Versatile use of ytterbium(III) triflate and acid washed molecular sieves in the activation of glycosyl trifluoroacetimidate donors. Assemblage of a biologically relevant tetrasaccharide sequence of Globo H

Article abstract of DOI:10.1021/jo050301x

The nonreducing tetrasaccharide terminus of Globo H has been assembled in good yield and excellent stereocontrol exclusively by using mild and moisture stable agents such as Yb(OTf)₃ and acid washed molecular sieves for the activation of glycos

Full text of DOI:10.1021/jo050301x

CHART 1. Target Compound and Building Blocks
Versatile Use of Ytterbium(III) Triflate and
Acid Washed Molecular Sieves in the
Activation of Glycosyl Trifluoroacetimidate
Donors. Assemblage of a Biologically
Relevant Tetrasaccharide Sequence of
Globo H
Matteo Adinolfi, Alfonso Iadonisi,*
Alessandra Ravida`, and Marialuisa Schiattarella
Dipartimento di Chimica Organica e Biochimica,
Via Cynthia 4, I-80126 Napoli, Italy
iadonisi@unina.it
Received February 17, 2005
form (2) that is suitable for the construction of novel
N-derivatized analogues, the 2-azido group functionality
representing a useful handle to this purpose. The whole
synthetic sequence relies exclusively on glycosidation
procedures based on the convenient moisture stable
promoters we recently reported. In particular the as-
semblage of the sequence demonstrates the efficacy of
both 4Å acid washed molecular sieves (4Å AW MS)5 and
ytterbium(III) triflate6 as activators of glycosyl N-phe-
nyltrifluoroacetimidate7 donors and their complementary
use in the stereocontrolled construction of three strategi-
cally different typologies of glycosidic linkages. In fact,
acid washed molecular sieves can both play the usual role
of drying agents and promote the stereocontrolled syn-
thesis of 1,2-trans glycosides with donors bearing par-
ticipating groups at position 2. In the absence of such a
group, either 1,2-cis or 1,2-trans selectivity can be
attained by the use of Yb(OTf)3 and the suitable choice
of the reaction solvent. N-Phenyltrifluoroacetimidate
donors were chosen for their lower propensity to give
undesired side products in the course of glycosidations,8
and their higher stability in storage than the correspond-
ing trichloroacetimidate analogues.7 The synthesis was
based on monosaccharide building blocks 3-6 as the
precursors of residues A-D, respectively. Donors 4 and
6 were prepared in high yield (76-99%) by reacting the
corresponding known hemiacetals with a slight excess
of N-phenyltrifluoroacetimidoyl chloride and NaH in
anhydrous dichloromethane. The use of a milder base
such as DIPEA resulted in sluggish reactions.6 Donor 5
was prepared as shown in Scheme 1.
The nonreducing tetrasaccharide terminus of Globo H has
been assembled in good yield and excellent stereocontrol
exclusively by using mild and moisture stable agents such
as Yb(OTf)3 and acid washed molecular sieves for the
activation of glycosyltrifluoroacetimidate donors in the gly-
cosylation steps.
In the last years the hexasaccharide Globo H (1, Chart
1) has been emerging as an important antigenic oligosac-
charide sequence for the development of vaccines against
some tumors.1 Several truncated versions of Globo H
have been prepared and biologically evaluated to define
synthetically simpler candidates as anticancer vaccines.2,3
These investigations led to the disclosure of a relevant
immunogenic activity associated with the tetrasaccha-
ridic nonreducing end of Globo H.4
In this paper an original and efficient approach is
proposed for the synthesis of this sequence in a protected
(1) (a) Danishefsky, S. J.; Allen, J. R. Angew. Chem., Int. Ed. 2000,
39, 836-863 and references therein. (b) Allen, J. R.; Allen, J. G.; Zhang,
X.-F.; Williams, L. J.; Zatorski, A.; Ragupathi, G.; Livingston, P. O.;
Danishefsky, S. J. Chem. Eur. J. 2000, 6, 1366-1375.
(2) (a) Lay, L.; Nicotra, F.; Panza, L.; Russo, G.; Adobati, E. Helv.
Chim. Acta 1994, 77, 509-514. (b) Lay, L.; Panza, L.; Russo, G.;
Colombo, D.; Ronchetti, F.; Adobati, E.; Canevari, S. Helv. Chim. Acta
1995, 78, 533-538. (c) Toma, L.; Colombo, D.; Ronchetti, F.; Panza,
L.; Russo, G. Helv. Chim. Acta 1995, 78, 636-646. (d) Kim, I. J.; Park,
T. K.; Hu, S.; Abrampah, K.; Zhang, S.; Livingston, P. O.; Danishefsky,
S. J. J. Org. Chem. 1995, 60, 7716-7717. (e) Adobati, E.; Panza, L.;
Russo, G.; Colnaghi, I.; Canevari, S. Glycobiology 1997, 7, 173-178.
(3) For other syntheses of the complete sequence see: (a) Park, T.
K.; Kim, I. J.; Hu, S.; Bilodeau, M. T.; Randolph, J. T.; Kwon, O.;
Danishefsky, S. J. J. Am. Chem. Soc. 1996, 118, 11488-11500. (b)
Lassaletta, J. M.; Schmidt, R. R. Liebigs Ann. 1996, 1417-1423. (c)
Zhu, T.; Boons, G.-J. Angew. Chem., Int. Ed. 1999, 38, 3495-3497. (d)
Burkhart, F.; Zhang, Z.; Wacowich-Sgarbi, S.; Wong, C.-H. Angew.
Chem., Int. Ed. 2001, 40, 1274-1277.
(5) Adinolfi, M.; Barone, G.; Iadonisi, A.; Schiattarella, M. Org. Lett.
2003, 5, 987-989.
(6) Adinolfi, M.; Barone, G.; Iadonisi, A.; Schiattarella, M. Tetra-
hedron Lett. 2002, 43, 5573-5577.
(4) (a) Panza, L.; Poletti, L.; Prosperi, D.; Canevari S.; Perico, M. E.
Eur. J. Org. Chem. 2001, 4331-4336. (b) Perico, M. E.; Mezzanzanica,
D.; Luison, E.; Alberti, P.; Panza, L.; Russo, G.; Canevari S. Cancer
Immunol. Immunother. 2000, 49, 296-304.
(7) (a) Yu, B.; Tao, H. Tetrahedron Lett. 2001, 42, 2405-2407. (b)
Yu, B.; Tao, H. J. Org. Chem. 2002, 67, 9099-9102.
(8) Tanaka, H.; Iwata, Y.; Takahashi, D.; Adachi, M.; Takahashi,
T. J. Am. Chem. Soc. 2005, 127, 1630-1631.
10.1021/jo050301x CCC: $30.25 © 2005 American Chemical Society
Published on Web 05/18/2005
5316
J. Org. Chem. 2005, 70, 5316-5319

SCHEME 1. Synthesis of Galactosyl Donor 5a
these conditions the â-anomer was obtained almost
exclusively. In contrast, compound 5 was obtained as an
approximately equimolar mixture of anomers when 10
was subjected to the procedure reported7b by Yu (reaction
with an excess of trifluoroacetimidoyl chloride in lab
grade non-anhydrous acetone and K2CO3 as the base). It
also should be noted that the whole synthetic sequence
to donor 5 requires eight chemical transformations but
only three chromatographic purifications.
With the required building blocks in hand, the linear
construction of the tetrasaccharide started with the
coupling (Scheme 2) of the known acceptor 312 with donor
4 (anomeric mixture), equipped with a 2-azido function-
ality. Despite the lack of participating ability of the azide
group, the reaction gave excellent results thanks to the
activation of catalytic ytterbium(III) triflate (0.1 equiv)
and the â-directing effect exerted by the acetonitrile
solvent.6,13 As a matter of fact, the â-linked disaccharide
11 was obtained in high yield (70-77%) and traces of the
R-linked disaccharide could be monitored only by a
careful inspection of the NMR spectrum of the crude
reaction mixture. Interestingly, this result was achieved
without resorting to the low temperatures required for
the corresponding TMSOTf promoted reactions of 2-azido
trichloroacetimidates.13,14 Moreover, 2-azido-3,4,6-O-
acetylated trichloroacetimidates were recently reported
to provide disappointing results in TMSOTf promoted
glycosidatons in nitrile solvents.14
Disaccharide 11 was submitted to a deacetylation-
benzylidenation sequence that readily provided the dis-
accharide acceptor 12 (80% yield over two steps) that was
then coupled with the galactose donor 5. In initial
attempts the use of commercially available 4Å acid
washed molecular sieves in the double role of activators
and drying agents led to satisfying yields (61-64%).5
Replacement of the 4Å with the 5Å AW MS afforded
slightly higher yields (65-70%) within a sensibly shorter
reaction time (ca. 24 h vs 48 h). A further improvement
(75% yield) was registered with a modified procedure that
entails the slow addition of donor 5 to a solution of
acceptor 12 in a dichloroethane/cyclohexane mixture
containing the 5Å sieves. The resulting trisaccharide 13
was easily deprotected with K2CO3 in methanol at 40 °C
to yield acceptor 14 (89%). The final sterecontrolled R-L-
fucosylation of the sterically encumbered 2-OH was
achieved by means of the recently15 described procedure
that combines the efficient activation of catalytic ytter-
bium(III) triflate with an R-directing solvent (a 4:1:1
dichloromethane/dioxane/diethyl ether mixture). Due to
the high reactivity of the perbenzylated fucosyl donor 6,
the reaction was conducted at low temperature (-30 °C)
to give the desired R-anomer 2 (66% yield). Hydrogenoly-
sis of 2 led to the removal of benzyl and benzylidene
groups and the concomitant reduction of the azide
functionality. The resulting compound 15 is expected to
be a useful building block for the planned synthesis of
a Reagents and conditions: (a) I2, Et3SiH, DCM reflux, 30 min,
then lutidine, ethanol, TBAB, from 0 °C to rt, overnight; (b) KOH,
toluene, reflux, then benzyl bromide, 1 h, 50-56% overall yield;
(c) allyl alcohol, acetyl chloride, 70 °C, 2 h, 88%; (d) TMEDA,
methyl chloroformate, DCM, 0 °C, 30 min, quant; (e) PdCl2,
methanol, 5 h, rt; (f) trifluoroacetimidoyl chloride, DIPEA, DCM,
36 h, rt, 73% from 8.
Ortho ester intermediate 7 (diastereoisomeric mixture)
was accessed starting from peracetylated galactopyranose
through a one-pot sequence of anomeric iodination, halide
promoted ortho esterification, deacetylation, and benzy-
lation followed by a chromatographic purification (50-
56% overall yield).9 7 was then exposed to allyl alcohol
at 70 °C in the presence of in situ generated HCl to
achieve simultaneous introduction of the anomeric allyl
group and deprotection of the 2-OH. Intermediate 8
(anomeric mixture) was readily purified by chromatog-
raphy and then protected in quantitative yield with a
methoxycarbonyl group. The choice of this unusual
protecting group was supported by our previous observa-
tions: (a) 2-O-methoxycarbonylated donors display less
propensity to yield ortho ester-like coupling products than
the more canonical acetylated or benzoylated counter-
parts, especially when glycosidations are conducted under
very mild activation conditions;10 (b) the product of the
TMEDA based methoxycarbonylation procedure11 is re-
covered pure in quantitative yield after a very short
reaction time by a simple extractive workup; and (c)
chemical conditions for the removal of this group are
comparable to those required by usual O-deacylations
(see below). Compound 9 was subjected to anomeric
deallylation with catalytic PdCl2. Crude compound 10,
isolated by a simple filtration, was directly converted into
the corresponding trifluoroacetimidate 5. In this case
DIPEA was a sufficiently strong base to accomplish the
reaction with N-phenyltrifluoroacetimidoyl chloride in
anhydrous DCM. It is worth pointing out that under
(12) Bazin, H. G.; Du, Y.; Polat, T.; Linhardt, R. J. J. Org. Chem.
1999, 64, 7254-7259.
(9) Adinolfi, M.; Iadonisi, A.; Ravida`, A.; Schiattarella, M. Tetrahe-
dron Lett. 2003, 44, 7863-7866.
(13) Schmidt, R. R.; Behrendt, M.; Toepfer, A. Synlett 1990, 694-
696.
(10) Adinolfi, M.; Barone, G.; Iadonisi, A.; Mangoni, L.; Schiattarella,
M. Tetrahedron Lett. 2001, 42, 5967-5969.
(14) Tsuda, T.; Nakamura, S.; Hashimoto, S. Tetrahedron 2004, 60,
10711-10737.
(11) Adinolfi, M.; Barone, G.; Guariniello, L.; Iadonisi, A. Tetrahe-
dron Lett. 2000, 41, 9305-9309.
(15) Adinolfi, M.; Iadonisi, A.; Ravida`, A.; Schiattarella, M. Synlett
2004, 257-258.
J. Org. Chem, Vol. 70, No. 13, 2005 5317

SCHEME 2. Construction of the Tetrasaccharidea
a Reagents and conditions: (a) Yb(OTf)3, acetonitrile, from 0 °C to rt, overnight, 70-77%; (b) 9:1 MeOH/aq ammonia (32%), 3 h, then
acetonitrile, benzaldehyde dimethyl acetal, camphorsulfonic acid (cat.) 70 °C, 3 h, 80% overall yield; (c) 5, 5Å AW MS, dichloroethane/
cyclohexane 5:1, rt, overnight, 75%; (d) K2CO3, methanol, 40 °C, 8 h, 89%; (e) 6, Yb(OTf)3, DCM/diethyl ether/dioxane 4:1:1, from -30 °C
to rt, 66%; (f) Pd(OH)2, H2, 3:3:1 DCM/MeOH/H2O, rt, 90%.
118.5, 114.5, 103.1, 102.7, 80.4, 79.2, 75.7, 75.3, 74.8, 73.7, 73.6,
70.9, 70.6, 68.8, 66.4, 61.4, 61.1, 55.6, 20.6. C46H51N3O14 calcd C
63.51, H 5.91, found C 63.23, H 5.68.
novel analogues (see above) whose preparation and
biological evaluation will be reported in due course.
In conclusion, the protected form 2 of the tetrasaccha-
ride extremity of Globo H was efficiently accessed resort-
ing exclusively to mild, moisture stable, and easy to
handle glycosidation promoters. In the absence of 2-O-
participating groups on the donor, ytterbium(III) triflate
proved efficient in promoting the synthesis of either 1,2-
cis or 1,2-trans glycosides, depending on the nature of
the adopted solvents. With donors equipped with ap-
propriate participating groups even the sole acid washed
molecular sieves can be used to conveniently perform 1,2-
trans glycosidations. This work demonstrates that a
practical alternative to the harsh and sensitive agents
adopted in standard glycosylation protocols is available
for the assemblage of nontrivial oligosaccharide se-
quences.
p-Methoxyphenyl 3,4,6-Tri-O-benzyl-2-O-methoxycarbo-
nyl-â-D-galactopyranosyl-(1f3)-4,6-O-benzylidene-2-deoxy-
2-azido-â-D-galactopyranosyl-(1f3)-2,4,6-tri-O-benzyl-â-D-
galactopyranoside (13). A solution of donor 5 (86 mg, 0.12
mmol) in 5:1 dichloroethane/cyclohexane (720 µL) was added in
6 h at room temperature by a syringe pump to a solution of
acceptor 12 (53 mg, 0.063 mmol) in 5:1 dichloroethane/cyclo-
hexane (1.2 mL) containing freshly activated 5Å AW molecular
sieves in pellets (1.1 g). After completion of the addition the
mixture was left under overnight stirring to ensure complete
consumption of the donor. The mixture was then filtered on a
cotton plug washed repeatedly with 9:1 dichloromethane/
methanol (with drops of pyridine). Silica gel chromatography of
the residue from the organic phase (eluent: petroleum ether/
ethyl acetate from 8:2 to 65:35) afforded pure trisaccharide 13
(63 mg, 75%) as an oil. [R]D -12.3 (c 1.2 in CH2Cl2). 1H NMR
(300 MHz, CDCl3) δ 7.50-6.80 (aromatic protons), 5.51 (1H, s),
5.23 (1H, dd, J ) 7.8 and 9.6 Hz), 5.10-4.30 (12H, 6 × AB, 6 ×
benzyl CH2), 4.84 (1H, d, J ) 7.5 Hz), 4.71 (1H, d, J ) 7.8 Hz),
4.68 (1H, d), 4.26-4.20 (2H), 4.12-4.04 (2H), 3.94-3.78 (4H),
3.77 and 3.74 (2 × 3H, 2 × s, 2 × -OCH3), 3.70-3.40 (7H), 3.24
(1H, s). 13C NMR (75 MHz, CDCl3) δ 155.1, 155.0, 151.6, 138.6,
138.5, 138.3, 138.0, 137.8, 137.8, 137.4, 128.6-126.3, 118.4,
114.4, 103.1, 103.0, 102.4, 100.6, 81.0, 80.5, 79.1, 78.1, 75.8, 75.6,
75.2, 74.7, 74.5, 73.9, 73.4, 73.0, 72.6, 69.2, 69.0, 66.5, 62.9, 55.6,
55.0. MALDI-TOF MS for C76H79N3O18 (m/z): Mr (calcd) 1321.54,
Mr (found) 1344.80 (M + Na)+. C76H79N3O18 calcd C 69.02, H
6.02, found C 68.88, H 6.21.
p-Methoxyphenyl 2,3,4-Tri-O-benzyl-r-L-fucopyranosyl-
(1f2)-3,4,6-tri-O-benzyl-â-D-galactopyranosyl-(1f3)-4,6-O-
benzylidene-2-deoxy-2-azido-â-D-galactopyranosyl-(1f3)-
2,4,6-tri-O-benzyl-â-D-galactopyranoside (2). Trisaccharide
14 (69 mg, 0.055 mmol) and the fucosyl donor 6 (99 mg, 0.16
mmol) were coevaporated three times in anhydrous toluene.
After adding 4Å AW 300 MS, the mixture was dissolved under
argon in 4:1 dichloromethane/diethyl ether (1.5 mL) and im-
mediately cooled to -30 °C. After the mixture was stirred for
15 min, a solution of ytterbium triflate (3.4 mg, 5.5 µmol) in
dioxane (300 µL) was added dropwise. After 3 h at -30 °C the
mixture was allowed to warm to room temperature to ensure
the consumption of residual amounts of the donor and the
reaction was then quenched with pyridine. The mixture was
filtered on a short plug of silica gel washed with 9:1 dichlo-
Experimental Section
Glycosidation Procedures: p-Methoxyphenyl 3,4,6-Tri-
O-acetyl-2-deoxy-2-azido-â-D-galactopyranosyl-(1f3)-2,4,6-
tri-O-benzyl-â-D-galactopyranoside (11). Donor 4 (246 mg,
0.49 mmol) and acceptor 312 (194 mg, 0.35 mmol) were coevapo-
rated three times with anhydrous toluene and kept for an hour
under vacuum. After the addition of freshly activated 4Å AW
300 MS (ca 400 mg in pellets), the mixture was dissolved under
argon in anhydrous acetonitrile (1.8 mL) at 0 °C. After 15 min
a solution of Yb(OTf)3 (21.7 mg, 0.035 mmol) in acetonitrile (1.1
mL) was added. The mixture was allowed to warm to room
temperature and left overnight under stirring to ensure complete
glycosidation. The reaction was quenched with a few drops of
pyridine and the mixture filtered on a short plug of silica gel
eluted with 9:1 dichloromethane/methanol (with a few drops of
pyridine). The residue was then chromatographed on a silica
gel column eluted with petroleum ether/ethyl acetate (from 8:2
to 7:3) to yield pure disaccharide 11 (211 mg, 70%). [R]D -32.6
(c 0.5 in CH2Cl2). 1H NMR (400 MHz, CDCl3) δ 7.40-6.80
(aromatic protons), 5.33 (1H, bd, J ) 3.4 Hz), 5.11-4.36 (6H, 3
× AB, 3 × benzyl CH2), 4.86 (2H, 2 × d, J ) 7.6 and 8.0 Hz),
4.76 (1H, dd, J ) 7.6 and 11.0 Hz), 4.22-4.06 (3H), 3.98-3.90
(2H), 3.78 (3H, s, -OCH3), 3.74-3.56 (5H), 2.16, 2.07, 2.00 (3 ×
3H, 3 × s, 3 × -COCH3). 13C NMR (50 MHz, CDCl3) δ 171.2,
170.2, 169.4, 155.3, 151.5, 138.5, 138.5, 137.8, 128.5-127.8,
5318 J. Org. Chem., Vol. 70, No. 13, 2005

romethane/methanol (with drops of pyridine). The residue was
then purified on a silica gel column eluted with toluene/ethyl
acetate (from 5:1 to 3:1) to yield tetrasaccharide 2 (61 mg, 66%)
(calcd) 1679.72, Mr (found) 1702.40 (M+Na)+. C101H105N3O20
calcd C 72.17, H 6.30, found C 71.90, H 6.45.
1
as the only detectable anomer. [R]D -38.6 (c 0.5 in CH2Cl2). H
Acknowledgment. This research was partially sup-
ported by MIUR (PRIN 2003 and 2004) and Regione
Campania (L.R. 5/2002). NMR and MS facilities of
CIMCF are acknowledged.
NMR (400 MHz, CDCl3) δ 7.50-6.80 (aromatic protons), 5.61
(1H, d, J ) 3.2 Hz), 5.18-4.40 (18 H, 9 × AB, 9 × benzyl CH2),
5.54 (1H, s), 4.89 (1H, d, J ) 7.6 Hz,), 4.78 (1H, d, J ) 8.0 Hz),
4.75 (1H, d, J ) 7.6 Hz), 4.34 (1H, bq, J ) 6.8 Hz), 4.28-4.15
(5H), 4.10-3.90 (4H), 3.79 (3H, s, -OCH3), 3.80-3.50 (9H), 3.26
(1H, s), 0.69 (3H, d, J ) 6.8 Hz). 13C NMR (50 MHz, CDCl3) δ
155.2, 151.6, 139.0, 139.0, 138.9, 138.6, 138.4, 138.3, 138.2, 138.0,
137.9, 137.9, 128.5-126.3, 118.5, 114.4, 103.6, 103.3, 102.9,
101.2, 97.8, 84.0, 81.2, 79.9, 79.1, 78.4, 76.2, 75.5, 75.4, 75.3,
74.9, 74.5, 74.0, 73.5, 73.0, 72.8, 72.6, 72.4, 71.4, 69.1, 68.9, 66.7,
66.4, 55.6, 16.1. MALDI-TOF MS for C101H105N3O20 (m/z): Mr
Supporting Information Available: Synthesis proce-
1
dures and characterization data, including H and 13C NMR
spectra, for all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
JO050301X
J. Org. Chem, Vol. 70, No. 13, 2005 5319