Towards the synthesis of aureolic acid analogue conjugates: Synthesis and glycosidation reactions of 3-o-acetyl-4-azido-2,4,6-trideoxy-L-glucopyranose derivatives

Article abstract of DOI:10.1071/CH01199

A stereoselective synthesis of 3-O-acetyl-4-azido-glucopyranose (6) is described. The derived anomeric acetate (13), and especially the thioglycoside (14), are demonstrated to be good donors for synthesis of α-glycosides of (6). Donor (14) was used in the

Full text of DOI:10.1071/CH01199

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Aust. J. Chem. 2002, 55, 141–146
Towards the Synthesis of Aureolic Acid Analogue Conjugates:
Synthesis and Glycosidation Reactions of
3-O-Acetyl-4-azido-2,4,6-trideoxy-L-glucopyranose Derivatives
William R. RoushA,B and Ray A. JamesA
A Department of Chemistry, University of Michigan, Ann Arbor, MI 48109, U.S.A.
B Author to whom correspondence should be addressed (e-mail: roush@umich.edu).
A stereoselective synthesis of 3-O-acetyl-4-azido-glucopyranose (6) is described. The derived anomeric acetate
(13), and especially the thioglycoside (14), are demonstrated to be good donors for synthesis of α-glycosides of (6).
Donor (14) was used in the synthesis of trisaccharide (21), which is targeted for use in the synthesis of the aureolic
acid trisaccharide conjugate analogue (5).
Manuscript received: 11 December 2001.
Final version: 8 March 2002.
C
H
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Jam
a
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s
Introduction
the GC-rich promoter regions of the c-myc protooncogene
and the dihydrofolate-reductase gene, thereby preventing
their translation.[9,10] Nuclear magnetic resonance (NMR)
studies by Patel[5,6] and Kahne[11,12] have established that the
C–D disaccharide of one aureolic acid monomer stacks on the
aromatic nucleus of the other aglycone in the 2 : 1 complexes
with Mg2+, both when bound to DNA,[5,6] and when free in
MeOH solution.[11,12] The intact C–D–E trisaccharide is
essential for stable dimer formation (in MeOH)[11,12] and
maximal DNA binding,[13,14] as well as for full activity as an
antitumour agent.[2–4] However, the contributions of the A–B
disaccharide and C3 polyoxygenated side chain to Mg2+
dimer formation and DNA binding are not yet known.
In the preceding paper[1] we described syntheses of
trisaccharide conjugates (4a) and (4b), which are simplified
analogues of olivomycin A (1) and other members of the
aureolic acid antitumour antibiotic family.[2–4] These
analogues were synthesized in order to assess the minimum
structural features required for biological activity. It is known
that the biological effects of the aureolic acids derive from
their ability to bind in the minor groove of double-stranded
deoxyribonucleic acid (DNA) as 2 : 1 antibiotic/Mg2+
complexes, with selectivity for guanine/cytosine (GC)-rich
sequences.[5–8] It is also known that mithramycin (3) binds to
MeO
Me
O
AcO
HO
Me
Me
Me
OMe OH
OMe OH
B
O
H
O
O
H
HO
HO
HO
O
O
Me
O
Me
O
A
B
A
O
O
OH
O
OH
R1
Me
O
OH OH
Me
O
OH OH
Me
O
HO
Me
Me
OH
Me
O
O
O
O
O
HO
HO
HO
HO
Me
O
O
O
O
D
C
Me
D
C
E
Me
R2CO2
O
E
(1) olivomycin A (R1 = H, R2 = Pri)
(2) chromomycin A3 (R1 = R2 = Me)
OH
(3) mithramycin
2S
2S
O
O
O
O
OH OH
Me
OH OH
Me
OH
Me
N3
Me
Me
O
O
O
O
O
HO
HO
HO
HO
O
O
O
O
OAc
C
D
C
D
Me
O
Me
PriCO2
Me
H2N
O
E
E
(
6)
(4a) (2S)-epimer
(4b) (2R)-epimer
(5)
(
(
OH
OH
© CSIRO 2002
10.1071/CH01199 0004-9425/02/0100141

142
W. R. Roush and R. A. James
One of our long-term goals is to develop aureolic acid
amino diol (11) in 62% yield, along with 33% of carbamate
(10) which was hydrolysed by treatment with LiOH to give
additional quantities of (11). At this stage we needed to
select a protecting group for the amino unit. The main
criterion that we applied in our selection process was that the
protecting group must be completely compatible with
anticipated glycosidation reactions. Carbamate protecting
groups were considered undesirable, owing to the propensity
for carbamates to participate in neighbouring-group assisted
reactions,[22,23] which could compromise the selectivity of
the α-glycosidation reactions planned in the synthesis of
(6).[24] These considerations prompted us to protect the
amino group of (11) as a non-nucleophilic and non-basic
azide. Thus, treatment of (11) with trifluoromethanesulfonyl
azide and 4-dimethylaminopyridine (DMAP) provided azide
(12) in good yield.[25] This intermediate undergoes a facile
internal dipolar cycloaddition reaction at ambient
temperature, so (12) was used immediately in the following
ozonolysis reaction, thereby giving the 4-azido
glucopyranose (6) in 65% yield from (11). Finally, treatment
of (6) with acetic anhydride and ZnCl2 provided glycosyl
acetate (13) almost exclusively as the α-anomer. Treatment
of (13) with thiophenyltrimethylsilane and ZnCl2 then
provided the thioglycoside (14) in 85% overall yield, as a
mixture of anomers.[26]
analogues with improved properties as DNA binding and
chemotherapeutic agents. It is believed that the preferred
DNA binding site of the aureolic acids is a tetranucleotide;[7]
however, the selectivity for binding among the various GC-
rich tetranucleotides is not great in the absolute.[15] One
established strategy to improve DNA-binding selectivity is to
couple the drug molecule of interest to another DNA-binding
element with well established DNA-binding selectivity
preferences.[15–19] The NMR structures of DNA-bound
aureolic acids reveals that the terminal E sugar makes strong
contacts with the floor of the minor groove, and that the C4
acyl substituent is in the middle and projects down the
middle of the minor groove. This suggested to us the
possibility that an aureolic acid analogue with a C4 amino
substituent could be used to couple with other known DNA
minor-groove binding agents, thereby leading to the
development of more effective and selective DNA binders,
and possibly also improved chemotherapeutic agents.
In initial steps towards this goal, we have developed and
describe herein a simple, stereoselective synthesis of the 4-
azido sugar (6). Glycosidation reactions with suitably
activated derivatives of (6) are also described, along with the
synthesis of protected forms of the trisaccharide unit [cf.
(21)–(23)] of aminotrisaccharide conjugate (5).
Results and Discussion
Br
Br
O
AcO
TMSO
The synthesis of 4-azido-2,4,6-trideoxy-L-glucopyranose
(6) originated from the readily available epoxy alcohol (7)
(Scheme 1).[20] Treatment of (7) with benzoyl isocyanate and
OAc
O
AcO
O
(15)
Me
N3
O
O
Me
N3
SnCl4, AgClO4, Et2O
OAc
(13)
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) provided
a
0˚C
46%
OAc
mixture of (8) and the benzoyl-transfer isomer (9) in 85%
combined yield.[21] Treatment of this mixture with LiOH in
aqueous tetrahydrofuran (THF) at reflux for 24 h provided
(16)
α-to-β ratio 4 : 1
OTs
OTs
O
O
ClAcO
TMSO
OAc
ClAcO
Me
O
Me
OH
Me
OR
BzNCO, DBU
CH2Cl2
Me
N3
(17)
O
O
Me
N3
O
SnCl4, AgClO4, Et2O
0–23˚C
85%
OAc
(13)
BzN
O
HN
O
0˚C
58%
OH
O
O
OAc
(18)
α-to-β ratio 4 : 1
)
(7
8)
(9) R = Bz
(10) R = H
(
Scheme 2
Me
OH
Me
OH
Initial glycosidation reactions were performed using
glycosyl acetate (13) as the donor (Scheme 2). Best results
for the glycosidation reactions of (13) with D-glycal
acceptors (15) and (17) were obtained by using SnCl4 and
AgClO4 as the activating agents.[27] These reactions provided
the indicated disaccharides (16) and (18) in 46 and 58%
yields, respectively, both as 4 : 1 α/β anomeric mixtures.
Ultimately, much better results were obtained by using
thioglycoside (14) as the substrate for the Kahne sulfoxide
glycosidation protocol.[28]
CF3SO2N3
LiOH, THF/H2O
CH3CN, 23˚C
reflux, 24 h
62%
H2N
N3
OH
OH
(11)
(12)
OH
X
O3, MeOH
Ac2O, ZnCl2
Me
N3
Me
N3
O
O
then Me2S
65% from (11)
OH
OAc
PhS−SiMe3
ZnCl2, 23˚C
85%
Treatment of the mixture of anomeric thioglycosides (14)
with m-chloroperoxybenzoic acid (m-CPBA) at –10°C
provided the glycosyl sulfoxides (19) in good yield (Scheme
3). In initial studies directed towards the synthesis of the
(6)
)
(13) X = OAc
(14) X = SPh
Scheme 1

3-O-Acetyl-4-azido-2,4,6-trideoxy-L-glucopyranose Derivatives
143
advanced intermediates in our syntheses of (4a,b)[1] and
olivomycin A,[31] especially the reductive removal of the
thiophenyl substituents, all future work on the synthesis of
aureolic acid analogues will utilize our newly introduced
method for synthesis of 2-deoxy-β-glycosides that involves
the glycosidation reactions of 2-deoxy-2-iodo glycosyl
donors.[32–34] Further efforts along these lines, including our
efforts to synthesize the targeted aminotrisaccharide
conjugate (5),* will be reported in due course.
O
Ph
SPh
S
m-CPBA
Me
N3
Me
N3
O
O
CH2Cl2, −10˚C
76%
OAc
(14)
OAc
(19)
+
Br
Br
Ac
O
O
O
AcO
HO
O
Experimental Section
SPh
(20)
General experimental details are provided in the accompanying
manuscript.[1]
(4R,5S,1´S)-4-[1´-Benzoyloxyethyl-5-(prop-2´-enyl)]oxazolidin-2-one
(9)
Br
Br
Ac
Tf2O
2,6-di-t-BuPy
O
O
O
O
AcO
O
O
Benzoyl isocyanate (19.2 mL, 152 mmol) was added dropwise over
10 min to a 0°C solution of the epoxy alcohol (7)[20] (16.3 g, 127 mmol)
in anhydrous CH2Cl2 (600 mL).[21] The resulting solution was allowed
to warm to ambient temperature over 3 h, after which DBU (4.83 g,
31.7 mmol) was added. The resulting mixture was stirred at 23°C for
16 h. Concentration of the reaction mixture, followed by purification of
the product by flash chromatography gave a mixture of (8) (unstable)
and (9) as a viscous oil (29.5 g, 85% yield). Data for (9): (Found
(HRMS): [M+H]+, 276.1234; [M – C7H5O2]+, 153.0836. Calc. for
C15HI8NO4: [M+H]+, 276.1312; [M – C7H5O2]+, 153.0785). Infrared
(IR) (CHC13) 3460, 3080, 2940, 1760, 1720, 1650, 1640, 1415, 1270,
1245, 1110, 1070 cm–1. 1H NMR (CDC13) δ 1.47, d, J 6.2 Hz, 3H;
2.52–2.46, m, 2H; 3.73, ddd (app. dt), J 4.0, 0.8 Hz, 1H; 4.56, qd, J 6.2,
4.0 Hz, 1H; 5.24–5.12, m, 3H; 5.80, m, 1H; 6.11, s, 1H, NH; 7.48–7.43,
m, 2H; 7.58, m, 1H; 8.03–7.99, m, 2H. 13C NMR (CDC13) δ 21.3, 34.8,
60.9, 73.5, 75.1, 119.4, 128.6, 129.3, 129.7, 131.8, 133.5, 158.8, 165.8.
EtCN, −65˚C
SPh
70%
Me
N3
OAc
(21)
Br
(i) PhSCl, CH2Cl2
(ii) Ag2CO3
THF/H2O
Br
Ac
O
O
O
O
AcO
O
O
(iii) Cl3CCN, NaH
57%
PhS
SPh
X
Me
N3
(22) X = OH
(23) X = OC(=NH)CCl3
OAc
Scheme 3
(4S,5S,6S)-5-Aminohept-1-ene-4,6-diol (11)
A mixture of oxazolidinones (8) and (9) (30.5 g, 111 mmol) and LiOH
(18.6 g, 443 mmol) in THF (250 mL) and H2O (100 mL) was heated at
reflux for 36 h. The reaction mixture was cooled to ambient temperature
and the solvent removed on a rotary evaporator to give a clear syrup.
Subsequent purification of the residue over silica gel (20 : 79 : 1 Et2O/
EtOH/NH4OH) afforded the desired product (11) as a white powder
(9.95 g, 62%), and oxazolidinone (10) (5.50 g, 29%) which could be
resubjected to the reaction conditions to give additional (11) (Found
(HRMS): [M+H]+, 146.1178; [(M+H) – H2O]+, 128.1075. C7H16NO2
requires [M+H]+, 146.1249; [(M+H) – H2O]+, 128.1135). IR (neat)
3700(br) (NH2, OH), 3100, 3060, 2980, 2920, 1640, 1580, 1430, 1375,
1260, 1060, 980, 920, 790 cm–1. 1H NMR (CDC13) δ 1.27, d, J 6.0 Hz,
3H; 2.41–2.10, m, 6H; 2.57, m, 1H; 3.91–3.87, m, 2H; 5.17–5.09, m,
2H; 5.83, br m, 1H. 13C NMR (CDC13) δ 19.6, 38.8, 57.6, 69.9, 70.6,
117.8, 134.6.
aureolic acid analogue (5), a mixture of (19) and the
disaccharide glycal (20) (which was prepared by desilylation
of the corresponding tert-butyldimethylsilyl (TBS) ether,[29]
see Experimental) was treated with triflic anhydride and 2,6-
di-tert-butylpyridine in propionitrile at –65°C to provide the
trisaccharide glycal (21) in 70% yield. The new glycosidic
linkage formed in this reaction was exclusively α; none of
the β-anomeric product was detected. Treatment of (21) with
PhSCl in CH2Cl2 and hydrolysis of the intermediate glycosyl
chloride with Ag2CO3 in wet THF provided trisaccharide
pyranose (22).[30] Finally, the trisaccharide trichloro-
acetimidate derivative (23) was obtained by exposure of (22)
to trichloroacetonitrile (as solvent) and NaH at –40°C.[29]
(3S,5S,6S)-5-Azidohept-1-ene-4,6-diol (12)
A solution of aminodiol (11) (247 mg, 1.70 mmol) and DMAP (914
mg, 7.48 mmol) in anhydrous CH3CN (35 mL) was treated with
freshly prepared triflic azide solution[25] (8.81 mL, 0.27 M in CH2Cl2),
added by syringe over 10 min. (NOTE: Triflic azide is potentially
hazardous and must be handled behind a safety shield.[35]) The
reaction mixture was stirred at 23°C for 2 h, and then the solvent was
removed on a rotary evaporator. The residue was filtered through a
small bed of silica gel to give azide (12) as a clear syrup (284 mg,
97%). IR (neat) 3100–3700(br) (OH), 3080, 2980, 2920, 2100, 1640,
1260, 920 cm–1. 1H NMR (CDC13) δ 1.34, d, J 6.4 Hz, 3H; 2.45–2.33,
m, 2H; 2.62, br s, 2H, OH; 3.17, dd, J 5.9, 3.0 Hz, 1H; 4.04, dq, J 5.9,
Conclusion
A stereoselective synthesis of 4-azido-glucopyranose (6) has
been developed. The anomeric acetate (13), and especially
the derived thioglycoside derivative (14), have been shown to
be good α-glycosyl donors. Donor (14) was used in the
synthesis of trisaccharide (21), which we initially targeted as
an intermediate in the synthesis of the aureolic acid
trisaccharide conjugate analogue (5). However, in view of the
difficulties encountered with the final deprotection of
* In unpublished preliminary investigations, we have coupled trichloroacetimidate (23) with the same aglycone as in analogues (4a) and (4b).
Unfortunately, all attempts to deprotect the trisaccharide conjugate to provide the targeted aminotrisaccharide (5) have also met with considerable
difficulty, paralleling the problematic thiophenyl group reduction that plagued our syntheses of (4a), (4b), and olivomycin A itself.

144
W. R. Roush and R. A. James
2.7 Hz, 1H; 4.12, dt (app. q), J 6.2 Hz, 1H; 5.18, m, 1H; 5.22, m, 1H;
5.81, m, 1H. 13C NMR (CDCl3, 300 MHz) δ 20.7, 39.2, 68.7, 69.4,
70.5, 119.3, 133.9.
Owing to the instability of (12), which undergoes an internal dipolar
cycloaddition reaction at ambient temperature, this compound was used
directly in the next step without further purification.
saturated aqueous NaHCO3 was added. Standard extractive workup
gave the crude disaccharide mixture, which was separated by flash
column chromatography (30% EtOAc/hexane). This provided
disaccharide (16) as a mixture of anomers (4 : 1 α/β). Further
purification by HPLC afforded the desired α-anomer (46%,
unoptimized). Partial data for α-anomer: IR (neat) 2978, 2937, 2350
(CO2), 2107, 1751, 1741, 1649, 1371, 1232, 1041, 997 cm–1. 1H NMR
(CDC13) δ 1.33, d, J 6.2 Hz, 3H, H6´; 1.71, ddd, J 12.9, 11.5, 3.6 Hz,
1H, H2´ax; 2.09, s, 3H; 2.11, s, 3H; 2.21, ddd, J 12.9, 5.1, 1.3 Hz, 1H,
H2´eq; 3.14, t, J 10.0 Hz, 1H, H4´; 3.54, dd, J 11.0, 8.5 Hz, 1H, H6;
3.62, dd, J 11.0, 5.6 Hz, 1H, H6; 3.74, m, 1H, H5´; 4.10, m, 1H, H3;
4.30, dt, J 7.6, 4.8 Hz, 1H, H5; 4.92, ddd, J 6.4, 4.0, 0.9 Hz, 1H, H2;
5.04, d, J 2.7 Hz, 1H, Hl´; 5.16, ddd, J 11.6, 9.9, 5.0 Hz, 1H, H3´; 5.21,
m, 1H; 6.46, dd, J 6.3, 0.8 Hz, 1H. 13C NMR (CDC13) δ 18.5, 21.0,
39.3, 35.4, 66.3, 66.5, 67.0, 69.2, 70.1, 72.5, 75.3, 93.7, 93.8, 97.6,
144.6, 169.9. Mass spectrum (FAB) m/z 470.0 ([M+Na]+, 100%).
Acetyl 3-O-Acetyl-4-azido-2,4,6-trideoxy-α-L-glucopyranoside (13)
A solution of azido diol (12) (284 mg, 1.66 mmol) in anhydrous MeOH
(15 mL) at –40°C was treated with a stream of ozone in O2 until (12)
was consumed as shown by TLC analysis. Excess O3 was removed with
a stream of N2, and then the ozonide solution was treated with dimethyl
sulfide (1.19 mL). The solvent was removed under vacuum and the
crude residue was treated with excess acetic anhydride (1.2 mL) in the
presence of ZnC12 (4.4 mg, 0.03 mmol) at 23°C for 23 h. Saturated
aqueous Na2CO3 solution (10 mL) was carefully added to the reaction
mixture and the resulting mixture was stirred at 23°C for 2 h. The
mixture was then diluted with EtOAc and the organic layer was
collected. The aqueous layer was extracted with EtOAc (3 × 10 mL),
washed with saturated aqueous Na2CO3, brine, dried (Na2SO4), and
concentrated under reduced pressure to give the crude diacetate. This
material consisted primarily of the α-anomer (384 mg, 90%) along with
a small amount of the β-anomer. Data for α-anomer (13) (Found
(HRMS): [(M+H) – C2H3O2]+, 198.0877; [(M+H) – C4H7N2O4]+,
110.0610. C10H14N3O5 requires [(M+H) – C2H3O2]+, 198.0865;
[(M+H) – C4H7N2O4]+, 110.0576). IR (neat) 2989, 2984, 2980, 2939,
2912, 2119, 1760, 1747, 1366, 1249, 1196, 1146, 1033, 993, 898 cm–1.
1H NMR (CDC13) δ 1.32, d, J 6.2 Hz, 3H; 1.83, ddd, J 13.4, 11.6,
3.5 Hz, 1H; 2.10, s, 3H; 2.11, s, 3H; 2.27, ddd, J 13.4, 5.1, 1.3 Hz, 1H,
H2eq; 3.21, t, J 9.9 Hz, 1H; 3.71, dq, J 10.0, 6.2 Hz, 1H; 5.19, ddd, J
11.3, 9.7, 5.1 Hz, 1H; 6.17, dd, J 3.4,1.3 Hz, 1H. 13C NMR (CDC13) δ
18.6, 21.1, 34.1, 66.1, 68.8, 69.8, 90.70, 90.72, 169.2, 170.0.
3-O-(3Ef-O-Acetyl-4´-azido-2´,6´-dideoxy-α,β-L-glucopyranosyl)-4-
O-chloroacetyl-6-bromo-6-deoxy-D-glucal (18)
A mixture of donor (13α) (58.8 mg, 0.22 mmol) and acceptor (17)
[108 mg, prepared by silylation of the corresponding secondary alcohol
by using the procedure outlined for (15)] in anhydrous Et2O (2.8 mL)
was added to a mixture of SnC14 (22 µL of a 1 M solution, 22 µmol)
and AgClO4 (9.0 mg, 44 mmol) suspended in Et2O (2.8 mL) in the dark
at 0°C using the procedure described for preparation of (16). The
disaccharide (18) was obtained as a mixture of anomers (4 : 1 α/β) in
58% overall yield after purification on silica gel (30% EtOAc/hexane)
and further purification by HPLC (30% EtOAc/hexane, 15 mL min–1).
Data for the α-anomer: IR (thin film, CDC13) 3080, 2960, 2940, 2870,
2110, 1745, 1730, 1650, 1595, 1520, 1410, 1305, 1235, 1180, 1155,
1065, 1030, 820 cm–1. 1H NMR (CDC13) δ 1.27–1.32, d, J 6.4 Hz, 3H,
H6´; 1.65–1.75, ddd, J 12.9,11.6, 3.8 Hz, 1H, H2´ax; 2.10, s, 3H; 2.15–
2.22, ddd, J 12.9, 4.1, 1.3 Hz, 1H, H2´eq; 2.45, s, 3H; 3.10–3.17, t, J 9.9
Hz, 1H, H4´; 3.55–3.65, m, 1H, H5´; 4.06, s, 2H; 4.10–4.15, m, 2H, H3
and H6; 4.25–4.31, dd, J 10.3, 6.9 Hz, 1H, H6; 4.31–4.35, m, 1H, H5;
4.86–4.90, ddd, J 6.8, 3.8, 0.6 Hz, 1H, H2; 5.02–5.04, br. d, J 2.7 Hz,
1H, Hl´; 5.04–5.11, m, 2H, H3´and H4; 6.33–6.39, dd, J 6.3, 0.9 Hz,
1H, H1; 7.34–7.40, d, J 8.6 Hz, 2H; 7.78–7.84, d, J 8.6 Hz, 2H. Mass
spectrum (fast atom bombardment, FAB) m/z 596.1 ([M+Na]+, 100%).
Partial data for β-anomer: 1H NMR (CDC13) δ 1.34, d, J 6.2 Hz, 3H,
H6´; 1.57, ddd, J 12.3, 11.8, 9.7 Hz, 1H, H2´ax; 2.12, s, 3H; 2.25, ddd,
J 12.5, 5.2, 2.0 Hz, 1H, H2´eq; 2.45, s, 3H; 3.12, t, J 9.7 Hz, 1H, H4´;
3.24, dd, J 9.8, 6.2 Hz, 1H, H5´; 4.06, s, 2H; 4.15, m, 2H, H3 and H6;
4.28, m, 2H, H5 and H6 [dd, J 10.2, 6.4 Hz]; 4.65, dd, J 9.8, 2.0 Hz, 1H,
H1´; 4.85, ddd, J 11.8, 9.7, 5.1 Hz, 1H, H3´; 4.92, ddd, J 6.2, 3.8, 0.8
Hz, 1H, H2; 5.07, t, J 5.1 Hz, 1H, H4; 6.27, dd, J 6.3, 1.1 Hz, 1H, H1;
7.34, d, J 8.3 Hz, 2H; 7.78, d, J 8.3 Hz, 2H.
Thiophenyl 3-O-Acetyl-4-azido-1,2,4,6-tetradeoxy-α-L-glucopyranose
(14)
A solution of the glycosyl acetate (13) (1.72 g, 6.69 mmol) in anhydrous
CH2Cl2 (50 mL) at 23°C was treated with thiophenyltrimethylsilane
(3.66 g, 3.7 mmol) followed by tetra-n-butylammonium iodide (2.97 g,
20.1 mmol) and zinc(II) iodide (6.41 g, 20.1 mmol).[26] The reaction
flask was covered with foil and stirred at 23°C for 1 h. The mixture was
then diluted with CH2Cl2 (100 mL) and washed with H2O (2 × 50 mL).
The layers were separated and the aqueous layer was extracted with
CH2Cl2 (1 × 50 mL). The combined organic layers were dried over
anhydrous Na2SO4, filtered, and concentrated under reduced pressure.
The resulting residue was purified by flash column chromatography
using 10% EtOAc/hexane as eluent. This gave thioglycoside (14)
(1.92 g, 93%) as a 3 : 1 mixture of α and β anomers. Data for the α-
anomer (14α) (Found: C, 54.8; H, 5.6; N, 12.6%; [M+NH4]+, 325.1324.
C14H21N4O3S requires C, 54.7; H, 5.6; N, 13.7%; [M+NH4]+,
325.1334). HPLC (10% EtOAc/hexane, 21 mm column, 12 mL min–1,
tR 9.0 min). 1H NMR (CDCl3) δ 1.35–1.30, d, J 6.4 Hz, 3H; 2.16–2.07,
m, 4H; 2.52–2.46, dd, J 13.2, 5.1 Hz, 1H; 3.23–3.16, t, J 9.9 Hz, 1H;
4.21–4.14, m, 1H; 5.27–5.16, m, 1H; 5.60–5.56, d, J 5.8 Hz, 1H; 7.33–
7.23, m, 3H; 7.46–7.42, d, J 1.2 Hz, 2H.
Phenylsulfoxide 3-O-Acetyl-4-azido-2,4,6-trideoxy-α,β-L-
glucopyranose (19)
A solution of the thioglycoside (14) (1.90 g, 6.2 mmol, 3 : 1 anomer
mixture) in anhydrous CH2Cl2 (100 mL) was cooled to –78°C and
treated dropwise with a solution of m-CPBA (1.09 g, 6.2 mmol) in
CH2Cl2 (15 mL) over 5 min. The reaction mixture was then allowed to
warm slowly to –10°C over 1.5 h. It was then quenched with
triethylamine (3 mL), diluted with CH2Cl2 (100 mL), and immediately
poured into a separatory funnel containing saturated aqueous NaHCO3
(100 mL). The layers were separated and the aqueous layer was extracted
with CH2Cl2 (2 × 50 mL). The combined organic layers were dried over
Na2SO4, filtered and concentrated under reduced pressure. The residue
was purified by flash column chromatography using 35% EtOAc/
hexane as eluent to give a mixture of anomeric sulfoxides (19) (1.52 g,
76%) as a clear viscous syrup. Data for the α-sulfoxide (19α) (Found:
C, 52.2; H, 5.4; N, 12.9%; [M+Na]+ (FAB), 346.3602. C14H17N3NaO4S
requires C, 52.0; H, 5.3; N, 13.0%; [M+Na]+, 346.3584). RF 0.27 (40%
EtOAc/hexane). IR (thin film, CHCl3) 3050, 2986, 2960, 2925, 2900,
2108, 1445, 1372, 1305, 1297, 1267, 1234, 1157, 1142, 1099, 1079,
1043, 998, 975, 898 cm–1. 1H NMR (CDCl3, 500 MHz) δ 1.33, d, J 6.0
Hz, 3H, H6; 1.89, ddd, J 14.4, 10.9, 6.0 Hz, 1H, H2ax; 2.12, s, 3H; 2.84,
4-O-Acetyl-3-O-(3-O-acetyl-4-azido-2,6-dideoxy-α-L-
glucopyranosyl)-6-bromo-6-deoxy-D-glucal (16α)
A solution of 4-O-acetyl-6-bromo-6-deoxy-D-glucal[29] (61.1 mg,
243 µmol) in anhydrous CH2Cl2 (350 µL) was treated with
bistrimethylsilyl urea (24.9 mg, 122 µmol) at 23°C for
1 h.
Concentration of the reaction mixture under reduced pressure followed
by high vacuum for 1 h afforded the trimethylsilyl-D-glucal (15) in
quantitative yield.
A solution of SnC14 (37.4 µL, 37.4 µmol, 1.0 M in CH2Cl2) was
added to AgClO4 (15.5 mg) suspended in anhydrous Et2O (1.9 mL) at
23°C. The mixture was shielded from light and stirred for 1 h. This
mixture was cooled to 0°C and then an ethereal solution (1.9 mL) of
glycal (15) (78.6 mg, 243 µmol) and glycosyl donor (13) (48.1 mg,
187 µmol) was added. The mixture was stirred for 3 h and then

3-O-Acetyl-4-azido-2,4,6-trideoxy-L-glucopyranose Derivatives
145
ddd, J 14.4, 5.3, 1.4, Hz, 1H, H2eq; 3.26, t, J 9.7 Hz, 1H, H4; 4.01, m,
1H, H5; 4.44, d, J 5.3 Hz, 1H, H1; 5.51, m, 1H, H3; 7.53, m, 3H; 7.62,
m, 2H. 13C NMR (CDCl3, 100 MHz) δ 18.6, 20.9, 26.9, 65.6, 72.6, 75.9,
92.1, 124.4, 129.2, 131.5, 140.1, 167.0.
(FAB, NaOAc and NBA matrix), 954.0587. C36H43Br2N3NaO12S2
requires C, 46.3; H, 4.6%; [M+Na]+ 954.0553). HPLC (30% EtOAc/
hexane, 21 mm column; 10 mL min–1, tR 23.2 min). 1H NMR (CDCl3,
500 MHz) δ 1.24, d, J 7 Hz, 3H; 1.61, m, 1H; 2.05, s, 3H; 2.11, s, 3H;
2.17, s, 3H; 2.30, m, 1H; 3.19–3.00, m, 4H; 3.27, m, 2H; 3.35, m, 1H;
3.44, m, 2H; 3.70–3.52, m, 3H; 4.25, m, 2H; 4.79, m, 1H; 4.86, m, 1H;
4.94, m, 1H; 5.08, m, 1H; 5.21, m, 1H; 5.28, m, 1H; 7.31–7.18, m, 8H;
7.40, m, 2H. 13C NMR (CDCl3, 100 MHz) δ 18.6, 21.0, 21.2, 31.8, 35.5,
55.2, 55.5, 66.7, 67.4, 69.6, 69.8, 71.4, 73.2, 73.6, 74.6, 79.6, 92.7,
94.4, 99.1, 102.6, 126.7, 127.5, 128.9, 129.1, 130.4, 131.8, 133.9,
169.7, 169.9, 193.7.
Partial data for the β-sulfoxide (19β): [α]D25 –20° (c, 1.04 in EtOH).
1H NMR (CDCl3, 500 MHz) δ 1.40–1.39, d, J 5.8 Hz, 3H; 2.14–1.92,
m, 5H; 3.33–3.18, m, 2H; 4.18–4.13, dd, J 11.0, 2.9 Hz, 1H; 4.90–4.82,
ddd, J 11.0, 9.5, 5.5 Hz, 1H; 7.54–7.50, m, 3H; 7.61–7.56, m, 2H. 13C
NMR (CDCl3, 100 MHz) δ 18.6, 20.9, 26.9, 65.6, 72.6, 75.1, 92.1,
124.4, 129.2, 131.5, 140.1, 170.0.
4-O-Acetyl-3-O-[4-O-acetyl-3-O-(3-O-acetyl-4-azido-2,4,6-trideoxy-
α-L-gluco-pyranosyl)-6-bromo-2,6-dideoxy-2-thiophenyl-β-D-
glucopyranosyl]-6-bromo-2,6-dideoxy-D-arabino-hex-1-enitol (21)
Trichloroacetamido 4-O-Acetyl-3-O-[4-O-acetyl-3-O-(3-O-acetyl-4-
azido-2,4,6-trideoxy-α-L-glucopyranosyl)-6-bromo-2,6-dideoxy-2-
thiophenyl-β-D-glucopyranosyl]-6-bromo-2,6-dideoxy-α-D-
glucopyranose (23)
A
solution of 4-O-acetyl-3-O-(4-O-acetyl-6-bromo-3-hydroxy-2,6-
dideoxy-2-thiophenyl-β-D-glucopyranosyl)-6-bromo-2,6-dideoxy-D-
arabino-hex-1-enitol (20) (487 mg, 0.72 mmol, prepared in 96% yield
by deprotection of the corresponding tert-butylsilyl ether[29] by using an
excess of Et3N/HF in CH3CN), sulfoxide (19) (400 mg, 1.23 mmol),
and 2,6-di-tert-butyl-4-methyl pyridine (386 mg, 1.64 mmol) was dried
by evaporation from anhydrous benzene (3 × 15 mL) under reduced
pressure. The residue was then dissolved in propionitrile (28 mL,
freshly distilled from P2O5), cooled to –78°C, and treated with
trifluoromethanesulfonic anhydride (245 µL, 1.48 mmol) in one
portion. The reaction was allowed to warm to –65°C by removal of dry
ice from the cooling bath over 10 min, at which time the appearance of
a voluminous precipitate signalled the end of the reaction. The reaction
was terminated by adding Et3N (1.5 mL), followed by addition of
saturated aqueous NaHCO3 solution (35 mL) and CH2Cl2 (30 mL). The
resulting mixture was poured into a separatory funnel and the organic
layer was collected. The aqueous layer was extracted with CH2Cl2
(2 × 30 mL). The combined organic layers were washed with brine and
dried over Na2SO4, and then concentrated under reduced pressure. The
residue was purified by flash chromatography using 40% EtOAc/
hexane to give a foam which was further purified by HPLC (30%
EtOAc/hexane, 20 mm column, 25 mL min–1). In this way, the desired
product (21) (644 mg, 70%) was obtained as a white foam (Found: C,
44.4; H, 4.6; N, 5.3%; [M+Na]+ (FAB, 3-nitrobenzyl alcohol (NBA)
matrix), 828.0425. C30H37Br2N3O11S requires C, 44.6; H, 4.6; N, 5.2%;
[M+Na]+, 828.0413). RF 0.32 (30% EtOAc/hexane, 20 mm column,
A solution of the azido trisaccharide lactol (22) (78 mg, 0.084 mmol) in
freshly distilled trichloroacetonitrile (2.2 mL) at –40°C was treated with
dry NaH powder (16 mg, 0.652 mmol) in portions. The mixture was
stirred at –40°C for 1 h and then sealed with Parafilm and placed in the
freezer at –20°C for 16–23 h. The resulting yellow solution was filtered
through a pad of Celite under N2 and the Celite pad was washed several
times with CH2Cl2. The filtrate was concentrated under reduced
pressure and the residue was purified by column chromatography (20%
EtOAc/hexane–1% Et3N) to give α-imidate (23) (67 mg, 74%). RF 0.16
(20% EtOAc/hexane). HPLC (20% EtOAc/hexane, 21 mm column, 12
mL min–1, tR 13.6 min). 1H NMR (CDCl3) δ 1.24, d, J 6.1 Hz, 3H, H6´;
1.60, dt, J 12.7, 3.8 Hz, 1H, H2´ax; 2.05, s, 3H; 2.11, s, 3H; 2.15, s, 3H;
2.29, ddd, J 12.7, 5.2, 1.2 Hz, 1H, H2´eq; 3.14–3.02, m, 2H; 3.25, dd, J
11.0, 10.2 Hz, 1H; 3.49–3.32, m, 4H; 3.61, m, 2H; 3.70, dd, J 10.5, 8.3
Hz, 1H; 4.13, m, 1H; 4.37, dd, J 11.2, 9.0 Hz, 1H; 4.75, dd, J 9.7, 8.4
Hz, 1H; 4.97, dd, J 10.3, 10.1 Hz, 1H; 5.05, d, J 8.8 Hz, 1H, H1´; 5.07,
m, 1H; 5.30, d, J 2.9 Hz, 1H, H1´; 6.39, d, J 3.2 Hz, 1H, H1; 7.24, m,
4H; 7.29, m, 2H; 7.45, m, 2H; 7.50, m, 2H; 8.72, s, 1H, NH. 13C NMR
(CDCl3, 100 MHz) δ 18.5, 21.0, 21.1, 21.2, 30.9, 31.2, 35.5, 52.8, 55.0,
55.7, 66.7, 67.3, 69.7, 71.0, 71.6, 73.4, 73.9, 74.1, 79.6, 95.7, 99.1,
102.5, 126.7, 127.9, 128.9, 129.1, 130.4, 132.7, 133.8, 135.2, 160.4,
169.0, 169.4, 169.5. Mass spectrum (FAB) m/z 1095.8 ([M+Na]+,
100%).
1
10 mL min–1, tR 23.8 min). H NMR (CDCl3, 400 MHz) δ 1.28–1.24,
Acknowledgments
d, J 6.3 Hz, 3H, H6; 1.72–1.62, ddd, J 12.6, 9.1, 3.6 Hz, 1H; 2.06, s, 3H;
2.07, s, 3H; 2.12, s, 3H; 2.46–2.39, ddd, J 12.6, 5.0, 1.3 Hz, 1H; 3.18–
3.06, m, 2H; 3.42–3.28, m, 3H; 3.59–3.48, m, 2H; 3.73–3.62, m, 2H;
4.08–4.04, m, 1H; 4.39–4.33, m, 1H; 4.64–4.60, d, J 8.8 Hz, 1H, H1´;
4.84–4.80, m, 1H; 4.89–4.84, dd, J 9.7, 8.7 Hz, 1H; 5.21–5.13, ddd,
11.3, 9.8, 4.9 Hz, 1H; 5.32–5.28, m, 2H; 6.46–6.43, dd, J 6.3, 0.6 Hz,
1H, H1; 7.39–7.26, m, 3H; 7.5, m, 2H. 13C NMR (CDCl3, 100 MHz) δ
18.6, 21.0, 21.2, 29.6, 31.3, 35.8, 56.3, 66.7, 67.5, 69.1, 69.7, 69.8,
72.9, 73.8, 75.4, 80.1, 97.6, 99.4, 102.2, 127.6, 129.1, 132.0, 134.1,
144.4, 169.6, 169.7, 169.9.
We thank the National Institutes of Health for financial
support of this research (GM 38907).
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