Multigram-scale synthesis of an orthogonally protected 2-acetamido-4-amino- 2,4,6-trideoxy-d-galactose (AAT) building block

Article abstract of DOI:10.1016/j.carres.2012.02.015

Reported is the gram-scale synthesis of tert-butyldiphenylsilyl 4-(N-benzyloxycarbonyl)-amino-2-azido-2,4,6-trideoxy-β-d-galactopyranoside, which represents an orthogonally protected 2,4-diamino-d-fucose building block, a common constituent of various zwi

Full text of DOI:10.1016/j.carres.2012.02.015

Carbohydrate Research 356 (2012) 282–287
Contents lists available at SciVerse ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Note
Multigram-scale synthesis of an orthogonally protected
2-acetamido-4-amino-2,4,6-trideoxy-D-galactose (AAT) building block
⇑
A.E. Christina, V.M. Blas Ferrando, F. de Bordes, W.A. Spruit, H.S. Overkleeft, J.D.C. Codée ,
G.A. van der Marel
⇑
Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 January 2012
Received in revised form 9 February 2012
Accepted 10 February 2012
Available online 20 February 2012
Reported is the gram-scale synthesis of tert-butyldiphenylsilyl 4-(N-benzyloxycarbonyl)-amino-2-azido-
2,4,6-trideoxy-b-
building block, a common constituent of various zwitterionic polysaccharides. The building block has been
synthesized from -glucosamine in 19% overall yield over 14 steps, requiring 5 chromatographic purifica-
D-galactopyranoside, which represents an orthogonally protected 2,4-diamino-D-fucose
D
tions. The key step in the synthesis is the introduction of the C-4 amino substituent, which has been
accomplished by a one-pot three step procedure, involving regioselective C-3-O-trichloroacetimidate
formation, C-4-O-triflation, and intramolecular substitution. The building block can be used as an acceptor
and is readily transformed into a donor glycoside.
Keywords:
Deoxysugar
Intramolecular substitution
Zwitterionic polysaccharide
Ó 2012 Elsevier Ltd. All rights reserved.
2-Acetamido-4-amino-2,4,6-trideoxy-
D
-galactose (AAT) is
a
employ the installment of a C-6 tosylate, which is subsequently
displaced by iodine prior to hydride substitution (Sharon 1974,12
Pozsgay 1997,13,7 Schmidt 201014). Introduction of the C-4 amino
functionality has most often been accomplished through the SN2-
type displacement of a C-4 mesylate,11,12 tosylate13 or triflate7,8,14
with azide15,16 or phthalimide.14
rare carbohydrate residue found in various polysaccharides, present
in infectious bacteria, such as Shigella sonnei,1 Streptococcus pneumo-
nia,2 Bacteroides fragilis,3 Streptococcus mitis4 and Proteus vulgaris.5
AAT represents an important constituent of many zwitterionic poly-
saccharides (ZPs), which are capable of eliciting a T-cell dependent
immune response.6 Key to this activity is the presence of both neg-
ative and positive charges on the polysaccharide backbone. The neg-
ative charges in these polysaccharides originate from either uronic
acid constituents or pyruvate moieties, where the positive charge
is often found on the C-4 amino function of the AAT-residues. To gain
insight into the role of these AAT-containing polysaccharides in
bacterial pathogenicity and immunogenicity, the availability of
(fragments of) pure polysaccharides is of importance and therefore
the synthesis of these polysaccharides has attracted ample atten-
tion.7–10 In these syntheses one of the obstacles is presented by
the procurement of sufficient amounts of a suitable AAT-building
block. Over the years several syntheses have been reported, most
of which start to form a glucosamine precursor, as summarized in
Scheme 1. Transformation of the glucosamine core (Scheme 1A) into
an AAT-building block requires deoxygenation of C-6 and introduc-
tion of the second amino functionality with concomitant inversion
at C-4. Lönngren and co-workers employed a di-mesylate to accom-
plish these two steps in the first synthesis of an orthogonally
protected AAT building block in 1984.11 Other syntheses typically
Syntheses starting from different precursors have also been
developed, as exemplified by the synthetic efforts of van Boom
and co-workers, who started from
Recently, Bundle reported an elegant procedure starting from
-glucal (Scheme 1C).18 Deoxygenation of C-6 was followed by
D
-mannose (Scheme 1B).17
D
the regioselective introduction of a C-3 benzyl carbamate. Intramo-
lecular displacement of the subsequently installed C-4 mesylate
led to a C-4-amino galactal, protected with a cyclic carbamate,
which was subjected to azidonitration to install the required C-2
azide functionality. Seeberger and co-workers employed a concep-
tually different approach and used Cbz-protected L-threonine as a
precursor to generate a Cbz-protected C-4-amino galactal interme-
diate in a de novo strategy (Scheme 1D).19 We have previously
reported on the use of an intramolecular displacement strategy
to obtain a suitably protected AAT-building block, featuring a
non-participating azide group at C-2.7 This strategy is based on
the regioselective installment of a C-3-O-imidate functionality,
followed by the introduction of a C-4-triflate and subsequent
oxazoline formation.20 We here describe an optimized synthetic
route, using this approach, for the multi-gram synthesis of AAT
building block 9.
⇑
Corresponding authors.
E-mail addresses: jcodee@chem.leidenuniv.nl (J.D.C. Codée), marel_g@chem.
leidenuniv.nl (G.A. van der Marel).
Our synthesis started from glucosamine hydrochloride (1) as de-
picted inScheme 2. Introduction ofthe requiredC-2azidewas accom-
plished by an azidotransfer reaction using imidazole-1-sulfonyl
Other abbreviations found in literature include: AATGal and D-FucNAc4N.
0008-6215/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2012.02.015

A. E. Christina et al. / Carbohydrate Research 356 (2012) 282–287
283
A
D-glucosamine
Sharon (1978)
AcHN
van der Marel (2007)
OTs
O
CbzHN
I
HO
BnO
O
O
O
HO
AcO
SPh
SPh
HO
AcO
AcHN
OH
OBn
AcHN
PhthN
N3
van der Marel (2007)
Lönngren (1984)
CbzHN
OMs
O
N3
O
TfO
O
MsO
BnO
OTBDMS
O
OTBDMS
HN
O
HO
HO
N3
OMs
AcHN
N3
OMe
Pozsgay (1997)
AcHN
N3
Bundle (2009)
CCl3
OMe
N3
OTs
O
O
HO
BnO
O
OMe
TsO
BnO
O
SEt
OMe
HO
AcHN
SEt
AcO
PhthN
AcHN
PhthN
Schmidt (2010)
Grindley (2004)
Br
O
OTs
O
N3
PhthN
AcO
HO
HO
AcO
AcO
O
O
SEt
BzO
OMP
AcHN
N3
OMP
OMe
PhthN
N3
B
D-mannose
C
D-glucal
Bn
N
Bundle (2010)
van Boom (1987)
O
O
MsO
O
AcHN
O
N3
OAn
O
O
O
O
O
N3
OH
HO
NHBn
N3
AcHN
OBn
D
de novo
van Boom (1992)
CbzHN
AcO
HO
O
O
N
Seeberger (2010)
CbzHN
AcO
CbzHN
MeO
O
O
O
O
OH
N3
SEt
O
OH
N3
Scheme 1. Previous syntheses of AAT-building blocks.
OH
O
OR
O
R
i
iii
O
HO
HO
RO
RO
HO
HO
OTBDPS
OTBDPS
H2N OH
N3
N3
•HCl
2 R = Ac
3 R = H
4 R = OTs
5 R = I
6 R = H
iv
v
ii
1
vi
CbzHN
ClAcO
CbzHN
RO
N
O
vii
xi
Cl3C
O
O
O
OTBDPS
7
OTBDPS
OTBDPS
N3
OR
N3
N3
11 R = H
12 R = C(=NPh)CF3
9 R = H
10 R = ClAc
+
x
viii
O
O
8
N3
N
Cl3C
Scheme 2. Reagents and conditions: (i) (1) Imidazole-1-sulfonyl azideꢀHCl, MeOH, CuSO4 (cat.); (2) pyridine, Ac2O; (3) piperidine, THF; (4) t-BuPh2SiCl, imidazole, DMF (60%,
4 steps); (ii) NaOMe (cat.), MeOH, DCM (quant.); (iii) tosylchloride, pyridine (83%); (iv) NaI, butanone (92%); (v) NaCNBH3, diethylene glycol diethyl ether, reflux (88%); (vi)
Cl3CCN, DBU, DCM, ꢁ13 °C then Tf2O, pyridine then DiPEA (I: 24%, J: 63%); (vii) (1) AcOH, H2O, EtOAc; (2) N-(benzyloxycarbonyloxy)succinimide, triethylamine, DCM (75%);
(viii) (ClAc)2O, pyridine, DCM, (quant.); (ix) triethylamineꢀ3HF, THF, (98%); (x) ClC(@NPh)CF3, Cs2CO3, H2O, acetone, (83%,
a/b 1:3).
azideꢀHCl, introduced by Goddard-Borger and Stick.21 Global acetyla-
tion was then followed by liberation of the anomeric hydroxyl
through the aegis of piperidine in THF. In our previous synthesis of
an AAT building block (see Scheme 1A) we employed a tert-
butyldimethylsilyl group to mask the anomeric hydroxyl.7 However,
we found that this silyl ether was not completely stable to the acidic

284
A. E. Christina et al. / Carbohydrate Research 356 (2012) 282–287
reaction conditions employed later on in the synthesis to cleave the
intermediate oxazoline and therefore we switched to the use of
the more acid stable tert-butyldiphenylsilyl ether.22 Introduction of
the anomeric TBDPS ether using TBDPS-Cl and imidazole in DCM
led to the fully protected crystalline glucosazide 2, which was ob-
tained in 60% yield over four steps without chromatographic purifica-
tion (300 mmol scale). Next, the three acetyl groups were removed
and a tosylate was regioselectively installed at the C-6-OH. Substitu-
tion of the tosylate for an iodine function then set the stage for the
crucial deoxygenation step, which required substantial optimization.
It was found that the use of NaBH4 as a reducing agent in DMSO led to
partial reduction of the azide functionality and we therefore switched
to the use of a milder reducing agent, NaCNBH3, at higher tempera-
ture. It was found that diethylene glycol was the optimal solvent for
the reaction and at reflux temperature iodide 5 was uneventfully re-
duced to give the key intermediate 6 in 88% yield. The required C-4
amino group was installed using an intramolecular displacement
strategy.20 Thus, in a one-pot three step procedure diol 6 was treated
with trichloroacetonitrile and a catalytic amount of DBU to give the
intermediate C-3-O-imidate. Next, triflic anhydride and pyridine
(5 equiv) were added to the reaction mixture to form the C-4
trifluoromethanesulfonyl ester. Finally treatment of this species with
an excess DiPEA furnished oxazoline 7,à which was isolated in 63%
yield. We also isolated the allo-configured oxazoline 8, formed from
the regioisomeric imidate, by C-3-O-triflation and intramolecular
substitution, in 23% yield. Hydrolysis of the oxazoline moiety in 7 with
acetic acid and water gave an intermediate amino alcohol, which was
directly transformed into benzylcarbamate 9. As anticipated the
anomeric TBDPS ether was unaffected during cleavage of the oxazoline
moiety. tert-Butyldiphenylsilyl4-(N-benzyloxycarbonyl)-amino-2-azi-
do-2,4,6-trideoxy-b-D-galactopyranoside 9 was obtained in 19% yield
from D-glucosamine in 14 steps, requiring 5 chromatographic purifica-
tions. AAT building block 9 was further converted into 1-hydoxyl donor
11 by installation of a chloroacetyl ester at the C-3-OH and subsequent
removal of the anomeric silyl group using HF.Et3N (98% over two steps).
Imidate donor 12 was obtained from this lactol by treatment with N-
phenyltrifluoroacetamidoyl chloride in acetone in the presence of
Cs2CO3 and a few drops of water. Buildingblocks 9, 11, and 12 represent
a set ofversatilebuilding blocksforthe construction ofvariousAATcon-
taining (zwitterionic) oligosaccharides.
gel (screening devices, 40–63
l
m
60 Å, www.screeningde-
vices.com) using technical grade, distilled solvents. NMR spectra
were recorded on a Bruker AV400. For solutions in CDCl3 chem-
ical shifts (d) are reported relative to tetramethylsilane (1H) or
CDCl3 (
13C). Peak assignments were made based on HH-COSY
and HSQC measurements. Optical rotation was measured using
a Propol automatic polarimeter. The IR absorbance was recorded
using a Shimadzu FTIR-83000 spectrometer. Mass analysis was
performed using a PE/SCIEX API 165 with an Electrospray Inter-
face (Perkin–Elmer).
1.2. tert-Butyldiphenylsilyl 3,4,6-tri-O-acyl-2-azido-2-deoxy-b-
D-glucopyranoside(2)
To a mixture of 107.8 g
D
-glucosamineꢀHCl (500 mmol, 1 equiv)
in 2 L MeOH was added 174 mL triethylamine (1.25 mol,
2.5 equiv), 1.25 g CuSO4ꢀH2O (5 mmol, 0.01 equiv), and 125.8 g
imidazole-1-sulfonyl azideꢀHCl22 (600 mmol, 1.2 equiv). The reac-
tion was stirred for 1.5 h and the solvents were evaporated. The
crude material was coevaporated with pyridine and subsequently
stirred overnight in 2 L pyridine/Ac2O (4/1 v/v). The solvent was
evaporated and the residue was partitioned between H2O and
EtOAc. The organic layer was washed with aq 1 M HCl solution,
satd aq NaHCO3 solution, and brine. The organic phase was dried
(MgSO4), filtered, and concentrated under reduced pressure. To a
mixture of the crude product in 1 L THF was added 117 mL piper-
idine (1.19 mol, 2.4 equiv) and the reaction was run for 2.5 h. The
mixture was diluted with 1.5 L EtOAc and washed with aq 1 M
HCl solution, satd aq NaHCO3 solution, and brine. The organic
phase was dried (MgSO4), filtered, and concentrated under reduced
pressure. The crude hemiacetal was coevaporated with toluene and
dissolved in 700 mL DMF. To this solution 47.5 g imidazole
(697 mmol, 1.4 equiv) and 135.6 mL t-BuPh2SiCl (523 mmol,
1.05 equiv) were added and the mixture was stirred for 2 h at
60 °C. Next, 1.5 L H2O was added and the mixture was extracted
with EtOAc. The combined organic layers were washed with aq
1 M HCl solution, satd aq NaHCO3 solution, and brine. The organic
phase was dried (MgSO4), filtered, and concentrated under reduced
pressure. Crystallization from EtOH yielded 171.9 g of the title
compound (2) (302.0 mmol, 60% over 4 steps). Spectral data were
in accordance with those reported in the literature.23
In conclusion we have described an optimized synthetic route
for the multi-gram synthesis of orthogonally protected AAT-build-
ing blocks starting from
D-glucosamine. Key steps in the synthesis
1.3. tert-Butyldiphenylsilyl 2-azido-2-deoxy-b-D-
include the deoxygenation of a C-6-iodo glucosazide and the
subsequent one-pot three step tethered nucleophilic inversion
procedure to introduce the C-4 amino functionality. The usefulness
of AAT synthons 9, 11, 12 in the construction of zwitterionic
oligosaccharides has previously been described in the context of
the assembly of all three repeating units of the ZP of S. pneumonia
Sp1.9
glucopyranoside(3)
To a solution of 66.11 g tert-butyldiphenylsilyl 3,4,6-tri-O-acet-
yl-2-azido-2-deoxy-b- -glucopyranoside 2 (116.1 mmol, 1 equiv)
D
in 500 ml methanol/DCM (9/1 v/v) was added 1.27 g NaOMe
(23.6 mmol, 0.2 equiv). The mixture was stirred until TLC indi-
cated complete conversion of the starting material to a single
lower running spot. The mixture was neutralized with Amberlite
H+ resin and filtered. The filtrate was evaporated to dryness yield-
ing 51.4 g of the title compound (115.8 mmol, quant.). Rf 0.25
(EtOAc/PE, 3/2, v/v); IR (neat, cmꢁ1) 3370 (br), 2932, 2860,
2110, 1428, 698; 1H NMR (400 MHz, CDCl3) d 7.73–7.66 (m, 4H,
1. Experimental section
1.1. General procedures
H
arom), 7.46–7.32 (m, 6H, Harom), 4.51 (d, J = 7.7 Hz, 1H, H-1),
All chemicals were used as received. Trifluoromethanesulfonic
anhydride (Tf2O) was distilled from P2O5 and stored in a Schlenk
flask. TLC analysis was conducted on silica gel-coated aluminum
TLC sheets (Merck, silica gel 60, F245). Compounds were visual-
ized by UV absorption (245 nm), by spraying with 20% H2SO4
in ethanol or with a solution of (NH4)6Mo7O24ꢀ4H2O 25 g/L,
(NH4)4Ce(SO4)4ꢀ2H2O10 g/L, 10% H2SO4 in H2O followed by char-
ring at ꢂ140 °C. Flash chromatography was performed on silica
4.22 (s, 2H, OH), 3.49–3.36 (m, 3H, H-6, H-4), 3.30 (dd, J = 10.0,
7.7 Hz, 1H, H-2), 3.19 (br t, J = 9.4 Hz, 1H, H-3), 2.85–2.78 (m,
1H, H-5), 1.90 (s, 1H, OH), 1.11 (s, 9H, CH3 t-Bu); 13C NMR
(101 MHz, CDCl3) d 135.7 (CHarom), 133.6, 132.4 (Cq), 130.1,
129.9, 127.7, 127.4 (CHarom), 96.9 (C-1), 75.0 (C-5), 74.6 (C-3),
69.6 (C-4), 68.6 (C-2), 61.4 (C-6), 26.7 (CH3t-Bu), 19.0 (Cqt-Bu);
½
a 2D2
ꢃ
+25 (c 1.0, CHCl3); HRMS [M+Na]+ calcd for C22H29N3O5SiNa
466.17687, found 466.17659.
à
Ring closure can also be effected by stirring overnight without addition of DiPEA.

A. E. Christina et al. / Carbohydrate Research 356 (2012) 282–287
285
1.4. tert-Butyldiphenylsilyl 2-azido-2-deoxy-6-O-tosyl-b-
glucopyranoside (4)
D
-
7.42–7.31 (m, 6H, Harom), 4.38 (d, J = 7.9 Hz, 1H, H-1), 4.11 (s, 1H,
OH), 3.83 (s, 1H, OH), 3.27 (t, J = 8.5 Hz, 1H, H-2), 3.14–3.02 (m,
2H, H-3, H-4), 2.88–2.81 (m, 1H, H-5), 1.12 (s, 9H, CH3 t-Bu), 1.02
(d, J = 6.9 Hz, 3H, H-6). 13C NMR (100 MHz, CDCl3, HH-COSY, HSQC)
d 135.9, 135.8 (CHarom), 133.2, 132.7 (CqPh), 129.8, 129.7, 127.5,
127.3 (CHarom), 96.5 (C-1), 75.4 (C-4), 74.7 (C-3), 71.4 (C-5), 68.9
(C-2), 26.8 (CH3 t-Bu), 19.1 (Cq t-Bu), 17.1 (C-6); HRMS [M+Na]+
calcd for C22H31N3O4SiNa 450.18195, found 450.18171.
8.45 g Tosylchloride (44.3 mmol, 3.0 equiv) was added to an
ice-cooled solution of 6.55 g tert-butyldiphenylsilyl 2-azido-2-
deoxy-b-D-glucopyranoside (3) (14.8 mmol, 1.0 equiv) in 75 mL
pyridine. The mixture was stirred for 2 h at 0 °C and quenched
by the addition of MeOH. After evaporation of the solvents the
crude mixture was partitioned between EtOAc and water and the
organic layer was washed with aq 1 M HCl solution, satd aq NaH-
CO3 solution, and brine. The organic phase was dried (MgSO4),
filtered, and concentrated under reduced pressure. Flash column
chromatography using EtOAc/ PE (3/7 ? 2/3) gave 7.31 g (12.2
mmol, 83%) of the title compound (4) as a colorless oil. Rf 0.29
1.7. 2-Trichloromethyl-4,5-dihydro-(2-azido-2,4,6-trideoxy-1-
O-tert-Butyldiphenylsilyl-b-
D-galactopyranoso)[4,3-d]-1,3-
oxazole (8) and 2-trichloromethyl-4,5-dihydro-(2-azido-2,4,6-
trideoxy-1-O-tert-butyldiphenylsilyl-b-D-allopyranoso)[3,4-d]-
1,3-oxazole (8)
(EtOAc/PE, 2/3, v/v); ½a D22
ꢃ
+11 (c 1.0, CH2Cl2); IR (neat, cmꢁ1
)
3400 (br), 2932, 2858, 2110, 1174, 812; 1H NMR (400 MHz, CDCl3,
HH-COSY, HSQC) d 7.68–7.62 (m, 6H, Harom), 7.44–7.21 (m, 8H, Har-
om), 4.36 (d, J = 7.6 Hz, 1H, H-1), 4.05 (dd, J = 10.5, 4.5 Hz, 1H, H-6),
3.85 (d, J = 10.5 Hz, 1H, H6), 3.77 (s, 1H, OH), 3.67 (s, 1H, OH), 3.46
(t, J = 9.2 Hz, 1H, H-4), 3.30 (dd, 1H, J = 9.2, 7.6 Hz, H-2), 3.20 (t,
J = 9.3 Hz, 1H, H-3), 2.99 (dd, J = 9.7, 4.2 Hz, 1H, H-5), 2.41 (s, 3H,
CH3Ts), 1.08 (s, 9H, CH3 t-Bu); 13C NMR (100 MHz, CDCl3, HH-COSY,
HSQC) d 144.9 (CqTs), 135.8, 135.7 (CHarom), 132.8, 132.4, 132.2
(CqPh), 129.9, 129.8, 129.7, 127.9, 127.5, 127.4 (CHarom), 96.7 (C-
1), 74.5 (C-3), 73.0 (C-5), 69.3 (C-4), 68.3 (C-2), 68.1 (C-6), 26.7
(CH3 t-Bu), 21.6 (CH3Ts), 19.0 (Cq t-Bu); HRMS [M+Na]+ calcd for
Diol (6) (3.49 g, 8.18 mmol, 1 equiv) and 984 lL Cl3CCN
(9.82 mmol,0 1.2 equiv) were dissolved in 80 mL DCM, stirred over
activated 3 ÅA molecular sieves, and cooled to ꢁ13 °C. After addition
of 122 lL DBU (818 lmol, 0.1 equiv) the reaction mixture was
allowed to stir for 1 h. Then 3.30 mL pyridine (40.9 mmol, 5 equiv)
and 1.64 mL triflic anhydride (9.82 mmol, 1.2 equiv) were added at
ꢁ30 °C and the reaction mixture was allowed to warm to ambient
temperature. Two hours later 13.52 mL DiPEA (81.8 mmol,
10 equiv) was injected and the mixture was stirred overnight.
H2O was added and the organic layer was separated from the
aqueous phase, which was extracted with DCM. Drying over
MgSO4, filtration, and concentration under reduced pressure, filtra-
tion over Celite (eluent: EtOAc/PE 1/99) and again removal of the
solvents gave a crude mixture. Purification was done by flash col-
umn chromatography (silica was pretreated with triethylamine/PE
(1/19 ? 0/1)) using Et2O/PE (0/1 ? 5/95) as eluent to furnish the
title compounds (8) (1.07 g, 1.94 mmol, 24%) Rf 0.80 (EtOAc/PE,
C29H35N3O7SSiNa 620.18572, found 620.18562.
1.5. tert-Butyldiphenylsilyl 2-azido-2,6-dideoxy-6-iodo-b-D-
glucopyranoside (5)
Tosylate 4 (7.22 g, 12.1 mmol, 1 equiv) was refluxed for 6 h in
60 mL butanone together with 3.98 g NaI (26.6 mmol, 2.2 equiv).
After cooling to room temperature EtOAc was added and the mix-
ture was washed with aq 1 M Na2S2O3 solution and H2O. The or-
ganic phase was dried (MgSO4), filtered, and concentrated under
reduced pressure. Flash column chromatography using EtOAc/PE
(1/4 ? 3/7) afforded 6.12 g of the title compound 5 (11.1 mmol,
1/9, v/v); ½a 2D2
ꢃ
ꢁ27 (c 1.0, CH2Cl2); IR (neat, cmꢁ1) 2932, 2860,
2108, 1653, 1427, 978, 698; 1H NMR (400 MHz, CDCl3, HH-COSY,
HSQC) d 7.72–7.65 (m, 4H, Harom), 7.45–7.33 (m, 6H, Harom), 4.87
(d, J = 5.8 Hz, 1H, H-1), 4.75 (dd, J = 8.5, 5.7 Hz, 1H, H-3), 4.59 (t,
J = 8.8 Hz, 1H, H-4), 3.93 (t, J = 5.7 Hz, 1H, H-2), 3.43–3.36 (m, 1H,
H-5), 1.14 (d, J = 6.2 Hz, 3H, H-6), 1.11 (s, 9H, CH3 t-Bu). 13C NMR
(100 MHz, CDCl3, HH-COSY, HSQC) d 163.7 (C@N), 135.7, 135.6
(CHarom), 132.8, 132.5(CqPh), 130.0, 129.8, 127.8, 127.7, 127.4
(CHarom), 94.6 (C-1), 84.0 (C-4), 68.8 (C-5), 66.5 (C-3), 61.4 (C-2),
26.7 (CH3 t-Bu), 19.0 (Cq t-Bu), 18.9 (C-6); HRMS [M+H]+ calcd
for C24H28Cl3N4O3Si 553.09908, found 553.09909; and (7) (2.83 g,
92%) as a yellow oil. Rf 0.46 (EtOAc/PE, 2/3, v/v); ½a D22
ꢃ
+8 (c 1.0,
CH2Cl2); IR (neat, cmꢁ1) 3400, 2858, 2110, 1078, 812; 1H NMR
(400 MHz, CDCl3, HH-COSY, HSQC) d 7.79–7.69 (m, 4H, Harom),
7.45–7.34 (m, 6H, Harom), 4.49 (d, J = 7.6 Hz, 1H, H-1), 3.51 (s, 1H,
OH), 3.41 (s, 1H, OH), 3.40 (t, J = 8.8 Hz, 1H, H-4), 3.37(dd, J = 9.6,
7.6, Hz, 1H, H-2), 3.28 (dd, J = 9.6, 8.8 Hz, 1H, H-3), 3.24 (dd,
J = 10.8, 4.8 Hz, 1H, H6), 3.15 (dd, J = 10.8, 2.8 Hz, 1H, H-6), 2.62–
2.53 (m, 1H, H-5), 1.13 (s, 9H, CH3 t-Bu). 13C NMR (100 MHz, CDCl3,
HH-COSY, HSQC) d 136.1, 135.9 (CHarom), 132.8, 132.5 (CqPh),
130.0, 129.7, 127.6, 127.5, 127.4, 127.3 (CHarom), 96.4 (C-1), 74.3
(C-3), 73.6 (C-4), 73.1 (C-5), 68.6 (C-2), 26.8 (CH3 t-Bu), 19.1 (Cq
t-Bu), 5.8 (C-6); HRMS [M+Na]+ calcd for C22H28IN3O4SiNa
576.07860, found 576.07845.
5.13 mmol, 63%) Rf 0.58 (EtOAc/PE, 1/9, v/v); ½a D22
ꢃ
+42 (c 1.0,
CH2Cl2); IR (neat, cmꢁ1) 2860, 2116, 1655, 1427, 1063, 700; 1H
NMR (400 MHz, CDCl3, HH-COSY, HSQC) d7.83–7.61 (m, 4H, Harom),
7.50–7.32 (m, 6H, Harom), 4.64 (t, J = 8.1 Hz, 1H, H-4), 4.31 (d,
J = 8.1 Hz, 1H, H-1), 3.90 (dd, J = 8.3, 3.2 Hz, 1H, H-4), 3.44–3.38
(m, 1H, H-5), 3.38 (t, J = 8.0 Hz, 1H, H-2), 1.32 (d, J = 6.3 Hz, 3H,
H-6), 1.13 (s, 9H, CH3 t-Bu). 13C NMR (100 MHz, CDCl3, HH-COSY,
HSQC) d 162.3 (C@N), 135.9, 135.8 (CHarom), 132.8, 132.5 (CqPh),
130.0, 129.8, 127.6, 127.4 (CHarom), 95.4 (C-1), 84.5 (C-3), 69.9
(C-5), 67.0 (C-4), 66.6 (C-2), 26.7 (CH3 t-Bu), 19.1 (Cq t-Bu), 17.3
(C-6); HRMS [M+H]+ calcd for C24H28Cl3N4O3Si 553.09908, found
553.09892.
1.6. tert-Butyldiphenylsilyl 2-azido-2,6-dideoxy-b-D-
glucopyranoside (6)
To a solution of 10.56 g (19.1 mmol, 1 equiv) tert-butyldiphenyl-
silyl 2-azido-2,6-dideoxy-6-iodo-b- -glucopyranoside 5 in 110 mL
D
1.8. tert-Butyldiphenylsilyl 4-(N-benzyloxycarbonyl)-amino-2-
diethylene glycol diethyl ether was added 11.9 g (190 mmol,
10 equiv) of NaCNBH3 and the mixture was refluxed for 7 h. After
cooling to room temperature, the mixture was diluted with 1 L of
EtOAc, washed with water and brine. dried (MgSO4), and concen-
trated in vacuo. Flash column chromatography using EtOAc/PE (1/
4 v/v) afforded 7.2 g of the title compound 6 (16.8 mmol, 88%). Rf
azido-2,4,6-trideoxy-b-D-galactopyranoside (9)
1.61 g Dihydro-oxazole (7) (2.92 mmol, 1 equiv) was stirred
overnight in 18 mL AcOH/H2O/EtOAc (4/1/1). The solvents were re-
moved and the residue was coevaporated with toluene. The crude
amine was dissolved in 15 mL of DCM and 526 lL triethylamine
0.38 (EtOAc/PE, 2/3, v/v); ½a D22
ꢃ
+22 (c 1.0, CH2Cl2); IR (neat, cmꢁ1
)
(3.79 mmol, 1.3 equiv) and 800 mg N-(benzyloxycarbonyloxy)suc-
cinimide (3.21 mmol, 1.1 equiv) were added. Stirring was allowed
for 45 min followed by quenching with MeOH. Product 9 (1.22 g,
3364, 2932, 2862, 2361, 2114, 1111, 1072, 818; 1H NMR
(400 MHz, CDCl3, HH-COSY, HSQC) d 7.70 (t, J = 7.9 Hz, 4H, Harom),

286
A. E. Christina et al. / Carbohydrate Research 356 (2012) 282–287
2.19 mmol, 75%) was obtained in pure form by flash column chro-
matography using EtOAc/PE (1/4 ? 1/3). Rf 0.59 (EtOAc/PE, 7/13, v/
HH-COSY, HSQC) d 166.9, 157.1, 157.0 (C@O), 136.1, 136.0 (CqPh),
128.6, 128.5, 128.3, 128.1, 127.9 (CHarom), 96.2 (C-1b), 91.8 (C-1 ),
74.6 (C-3b), 72.2 (C-3 ), 69.2 (C-5b), 67.3, 67.2 (CH2Cbz), 64.1 (C-
), 61.8 (C-2b), 58.0 (C-2 ), 52.5 (C-4 ), 51.8 (C-4b), 40.6, 40.5
(CH2ClAc), 16.4 (C-6b), 16.3 (C-6
); HRMS [M+H]+ calcd for
16H20ClN4O6 399.10659, found 399.10647.
a
v); ½a 2D2
ꢃ
+19 (c 1.0, CH2Cl2); IR (neat, cmꢁ1) 3410 (br), 2939, 2862,
a
2114, 1705, 1512, 1111, 1065; 1H NMR (400 MHz, CDCl3, HH-COSY,
HSQC) d7.71–7.68 (m, 4H, Harom), 7.44–7.31 (m, 11H, Harom), 5.17–
5.06 (m, 2H, CH2Cbz), 4.97 (d, J = 9.4 Hz, 1H, NH), 4.37 (d, J = 7.8 Hz,
1H, H1), 3.83 (dd, J = 9.3, 3.4 Hz, 1H, H-4), 3.51 (dd, J = 10.0, 2.8 Hz,
1H, H-3), 3.31–3.20 (m, 2H, H-2, H-5), 3.17 (s, 1H, OH), 1.10 (s, 9H,
CH3 t-Bu), 0.96 (d, J = 6.4 Hz, 3H, H-6); 13C NMR (100 MHz, CDCl3,
HH-COSY, HSQC) d 157.8 (C@O Cbz), 135.9 (CqPh), 135.8, 135.7
(CHarom), 133.2, 132.7 (CqPh), 129.8, 129.7, 128.5, 128.2, 128.1,
127.4, 127.3 (CHarom), 97.0 (C-1), 72.2 (C-3), 69.3 (C-5), 67.3
(CH2Cbz), 66.8 (C-2), 54.8 (C-4), 26.7 (CH3 t-Bu), 19.0 (Cq t-Bu),
16.1 (C-6). HRMS [M+H]+ calcd for C30H37N4O5Si 561.25277, found
561.25250.
5
a
a
a
a
C
1.11. 4-(N-Benzyloxycarbonyl)-amino-2-azido-3-O-chloroacetyl-
2,4,6-trideoxy- /b- -galactopyranosyl (N-
a
D
phenyl)trifluoroacetimidate (12)
To a solution of 511 mg hemiacetal 11 (1.28 mmol, 1 equiv) in
6.1 mL acetone and 0.3 mL H2O were added 460 mg Cs2CO3
(1.41 mmol, 1.1 equiv) and 532 mg ClC(C@NPh)CF3 (2.56 mmol,
2 equiv). When TLC analysis showed complete consumption of
the starting material, the mixture was coevaporated with toluene.
Purification by flash column chromatography using EtOAc/PE (1/9
1.9. tert-Butyldiphenylsilyl 4-(N-benzyloxycarbonyl)-amino-2-
azido-3-O-chloroacetyl-2,4,6-trideoxy-b-
(10)
D
-galactopyranoside
? 3/7) yielded 606 mg of imidate 12 (1.06 mmol, 83%, anomers a/b
1:3). Rf 0.54 (EtOAc/PE, 1/3, v/v); IR (neat, cmꢁ1) 2116, 1717, 1524,
1211, 1163, 1072, 696; 1H NMR (400 MHz, CDCl3, HH-COSY, HSQC)
(T = 333 K) d 7.41–7.23 (m, 9.3H, Harom), 7.09 (m, 1.4H, Harom), 6.83
To a mixture of alcohol 9 (860 mg, 1.54 mmol, 1 equiv), 5 mL
DCM and 607 pyridine (7.68 mmol, 5 equiv) was added
525 mg chloroacetic anhydride (3.07 mmol, 2 equiv). After 1 h,
500 l H2O was added and the mixture was stirred for another
l
l
(m, 2.7H, Harom), 6.35 (s, 0.3H, H-1
5.28 (dd, J = 11.1, 3.5 Hz, 0.3H, H-3
NH), 4.81 (dd, J = 10.7, 3.9 Hz, 1H, H-3b), 4.38–4.26 (m, 0.7H, H-
, H-5 ), 4.16 (dd, J = 9.7, 3.1 Hz, 1H, H-4b), 3.89 (s, 2.7H,
a), 5.49 (d, J = 7.5 Hz, 1H, H-1b),
a
), 5.20–4.96 (m, 4H, CH2Cbz,
l
15 min. After evaporation the residue was taken up in EtOAc and
washed with aq 1 M HCl, satd aq NaHCO3 and brine. The organic
phase was dried over MgSO4, filtered, and evaporated to dryness
yielding title compound 10 (984 mg, 1.54 mmol, quant.). Rf 0.79
4a
a
CH2ClAc), 3.81 (dd, J = 10.9, 3.9 Hz, 0.3H, H-2a), 3.75–3.59 (m,
2H, H-2b, H-5b), 1.20 (m, 4H, H-6). 13C NMR (100 MHz, CDCl3,
HH-COSY, HSQC) (T = 333 K) d 166.4, 156.7 (C@O), 143.1, 143.0,
136.3 (CqPh), 128.8, 128.6, 128.3, 128.0, 124.7, 124.6, 119.3,
(EtOAc/PE, 1/3, v/v); ½a D22
ꢃ
ꢁ9 (c 1.0, CH2Cl2); IR (neat, cmꢁ1
)
2114, 1713, 1504, 1165, 1057, 733. 1H NMR (400 MHz, CDCl3,
119.2 (CHarom), 95.9 (C-1b), 93.7 (C-1
a
), 74.6 (C-3b), 72.2 (C-3
a
a
),
),
HH-COSY, HSQC) d 7.71–7.68 (m, 4H, Harom), 7.46–7.25 (m, 11H,
70.6 (C-5b), 67.5 (C-5 ), 67.4 (CH2Cbz), 60.2 (C-2b), 57.2 (C-2
a
H
arom), 5.14 (d, J = 12.2 Hz, 1H, CH2Cbz), 5.02 (d, J = 12.2 Hz, 1H,
52.4 (C-4a), 51.8 (C-4b), 40.3 (CH2ClAc), 16.2 (C-6); HRMS
CH2Cbz), 4.93 (d, J = 9.5 Hz, 1H, NH), 4.64 (dd, J = 10.7, 3.7 Hz, 1H,
H-3), 4.44 (d, J = 7.7 Hz, 1H, H-1), 4.00 (dd, J = 9.5, 3.4 Hz, 1H, H-
4), 3.95–3.81 (m, 2H, CH2, ClAc), 3.48 (dd, J = 10.4, 8.0 Hz, 1H, H-
2), 3.36 (q, J = 6.2 Hz, 1H, H-5), 1.11 (s, 9H, CH3 t-Bu), 0.97 (d,
J = 6.3 Hz, 3H, H-6). 13C NMR (100 MHz, CDCl3, HH-COSY, HSQC)
d 166.5, 156.45 (C@O), 136.2 (CqPh), 135.7 (CHarom), 132.9, 132.4
(CqPh), 129.9, 128.5, 128.3, 128.1, 127.5, 127.4 (CHarom), 97.0 (C-
1), 74.6 (C-3), 68.9 (C-5), 67.1 (CH2Cbz), 63.5 (C-2), 51.6 (C-4),
40.5 (CH2ClAc), 26.7 (CH3 t-Bu), 19.0, (Cq t-Bu), 16.0 (C-6); HRMS
[M+Na]+ calcd for C32H37ClN4O6SiNa 659.20631, found 659.20672.
[Mꢁ(C(N@Ph)CF3)+H+Na]+ calcd for C16H19ClN4O6 421.08853,
found 421.08845.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.carres.2012.02.015.
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treated with 527
ll N3Etꢀ3HF (3.23 mmol, 2 equiv) and the mixture
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). 13C NMR (100 MHz, CDCl3,
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