGly-PheOEt (6)
BocGly-PheOEt (6)
BocGly-PheOEt (6)
BocGly-PheOEt (6)
BzLeu-GlyOEt (7)[c]
ZGlyPhe-ValOMe (8)[d]
BocAib-PheOEt (9)
BocAib-ValOMe (10)
MeCN
MeCN
DCM
DCM
DCM
DCM/MeCN (1:1)
DCM
TEA
pyridine
TEA
pyridine
DIPEA
DIPEA
TEA
74
68
80
78
82
80
75
76
DCM
TEA
[a]
[b]
[c]
[d]
The reactions were performed at room temp. for 18 h.
detectable epimerization (1H NMR, 400 MHz).
Isolated pure peptides.
Young’s test: ee ϭ 53%.
Anteunis’s test: no
10%MeCN up to 10%H2O/90%MeCN within 15 min at 0.2 mL/
peptide generated by coupling Bz--Leu-OH and H-Gly-
OEt (Young’s test), showed 23.5% of the -isomer, whereas
unsupported BOP reagent is reported to induce 60% racem-
ization when the mentioned test is carried out.[10] Similar to
P-TBTU,[5] no epimerization was detected in the tripeptide
generated by coupling Z-Gly-Phe-OH and H-Val-OMe
(Anteunis’s test) as shown by 1H NMR analysis (400 MHz).
Contrary to the results reported with the polymer-sup-
ported TBTU reagent,[5] heating was not required and Z-
1
min flow rate was used. H and 13C NMR spectra were recorded
with a Bruker ARX250 spectrometer at 250 MHz, respectively
63 MHz. Gel-phase 1H NMR spectra were recorded with a Bruker
Avance spectrometer at 400 MHz, equipped with an HRMAS pro-
behead. Chemical shifts δ are given in ppm relative to TMS; coup-
ling constants J are reported in Hz. IR spectra were recorded using
a Bruker Vector 22 spectrometer coupled with a microscope. El-
emental analyses were determined by Service Central d’Analyse,
CNRS, Vernaison, France. Analytical TLC plates (Silica gel 60 F254
Gly-Phe-Val-OMe was obtained in higher yield using P- on glass) were purchased from Merck. Silica gel 60 (40Ϫ63 µm,
Merck) was used for flash chromatography.
BOP (2). Moreover, good yields were obtained for the diffi-
cult couplings involving the sterically hindered Aib
(Table 2, Entries 7 and 8), showing that P-BOP (2) pre-
serves this very important property of the BOP reagent.
Synthesis of P-BOP (2): Brop (1.94 g, 5 mmol, 5 equiv.) was added
to a suspension of P-HOBt (1 mmol/g, 1.0 g, 1 mmol, 1 equiv.) in
MeCN (5 mL), followed by addition of TEA (0.5 g, 0.7 mL,
5 mmol, 5 equiv.). The suspension was shaken at room temperature
for 18 h. At this time the polymer was filtered, washed with MeCN
(4 ϫ 10 mL), MeOH (2 ϫ 10 mL), DCM (4 ϫ 10 mL), dry Et2O
(4 ϫ 10 mL), and dried at 50 °C in vacuo (6 mbar), to furnish a
light-brown resin. IR (resin bead deposited on NaCl window): ν˜ ϭ
3358 (NH), 3058, 3025, 2924, 2850, 1601, 1492, 1452, 1319
(PϪNMe2), 1161 (SO2ϪN), 1064 (PϪNMe2), 1019, 842 (PϪF)
Conclusions
In conclusion, P-BOP (2) is a promising new polymer-
supported peptide coupling reagent, inducing an acceptable
degree of epimerization during synthesis of standard pep-
tides. Moreover, these preliminary results indicate that P-
BOP (2) is also a suitable activating reagent for difficult
peptide coupling reactions involving α,α-dialkyl amino
acids.
cmϪ1 1H NMR (400 MHz, CD2Cl2, gel phase): δ ϭ 6.20Ϫ7.50
.
(br., aromatic), 2.90 (br. PN(CH3)2], 2.60Ϫ2.80 (m, CH2) ppm. El-
emental analysis: found C 62.68, H 6.50, N 8.84, P 3.90, S 2.98.
General Procedure for the Determination of the Effective Loading of
P-BOP (2): Fmoc-βAla-OH (31.2 mg, 0.1 mmol, 2 equiv.) was ad-
ded to suspension of P-BOP (theoretical loading 1 mmol/g, 50 mg,
0.05 mmol, 1 equiv.) in the corresponding solvent (3 mL), followed
by the organic base (0.1 mmol, 2 equiv.). The mixture was shaken
at room temperature for 15 min, then iPrNH2 (6.5 mg, 9.42 µL,
0.011 mmol, 2.2 equiv.) was added and the shaking continued for
16 h. At this time the polymer was filtered and washed with MeCN
(5 mL) and DCM (5 mL). The filtrate and the washings were com-
bined, and the solvents were removed by evaporation. The crude
reaction mixture was dissolved in MeCN (10 mL) and MeOH
(3 mL), and submitted to the HPLC analysis.
Experimental Section
General Remarks: All reactions were carried out in 10-mL polypro-
pylene reactors (MultiSynTech GMBH). All commercial reagents
and solvents were used without further purification. The polymer-
supported HOBt [PS-HOBt(HL), 1.0 mmol/g] was purchased from
Argonaut Technologies. The Brop was purchased from Fluka.
HPLC analyses were performed using a 4.6 ϫ 50 mm reversed
phase Waters Symmetry Shield RP18 5µm column, with
a
HewlettϪPackard 1050 HPLC system. A gradient starting from
General Procedure for Peptide Coupling Using P-BOP (2): The N-
100%H2O/0.1%TFA up to 100%MeCN/0.1%TFA within 14 min at
protected amino acid (0.15 mmol, 1 equiv.) was added to a suspen-
1 mL/min flow rate was used. The chiral HPLC analyses were per- sion of P-BOP (0.76 mmol/g, 0.30 g, 0.23 mmol, 1.5 equiv.) in the
formed using a 4.6 ϫ 150 mm Chiracel OD-R 5µm column, under
isocratic conditions (0.5  NaClO4/MeCN, 60:40) and UV detec-
corresponding solvent (3 mL), followed by the amino ester hydro-
chloride (0.18 mmol, 1.2 equiv.) and the base (0.60 mmol, 4 equiv.).
tion at 214 nm, with a HewlettϪPackard 1050 HPLC system. The The mixture was shaken at room temperature for 18 h, the resin
LC-MS analyses were performed using a 2.1 ϫ 50 mm reversed
phase Intercom C18 3µm column, with a HewlettϪPackard 1100
LC system coupled with a MS Micromass Platform LC mass ana-
lyser, operating at 3.5 kV. A gradient starting from 90%H2O/
was filtered and washed with DCM (3 ϫ 5 mL). After evaporation
of the solvent, the crude reaction mixture was dissolved in EtOAc,
washed with NaHCO3 saturated aqueous solution (3 ϫ 15 mL),
KHSO4 saturated aqueous solution (3 ϫ 15 mL) and dried with
1938
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Org. Chem. 2004, 1936Ϫ1939

Peptide Bond Formation Using Polymer-Bound BOP
FULL PAPER
MgSO4. The resulting oil showed to be pure by TLC. However, a
sample was purified by flash chromatography on silica gel (DCM/
MeOH, 8:1) for analytical purposes.
1 H, NH), 6.80 (br. s, 1 H, NH), 7.05Ϫ7.28 (m, 5 H, ArH) ppm.
13C NMR (63 MHz, CDCl3): δ ؍ 13.9 (CH3), 25.3 (CH3-Aib), 28.1
(tBu), 28.6 (CH3-Aib), 38.0 (CH2-Phe), 53.2 (CH-Phe), 56.5 (C-
Aib), 61.2 (CH2), 80.0 (C-tBu), 126.8 (Ar), 128.3 (Ar), 129.2 (Ar),
135.9 (Ar), 154.4 (Ar), 171.4 (CϭO), 174.0 (CϭO) ppm.
Boc-Gly-Phe-OEt (6):[3] Light-yellow oil. Yield 68Ϫ80% (see
Table 2). Rf ϭ 0.57 (DCM/MeOH, 8:1). 1H NMR (250 MHz,
Boc-Aib-Val-OMe (10):[13] White solid. Yield 76% (36 mg). Rf ϭ
3
CDCl3): δ ϭ 1.20 (t, JH,H ϭ 7.2 Hz, 3 H, CH3CH2), 1.33 (s, 9 H,
1
3
0.60 (DCM/MeOH, 8:1). H NMR (250 MHz, CDCl3): δ ϭ 0.85
tBu), 3.05 (d, JH,H ϭ 6.8 Hz, 2 H, CH2Ph), 3.67Ϫ3.78 (m, 2 H,
3
3
3
[d, JH,H ϭ 7.1 Hz, 3 H, (CH3)2CH], 0.95 [d, JH,H ϭ 7.1 Hz, 3 H,
(CH3)2CH], 1.37 (s, 9 H, tBu), 1.42 (s, 3 H, CH3-Aib), 1.48 (s, 3 H,
CH3-Aib), 2.12 [m, 1 H, CH(CH3)2], 3.64 (OCH3), 4.45 (dd,
3JH,H ϭ 8, 3JH,NH ϭ 5 Hz, 1 H, HNCH-iPr), 4.90 (br. s, 1 H, NH),
6.95 (br. s, 1 H, NH) ppm. 13C NMR (63 MHz, CDCl3): δ ؍ 17.4
(CH3-Val), 18.8 (CH3-Val), 26.0 (CH3-Aib), 28.1 (tBu), 29.5 (CH-
Val), 31.2 (CH3-Aib), 51.9 (OCH3), 56.7 (C-Aib), 57.0 (NHCH-
Val), 80.0 (C-tBu), 172.4 (CϭO), 174.3 (CϭO) ppm.
NHCH2), 4.10 (q, JH,H ϭ 7.2 Hz, 2 H, CH3CH2), 4.79 (m, 1 H,
CH-Bn), 5.04 (br. s, 1 H, NH), 6.42 (br. s, 1 H, NH), 7.05Ϫ7.28
(m, 5 H, ArH) ppm. 13C NMR (63 MHz, CDCl3): δ ؍ 13.0 (CH3),
27.2 (tBu), 36.9 (CH2-Phe), 44.0 (CH2-Gly), 52.0 (CH-Phe), 60.5
(CH2), 79.5 (C), 126.1 (Ar), 127.5 (Ar), 128.2 (Ar), 134.6 (Ar),
167.8 (CϭO), 167.9 (CϭO), 170.2 (CϭO) ppm.
Bz-Leu-Gly-OEt (7):[11] White solid. Yield 82% (63 mg). Rf ϭ 0.51
(DCM/MeOH, 8:1). tR (chiral HPLC) ϭ 13.32 (major), 14.79
1
3
(minor) min. H NMR (250 MHz, CDCl3): δ ϭ 0.91 [d, JH,H
ϭ
3
6.1 Hz, 6 H, (CH3)2CH], 1.20 (t, JH,H ϭ 7.2 Hz, 3 H, CH3CH2),
1.58Ϫ1.80 (m, 3 H, CH2CH, (CH3)2CH], 3.90Ϫ4.02 (m, 2 H,
CH2NH), 4.10 (q, JH,H ϭ 7.2 Hz, 2 H, CH3CH2), 4.74 (m, 1 H,
Acknowledgments
The financial support of Bayer CropScience is gratefully acknowl-
edged. We thank Na¨ıma Heude and Jean-Pierre Marquet from
Bayer Cropscience, Lyon, for the IR and gel-phase 1H NMR meas-
urements, respectively.
3
CHNH), 5.60 (br. s, 1 H, NH), 6.70 (br. s, 1 H, NH), 7.30Ϫ7.81
(m, 5 H, ArH) ppm. 13C NMR (63 MHz, CDCl3): δ ؍ 13.9 (CH3),
22.0 (CH3-Leu), 22.0 (CH3-Leu), 24.7 (CH-Leu), 41.0 (CH2-Leu),
41.2 (CH2-Gly), 51.8 (NHCH-Leu), 61.3 (CH2), 127.0 (Ar), 128.4
(Ar), 131.6 (Ar), 133.5 (Ar), 167.4 (CϭO), 169.4 (CϭO), 172.5 (Cϭ
O) ppm. LC-MS: m/z ϭ 321 [M ϩ H]ϩ.
[1]
M. C. Desai, L. M. Stephens Stramiello, Tetrahedron Lett.
1993, 34, 7685Ϫ7688.
[2]
´
I. E. Pop, B. P. Deprez, A. L. Tartar, J. Org. Chem. 1997, 62,
Z-Gly-Phe-Val-OMe (8):[12] White solid. Yield 80% (94 mg). Rf ϭ
0.62 (DCM/MeOH, 8:1). tR (HPLC) ϭ 7.62 (minor), 8.65 (major)
2594Ϫ2603.
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
A. Berrada, F. Cavelier, R. Jacquier, J. Verducci, Bull. Soc.
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1
3
min. H NMR (250 MHz, CDCl3): δ ϭ 0.71 [d, JH,H ϭ 7.1 Hz, 3
F. Cavelier, F. Guendouz, R. Jacquier, J. Verducci, Bull. Soc.
Chim. Fr. 1992, 129, 463Ϫ467.
3
H, (CH3)2CH], 0.79 [d, JH,H ϭ 7.1 Hz, 3 H, (CH3)2CH], 1.99 [m,
1 H, CH(CH3)2], 2.99 (m, 2 H, CH2Ph), 3.62 (s, 3 H, CH3O), 3.79
´
R. Chinchilla, D. J. Dodsworth, C. Najera, J. M. Soriano,
3
3
(d, JH,NH ϭ 5.4 Hz, 2 H, HNCH2CO), 4.35 (dd, JH,H ϭ 8,
Tetrahedron Lett. 2000, 41, 2463Ϫ2466.
A. Fournier, W. Danho, A. M. Felix, Int. J. Peptide Protein
Res. 1989, 33, 133Ϫ139.
3JH,NH ϭ 5 Hz, 1 H, HNCH-iPr), 4.64 (m, 1 H, CH-Bn), 5.04 (s,
2 H, CH2OCONH), 5.33 (br., 1 H, NHCH2), 6.25 (d, JH,NH
3
ϭ
3
B. Castro, J.-R. Dormoy, G. Evin, C. Selve, Tetrahedron Lett.
1975, 14, 1219Ϫ1222.
7.2 Hz, 1 H, NHCH-Bn), 6.63 (d, JH,NH ϭ 7.2 Hz, 1 H, NHCH-
iPr), 7.10Ϫ7.35 (m, 10 H, ArH) ppm. 13C NMR (63 MHz, CDCl3):
δ ؍ 17.6 (CH3-Val), 18.7 (CH3-Val), 20.6 (CH-Val), 38.3 (CH2-
Phe), 44.1 (CH2-Gly), 52.0 (OCH3), 54.4 (CH-Phe), 57.3 (NHCH-
Val), 66.9 (OCH2), 126.8 (Ar), 127.9 (Ar), 128.0 (Ar), 128.4 (Ar),
129.2 (Ar), 136.1 (Ar), 156.5 (Ar), 169.3 (CϭO), 171.2 (CϭO),
171.6 (CϭO), 175.1 (CϭO) ppm. LC-MS: m/z ϭ 470 [M ϩ H]ϩ.
´
E. Frerot, J. Coste, A. Pantaloni, M.-N. Dufour, P. Jouin,
Tetrahedron 1991, 47, 259Ϫ270.
V. Lejeune, PhD Thesis, University of Montpellier, France,
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3315Ϫ3317.
[11]
[12]
Boc-Aib-Phe-OEt (9): White solid. Yield 75% (43 mg). Rf ϭ 0.62
(DCM/MeOH, 8:1). 1H NMR (250 MHz, CDCl3): δ ϭ 1.20 (t,
3JH,H ϭ 7.2 Hz, 3 H, CH3CH2), 1.40 (s, 3 H, CH3-Aib), 1.43 (s, 12
[13]
3
H, tBu, CH3-Aib), 3.03 (d, JH,H ϭ 6.8 Hz, 2 H, CH2Ph), 4.05 (q,
3JH,H ϭ 7.2 Hz, 2 H, CH3CH2), 4.78 (m, 1 H, CH-Bn), 4.86 (br. s,
Received December 19, 2003
Eur. J. Org. Chem. 2004, 1936Ϫ1939
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1939