Synthesis of (1S,2S,3R,8S,8aR)-1,2,3,8-tetrahydroxy-6-oxa-5-thioxoindolizidine: A stable reducing swainsonine analog with controlled anomeric configuration

Article abstract of DOI:10.1055/s-2003-37109

The title indolizidine, a ring-modified analog of the potent mannosidase inhibitor (-)-swainsonine, has been prepared by intramolecular nucleophilic addition of the sp²-type nitrogen atom of a preformed 1,3-oxazine-2-thione heterocycle to the masked aldehyde group in a D-mannose precursor; the stereochemistry of the generated aminoacetalic stereocenter is governed by the generalized anomeric effect, the pseudoanomeric hydroxyl group adopting an axial orientation that mimics that of α-D-mannopyranoside aglycons.

Full text of DOI:10.1055/s-2003-37109

LETTER
341
Synthesis of (1S,2S,3R,8S,8aR)-1,2,3,8-Tetrahydroxy-6-oxa-5-
thioxoindolizidine: A Stable Reducing Swainsonine Analog with Controlled
Anomeric Configuration
Synthesis of
a
.
ReducingSw
D
ainsonine Analogíaz-Pérez,a M. I. García-Moreno,a C. Ortiz Mellet,*a J. M. García Fernández*b
a
Departamento de Química Orgánica, Facultad de Química, Universidad de Sevilla, Aptdo. 553, 41071 Sevilla, Spain
Fax +34(954)624960; E-mail: mellet@us.es
b
Instituto de Investigaciones Químicas, CSIC, Américo Vespucio s/n, Isla de la Cartuja, E-41092 Sevilla, Spain
Fax +34(954)460565; E-mail: jogarcia@cica.es
Received 10 January 2003
The glycosidase inhibitory activity of swainsonine has
been attributed to the arrangement of functional groups
putatively mimicking the stereochemistry as well as
charge distribution of a D-mannopyranosyl oxocarbenium
Abstract: The title indolizidine, a ring-modified analog of the po-
tent mannosidase inhibitor (–)-swainsonine, has been prepared by
intramolecular nucleophilic addition of the sp2-type nitrogen atom
of a preformed 1,3-oxazine-2-thione heterocycle to the masked al-
dehyde group in a D-mannose precursor; the stereochemistry of the ion,5 the postulated intermediate in the enzymatic hydrol-
generated aminoacetalic stereocenter is governed by the generalized
anomeric effect, the pseudoanomeric hydroxyl group adopting an
ysis of mannopyranosides. One could expect an increase
in the inhibition potency and specificity towards α-man-
axial orientation that mimics that of α-D-mannopyranoside agly-
nosidases by setting up structural modifications aimed to
cons.
mimic the transition state leading to this intermediate,
which demands: (i) sp2 hybridation at the anomeric centre,
Key words: carbohydrates, cyclizations, glycosidase inhibitors,
indolizidines, swainsonine
(ii) analogous charge delocalization (formal charge is not
essential) and (iii) a pseudoanomeric group with the cor-
rect orientation to mimic the natural aglycon.6 We have
recently found that a subtle change in the structure of aza-
(1S,2R,8R,8aR)-1,2,8-Trihydroxyindolizidine, (–)-swain-
sugars, by replacing the imino sp3 nitrogen atom by a
pseudoamide-type nitrogen, with substantial sp2 charac-
ter, is a valuable approach to achieve the above require-
ments.7 This concept has already been proved by
preparing the reducing indolizidine derivative 3, a stereo-
chemical mimic of α-D-glucopyranose which turned to be
a strong inhibitor of yeast α-glucosidase, exhibiting a
10000-fold increase in anomer selectivity as compared
sonine 1 (Figure 1), isolated from the fungus Rhizoctonia
leguminicola in 1973,1 was the first of the polyhydroxyl-
ated bicyclic azasugar mimics discovered. A diverse
plethora of biological properties have been ascribed to 1
as a consequence of its potent and specific inhibition of
both lysosomal α-mannosidase and mannosidase II, in-
cluding antimetastatic, antitumor-proliferative and anti-
cancer activities,2 being the first glycosidase inhibitor
selected for clinical testing as an anticancer drug.3 Due to
its biological significance, swainsonine has been the focus
of extensive synthetic efforts, and a number of analogs
have been reported by inversion of the configuration, re-
moval, replacement or selective protection of the hydrox-
yl groups.4 All of these modifications, however, resulted
in a decreased α-mannosidase activity.
with the related alkaloid (+)-castanospermine
2
(Figure 1).8 Here we report on the synthesis of the title 3-
hydroxy-6-oxa-5-thioxoindolizidine derivative 4, the first
example of a ring-modified swainsonine analog bearing a
pseudoaglyconic substituent.9
A retrosynthetic analysis revealed that the bicyclic skele-
ton of 6-oxa-5-thioxoindolizidine can be constructed by
intramolecular nucleophilic addition of the nitrogen atom
of a six-membered cyclic thiocarbamate to a suitably lo-
cated carbonyl group, with simultaneous generation of the
aminoacetal function (Scheme 1). Our synthetic route re-
lies on the ability of the masked aldehyde group of a
monosaccharide to act as the electrophile target for azole
heterocycles through the open-chain tautomeric form (5).
A hydroxylation profile analogous to that of (–)-swainso-
nine requires a precursor having the D-manno configura-
tion (6).
S
O
N
N
OH
OH
HO
N
H
H
HO
H
HO
OH
HO
HO
OH
OH
OH
(+)-castanospermine (2)
(−)-swainsonine (1)
3
Figure 1
On principle, the 1,3-oxazine-2-thione heterocycle can be
generated by base-catalyzed intramolecular cyclization of
γ-hydroxyisothiocyanates.10,11 In a first approach, a syn-
thesis of 6 from the selectively protected methyl 4-azido-
4-deoxy-α-D-mannopyranoside derivative 7, available in
Synlett 2003, No. 3, Print: 19 02 2003.
Art Id.1437-2096,E;2003,0,02,0341,0344,ftx,en;D27303ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214

342
P. Díaz-Pérez et al.
LETTER
route A
O
route B
O
S
S
O
O
6
5
1
7
HO
N3
6
HO
N3
8
N
5
8a
NH
1
OH
4
3
4
O
HO
HO
e
OMe
2
3 2
OH
H
H
HO
OH
HO
OH
5
O
O
HO
OH
4
11
O
7
a
S
O
f
HN
OH
HO
TBDMSO
N3
O
O
HO
OH
H2N
OMe
OAc
OAc
6
Scheme 1
O
O
AcO
12
8
b
g
5 steps (57% overall yield) from commercial methyl α-D-
mannopyranoside,12 was devised. Reduction of the azido
group followed by isothiocyanation of the resulting amine
8 with thiophosgene afforded the requested γ-hydroxy-
isothiocyanate 9, which was subsequently transformed
into the corresponding cyclic thiocarbamate 10 by treat-
ment with triethylamine in N,N-dimethylformamide. Si-
multaneous acid-catalyzed removal of the isopropylidene
and aglyconic groups provided the target compound 4,
though in disappointingly low yield, probably due to ex-
tensive dehydration reactions under the harsh conditions
needed for glycoside hydrolysis (Scheme 2, route A).13
HO
TBDMSO
H2N
O
O
SCN
OMe
OAc
OAc
O
O
AcO
13
9
c
h
O
TBDMSO
SCN
S
O
O
HN
OMe
OAc
OAc
In view of the above observation, a different synthetic
route was disclosed starting from the reducing 4-azido-
mannose derivative 11.12 Selective protection of the pri-
mary hydroxyl group as its tert-butildimethylsilyl ether
and further acetylation led to the corresponding triacetate
12 as a mixture of the corresponding α and β-anomers.
Azide reduction and isothiocyanation of the resulting
amine 13 provided the 4-deoxy-4-isothiocyanato-D-man-
nopyranose derivative 14. Fluorolysis of the silyl ether
protecting group generated the requested hydroxiisothio-
cyanate segment, which underwent intramolecular cy-
clization upon treatment with triethylamine to give the
bicyclic triacetate 15. Conventional sodium methoxide-
catalyzed deacetylation resulted in spontaneous re-
arrangement into the target swainsonine analog 4
(Scheme 2, route B).13
O
O
AcO
O
14
10
i
d
S
O
j
HN
AcO
OAc
OAc
4
15
Scheme 2 Reagents and conditions: a) H2, 10% Pd/C, MeOH, 3 h;
b) CSCl2, CaCO3, H2O–acetone, 40% over two steps; c) Et3N, DMF,
80 ºC, 30 min, 85%; d) 1:1 TFA–H2O, 100 ºC, 48 h, 18%; e) 1:1 TFA–
H2O, dioxane, 100 ºC, 48 h, 80% (ref.12); f) TBDMSCl, pyridine, 45
min, them Ac2O, 62%; g) H2, 10% Pd/C, MeOH, 2 h; h) CSCl2,
CaCO3, CH2Cl2–H2O, 0 ºC, 10 min, 67% over two steps; i) TBAF,
THF, then Et3N, dioxane, 45 min (73%).
Compound 4 existed in D2O solution as a single diastereo-
mer with high stability at basic, neutral or slightly acidic
pH, which is notably different from that known for sp3 re-
ducing azasugars. Actually, to the best of our knowledge,
reducing azasugar glycomimetics of the pyrrolidine fami-
ly are unknown. The high field shift of the C-3 resonance,
as compared with the anomeric carbon C-1 in the D-man-
nose precursors, confirmed the aminoacetalic bicyclic
structure. The vicinal 3JHH coupling constants around the
1,3-oxazine ring13 point to a conformation closed to a
chair, probably flattened at the region of the thiocarbonyl
group, which is in agreement with molecular dynamics
calculations. The R configuration at the aminoacetalic
center is supported by the absence of NOE or ROE con-
tacts between H-3 and H-8a, which should be present in
the (3S) diastereomer, while the corresponding vicinal
coupling constants at the pyrrolidine ring are indicative of
a conformation similar to that encountered in the natural
compound 1.14 This situation is compatible with a pseudo-
axial disposition of the hydroxyl group at C-3, fitting the
anomeric effect, in agreement with the existence of a very
strong interaction between the π-type lone pair of the ni-
trogen atom of the thiocarbamate unit and the σ* anti-
bonding orbital of the contiguous C–O bond (Figure 2).
Synlett 2003, No. 3, 341–344 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Synthesis of a Reducing Swainsonine Analog
343
Fuentes, J. Synlett 1998, 316. (d) Jiménez Blanco, J. L.;
Ortiz Mellet, C.; Fuentes, J.; García Fernández, J. M.
Tetrahedron 1998, 54, 14123.
OH
OH
HO
HO
O
S
OH
O
N
S
HO
(8) (a) Díaz Pérez, V. M.; García-Moreno, M. I.; Ortiz Mellet,
C.; Fuentes, J.; García Fernández, J. M.; Díaz Arribas, J. C.;
Cañada, F. J. J. Org. Chem. 2000, 65, 136. (b) Jiménez
Blanco, J. L.; Díaz Pérez, V. M.; Ortiz Mellet, C.; Fuentes,
J.; García Fernández, J. M.; Díaz Arribas, J. C.; Cañada, F.
J. Chem. Commun. 1997, 1969.
(9) Notice that, although the stereochemical notation at C-2 and
C-8 changes from 1 to 4, the absolute stereochemistry
remains identical at these centers.
N
HO
H
H
H
H
OH
(3S)-4
(3R)-4
Figure 2 Conformation of the (3R)-diastereomer of 4, the only one
detected in D2O solution, showing the favored π→σ* orbitalic in-
teraction. The expected NOE contact between H-3 and H-8a in the
(3S)-diastereomer is also indicated.
(10) García Fernández, J. M.; Ortiz Mellet, C. Adv. Carbohydr.
Chem. Biochem. 1999, 55, 35.
In summary, the reducing (–)-swainsonine analog 2, hav-
ing an sp2-azasugar-type structure, has been prepared
from a D-mannose precursor and shown to exist in water
solution as a single diastereomer that mimics the configu-
ration of the anomeric center in α-mannosides. A variety
of analogs for structure-activity studies should now be-
come available by modifying both the starting monosac-
charide template and the nature of the pseudoamide group
(e.g., carbamate, urea, isourea).15 Work in this direction is
currently under way in our laboratories.
(11) (a) García Fernández, J. M.; Ortiz Mellet, C.; Jiménez
Blanco, J. L.; Fuentes Mota, J.; Gadelle, A.; Coste-Sarguet,
A.; Defaye, J. Carbohydr. Res. 1995, 268, 57. (b) García
Fernández, J. M.; Ortiz Mellet, C.; Jiménez Blanco, J. L.;
Fuentes, J. J. Org. Chem. 1994, 59, 1565. (c) García
Fernández, J. M.; Ortiz Mellet, C.; Fuentes, J. J. Org. Chem.
1993, 58, 5192.
(12) Davis, B. J.; Nash, R. J.; Watson, A. A.; Smith, C.; Fleet, G.
W. J. Tetrahedron 1999, 55, 4501.
(13) (i)Preparation of the reducing (–)-swainsonine derivative 4
from 7. A solution of 7 (285 mg, 1.1 mmol) in MeOH (2 mL)
was hydrogenated in the presence of Pd/C (10%, 110.4 mg)
for 3 h to give amine 8 as a hygroscopic solid that was used
in the next step without further purification.
Acknowledgement
Isothiocyanation of 8 (254 mg, 1.07 mmol) by reaction with
CSCl2 (84 µL, 1.1 equiv) and CaCO3 (214 mg, 2 equiv) in
H2O–acetone (3:2, 3.5 mL) afforded, after column
chromatography (EtOAc–hexanes, 1:4), the hydroxyiso-
thiocyanate 9 (118 mg, 40%). To a solution of 9 (50 mg, 0.18
mmol) in DMF (3.5 mL) Et3N (8.5 µL) was added and the
reaction mixture was stirred at 80 ºC for 30 min, and then
concentrated. Column chromatography (EtOAc–hexanes,
1:2) yielded the corresponding cylic carbamate 10 (42 mg,
85%). Treatment of 10 with TFA–H2O (1:1, 3 mL) at 100 ºC
for 48 h led to mixture of compounds from which the target
oxaindolizidine 4 (6 mg, 18%) was isolated by column
chromatography (EtOAc–EtOH–H2O, 45:5:3). Compounds
9, and 10 gave satisfactory microanalytical, NMR (1H and
13C) and FAB-mass spectral data in agreement with the
proposed structures.
(ii) From 11. To a solution of 11 (138 mg, 0.67 mmol) in
pyridine (3 mL) t-butyldimethylsilyl chloride (101 mg, 0.67
mmol) was added. The mixture was stirred at r.t. for 45 min,
then Ac2O (2 mL) was added and the mixture stirred for an
additional 1 h. Conventional work up afforded 12 (256 mg,
62%), which was reduced to the corresponding amine 13 by
hydrogenation (10% Pd/C, 46 mg) in MeOH (5 mL) and
used in the next step without further purification. To a
heterogeneous mixture of 13 (180 mg, 0.43 mmol) and
CaCO3 (97 mg, 2 equiv) in CH2Cl2–H2O (1:1, 6.6 mL) at
0 ºC CSCl2 (36 µL, 1.1 equiv) was added. The reaction
mixture was vigorously stirred for 10 min, the organic phase
was separated, washed with water and concentrated. Column
chromatography (EtOAc–hexanes, 1:5) of the residue
yielded isothiocyanate 14 (132 mg, 67%). To a solution of
14 (102 mg, 0.22 mmol) in THF (5 mL) under Ar, TBAF (1
M in THF, 0.26 mL, 1.1 equiv) was added and the mixture
was adjusted to pH 7 by addition of glacial HOAc. After 45
min the solvents were removed, the residue was dissolved in
dioxane (7 mL) and a catalytic amount of Et3N was added.
The mixture was stirered for 45 min, concentrated and
chromatographed (EtOAc–hexanes, 1:2) to give the cyclic
thiocarbamate 15 (55 mg. 73%). Deacetylation of 15 with
We thank the Ministerio de Ciencia y Tecnología for financial
support (grant no. BMC2001-2366-CO3-03) and the Fundación
Cámara for a doctoral fellowship (M. I. G.-M.).
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P. Díaz-Pérez et al.
LETTER
methanolic NaOMe, neutralization of the mixture with solid
CO2 and purification of the resulting residue by column
chromatography (EtOAc→EtOAc–EtOH–H2O, 45:5:3)
afforded 4 (21 mg, 79%). Compounds 11, 12, 14, and 15
gave satisfactory microanalytical, NMR (1H and 13C) and
FAB-mass spectral data in accord with the proposed
structures.
Data for 4: Rf (EtOAc–EtOH–H2O, 45:5:3) 0.53; [α]D +12.5
(c 1.0, H2O); 1H NMR (500 MHz, CD3OD): δ (ppm) = 5.41
(d, 1 H, H-3), 4.32 (dd, 1 H, H-7a), 4.18 (dd, 1 H, H-1), 4.14
(ddd, 1 H, H-8), 4.08 (dd, 1 H, H-2), 3.95 (t, 1 H, H-7b), 3.64
(dd, 1 H, H-8a), J1,2 = 4.2 Hz, J1,8a = 2.0 Hz, J2,3 = 6.0 Hz,
J
7a,8 = 4.6 Hz, J7b,8 = 10.3 Hz, J8,8a = 8.5 Hz; 13C NMR
(125.7 MHz, CD3OD): δ (ppm) = 189.2 (C-5), 93.8 (C-3),
80.6 (C-2), 74.1 (C-7), 72.1 (C-1), 69.0 (C-8a), 61.9 (C-8);
FABMS m/z 222 (100%, [M+H]+); FABMSHR m/z
221.035640 (221.035794 Calcd for C7H11NO5S).
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Synlett 2003, No. 3, 341–344 ISSN 0936-5214 © Thieme Stuttgart · New York