Synthesis of α-CF2-mannosides and their conversion to fluorinated pseudoglycopeptides

Article abstract of DOI:10.1021/jo702466h

(Chemical Equation Presented) A methodology allowing the synthesis α-CF₂-mannosides, based on the addition of a difluoroenoxysilane to a glycal followed by a dihydroxylation reaction, is described. The resulting 2,2-difluoro-2-mannosylacetate is converted into two pseudoglycopeptides which may act as E- and P-selectin inhibitors.

Full text of DOI:10.1021/jo702466h

synthesis of these so-called CF2-glycopyranosides was pioneered
by Motherwell and further investigated by others.6 Our group
aimed at providing methodologies which allowed the synthesis
of 2,2-difluoro-2-glycosylacetates such as 1 from commercially
available and easy-to-handle ethyl bromodifluoroacetate. Com-
pound 1 indeed features a valuable ester function which has
already proven useful for further synthetic elaboration.6g,h
Moreover, we wished to develop different strategies allowing
the synthesis of these analogues for many carbohydrate series
(glucose, mannose, and galactose) and for any pseudoanomeric
center configuration (R or â).
Synthesis of r-CF2-mannosides and Their
Conversion to Fluorinated Pseudoglycopeptides
Florent Poulain, Anne-Lise Serre, Je´roˆme Lalot,
Eric Leclerc,* and Jean-Charles Quirion*
UniVersite´ et INSA de Rouen, CNRS, UMR 6014
C.O.B.R.A.-IRCOF, 1 rue Tesnie`re, 76821 Mont Saint-Aignan
Cedex, France
eric.leclerc@insa-rouen.fr; quirion@insa-rouen.fr
ReceiVed NoVember 17, 2007
A methodology allowing the synthesis R-CF2-mannosides,
based on the addition of a difluoroenoxysilane to a glycal
followed by a dihydroxylation reaction, is described. The
resulting 2,2-difluoro-2-mannosylacetate is converted into
two pseudoglycopeptides which may act as E- and P-selectin
inhibitors.
FIGURE 1. R- and â-pseudomannopeptides for E- and P-selectin
inhibition.
One of our goals for this project was to synthesize the
fluorinated analogues of CH2-glycopeptides 2 prepared by Wong
and Kaila for E- and P-selectin inhibition (Figure 1).7 The R
compounds were designed by Wong on the basis of a study of
the host-enzyme interaction, and these small mannose-derived
pseudoglycopeptides indeed acted as sialyl Lewis X mimics.7a
A future evaluation of their fluorinated counterparts for E- and
P-selectin inhibition will then be an opportunity to assess the
influence of a CF2 group in pseudoanomeric position. However,
The incorporation of fluorine atoms onto biomolecules or
synthetic biologically active molecules has become an intense
research field in the two or three past decades, especially for
drug development purposes.1 This widespread interest stems
from the unique properties of the fluorine atom and of the C-F
bond.1,2 When a fluorine atom is introduced into a biologically
active molecule, its small size and its strong electronegativity,
along with the strength and short length of the C-F bond, allow
induction of alterations of crucial biological properties (such
as substrate recognition, metabolic stability, drug transport, etc.)
within limited structural modifications.2b Due to the implication
in many biological events of their natural counterparts, fluori-
nated carbohydrate derivatives have thus received careful
attention whatever the nature of the fluorinated substitution or
its position on the sugar backbone.3 In addition, one major
drawback in the use of O-glycosides and O-glycoconjugates for
drug development remains the low metabolic stability of the
anomeric bond, highly cleavable in vivo, which erodes the
bioavailability of carbohydrate-based drugs. It thus appeared
interesting to combine the hydrolytic stability of carbohydrate
analogues such as C-glycosides with the interesting properties
of fluorine to give rise to a new class of glycomimetics.4,5 The
(4) For a review on C-glycoside synthesis, see: (a) C-Oligosaccharide
synthesis: Skrydstrup, T.; Vauzeilles, B.; Beau, J. M. In Glycoscience:
Chemistry and Chemical Biology; Fraser Reid, B., Tatstuta, K., Thiem, J.,
Eds.; Springer: Heidelberg, 2001; Vol. 3, p 2679. For a recent and powerful
application of a CH2-glyconjugate analogue, see: (c) Yang, G.; Schmieg,
J.; Tsuji, M.; Franck, R. W. Angew. Chem., Int. Ed. 2004, 39, 3818.
(5) For potent nonhydrolyzable CF2-analogues of bioactive molecules,
see: (a) Burke, T. R., Jr.; Lee, K. Acc. Chem. Res. 2003, 36, 426. (b) Burke,
T. R., Jr.; Smyth, M. S.; Otaka, A.; Nomizu, M.; Roller, P. P.; Wolf, G.;
Case, R.; Shoelson, S. E. Biochemistry 1994, 33, 6490. (c) Blackburn, G.
M.; Jakeman, D. L.; Ivory, A. J.; Williamson, M. P. Bioorg. Med. Chem.
Lett. 1994, 4, 2573 and references cited therein.
(6) (a) Houlton, J. S.; Motherwell, W. B.; Ross, B. C.; Tozer, M. J.;
Williams, D. J.; Slawin, A. M. Z. Tetrahedron 1993, 49, 8087. (b) Herpin,
T. F.; Motherwell, W. B.; Weibel, J.-M. Chem. Commun. 1997, 923. (c)
Brigaud, T.; Lefebvre, O.; Plantier-Royon, R.; Portella, C. Tetrahedron Lett.
1996, 37, 6115. (d) Berber, H.; Brigaud, T.; Lefebvre, O.; Plantier-Royon,
R.; Portella, C. Chem. Eur. J. 2001, 7, 903. (e) Wegert, A.; Miethchen, R.;
Hein, M.; Reinke, H. Synthesis 2005, 1850. (f) Wegert, A.; Hein, M.; Reinke,
H.; Hoffmann, N.; Miethchen, R. Carbohydr. Res. 2006, 341, 2641. (g)
Karche, N. P.; Pierry, C.; Poulain, F.; Oulyadi, H.; Leclerc, E.; Pannecoucke,
X.; Quirion, J.-C. Synlett 2007, 123. (h) Moreno, B.; Quehen, C.; Rose-
He´le`ne, M.; Leclerc, E.; Quirion, J.-C. Org. Lett. 2007, 9, 2477. (i) Hirai,
G.; Watanabe, T.; Yamaguchi, K.; Miyagi, T.; Sodeoka, M. J. Am. Chem.
Soc. 2007, 129, 15420.
(1) Kirsch, P. Modern Fluoroorganic Chemistry; Wiley-VCH: Wein-
heim, Germany, 2004.
(2) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441. (b) Biffinger, J. C.;
Kim, H. W.; DiMagno, S. G. ChemBioChem 2004, 5, 622 and references
cited therein.
(3) For reviews on fluorinated carbohydrates and fluorinated sugar
mimics, see: (a) Dax, K.; Albert, M.; Ortner, J.; Paul, B. J. Carbohydr.
Res. 2000, 327, 47. (b) Pankiewicz, K. W. Carbohydr. Res. 2000, 327, 87.
(c) Plantier-Royon, R.; Portella, C. Carbohydr. Res. 2000, 327, 119.
(7) (a) Wong, C.-H.; Moris-Varas, F.; Hung, S.-C.; Marron, T. G.; Lin,
C.-C.; Gong, K. W.; Weitz-Schmidt, G. J. Am. Chem. Soc. 1997, 119, 8152.
(b) Kaila, N.; Thomas, B. E.; Thakker, P.; Alvarez, J.; Camphausen, R. T.;
Crommie, D. Bioorg. Med. Chem. Lett. 2001, 11, 151.
10.1021/jo702466h CCC: $40.75 © 2008 American Chemical Society
Published on Web 02/15/2008
J. Org. Chem. 2008, 73, 2435-2438
2435

SCHEME 1. Retrosynthesis of the r-Mannoside
Intermediate
SCHEME 2. Addition of 4 to Glycosyl Donors and D-Glucal
if the synthesis of the â-CF2-glycopeptides â-3a and â-3b has
already been reported,6g no method was available at that time
for the synthesis of the required R-CF2-mannopyranoside
intermediates.8 We wish to report herein the development of
such a methodology and the synthesis of the targeted R-glyco-
peptides R-3a and R-3b.
SCHEME 3. Diastereomer Separation and Dihydroxylation
Reactions
The strategy we chose to investigate to achieve this goal relied
on the addition of silyl enol ether 4 to a glycosyl donor. If the
direct addition of 4 to an activated glycoside appeared as the
most straightforward, but also most challenging route, the use
of tri-O-acetyl-D-glucal 5 as the electrophile was a safe
alternative as the subsequent dihydroxylation reaction was
expected to lead to a mannose derivative (Scheme 1). An elegant
generation of a ketone-derived difluoroenoxysilane and its
efficient addition to 5 or to 2-deoxyglycosyl donors has already
been reported by Portella.6c,d To the best of our knowledge, no
double bond functionnalization of the resulting difluoro ketone
derivatives has been described by this group.
Our first task was to identify an appropriate salt-free source
of difluoroketene silyl acetal 4 and determine if it was a suitable
nucleophile for glycosylation reactions. Reagent 4 is usually
prepared from ethyl bromodifluoroacetate, zinc, and TMSCl in
THF, and its isolation has been described by Iseki.9 This
procedure involved repeated precipitations of zinc salts with
n-pentane and a final distillation of this moisture-sensitive
difluoroenoxysilane. However, the unsatisfactory yields we
obtained using this method made clear that the isolation of 4
was not suitable for our purposes. The Reformatsky reagent was
finally prepared in acetonitrile, solvent in which the amount of
homocoupling product is limited, and quenched with TMSCl.10
Extractions with n-pentane eventually provided a salt-free
solution of 4 which was used directly for glycosylation reactions.
Despite our efforts and in agreement with Portella’s report,
this nucleophile failed to react with activated glycosides.6d The
nature of the protecting group (benzyl, acetates), of the leaving
group (acetate, trifluoroacetate, trichloroacetimidate, bromide),
and of the Lewis acid (BF3‚Et2O, TiCl4, TMSOTf, etc.) had
little influence on the outcome of the reaction (Scheme 2). A
similar approach using glycal 5 as the glycosyl donor proved
to be much more attractive since significant amounts of the SN2′
products 6 were isolated in our first attempts. Under optimized
reaction conditions, the addition of difluoroketene silyl acetal
4 to D-glucal 5 promoted by BF3‚Et2O in CH2Cl2 at -40 °C
afforded 6 in 72% yield and in a 6:4 R/â ratio (Scheme 2).
Despite our efforts, no improvement in the diastereoselectivity
could be obtained without substantial erosion of conversion and
yield.
We then decided to submit this unseparable mixture of
diastereomers to a dihydroxylation reaction. The use of Upjohn
conditions (cat. OsO4-NMO) using different solvents never led
to any conversion of the starting material, suggesting that
acetate-protected CF2-glycosylester 6 was not a good substrate
for such a transformation.11 Protecting group modification, on
the other hand, appeared appropriate since complete deacety-
lation and selective protection of the C-6 alcohol with a TBDPS
group allowed the separation of the two diastereomers R-7 and
â-7 (Scheme 3). Moreover, the presence of a free hydroxy group
at C-4 now allowed the dihydroxylation reaction to occur when
compounds 7 were subjected to the OsO4-NMO oxidation
system. CF2-Glycosides 8 and 9 were indeed obtained from â-7
and R-7 in 68% and 62% yield, respectively, and only one
diastereomer could be detected in each case by 1H and 19F NMR
of the crude mixture (Scheme 3).
(8) A five-step synthesis of such an analog and its conversion to
disaccharide mitetics has been reported during the course of this study: (a)
Tony, K. A.; Denton, R. W.; Dilhas, A.; Jime´nez-Barbero, J.; Mootoo, D.
R. Org. Lett. 2007, 9, 1441. (b) Denton, R. W.; Tony, K. A.; Hernandez-
Gay, J. J.; Canada, F. J.; Jimenez-Barbero, J.; Mootoo, D. R. Carbohydr.
Res. 2007, 342, 1624-1635.
(9) Iseki, K.; Kuroki, Y.; Asada, D.; Takahashi, M.; Kishimoto, S.;
Kobayashi, Y. Tetrahedron 1997, 30, 10271.
(10) Britton, T. C. US Patent 5,618,951, 1997.
The relative configuration of these compounds was deter-
mined on the basis of NMR data. An HOESY experiment
performed on compound 9 showed a strong F-H5 correlation,
proving that 9, and thus R-7, were indeed R anomers. Moreover,
(11) VanRheenen, V.; Kelly, R. C.; Cha, D. Y. Tetrahedron Lett. 1976,
17, 1973.
2436 J. Org. Chem., Vol. 73, No. 6, 2008

SCHEME 4. Chemical Correlation Experiments
nosides. This methodology was applied to the synthesis of
fluorinated analogues of pseudoglycopeptides designed by Wong
for E- and P-selectin inhibition. A biological evaluation of
compounds 3a and 3b and other fluorinated glycosides is in
preparation.
Experimental Section
Ethyl Trimethylsilyl-2,2-difluoroketene Acetal (4). Zinc dust
(3 g, 46 mmol) in a Schlenk tube was dried by heating under
vacuum and cooled to room temperature under argon atmosphere.
Acetonitrile (16 mL) was added, and the zinc was activated under
vigorous stirring by adding dibromoethane (3% mol) and TMSCl
(5% mol). The mixture was cooled to room temperature, and a
solution of ethyl bromodifluoroacetate (3.75 mL, 27 mmol) in
acetonitrile (6 mL) was added dropwise over 1 h. The mixture was
then stirred for an additional 1 h after the end of the addition, and
TMSCl (5 mL, 40mmol) was then added. After 1 h, the reaction
mixture was then cooled to 4 °C and extracted with pentane (8 mL
+ 3 × 4 mL) under argon atmosphere. Care should be taken to
avoid moisture contact with 4, which is used as a pentane solution
without further purification.
F-H3 and F-H2 correlations observed with the same experi-
ment clearly established the R-mannoside (4C1 chair conforma-
tion) configuration of 9.12 This stereoselectivity was predictible
since the OH group at C4 and the CF2CO2Et group at C1 produce
a cooperative effect for an anti orientation of the OsO4
addition.13
The â configuration of â-7 (and therefore of 8) was confirmed
by a NOESY correlation between H1 and H5. Unfortunately,
an overlap between H2 and H3 signals in the NMR spectrum of
8 did not allow us to draw any conclusion concerning C2 and
C3 configurations from coupling constants or NOESY data. A
synthesis of â-CF2-mannopyranoside 10 recently developed by
our group prompted us to perform chemical correlation
experiments.6g Indeed, the syn-addition process of the dihy-
droxylation reaction implied that 8 had to be a â-CF2-
mannopyranoside or a â-CF2-allopyranoside. Since the deben-
zylation of compound 10 and the protodesilylation of 8 afforded
Ethyl 2-(4,6-Di-O-acetyl-2,3-dideoxy-D-erythro-hex-2-enopy-
ranosyl)-2,2-difluoroacetate (6). To a solution of D-glucal 5 (2 g,
7.35 mmol) in dichloromethane (22 mL) at -40 °C was added a
solution of 4 (24 mmol) in pentane and boron trifluoride diethyl
etherate (1 mL, 7.9 mmol). The solution was stirred at this
temperature for 40 min and neutralized by adding saturated aqueous
NaHCO3 (15 mL) and water (15 mL). The organic layer was
separated and the aqueous phase extracted with ether (2 × 15 mL).
The combined organic extracts were washed with brine (20 mL),
dried over magnesium sulfate, and concentrated under reduced
pressure. Purification by column chromatography over silica gel
(30% ethyl acetate in hexane) afforded 6 as a colorless oil and as
a mixture of anomers (1.78 g, 72%). Fractions of reasonably pure
R anomer can, however, be isolated: Rf ) 0.23 (30% EtOAc in
1
two compounds with totally different H and 19F NMR data,
the allopyranoside configuration was attributed to 8 (Scheme
4). This result was somehow disappointing as it was hoped that
the strong anti selectivity generally induced by OH groups for
dihydroxylation reactions would afford a â-CF2-mannopyrano-
side when â-7 was used as the starting material.13 However,
the CF2 moiety generates apparently greater steric and electro-
static repulsions than the OH group and induces a complete
and noteworthy syn selectivity to produce 8.
The synthesis of the targeted fluorinated pseudoglycopeptides
R-3a and R-3b was completed using standard chemistry
(Scheme 5). Benzylation of 9 under acidic conditions afforded
the common intermediate 12, which was saponified and coupled
to either benzylated l-glutamic acid or ethyl 3-aminobenzoate.
Compounds 13 and 14 were then fully deprotected to afford
the desired fluorinated analogues of Wong’s glycopeptides R-3a
and R-3b.
1
cyclohexane); H NMR (300 MHz, CDCl3) δ 6.10 (dt, J ) 10.5
Hz, J ) 2.2 Hz, 1H), 6.01-5.94 (m, 1H), 5.24-5.19 (m, 1H),
4.74-4.61 (m, 1H), 4.36 (q, J ) 7.2 Hz, 2H), 4.22 (dd, J ) 12.1
Hz, J ) 5.6 Hz), 4.11 (dd, J ) 12.1 Hz, J ) 2.7 Hz), 4.05-3.98
(m, 1H), 2.08 (s, 3H), 2.06 (s, 3H), 1.34 (t, J ) 7.2 Hz, 3H); 19F
NMR (282.5 MHz, CDCl3) δ -110.5 (d, J ) 260.7 Hz, 1F), -117.7
(dd, J ) 260.7 Hz, J ) 22.6 Hz, 1F); 13C NMR (75.5 MHz, CDCl3)
δ 170.8, 170.3, 163.1 (t, J ) 30.2 Hz), 114.1 (t, J ) 254.7 Hz),
130.0, 122.3, 71.9 (dd, J ) 30.5 Hz, J ) 24.2 Hz), 71.2, 64.0,
63.3, 62.5, 21.0, 20.8, 14.0. The following analytical data were
obtained from the mixture of anomers: IR (neat) νmax 2987, 1764,
1742 cm-1; MS (ESI+) m/z ) 359 ([M + Na]+). Anal. Calcd for
C14H18F2O7: C, 50.04; H, 5.40. Found: C, 50.05; H, 5.22.
Ethyl 2-(6-O-(tert-Butyldiphenylsilyl)-R-D-mannopyranosyl)-
2,2-difluoroacetate (9). To a vigorously stirred solution of R-7
(240 mg, 0.49 mmol) in CH2Cl2 (3 mL) was added NMO (250
In summary, we have developed a concise method for the
preparation of functionalized CF2 analogues of R-mannopyra-
SCHEME 5. Synthesis of the Fluorinated Pseudoglycopeptides from r-Mannoside 9
J. Org. Chem, Vol. 73, No. 6, 2008 2437

mg, 2.1 mmol), a few drops of water, and a solution of osmium
tetraoxide (140 µL, 4% in water, 0.02 mmol). The reaction was
monitored by TLC (reaction time ∼60 h), and after complete
consumption of the starting material, the emulsion was dried over
magnesium sulfate and directly transferred over a silica gel packed
column for purification (0-10% MeOH in CH2Cl2) to afford 9 as
a colorless oil (159 mg, 62%): [R]D +0.50 (c 0.5, CHCl3); 1H NMR
(300 MHz, CDCl3) δ 7.69-7.63 (m, 4H), 7.48-7.36 (m, 6H), 4.40
(ddd, J ) 27.1, 6.7, 1.5 Hz, 1H), 4.31 (bs, 1H), 4.23 (q, J ) 6.9
Hz, 2H), 4.00-3.94 (m, 2H), 3.90-3.79 (m, 2H), 3.79-3.74 (m,
1H), 3.20 (bs, 1H), 3.10 (bs, 1H), 2.91 (bs, 1H), 1.21 (t, J ) 6.9
Hz, 3H), 1.06 (s, 9H); 19F NMR (282.5 MHz, CDCl3) δ -108.5
(dd, J ) 258.6, 6.4 Hz, 1F), -114.2 (dd, J ) 258.6, 26.8 Hz, 1F);
13C NMR (75.5 MHz, CDCl3) δ 162.8 (dd, J ) 32.6, 28.6 Hz),
135.7, 132.6, 130.2, 128.0, 115.2 (t, J ) 256.4 Hz), 77.0 (dd, J )
21.6, 29.6 Hz), 75.0, 71.8, 69.9, 66.2, 65.5, 63.4, 26.9, 19.3, 14.0;
IR (neat) νmax 3400, 3072, 2932, 2858, 1771 cm-1; MS (ESI+)
m/z ) 547 ([M + Na]+). Anal. Calcd for C26H34F2O7Si: C, 59.52;
H, 6.53. Found: C, 59.32; H, 6.35.
Di-O-benzyl N-[2-(2,3,4-Tri-O-benzyl-6-O-(tert-butyldiphe
nylsilyl)-R-D-mannopyranosyl)-2,2-difluoroacetyl]-L-gluta- mate
(13). To a solution of 12 (231 mg, 0.29 mmol, 1 equiv) in THF (2
mL) was added LiOH (14.3 mg, 0.59 mmol, 2 equiv) dissolved in
water (1 mL). The reaction was stirred at rt and monitored by TLC,
and more LiOH in water was added if necessary. CH2Cl2 (10 mL)
was added, and the reaction mixture was acidified to pH 1 by
addition of HCl (1 N). The aqueous layer was extracted with CH2-
Cl2 (3 × 5 mL), and the collected organics were washed with water
and evaporated under reduced pressure. The crude acid was obtained
as a colorless oil (210 mg, 95%) after drying under vacuum and
was used for the next step without further purification. This residue
was dissolved in CH2Cl2 (2 mL), and to this solution were added
HGlu(OBn)2‚TsOH (170 mg, 0.34 mmol), HOBt (49 mg, 0.29
mmol), NMM (80 µL, 0.72 mmol), and EDC (70 mg, 0.36 mmol).
The mixture was stirred for 36 h and then diluted with EtOAc (10
mL). The solution was washed with 10% aq citric acid (10 mL),
satd NaHCO3 (10 mL), and brine (10 mL). The organic layer was
dried over MgSO4 and evaporated under reduced pressure. Purifica-
tion by column chromatography (10% EtOAc in cyclohexane)
afforded 13 as a colorless oil (183 mg, 62%): [R]D +3.2 (c 1.5,
CHCl3); 1H NMR (300 MHz, CDCl3) δ 7.66-7.61 (m, 4H), 7.42-
7.12 (m, 31H), 7.04 (d, J ) 7.6 Hz, 1H), 5.04 (s, 2H), 4.99 (d, J
) 12.2 Hz, 1H), 4.89 (d, J ) 12.2 Hz, 1H), 4.79-4.44 (m,
7H), 4.13-4.06 (m, 2H), 3.95-3.70 (m, 4H), 2.40-2.12 (m, 3H),
2.01-1.89 (m, 1H), 0.99 (s, 9H); 19F NMR (282.5 MHz, CDCl3)
δ -110.6 (d, J ) 258.6 Hz, 1F), -117.0 (dd, J ) 258.6, 20.4 Hz,
1F); 13C NMR (75.5 MHz, CDCl3) δ 172.5, 170.5, 162.9 (t, J )
27.0 Hz), 138.5, 138.3, 138.0, 136.0, 135.7, 135.6, 135.0, 133.8,
133.3, 129.7, 129.0, 128.8-127.7 (CHAr), 116.2 (t, J ) 258.2 Hz),
77.7, 77.4, 73.8, 73.7, 72.8 (t, J ) 22.0 Hz), 72.6, 72.2, 72.1, 67.6,
66.7, 62.3, 51.8, 29.9, 27.1, 26.9, 19.4; IR (neat) νmax 3351, 3032,
2930, 2857, 1738, 1710 cm-1; MS (ESI+) m/z ) 1098 ([M +
Na]+). Anal. Calcd for C64H67F2NO10Si : C, 71.42; H, 6.27; N,
1.30. Found: C, 71.34; H, 6.31; N, 1.34.
N-(2,2-difluoro-2-(R-D-mannopyranosyl)acetyl)-L-glutamic Acid
(R-3a). To a solution of 13 (92 mg, 0.085 mmol) in THF (3 mL)
was added TBAF (1 M solution in THF, 0.1 mL, 0.1 mmol). The
mixture was stirred at rt for 3 h and then quenched with satd NH4-
Cl. The aqueous layer was extracted with EtOAc (3 × 5 mL), and
the collected organics were washed with brine (5 mL), dried over
MgSO4, and evaporated under reduced pressure. Purification by
column chromatography (20% EtOAc in cyclohexane) afforded the
desired deprotected glycopeptide as a colorless oil (45 mg, 63%).
To a solution of this compound (45 mg, 0.054 mmol) in THF (2
mL) were added water (1 mL), a few drops of concd HCl, and
Pd/C (tip of spatula). The reaction mixture was purged with H2
and stirred under an H2 atmosphere for 24 h. The suspension was
then filtered and washed with water, and the filtrate was evaporated
under reduced pressure. The residue was dried by successive
coevaporations with toluene and then redissolved in water, filtered,
and lyophilized to afford R-3a (20 mg, quant) as a white solid:
mp 118-120 °C; [R]D +15.55 (c 0.6, H20, λ ) 436 nm); 1H NMR
(300 MHz, D2O) δ 4.38 (ddd, J ) 21.9, 11.8, 2.4 Hz, 1H), 4.25 (t,
J ) 2.7 Hz, 1H), 4.16 (dd, J ) 4.6, 8.8 Hz, 1H), 3.91-3.86 (m,
1H), 3.79-3.70 (m, 4H), 2.23 (app t, J ) 7.7 Hz, 2H), 2.14-2.02
(m, 1H), 1.99-1.86 (m, 1H); 19F NMR (282.5 MHz, D2O) δ -110.2
(dd, J ) 259.1, 11.8 Hz, 1F), -114.53 (dd, J ) 259.1, 21.5 Hz,
1F); 13C NMR (75.5 MHz, D2O) δ 182.5, 177.9, 164.2 (t, J ) 27.9
Hz), 116.4 (t, J ) 259.3 Hz), 78.2, 76.8 (dd, J ) 28.5, 22.2 Hz),
71.3, 66.7, 66.6, 61.2, 56.1, 34.2, 28.0; HRMS (TOF-) calcd for
C13H18F2NO10 m/z (M - H)- 386.0898, found 386.0882.
Acknowledgment. We thank Perigene, Inc., for a PhD grant
to F.P., the Association pour la Recherche contre le Cancer for
financial support, and Dr. Hassan Oulyadi (LRMN, UMR 6014,
Mont Saint-Aignan, France) for the 400 MHz NMR experi-
ments.
(12) This experiment also allowed to establish that the “exo/anti”
conformation of the C1-CF2 bond was the most populated one (see the
Supporting Information for details). This terminology was used for O- and
CH2-glycosides: Goekjian, P. G.; Wei, A.; Kishi, Y. In Carbohydrate-
Based Drug DiscoVery; Wong, C.-H., Ed.; Wiley-VCH: Weinheim,
Germany, 2003; Vol. 1, pp 305-340.
Supporting Information Available: Complete experimental
details and compound characterization data, as well as copies of
1H, 19F, and 13C spectra. COSY, HMQC, and NOESY or HOESY
(400 Mhz) spectra for â-7 and 9. This material is available free of
charge via the Internet at http://pubs.acs.org.
(13) (a) Cha, J. K.; Christ, W. J.; Kishi, Y. Tetrahedron 1984, 40, 2247.
(b) Harding, M.; Nelson, A. Chem. Commun. 2001, 695.
JO702466H
2438 J. Org. Chem., Vol. 73, No. 6, 2008