Synthesis and biological evaluation of 6-oxa-nor-tropane glycomimetics as glycosidase inhibitors

Article abstract of DOI:10.1016/j.tet.2007.05.085

The preparation of polyhydroxylated 6-oxa-nor-tropane glycomimetics structurally related to the glycosidase inhibitor family of the calystegines is reported. The synthetic strategy involves the furanose→piperidine rearrangement of 5-deoxy-5-ureido-l-idose

Full text of DOI:10.1016/j.tet.2007.05.085

Tetrahedron 63 (2007) 7879–7884
Synthesis and biological evaluation of 6-oxa-nor-tropane
glycomimetics as glycosidase inhibitors
a
M. Isabel Garcıa-Moreno, Carmen Ortiz Mellet and Jose M. Garcıa Fernandez
a,
b,
*
*
ꢀ
ꢀ
ꢀ
ꢀ
aDepartamento de Quꢀımica Orgaꢀnica, Facultad de Quꢀımica, Universidad de Sevilla,
Profesor Garcꢀıa Gonzaꢀlez 1, E-41012 Sevilla, Spain
b
´
Instituto de Investigaciones Quꢀımicas, CSIC—Universidad de Sevilla, Americo Vespucio 49, Isla de la Cartuja, E-41092 Sevilla, Spain
Received 2 April 2007; revised 18 May 2007; accepted 22 May 2007
Available online 26 May 2007
Abstract—The preparation of polyhydroxylated 6-oxa-nor-tropane glycomimetics structurally related to the glycosidase inhibitor family of
the calystegines is reported. The synthetic strategy involves the furanose/piperidine rearrangement of 5-deoxy-5-ureido-L-idose precursors,
followed by intramolecular glycosylation involving the primary hydroxyl group. Inversion of the configuration at C-3 in the resulting 6-oxa-
(+)-calystegine B2 analogue allows accessing the elusive 3-epi-6-oxa-(+)-calystegine B2 skeleton. Acid-catalyzed opening of the nor-tropane
bicycle was observed, however, which could be avoided by careful neutralization of the reaction mixture. The inhibition results suggest that
(+)-calystegine B2 derivatives and the corresponding C-3 epimers can be seen as glucomimetics and galactomimetics, respectively, pointing to
a 1-azasugar mode of action for this family of alkaloids.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
glycosyloxocarbenium cation involved in the case of
a-glucosidases.11
The polyhydroxy-nor-tropane alkaloids of the calystegine
family1–3 are the most recently discovered members of the
iminosugar (azasugar4) glycosidase inhibitor family. They
were first isolated from the root extrudates of Calystegia
sepium in 19885 and further encountered in other plant or-
gans as well as other plant families, including edible vegeta-
bles such as potato, egg plant or cabbage.1,6 Contrary to
other well-studied azasugar glycomimetics,7 the structural
basis for glycosidase inhibition by calystegines is poorly un-
derstood.8 (+)-Calystegine B2 (1), for instance, is a bicyclic
amine that combines a pyrrolydine and a piperidine ring in
the structure, with a hydroxylation profile that bears close
similarities with that of 1-deoxynojirimycin (2) and castano-
spermine (3). Notwithstanding, the biological properties are
totally different: while 2 and 3 are potent inhibitors of a-
glucosidases, 1 behaves as a potent and specific inhibitor
of b-glucosidases.9 In this respect, calystegine B2 resembles
the 1-azasugar glucomimetic isofagomine (4).10 The loca-
tion of the basic nitrogen atom in 4 at the homologous posi-
tion of the anomeric carbon is postulated to mimic the
situation encountered in the transition state of b-glucosidase
hydrolysis, closer to an anomeric carbocation than to the
OH
NH
HO
HO
HO
HO
NH
OH
HO
HO
N
HO
HO
OH
OH
2
3
1
6
7
OH
HO 4
5
HO
3
HO
HO
HO
NH
1 OH
2
NH
4
1
We have recently reported a new family of highly selective
glycosidase inhibitors in which the sp3 amine-type nitrogen
typical of azasugars is replaced by a pseudoamide-type
(urea, thiourea, carbamate, thiocarbamate, isourea) nitrogen
atom, with a substantial sp2-character12 (sp2-azasugars).13
This subtle structural change has important consequences
on the stability of the resulting glycomimetics, favoring dis-
positions that fulfill the anomeric effect. Interestingly, the
neutral sp2-azasugars 5 and 6, with 1-deoxy-6-oxa-N-(thio)-
carbamoyl-(+)-calystegine B2 structure, exhibited very
selective and strong inhibitory activity against the mamma-
lian cytosolic b-glucosidase/b-galactosidase (bovine liver).
Actually, the corresponding inhibition constant (Ki) values
(2.5 and 30 mM, respectively) were indicative of a more po-
tent inhibition for this particular enzyme than the natural
compound 1 (Ki¼45 mM),14 suggesting a 1-azasugar inhibi-
tion mode. If this hypothesis is correct, the corresponding
Keywords: Calystegines; Glycosidase inhibitors; Iminosugars; sp2-Azasu-
gars; b-Glucosidase/b-galactosidase.
* Corresponding authors. Tel.: +34 954551519; fax: +34 954624960
(C.O.M.); tel.: +34 954489559; fax: +34 460565 (J.M.G.F.); e-mail
addresses: mellet@us.es; jogarcia@iiq.csic.es
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.05.085

7880
M. I. Garcꢀıa-Moreno et al. / Tetrahedron 63 (2007) 7879–7884
OR
OTr
epimers at C-3 should act as galactomimetics and, conse-
quently, inhibit the b-glucosidase/b-galactosidase. Previous
attempts to synthesize compounds 7 and 8 by furanose/
piperidine rearrangement of hexofuranose precursors failed,
however, the L-talofuranose forms 9 and 10, respectively,
being the major species in solution (nor-tropane–furanose
ratio 5:95).15 We reasoned that the nor-tropane structure
might be trapped by preforming the bicyclic skeleton prior
to C-3 epimerization. This has now been translated into
the preparation of 8, the first example of a 3-epi-(+)-calyste-
gine B2 derivative, in pure form. The reactivity of the inter-
mediates and the biological evaluation of the final compound
are discussed.
O
O
O
O
C
b
N3
N3
HO
O
BnO
O
c
13
11, R = H
12, R = Tr
a
OTr
OTr
O
O
O
O
d
O
Ph
PhN
N
N
H
N
H
BnO
O
BnO
O
14
15
e
OH
O
HO
OH
N
O
O
O
HO
X
OH
OH
HO
Ph
N
N
BnO
BnO
HO
HO
H
H
HO
NHPh
NHPh
N
NHPh
BnO
5, X = S
6, X = O
OH
17
16
X
HO
HO
O
O
OH
OH
Ph
X
N
H
N
H
O
HO
N
NHPh
O
HO
9, X = S
10, X = O
AcO
7, X = S
8, X = O
AcO
N
OH
HO
O
18
O
OH
OH
Ph
N
N
f
H
H
2. Results and discussion
AcO
O
10
OR
O
h
+
Our synthetic approach starts from 5-azido-5-deoxy-1,2-
O-isopropylidene-b-L-idofuranose (11), readily accessible
from commercial glucuronolactone.16 Regioselective trity-
lation of the primary hydroxyl (/12) followed by benzyla-
tion of the remaining hydroxyl afforded the key idofuranose
precursor 13. The urea functionality at C-5 was introduced
through a two-step sequence that avoids the use of highly
toxic isocyanate reagents, involving (i) formation of carbo-
AcO
N
NHPh
g
OH
19, R = H
20, R = Ac
O
O
OMe
OH
Ph
N
N
H
H
i
HO
HO
21
O
OH
O
HO
N
NHPh
8
€
diimide 14 by tandem Staudinger–aza-Wittig reaction of
Scheme 1. Reagents: (a) TrCl, pyridine, rt, 24 h (70%); (b) NaH, BnBr,
DMF, rt, 40 min (80%); (c) PhNCS, Ph3P, toluene, 80 ꢀC, 2 h (70%); (d)
1% aq TFA, 2:1 acetone–water, rt, 18 h (70%); (e) (1) 90% TFA-water,
0 ꢀC, 30 min; (2) Ac2O–pyridine (1:1), rt, 6 h (75%); (f) (1) H2, Pd(OH)2,
EtOH, rt, 1 h; (2) Tf2O, pyridine, CH2Cl2, ꢁ25 ꢀC, 30 min; (3) NaNO2,
DMF, rt, 18 h (69% overall); (g) Ac2O–pyridine (1:1), rt, 6 h (90%); (h)
(1) NaMeO, MeOH, rt, 30 min; (2) AmberliteÒ IR-120 (H+) (10, 55%;
21, 25%); (i) (1) NaMeO, MeOH, rt, 30 min; (2) solid CO2 (86%).
azide 13 with triphenylphosphine and phenyl isothiocya-
nate17 and (ii) acid-catalyzed addition of water to the heter-
ocumulene group of 14 (/15). Simultaneous hydrolysis of
the trityl and isopropylidene groups with 90% aqueous tri-
fluoroacetic acid provided the corresponding 5-ureido-L-
idofuranose species 16, which on elimination of the acid
by coevaporation with water, underwent spontaneous nucleo-
philic addition of the urea nitrogen to the masked carbonyl
group through the open chain form of the sugar. The result-
ing transient piperidine 17 experienced in situ intramolecu-
lar attack of the primary hydroxyl to the pseudoanomeric
hemiaminal center, zipping up the bicyclic nor-tropane
core. After conventional acetylation, the corresponding di-
acetate 18 was isolated in 75% overall yield (Scheme 1).
Attempts to prepare the target fully unprotected 3-epi-(+)-
calystegine B2 derivative 8 by catalytic transesterification
of 20 with methanolic sodium methoxide followed by neu-
tralization with AmberliteÒ IR-120 (H+) ion-exchange resin
failed, however, resulting in reversion to the L-talofuranose
ureidosugar 10. Formation of the corresponding methyl a-
L-talofuranoside 21 as a minor compound was also observed
under these conditions (Scheme 1). It seems that the pres-
ence of the three axially-oriented substituents at the six-
membered ring is hardly compatible with the existence of
an aminoacetal center, which probably accounts for the
fact that 3-epi-(+)-calystegine B2 is the only diastereomer
missing in the calystegine B natural compound series.
Compound 18 exhibits a configurational pattern identical to
that of (+)-calystegine B2 at the C-2–C-3–C-4 segment, with
the hydroxyl group at C-3 purposely differentiated. Inver-
sion of the configuration at this position was effected by
sequential catalytic hydrogenolysis of the benzyl group,
trifluoromethanesulfonylation of the resulting alcohol and
nucleophilic displacement of the triflate ester by nitrite an-
ion. Concomitant migration of the equatorial acetyl group
at O-2 to the axial hydroxyl at O-3 occurred under these
conditions, affording the diacetate 19 in 69% overall yield.
Conventional acetylation provided the corresponding tri-
O-acetate 20 (Scheme 1).
Our previous results in the synthesis of sp2-azasugar glyco-
mimetics point to the anomeric effect as the driving force for
the furanose/piperidine rearrangement. The p symmetry
of the orbital hosting the lone pair in the endocyclic

M. I. Garcꢀıa-Moreno et al. / Tetrahedron 63 (2007) 7879–7884
7881
pseudoamide nitrogen atom results in a very efficient
overlapping with the s* antibonding orbital of the axially-
oriented vicinal C–O bond. Orbitalic interactions addition-
ally stabilize transient azacarbenium cations, thereby
facilitating acid-promoted intra- as well as intermolecular
glycosylation processes. In the case of 8, the unfavorable
steric interactions probably overcome the anomeric effect
stabilization. Formation of a reducing piperidine or the cor-
responding methyl glycoside (22) occurs then at the surface
of the acid resin. Both compounds can undergo conversion
into furanose derivatives through open chain intermediates
(23), the driving force being in this case the release of the
steric constrain. The fact that the 5-ureido talofuranose de-
rivative 10 did not form the methyl glycoside 21 under iden-
tical conditions supported this reaction pathway (Scheme 2).
Even though the reversion process precludes the direct mea-
surement of the inhibition constant values for 8, comparison
of the results obtained for mixtures of 8 and 10 with pure 10
and 21 allowed an indirect determination. Neither of the
furanose derivatives inhibited the bovine b-glucosidase/
b-galactosidase. Any inhibitory activity observed for sam-
ples containing 8 and 10 must be related, therefore, to the
existing concentration of the nor-tropane derivative 8. In
control experiments, a 1:10 (8/10) relative proportion was
determined after the standard incubation periods for deter-
mination of the glycosidase inhibitory properties (phosphate
buffer, pH 7.3). From the observed Ki value for this mixture
(750ꢂ60 mM) a Ki value of 68ꢂ6 mM can be estimated for 8,
in the same order of magnitude as compared with the value
previously found for the glucomimetic epimer 6.14 No inhi-
bition was observed against b-glucosidase from almonds, a-
glucosidase from yeast or a-galactosidase from green coffee
beans, which is in agreement with the high enzyme selectiv-
ity already encountered in the calystegine-type sp2-azasugar
series.14,15 This result confirms that compound 8 behaves as
a galactomimetic and strongly supports a 1-azasugar mode
of action for calystegine glycosidase inhibitors, in line
with recent crystallographic evidence for the natural com-
pound 1.20 In this orientation, the nitrogen substituent prob-
ably projects into the aglyconic binding site of the enzyme,
providing additional interactions, which offers a possibility
for further improvement of the molecular design. Research
in that direction is currently sought in our laboratories.
PhHN
O
O
PhHN
O
H
H2O or
MeOH
N
N
H+
HO
HO
8
HO
OH
OH
OH
OH
PhHN
O
PhHN
O
OH
OR
N
OR
OH
NH
-H2O
H2O
HO
HO
HO
HO
10
21
+
OH
OH
OH
23 R = H or Me
22 (R = H or Me)
Scheme 2. Probable acid-catalyzed mechanism for the reversion reaction of
the 3-epi-6-oxacalystegine derivative 8 into the L-talofuranose derivatives
10 and 21.
3. Experimental
3.1. General methods
According to the above mechanistic proposal, reversion of
the preformed 3-epi-(+)-calystegine B2 bicyclic system
must imply prior protonation at the endocyclic oxygen. In
order to carefully control the neutralization step after deace-
tylation, we replaced the sulfonic acid resin by solid carbon
dioxide. We were delighted to see that using these conditions
the elusive trihydroxylated nor-tropane structure 8 could be
isolated in 86% yield (Scheme 1).
Optical rotations were measured at room temperature in
1 cm or 1 dm tubes. IR spectra were recorded on a FTIR in-
1
strument. H (and 13C) NMR spectra were recorded at 500
(125.7) and 300 (75.5) MHz. 2D COSY and HMQC experi-
ments were carried out to assist in signal assignment. In the
FABMS spectra, the primary beam consisted Xe atoms with
a maximum energy of 8 keV. The samples were dissolved in
m-nitrobenzyl alcohol or thioglycerol as the matrixes and the
positive ions were separated and accelerated over a potential
of 7 keV. NaI was added as cationizing agent. TLC was
performed with E. Merck precoated TLC plates, silica gel
30F-245, with visualization by UV light, and by charring
with 10% H2SO4 or 0.2% w/v cerium (IV) sulfate–5% am-
monium molybdate in 2 M H2SO4. Column chromatography
was carried out with Silica Gel 60 (E. Merk, 230–400 mesh).
Microanalyses were performed by Instituto de Investiga-
The structures of the new compounds were confirmed by
NMR spectroscopy, mass spectrometry, and microanalytical
data. The 13C resonance for the aminoketal carbon atom C-5
in nor-tropane derivatives was found at 88–85 ppm, while
the anomeric carbon in furanose compounds resonated at
108–104 ppm, allowing unequivocal structural assignment.
The lowfield shift of the C-5 resonance in the latter (54–
52 ppm) accounts for the presence of the nitrogen function-
ality at this position. The anomeric configuration was
attributed by comparison of the 13C NMR spectra with
data for D-talofuranose derivatives.18,19 For example, C-2
resonates atw76–74 ppm in a-talofuranose derivatives and
ꢀ
ciones Quımicas (Sevilla, Spain).
The glycosidases a-glucosidase (from yeast), b-glucosidase
(from almonds), b-glucosidase/b-galactosidase (from bo-
vine liver, cytosolic), and a-galactosidase (from green coffee
beans), used in the inhibition studies, as well as the corre-
sponding o- and p-nitrophenyl glycoside substrates were
purchased from Sigma Chemical Co. Inhibitory potencies
were determined by spectrophotometrically measuring
the residual hydrolytic activities of the glycosidases against
the respective o- (for b-glucosidase/b-galactosidase from
bovine liver) or p-nitrophenyl a- or b-D-glycopyranoside,
in the presence of the corresponding calystegine or
3
is about 5 ppm highfield shifted for b-anomers. The JH,H
values around the six-membered ring in calystegine B2 and
3-epi-calystegine B2 derivatives were in agreement with
the all-equatorial and equatorial–axial–equatorial arrange-
ment of the oxygen substituents, respectively.
Compound 8 was stable for several days in D2O solution at
4 ꢀC at neutral pH. Slow conversion into the talofuranose
tautomer 10 was observed, however, at room temperature.

7882
M. I. Garcꢀıa-Moreno et al. / Tetrahedron 63 (2007) 7879–7884
L-talofuranose derivative. Each assay was performed in
phosphate buffer at the optimal pH for each enzyme. The
Km values for the different glycosidases used in the tests
and the corresponding working pHs are listed herein: a-
glucosidase (yeast), Km¼0.35 mM (pH 6.8); b-glucosidase
(almonds), Km¼3.5 mM (pH 7.0); b-glucosidase/b-galacto-
sidase (bovine liver), Km¼1.8 mM (pH 7.3); a-galactosidase
(coffee beans), Km¼2.02 mM (pH 6.8). The reactions were
initiated by addition of enzyme to a solution of the substrate
in the absence or presence of various concentrations of
inhibitor. After the mixture was incubated for 10–30 min
at 37 ꢀC the reaction was quenched by addition of 1 M
Na2CO3. The absorbance of the resulting mixture was deter-
mined at 405 nm. The Ki value and enzyme inhibition mode
were determined from the slope of Lineweaver–Burk plots
and double reciprocal analysis.
82.1 (C-3), 81.7 (C-2), 79.6 (C-4), 71.6 (CH2Ph), 62.9 (C-
6), 61.2 (C-5), 26.6, 26.3 (CMe2); FABMS: m/z 578 (20,
[M+H]+). Anal. Calcd for C35H35N3O5: C, 72.77; H, 6.11;
N, 7.27. Found: C, 72.73; H, 5.97; N, 7.20.
3.1.3. 3-O-Benzyl-5-deoxy-1,2-O-isopropylidene-5-(N0-
phenylcarbodiimido)-6-O-trityl-a-D-glucofuranose (14).
To a solution of 13 (1 g, 1.73 mmol) in toluene (18 mL)
under N2 at room temperature, phenyl isothiocyanate
(0.23 mL, 2.08 mmol, 1.2 equiv) and a solution of PPh3
(499 mg, 1.90 mmol, 1.1 equiv) in toluene (5 mL) were suc-
cessively added. The resulting solution was stirred at 80 ꢀC
for 2 h then concentrated and the resulting residue purified
by column chromatography (1:5 EtOAc–petroleum ether)
to afford 14 (790 mg, 70%). Rf¼0.44 (1:3 EtOAc–petroleum
ether); [a]D¼ꢁ10.8 (c 1.02, CH2Cl2); IR (KBr) nmax 2988,
2128, 1593, 1501, 1483, 1380, 1262, 1094 cmꢁ1; 1H NMR
(300 MHz, CDCl3) d 7.50–7.03 (m, 20H, Ph), 5.93 (d, 1H,
J1,2¼3.8 Hz, H-1), 4.56 (dd, 1H, J4,5¼8.5 Hz, J3,4¼3.3 Hz,
H-4), 4.51 (d, 1H, H-2), 4.27 (d, 1H, 2JH,H¼11.3 Hz, CHPh),
4.03 (ddd, 1H, J5,6b¼4.6 Hz, J5,6a¼2.6 Hz, H-5), 3.76 (d,
3.1.1. 5-Azido-5-deoxy-1,2-O-isopropylidene-6-O-trityl-
b-L-idofuranose (12). Trityl chloride (1.2 g, 4.3 mmol,
1.3 equiv) was added to a solution of 5-azido-5-deoxy-
1,2-O-isopropylidene-b-L-idofuranose16
(11,
781 mg,
2
3.2 mmol) in pyridine (7 mL) and the solution was stirred
at room temperature for 24 h. The reaction mixture was
poured into ice-water (30 mL) and the resulting solid was
dissolved in toluene (15 mL) and washed with iced 10%
aq AcOH (6 mL), saturated aq NaHCO3 (6 mL), dried
(MgSO4), and concentrated under reduced pressure. The res-
idue was purified by column chromatography (1:3 EtOAc–
petroleum ether) to furnish 12 (1.1 g, 70%). Rf¼0.57 (1:1
EtOAc–petroleum ether); [a]D¼ꢁ10.8 (c 1.02, CH2Cl2);
IR (KBr) nmax 3449, 3059, 2988, 1489, 1379, 1262,
1H, JH,H¼11.3 Hz, CHPh), 3.54 (d, 1H, H-3), 3.51 (dd,
1H, J6a,6b¼9.5 Hz, H-6a), 3.09 (dd, 1H, H-6b), 1.46, 1.30
(2s, 6H, CMe2); 13C NMR (125.7 MHz, CDCl3) d 140.3
(NCN), 143.5–127.0 (Ph), 111.7 (CMe2), 104.8 (C-1), 86.7
(CPh3), 82.1 (C-2, C-3), 80.5 (C-4), 71.7 (CH2Ph), 63.5
(C-6), 57.6 (C-5), 26.7, 26.2 (CMe2); FABMS: m/z 675
(100, [M+Na]+), 653 (30, [M+H]+). Anal. Calcd for
C42H40N2O5: C, 77.27; H, 6.17; N, 12.25. Found: C,
77.34; H, 6.15; N, 4.29.
1
1097 cmꢁ1; H NMR (300 MHz, CDCl3) d 7.45–7.26 (m,
3.1.4. 3-O-Benzyl-5-deoxy-1,2-O-isopropylidene-5-(N0-
phenylureido)-6-O-trityl-a-D-glucofuranose (15). To a
solution of carbodiimide 14 (423 mg, 1.2 mmol) in ace-
tone–water (2:1, 18 mL), TFA (0.2 mL) was added. The re-
action mixture was stirred at room temperature for 18 h, then
the solvents were evaporated under vacuum and the resulting
residue was purified by column chromatography (1:6
EtOAc–petroleum ether/EtOAc) to give 15 (296 mg,
70%). Rf¼0.14 (1:4 EtOAc–petroleum ether); [a]D¼ꢁ5.8
(c 1.04, CH2Cl2); IR (KBr) nmax 3393, 3059, 2963, 1657,
15H, 3Ph), 5.93 (d, 1H, J1,2¼3.7 Hz, H-1), 4.47 (d, 1H, H-
2), 4.15–4.03 (m, 2H, H-3, H-4), 3.69 (dt, 1H, J5,6a
¼
J5,6b¼5.6 Hz, J4,5¼7.7 Hz, H-5), 3.39 (d, 2H, H-6a, H-6b),
1.48, 1.29 (2s, 6H, CMe2); 13C NMR (75.5 MHz, CDCl3)
d 142.9–127.3 (Ph), 111.7 (CMe2), 104.4 (C-1), 87.0 (CPh3),
84.9 (C-2), 80.8 (C-4), 75.0 (C-3), 63.6 (C-6), 60.7 (C-5),
26.6, 26.1 (CMe2); FABMS: m/z 510 (100, [M+Na]+).
Anal. Calcd for C28H29N3O5: C, 68.98; H, 6.00; N, 8.62.
Found: C, 68.95; H, 5.78; N, 8.52.
1
1545, 1445, 1379, 1213, 1092 cmꢁ1; H NMR (500 MHz,
3.1.2. 5-Azido-3-O-benzyl-5-deoxy-1,2-O-isopropyl-
idene-6-O-trityl-b-L-idofuranose (13). To a solution of 12
(1.1 g, 2.3 mmol) in DMF (10 mL) under Ar at 0 ꢀC, NaH
(60% in mineral oil, 230 mg, 5.75 mmol, 2.5 equiv) was
slowly added and the mixture was stirred for 10 min. Benzyl
bromide (0.6 mL, 4.6 mmol, 2 equiv) was added dropwise
and the reaction mixture was further stirred at room temper-
ature for 40 min, then quenched by addition of MeOH
(1 mL) and concentrated under reduced pressure. The resi-
due was purified by column chromatography (1:3 EtOAc–
petroleum ether) to give 13 (1.1 g, 80%). Rf¼0.52 (1:2
EtOAc–petroleum ether); [a]D¼ꢁ20.7 (c 1.06, CH2Cl2);
IR (KBr) nmax 3061, 2988, 1603, 1487, 1381, 1262,
CDCl3) d 7.40–7.00 (m, 20H, Ph), 6.65 (bs, 1H, N0H),
5.92 (d, 1H, J1,2¼3.8 Hz, H-1), 5.29 (d, 1H, J5, NH¼7.8 Hz,
NH), 4.59 (dd, 1H, J4,5¼7.4 Hz, J3,4¼3.1 Hz, H-4), 4.54 (d,
1H, H-2), 4.38 (d, 1H, 2JH,H¼11.3 Hz, CHPh), 4.33 (m, 1H,
H-5), 4.02 (d, 1H, CHPh), 3.64 (d, 1H, H-3), 3.43 (dd, 1H,
J6a,6b¼9.5 Hz, J5,6a¼4.5 Hz, H-6a), 3.09 (dd, 1H, J5,6b
¼
3.6 Hz, H-6b), 1.52, 1.27 (2s, 6H, CMe2); 13C NMR
(125.7 MHz, CDCl3) d 156.4 (CO), 146.9–120.4 (Ph),
111.7 (CMe2), 104.6 (C-1), 86.6 (CPh3), 82.3 (C-2), 82.0
(C-3), 79.0 (C-4), 71.9 (CH2Ph), 63.4 (C-6), 52.1 (C-5),
26.7, 26.2 (CMe2); FABMS: m/z 693 (40, [M+Na]+). Anal.
Calcd for C42H42N2O6: C, 75.20; H, 6.31; N, 4.17. Found:
C, 75.12; H, 6.28, N, 4.13.
1
1097 cmꢁ1; H NMR (300 MHz, CDCl3) d 7.50–7.02 (m,
20H, 4Ph), 5.94 (d, 1H, J1,2¼3.8 Hz, H-1), 4.50 (d, 1H, H-2),
4.48 (dd, 1H, J4,5¼9.1 Hz, J3,4¼3.2 Hz, H-4), 4.27 (d, 1H,
2JH,H¼11.2 Hz, CHPh), 3.85 (ddd, 1H, J5,6b¼5.0 Hz,
J5,6a¼2.5 Hz, H-5), 3.75 (d, 1H, CHPh), 3.53 (d, 1H, H-3),
3.44 (dd, 1H, J6a,6b¼9.7 Hz, H-6a), 3.07 (dd, 1H, H-6b),
1.54, 1.31 (2s, 6H, CMe2); 13C NMR (75.5 MHz, CDCl3) d
143.3–127.0 (Ph), 111.7 (CMe2), 104.7 (C-1), 86.6 (CPh3),
3.1.5. (1S,2R,3S,4R,5R)-2,4-Diacetoxy-3-benzyloxy-N-
(N0-phenylcarbamoyl)-6-oxa-nor-tropane (18). A solution
of urea 15 (354 mg, 0.78 mmol) in a mixture of TFA–H2O
(9:1, 4 mL) was stirred at 0 ꢀC for 30 min. The solvent
was then removed under vacuum and the residue was coeva-
porated several times with water. Conventional acetylation
of the resulting residue by treatment with 1:1 Ac2O–pyridine

M. I. Garcꢀıa-Moreno et al. / Tetrahedron 63 (2007) 7879–7884
7883
1
(3 mL) at room temperature for 6 h and purification of the
crude reaction mixture by column chromatography (1:3
EtOAc–petroleum ether) gave 18 (266 mg, 75%). Rf¼0.54
(1:1 EtOAc–petroleum ether); [a]D¼+46.3 (c 1.08,
CH2Cl2); IR (KBr) nmax 3030, 2963, 1746, 1661, 1601,
1537, 1373, 1225, 1094 cmꢁ1; 1H NMR (300 MHz, CDCl3)
d 7.42–7.08 (m, 10H, Ph), 5.67 (d, 1H, J4,5¼1.6 Hz, H-5),
5.08 (ddd, 1 H, J2,3¼8.3 Hz, J1,2¼4.1 Hz, J2,7b¼1.1 Hz, H-
1092 cmꢁ1; H NMR (300 MHz, CDCl3) d 7.40–7.03 (m,
5H, Ph), 5.67 (t, 1H, J2,3¼J3,4¼4.9 Hz, H-3), 5.65 (d, 1H,
J4,5¼1.6 Hz, H-5), 5.15 (t, 1H, J1,2¼4.9 Hz, H-2), 5.00
(dd, 1H, H-4), 4.59 (t, 1H, J1,7b¼4.9 Hz, H-1), 4.40 (d, 1H,
J7a,7b¼7.8 Hz, H-7a), 3.79 (dd, 1H, H-7b), 2.15, 2.05, 2.02
(3s, 9H, MeCO); 13C NMR (75.5 MHz, CDCl3) d 170.2,
170.1, 169.3 (CO ester), 153.4 (CO urea), 137.8–119.9
(Ph), 85.8 (C-5), 70.3 (C-4), 67.5 (C-2), 66.5 (C-3), 66.0
(C-7), 54.3 (C-1), 20.6, 20.5, 20.4 (MeCO); FABMS: m/z
429 (100%, [M+Na]+). Anal. Calcd for C19H22N2O8: C,
56.15; H, 5.46; N, 6.89. Found: C, 56.12; H, 5.45; N, 6.89.
2), 4.89 (dd, 1H, J3,4¼8.3 Hz, H-4), 4.71 (t, 1H, J1,7b
¼
¼
4.1 Hz, H-1), 4.67 (s, 2H, CH2Ph), 4.00 (d, 1H, J7a,7b
8.2 Hz, H-7a), 3.92 (t, 1H, H-3), 3.74 (ddd, 1H, H-7b),
2.09, 2.04 (2s, 6H, MeCO); 13C NMR (75.5 MHz, CDCl3)
d 170.6, 169.7 (CO ester), 153.7 (CO urea), 137.8–123.8
(Ph), 85.6 (C-5), 77.9 (C-3), 77.0 (C-4), 74.5 (CH2Ph),
73.1 (C-2), 65.6 (C-7), 54.6 (C-1), 20.8, 20.7 (MeCO);
FABMS: m/z 477 (100, [M+Na]+). Anal. Calcd for
C24H26N2O7: C, 63.43; H, 5.77; N, 6.16. Found: C, 63.33;
H, 5.63; N, 6.14.
3.1.8. 5-Deoxy-5-(N0-phenylureido)-L-talofuranose and
methyl 5-deoxy-5-(N0-phenylureido)-a-L-talofuranoside
(10 and 21). To a solution of 20 (100 mg, 0.246 mmol) in
dry MeOH (5 mL) methanolic NaOMe (1 M, 0.1 equiv per
mole of acetate) was added and the reaction mixture was
stirred at room temperature for 30 min. Neutralization with
Amberlite IR-120 (H+) ion-exchange resin and column chro-
matography (EtOAc–EtOH–H2O 45:5:3) of the resulting
mixture afforded, sequentially, the L-talofuranoside 21
(19.2 mg, 25%) and the L-talofuranose derivative 10
(40.4 mg, 55%). Compound 10 showed spectroscopic and
physicochemical data identical to those previously re-
ported.14 Compound 21 had Rf¼0.38 (EtOAc–EtOH–H2O
3.1.6. (1S,2R,3R,4R,5R)-3,4-Diacetoxy-2-hydroxy-N-(N0-
phenylcarbamoyl)-6-oxa-nor-tropane (19). A solution of
18 (238 mg, 0.52 mmol) in EtOH (7 mL) was hydrogenated
at atmospheric pressure for 1 h using 10% Pd(OH)2
(253 mg) as heterogeneous catalyst. The suspension was fil-
tered through Celite, concentrated and the resulting residue
was dissolved in CH2Cl2 (2 mL) and cooled at ꢁ25 ꢀC. Tri-
fluoromethanesulfonic anhydride (0.66 mmol, 0.12 mL) and
pyridine (0.1 mL) were added under N2. The reaction mix-
ture was stirred for 30 min at the same temperature, diluted
with CH2Cl2 (5 mL), washed with saturated aq NaHCO3
(4 mL), dried (MgSO4), and concentrated. The resulting
crude triflate ester was dissolved in DMF (1.4 mL),
NaNO2 (168 mg, 2.52 mmol, 5 equiv) was added and the
reaction mixture was stirred at room temperature for 18 h.
The solvent was removed under reduced pressure and the
resulting residue was dissolved in CH2Cl2 and washed
with water. The organic extract was dried (MgSO4) and
concentrated to give a solid, which was purified by column
chromatography (1:1 EtOAc–petroleum ether) to furnish
19 (130.7 mg, 69%). Rf¼0.29 (2:1 EtOAc–petroleum ether);
[a]D¼+26.7 (c 1.05, CH2Cl2). IR (KBr) nmax 3370, 3061,
2963, 1748, 1627, 1603, 1537, 1445, 1380, 1260,
1
45:5:3); [a]D¼ꢁ22.5 (c 1.0, H2O); H NMR (500 MHz,
D2O) d 7.22–7.06 (m, 5H, Ph), 4.74 (s, 1H, H-1), 4.08 (dd,
1H, J3,4¼7.5 Hz, J2,3¼4.6 Hz, H-3), 3.99 (d, 1H, J4,5
¼
3.3 Hz, H-4), 3.95 (m, 1H, H-5), 3.89 (d, 1H, H-2), 3.63
(dd, 1H, J6a,6b¼11.6 Hz, J5,6a¼5.7 Hz, H-6a), 3.56 (dd, 1H,
J5,6b¼7.2 Hz, H-6b), 3.24 (s, 3H, OMe); 13C NMR
(75.5 MHz, D2O) d 158.2 (CO), 137.9–122.1 (Ph), 108.3
(C-1), 81.3 (C-4), 74.2 (C-2), 71.2 (C-3), 62.2 (C-6), 55.7
(OMe), 52.3 (C-5); FABMS: m/z 335 (100, [M+Na]+).
Anal. Calcd for C14H20N2O6: C, 53.84; H, 6.45; N, 8.97.
Found: C, 53.71; H, 6.11; N, 8.87.
3.1.9. (1S,2R,3R,4R,5R)-N-(N0-Phenylcarbamoyl)-2,3,4-
trihydroxy-6-oxa-nor-tropane (8). To a solution of 20
(77 mg, 0.211 mmol) in dry MeOH (5 mL), methanolic
NaOMe (1 M, 0.1 equiv per mole of acetate) was added.
The reaction mixture was stirred at room temperature for
30 min, then neutralized with solid CO2, and concentrated.
The resulting residue was purified by column chromato-
graphy (45:5:3 EtOAc–EtOH–H2O) to give 8 (51 mg,
86%). Rf¼0.53 (45:5:3 EtOAc–EtOH–H2O); [a]D¼+51.7
1
1094 cmꢁ1; H NMR (500 MHz, CDCl3) d 7.53–7.03 (m,
5H, Ph), 5.66 (s, 1H, H-5), 5.48 (t, 1H, J2,3¼J3,4¼2.5 Hz,
H-3), 5.10 (t, 1H, J1,2¼2.5 Hz, H-2), 4.59 (t, 1H, J1,7b
¼
2.5 Hz, H-1), 4.22 (d, 1H, J7a,7b¼4.7 Hz, H-7a), 3.89 (d,
1H, H-4), 3.64 (dd, 1H, H-7b), 2.13, 1.99 (2s, 6H, MeCO);
13C NMR (125.7 MHz, CDCl3) d 170.6, 169.4 (CO ester),
154.5 (CO urea), 138.6–123.4 (Ph), 88.1 (C-5), 69.4 (C-4),
68.5 (C-3), 67.8 (C-2), 65.3 (C-7), 54.0 (C-1), 20.7, 20.5
(MeCO); FABMS: m/z 387 (100%, [M+Na]+). HRFABMS:
calculated for C17H21N2O7 (365.1348). Found: 365.1339.
Anal. Calcd for C17H20N2O7: C, 56.04; H, 5.53; N, 7.69.
Found: C, 55.83; H, 5.47; N, 7.57.
1
(c 1.0, H2O); H NMR (500 MHz, D2O) d 7.36–7.15 (m,
5H, Ph), 5.59 (d, 1H, J4,5¼1.8 Hz, H-5), 4.46 (dd, 1H,
J1,7b¼5.2 Hz, J1,2¼4.1 Hz, H-1), 4.32 (d, 1H, J1a,1b¼7.9 Hz,
H-7a), 4.11 (dd, 1H, J3,4¼5.0 Hz, J2,3¼4.2 Hz, H-3), 3.98 (t,
1H, H-2), 3.78 (dd, 1H, H-4), 3.63 (dd, 1H, H-7b); 13C NMR
(125.7 MHz, D2O) d 158.2 (CO), 138.0–123.9 (Ph), 87.8 (C-
5), 70.7 (C-4), 70.1 (C-3), 68.8 (C-3), 66.4 (C-7), 57.4 (C-1);
FABMS: m/z 303 (100%, [M+Na]+). Anal. Calcd for
C13H16N2O5: C, 55.71; H, 5.75; N, 9.99. Found: C,
55.149; H, 5.58; N, 9.77.
3.1.7. (1S,2R,3R,4R,5R)-2,3,4-Triacetoxy-N-(N0-phenyl-
carbamoyl)-6-oxa-nor-tropane (20). Conventional acet-
ylation of 19 (200 mg, 0.547 mmol) with 1:1 Ac2O–
pyridine at room temperature for 6 h and purification of
the crude reaction mixture by column chromatography
gave 20 (199 mg, 90%). Rf¼0.33 (1:1 EtOAc–petroleum
ether); [a]D¼+41.0 (c 1.0, CH2Cl2); IR (KBr) nmax 3310,
3090, 2905, 1751, 1653, 1541, 1445, 1377, 1231,
Acknowledgements
ꢀ
We thank the Spanish Ministerio de Educacion y Ciencia for
financial support (contracts number CTQ2004-05854/BQU
and CTQ2006-15515-C02-01/BQU).

7884
M. I. Garcꢀıa-Moreno et al. / Tetrahedron 63 (2007) 7879–7884
Inhibitors; St€utz, A. E., Ed.; Wiley-VCH: Weinheim, 1999;
pp 253–397.
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