C-Glycosylated Phenylalanine Synthesis by Palladium-Catalyzed Cross-Coupling Reactions

Article abstract of DOI:10.1055/s-2003-41416

A new and convergent synthesis of a C-glycosylated phenylalanine derivative using palladium-catalyzed Stille and Neghishi cross-coupling reactions is described. The coupling product constitutes a precursor of a natural glycosylated tyrosine mime.

Full text of DOI:10.1055/s-2003-41416

1834
LETTER
C-Glycosylated Phenylalanine Synthesis by Palladium-Catalyzed Cross-
Coupling Reactions
C-Glycosylated
P
h
a
enylalanine
l
Synth
é
esis rie Boucard, Karen Larrieu, Nadège Lubin-Germain,* Jacques Uziel, Jacques Augé*
UMR 8123 CNRS-UCP-ESCOM, 5 Mail Gay-Lussac, Neuville sur Oise, 95031 Cergy-Pontoise Cedex, France
Fax +33(1)34257067; E-mail: jacques.auge@chim.u-cergy.fr
Received 16 May 2003
methoxycarbonylsuccinimide (FmocOSu) leading to the
Abstract: A new and convergent synthesis of a C-glycosylated
phenylalanine derivative using palladium-catalyzed Stille and
Neghishi cross-coupling reactions is described. The coupling prod-
uct constitutes a precursor of a natural glycosylated tyrosine mime.
Fmoc-Phe-(4-I)-OHep (4) in 25% overall yield, due es-
sentially to the difficulty of the iodination step. Although
the class10 of compound 4 was previously prepared, in our
case this pathway allowed to obtain the compound 4 in
large scale.
Key words: tin, zinc, carbohydrates, amino acids, cross-coupling,
palladium
t-Bu
Si
OAc
a,b,c
O
The importance of glycoproteins in number of biological
processes as cell adhesion, cell differentiation or regula-
tion of cell growth is now well established.1 Almost all of
the naturally occurring glycosidic linkages can be divided
into N-glycosides, mostly attached to asparagine and
O-glycosides often attached to serine or threonine. How-
ever O-glycosylation of phenolic units is also frequently
encountered.2 For example a tyrosine residue was glyco-
sylated in glycopeptide antibiotics such as mannopepti-
mycin or vancomycin.3 In order to understand the
biological mechanisms it is always attractive to have C-
glycosylated aminoacids4 due to their stability. They can
be used as tools incorporated into more relevant frame-
works through supported peptidic synthesis.
O
t-Bu
O
AcO
AcO
O
TIPSO
1
2
Scheme
1
(a) K2CO3 (0.01 equiv), MeOH (99%); (b) (t-
Bu)2Si(OTf)2 (1 equiv), 2,6-lutidine (2.6 equiv), DMF, –20 °C to 0 °C
(64%); (c) TIPSCl (1.1 equiv), imidazole (1.7 equiv), DMF (75%).
NH2·TsOH
a, b
L-Phe
I
COO(CH2)6CH3
3
Few syntheses of C-glycosylated tyrosine or analogues
are reported.2c,5 We chose to use palladium-catalyzed
cross-coupling reactions in order to achieve the carbon-
carbon bond formation. We describe herein a convergent
synthesis of such compounds involving organometallic
species produced from the silylated glucal 2 and the pro-
tected iodophenylalanine 4.
c
NHFmoc
I
COO(CH2)6CH3m
4
Protective groups on both compounds were chosen in or-
der to be orthogonal and compatible with a future solid-
phase peptide synthesis. Sugar protection by silylated
groups was proved to be adequate with further C-1 depro-
tonation.6 In addition, the protection of the 4,6-position as
di-tert-butylsilylidene liberates the anomeric position for
sterical and conformational reasons.7 Thus, derivative 2
was prepared from tri-O-acetyl-D-glucal 1 according to
Scheme 1.
Scheme 2 (a) I2 (0.4 equiv), NaIO3 (0.2 equiv), H2SO4 (2.2 equiv),
HOAc, 70 °C (50%); (b) CH3(CH2)6OH (5 equiv), TsOH (1.5 equiv),
benzene, reflux (50%); (c) NaHCO3 then FmocOSu (0.83 equiv),
Na2CO3 (2 equiv), dioxane–water (98%).
The coupling of the sugar and amino acid moieties was
initially investigated using Stille conditions. The glucal 2
was thus transformed into the stannylated derivative 2¢a
(Z = SnBu3) by deprotonation with t-BuLi and then
quenching with tributyltin chloride. As the coupling reac-
tion of a stannylated glucal with simple aryl bromides had
been described,11 we tested some similar catalytic condi-
tions involving the iodophenylalanine derivative 4
(Scheme 3).
On the other hand, 4-iodophenylalanine derivative 4 was
prepared from L-phenylalanine (Scheme 2). Iodination
was performed as described by Schwabacher et al.8 and
the resulting product was then esterified with heptanol.9
Finally, the amino group was protected with 9-fluorenyl-
Synlett 2003, No. 12, Print: 29 09 2003. Web: 28 08 2003.
Art Id.1437-2096,E;2003,0,12,1834,1837,ftx,en;G10903ST.pdf.
DOI: 10.1055/s-2003-41416
In order to establish the best reaction conditions, we first
tested different solvents with Pd(PPh3)4 as the catalyst. As
it is shown in Table 1, toluene proved to be the best sol-
© Georg Thieme Verlag Stuttgart · New York

LETTER
C-Glycosylated Phenylalanine Synthesis
1835
t-Bu
Si
t-Bu
t-Bu
NHFmoc
COO(CH2)6CH3
Si
Si
O
O
t-Bu
O
t-Bu
t-Bu
O
O
O
O
TIPSO
O
O
TIPSO
+
I
OTIPS
Z
6
2'
4
Figure 1
Pd catalyst
using a bulky phosphine (entries 5 and 6) or triphenyl-
arsine (entries 7 and 8) allowed us to avoid the formation
of the dimer, but the coupled product 5 was obtained in
lower yields than in toluene.
t-Bu
Si
NHFmoc
COO(CH2)6CH3m
O
t-Bu
O
O
TIPSO
Then we decided to change the metal at glucal 2 choosing
to use an organozinc derivative in a Neghishi type cou-
pling reaction with iodophenylalanine derivative 4. For
this purpose 2¢b (Z = ZnCl) was formed in situ by depro-
tonation with t-BuLi and reaction with zinc dichloride and
directly engaged in the reaction with Fmoc-Phe-(4-I)-
OHep 4 in presence of a catalytic amount of palladium.
When PdCl2(PPh3)2 in the presence of DIBAL was used14
in THF at room temperature the coupled product 5 was
isolated in 30% yield. In order to increase this yield, the
reaction was performed in a 1/1 THF/DMA mixture.15 In
this case the coupling led to the unprotected compound 7
in 61% yield (Figure 2). More interestingly, the desired
product was obtained in 69% yield when a complex ob-
tained from Pd2dba3·CHCl3 and the bulky tri-o-tolylphos-
phine was used.16 It is noteworthy that in all these
experiments realized with the glucal zinc derivative no
dimer from the homocoupling of the sugar moiety was
detected.
5
Scheme 3
vent especially when a catalytic amount of copper iodide
is added in the medium. The use of copper salts favoring
the Sn/Cu transmetallation was often described as
efficient12 especially in the case of Stille type reactions in-
volving bulky stannanes.13 In our case the desired coupled
product 5 is obtained along with a byproduct correspond-
ing to the dimer 6 (Figure 1), which can be easily separat-
ed by flash chromatography (entry 2).
The use of a more polar solvent such as DMF or N-meth-
ylpyrrolidinone, which are known to favor this type of
couplings12 failed, leading exclusively to the formation of
6 (entries 9 and 10). Changing the source of palladium and
Table 1 Catalyzed Coupling Reaction between 2¢ and 4
Entry
1
Z
Catalysta
Additive
Solvent
Toluene
T (°C)
90
t (h)
24
16
48
48
16
16
16
16
16
16
16
5
5 Yield (%)b 6 Yield (%)
SnBu3
SnBu3
SnBu3
SnBu3
SnBu3
SnBu3
SnBu3
SnBu3
SnBu3
SnBu3
ZnCl
Pd(PPh3)4
38
50
38
27
24
35
29
35
0
0
2
Pd(PPh3)4
CuI 0.2 equiv Toluene
THF
90
42
24
45
0
3
Pd(PPh3)4
reflux
reflux
50
4
Pd(PPh3)4
LiCl 3 equiv
THF
5
Pd2(dba)3·CHCl3, (o-tol)3P
Pd2(dba)3·CHCl3, (o-tol)3P
Pd2(dba)3·CHCl3, Ph3As
Pd2(dba)3·CHCl3, Ph3As
Pd(PPh3)4
THF
6
Dioxane
THF
70
0
7
50
0
8
Dioxane
DMF
70
0
9
60
48
48
0
10
11
12
13
Pd(PPh3)4
NMP
60
0
PdCl2(PPh3)2, DIBAL
PdCl2(PPh3)2, DIBAL
Pd2(dba)3.CHCl3, (o-tol)3P
THF
20
30
61c
69
ZnCl
THF/DMA
THF
20
0
ZnCl
20
3
0
a 10 mol%.
b Yield after purification by flash chromatography.
c Yield of the deprotected product 7.
Synlett 2003, No. 12, 1834–1837 © Thieme Stuttgart · New York

1836
V. Boucard et al.
LETTER
The mixture was stirred at r.t. and the reaction was monitored by
TLC [Rf = 0.49 (HOAc/toluene 1/9)].
t-Bu
Si
NH2
COO(CH2)6CH3
O
t-Bu
O
The mixture was diluted in H2O (50 mL) and extracted with EtOAc
(3 × 50 mL). The combined organic layers were washed with brine
(50 mL), dried on Na2SO4 and evaporated. N-Fmoc-4-iodo-L-phe-
nylalanine heptyl ester 4 was obtained as a white solid (8.34 g,
86%), mp 54–55 °C. [a]D20 +22.8 (c 1.1, CHCl3). IR (KBr): 1690,
1726, 2924, 3321 cm–1. 1H NMR (250 MHz, CDCl3): d = 0.90 [t, 3
H, J = 6.4 Hz, CH3-(CH2)5], 1.30 [m, 8 H, (CH2)5], 1.60 [m, 2 H,
(CH2)5], 3.00 (m, 2 H, CH2-CH), 4.06–4.10 (m, 2 H, O-CH2 Fmoc),
4.13–4.22 (m, 1 H, H-9 Fmoc), 4.36–4.43 (m, 2 H, CH2-OCO), 4.5
(m, 1 H, CH2-CH), 5.30 (d, 1 H, NH, J = 7.8 Hz), 6.80 (d, 2 H, Har,
J = 8 Hz), 7.25–7.50 (m, 4 H, Har Fmoc), 7.55 (d, 2 H, J = 7 Hz, Har
Fmoc), 7.65 (d, 2 H, J = 8 Hz, Har), 7.70 (d, 2 H, J = 7.4 Hz, Har
Fmoc). 13C NMR (62.9 MHz, CDCl3): d = 14.0 [CH3-(CH2)5], 22.5,
25.7, 28.4, 28.8, 31.6 [(CH2)5], 37.8 (CH2-CH), 47.1 (CH C-9
Fmoc), 54.5 (CH2-CH), 65.8 (CH2-OCO), 66.8 (O-CH2 Fmoc),
92.5, 119.9, 127.7–127.0-125.0, 131.3, 135.4, 137.5, 141.3, 143.7
(Car), 155.4 (CON), 171.2 (COO). Anal. Calcd for C31H34NO4I: C,
60.89; H, 5.60; N, 2.29. Found: C, 60.94; H, 5.68; N, 2.31.
O
TIPSO
7
Figure 2
In conclusion, we have developed a convergent synthesis
of C-glycosylated aminoacids by carbon-carbon bond for-
mation according to a palladium catalyzed cross-coupling
procedure involving a stannyl or a zinc glucal and a phe-
nylalanine derivative. The functionalization of the cou-
pled product in order to obtain the C-glycosylated tyrosine
mime is under investigation in the laboratory.
Preparation of 4-Iodo-L-phenylalanine.
Preparation of (1,5-Anhydro-2-deoxy-4,6-tri-O-di-(tert-bu-
tyl)silanediyl-3-O-triisopropylsilyl-d-arabino-hex-1-enitolyl)-
N-(9-fluorenylmethoxycarbonyl)-l-phenylalanine Heptyl Ester
(5).
In a solution of L-phenylalanine (10 g, 60 mmol) dissolved in acetic
acid (55 mL) and concentrated sulfuric acid (7 mL), iodine (6.15 g,
24 mmol) and sodium iodate (2.54 g, 6.29 mmol) were added. The
mixture was stirred vigorously and heated at 70 °C during 21 h.
Then NaIO4 was introduced (2 × 0.25 g). At the end of the reaction,
the solution became orange. Acetic acid was removed in vacuo and
the crude was diluted in H2O (100 mL). The aqueous layer was ex-
tracted with Et2O (2 × 25 mL) and CH2CL2 (2 × 25 mL). The com-
bined organic layers were decolored by norite (1.25 g), then
filtrated. pH was increased to 5 with aq concd NaOH. The precipi-
tate was filtered, washed with H2O (200 mL) and EtOH (75 mL)
yielding 4-iodophenylalanine as a white solid, 9.4 g (53%), mp
255–265 °C, used in the next step without purification.
Tin-mediated Procedure: In a Schlenk tube, t-BuLi (1.7 M, 1.6
mL) was slowly added to a solution of the protected glucal 2 (300
mg, 0.68 mmol) in freshly distilled THF (2.7 mL) stirred at –78 °C.
The solution was then stirred during 30 min at 0 °C. Tributyltin
chloride (0.46 mL, 1.7 mmol) was added at –78 °C and the mixture
was stirred for 1 h at 0 °C. After hydrolysis at 0 °C, the aqueous
phase was extracted with Et2O. Combined organic extracts were
washed with H2O and brine, then dried over MgSO4. Purification by
flash chromatography (pure hexane) afforded the product 2¢a (260
mg, 52%). This compound was dissolved in anhydrous toluene (4
mL) and iodophenylalanine 4 (324 mg, 0.53 mmol) was added. This
mixture was then transferred by cannula into a solution of Pd(PPh3)4
(34 mg, 0.04 mmol) and CuI (13 mg, 0.07 mmol) in toluene (4 mL)
and heated at 90 °C for 16 h. After cooling at r.t. the solvent was re-
moved and the residue was purified by flash chromatography (pure
hexane) affording a mixture of 5 (164 mg, 50%) and 6 (130 mg,
42%).
Preparation of 4-Iodo-L-phenylalanine Heptyl Ester p-Toluene-
sulfonate (3).
A solution of 4-iodophenylalanine (7.5 g, 25.7 mmol) and p-tolue-
nesulfonic acid (7.3 g, 38.5 mmol) in benzene (125 mL) and hep-
tanol (18.7 mL) was refluxed until H2O was separated. The solution
was concentrated in vacuo and the product was crystallized in Et2O.
The filtration furnished the compound as a white solid, 7.817 g
Zinc-mediated Procedure: In a Schlenk tube, t-BuLi (1.7 M, 540
mL) was slowly added to a solution of the protected glucal 2 (216
mg, 0.49 mmol) in freshly distilled THF (2.7 mL) stirred at –78 °C.
The solution was then stirred during 30 min at r.t. In another
Schlenk tube, ZnCl2 (164 mg, 1.2 mmol) was heated under reduced
pressure during 15 min. Then, THF (2.7 mL) was added. This sec-
ond solution was transferred into the first one and was stirred 1 h at
r.t. In another Schlenk tube, a solution of the catalyst was prepared
by dissolving Pd2(dba)3·CHCl3 (14 mg, 0.014 mmol), tri-(o-
tolyl)phosphine (30 mg, 0.1 mmol) in THF (2.3 mL). A solution of
iodophenylalanine 4 (160 mg, 0.26 mmol) in THF (2.3 mL) was
slowly added at r.t. Finally, the solution of the organozinc com-
pound was transferred by cannula into the solution containing the
catalyst and the aryl iodide. The mixture was stirred during 3 h. The
mixture was diluted in brine (6 mL) and extracted with EtOAc
(3 × 10 mL). The combined organic layers were washed with brine
(6 mL), dried on Na2SO4 and evaporated under reduced pressure.
Purification by flash chromatography (cyclohexane–EtOAc 98/2)
afforded the product 5 (166 mg, 69%) mp 58–62 °C. [a]D20 +5.6 (c
1, CHCl3). Rf = 0.41 (cyclohexane/EtOAc 9/1). IR (KBr): 1732,
20
(53%), mp 118–121 °C. [a]D +19.1 (c 1, CHCl3). Rf = 0.92
(CH2Cl2/MeOH 8/2), ninhydrine. IR (KBr): 1744, 3015, 3403 cm–
1. 1H NMR (250 MHz, CDCl3): d = 0.75 [t, 3 H, CH3-(CH2)5], 1.01–
1.27 [m, 10 H, (CH2)5], 2.3 (s, 3 H, CH3-Ph), 2.90–3.00 (m, 1 H,
CH2-CH), 3.10 (m, 1 H, CH2-CH), 3.80 (m, 2 H, CH2-O), 4.20 (s, 1
H, CH2-CH), 6.80 (d, 2 H, Har, J = 10 Hz), 7.10 (d, 2 H, Har J = 7.5
Hz), 7.40 (d, 2 H, Har, J = 7.5 Hz), 7.60 (d, 2 H, Har, J = 10 Hz),
8.20 (brs, 3 H, NH3). 13C NMR (62.9 MHz, CDCl3): d = 14 [CH3-
(CH2)5], 21.3 (CH3-Ph), 22.6, 25.5, 28.0, 28.8, 31.6 [(CH2)5], 35.7
(CH2-CH). 53.8 (CH2-CH), 66.3 (CH2-O), 92.0, 126.0, 128.9,
131.2, 134.1, 136.9, 137.4, 137.5, 141.2 (Car), 174.8 (CO).
Preparation of N-Fmoc-4-iodo-L-phenylalanine Heptyl Ester
(4).
A solution of 4-iodo-L-phenylalanine heptyl ester, TsOH 3 (7.8 g,
13.5 mmol) in CH2Cl2 (100 mL) was washed with 10% NaHCO3
(2 × 100 mL). The organic layer was dried on Na2SO4. After filtra-
tion and evaporation under reduced pressure, 4-iodo-L-phenylala-
nine heptyl ester was obtained as a colorless syrup (6.33 g, 100%).
Rf = 0.85 (CH2Cl2/MeOH 8/2).
1
2933, 3356 cm–1. H NMR (250 MHz, CDCl3): d = 0.80–1.40 [m,
4-Iodo-L-phenylalanine heptyl ester (6.16 g, 16 mmol) was added in
a 10% Na2CO3 solution cooled at 0 °C. The mixture was stirred dur-
ing 15 min and a solution of 9-fluorenylmethoxycarbonyl succinim-
ide (4.512 g, 13.4 mmol) in dioxane (37.5 mL) was added dropwise.
52 H, CH3-(CH2)5, (CH3)3C, (CH3)2CH)], 3.10 (m, 2 H, CH2-CH),
4.00–4.18 (7 H, m, H4, H5, H6,6¢, O-CH2 Fmoc, H-9 Fmoc), 4.20–
4.30 [m, 1 H, O-CHH-(CH2)4], 4.27–4.40 [m, 1 H, O-CHH-(CH2)4],
4.60 (dd, 1 H, H3, J = 2.3 Hz, J = 6.5 Hz), 4.60–4.80 (m, 1 H, CH2-
Synlett 2003, No. 12, 1834–1837 © Thieme Stuttgart · New York

LETTER
C-Glycosylated Phenylalanine Synthesis
1837
CH), 5.20 (d, 1 H, H2, J = 2.3 Hz), 5.30 (d, 1 H, NH, J = 8 Hz), 7.00
(d, 2 H, Har, J = 8 Hz), 7.20–7.4 (m, 6 H, Har Fmoc), 7.50 (d, 2 H,
Har, J = 8 Hz), 7.70 (d, 2 H, J = 7.4 Hz, Har Fmoc). 13C NMR (62.9
MHz, CDCl3): d = 12.4 [(CH3)2CH], 13.9 [CH3-(CH2)5], 18.0
[(CH3)2CH], 19.7 [(CH3)3C], 22.4 (CH2-CH3), 22.6 [(CH3)3C], 25.7
[(CH2)5], 26.8–27.4 [(CH3)3C], 28.3, 28.7, 31.5 [(CH2)5], 37.9 (Ph-
CH2-CH), 47.0 (CH C-9 Fmoc), 54.6 (Ph-CH2-CH), 65.6 (C6), 66
(CH2-OCO), 66.8 (O-CH2 Fmoc), 71.6 (C5), 72.8 (C4), 77.4 (C3),
101.0 (C2), 119.9, 124.9, 125.0, 126.9, 127.6, 129.0, 132.0, 136.0,
141.0, 143.0 (Car), 150.5 (C1), 155.4 (CON), 171.4 (COO). Anal.
Calcd for C54H79NO8Si2: C, 70.01; H, 8.60; N, 1.51. Found: C,
70.02; H, 8.56; N, 1.48.
Homocoupling product 6: 1H NMR (250 MHz, CDCl3): d = 0.90–
1.10 [m, 78 H, (CH3)3C, (CH3)2CH], 3.92 (2 H, ddd, H5¢ J = 2.5 Hz,
J = 4.9 Hz, J = 9.9 Hz), 4.00–4.15 (4 H, m, H4, H6), 4.25 (2 H, dd,
H6, J = 10.5 Hz), 4.50 (2 H, dd, H3, J = 6.6 Hz, J = 2.4 Hz), 5.17 (2
H, d, H2). 13C NMR (62.9 MHz, CDCl3): d = 12.5 [(CH3)2CH], 18.2
[(CH3)2CH], 19.8 [(CH3)3C], 22.7 [(CH3)3C], 26.9 [(CH3)3C], 29.6
[(CH3)3C], 66.1 (C6), 71.7 (C5), 72.9 (C4), 76.5 (C3), 101.2 (C2),
151.0 (C1).
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Synlett 2003, No. 12, 1834–1837 © Thieme Stuttgart · New York