Chemical investigations in the synthesis of O-serinyl aminoribosides

Article abstract of DOI:10.1016/j.tetasy.2005.11.019

Glycosylation involving d-ribose derivatives and various N-protected tert-butyl l-serinates can be achieved efficiently by careful choice of the activation method at the anomeric position and of the Lewis acid promoter. The conditions described allow the major formation of the β-anomer required for further elaboration to liposidomycin and caprazamycin analogues.

Full text of DOI:10.1016/j.tetasy.2005.11.019

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 17 (2006) 142–150
Chemical investigations in the synthesis of O-serinyl aminoribosides
Maryon Ginisty, Christine Gravier-Pelletier* and Yves Le Merrer*
Universite´ Paris Descartes, UMR 8601-CNRS, Laboratoire de Chimie et Biochimie Pharmacologiques et
`
Toxicologiques, 45 rue des Saints-Peres, 75270 Paris Cedex 06, France
Received 10 November 2005; accepted 22 November 2005
Available online 9 January 2006
Abstract—Glycosylation involving D-ribose derivatives and various N-protected tert-butyl L-serinates can be achieved efficiently by
careful choice of the activation method at the anomeric position and of the Lewis acid promoter. The conditions described allow the
major formation of the b-anomer required for further elaboration to liposidomycin and caprazamycin analogues.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
side chains, eventually substituted by a fucosyl deriva-
tive in the case of caprazamycins. In the context of an
ongoing program dealing with the synthesis of liposid-
omycins analogues using the diazepanone core as a
versatile scaffold with high and orthogonal functionali-
zation, we aimed at synthesizing O-serinyl amino-
riboside for further transformation into O-serinyl
aminoriboside diazepanones. Indeed, it has been shown7
that the aminoribose moiety of the liposidomycins is
essential for biological activity. Although numerous
methods of glycosylation8 related to the synthesis of
pyranosyl aminoacids9 have been described, only few
examples are dedicated to the apparented O-glycoside
derivatives of furanose.10 Furanosyl derivatives are
mainly involved in N-glycosylation reactions with either
purine or pyrimidine moieties to afford nucleosidic com-
pounds. The synthesis of such O-linked furanosyl
aminoacids is complicated both by the acid lability of
glycosides in general, and by the base sensitivity of the
O-serinyl glycosides, involving retro-Michael reaction,
in particular.11 In this context, we embarked on the syn-
thesis of O-serinyl aminoriboside according to efficient
routes. Part of our results involving commercially avail-
able O-protected derivatives of D-ribose has already
been published in a preliminary form.12 Herein we
report the totality of our work in this field, notably
including access to an O-serinyl azidoriboside, a key
building block for further elaboration.
A concise and efficient synthesis of glycoconjugates is
a significant challenge for the better understanding of
biological events, such as cell–cell recognition, cell
adhesion, host pathogen interactions, and tumor cell
metastasis.1 Furthermore, glycosyl aminoacids can also
be part of complex natural products.2 For example, O-
serinyl aminoriboside is a key fragment of the naturally
occurring family of lipo-uridinyl antibiotics, such as
liposidomycins3 and caprazamycins4 (Fig. 1), for which
several synthetic approaches have been studied.5,6 These
biologically active compounds are formed from com-
mon structural fragments: 1,4-diazepan-3-one hetero-
cycle, aminoribosyl, and uridinyl moieties and lipophilic
NH2
HO
HO
Me
O
O
R2
O
N
O
O
NH
O
O
O
HO2C
R1O
N
Me
O
H
HO
O
OH
liposidomycins : R1 = H, R2 = alkyl or alkenyl chain
caprazamycins : R1 = fucose derivative, R2 = alkyl chain
Figure 1. Structure of liposidomycins and caprazamycins, naturally
occurring compounds with antibacterial activity.
Retrosynthetic analysis of the targeted compound
(Fig. 2) involves a tert-butyl N-protected-L-serinate as
a glycosyl acceptor and various D-ribose derivatives acti-
vated at the anomeric position as glycosyl donnors. For
this purpose, access to orthogonally N- and O-protected
*
Corresponding authors. Tel.: +33 1 4286 2176; fax: +33 1 4286 8387
(Y.L.M.); e-mail addresses: Christine.Gravier-Pelletier@univ-paris5.fr;
Yves.Le-Merrer@univ-paris5.fr
0957-4166/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.11.019

M. Ginisty et al. / Tetrahedron: Asymmetry 17 (2006) 142–150
143
O
CO2H
NH2
O
O
X
H2N
PHN
CO2P
NHP
HO
+
HO
OH
PO
OP
Figure 2. Retrosynthetic analysis.
derivatives with predominant or exclusive b-O-glyco-
sidic linkage has been targeted.
described for the preparation of 4b, however in this case
O-benzyl hydrogenolysis occurred in a lower yield
(29%). The second method involving direct esterification
of the N-Fmoc serine 8 in the presence of tert-butyl tri-
chloroacetimidate17 was achieved in an improved 84%
yield.
2. Results and discussion
Before studying the glycosylation reaction, various
derivatives of both counterparts have been synthesized.
With the goal of further elaboration of liposidomycin
analogues in mind, we focused on the preparation of
an azidoribose derivative for which the secondary alco-
hol functions at the 2- and 3-positions were protected as
an acetonide. Thus, the commercially available D-ribose
was first submitted to concomitant protection of the
anomeric hydroxy group and secondary alcohols
(Scheme 1) to give 1 in 75% yield.13 Subsequent Mitsu-
nobu reaction14 with hydrazoic acid afforded the corre-
sponding methyl 5-azidofuranoside derivative 2 in 98%
yield. Further acidic hydrolysis of the protecting groups
at 85 ꢁC, followed by selected protection of the 2,3-diol
by 2,2-dimethoxypropane in acetone in the presence of
camphor sulfonic acid yielded the expected precursor 3
(40%).
CO2H
NHZ
CO2P
HO
BnO
d
NHFmoc
a
e
7a : P = H
CO2tBu
NHP
5
HO
t
7b : P = Bu
4a : P = Z
c
f
CO2P
CO2H
NHFmoc
8
4b : P = Boc
4c : P = Fmoc
BnO
b
HO
NHBoc
6a : P = H
t
6b : P = Bu
Scheme 2. Reagents and conditions: (a) tBuBr, K2CO3, BnEt3NCl,
CH3CN, 50 ꢁC, 88%; (b) Cl3CC(@NH)OtBu, BF3ÆOEt2, C6H12
CH2Cl2, 96%; (c) H2, Pd(OH)2/C, EtOH, AcOH, 99%; (d)
Cl3C(@NH)OtBu, CH2Cl2, C6H12, 40 ꢁC, 100%; (e) H2, Pd/C, EtOH,
AcOH, 29%; (f) Cl3C(@NH)OtBu, C6H12, EtOAc, 84%.
,
OMe
OH
O
O
HO
HO
a
With these building blocks in hand, the key glycosyla-
tion step was then studied (Scheme 3). Due to the possi-
ble anchimeric assistance of a participating group at the
2-position of the glycosyl derivatives to direct the reac-
tion toward a single anomer, commercially available
D-ribose derivatives displaying either an acetyl 9 or a
benzoyl 10 group were included herein. Moreover, in
order to examine the diastereoselectivity, the glycosyla-
tion was also carried out with the C2 O-benzyl derivative
11, which was easily available from 12 by acetylation in
anomeric position (Ac2O, DMAP, CH2Cl2, 93%). In all
cases, the glycosylation was carried out through activa-
tion of the anomeric position as a halide. On one hand,
bromide derivatives of C2-O-acetyl 9, -benzoyl 10 and
-benzyl 11 were prepared by reaction with an excess of
trimethylsilyl bromide18 in dichloromethane. Due to
the instability of these bromides19 they could not be puri-
fied by flash chromatography. Nevertheless 1H NMR of
the crude showed the major formation of the expected
b-anomer (over 95%). On the other hand, fluorination
by DAST20 [(diethylamino)sulfur trifluoride] was carried
out either on 12 or on azidoacetonide 3. In these cases,
the resulting b/a mixture of fluorinated compounds
16 and 17 were more stable, and could be isolated by
flash chromatography in about a 5.3 ratio in 84% yield.
Careful choice of the Lewis acid used to promote the
glycosylation8 revealed that the best conditions were
either silver triflate in dichloromethane at ꢀ15 ꢁC for
compounds activated as a bromide, or stannous dichlo-
ride in the presence of silver perchlorate from ꢀ15 ꢁC
HO
b
OH
O
O
1
OH
OMe
O
O
N3
N3
c
O
O
O
O
3
2
Scheme 1. Reagents and conditions: Me2CO, MeOH, HClg, 75%; (b)
HN3, Ph3P, DIAD, THF, 0 ꢁC, 98%; (c) (i): H2SO4, 85 ꢁC, (ii):
Me2C(OMe)2, Me2CO, CSA, 50 ꢁC, 40% (b/a = 9:1).
We next turned our attention to the preparation of var-
iously protected L-serine derivatives. Thus, the corre-
sponding N-Cbz, N-Boc and N-Fmoc compounds 4
were prepared (Scheme 2) from commercially available
reagents. The N-Cbz compound 4a was readily obtained
in 88% yield by esterification of N-Cbz serine with tert-
butyl bromide in the presence of potassium carbonate
and benzyl triethylammonium chloride.15 The related
N-Boc derivative 4b was prepared in two steps from
the O-benzyl N-Boc serine 6a. Esterification with tert-
butyl trichloroacetimidate in the presence of boron tri-
fluoride etherate afforded the corresponding tert-butyl
ester in 96% yield and was followed by O-benzyl hydro-
genolysis in the presence of Pearlmanꢀs catalyst in a quan-
titative yield.16 Finally, the synthesis of the N-Fmoc
derivative 4c could be achieved according to two differ-
ent ways. The first one involved a similar strategy as that

144
M. Ginisty et al. / Tetrahedron: Asymmetry 17 (2006) 142–150
CO2tBu
O
O
O
AcO
AcO
NHZ
18
OAc
O
CO2tBu
O
Br
BzO
Y
glycosylation
NHZ
19
P1O
OP1
BzO
OBz
O
OP2
OP1
O
13 : Y= OAc, P1 = Ac
14 : Y= OBz, P1 = Bz
15 : Y= OBn, P1 = Bn
Y
CO2tBu
O
O
CO2tBu
NHP3
activation
BnO
N3
HO
NHBoc
P1O
20
BnO
O
OBn
O
4a, 4b or 4c
(β:α = 0.3 to 1.2)
CO2tBu
9 : Y= OAc, P1 = Ac, P2 = Ac
10 : Y= OBz, P1 = Bz, P2 = Ac
11 : Y= OBn, P1 = Bn, P2 = Ac
12 : Y= OBn, P1 = Bn, P2 = H
3 : Y= N3, P1 = C(Me)2, P2 = H
O
F
Y
NHBoc
21
(β:α = 2.15)
P1O
OP1
O
16 : Y= OBn, P1 = Bn
17 : Y= N3, P1 = C(Me)2
CO2tBu
O
O
N3
NHFmoc
22
(β:α = 1.7)
O
O
Scheme 3. Reagents and conditions: see text and Table 1.
to rt, for derivatives activated as a fluoride. In each
case, the success of the reaction depends on the presence
of a large excess of molecular sieves.21 According to
the nature of both the ribosyl and serinyl derivatives,
the yield and the diastereoselectivity of the reaction are
gathered in Table 1. As expected, with a participating
group at the C2 position of the ribose (entries 1 and 2)
the exclusive formation of the b-anomer was observed.
Glycoside 19 was isolated in an excellent overall yield
(92%) after flash chromatography, while its acetate
analogue 18 was isolated in a lower yield (32%). In the
latter case, 2,3,5-tri-O-acetyl ribose was isolated in yields
of up to 62% revealing in situ hydrolysis of the bromide
intermediate 13. For the C2 benzyl or acetonide deriva-
tive (unable to promote anchimeric assistance) the for-
mation of a mixture of a,b-anomers was observed. For
benzyl derivatives 11 and 12 (entries 3 and 4) according
to the nature of both the halide in anomeric position
(15 and 16) and the Lewis acid used in the glycosylation
step, the yield and diastereoselectivity for the formation
of 20 varied. As expected with the non-participating
group and with bromide activation (entry 3), the major
formation of the a-anomer was observed (b/a = 0.3,
69% yield of 20). Nevertheless, fluoride activation
allowed the inversion of the diastereoselectivity to give
the major formation of the b-anomer (b/a = 1.2, entry
4). However in that case, the yield of 20 decreased to
44%. With regards to azidoribose compound 3 (entries
5 and 6), its fluoride derivative was involved in glycosyl-
ation with either tert-butyl N-Boc or N-Fmoc serinate,
4b or 4c, to afford the glycosylated compound 21 or
22, respectively, in good to excellent yields. In each case,
the major formation of the b-anomer was observed and
was unambiguously assigned by 1H NMR analysis
(Table 2), singlet or doublet for H1 in the b- or a-anomer,
respectively. Indeed, we were delighted to observe that
for entries 4–6, the experimental conditions allowed
the predominant formation of the desired b-anomer.
In these cases, increasing both the temperature and
duration of the reaction increased the b-anomer forma-
tion as observed by TLC. These observations seem to be
in agreement with the formation of the a-anomer as a
kinetic product, and b-anomer as a thermodynamic
product. Unfortunately, exclusive formation of either
one or the other anomer could not be achieved under
these conditions.
Table 1. Synthesis of O-serinyl riboside derivatives via Scheme 3
Entry Compd Activation
Compd Yield Aminoacid Glycosylation
Compd Ratio b:a Yieldb
1
2
3
4
5
6
9
TMSBr, CH2Cl2, ꢀ40 ꢁC to rt 13a
TMSBr, CH2Cl2, ꢀ40 ꢁC to rt 14a
TMSBr, CH2Cl2, ꢀ40 ꢁC to rt 15a
—
—
—
84
95
95
4a
4a
4b
4b
4b
4c
AgOTf, CH2Cl2, ꢀ15 ꢁC
AgOTf, CH2Cl2, ꢀ15 ꢁC
AgOTf, CH2Cl2, ꢀ15 ꢁC
18
19
20
b: 100%
b: 100%
0.3
1.2
2.15
32b,c
92b
69b
44
64
100
10
11d
12
3
DAST, THF, ꢀ30 ꢁC to rt
DAST, THF, ꢀ30 ꢁC to rt
DAST, THF, ꢀ30 ꢁC to rt
16
17
17
SnCl2, AgClO4, ꢀ15 ꢁC to rt 20
SnCl2, AgClO4, ꢀ15 ꢁC to rt 21
SnCl2, AgClO4, ꢀ15 ꢁC to rt 22
3
1.7
a Unstable compound.
b Overall yield for the two steps.
c Corrected yield.
d Available from 12 by acetylation: Ac2O, DMAP, CH2Cl2, 93%.

M. Ginisty et al. / Tetrahedron: Asymmetry 17 (2006) 142–150
145
Table 2. 1H NMR data, chemical shift and coupling constants, for H1 of a and b-anomers of the fluorinated derivatives 16 and 17 and the
glycosylated products 18–22
16
17
18
19
20
21
22
a
b
d
5.61
5.70
—
—
—
—
4.85
5.09
4.99
multiplicity
dd (2JH1,F = 64,
3JH1,H2 = 3.1)
dd (2JH1,F = 64,
3JH1,H2 = 3.5)
d (3JH1,H2 = 3.1)
d (3JH1,H2 = 4.4)
d (3JH1,H2 = 4.0)
d
5.84
d (2JH1,F = 61)
5.83
d (2JH1,F = 61)
4.98
s
5.30
s
5.02
s
5.01
s
5.13
s
multiplicity
3. Conclusion
expected methyl azido-ribofuranoside 2 (5.54 g) as a yel-
1
low oil in 98% yield. [a]D = ꢀ53 (c 1.0, CH2Cl2); H
The glycosylation involving ribose derivatives and
variously N-protected tert-butyl L-serinate could be
achieved efficiently by careful choice of the activation
method at the anomeric position and of the Lewis acid
promoter. The conditions described allow the major for-
mation of the b-anomer required for further elaboration
to liposidomycin and caprazamycin analogues. Their
obtention will take advantage of the orthogonally pro-
tected functional groups of the synthesized O-serinyl
aminoribosides. These results make a contribution to
the already reported methods for the obtention of O-gly-
cosylated aminoacids which are, to the best of our
knowledge, seldom described in furanosyl series.
NMR d 4.98 (s, 1H, H1), 4.56 (s, 2H, H2, H3), 4.27
(dd, 1H, JH4–H5a = 7.6 Hz, JH4–H5b = 6.8 Hz H4), 3.43
(dd, 1H, JH5a–H4 = 7.6 Hz, JH5a–H5b = 12.6 Hz, H5a),
3.36 (s, 3H, OMe); 3.22 (dd, 1H, JH5b–H4 = 6.8 Hz,
JH5b–H5a = 12.6 Hz, H5b), 1.47, 1.30 (2s, 6H,
CMe2); 13C NMR d 112.7 (CMe2), 109.8 (C1), 85.4
(C4), 85.1 (C2), 82.1 (C3), 55.2 (MeO), 53.8 (C5), 26.4,
24.9 (CMe2); MS (CI, NH3): 247 (M+NH4)+; HMRS
for C9H19N4O4 (M+NH4)+: calcd 247.1406; found
247.1407.
4.2. 5-Azido-5-deoxy-2,3-O-isopropylidene-D-ribo-
furanose 3
To azidoribofuranoside 2 (150 mg, 1 equiv, 0.66 mmol)
was added a 0.1 M solution of sulfuric acid (3.1 mL)
and the mixture stirred at 85 ꢁC for 4 h. After cooling
to 20 ꢁC, the solution was neutralized by the addition
of 1X8-400 Dowexꢂ resin until pH ꢁ 7.5. After filtration
and concentration in vacuo, the residual water was azeo-
tropically removed with toluene and the residue concen-
trated in vacuo. To the resulting residue in acetone
(2.7 mL) were then added camphor sulfonic acid
(15.5 mg, 0.1 equiv, 0.067 mmol) and 2,2-dimethoxypro-
pane (0.535 mL, 6.7 equiv, 4.39 mmol). After heating at
50 ꢁC for 30 min, the temperature was decreased to
20 ꢁC and a saturated aqueous solution of sodium bicar-
bonate added. Evaporation of the acetone was followed
by the addition of ethyl acetate and the organic layer
successively washed with water and brine, then dried
over MgSO4, filtered and concentrated in vacuo. Flash
chromatography of the residue (cyclohexane/EtOAc
7:3) gave an unseparable b/a-anomers mixture (9:1) of
the ribofuranose 3 (57 mg) as a colorless oil in 40%
4. Experimental
1H NMR (250 or 500 MHz) and 13C NMR (63 MHz)
spectra were recorded on a Bruker AM250 in CDCl3
(unless indicated). Chemical shifts (d) are reported in
ppm and coupling constants given in Hz. Optical rota-
tions were measured on a Perkin–Elmer 241C polari-
meter with a sodium (589 nm) or mercury (365 nm) lamp
at 20 ꢁC. Mass spectra, chemical ionization (CI), and
high resolution (HRMS) were recorded by the Service
de Spectrometrie de Masse, Ecole Normale Superieure,
Paris. All reactions were carried out under an argon
atmosphere, and monitored by thin-layer chromato-
graphy with Merck 60F-254 precoated silica (0.2 mm)
on glass. Unless indicated, flash chromatography was
performed with Merck Kieselgel 60 (0.2–0.5 mm); the
solvent system was given v/v. Spectroscopic (1H and
13C NMR, MS) and/or analytical data were obtained
using chromatographically homogeneous samples.
´
´
1
yield. H NMR d 5.49 (s, 1H, H1b), 5.43 (sl, 1H, H1a),
4.1. Methyl 5-azido-5-deoxy-2,3-O-isopropylidene-b-D-
ribofuranoside 2
4.65 (m, 2H, H2, H3), 4.35 (ddd, 1H, JH4–H3 = 0.7 Hz,
JH4–H5a = 7.1 Hz, JH4–H5b = 5.7 Hz, H4), 3.58 (dd, 1H,
JH5a–H4 = 7.1 Hz, JH5a–H5b = 8.9 Hz, H5a), 3.41 (dd,
1H, JH5b–H4 = 5.7 Hz, JH5b–H5a = 8.9 Hz, H5b), 1.50,
1.34 (2s, 6H, CMe2); 13C d 112.8 (CMe2), 103.4 (C1),
86.0 (C2), 85.4 (C4), 82.1 (C3), 53.9 (C5), 26.2, 24.7
(CMe2); MS (CI, NH3): 215 (MꢀH2O+NH4+); HMRS
for C8H13N3O4 (M+ꢀH2O+NH4+): calcd 215.1144;
found 215.1148.
To a solution of triphenylphosphine, dried in vacuo
(14.26 g, 2.2 equiv, 54.4 mmol), in THF (120 mL) at
0 ꢁC, was added dropwise diisopropyl azodicarboxylate
(10.52 mL, 2.2 equiv, 54.4 mmol) and the mixture stirred
for 5 min prior to the sequential dropwise addition of a
benzene solution of hydrazoic acid22 (2.8 M, 69.3 mL,
2.1 equiv, 52.6 mmol) and a solution of methyl 2,3-
O-isopropylidene-b-D-ribofuranoside 1 (5 g, 1 equiv,
24.6 mmol) in THF (10 mL). After 3 h stirring, the mix-
ture was concentrated in vacuo and flash chromatogra-
phy (cyclohexane/EtOAc 9:1) of the residue afforded the
4.3. tert-Butyl N-benzyloxycarbonyl-L-serinate 4a
To a solution of the commercially available N-benzyl-
oxycarbonyl-L-serine (500 mg, 1 equiv, 2.09 mmol) in

146
M. Ginisty et al. / Tetrahedron: Asymmetry 17 (2006) 142–150
acetonitrile (16 mL) were successively added benzyl tri-
ethylammonium chloride (477 mg, 1 equiv, 2.09 mmol),
potassium carbonate (7.50 g, 26 equiv, 54.3 mmol) and
finally tert-butyl bromide (11.3 mL, 48 equiv, 0.1 mol)
at 20 ꢁC. After 24 h stirring at 48 ꢁC and cooling to
20 ꢁC, cold water (2.1 mL) was added to the mixture
which was then concentrated in vacuo. The resulting res-
idue was extracted with ethyl acetate, (5 · 50 mL), dried
(MgSO4), filtered, and concentrated in vacuo. Flash
chromatography of the residue (Merck Kieselgel 60H,
cyclohexane/EtOAc 1:1) furnished the N-Cbz-protected
serinate 4a (539 mg) as a white solid in 88% yield.
[a]D = ꢀ16 (c 1.1, EtOH abs.), Lit.15 [a]D = ꢀ16.0 (c
1.1, EtOH), Lit23: [a]D = ꢀ16.3 (c 1.03, EtOH); Mp
H3b), 1.46, 1.43 (2s, 18H, tBu); MS (FAB): 284
(M+Na)+; HMRS for C12H24NO5 (M+H+): calcd
262.1654; found 262.1651.
4.5. tert-Butyl N-fluorenylmethoxycarbonyl-L-serinate 4c
To a solution of the commercially available N-Fmoc-
serine (350 mg, 1 equiv, 1.07 mmol) in ethyl acetate
(10 mL) was dropwise added a solution of tert-butyl tri-
chloroacetimidate (0.766 mL, 4 equiv, 4.28 mmol) in
cyclohexane (1 M, 4.3 mL) and the mixture was stirred
for 24 h at 20 ꢁC. Concentration in vacuo was then fol-
lowed by flash chromatography of the residue (Merck
Kieselgel 60H, cyclohexane /EtOAc 6:4) to give the
tert-butyl N-Fmoc-L-serinate 4c (344 mg) in 84% yield.
[a]D = +2 (c 1.0, CH2Cl2); Mp 130 ꢁC; 1H NMR d
7.81–7.21 (m, 8H, Ph), 5.78 (d, 1H, JNH–H2 = 5.7 Hz,
1
95 ꢁC, Lit.15 Mp 94 ꢁC, Lit.23 Mp 93–95 ꢁC; H NMR
d 7.29 (s, 5H, Ph), 5.92 (d, 1H, JNH–H2 = 7.4 Hz, NH),
5.06 (s, 2H, CH2Ph), 4.27 (m, 1H, H2), 3.85 (m, 2H,
H3), 3.20 (s, 1H, OH), 1.42 (s, 9H, tBu); 13C NMR d
169.7 (C1), 156.4 (NHCO), 136.2–128.1 (Car), 82.5
(tBu), 67.0, 63.3 (C3, CH2Ph), 55.0 (C2), 27.9 (tBu);
MS (CI, NH3): 296 (M+H)+, 295 (MꢀH2O+NH4+);
HRMS for C15H22NO5 (M+H)+: calcd 296.1498; found
296.1502; for C15H23N2O4 (MꢀH2O+NH4+): calcd
295.1658; found 295.1660; Anal. Calcd for
C15H21NO5: C, 61.00; H, 7.17; N, 4.74. Found C,
61.26; H, 7.26; N, 4.86.
0
0
0
NH), 4.45 (d, 2H, JH1 –H2 = 6.9 Hz, H1 ), 4.35 (m,
0
0
0
1H, H2), 4.25 (t, 1H, JH2 –H1 = 6.9 Hz, H2 ), 3.98–3.94
(m, 2H, H3), 1.52 (s, 9H, tBu); 13C NMR d 170.0 (C1),
156.8 (NHCO), 144.1–120.4 (Car), 83.3 (tBu), 67.6
0
0
(C1 ), 64.0 (C3), 57.1 (C2), 47.5 (C2 ), 28.4 (tBu); MS
(CI, NH3): 401 (M+NH4+); HMRS for C22H29N2O5
(M+NH4+): calcd 401.2076; found 401.2079; Anal.
Calcd for C22H29N2O5: C, 68.91; H, 6.59; N, 3.65.
Found: C, 68.41; H, 6.58; N, 3.58.
4.4. tert-Butyl N-tert-butyloxycarbonyl-L-serinate 4b
4.6. Acetyl 2,3,5-tri-O-benzyl-D-ribofuranoside 11
To a solution of the commercially available N-Boc-O-
benzyl-L-serine (2 g, 1 equiv, 6.77 mmol) in dichloro-
methane (7 mL) were added a solution of tert-butyl
trichloroacetimidate (2.95 g, 2 equiv, 13.54 mmol) in
cyclohexane (14 mL) and boron trifluoride etherate
(0.136 mL, 0.16 equiv, 1.08 mmol) and the mixture stir-
red for 20 h at 20 ꢁC. Neutralization by the addition of
solid sodium hydrogen carbonate, was followed by con-
centration in vacuo and flash purification of the residue
(Merck Kieselgel 60H, toluene/EtOAc 9:1) to afford the
corresponding ester 6b (2.28 g) in 96% yield. [a]D = ꢀ4
(c 1.05, EtOH abs.); 1H NMR d 7.36–7.23 (m, 5H,
Ph), 5.35 (d, 1H, JNH–H2 = 8.1 Hz, NH), 4.55,
4.45 (AB, 2H, JAB = 12.1 Hz, CH2Ph), 4.30 (dd, 1H,
JH2–H3a ꢁ JH2–H3b ꢁ 3.2 Hz, H2), 3.83 (dd, 1H,
JH3a–H3b = 9.3 Hz, JH3a–H2 = 3.2 Hz, H3a), 3.64 (dd,
1H, JH3b–H3a = 9.3 Hz, JH3b–H2 = 3.2 Hz, H3b), 1.45–
1.33 (m, 18H, tBu); 13C NMR d 169.6 (C1), 155.5
(NHCO), 137.8, 137.6, 129.0, 128.3, 128.2, 127.7,
127.6, 125.4 (CPh), 81.6, 79.4 (2 tBu), 73.2 (CH2Ph),
70.4 (C3), 54.6 (C2), 28.3, 27.9, 20.6 (2 · tBu).
To a solution of the commercially available 2,3,5-tri-O-
benzyl-D-ribofuranose (100 mg, 1 equiv, 0.24 mmol) and
4,4-dimethylaminopyridine (87.5 mg, 3 equiv, 0.71 mmol)
in dichloromethane (1 mL) at 20 ꢁC was added acetic
anhydride (0.068 mL, 3 equiv, 0.71 mmol). After 3 h
stirring, a saturated aqueous solution of ammonium
chloride (3 mL) was added and the aqueous layer
extracted with dichloromethane (3 · 10 mL). The com-
bined extracts were washed with brine, dried over
MgSO4, filtered, and concentrated in vacuo. Flash chro-
matography of the crude (cyclohexane/EtOAc 1:1) affor-
ded an unseparable b/a-anomers mixture (4:1) of the
acetyl ribofuranoside 11 (101.9 mg) in 93% yield, as a
colorless oil.
11b: 1H NMR d 7.39–7.25 (m, 15H, Ph), 6.27 (s,
1H, H1), 4.73–4.55 (m, 6H, CH2Ph), 4.39 (ddd, 1H,
JH4–H3 = 5.6 Hz, JH4–H5 = 5.2 Hz, H4), 4.11 (d, 1H,
JH2–H3 = 1.9 Hz, H2), 4.00 (dd, 1H, JH3–H2 = 1.9 Hz,
JH3–H4 = 5.6 Hz, H3), 3.64 (d, 2H, JH5–H4 = 5.1 Hz,
H5), 2.05 (s, 3H, OAc); 13C NMR d 170.0 (OAc),
138.0, 137.7, 137.3 (3Cq (ar)), 128.5–127.7 (Car), 100.6
(C1), 87.1 (C2), 83.8 (C3), 83.4 (C4), 73.5, 72.2, 72.0
(CH2Ph), 69.7 (C5), 21.3 (OMe).
Hydrogenolysis of the O-benzyl protective group of 6b
was then carried out by adding to a solution of the ester
(2.28 g, 1 equiv, 6.51 mmol) in a 5:1 mixture of ethanol/
acetic acid (24 mL) the Pearlmanꢀs catalyst (228 mg,
10% w/w) in the presence of dihydrogen. After 36 h stir-
ring at 20 ꢁC, filtration through a Celite pad and concen-
tration in vacuo, the expected tert-butyl N-Boc-serinate
4b (1.67 g) was isolated in 99% yield as a white solid.
[a]D = ꢀ22 (c 1.8, EtOH abs.), Lit16: [a]D = ꢀ22.5 (c
1.8, EtOH abs.), Lit24: [a]D = ꢀ20.0 (c 1.8, EtOH); Mp
1
11a: H NMR d 7.39–7.27 (m, 15H, Ph), 6.31 (d, 1H,
JH1–H2 = 3.5 Hz, H1), 4.52 (m, 6H, CH2Ph), 4.22–4.17
(m, 3H, H2, H3, H4), 3.59 (d, 2H, JH5–H4 = 4.5 Hz,
H5), 1.99 (s, 3H, OAc); 13C NMR d 170.0 (OAc),
138.0, 137.7, 137.3 (3Cq (ar)), 128.5–127.7 (Car), 100.6
(C1), 87.1 (C2), 83.8 (C3), 83.4 (C4), 73.5, 72.2, 72.0
(CH2Ph), 69.7 (C5), 21.3 (OMe); MS (CI, NH3): 480
(M+NH4)+; HRMS for C28H34NO6 (M+NH4)+: calcd
480.2386; found: 480.2383.
1
80 ꢁC, Lit16: 80 ꢁC, Lit24: 76–78 ꢁC; H NMR d 5.40
(br s, 1H, NH), 4.22 (m, 1H, H2), 3.87 (s, 2H, H3a,

M. Ginisty et al. / Tetrahedron: Asymmetry 17 (2006) 142–150
147
4.7. 2,3,5-Tri-O-benzyl-1-fluoro-D-ribofuranose 16
C8FH16N4O3 (M+NH4)+: calcd 235.1206; found
235.1210.
To a solution of the commercially available 2,3,5-tri-O-
benzyl-D-ribofuranose (291 mg, 1 equiv, 0.69 mmol) in
THF (5 mL) at ꢀ30 ꢁC was rapidly added (diethyl-
amino)sulfur trifluoride (0.11 mL, 1.2 equiv, 0.83 mmol).
The cooling bath was then immediately removed and
TLC analysis (cyclohexane/EtOAc 9:1) revealed that
the reaction was complete within 30 min. The mixture
was again cooled to ꢀ30 ꢁC prior to the addition of
methanol (0.470 mL). After concentration in vacuo,
the resulting residue was taken up in dichloromethane
and the organic layer washed with brine. The aqueous
layer was further extracted with dichloromethane
(3 · 20 mL) and the combined extracts dried over
MgSO4, filtered, and concentrated in vacuo. Flash chro-
matography of the crude (Merck Kieselgel 60H, cyclo-
hexane/EtOAc 9:1) afforded a b/a-anomers mixture
(95:5 to 99:1) of the fluoro ribofuranose 16 (276.4 mg)
in 95% yield, as a pale oil.
1
17a: [a]D = +98 (c 1.0, CH2Cl2); H NMR d 5.70 (dd,
1H, JH1–F = 64.1 Hz, JH1–H2 = 3.5 Hz, H1), 4.76 (ddd,
1H, JH2–H1 = 3.5 Hz, JH2–F = 15.6 Hz, H2), 4.64 (dd, 1H,
JH3–H2 = 6.8 Hz, JH3–H4 = 2.3 Hz, H3), 4.57 (ddd, 1H,
JH4–H5a = JH4–H5b = 3.5 Hz, JH4–H3 = 2.3 Hz, H4), 3.66
(dd, 1H, JH5a–H4 = 3.5 Hz, JH5a–H5b = 13.3 Hz, H5a),
3.45 (dd, 1H, JH5b–H4 = 3.5 Hz, JH5b–H5a = 13.3 Hz,
H5b), 1.59, 1.40 (2s, 6H, C(Me2)); 13C d 116.5 (CMe2),
110.4, 106.6 (d, JC1–F = 236 Hz, C1), 83.0 (C4), 81.7,
81.4 (d, JC1–F = 20.3 Hz, C2), 80.1 (C3), 52.6 (C5), 26.1
(CMe2); HMRS for C8FH16N4O3 (M+NH4)+: calcd
235.1206; found 235.1211.
4.9. General procedure for glycosylation
4.9.1. Glycosylation according to path A
4.9.1.1. Preparation of the bromide derivatives 13, 14,
and 15. To a solution of 1-acetyl ribofuranose 9, 10, or
11 (1.57 mmol) in dichloromethane (2 mL), at ꢀ40 ꢁC,
was dropwise added trimethylsilyl bromide (0.735 mL,
3.7 equiv, 5.8 mmol) and the temperature raised to
20 ꢁC. Monitoring of the reaction by thin layer chroma-
tography often revealed incomplete bromination. As a
result, the mixture was again cooled to ꢀ40 ꢁC prior
to further addition of TMSBr followed by increasing
the temperature to 20 ꢁC and stirred again. Several
successive additions of TMSBr were usually required
to obtain total transformation into the corresponding
1-bromo derivative. The excess of TMSBr and the tri-
methylsilyl acetate were then removed by concentration
in vacuo and the resulting crude bromide 13, 14, or 15
was then used for the next glycosylation step without
purification.
1
16b: [a]D = +22 (c 1.0, CH2Cl2); H NMR d 7.44–7.33
(m, 15H, Ph), 5.84 (d, 1H, JH1–F = 61.5 Hz, H1), 4.60–
4.50 (m, 7H, H4, CH2Ph), 4.22 (dd, 1H, JH2–H3
=
1.9 Hz, JH2–F = 9.2 Hz, H2), 4.05 (dd, 1H, JH3–H2 = 1.9
Hz, JH3–H4 = 5.1 Hz, H3), 3.67 (d, 2H, JH5–H4
=
4.8 Hz, H5); 13C NMR d 137.9, 137.5, 137.0 (3Cq (ar)),
128.6–127.8 (Car), 113.6 (d, JC1–F = 225 Hz, C1), 86.9 (d,
JC2–F = 33.7 Hz, C2), 84.2 (C4), 82.6 (C3), 73.5–72.2
(CH2Ph), 69.4 (C5), MS (CI, NH3): 440 (M+NH4)+;
HRMS for C26FH31NO4 (M+NH4)+: calcd: 440.2237;
found 440.2231.
1
16a: H NMR d 7.34–7.18 (m, 15H, Ph), 5.61 (dd, 1H,
JH1–F = 64 Hz, JH1–H2 = 3.1 Hz, H1), 4.85–4.50 (m,
7H, H4, CH2Ph), 4.22–4.06 (m, 2H, H2, H3), 3.58 (m,
2H, H5); MS (CI, NH3): 440 (M+NH4)+; HRMS for
C26FH31NO4 (M+NH4)+: calcd 440.2237; found
440.2235.
4.9.1.2. Glycosylation. To a solution of tert-butyl N-
carbamoyl-serinate (1.2 mmol, 0.76 equiv related to the
1-acetyl ribofuranose) in dichloromethane (3 mL), in
˚
4.8. 5-Azido-5-deoxy-2,3-O-isopropylidene-1-fluoro-D-
ribofuranose 17
the presence of 4 A molecular sieves (0.465 g, Lancas-
terꢂ) at ꢀ20 ꢁC, was added silver triflate (398 mg,
1 equiv). After 1 h stirring, a solution of the bromide
derivative 13, 14, or 15 (1.57 mmol) in dichloromethane
(2 mL) was slowly added at ꢀ10 ꢁC and the mixture stir-
red for 24 h at ꢀ10 ꢁC. Triethylamine (2 mL) was then
added and the mixture filtered through a Celiteꢂ pad.
A saturated aqueous solution of sodium hydrogeno-
carbonate was then added and the aqueous layer
was extracted with dichloromethane (4 · 25 mL). The
combined organic extracts were dried over MgSO4 and
concentrated in vacuo prior to flash chromatographic
purification. According to the starting compounds, the
results are given below.
The preparation of fluoro derivative 17 was carried out
from the 5-azido-5-deoxy-2,3-O-isopropylidene-D-ribo-
furanose 3 (2.38 g, 1 equiv, 11 mmol) according to the
experimental procedure described above for the prepa-
ration of the fluoro compound 16. Flash chromato-
graphic purification of the crude (cyclohexane/EtOAc
9:1) gave a b/a-anomers mixture (84:16) of the fluoro
ribofuranose 17 (2.06 g) in 86% yield. Each anomer
could be isolated as a pure compound.
17b: [a]D = +24 (c 1.0, CH2Cl2); 1H NMR d 5.83 (d, 1H,
JH1–F = 61.3 Hz, H1), 4.83 (dd, 1H, JH3–H2 = JH3–H4
5.9 Hz, H3), 4.69 (d, 1H, JH2–H3 = 5.9 Hz, H2), 4.48
(ddd, 1H, JH4–H5a = 7.5 Hz, JH4–H5b = 6.7 Hz, JH4–H3
5.9 Hz, H4), 3.53 (dd, 1H, JH5a–H4 = 7.5 Hz, JH5a–H5b
12.9 Hz, H5a), 3.26 (dd, 1H, JH5b–H4 = 6.7 Hz,
JH5b–H5a = 12.9 Hz, H5b), 1.61, 1.36 (2 s, 6H, CMe2);
13C d 115.3 (d, JC1–F = 245 Hz, C1), 113.7 (CMe2), 87.8
(C4), 84.9 (C3), 81.4 (C2), 53.6 (C5), 26.6, 25.7
(CMe2); MS (CI, NH3): 235 (M+NH4)+; HMRS for
=
4.9.1.3. (tert-Butyl N-benzyloxycarbonyl-L-serinate-30-
yl)2,3,5-tri-O-acetyl-b-D-ribofuranoside 18. From the
tert-butyl N-benzyloxycarbonyl-L-serinate 4a (354 mg,
1.2 mmol) and the 2,3,5-tri-O-acetyl-1-bromo-D-ribo-
furanose 13 (1.57 mmol), the described procedure followed
by flash chromatography (cyclohexane/EtOAc 1:1)
afforded the target glycosylated compound 18 (275 mg)
as a colorless oil in 32% yield. Rf 0.24 (cyclohexane/
=
=

148
M. Ginisty et al. / Tetrahedron: Asymmetry 17 (2006) 142–150
EtOAc 1:1); [a]D = ꢀ20 (c 1.0, CH2Cl2); 1H NMR d 7.33–
7.24 (m, 5H, Ph), 5.57 (d, JNH–H2 = 8.0 Hz, 1H, NH),
3.2 Hz, H2), 3.92 (dd, 1H, JH3–H2 = 3.2 Hz, JH3–H4
=
=
0
6.5 Hz, H3), 3.89 (m, 2H, H3 ), 3.61 (dd, 1H, JH5a–H4
5.18 (m, 2H, H2, H3), 5.10, 5.08 (AB, 2H, JAB
=
4.1 Hz, JH5a–H5b = 10.9 Hz, H5a), 3.59 (dd, 1H,
0
0
11.8 Hz, H4 ), 4.98 (s, 1H, H1), 4.36 (ddd, JH2 –NH =
JH5b–H4 = 4.8 Hz, JH5b–H5a = 10.9 Hz, H5b), 1.44, 1.43
13
0
0
0
0
8.0 Hz, JH2 –H = 5.5 Hz, JH2 –H = 2.5 Hz, H2 ), 4.29–
4.19 (m, 2H, JH4–H5a = 4.5 Hz, H5a, H4), 4.14–4.09 (m,
(s, 18H, tBu); C NMR d 169.7 ðC1 Þ, 155.6 (NHCO),
30 a
30 b
138.1, 137.8, 137.4 (Cq (ar)), 129.6, 128.5, 128.4, 127.9,
127.8, 127.6, 126.6 (Car), 106.3 (C1), 88.0 (C2), 83.8
(C3), 81.9 (CO2tBu), 81.1 (C4), 79.6 (tBu), 73.4, 72.2,
0
2H, JH3 a–H = 5.5 Hz, JH5b–H4 = 2.4 Hz, JH5b–H5a = 12
20
0
0
0
Hz, H5b, H3 a), 3.64 (dd, JH3 b–H = 9.8 Hz, JH3 b–H
=
30 a
20
0
0
0
2.5 Hz, H3 b), 2.07, 2.05, 2.02 (3 s, 9H, OAc), 1.44
71.9 (CH2Ph), 69.5 (C5), 68.1 ðC3 Þ, 54.7 ðC2 Þ, 28.4;
28.1 (tBu); MS (CI, NH3): 664 (M+H)+; HRMS for
C38H50NO9 (M+H)+: calcd 664.3486; found 664.3484.
13
0
(s, 9H, tBu); C NMR d 170.5 (C1 ), 169.8, 168.8,
168.7 (O-COCH3), 155.9 (NHCO), 136.3, 128.5, 128.0,
124.4, 124.2 (Car), 104.1 (C1), 82.3 (tBu), 78.2 (C4), 76.1
0
(C3), 72.1 (C2), 70.5 (C3 ), 66.8 (CH2-Ph), 62.2 (C5),
20a: Rf 0.56 (cyclohexane/EtOAc 8:2); [a]D = ꢀ27 (c 1.0,
1
0
54.6 (C2 ), 27.8 (tBu), 22.9, 22.4, 20.6 (OAc); MS (CI,
CH2Cl2); H NMR d 7.39–7.24 (m, 15H, Ph); 5.60 (d,
NH3): 554 (M+H)+: HRMS for C26H36NO12 (M+H)+:
1H, JNH–H2 ¼ 8:1 Hz, NH), 4.85 (d, 1H, JH1–H2
=
0
calcd 554.2238; found 554.2237.
3.1 Hz, H1), 4.64, 4.61, 4.59 (3AB, 6H, JAB = 11.3 Hz,
0
0
00 00
JA B ¼ 12:1 Hz, JA B ¼ 14:3 Hz, CH2Ph), 4.33 (m,
4.9.1.4. (tert-Butyl N-benzyloxycarbonyl-L-serinate-30-
yl)2,3,5-tri-O-benzoyl-b-D-ribofuranoside 19. From the
tert-butyl N-benzyloxycarbonyl-L-serinate 4a (190 mg,
0.644 mmol) and the 2,3,5-tri-O-benzoyl-1-bromo-D-
ribofuranose 14 (0.5 mmol), the described procedure fol-
lowed by flash chromatography (cyclohexane/EtOAc/
Et3N 8:2:3) afforded the targeted glycosylated compound
19 (338.2 mg) as a white foam in 92% yield. Rf 0.25
(cyclohexane/EtOAc 8:2); [a]D = +18 (c 1.0, CH2Cl2);
1H NMR d 8.04–7.28 (m, 20H, Ph), 5.79 (d, 1H,
1H, JH2 –H3 a ¼ 4:3 Hz, JH2 ꢀNH ¼ 8:1 Hz, H2 ), 4.07 (m,
0
0
0
0
0
0
0
4H, H2, H3 a, H3, H4), 3.62 (dd, 1H, JH3 b–H2 ¼ 3:7 Hz,
13
0
0
0
JH3 b–H3 a ¼ 10:2 Hz, H3 b), 3.53 (m, 2H, H5a, H5b),
0
1.43, 1.42 (2s, 18H, tBu); C NMR d 169.4 ðC1 Þ,
155.6 (NHCO), 138.2, 138.1, 137.8 (Cq (ar)), 128.5,
128.4, 128.0, 127.9, 127.8 (Car), 101.1 (C1), 84.2 (C2),
82.3 (C3), 81.9 (CO2tBu), 80.1 (C4), 79.6 (tBu), 73.2,
0
0
72.4, 71.7 (CH2Ph), 69.5 (C5), 68.4 ðC3 Þ, 54.5 ðC2 Þ,
28.4, 28.0 (tBu); MS (CI, NH3): 664 (M+H)+; HRMS
for C38H50NO9 (M+H)+: calcd 664.3486; found
664.3496.
0
JNH–H2 ¼ 7:5 Hz, NH), 5.70 (d, 1H, JH2–H3 = 4.7 Hz,
H2), 5.68 (dd, 1H, JH3–H2 = 4.7 Hz, JH3–H4 = 5.6 Hz,
H3), 5.30 (s, 1H, H1), 5.08, 5.06 (AB, 2H, JAB = 13.9 Hz,
4.9.2. Glycosylation according to path B. To a solution
of tert-butyl N-carbamoyl-L-serinate (0.85 equiv, 0.29
mmol), stannous chloride (65.4 mg, 1 equiv, 0.34 mmol)
and silver perchlorate (71 mg, 1 equiv, 0.34 mmol) in
0
2H4 ), 4.75 (ddd, 1H, JH4–H3 = 5.6 Hz, JH4–H5b = 5.8 Hz,
JH4–H5a = 9.4 Hz, H4), 4.60 (dd, 1H, JH5a–H4 = 9.4 Hz,
JH5a–H5b = 14.7 Hz, H5a), 4.50 (dd, 1H, JH5b–H4
=
˚
0
5.8 Hz, JH5b–H5a = 14.7 Hz, H5b), 4.48 (m, 1H, H2 ),
ether (5 mL) at ꢀ15 ꢁC, in the presence of 4 A molecular
0
0
0
0
4.25 (dd, 1H, JH3 a–H2 ¼ 2:4 Hz, JH3 a–H3 b ¼ 9:6 Hz,
sieves (1.5 g, Lancasterꢂ) was added a solution of fluo-
ride derivative 16 or 17 (1 equiv, 0.34 mmol) in ether
(5 mL). After 2 h stirring, the temperature was raised
to 20 ꢁC and stirring continued for 48–72 h. The mixture
was then filtered through a Celiteꢂ pad and a saturated
aqueous solution of sodium hydrogen carbonate added.
The aqueous layer was extracted with dichloromethane
(4 · 25 mL) and the combined organic extracts dried
over MgSO4 and concentrated in vacuo prior to flash
chromatographic purification. According to the starting
compounds, the results are given below.
0
0
0
0
0
H3 a), 3.80 (dd, 1H, JH3 b–H2 ¼ 2 Hz, JH3 b–H3 a
¼
9:6 Hz, H3 b), 1.49 (s, 9H, tBu); 13C NMR d 168.7
0
0
ðC1 Þ, 166.1, 165.3, 165.2 (PhCO), 156.0 (NHCO),
136.4–128.1 (Car), 106.0 (C1), 82.8 (tBu), 79.2 (C4), 75.4
0
0
(C3), 72.8 (C2), 68.8 (C3 ), 67.0 ðC4Þ, 63.7 (C5), 54.7
+
0
ðC2 Þ, 28.0 (tBu); MS (CI, NH3): 757 (M+NH4) ;
HRMS for C41H45N2O12 (M+NH4)+: calcd 757.2973;
found 757.2980; Anal. Calcd for C41H41NO12: C,
66.57; H, 5.59; N, 1.89. Found C, 66.66; H, 5.63; N, 1.95.
4.9.1.5. (tert-Butyl N-tert-butyloxycarbonyl-L-serinate-
20. From
30-yl)2,3,5-tri-O-benzyl-b-D-ribofuranoside
4.9.2.1. (tert-Butyl N-tert-butyloxycarbonyl-L-serinate-
20. From
the tert-butyl N-tert-butyloxycarbonyl-L-serinate 4b (42
mg, 0.16 mmol) and the 2,3,5-tri-O-benzyl-1-bromo-D-
ribofuranose 15 (0.18 mmol), the described procedure
followed by flash chromatography (cyclohexane/EtOAc
8:2) afforded the targeted glycosylated compound 20 in
69% overall yield as a mixture of a-epimer (62 mg, oil)
and b-epimer (19 mg, oil), which could be isolated as
pure compounds.
30-yl)2,3,5-tri-O-benzyl-b-D-ribofuranoside
the tert-butyl N-tert-butyloxycarbonyl-L-serinate 4b (76
mg, 0.29 mmol) and the 2,3,5-tri-O-benzyl-1-fluoro-D-
ribofuranose 16 (145 mg, 0.34 mmol), the described pro-
cedure followed by flash chromatography (cyclohexane/
EtOAc 8:2) afforded the target glycosylated compound
20 in 44% overall yield as a mixture of the a-epimer
(47 mg, oil) and the b-epimer (54 mg, oil), which could
be isolated as pure compounds. For physical data of
20a and 20b, see above.
20b: Rf 0.61 (cyclohexane/EtOAc 8:2); [a]D = +31 (c 1.0,
CH2Cl2); 1H NMR d 7.37–7.18 (m, 15H, Ph), 5.52 (d, 1H,
0
JNH–H2 ¼ 9 Hz, NH), 5.02 (s, 1H, H1), 4.60–4.44 (3AB,
4.9.2.2. (tert-Butyl N-tert-butyloxycarbonyl-L-serinate-
30-yl)5-azido-5-deoxy-2,3-O-isopropylidene-D-ribofurano-
side 21. From the tert-butyl N-tert-butyloxy- carbonyl-
L-serinate 4b (204 mg, 0.78 mmol) and the 5-azido-
5-deoxy-2,3-O-isopropylidene-1-fluoro-D-ribofuranose
0
0
00 00
6H, JAB = 14.3 Hz, J
¼ 13:1 Hz, JA B ¼ 11:9 Hz,
CH2Ph), 4.30 (m, 1H, JA B
¼ 9 Hz, JH2 –H3 ¼ 2:6 Hz,
H20–NH
0
0
0
H2 ), 4.19 (ddd, 1H, JH4–H5a = 4.1 Hz, JH4–H5b
4.8 Hz, JH4–H3 = 6.5 Hz, H4), 3.96 (d, 1H, JH3–H2
=
=

M. Ginisty et al. / Tetrahedron: Asymmetry 17 (2006) 142–150
149
17 (200 mg, 0.92 mmol), the described procedure fol-
lowed by flash chromatography (cyclohexane/EtOAc
9:1) afforded the targeted glycosylated compound 21 in
64% overall yield as a mixture of a-epimer (86 mg, oil)
and b-epimer (184 mg, oil), which could be isolated as
pure compounds.
JH5b–H5a = 12.5 Hz, H5b), 1.51 (s, 12H, tBu, CMe2),
13
0
1.35 (s, 3H, CMe2); C NMR d 169.6 ðC1 Þ, 156.3
(NHCO), 144.2, 141.7, 128.1, 127.5, 125.5, 120.4 (Car.),
113.3 (CMe2), 109.8 (C1), 85.9 (C4), 85.5 (C3), 82.4
0
0
0
(tBu), 82.4 (C2), 68.9 ðC3 Þ, 67.6 ðC4 Þ, 54.9 ðC2 Þ, 53.7
0
(C5), 47.6 ðC5 Þ, 28.4 (tBu), 26.8, 25.3 (CMe2); MS (CI,
NH3): 598 (M+NH4)+; HRMS for C30H40N5O8
21b: Rf 0.49 (cyclohexane/EtOAc 7:3); [a]D = ꢀ15 (c 1.0,
(M+NH4)+: calcd 598.2877; found 598.2872.
CH2Cl2); 1H NMR d 5.19 (d, 1H, JNH–H2 ¼ 7:9 Hz,
0
NH), 5.01 (s, 1H, H1), 4.58 (dd, 1H, JH3–H2
=
22a: Rf 0.31 (cyclohexane/EtOAc 7:3); [a]D +21 (c 1.0,
1
JH3–H4 = 6 Hz, H3), 4.50 (d, 1H, JH2–H3 = 6 Hz, H2),
CH2Cl2); H NMR d 7.93–7.29 (m, 8H, Har.), 6.03 (d,
3
0
0
0
4.26–4.20 (m, 2H, JH4–H5 = 7.5 Hz, H2 , H4 ), 3.99 (dd,
1H, JNH–H2 ¼ 8:4 Hz, NH), 4.99 (d, 1H, JH1–H2
=
0
0
0
0
0
1H, JH3 a–H2 ¼ 3:5 Hz, JH3 a–H3 b ¼ 10 Hz, H3 a), 3.53
4.0 Hz, H1), 4.69 (dd, 1H, JH2–H3 = 6.9 Hz, JH2–H1 = 4.0
0
0
0
0
0
(dd, 1H, JH3 b–H2 ¼ 3:5 Hz, JH3 b–H3 a ¼ 10 Hz, H3 b),
Hz, H2), 4.59 (dd, 1H, JH3–H4 = 2.9 Hz, JH3–H2
=
0
0
0
3.34 (dd, 1H, JH5a–H4 = 7.4 Hz, JH5a–H5b = 12.6 Hz,
6.9 Hz, H3), 4.48–4.31 (m, 3H, JH4 –H5 ¼ 7 Hz, H2 ,
0
0
0
0
H5a), 3.11 (dd, 1H, JH5b–H4 = 7.5 Hz, JH5b–H5a
=
H4 a, H4 b), 4.30–4.10 (m, 3H, JH5 –H4 a ¼ 7 Hz, JH4–H5b
=
0
0
0
0
12.6 Hz, H5b), 1.41, 1.39, 1.33 (3s, 24H, tBu, CMe2);
3.2 Hz, H4, H5 , H3 a), 4.05 (dd, 1H, JH3 b–H2 ¼ 3:2 Hz,
13
0
0
0
0
C NMR d 169.6 ðC1 Þ, 155.7 (NHCO), 113.2 (CMe2),
JH3 b–H3 a ¼ 11 Hz, H3 b), 3.55 (dd, 1H, JH5a–H4
=
¼
0
109.6 (C1), 85.9 (C4), 85.4 (C3), 82.8 (CO2tBu), 82.4
2.6 Hz, JH5a–H5b = 13 Hz, H5a), 3.35 (dd, 1H, JH bꢀH4
0
0
(C2), 80.3 (tBu), 69.1 ðC3 Þ, 54.4 ðC2 Þ, 53.7 (C5), 28.7;
28.4 (tBu), 26.7, 25.3 (CMe2); MS (CI, NH3): 459
(M+H)+; HRMS for C20H35N4O8 (M+H)+: calcd
459.2455; found 459.2453.
3:2 Hz, JH5b–H5a = 13 Hz, H5b), 1.57, 1.38 (2s, 6H,
13
0
CMe2), 1.51 (s, 9H, tBu); C NMR d 169.6 ðC1 Þ,
156.5 (NHCO), 144.2, 141.7, 128.0, 127.5, 125.5, 120.4
(Car.), 116.3 (CMe2), 102.6 (C1), 82.9 (tBu), 81.1 (C2,
0
0
0
C3), 80.6 (C4), 70.0 ðC3 Þ, 67.5 ðC4 Þ, 55.3 ðC2 Þ, 52.7
0
21a: Rf 0.39 (cyclohexane/EtOAc 7:3); [a]D = +32 (c 1.0,
(C5), 47.5 ðC5 Þ, 28.4 (tBu), 26.3, 26.1 (CMe2); MS (CI,
CH2Cl2); 1H NMR d 5.73 (d, 1H, JNH–H2 ¼ 8:6 Hz,
NH3): 598 (M+NH4)+; HRMS for C30H40N5O8
0
NH), 5.09 (d, 1H, JH1–H2 = 4.4 Hz, H1), 4.80 (dd,
1H, JH2–H1 = 4.4 Hz, JH2–H3 = 7.2 Hz, H2), 4.70 (dd,
1H, JH3–H2 = 7.2 Hz, JH3–H4 = 3.2 Hz, H3), 4.50–4.41
(M+NH4)+: calcd 598.2877, found 598.2871.
0
(m, 2H, JH4–H3 = 3.2 Hz, H2 , H4), 4.20 (dd, 1H,
Acknowledgements
0
0
0
0
0
JH3 a–H2 ¼ 3:3 Hz, JH3 a–H3 b ¼ 10:7 Hz, H3 a), 4.07 (dd,
0
0
0
0
0
1H, JH3 bꢀH2 ¼ 3:1 Hz, JH3 bꢀH3 a ¼ 10:7 Hz, H3 b), 3.73
(dd, 1H, JH5a–H4 = 3.8 Hz, JH5a–H5b = 8.8 Hz, H5a),
3.52 (dd, 1H, JH5b–H4 = 4 Hz, JH5b–H5a = 8.8 Hz, H5b),
1.66, 1.61, 1.59, 1.48 (4s, 24H, tBu, CMe2); 13C NMR
We gratefully acknowledge the European Community
for the financial support of the Eur-INTAFAR inte-
grated project within the 6th PCRDT framework (con-
tract No. LSHM-CT-2004-512138).
0
d 169.8 ðC1 Þ, 156.0 (NHCO), 116.4 (CMe2), 102.5
(C1), 82.4 (CO2tBu), 81.1 (C2), 80.5 (C3), 80.0 (C4),
0
0
70.1 ðC3 Þ, 54.9 ðC2 Þ, 52.7 (C5), 28.7; 28.4 (tBu), 26.4,
26.2 (CMe2); MS (CI, NH3): 459 (M+H)+; HRMS for
C20H35N4O8 (M+H)+: calcd 459.2455; found 459.2461.
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