A stereoselective synthesis of (2S,3R)-β-methoxyphenylalanine: A component of cyclomarin A

Article abstract of DOI:10.1016/j.tetasy.2005.10.029

A stereoselective synthesis of (2S,3R)-β-methoxyphenylalanine, an amino acid contained within the cyclic peptide cyclomarin A, was successfully synthesized from Lajoie's serine aldehyde.

Full text of DOI:10.1016/j.tetasy.2005.10.029

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 3963–3969
A stereoselective synthesis of (2S,3R)-b-methoxyphenylalanine:
a component of cyclomarin A
*
´
Darren B. Hansen and Madeleine M. Joullie
Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, PA 19104, USA
Received 20 June 2005; revised 23 September 2005; accepted 4 October 2005
Abstract—A stereoselective synthesis of (2S,3R)-b-methoxyphenylalanine, an amino acid contained within the cyclic peptide
cyclomarin A, was successfully synthesized from LajoieÕs serine aldehyde.
Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction
was deduced using a variety of spectroscopic methods.
The number of unusual amino acids contained with
cyclomarin A has attracted significant interest from
the synthetic community,6–9 culminating in the synthesis
of cyclomarin C10,11 a close congener of cyclomarin A
and two separate syntheses of (2S,3R)-b-methoxyphen-
ylalanine. The first synthesis of the desired b-methoxy-
phenylalanine was based on the Scho¨llkopfÕs chiral
bis-lactam,6 while the second is based on a stereoselec-
tive radical bromination.10 En route to a total synthesis
of cyclomarin A, we herein report the synthesis of
(2S,3R)-b-methoxyphenylalanine based on LajoieÕs ser-
ine aldehyde.
The presence of b-methoxy amino acids in biologically
significant natural products1–5 requires the stereoselec-
tive syntheses of these amino acids. Cyclomarin A is a
novel cyclic peptide isolated from aestuarine actinomy-
cete, cultured from a sediment sample collected in Mis-
sion Bay, California5 (Fig. 1). Cyclomarin A is cytotoxic
toward cancer cells with a mean IC50 of 2.6 lM against a
panel of human cancer cell lines. Of greater interest is
cyclomarin AÕs potent anti-inflammatory activity both
in vitro and in vivo. The structure of cyclomarin A
2. Results and discussion
N
O
LajoieÕs serine aldehyde has proven to be a powerful
intermediate in the synthesis of b-hydroxy and b-meth-
oxy amino acids.12–14 The method is general and allows
for the generation of both threo and erythro diastereo-
mers in a straightforward fashion. The serine aldehyde
was prepared according to the literature procedures.14,15
The addition of phenyl magnesium bromide to serine
aldehyde afforded b-hydroxy phenylalanine intermedi-
ate 3 (Scheme 1). Methylation of the hydroxyl group uti-
O
H
N
HO
N
H
O
N
O
N
O
O
HO
O
HN
H
N
H2N
HN
OH
O
O
MeO
MeO
lized a combination of Me3OþBFꢀ and proton sponge
1
4
to give intermediate 4.16 Hydrogenolysis of the Cbz
group gave amine 5, which was amenable to chromato-
graphic separation of the small amount of undesired
diastereomer. Exposure to LajoieÕs deprotection12
conditions followed by ion-exchange chromatography
allowed for the isolation of (2S,3R) b-methoxyphenyl-
alanine 1.
(2S,3R) β-Methoxyphenylalanine
Cyclomarin A
Figure 1. Cyclomarin A and (2S,3R) b-methoxyphenylalanine.
*
Corresponding author. Tel.: +1 215 898 3158; fax: +1 215 573
9711; e-mail: mjoullie@sas.upenn.edu
0957-4166/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.10.029

3964
D. B. Hansen, M. M. Joullie´ / Tetrahedron: Asymmetry 16 (2005) 3963–3969
O
H
O
HO
b
a
O
O
O
O
ZHN
ZHN
O
2
3
MeO
ZHN
MeO
H2N
c
O
O
O
O
O
O
5
4
MeO
H2N
d
CO2H
1
Scheme 1. Reagents and conditions: (a) PhMgBr 61% yield, 90% de; (b) Me3OþBFꢀ, proton spongeÒ, 96%; (c) 10% Pd/C, H2; (d) (1) 0.1% aqueous
4
TFA, (2) 10% aqueous Cs2CO3, 88%.
To further extend our work toward the synthesis of b-
methoxyphenylalanine, we sought to synthesize the
opposite diastereomer of 1 to showcase the power of this
method. Dess–Martin oxidation of 3 provided b-keto
intermediate 4 (Scheme 2). Reduction of the carbonyl
with LiBH4 under Felkin–Anh control gave clean inver-
sion of the hydroxyl group to give diastereomer 7. Meth-
ylation was accomplished as described earlier to afford
8. Hydrogenolysis of the Cbz group then provided
amine 9. Exposure of amine 9 to LajoieÕs deprotection
conditions followed by ion-exchange chromatography
allowed for the isolation of (2S,3S)-b-methoxyphenyl-
alanine 10.
ester could be cleaved, the resulting acid could then be
coupled with other amino acids, extending LajoieÕs
methodology and shortening the total synthesis of
cyclomarin A. Dipeptide 13 was prepared by coupling
Z-Val with N(Me)-Leu-OTMSE. The ortho-ester inter-
mediate 4 was deprotected with aqueous acetic acid to
give crude a diol-ester, which was then saponified with
aqueous lithium hydroxide to give the crude free acid.
Hydrogenolysis of the Cbz group from 13 gave an
amine, which was directly coupled with 4 to give a mod-
erate yield of tripeptide 14 (Scheme 4).
3. Conclusion
Having synthesized both diastereomers of b-methoxy-
phenylalanine (b-MeO-Phe), we investigated whether it
would participate in the initial coupling reactions neces-
sary for the synthesis of cyclomarin A. (2S,3R)-b-Meth-
oxyphenylalanine 1 was suitably protected in a two-step
procedure. First the nitrogen was protected as a Cbz
carbamate under Schotten–Baumen conditions. The
crude Z-protected b-MeO-Phe was esterified with 2-
(trimethylsilyl)ethanol (TMSE) to give the desired 2-
(trimethylsilyl)ethyl ester 11 (Scheme 3) in good yield.
Removal of the Cbz group proceeded without incident,
and the crude amine was then coupled Cbz-Ala to pro-
vide dipeptide 12 in good yield.
An efficient synthesis of (2S,3R)-b-methoxyphenylala-
nine 1 was achieved. The power of LajoieÕs serine alde-
hyde was particularly evident in that it ultimately
could be exploited to the synthesis of (2S,3S) diastereo-
mer of b-methoxyphenylalanine 10 as well. The ortho-
ester protecting group can be selectively removed to
allow for the formation of tripeptide 14 found within
cyclomarin A.
4. Experimental
Reactions requiring air-sensitive manipulations were
conducted under an argon atmosphere. Methylene chlo-
ride was distilled from calcium hydride, tetrahydrofuran
and diethyl ether were distilled from sodium/benzophe-
none. Analytical TLC was performed on 0.25 mm E.
silica gel 60 F254 plates. Silica gel (60, particle size
0.040–0.063 mm) was used for flash column chromato-
A different approach was investigated utilizing the cou-
pling of the carboxyl group of the b-MeO-Phe 1. In our
previous synthesis of b-methoxytyrosine,14 we had
shown that the ortho-ester could be opened in the pres-
ence of the Cbz protected nitrogen without any accom-
panying racemization of the a-stereocenter. If the diol-

D. B. Hansen, M. M. Joullie´ / Tetrahedron: Asymmetry 16 (2005) 3963–3969
3965
HO
O
b
a
O
O
O
O
ZHN
ZHN
O
O
3
6
MeO
ZHN
HO
d
c
O
O
O
O
ZHN
O
O
8
7
MeO
H2N
MeO
H2N
e
O
O
CO2H
O
9
10
Scheme 2. Reagents and conditions: (a) Dess–Martin periodinane, 89%; (b) LiBH4, 74% > 95% de; (c) Me3OþBFꢀ, proton spongeÒ, 73%; (d) 10%
4
Pd/C, H2; (e) (1) 0.1% aqueous TFA, (2) 10% aqueous Cs2CO3, 62% three steps.
points were determined in an open capillary tube and
were uncorrected.
MeO
ZHN
MeO
H2N
a
4.1. 2-(40-Methyl-20,60,70-trioxabicyclo[2.2.2]octan-10-yl)-
2-benzyloxycarbonylaminoethanal (LajoieÕs serine
aldehyde) 2
OTMSE
CO2H
O
11
Prepared according to the literature procedures.14,15
1
4.2. Benzyl-(1S,2R)-2-hydroxy-1-(40-methyl-20,60,70-tri-
oxabicyclo[2.2.2]octan-10-yl)-2-phenylethylcarbamate 3
MeO
O
Serine aldehyde 2 (30.9 mmol) was dissolved in anhy-
drous CH2Cl2 (300 mL), diluted with anhydrous Et2O
(300 mL) under argon and cooled to 0 °C. PhMgBr
(3.0 M Et2O, 35 mL, 105 mmol) was added and the reac-
tion was allowed to stir for 5 min. The reaction was
quenched at 0 °C with 5% aqueous NH4Cl, and diluted
with Et2O (500 mL). The organic layer was separated
and washed with water, then brine, dried over MgSO4,
and reduced in vacuo. The residue was purified by flash
silica gel chromatography, eluting with 30–50% EtOAc–
Hex to give a white powder (7.50 g, 61% yield, 90% de
b
ZHN
OTMSE
N
H
O
12
Scheme 3. Reagents and conditions: (a) (1) Cbz-Cl, NaHCO3; (2)
EDAC, NEt3, TMSE, DMAP, 79% two steps; (b) (1) 10% Pd/C, H2;
(2) Z-Ala, EDAC, NEt3, 62% two steps.
graphy. NMR spectra were recorded on a 500 MHz
spectrometer and calibrated by using residual undeuter-
ated solvent or TMS as an internal reference. Chemical
shifts (d) were measured in parts per million, and cou-
pling constants (J values) in Hertz (Hz). Infrared spectra
(IR) were recorded on an FT-IR spectrometer. High-res-
olution mass spectra (HRMS) were recorded using elec-
trospray ionization (ESI). Optical rotations were
recorded on a polarimeter at the sodium D line. Melting
determined by integration of the a proton) : mp: 132–
20
134 °C; ½aꢁD ¼ ꢀ35:3 (c 0.6, CHCl3); TLC (50%
1
EtOAc–Hex), Rf = 0.36; H NMR (CDCl3, 500 MHz)
d 7.37–7.20 (m, 10H), 5.42 (d, J = 10.3, 1H), 5.34 (s,
1H), 5.00–4.91 (m, 1H), 4.86–4.79 (m, 1H), 4.09 (d,
J = 10.1, 1H), 3.99 (s, 6H), 3.35 (s, 1H), 0.85 (s, 3H);
13C NMR (CDCl3, 125 MHz) d 156.3, 140.0, 136.7,
128.4, 128.1, 127.8, 127.7, 127.3, 125.9, 108.8, 72.9,

3966
D. B. Hansen, M. M. Joullie´ / Tetrahedron: Asymmetry 16 (2005) 3963–3969
O
O
a
OTMSE
+
ZHN
OTMSE
ZHN
HN
N
OH
O
O
13
O
O
ZHN
OTMSE
b
H2N
OTMSE
N
N
O
O
13
MeO
ZHN
O
d
H
N
OTMSE
N
O
O
14
MeO
ZHN
MeO
ZHN
c
O
O
OH
O
O
4
Scheme 4. Reagents and conditions: (a) DCC, 48%; (b) Pd/C, H2; (c) (1) aqueous HOAc; (2) LiOH; (d) EDAC, DIEA 45% over four steps.
70.8, 66.5, 58.6, 30.7, 14.3; IR (thin film) 3506 w, 3448 w,
2942 w, 2884 w, 1725 s br, 1513 m, 1448 w, 1396 w, 1331
w, 1290 m, 1225 m, 1196 m, 1090 m, 1049 s, 1020 s, 984
m, 726 m, 696 m; HRMS (EI): m/z calcd for
C22H25O6NNa 422.1580, found 422.1564.
m, 1351 m, 1302 m, 1295 m, 1202 m, 1105 m, 1052 s,
1012 s, 913 w, 769 w, 726 m, 699 m, 627 m; HRMS
(EI): m/z calcd for C23H27O6NNa 436.1736, found
436.1750.
4.4. (1S,2R)-2-Methoxy-1-(40-methyl-20,60,70-trioxa-
bicyclo[2.2.2]octan-10-yl)-2-phenylethanamine 5
4.3. Benzyl-(1S,2R)-2-methoxy-1-(40-methyl-20,60,70-tri-
oxabicyclo[2.2.2]octan-10-yl)-2-phenylethylcarbamate 4
Compound 4 (1.09 g, 2.64 mmol) was dissolved in
EtOAc (40 mL), and diluted with EtOH (40 mL). After
the addition of 10% Pd(OH)2/C (100 mg), the reaction
was placed in a Parr bottle. The system was evacuated,
filled with hydrogen gas (45 psi) and allowed to shake
for 4 h. After the hydrogen gas was vented, the reaction
was filtered through Celite, and the Celite pad was
washed with EtOAc. The filtrate was reduced in vacuo
and the residue purified by column flash silica gel chro-
matography, eluting with 50% acetone–Hex to give two
products; a trace amount of the undesired diastereomer
Compound 3 (7.80 g, 19.6 mmol) was dissolved in anhy-
drous CH2Cl2 (250 mL), under argon, and to the solu-
˚
tion was added 4 A mol. sieves (20.00 g), and proton
spongeÒ (16.80 g, 78.00 mmol), followed by MeOþBF4ꢀ
(10.00 g, 67.6 mmol). The reaction was allowed to stir
under argon vigorously for 5 h. When the reaction was
shown to be complete by TLC, it was diluted with
EtOAc (750 mL) and filtered. The filtrate was washed
sequentially with 10% aqueous CuSO4, 5% aqueous
NH4Cl, satd NaHCO3, and brine. It was then dried over
MgSO4 and the solution reduced in vacuo. The residue
was purified by flash silica gel chromatography, eluting
(Rf = 0.27, 50% acetone–Hex) and the major diastereo-
20
mer (0.53 g, 72%) as a viscous oil: ½aꢁD ¼ ꢀ16:8 (c 1.5,
1
with 30% EtOAc–Hex to give a pale pink oil (7.71 g,
CHCl3); TLC (50% acetone–Hex), Rf = 0.15; H NMR
20
96% yield): ½aꢁD ¼ ꢀ15:0 (c 0.9, CHCl3); TLC (50%
(CDCl3, 500 MHz) d 7.33–7.28 (m, 5H), 4.48 (d,
J = 3.9, 1H), 3.86 (s, 6H), 3.27 (s, 3H), 2.95 (d,
J = 3.9, 1H), 0.78 (s, 3H); 13C NMR (CDCl3,
125 MHz) d 141.1, 128.0, 127.2, 127.1, 108.7, 81.7,
72.6, 61.2, 57.2, 30.5, 14.4; IR (thin film) 2930 m,
2877 m, 1731 w, 1455 w, 1398 w, 1351 w, 1278 w,
1195 w, 1096 s, 1049 s, 1013 s, 987 s, 701 s; HRMS
(EI): m/z calcd for C15H22O4N 280.1549, found
280.1557.
1
EtOAc–Hex), Rf = 0.44; H NMR (CDCl3, 500 MHz)
d 7.34–7.19 (m, 10H), 5.47 (d, J = 10.3, 1H), 5.06 (d,
J = 12.5, 1H), 4.93 (d, J = 12.5, 1H), 4.66 (s, 1H), 3.97
(d, J = 10.5, 1H), 3.93 (s, 6H), 3.28 (s, 3H), 0.80 (s,
3H); 13C NMR (CDCl3, 125 MHz) d 156.2, 140.0,
136.9, 128.3, 128.2, 127.7, 127.5, 126.5, 108.4, 79.6,
72.7, 66.3, 59.5, 57.4, 30.7, 14.4; IR (thin film) 3455 w,
3360 w, 2932 m, 2878 m, 1729 s, 1511 s, 1454 m, 1396

D. B. Hansen, M. M. Joullie´ / Tetrahedron: Asymmetry 16 (2005) 3963–3969
3967
4.5. (2S,3R)-2-Amino-3-methoxy-3-phenylpropanoic acid
(b-methoxyphenylalanine) 1
allowed to warm to room temperature. The organic
layer was separated and dried over MgSO4 and reduced
in vacuo. The residue was purified by flash silica gel
Amine 5 (180 mg, 0.64 mmol) was dissolved in water
(10 mL) and TFA (0.1 mL) then added. The reaction
was allowed to stir at room temperature for 45 min.
The solution was reduced in vacuo, the remaining residue
redissolved in water (10 mL), Cs2CO3 (1.00 g, 3.04
mmol) was added, and the reaction allowed to stir
for 16 h. The reaction was acidified to pH ꢂ 2 with
6.0 M HCl. The total solution was loaded on approxi-
mately 15 cm2 of IRC-50 (strong acid exchange resin,
H+ form, Rohm and Haas Corp.), washed with water
until the pH of the column eluant was greater than 5.0.
The column was eluted with 4% aqueous NH4OH, all
ninhydrin positive fractions were collected and reduced
chromatography, eluting with 40% EtOAc–Hex to give
20
a pale oil (660 mg, 74% yield): ½aꢁD ¼ ꢀ15:5 (c 0.7,
1
CHCl3); TLC (50% EtOAc–Hex), Rf = 0.32; H NMR
(CDCl3, 500 MHz) d 7.36 (d, J = 7.1, 2H), 7.29–7.23
(m, 6H) 7.11 (d, J = 7.2, 2H), 4.97 (d, J = 12.4, 1H),
4.85–4.82 (m, 2H), 4.22–4.18 (m, 1H), 4.10 (s, 1H),
3.96 (s, 6H), 0.84 (s, 3H); 13C NMR (CDCl3,
125 MHz) 155.7, 140.1, 136.5, 128.3, 127.8, 127.7,
108.7, 74.3, 72.8, 66.5, 58.7, 30.7, 14.2; IR (thin film)
3495 m, 3342 m, 2896 m, 1719 s, 1531 s, 1243 m, 991
s, 697 m; HRMS (EI): m/z calcd for C22H25O6NNa
422.1580, found 422.1583.
in vacuo to give a tan powder (110 mg, 88%): mp =
4.8. Benzyl-(1S,2S)-2-methoxy-1-(40-methyl-20,60,70-tri-
20
185–189 °C; ½aꢁD ¼ ꢀ18:4 (c 0.4, H2O); 1H NMR
oxabicyclo[2.2.2]octan-10-yl)-2-phenylethylcarbamate 8
(D2O, 500 MHz) d 7.50–7.42 (m, 1H), 4.78 (d, J =
5.5, 1H), 3.85 (d, J = 5.5, 1H), 3.32 (s, 3H); 13C NMR
(D2O, 125 MHz) d 172.4, 136.8, 129.7, 129.6, 127.5,
81.7, 67.2, 61.1, 57.8; IR (KBr pellet) 3063 br s, 1617 s,
1392 s, 1362 m, 1089 m, 703 m; HRMS (EI): m/z calcd
for C10H13O3NNa 218.0793, found 218.0802.
Compound 7 (2.68 g, 6.72 mmol) was dissolved in anhy-
drous CH2Cl2 (175 mL), proton spongeÒ (3.60 g,
16.8 mmol) was added, followed by Me3OþBFꢀ
4
(1.99 g, 13.4 mmol). The reaction was allowed to stir
for 8 h at room temperature under argon. The reaction
was diluted with EtOAc (500 mL) and filtered. The fil-
trate was washed with 10% aqueous CuSO4, 5% aqueous
NH4Cl, satd NaHCO3, brine, dried over MgSO4, and
reduced in vacuo. The residue was purified by flash silica
4.6. (S)-Benzyl-1-(40-methyl-20,60,70-trioxabicyclo-
[2.2.2]octan-10-yl)-2-oxo-2-phenylethyl-carbamate 6
gel chromatography, eluting with 40% EtOAc–Hex to
Compound 3 (1.00 g, 2.51 mmol) was dissolved in
CH2Cl2 (50 mL), and Dess–Martin periodinane
(1.60 g, 3.76 mmol) added. The reaction was allowed
to stir at room temperature for 15 min. When the reac-
tion was shown to be complete by TLC, it was quenched
with satd NaHCO3 (100 mL) and satd Na2S2O3
(100 mL) and the mixture was stirred for 15 min until
both layers were clear. The organic layer was separated,
dried over MgSO4, and reduced in vacuo. The residue
was purified by flash silica gel chromatography, eluting
20
give a pale pink oil (2.03 g, 73% yield): ½aꢁD ¼ þ7:2 (c
0.6, CHCl3); TLC (50% EtOAc–Hex), Rf = 0.52; 1H
NMR (CDCl3, 500 MHz) d 7.38–7.23 (m, 5H), 5.15–
5.06 (m, 2H), 4.73 (d, J = 10.7, 1H), 4.65 (d, J = 3.7,
1H), 4.47 (dd, J = 3.7, 10.7, 1H), 3.85–3.78 (m, 6H),
3.28 (s, 3H), 0.77 (s, 3H); 13C NMR (CDCl3,
125 MHz) d 156.3, 138.2, 136.8, 128.3, 128.3, 127.9,
127.8, 127.5, 127.4, 107.8, 81.3, 72.5, 66.6, 57.6, 56.8,
30.5, 14.3; IR (thin film) 2933 m, 2878 m, 1732 s, 1514
m, 1397 m, 1315 m, 1218 m, 1100 m, 1012 s, 731 m,
700 m; HRMS (EI): m/z calcd for C23H28O6N
414.1917, found 414.1900.
with 40% EtOAc–Hex to give a pale oil (891 mg, 89%
20
yield): ½aꢁD ¼ ꢀ14:1 (c 0.4, CHCl3); TLC (50%
1
EtOAc–Hex), Rf = 0.48; H NMR (CDCl3, 500 MHz)
d 8.06 (d, J = 7.5, 2H), 7.56 (t, J = 7.4, 1H), 7.45 (t,
J = 7.8, 2H), 7.35 (m, 5H), 5.92 (d, J = 9.1, 1H), 5.59
(d, J = 9.2, 1H), 5.10 (s, 2H), 3.86 (s, 6H), 0.75 (s,
3H); 13C NMR (CDCl3, 125 MHz) d 195.1, 155.9,
136.7, 136.3, 133.2, 129.4, 128.9, 128.4, 128.2, 128.1,
107.3, 72.9, 67.0, 57.5, 30.7, 14.2; IR (thin film) 3436
w, 3366 w, 2954 w, 2884 m, 1725 s, 1690 s, 1590 w,
1507 s, 1467 w, 1449 m, 1355 m, 1320 m, 1296 m,
1214 s, 1049 s, 1002 s, 691 m; HRMS (EI): m/z calcd
for C22H23O6NNa, 420.1423 found 420.1413.
4.9. (1S,2S)-2-Methoxy-1-(40-methyl-20,60,70-trioxabi-
cyclo[2.2.2]octan-10-yl)-2-phenylethanamine 9
Compound 8 (2.04 g, 7.30 mmol) was dissolved in
EtOAc (25 mL) and diluted with EtOH (25 mL). After
addition of 10% Pd(OH)2/C (0.20 g), the mixture was
placed in a Parr bottle. The system was evacuated and
filled with hydrogen gas (45 psi), and the reaction mix-
ture was shaken for 4 h. The Parr bottle was vented
and the reaction mixture was filtered through Celite,
the Celite pad was then washed with EtOAc. The filtrate
was reduced in vacuo to give a clear viscous oil that was
4.7. Benzyl-(1S,2S)-2-hydroxy-1-(40-methyl-20,60,70-tri-
oxabicyclo[2.2.2]octan-10-yl)-2-phenylethylcarbamate 7
recrystallized from EtOAc. (1.75 g, 86% yield): mp =
20
96–98 °C; ½aꢁD ¼ ꢀ6:5 (c 1.1, CHCl3); TLC (50%
1
Ketone 6 (891 mg, 2.25 mmol) was dissolved in CH2Cl2
(25 mL) and the solution diluted with MeOH (25 mL)
under argon, and cooled to ꢀ78 °C. Then a solution
of LiBH4 (2.0 M in THF, 1.3 mL, 2.6 mmol) was added
and the reaction allowed to stir at ꢀ78 °C for 6 h. When
the reaction was complete by TLC, it was quenched with
5% aqueous NH4Cl, diluted with CH2Cl2 (200 mL), and
EtOAc–Hex), Rf = 0.20; H NMR (CDCl3, 500 MHz)
d 7.46–7.27 (m, 5H), 4.80 (d, J = 4.1, 1H), 3.82 (s,
6H), 3.77 (d, J = 4.2, 1H), 3.30 (s, 3H), 0.76 (s, 3H);
13C NMR (CDCl3, 125 MHz) 135.4, 128.3, 128.3,
128.1, 106.2, 79.1, 58.2, 57.1, 30.6, 14.1; IR (thin film)
3370 m, 2936 s, 2881 s, 1744 m, 1593 m, 1496 m, 1456
w, 1354 w, 1273 w, 1209 w, 1099 s, 1048 s, 1026 s, 728

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D. B. Hansen, M. M. Joullie´ / Tetrahedron: Asymmetry 16 (2005) 3963–3969
w, 704 m; HRMS (EI): m/z calcd for C15H22O4N
280.1549, found 280.1537.
4.12. (S)-N-Cbz-Ala-(2S,3R)-N-Cbz-b-methoxyphenyl-
alanine(OTMSE)-ester 12
4.10. (2S,3S)-2-Amino-3-methoxy-3-phenylpropanoic
acid (b-methoxyphenylalanine) 10
Cbz-b-MeO-Phe-(OTMSE) ester 11 (50.0 mg, 0.11
mmol) was dissolved in EtOAc (5 mL) and diluted with
EtOH (5 mL) in a Parr hydrogenation flask. After
the addition of 10% Pd/C (10.0 mg), flask was evacu-
ated, then filled with hydrogen (1 atm) and stirred for
4 h. The reaction was evacuated and the mixture filtered
through Celite, the Celite pad was then washed with
EtOAc. The filtrate was reduced in vacuo and the resi-
due was combined with CH2Cl2 (5 mL), (S)-N-Cbz-Ala
(38.0 mg, 0.17 mmol), followed by NEt3 (0.031 mL,
0.22 mmol), and DMAP (5.0 mg). The reaction was
placed under argon and cooled to 0 °C. EDAC
(33.0 mg, 0.17 mmol) was then added and the reaction
allowed to slowly warm to room temperature and stirred
overnight. The reaction was diluted with Et2O (30 mL),
washed with 1.0 M aqueous citric acid, water, satd
NaHCO3, brine, dried over MgSO4, and reduced in
vacuo. The residue was purified by flash silica gel chro-
Amine 9 (1.75 g, 6.23 mmol) was dissolved in water
(50 mL), TFA (0.5 mL) was added and the reaction
allowed to stir at room temperature for 45 min. The
solution was reduced in vacuo. The remaining residue
was redissolved in water (10 mL), and Cs2CO3 (3.00 g,
9.20 mmol) added. The reaction was allowed to stir for
16 h and acidified to pH ꢂ 2 with 6.0 M HCl. The total
solution was loaded on approximately 50 cm2 of IRC-50
(strong acid exchange resin, H+ form, Rohm and Haas
Corp.), washed with water until the pH of the column
eluant was greater than 5.0. The column was eluted with
4% aqueous NH4OH, all ninhydrin positive fractions
were collected and reduced in vacuo to give a tan pow-
20
der (884 mg, 72%): ½aꢁD ¼ þ34:5 (c 0.6, H2O); 1H NMR
(D2O, 500 MHz) d 7.49–7.34 (m, 5H), 4.91 (d, J = 3.9,
1H), 4.16 (d, J = 3.9, 1H), 3.39 (s, 3H); 13C NMR
(D2O, 125 MHz) d 171.5, 135.1, 129.7, 129.5, 127.6,
67.2, 59.4, 57.4; IR (KBr pellet) 3142 br, 1734 m, 1617
s, 1529 s, 1455 m, 1309 s, 1273 m, 1098 s, 1075 m, 967
m, 749 s, 704 s, 570 m; HRMS (EI): m/z calcd for
C10H13O3NNa 218.0793, found 218.0797.
matography, eluting with 20–30% EtOAc–Hex to give a
20
clear oil (34 mg, 62% yield): ½aꢁD ¼ ꢀ12:0 (c 2.0,
1
CHCl3); TLC (20% EtOAc–Hex), Rf = 0.11; H NMR
(CDCl3, 500 MHz) d 7.37–7.25 (m, 10H), 6.58 (d,
J = 8.9, 1H), 5.29 (br s, 1H), 5.13 (s, 2H), 4.83 (s, 1H),
4.74 (d, J = 9.1, 1H), 4.27–4.24 (m, 3H), 3.28 (s, 3H),
1.34–1.30 (m, 3H), 1.03–0.99 (m, 2H), 0.11–0.00 (m,
9H); 13C NMR (CDCl3, 125 MHz) d 171.8, 169.8,
155.6, 136.9, 136.2, 128.5, 128.3, 128.1, 128.1, 128.0,
126.6, 82.3, 66.8, 64.0, 57.7, 57.5, 50.2, 18.6, 17.3,
ꢀ1.7; IR (thin film) 3315 br m, 2953 m, 1729 s, 1669 s,
1507 s, 1454 m, 1250 s, 1097 m, 860 m, 838 m, 699 m;
HRMS (EI): m/z calcd for C26H36O6N2NaSi 523.2240,
found 523.2238.
4.11. (2S,3R)-N-Cbz-b-Methoxyphenylalanine(OTMSE)
ester 11
b-Methoxyphenylalanine 1 (1.17 g, 6.02 mmol) was dis-
solved in satd NaHCO3 (60 mL), NaHCO3 (0.51 g,
6.1 mmol) added, followed by Cbz-Cl (1.3 mL,
9.2 mmol). The reaction was allowed to stir overnight
at room temperature. The reaction was washed twice
with Et2O and the aqueous layer was acidified to
pH ꢂ 2 with 6.0 M HCl. The reaction was extracted
three times with EtOAc. The organic extracts were com-
bined and washed with brine, dried over MgSO4, and
reduced in vacuo to give a pale oil that was dissolved
in CH2Cl2 and cooled to 0 °C under argon. Then
NEt3 (1.7 mL, 12 mmol), DMAP (100 mg), EDAC
(2.30 g, 12.0 mmol), and 2-trimethylsilylethanol (3.0
mL, 12 mmol) were added. The reaction was allowed
to slowly warm to room temperature and stirred over-
night. The reaction was diluted with Et2O (250 mL),
washed with 1.0 N HCl, water, satd NaHCO3, and
brine. The solution was dried over MgSO4 and reduced
in vacuo. The residue was purified by flash silica gel
4.13. (S)-N-Cbz-Val-(S)-N-Me-Cbz-Leu(OTMSE) ester
13
(S)-Cbz-Val (2.60 g, 10.33 mmol) was dissolved in
CH2Cl2 (10 mL) under argon and cooled to 0 °C.
DCC (2.13 g, 10.33 mmol) was added and the reaction
allowed to stir for 1 min before adding DMAP
(50.0 mg), and (S)-N-Me-leucine(OTMSE) ester (1.05 g,
6.88 mmol). The reaction was allowed to stir at
0 °C for 1 h and warmed to room temperature and
stirred overnight. The reaction was diluted with Et2O
(50 mL), and placed in the freezer (ꢀ20 °C) overnight.
The mixture was filtered at low temperature, and the fil-
trate then washed with 1.0 M HCl, water, satd NaH-
CO3, brine, dried over MgSO4, and reduced in vacuo.
The residue was purified by flash silica gel chromatogra-
chromatography, eluting with 10% EtOAc–Hex to give
20
a clear oil (2.04 g, 79% yield): ½aꢁD ¼ ꢀ15:2 (c 0.7,
1
CHCl3); TLC (10% EtOAc–Hex), Rf = 0.22; H NMR
phy, eluting with 10–20% EtOAc–Hex to give a clear oil
20
(CDCl3, 500 MHz) d 7.34–7.26 (m, 10H), 5.57 (d,
J = 9.3, 1H), 5.00 (s, 2H), 4.82 (d, J = 2.7, 1H), 4.52
(dd, J = 2.9, 9.4, 1H), 4.30–4.27 (m, 2H), 3.27 (s, 3H),
1.03–0.95 (m, 2H), 0.12–0.02 (m, 9H); 13C NMR
(CDCl3, 125 MHz) d 170.2, 156.0, 137.0, 136.3, 128.3,
127.9, 127.8, 127.7, 126.7, 82.5, 66.6, 63.9, 59.8, 57.4,
17.2; IR (thin film) 3355 w, 2953 m, 1730 s, 1508 m,
1250 m, 1103 m, 1059 m, 838 m, 698 m; HRMS (EI):
m/z calcd for C23H31O5NNaSi 452.1869, found
452.1851.
(1.60 g, 62% yield): ½aꢁD ¼ ꢀ16:1 (c 0.4, CHCl3); TLC
(10% EtOAc–Hex), Rf = 0.16; 1H NMR (CDCl3,
500 MHz) d 7.36–7.29 (m, 5H), 5.51 (d, J = 9.1, 1H),
5.30 (dd, J = 5.0, 10.7, 1H), 5.09 (s, 2H), 4.54 (dd,
J = 6.1, 9.1, 1H), 4.20–4.15 (m, 2H), 3.00 (s, 3H),
2.08–2.04 (m, 1H), 1.78–1.64 (m, 2H), 1.48–1.45 (m,
1H), 1.03–0.86 (m, 14H), 0.04–0.00 (m, 9H); 13C NMR
(CDCl3, 125 MHz) d 172.7, 171.7, 156.4, 136.4, 128.5,
128.0, 127.9, 66.8, 63.5, 55.9, 54.7, 36.9, 31.2, 31.2,
24.8, 23.2, 21.4, 19.5, 17.4, 17.3, ꢀ1.6; IR (thin film)

D. B. Hansen, M. M. Joullie´ / Tetrahedron: Asymmetry 16 (2005) 3963–3969
3969
3304 br m, 2956 s, 1732 s, 1645 s, 1524 m, 1469 m, 1410
m, 1251 m, 1176 m, 1026 w, 860 m, 838 m, 697 w;
HRMS (EI): m/z calcd for C25H42O5N2NaSi 501.2761,
found 501.2770.
IR (thin film) 3316 br w, 2955 m, 1732 s, 1641 s, 1506
m, 1409 w, 1250 m, 1221 m, 1100 w, 1059 w, 858 w,
839 w, 698 w; HRMS (EI): m/z calcd for C35H53O7N3-
NaSi 678.3351, found 678.3562.
4.14. (2S,3R)-N-Cbz-b-Methoxyphenylalanine-(S)-
Val-(S)-N-Me-Leu(OTMSE) ester 14
Acknowledgments
Compound 4 (1.00 g, 2.41 mmol) was dissolved in diox-
ane (15 mL), and AcOH (12 mL) was added followed by
water (13 mL). The reaction was allowed to stir for
10 min and the solution reduced in vacuo. The residue
was redissolved in toluene (30 mL) and reduced in vacuo
twice. The remaining residue was dissolved in dioxane
(10 mL), diluted with MeOH (10 mL), and water
(10 mL), cooled to 0 °C followed by the addition of
1.0 M aqueous LiOH (3.6 mL, 3.61 mmol). The reaction
was allowed to warm slowly to room temperature and
stir overnight. The reaction was acidified with 6.0 M
aqueous HCl until the pH was 2–3 and then extracted
three times with EtOAc. The organic layers were
combined and washed with brine, dried over MgSO4,
and reduced in vacuo to give the crude acid. In a Parr
flask, dipeptide 13 (1.20 g, 2.50 mmol) was dissolved in
MeOH (20 mL) and 10% Pd/C (220 mg) was added to
the solution. The flask was evacuated and filled with
hydrogen (1 atm) and allowed to stir for 3 h. The mix-
ture was filtered through Celite, the Celite pad washed
with EtOAc, the filtrate was combined and reduced in
vacuo to give the crude dipeptide amine. The crude
dipeptide amine and crude acid were combined in
CH2Cl2 (10 mL), DMAP (10 mg), EDAC (647 mg,
3.37 mmol), DIEA (0.84 mL, 4.82 mmol) were added
and the reaction allowed to stir overnight under argon.
The reaction was diluted with EtOAc (100 mL) and
washed with 1.0 M HCl, water, satd NaHCO3, brine.
The solution was dried over MgSO4 and reduced in
vacuo. The residue was purified by flash silica gel chroma-
We thank NSF (CHEM-0130958) and the University of
Pennsylvania for their support.
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20
oil (710 mg, 45% yield four steps): ½aꢁD ¼ ꢀ9:5 (c 0.8,
1
CHCl3); TLC (20% EtOAc–Hex), Rf = 0.20; H NMR
(CDCl3, 500 MHz) d 7.47–7.26 (m, 10H), 5.53 (d,
J = 8.2, 1H), 5.36 (dd, J = 4.9, 10.6, 1H), 5.06 (d,
J = 12.1, 1H), 4.97 (d, J = 12.2, 1H), 4.19–4.84 (m,
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3.31 (s, 3H), 3.01 (s, 3H), 2.17–2.09 (m, 1H), 1.80–1.62
(m, 2H), 1.53–1.36 (m, 1H), 1.03–0.86 (m, 14H), 0.04
(s, 9H); 13C NMR (CDCl3, 125 MHz) d 172.0, 171.6,
168.7, 155.9, 126.8, 136.2, 128.4, 128.3, 128.0, 127.9,
126.7, 126.6, 81.8, 66.9, 63.5, 59.8, 57.4, 54.5, 54.0,
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