Synthesis of a tetrasaccharide fragment of mycobacterial arabinogalactan

Article abstract of DOI:10.1016/j.carres.2008.01.024

The arabinogalactan of mycobacteria contains both monosaccharides in the furanose ring form, which are absent in mammals. We report here the first synthesis of the tetrasaccharide fragment α-d-Araf-(1→5)-β-d-Galf-(1→5)-β-d-Galf-(1→6)-d-Galf, conveniently derivatized for further elongation. The strategy relied on the use of suitably substituted d-galactono-1,4-lactones as precursors for the galactofuranose units. Reduction of lactone tetrasaccharide 9 with disiamylborane afforded the tetrasaccharide synthon 1. The tetrasaccharide contains the linker unit of the arabinan to the galactan.

Full text of DOI:10.1016/j.carres.2008.01.024

Available online at www.sciencedirect.com
Carbohydrate Research 343 (2008) 1870–1875
Note
Synthesis of a tetrasaccharide fragment
of mycobacterial arabinogalactan
*
´
´
Lucıa Gandolfi-Donadıo, Carola Gallo-Rodriguez and Rosa M. de Lederkremer
CIHIDECAR, Departamento de Quı´mica Orga´nica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires,
Ciudad Universitaria, Pabello´n II, 1428 Buenos Aires, Argentina
Received 21 December 2007; received in revised form 15 January 2008; accepted 19 January 2008
Available online 29 January 2008
Abstract—The arabinogalactan of mycobacteria contains both monosaccharides in the furanose ring form, which are absent in
mammals. We report here the first synthesis of the tetrasaccharide fragment a-D-Araf-(1?5)-b-D-Galf-(1?5)-b-D-Galf-(1?6)-D-
Galf, conveniently derivatized for further elongation. The strategy relied on the use of suitably substituted D-galactono-1,4-lactones
as precursors for the galactofuranose units. Reduction of lactone tetrasaccharide 9 with disiamylborane afforded the tetrasaccharide
synthon 1. The tetrasaccharide contains the linker unit of the arabinan to the galactan.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Mycobacteria; Arabinogalactan; Galactofuranose; Arabinofuranose; Galactonolactone
Mycobacterium tuberculosis, the most studied species of
mycobacteria, has received much attention in the last
years.1 The appearance of multidrug-resistant strains
points to the search of new targets for chemotherapy.2
The integrity of mycobacterial cell wall is essential for
the viability of mycobacteria.3 A major structure com-
ponent of the cell wall is an arabinogalactan.4 This poly-
saccharide, entirely formed by furanose residues, is a
good candidate for carbohydrate-based vaccines,
because the furanose constituents are absent in mammals.
Also, synthesis of the oligosaccharides is an attractive
challenge for carbohydrate chemists, considering the
lability of the furanosidic as compared to pyranosidic
linkages. All the galactofuranoses are b-linked in the
galactan. However, the a-D-arabinan chains are sub-
stituted at nonreducing ends with units of b-D-Araf. A
domain of 22 Araf residues has been recently synthe-
sized by Lowary and co-workers.5 Several reports from
other and our laboratories described the chemical syn-
thesis of the galactan fragments.6–9 Few reports on the
synthesis of oligosaccharides containing both units have
been published. In this regard, we have recently synthe-
sized the disaccharide linker unit of the arabinan to
the galactan,10 and glycosides of the disaccharide were
also synthesized.11 The synthesis of the branched trisac-
charide a-D-Araf-(1?5)-[b-D-Galf-(1?6)]-D-Galf was
reported by two groups.12,13 We now describe the
preparation for the first time of tetrasaccharide a-D-
arabinofuranosyl-(1?5)-b-D-galactofuranosyl-(1?5)-b-
D-galactofuranosyl-(1?6)-D-galactofuranose 1 conve-
niently derivatized for further elongation of the chain.
Synthetic oligosaccharide fragments of the mycobacte-
rial cell wall are useful for studies on the biosynthetic
pathways used for polysaccharide assembly. Thus, by
using the galactofuranose trisaccharides containing b-
(1?6) and b-(1?5) linkages the characterization of a
bifunctional galactofuranosyl transferase was possible.14
Very recently,15 the galactofuranosyl transferase (GlfT1)
responsible for attaching the first unit of Galf to deca-
prenyl-P-P-GlcNAc-Rha was characterized using syn-
thetic acceptors.
Our synthetic strategy relied on the glycosyl aldonolac-
tone approach6–8,16 for the introduction of the galacto-
furanose residues. Thus, after glycosylation, the lactone
disaccharide is selectively reduced with diisoamylborane
(DSB) followed by further activation of the anomeric
*
Corresponding author. Tel./fax: +54 11 4576 3352; e-mail: lederk@
qo.fcen.uba.ar
0008-6215/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2008.01.024

L. Gandolfi-Donadı´o et al. / Carbohydrate Research 343 (2008) 1870–1875
1871
center. The synthesis began with disaccharide imidate 2,
which was recently described10 (Scheme 1). For the intro-
duction of the terminal nonreducing arabinofuranose in
2, tin(IV) chloride-promoted glycosylation has been
conveniently used.10 Elongation of the chain with galacto-
furanoses was achieved using the milder trichloroace-
timidate method.8,17 D-Galactono-1,4-lactone, besides
being a stable precursor for the reducing sugar, can be
selectively substituted by acylation reactions.18 Thus,
2,6-di-O-pivaloyl-D-galactono-1,4-lactone (3),8 obtained
in one step from D-galactono-1,4-lactone, was used for
the introduction of the two internal galactose residues
in 1 (Scheme 1). For the incorporation of the galactose
in the reducing end, the easily obtained 2,3,5-tri-O-
benzoyl-D-galactono-1,4-lactone (8)16 was used as the
precursor.
Selective glycosylation of the exocyclic OH-5 of lac-
tone derivative 3 with 2 gave trisaccharide lactone 4 in
74% yield after purification by column chromatography.
The same regioselectivity was found on glycosylation of
the lactone with other imidates8 or by using the per-
benzoyl sugar and tin(IV) chloride as promoter.10 The
structure of 4 was confirmed by one- and two-dimen-
sional NMR spectroscopy. The 13C NMR spectrum
O
OH
O
O
OR
O
BzO
OBz
OPiv
O
OH
O
BzO
OBz
OPiv
O
O
CCl3
OAc
OPiv
O
O
OAc
3
OPiv
DSB
NH
OBz
OPiv
TMSOTf, CH2Cl2, 4 Å MS, -10 ºC
(73%)
(73%)
O
OPiv
OBz
OPiv
O
2
OPiv
4 R = H
Ac2O/ C5H5N
(99%)
5 R = Ac
O
OBz
O
OBz
O
O
O
OR
OAc
OBz
OBz
O
OBz
OBz
OH
8
O
BzO
OBz
OPiv
O
OAc
O
OPiv
O
OAc
DSB
(54%)
TMSOTf, 4 Å MS, BzO
CH2Cl2, -10 ºC
OPiv
OBz
O
OBz
O
OPiv
O
OAc
OPiv
O
(67%)
OPiv
OBz
OPiv
O
OPiv
6 R = H
Cl3CCN,
DBU,CH2Cl2
(84%)
7 R = C(NH)CCl3
9
O
OBz
OH
OBz
OBz
O
O
OAc
BzO
OPiv
OBz
O
O
O
OAc
OPiv
OBz
OPiv
O
OPiv
1
Scheme 1.

1872
L. Gandolfi-Donadı´o et al. / Carbohydrate Research 343 (2008) 1870–1875
showed C-100 and C-10 at 106.1 and 105.6 ppm, charac-
5% (v/v) sulfuric acid in EtOH and charring. Column
chromatography was performed on Silica Gel 60 (230–
400 mesh, Merck). NMR spectra were recorded with a
Bruker AVANCE II 500 spectrometer at 500 MHz
(1H) and 125.8 MHz (13C), or with a Bruker AC 200
at 200 MHz (1H) and 50.3 MHz (13C). Chemical shifts
are given relative to the signal of internal acetone stan-
teristic of a-D-arabinofuranosyl and b-D-galactofuran-
1
osyl linkages. In the H NMR spectrum, H-100 and H-10
appeared as broad singlets at 5.64 and 5.21 ppm and
H-200 and H-20 as doublets, with J2,3 < 1.5 Hz, at 5.38
and 5.13 ppm. Acetylation of compound 4 at OH-3
was performed before reduction of the lactone to sugar
1
1
6. In the H NMR spectrum of 5, the signal of H-3 was
dard at 2.16 ppm and 30.8 ppm for H NMR and 13C
shifted ꢀ1 ppm downfield with respect to 4. Reduction
of the trisaccharide lactone derivative 5 with DSB
yielded the furanosidic derivative 6 as a 4.6:5.4 a:b ano-
NMR spectra when recorded in D2O. 1H and 13C
assignments were supported by DEPT 135, homo-
nuclear COSY and HSQC experiments. High resolution
mass spectra (HRMS) were recorded on Agilent
LCTOF (2006) equipped with a high resolution TOF
analyzer with Windows XP based OS and APCI/ESI
ionization.
1
meric mixture as indicated by the H NMR spectrum.
The anomeric protons appeared at 5.34 (H-1b) as a
doublet (J = 3.6 Hz) and at 5.44 ppm as a doublet of
doublets (H-1a, J = 4.8, 9.4 Hz). After D2O exchange,
the signal for H-1b appeared as a broad singlet and
the signal for H-1a was simplified to a doublet
(J = 4.8 Hz). The 13C NMR spectrum showed all the
anomeric signals for both anomers, and could be
assigned with the aid of COSY and HSQC experiments.
The anomeric carbons for the reducing unit appeared at
100.4 (C-1b) and 95.0 ppm (C-1a).
1.2. 2,3,5-Tri-O-benzoyl-a-D-arabinofuranosyl-(1?5)-3-
O-acetyl-2,6-di-O-pivaloyl-b-D-galactofuranosyl-(1?5)-
2,6-di-O-pivaloyl-D-galactono-1,4-lactone (4)
A vigorously stirred solution of dried trichloroacet-
imidate 210 (1.07 g, 1.09 mmol), 2,6-di-O-pivaloyl-D-
galactono-1,4-lactone (3,8 486 mg, 1.40 mmol), and acti-
Activation of the anomeric center in 6 was performed
by the trichloroacetimidate method.19 Imidate 7 was
obtained as a 1:9 a:b mixture in 84% yield, by treatment
of 6 with Cl3CCN and DBU. Glycosylation of 7 with the
lactone derivative 8 afforded the tetrasaccharide lactone
9 in 73% yield. Formation of the new b-galactofurano-
˚
vated 4 A powdered molecular sieves (0.7 g) in anhyd
CH2Cl2 (60 mL) was cooled to ꢁ10 °C, and TMSOTf
(79 lL, 0.44 mmol) was slowly added. After 1 h of stir-
ring, the mixture was filtered into satd aq NaHCO3
(150 mL)
and
then
extracted
with
CH2Cl2
1
side linkage was indicated by the H NMR spectrum
(2 ꢂ 200 mL). The organic layer was washed with water
(3 ꢂ 150 mL), dried (Na2SO4), filtered, and concen-
trated under reduced pressure. The residue was purified
by column chromatography (15:1 toluene–EtOAc) afford-
ing 2,3,5-tri-O-benzoyl-a-D-arabinofuranosyl-(1?5)-3-
O-acetyl-2,6-di-O-pivaloyl-b-D-galactofuranosyl-(1?5)-
2,6-di-O-pivaloyl-D-galactono-1,4-lactone (4, 933 mg,
73%) as a syrup: Rf = 0.37 (5:1, toluene–EtOAc); [a]D
of 9. The H-10 appeared as a singlet at 5.05 ppm and a
doublet was observed at 6.08 ppm with J2,3 = 5.8 Hz
characteristic of the H-2 for acylated D-galactono-1,4-
lactone. Reduction of the tetrasaccharide lactone 9
afforded the furanose derivative 1 in 54% yield. The
13C NMR spectrum of 1 showed the signals for the
two anomers at 100.5 ppm (C-1b) and 95.3 ppm (C-1a).
In summary, we have constructed a synthon contain-
ing the two galactofuranosyl linkages present in the
arabinogalactan, substituted at the nonreducing end
by a D-arabinofuranose unit. Once again, we proved
the efficiency of the glycosyl-lactone approach followed
by DSB reduction. Synthon 1 is very appropriate for
further coupling as subsequent deprotection could be
achieved in one step.
1
ꢁ29.7 (c 1.0, CHCl3); H NMR (CDCl3, 500 MHz): d
8.08–8.04 (m, 15H, aromatic), 5.65 (dd, 1H, J = 1.2,
5.0 Hz, H-300), 5.64 (br s, 1H, H-100), 5.56 (d, 1H,
J = 8.7 Hz, H-2), 5.38 (d, 1H, J = 1.2 Hz, H-200), 5.21
(br s, 1H, H-10), 5.18 (dd, 1H, J = 1.3, 4.2 Hz, H-30),
5.13 (d, 1H, J = 1.3 Hz, H-20), 4.83 (dt, 1H, J = 4.2,
8.2 Hz, H-3), 4.80 (dd, 1H, J = 3.5, 11.4 Hz, H-5a00),
4.77 (d, 1H, J = 4.2 Hz, OH), 4.75 (m, 1H, H-400), 4.69
(dd, 1H, J = 4.7, 11.4 Hz, H-5b00), 4.49 (dd, 1H,
J = 5.6, 14.0 Hz, H-6a0), 4.42 (dd, 1H, J = 4.2, 7.1 Hz,
H-40), 4.33 (dd, 1H, J = 4.3, 7.7 Hz, H-4), 4.32–4.30
(m, 2H, H-6a, H-6b), 4.19–4.13 (m, 3H, H-5, H-50,
H-6b0), 2.07 (s, 3H, CH3), 1.19, 1.18, 1.17, 1.16 (4s, 36H,
(CH3)3CCO); 13C NMR (CDCl3, 125.8 MHz): d 178.2,
177.8, 177.4, 176.7 ((CH3)3CCO), 170.0 (C-1), 168.9–
165.7 (CH3CO and COPh), 134.4–128.2 (aromatic),
106.1 (C-100), 105.6 (C-10), 84.9 (C-40), 83.2 (C-200), 80.9
(C-400), 80.2 (C-20), 79.2 (C-4), 77.4 (C-300), 76.2 (C-30),
75.5 (C-50or C-5), 75.0 (C-2), 72.2 (C-5 or C-50), 71.4
(C-3), 64.2 (C-60), 63.5 (C-500), 63.0 (C-6), 38.7
1. Experimental
1.1. General methods
Melting points were determined with a Thomas-Hoover
apparatus and are uncorrected. Optical rotations were
measured with a Perkin–Elmer 343 polarimeter at
25 °C. TLC was performed on 0.2 mm Silica Gel 60
F254 (Merck) aluminum-supported plates. Detection
was effected by exposure to UV light or by spraying with

L. Gandolfi-Donadı´o et al. / Carbohydrate Research 343 (2008) 1870–1875
1873
((CH3)3CCO ꢂ 4), 27.1, 27.0, 26.9, 26.8 ((CH3)3CCO),
20.6 (CH3); HRMS (ESI/APCI) m/z: [M+Na]+ calcd
for C60H74O23Na, 1185.4519; found, 1185.4495.
ture and then processed as previously described.20 The
organic layer was washed with water, dried (Na2SO4),
and concentrated. Boric acid was eliminated by coeva-
poration with MeOH (5 ꢂ 3 mL) at room temperature.
The residue was purified by column chromatography
(7:1, toluene–EtOAc) to give 107 mg (73%) of syrupy 6
as a 0.46:0.54 a/b anomeric mixture: Rf = 0.43 (3:1,
1.3. 2,3,5-Tri-O-benzoyl-a-D-arabinofuranosyl-(1?5)-3-
O-acetyl-2,6-di-O-pivaloyl-b-D-galactofuranosyl-(1?5)-
3-O-acetyl-2,6-di-O-pivaloyl-D-galactono-1,4-lactone (5)
1
toluene–EtOAc); [a]D ꢁ17.3 (c 1.0, CHCl3); H NMR
To a solution of 4 (894 mg, 0.77 mmol) in dry pyridine
(6.1 mL), cooled at 0 °C, was added Ac2O (6.1 mL)
dropwise and the mixture was stirred at room tempera-
ture for 30 min. After cooling to 0 °C, the reaction was
quenched by the slow addition of MeOH (2.3 mL) and
the stirring continued for 30 min at room temperature.
The solution was diluted with CH2Cl2 (150 mL) and
then sequentially washed with 10% HCl (2 ꢂ 150 mL),
water (150 mL), satd aq NaHCO3 (150 mL), water
(2 ꢂ 150 mL), dried (Na2SO4), filtered, and then concen-
trated to give 5 (915 mg, 99%) as a chromatographically
pure syrup; Rf = 0.48 (5:1, toluene–EtOAc); [a]D ꢁ40.2
(CDCl3, 500 MHz), only the assigned d are listed: 5.66
(br s, 0.46H, H-100 a anomer), 5.64 (br s, 0.54H, H-100
b anomer), 5.61 (dd, 0.54H, J = 1.4, 4.8 Hz, H-300),
5.58 (dd, 0.46H, J = 1.2, 4.8 Hz, H-300), 5.56 (d, 0.46H,
J = 1.2 Hz, H-200), 5.55 (d, 0.56H, J = 1.4 Hz, H-200),
5.44 (dd, 0.46H, J = 4.8, 9.4 Hz, H-1 a anomer), 5.34
(d, 0.54H, J = 3.6 Hz, H-1 b anomer), 5.32 (br s,
0.46H, H-10), 5.25 (m, 0.56H, H-30), 5.23 (br s, 0.56H,
H-10), 5.21 (dd, 0.46H, J = 2.0, 6.5 Hz, H-30), 3.86 (d,
1H, J = 9.4 Hz, OH), 3.81 (d, 1H, J = 3.6 Hz, OH);
13C NMR (CDCl3, 125.8 MHz): d 178.2, 177.9, 177.6,
176.9 ((CH3)3CCO), 170.0, 169.9, 169.8, 169.7
(CH3CO), 166.1–165.6 (COPh), 133.6–129.0 (aromatic),
106.1 (C-100 a anomer), 106.0 (C-100 b anomer), 105.2
(C-10 b anomer), 105.1 (C-10 a anomer), 100.4 (C-1 b
anomer), 95.0 (C-1 a anomer), 82.9, 82.2, 82.3, 81.6,
81.5, 81.4, 81.2, 81.0, 79.7, 77.9, 77.8, 77.7, 76.8, 76.5,
76.0, 75.1, 74.9, 74.8, 73.5, 64.4, 64.2, 63.7, 63.6, 63.5,
63.3, 38.7, 38.6 ((CH3)3CCO); 27.1, 27.0, 26.9
((CH3)3CCO); 20.7, 20.6 (CH3); HRMS (ESI/APCI)
m/z: [M+Na]+ calcd for C62H78O24Na, 1229.4781;
found, 1229.4758.
1
(c 1.0, CHCl3); H NMR (CDCl3, 500 MHz) d: 8.07–
7.16 (m, 15H, aromatic), 5.80 (t, 1H, J = 8.0 Hz, H-3),
5.67 (d, 1H, J = 8.0 Hz, H-2), 5.61 (s, 1H, H-100), 5.58
(dd, 1H, J = 1.2, 4.7 Hz, H-300), 5.51 (d, 1H,
J = 1.2 Hz, H-200), 5.22 (s, 1H, H-10), 5.20 (dd, 1H,
J = 1.7, 5.0 Hz, H-30), 5.13 (d, 1H, J = 1.7 Hz, H-20),
4.79 (dd, 1H, J = 3.0, 11.2 Hz, H-5a00), 4.71 (ddd, 1H,
J = 3.0, 4.7, 5.0 Hz, H-400), 4.68 (dd, 1H, J = 5.0,
11.2 Hz, H-5b00), 4.51 (dd, 1H, J = 3.5, 7.7 Hz, H-4),
4.48 (dd, 1H, J = 5.0, 7.0 Hz, H-40), 4.40 (dd, 1H,
J = 3.0, 11.7 Hz, H-6a0), 4.36 (dd, 1H, J = 3.7,
10.7 Hz, H-6a), 4.24 (m, 1H, H-5), 4.22 (dd, 1H,
J = 6.1, 10.7 Hz, H-6b), 4.18 (ddd, 1H, J = 3.0, 6.2,
7.0 Hz, H-50), 4.12 (dd, 1H, J = 6.2, 11.7 Hz, H-6b0);
2.10, 2.03 (2s, 6H, CH3); 1.17, 1.16, 1.15, 1.14 (4s,
36H, (CH3)3CCO); 13C NMR (CDCl3, 125.8 MHz) d:
178.0, 177.7, 177.0, 176.8 ((CH3)3CCO), 170.0 (C-1);
169.4, 168.0 (CH3CO), 166.1–165.2 (COPh), 137.8–
128.2 (aromatic), 106.0 (C-100), 104.4 (C-10), 83.5
(C-40), 82.0 (C-200), 81.2 (C-400), 80.0 (C-20), 77.8 (C-300),
77.0 (C-4), 76.5 (C-30), 75.0 (C-50), 72.2 (C-2), 72.0
(C-3), 71.2 (C-5), 64.2 (C-60), 63.6 (C-500), 62.2 (C-6);
38.7, 38.6, 38.5 ((CH3)3CCO); 27.1, 27.0, 26.9, 26.8
((CH3)3CCO); 20.7, 20.4 (CH3); HRMS (ESI/APCI)
m/z: [M+Na]+ calcd for C62H76O24Na, 1227.4624;
found, 1227.4597.
1.5. 2,3,5-Tri-O-benzoyl-a-D-arabinofuranosyl-(1?5)-3-
O-acetyl-2,6-di-O-pivaloyl-b-D-galactofuranosyl-(1?5)-
3-O-acetyl-2,6-di-O-pivaloyl-b-D-galactofuranosyl-(1?6)-
2,3,5-tri-O-benzoyl-D-galactono-1,4-lactone (9)
To a stirred solution of 6 (86 mg, 0.071 mmol) and tri-
chloroacetonitrile (0.036 mL, 0.36 mmol) in CH2Cl2
(5 mL), cooled to 0 °C, DBU (5 mL, 0.036 mmol) was
slowly added. After 1 h, TLC monitoring showed the
consumption of the starting material. The solution was
concentrated at room temperature under reduced
pressure, and the residue was purified by column chro-
matography (20:1:0.21, toluene–EtOAc–TEA) to give
O-(2,3,5-tri-O-benzoyl-a-D-arabinofuranosyl-(1?5)-3-
O-acetyl-2,6-di-O-pivaloyl-b-D-galactofuranosyl-(1?5)-
3-O-acetyl-2,6-di-O-pivaloyl-b-D-galactofuranosyl) tri-
chloroacetimidate (7, 80.2 mg, 84%) as a syrup. Com-
pound 7 was stable for 1 day at ꢁ20 °C: Rf = 0.60
(b-anomer), 0.50 (a-anomer) (5:1:0.06, toluene–
1.4. 2,3,5-Tri-O-benzoyl-a-D-arabinofuranosyl-(1?5)-3-
O-acetyl-2,6-di-O-pivaloyl-b-D-galactofuranosyl-(1?5)-
3-O-acetyl-2,6-di-O-pivaloyl-D-galactofuranose (6)
1
EtOAc–TEA); H NMR (CDCl3, 200 MHz) d for the
A solution of bis(2-butyl-3-methyl)borane (2.3 mmol) in
anhyd THF (0.63 mL) cooled to 0 °C and under an
argon atmosphere was added to a flask containing com-
pound 5 (148 mg, 0.12 mmol) previously dried. The
resulting solution was stirred for 22 h at room tempera-
b anomer: 8.72 (s, 0.1H, NH a anomer), 8.59 (s, 0.9H,
NH), 8.08–7.15 (m, 15H, aromatic), 6.51 (d, 0.1H,
J = 4.0 Hz, H-1 a anomer), 6.25 (s, 0.9H, H-1 b ano-
mer), 5.62 (br s, 0.9H, H-100), 5.60 (m, 0.9H, H-300),
5.52 (d, 0.9H, J = 1.2 Hz, H-200), 5.36–5.31 (m, 1.8H,

1874
L. Gandolfi-Donadı´o et al. / Carbohydrate Research 343 (2008) 1870–1875
H-30, H-10), 5.26–5.24 (m, 1.8H, H-2, H-3), 5.15 (d,
0.9H, J = 1.6 Hz, H-200), 4.84–4.64 (m, 3H, H-400,
H-5a00, H-5b00), 4.50–4.13 (m, 8H, H-4, H-40, H-5, H-50,
H-6a, H-6b, H-6a0, H-6b0); 2.06, 2.02 (2s, 6H, CH3);
1.26, 1.23, 1.18, 1.17 (4s, 36H, (CH3)3CCO); 13C
NMR (CDCl3, 50.3 MHz) d for the b anomer: 178.0,
176.9 ((CH3)3CCO); 170.0, 169.6 (CH3CO), 166.1–
165.1 (COPh), 160.1 (NHCOCl3), 133.5–125.2 (aro-
matic), 105.6 (C-100 b anomer), 104.6 (C-10 b anomer),
102.7 (C-1 b anomer), 90.8 (C-1 a anomer), 84.8,
83.3, 82.1, 81.5, 81.4, 81.3, 81.1, 80.4, 77.8, 76.7, 75.5,
74.0, 72.0, 64.2, 63.6, 63.5, 28.7, 38.6, 38.5 ((CH3)3-
CCO); 27.1, 27.0, 26.9, 26.8 ((CH3)3CCO); 20.7, 20.5
(CH3).
1.6. 2,3,5-Tri-O-benzoyl-a-D-arabinofuranosyl-(1?5)-3-
O-acetyl-2,6-di-O-pivaloyl-b-D-galactofuranosyl-(1?5)-
3-O-acetyl-2,6-di-O-pivaloyl-b-D-galactofuranosyl-(1?6)-
2,3,5-tri-O-benzoyl-D-galactose (1)
Lactone 9 (26 mg, 0.015 mmol) was reduced with a solu-
tion of bis(2-butyl-3-methyl)borane (2.16 mmol) in
anhyd THF (0.63 mL) as described for compound 5.
Purification by column chromatography (3:1, hexane–
EtOAc) gave 14 mg (54%) of syrupy 1 as a 0.1:0.9 a/b
anomeric mixture: Rf = 0.63 (4:1, toluene–EtOAc); [a]D
1
ꢁ30.6 (c 1.0, CHCl3); H NMR (CDCl3, 500 MHz) d
for the b anomer: 8.06–7.21 (m, 30H, aromatic), 5.78–
5.71 (m, 1H, H-5, H-1 a anomer), 5.72 (d, 0.9H,
J = 4.6 Hz, H-1 b anomer), 5.66 (br s, 0.9H, H-1000),
5.59 (dd, 0.9H, J = 1.4, 5.0 Hz, H-3000), 5.54 (d, 0.9H,
J = 1.2 Hz, H-2), 5.51 (dd, 0.9H, J = 1.2, 5.5 Hz, H-3),
5.48 (d, 0.9H, J = 1.4 Hz, H-2000), 5.38 (br s, 0.9H,
H-100), 5.25 (dd, 0.9H, J = 1.9, 6.0 Hz, H-300), 5.23 (dd,
0.9H, J = 1.9, 6.0 Hz, H-30), 5.18 (d, 0.9H, J = 1.9 Hz,
H-20), 5.04 (d, 0.9H, J = 1.9 Hz, H-200), 4.98 (s, 0.9H,
H-10), 4.91 (d, 0.9H, J = 4.6 Hz, OH), 4.82 (dd, 0.9H,
J = 3.4, 11.5 Hz, H-5a000), 4.79 (dd, 0.9H, J = 1.9,
5.5 Hz, H-4), 4.74–4.67 (m, 1.8H, H-4000, H-6a0 or
H-6a00), 4.68 (dd, 0.9H, J = 4.6, 11.5 Hz, H-5b000), 4.46
(t, 0.9H, J = 5.7 Hz, H-40), 4.41 (dd, 0.9H, J = 3.4,
6.0 Hz, H-400), 4.33–4.27 (m, 2.7H, H-50, H-500, H-6a00 or
H-6a0), 4.21–4.15 (m, 1.8H, H-6b0, H-6b00), 4.07 (t,
0.9H, J = 8.4 Hz, H-6a), 4.78 (dd, 0.9H, J = 5.6,
8.3 Hz, H-6b); 2.04, 1.83 (2s, 5.4H, CH3); 1.18, 1.17,
1.13, 1.11 (4s, 32.4H, (CH3)3CCO); 13C NMR (CDCl3,
A vigorously stirred suspension of dried trichloroacet-
imidate 7 (55 mg, 0.040 mmol), 2,3,5-tri-O-benzoyl-D-
galactono-1,4-lactone (8,16 24 mg, 0.048 mmol), and
˚
dried 4 A powdered molecular sieves (0.2 g) in anhyd
CH2Cl2 (4 mL) was cooled to ꢁ15 °C. After 10 min of
stirring, TMSOTf (3 lL, 0.016 mmol) was slowly added.
After 1 h, TLC monitoring showed the consumption of
imidate 7, the mixture was rapidly filtered into satd aq
NaHCO3 (25 mL) and then extracted with CH2Cl2
(2 ꢂ 25 mL). The organic phase was separated and
washed with water (3 ꢂ 50 mL), dried (MgSO4), and
concentrated. The oily residue was purified by column
chromatography (12:1, toluene–EtOAc) to give 44 mg
of 9 (67% yield) as an amorphous solid: Rf = 0.69 (4:1,
1
toluene–EtOAc); [a]D ꢁ27.2 (c 1.0, CHCl3); H NMR
(CDCl3, 500 MHz): d 8.02–7.16 (m, 30H, aromatic),
6.08 (d, 1H, J = 5.8 Hz, H-2), 5.82 (t, 1H, J = 5.5 Hz,
H-3), 5.76 (ddd, 1H, J = 2.2, 6.2, 7.7 Hz, H-5), 5.64 (s,
1H, H-1000), 5.60 (dd, 1H, J = 1.2, 4.8 Hz, H-3000), 5.52
(d, 1H, J = 1.2 Hz, H-2000), 5.31–5.29 (m, 2H, H-300,
H-100), 5.26 (dd, 1H, J = 1.6, 5.5 Hz, H-30), 5.17 (d, 1H,
J = 1.6 Hz, H-20), 5.06 (dd, 1H, J = 2.2, 5.5 Hz, H-4),
5.05 (br s, 1H, H-10), 5.02 (d, 1H, J = 2.1 Hz, H-200),
4.80 (dd, 1H, J = 3.4, 11.5 Hz, H-5a000), 4.72 (m, 1H,
H-4000), 4.67 (dd, 1H, J = 4.7, 11.5 Hz, H-5b000), 4.42 (t,
1H, J = 5.5 Hz, H-40), 4.40–4.33 (m, 2H, H-6a0,
H-6a00), 4.30 (dd, 1H, J = 3.7, 6.1 Hz, H-400), 4.27 (m,
1H, H-500), 4.25–4.17 (m, 3H, H-50, H-6b0, H-6b00), 4.03
(dd, 1H, J = 7.7, 10.3 Hz, H-6a), 3.92 (dd, 1H,
J = 6.2, 10.3 Hz, H-6b); 2.03, 1.94 (2s, CH3); 1.16,
1.13, 1.12 (4s, 36H, (CH3)3CCO); 13C NMR (CDCl3,
125.8 MHz) d: 177.8, 177.3 ((CH3)3CCO); 170.0, 169.7
(CH3CO), 168.9 (C-1), 165.7–165.1 (COPh), 137.8–
128.3 (aromatic), 106.1 (C-1000), 105.4 (C-100), 104.5
(C-10), 82.1 (C-2000), 82.0 (C-40), 81.5 (C-200), 81.4 (C-20),
81.2 (C-400), 81.1 (C-4000), 78.9 (C-4), 77.8 (C-3000), 76.8
(C-30), 75.8 (C-300), 74.5 (C-500), 74.1 (C-3), 72.4 (C-2),
71.4 (C-50), 70.1 (C-5), 64.2, 63.8 (C-60, C-600), 63.6
(C-5000, C-6); 38.6, 38.5 ((CH3)3CCO); 27.1, 27.0,
26.9, 26.8 ((CH3)3CCO); 20.7, 20.5 (CH3); HRMS
(ESI/APCI) m/z: [M+Na]+ calcd for C89H98O32Na,
1701.5939; found, 1701.5944.
125.8 MHz)
d for the b anomer: 178.2, 177.0
((CH3)3CCO), 169.8 (CH3CO ꢂ 2), 168.9 (C-1), 166.7–
164,1 (COPh), 137.8–128.3 (aromatic), 105.9 (C-1000),
104.0 (C-01000, C-100), 100.5 (C-1), 95.3 (C-1a anomer),
83.4 (C-2 ), 82.1 (C-2), 81.9, 81.8 (C-40, C-400), 81.6
(C-20, C-200, C-4000), 80.5 (C-4), 78.0 (C-3000), 77.6 (C-3),
76.9 (C-300), 75.7 (C-30), 74.4, 70.2 (C-50, C-500), 69.5
(C-5); 66.9, 64.0 (C-60, C600); 63.6 (C-5000), 62.4 (C-6),
38.7, 38.6 ((CH3)3CCO); 27.1, 27.0, 26.9 (ꢂ2)
((CH3)3CCO); 20.7, 20.4 (CH3); HRMS (ESI/APCI)
m/z: [M+Na]+ calcd for C89H100O32Na, 1703.6095,
found, 1703.6091.
Acknowledgments
This work was supported by grants from Agencia Nac-
´
´
´
ional de Promocion Cientıfica y Tecnologica, Universi-
dad de Buenos Aires and CONICET. R.M.L. and
C.G.-R. are research members of CONICET.
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