Allyl deprotection of galacturonic acid derivatives: mechanistic aspects of mercuric-catalyzed prop-1-enyl acetal cleavage

Article abstract of DOI:10.1016/j.carres.2007.08.019

Different deallylation methods were assayed for selective deprotection of allyl galactopyranosiduronic acid derivatives. A two-step procedure using DABCO and (Ph₃P)₃RhCl followed by mercuric-assisted cleavage gave quantitative yields. Reaction in the presence of [¹⁸O]water allowed us to obtain evidence about the mechanism of prop-1-enyl cleavage.

Full text of DOI:10.1016/j.carres.2007.08.019

Available online at www.sciencedirect.com
Carbohydrate Research 342 (2007) 2635–2640
Note
Allyl deprotection of galacturonic acid derivatives:
mechanistic aspects of mercuric-catalyzed prop-1-enyl acetal cleavage
*
´
Maximilien Barbier, Eric Grand and Jose Kovensky
Laboratoire des Glucides UMR CNRS 6219, Universite´ de Picardie Jules Verne, 33 Rue Saint-Leu, 80039 Amiens, France
Received 3 May 2007; received in revised form 17 August 2007; accepted 19 August 2007
Available online 31 August 2007
Abstract—Different deallylation methods were assayed for selective deprotection of allyl galactopyranosiduronic acid derivatives. A
two-step procedure using DABCO and (Ph3P)3RhCl followed by mercuric-assisted cleavage gave quantitative yields. Reaction in the
presence of [18O]water allowed us to obtain evidence about the mechanism of prop-1-enyl cleavage.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Allyl deprotection; Rhodium complex; Mechanism; Galacturonic acid
Several methods for deprotection of allyl glycosides are
described1,2 that tolerate a diversity of functional
groups. Nonetheless, cleavage of allyl protection of
polyfunctional allyl protected carbohydrate acetals is
still problematic. Selective cleavage3 using PdII has been
previously used on oligosaccharides bearing different
protecting groups such as esters, acids, amines, amides,
phosphates and aromatic substituents.4–6 Compound 1
was reacted in the conditions previously described4
(PdCl2/AcOH/AcONa/H2O). After 24 h, mass spec-
trometry analysis (ESI+) showed the presence of start-
ing material 1 at [M+Na]+ = 571, and the desired
product 2 at [M+Na]+ = 531. A third unexpected
compound was detected at [M+Na]+ = 587. The differ-
ence of +16 mass units compared to the starting com-
pound 1 suggested the presence of oxidation product 3
(Scheme 1).
All attempts to achieve chromatographic separation
of compounds 2 and 3 were unsuccessful, and the mix-
ture was analyzed by NMR. The typical allyl reso-
nances, a dddd for H20 at d = 5.9 ppm, and two ddt
1
for H30 at d = 5.3 ppm in the H NMR spectrum, and
at d = 133.5 ppm (C20) and d = 118.1 ppm (C30) in the
13C NMR spectrum, were completely absent. Instead,
new resonances were observed at 2.12 ppm and
205.6 ppm, respectively, confirming the formation of
ketone 3. From the NMR spectra of the mixture, 2/3
ratio was estimated as 60/40. As by-product 3 arises
from a Wacker-type oxidation of the double bond, the
presence of an oxidant in the reaction mixture was sus-
pected. The reaction was repeated using degassed water
(by bubbling He), but the same result was obtained.
As the problem could be related to the substrate struc-
ture, and compound 1 required multistep synthesis,
PMBO
PMBO
BnO
PMBO
BnO
CO Me
2
CO Me
2
CO Me
2
O
O
O
PdCl2, H2O
BnO
+
O
AcOH, AcONa
24 h
BnO
BnO
BnO
OH
O
O
1
3
2
Scheme 1.
*
Corresponding author. Tel.: +33 322827567; fax: +33 322827568; e-mail: jose.kovensky@u-picardie.fr
0008-6215/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2007.08.019

2636
M. Barbier et al. / Carbohydrate Research 342 (2007) 2635–2640
OAc
OAc
OAc
AcO
AcO
AcO
AcO
AcO
AcO
O
O
O
+
O
AcO
AcO
AcO
OH
O
O
4
6
5
Scheme 2.
different conditions were tested on a simpler starting
material, allyl 2,3,4,6-tetra-O-acetyl-a-D-galactopyrano-
side7 (4). Firstly, the Pd(II) reaction was performed on 4
in the same conditions. Again, the expected product 5
was obtained, together with the oxidized compound 6
(Scheme 2), which was the major product as estimated
from NMR spectra of the 5/6 mixture.
Different modifications of the reaction were assayed.
The results are summarized in Table 1. Changing the
ligands of the Pd(II), by replacing PdCl2 by Pd(OAc)2
(entry 2), did not make any difference. In some exam-
ples5c the reaction was performed in methanol, in our
hands the 5/6 ratio was not improved, either using
Pd(OAc)2 or PdCl2 (entries 3 and 4). When a non-protic
solvent such as dichloromethane was used (entry 5), no
reaction occurred. A bulky alcohol (entry 6) or an amine
(entry 7) did not work. A method using a catalytic
amount of PdCl2, which is regenerated by CuI, which
in turn is obtained by bubbling oxygen into the solu-
tion,8 was assayed (entry 8) with similar results.
Alternative methods have been reported. Pd/C
seemed to be only effective on O-allyl phenols.9 Reac-
tions using DDQ10 or NBS11 are not applicable to our
original substrate 1 bearing p-methoxybenzyl groups.
On the other hand, the use of organomagnesians12 is
precluded for uronic acids derivatives, due to the alka-
line hydrolysis of the ester function, and the acidity of
the H-5 that could lead to epimerisation. A reported
method13 for allyl cleavage, using NaBH4 and I2, was
tested (entry 9), but no reaction was observed. Finally,
a direct allyl deprotection of 4 was assayed in the pres-
ence of ZnCl2, Pd(PPh3)4 and Bu3SnH14 (entry 10). Here
a reductive medium is used, and as expected, the
oxidized product was not detected; however, the reac-
tion did not go to completion.
Two-step procedures involve an initial isomerisation
of allyl into prop-1-enyl glycoside, followed by a smooth
cleavage of the last one by mercuric compounds.15 The
isomerisation can be performed by a diversity of
reagents. The most often employed is tBuOK,16 but this
strong base is not adapted for uronic acid derivatives.
Milder methods employing iridium17 or rhodium18 com-
plex have been described. In both cases, catalytic
amounts of reagents are used. Cycloocta-1,5-diene-
bis[methyldiphenylphosphine]iridium hexafluorophos-
phate was tested (entry 11), but after 16 h no evolution
was observed. Finally, compound 4 was reacted with
DABCO and (Ph3P)3RhCl (entry 12). After 48 h only
the propenyl compound was present. Moreover, the
reaction can be easily followed by the colour change
of the reaction mixture, from red to colourless. The
crude propenyl was treated with HgO and HgCl2 in
acetone–water, leading to the desired compound 6 within
25 min. These conditions were applied to galacturonic
acid derivatives 1 and 7 (Scheme 3). After the isomerisa-
tion in the presence of rhodium catalyst (16 h) and mer-
curic-assisted propenyl cleavage, compounds 2 and 8
were obtained in 100% yield.
Compounds 2 and 8 could then be conveniently acti-
vated as glycosyl donors as, for example, trichloroace-
timidates or bromides. Considering the nature of the
intermediate prop-1-enyl glycoside (I), the use of depro-
tection conditions to obtain disaccharides directly was
investigated. In the cleavage step, the anomeric hydr-
oxyl group is regenerated with water in the presence
of mercuric ions, and water might be replaced by a
Table 1. Deallylation of compound 4
Entry
Reaction conditions
Conversion (%)
5/6 Ratio
1
2
3
4
5
6
7
8
9
PdCl2, AcONa, AcOH, H2O
Pd(OAc)2, AcONa, AcOH, H2O
PdCl2, MeOH
Pd(OAc)2, MeOH
PdCl2, CH2Cl2
91
90
83
88
0
0
0
92
0
55
0
38/62
42/58
41/59
33/67
No reaction
No reaction
No reaction
39/61
No reaction
100/0
PdCl2, tBuOH
PdCl2, BuNH2
PdCl2, CuI, DMF, H2O, O2
NaBH4, I2
Pd(PPh3)4, ZnCl2, Bu3SnH, THF
(i) Ir complex; (ii) HgO, HgCl2
(i) Rh complex, DABCO; (ii) HgO, HgCl2
10
11
12
No reaction
100/0
100

M. Barbier et al. / Carbohydrate Research 342 (2007) 2635–2640
2637
PMBO
PMBO
BnO
CO2Me
CO2Me
O
O
(i)
BnO
100%
BnO
BnO
OH
OH
OAll
1
2
PMBO
BnO
PMBO
BnO
CO2Bn
CO2Bn
O
O
(i)
100%
BnO
7
BnO
OAll
8
Scheme 3. Reagents and conditions: (i) (a) (Ph3P)3RhCl, DABCO, EtOH, PhMe, H2O, 16 h; (b) HgCl2, HgO, acetone, H2O, 1 h 30 min.
suitable protected sugar derivative having a free hydr-
oxyl group.
water molecule attacks the anomeric carbon atom (route
1), the isotopic labelling will be found at the anomeric
position, and this is the only mechanism allowing a
glycosylation reaction to be envisaged.21 On the other
hand, the nucleophile attack may occur at the sp2 car-
bon atom linked to the oxygen (route 2) or at the other
unsaturated carbon atom (route 3).
ESI-MS analysis of the reaction mixture indicated
that no 18O was incorporated at the anomeric position
of the galacturonic acid derivative, only a peak at m/z
607 (and not at m/z 609) was detected. This finding sug-
gested that the glycosidic C–O bond is not cleaved dur-
ing the reaction, the mechanism must follow either route
2 or route 3.
Substituted isopropenyl glycosides (II–IV) have been
previously activated in the presence of electrophiles as
glycosylation donors.19 NIS/triflic acid, TMSOTf and
BF3ÆEt2O in acetonitrile are the systems typically used
for activationl; however, only a few examples gave both
very good yields and a/b selectivity.
O
O
O
R
R
R
O
R
I
II
III
IV
Therefore, compound 7 was isomerised as before in
the presence of Wilkinson catalyst, then the propenyl
compound was quickly purified by flash chromatogra-
phy and reacted in the conditions of propenyl cleavage
(HgCl2, HgO, acetone) with 1,2:3,4-di-O-isopropylid-
ene-a-D-galactopyranose. Unfortunately, no glycosyla-
tion was observed, and the addition of TMSOTf did
not make any difference. In view of these disappointing
results, the mechanistic pathway of the reaction was fur-
ther investigated.
In the case of isopropenyl glycosides, it is clear that
the nucleophile goes to the carbon atom linked to the
oxygen which can better stabilize the positive charge.
However, in our case this position is not substituted,
and the possibility of introduction of the water molecule
at the b-position must also be considered. As shown in
Scheme 5, the incorporation of [18O]water at C-1 of
the alkenyl moiety would result in the formation of
[18O]propanal, while the final product of C-2 attack
would be [18O]acetone.
Chenault and Chafin20 obtained spectroscopic evi-
dence on the mechanism of acid hydrolysis of isopro-
penyl glucopyranosides. Using [18O]water, they could
prove unequivocally that during hydrolysis, the vinyl
ether C–O bond is cleaved, and not the glycosidic C–O
bond (Scheme 4).
Therefore, the reaction of prop-1-enyl glycoside 70
was performed in the same conditions (HgO, HgCl2,
acetone) but using 95% 18OH2. The possible mechanisms
are depicted in Scheme 5. If, as previously expected, the
To prove unequivocally the position of water incorpo-
ration, the solvent was distilled from the reaction mixture
and analyzed by NMR spectroscopy. Changes in 13C
NMR chemical shifts upon isotopic substitution with
18O have been well documented.18,22 In the 13C NMR
spectrum (solvent CDCl3) of the distilled solvent, no sig-
nal arising from propanal was detected. A resonance at d
207.43 corresponding to acetone C@O was observed, and
a second resonance shifted 0.05 ppm upfield appeared at
d 207.38, and can be assigned to the [18O]acetone pro-
OH
OH
H218O
Chloroacetate buffer
O
O
HO
HO
HO
HO
+
OH
18O
OH
OH
O
Scheme 4.

2638
M. Barbier et al. / Carbohydrate Research 342 (2007) 2635–2640
H
18O
PMBO
BnO
PMBO
CO2Bn
CO2Bn
O
O
H
H
H
HO
O
Route 1
Route 2
Route 3
+
BnO
18OH
BnO
BnO
[M+Na]+ = 609
O
O
O
7'
Hg
H
18O
PMBO
BnO
PMBO
CO2Bn
CO2Bn
O
O
H18
O
+
BnO
BnO
BnO
OH
[M+Na]+ = 607
18O
Hg
H
18O
PMBO
BnO
PMBO
CO2Bn
CO2Bn
18OH
O
O
+
BnO
BnO
BnO
[M+Na]+ = 607
OH
18O
Hg
Scheme 5.
duced by propenyl cleavage. As a consequence, the water
molecule attacks the C-2 of the alkenyl moiety.
4.92 mmol), KHCO3 (3.10 g, 31.0 mmol) and Bu4NI
(182 mg, 0.49 mmol) in anhydrous DMF (180 mL),
MeI (1.62 mL, 26.1 mmol) was added under argon.
After stirring for 14 h at rt, the solvent was evaporated
under diminished pressure. The residue was dissolved
in Et2O, and the solution was washed with brine. The
organic layer was separated, dried (Na2SO4), filtered
and the solvent evaporated. Flash chromatography on
silica gel (7:3 hexane–EtOAc) afforded pure 1 (2.46 g,
In conclusion, allyl deprotection of galacturonic acid
derivatives was performed by a two-step procedure
involving rhodium-assisted double bond isomerisation
followed by mercuric-catalyzed prop-1-enyl cleavage.
Furthermore, during the second step the vinyl ether
C–O bond is cleaved, and not the glycosidic C–O bond.
The lack of substitution at C-1 of the alkenyl moiety
makes the b carbon atom more electrophilic.
29
91% yield) as a colourless gum: ½aꢁD +34 (c 0.27,
1
CHCl3); H NMR (CDCl3, 300 MHz): d 7.47–7.28 (m,
10H, HAr–Bn), 7.20 (d, 2H, J0 8.6 Hz, HAr–PMB),
6.81 (d, 2H, J0 8.6 Hz, HAr–PMB), 5.93 (dddd, 1H,
1. Experimental
0
0
0
0
0
0
0
0
J2 ;3 trans 17.0 Hz, J2 ;3 cis 10.4 Hz, J1 a;2 5.2 Hz, J1 b;2
0
0
1.1. General methods
6.4 Hz, H-20), 5.34 (dq, 1H, J2 ;3 trans 17.0 Hz, Jgem
1.5 Hz, J1 ;3 a 1.5 Hz, H-30a), 5.23 (ddt, 1H, J2 ;3 cis
0
0
0
0
10.4 Hz, Jgem 1.5 Hz, J1 ;3 b 1.2 Hz, H-30b), 5.02 (d, 1H,
J1,2 3.4 Hz, H-1), 4.89 (d, 1H, Jgem 11.7 Hz, CHHPh),
4.88 (d, 1H, Jgem 11.4 Hz, CHHPMP), 4.84 (d, 1H, Jgem
12.1 Hz, CHHPh), 4.78 (d, 1H, Jgem 11.7 Hz, CHHPh),
4.68 (d, 1H, Jgem 12.1 Hz, CHHPh), 4.56 (d, 1H, Jgem
11.4 Hz, CHHPMP), 4.45 (d, 1H, J4,5 1.5 Hz, H-5),
4.32 (dd, 1H, J3,4 2.7 Hz, J4,5 1.5 Hz, H-4), 4.19 (dddd,
0
0
Melting points were determined on a Buchi 535. Optical
¨
rotations were measured with a Perkin Elmer Model 343
polarimeter using a sodium lamp at 20 ꢁC. [a]D values
are given in 10ꢀ1 deg cm2 gꢀ1. High-resolution electro-
spray mass spectra in the positive ion mode were ob-
tained on a Q-TOF Ultima Global hybrid quadrupole/
time-of-flight instrument (Waters-Micromass, Manches-
ter, UK), equipped with a pneumatically-assisted elec-
trospray (Z-spray) ion source and an additional
0
0
0
0
0
0
1H, Jgem 13.0 Hz, J1 a;2 5.2 Hz, J1 a;3 a 1.5 Hz, J1 a;3 b
1.2 Hz, H-10a), 4.13 (dd, 1H, J1,2 3.4 Hz, J2,3 10.4 Hz,
1
0
0
sprayer (Lock Spray) for the reference compound. H
H-2), 4.06 (dddd, 1H, Jgem 13.0 Hz, J1 b;2 6.4 Hz,
and 13C NMR spectra were recorded with a Bruker
AC 300 spectrometer.
J1 b;3 a 1.5 Hz, J1 b;3 b 1.2 Hz, H-10b), 4.05 (dd, 1H, J2,3
10.4 Hz, J3,4 2.7 Hz, H-3), 3.80 (s, 3H, Ar–OCH3),
3.68 (s, 3H, CO2CH3). 13C NMR (CDCl3, 75.5 MHz):
d 169.2 (C-6), 159.0 (Ci–OMe), 138.5 (Ci–CH2Bn),
138.3 (Ci–CH2Bn), 133.5 (C-20), 130.4 (Ci–CH2PMB),
129.5 (CAr–PMB), 128.3–127.3 (CAr–Bn), 118.1 (C-30),
113.4 (CAr–PMB), 96.6 (C-1), 78.2 (C-3), 76.1 (C-4),
75.7 (C-2), 74.2 (CH2PMP), 73.4 (CH2Ph), 73.3
0
0
0
0
1.2. Methyl (allyl 2,3-di-O-benzyl-4-O-p-methoxybenzyl-
a-D-galactopyranosid)uronate (1)
To a solution of allyl 2,3-di-O-benzyl-4-O-p-methoxy-
benzyl-a-D-galactopyranosiduronic
acid23
(2.63 g,

M. Barbier et al. / Carbohydrate Research 342 (2007) 2635–2640
2639
(CH2Ph), 70.8 (C-5), 68.7 (C-10), 55.1 (Ar–OCH3), 52.1
(CO2CH3); HRESIMS (m/z): [M+Na]+ calcd for
C32H36O8Na, 571.2308; found, 571.2330.
1, but replacing MeI by benzyl bromide. Flash chroma-
tography on silica gel (9:1 hexane–EtOAc) afforded pure
29
7 (2.31 g, 92% yield) as a colourless gum: ½aꢁD +17 (c
0.25, CHCl3). 1H NMR (CDCl3, 300 MHz): d 7.45–
1.3. Methyl 2,3-di-O-benzyl-4-O-p-methoxybenzyl-D-
galactopyranuronate (2)
7.28 (m, 15H, HAr–Bn), 7.15 (d, 2H, J0 8.7 Hz, HAr
PMB), 6.73 (d, 2H, J0 8.7 Hz, HAr–PMB), 5.94 (dddd,
–
0
0
0
0
0
0
1H, J2 ;3 trans 17.0 Hz, J2 ;3 cis 10.3 Hz, J1 a;2 5.2 Hz,
0
0
0
0
A
solution of compound
1
(1.01 g, 1.82 mmol),
J1 b;2 6.5 Hz, H-20), 5.35 (dq, 1H, J2 ;3 trans 17.0 Hz, Jgem
0
0
0
0
(Ph3P)3RhCl (138 mg, 0.15 mmol) and DABCO
(42 mg, 0.36 mmol) in 7:3:1 EtOH–toluene–H2O
(110 mL) was stirred at reflux for 16 h. After filtration
through Celite and solvent evaporation, the crude pro-
penyl derivative was dissolved in 9:1 acetone–H2O
(25 mL). HgCl2 (1.52 g, 5.52 mmol) and red HgO
(40 mg, 0.18 mmol) were added. After stirring for 1 h
30 min at rt, acetone was evaporated and the residue
was dissolved in EtOAc, and the solution washed with
brine. The organic layer was separated, dried (Na2SO4),
filtered and the solvent evaporated. Flash chromatogra-
phy on silica gel (2:3 hexane–EtOAc) afforded pure 2
(936 mg, 100% yield, a/b ratio 64:36) as a colourless
gum. 1H NMR (CDCl3, 300 MHz): d 7.86 (m, 10H,
HAr–Bn), 7.19 (m, 2H, HAr–PMB), 6.88 (m, 2H, HAr–
PMB), 5.44 (br s, 0.6H, H-1a), 5.00 (d, 0.4H, Jgem
10.6 Hz, CHHAr), 4.88 (d, 0.6H, Jgem 11.5 Hz,
CHHAr), 4.86 (d, 0.8H, Jgem 11.6 Hz, CHHAr), 4.85
(d, 0.7H, Jgem 12.4 Hz, CHHAr), 4.83 (d, 0.8H, Jgem
10.6 Hz, CHHAr), 4.78 (d, 0.5H, Jgem 11.6 Hz,
CHHAr), 4.76 (d, 0.4H, Jgem 12.2 Hz, CH2Ar), 4.75
(d, 0.6H, Jgem 12.4 Hz, CHHAr), 4.69 (d, 0.4H, J1b,2b
6.9 Hz, H-1b), 4.66 (br s, 0.6H, H-5a), 4.60 (d, 1.2H,
Jgem 11.5 Hz, CHHAr), 4.36 (m, 0.6H, H-4a), 4.26 (m,
0.4H, H-4b), 4.13 (dd, 0.6H, J1a,2a 3.3 Hz, J2a,3a
9.9 Hz, H-2a), 4.07 (m, 0.4H, H-5b), 4.05 (d, 0.6H,
J2a,3a 9.9 Hz, J3a,4a 2.3 Hz, H-3a), 3.95 (br s, 1H, OH),
3.86 (d, 0.4H, J1b,2b 6.9 Hz, J2b,3b 9.5 Hz, H-2b), 3.82
(s, 1.8H, OCH3PMB a), 3.80 (s, 1.2H, OCH3PMB b),
3.69 (s, 1.8H, CO2CH3 b), 3.68 (s, 1.2H, CO2CH3 a),
3.61 (dd, 0.4H, J2b,3b 9.5 Hz, J3b,4b 2.8 Hz, H-3b). 13C
NMR (CDCl3, 75.5 MHz): d 169.4 (C-6a), 168.7 (C-
6b), 159.0 (Ci–OMe), 138.3 and 138.0 (Ci–CH2Bn),
130.3 and 130.0 (Ci–CH2PMB), 129.7 and 129.6 (CAr–
PMB), 128.3–127.4 (CAr–Bn), 113.1 (CAr–PMB), 97.5
(C-1b), 91.8 (C-1a), 81.2 (C-3b), 80.0 (C-2b), 77.9 (C-
3a), 75.7 (C-2a), 75.5 (C-4a), 74.9 (CH2Ph), 74.5 (C-
4b), 74.1 (CH2Ph), 74.0 (CH2Ph), 73.8 (C-5b), 73.3
(CH2Ph), 72.9 (CH2Ph), 70.5 (C-5a), 55.1 (OCH3PMB),
52.3 (CO2CH3 b), 52.1 (CO2CH3 a); HRESIMS (m/z):
[M+Na]+ calcd for C29H32O8Na, 531.1995; found,
531.1991.
1.5 Hz, J1 ;3 a 1.5 Hz, H-30a), 5.24 (ddt, 1H, J2 ;3 cis
10.3 Hz, Jgem 1.5 Hz, J1 ;3 b 1.1 Hz, H-30b), 5.20 (d, 1H,
Jgem 12.1 Hz, CO2CHHPh), 5.05 (d, 1H, J1,2 3.3 Hz,
H-1), 5.02 (d, 1H, Jgem 12.1 Hz, CO2CHHPh), 4.90 (d,
1H, Jgem 11.7 Hz, CHHPh), 4.85 (d, 1H, Jgem 11.2 Hz,
CHHPMP), 4.84 (d, 1H, Jgem 12.0 Hz, CHHPh), 4.77
(d, 1H, Jgem 11.7 Hz, CHHPh), 4.69 (d, 1H, Jgem
12.0 Hz, CHHPh), 4.50 (d, 1H, J4,5 1.6 Hz, H-5), 4.48
(d, 1H, Jgem 11.2 Hz, CHHPMP), 4.33 (dd, 1H, J3,4
2.7 Hz, J4,5 1.6 Hz, H-4), 4.21 (dddd, 1H, Jgem
0
0
0
0
0
0
0
0
13.1 Hz, J1 a;2 5.2 Hz, J1 a;3 a 1.5 Hz, J1 a;3 b 1.1 Hz, H-
10a), 4.16 (dd, 1H, J1,2 3.4 Hz, J2,3 10.2 Hz, H-2), 4.08
0
0
0
0
(dddd, 1H, Jgem 13.1 Hz, J1 b;2 6.5 Hz, J1 b;3 a 1.5 Hz,
J1 b;3 b 1.1 Hz, H-10b), 4.06 (dd, 1H, J2,3 10.2 Hz, J3,4
2.7 Hz, H-3), 3.79 (s, 3H, OMe). 13C NMR (CDCl3,
75.5 MHz): d 168.4 (C-6), 158.9 (Ci–OMe), 138.5 (Ci–
CH2Bn), 138.3 (Ci–CH2Bn), 135.1 (Ci–CH2CO2Bn),
133.5 (C-20), 130.5 (Ci–CH2PMB), 129.3 (CAr–PMB),
128.4–127.3 (CAr–Bn), 118.1 (C-30), 113.4 (CAr–PMB),
96.6 (C-1), 78.3 (C-3), 76.0 (C-4), 75.7 (C-2), 74.0
(CH2PMP), 73.4 (CH2Ph), 73.3 (CH2Ph), 70.7 (C-5),
68.7 (C-10), 66.8 (CO2CH2Ph), 55.1 (OMe); HRESIMS
(m/z): [M+Na]+ calcd for C38H40O8Na, 647.2621;
found, 647.2618.
0
0
1.5. Benzyl 2,3-di-O-benzyl-4-O-p-methoxybenzyl-D-
galactopyranuronate (8)
Compound 8 was prepared from 7 (957 mg, 1.53 mmol)
as for compound 2. Flash chromatography on silica gel
(2:3 hexane–EtOAc) afforded pure 8 (896 mg, 100%
1
yield, a/b ratio 78:22) as a colourless gum. H NMR
(CDCl3, 300 MHz): d 7.36 (m, 15H, HAr–Bn), 7.15 (m,
2H, HAr–PMB), 6.82 (m, 2H, HAr–PMB), 5.46 (dd,
0.8H, J1a,OH 3.0 Hz, J1a,2a 3.4 Hz, H-1a), 5.21 (d,
0.8H, Jgem 12.1 Hz, CO2CHHPh a), 5.03 (d, 0.8H, Jgem
12.1 Hz, CO2CHHPh a), 4.99 (d, 0.8H, Jgem 11.9 Hz,
CHHAr a), 4.98 (d, 0.2H, Jgem 10.9 Hz, CO2CHHPh
b), 4.93 (d, 0.2H, Jgem 10.9 Hz, CO2CHHPh b), 4.86
(d, 0.2H, Jgem 10.0 Hz, CHHAr b), 4.84 (d, 0.8H, Jgem
12.8 Hz, CHHAr a), 4.83 (d, 0.8H, Jgem 11.0 Hz,
CHHAr a), 4.82 (d, 0.8H, Jgem 11.9 Hz, CHHAr a),
4.80 (d, 0.2H, Jgem 10.6 Hz, CHHAr b), 4.79 (d, 0.2H,
Jgem 10.0 Hz, CHHAr b), 4.73 (d, 0.8H, Jgem 12.8 Hz,
CHHAr a), 4.72 (d, 0.2H, Jgem 11.8 Hz, CHHAr b),
4.68 (d, 1H, J4a,5a 1.4 Hz, H-1b-5a), 4.63 (d, 0.2H, Jgem
11.8 Hz, CHHAr b), 4.50 (d, 0.8H, Jgem 11.0 Hz,
CHHAr a), 4.46 (d, 0.2H, Jgem 10.6 Hz, CHHAr b),
1.4. Benzyl (allyl 2,3-di-O-benzyl-4-O-p-methoxybenzyl-
a-D-galactopyranosid)uronate (7)
Compound 7 was prepared from the corresponding
galacturonic acid (2.15 g, 4.02 mmol) as for compound

2640
M. Barbier et al. / Carbohydrate Research 342 (2007) 2635–2640
Res. 2002, 337, 1367–1371; (c) Zeng, Y.; Zhang, J.; Ning,
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hydr. Chem. 2000, 19, 891–921.
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9. Ishizaki, M.; Yamada, M.; Watanabe, S.; Hoshino, O.;
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Tetrahedron 2004, 60, 7973–7981.
4.35 (dd, 0.8H, J3a,4a 2.6 Hz, J4a,5a 1.4 Hz, H-4a), 4.29
(m, 0.2H, OH b), 4.25 (m, 0.2H, H-4b), 4.13 (dd,
0.8H, J1a,2a 3.4 Hz, J2a,3a 9.9 Hz, H-2a), 4.09 (d, 0.2H,
J4b,5b 1.2 Hz, H-5b), 4.04 (dd, 0.8H, J2a,3a 9.9 Hz,
J3a,4a 2.6 Hz, H-3a), 3.87 (dd, 0.2H, J1b,2b 7.5 Hz,
J2b,3b 9.5 Hz, H-2b), 3.80 (s, 2.4H, OCH3PMB a), 3.78
(s, 0.6H, OCH3PMB b), 3.69 (d, 0.8H, J1a,OH 3.0 Hz,
OH a), 3.61 (dd, 0.2H, J2b,3b 9.5 Hz, J3b,4b 2.9 Hz, H-
3b). 13C NMR (CDCl3, 75.5 MHz): d 168.6 (C-6a),
167.8 (C-6b), 158.8 (Ci–OMe), 138.1 (Ci–CH2Bn),
137.8 (Ci–CH2Bn), 134.8 (Ci–CH2CO2Bn), 130.2 (Ci–
CH2PMB a), 129.9 (Ci–CH2PMB b), 129.3 (CAr–PMB
b), 129.2 (CAr–PMB a), 128.4–127.2 (CAr–Bn), 113.3
(CAr–PMB), 97.4 (C-1b), 91.7 (C-1a), 81.1 (C-3b), 79.8
(C-2b), 77.8 (C-3a), 75.6 (C-2a), 75.3 (C-4a), 74.7
(CH2Ph), 74.3 (C-4b), 73.8–73.7 (C-5b, 2 · CH2Ph),
73.2 (CH2Ph), 72.8 (CH2Ph), 70.4 (C-5a), 69.5 (CH2Ph),
66.9 (CO2CH2Ph b), 66.8 (CO2CH2Ph a), 54.9
(OCH3PMB); HRESIMS (m/z): [M+Na]+ calcd for
C35H36O8Na, 607.2307; found, 607.2302.
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´
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This work was carried out with the financial contribu-
´
tion of the Conseil Regional de Picardie (Alternatives
´ ´
´
Vegetales). M.B. thanks the Conseil Regional de Picar-
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