# Syntheses of 5-Thio-D-Mannose from Petrochemicals and  a Disaccharide

Authors: Hideya Yuasa; Masatake Jouyabu; Nobuyuki Mitsuhashi; Hironobu Hashimoto
Journal: Research Letters in Organic Chemistry
 (2008)

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## Abstract

Per-O-acetyl-5-thio-DL-mannose was synthesized from
petrochemicals in six steps and 9% overall yield. It was then derivatized into glycosyl
trichloroacetimidate and subjected to glycosidation reaction with a mannosyl acceptor to
give a separatable mixture of disaccharides with 5-thio-D- and L-mannosides. This is the
first synthesis of an enantiomerically pure 5-thiosugar derivative from racemic chemicals.
The D-glycoside was derivatized into methyl
(5-thio-α-D-mannopyranosyl)-2-O-α-D-mannopyranoside 6-phosphate as a potential inhibitor
of a golgi α-1,2-mannosidase.


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## Body

## 1. Introduction

5-thiosugars
are aldopyranose analogs with a sulfur atom in the pyranose ring [1–3] and it
has been demonstrated that oligosaccharide analogs with a 5-thiosugar at the
nonreducing end are exoglycosidase-resistant [4–6]. The resistance to hydrolases
is a property desired for potential drugs [7], allowing for a practical
duration of potency, hence a number of 5-thiosugars have been synthesized [2]
and their behaviors against glycosidases studied [8–10]. Although most of
5-thiosugars have been synthesized from natural carbohydrates in not less than
10 steps, a few trials were made for the construction of the
thiopyran structure through
hetero Diels-Alder reaction from petrochemicals [11–13]. With this strategy, however,
5-thiosugars are obtained necessarily in racemic form and no analogs for
mammalian monosaccharides have been synthesized. One of the racemic
5-thiosugars obtained in this method is 5-thiomannuronate, a potential
intermediate for the synthesis of 5-thio-D-mannose. 5-thio-D-mannose is the only 5-thiosugar that has been
isolated from nature [14], awaiting studies for its biological significance. We
thus report here on the synthesis of racemic 5-thiomannose (5SMan) from the
product of a hetero Diels-Alder reaction reported by  Prabhakaran et al. [12] and
then on the glycosidation reaction of the 1-O-trichloroacetimidate
derivative of 5SMan, which gave a diastereomeric mixture of 5SMan-containing
disaccharides, and they were easily separated by column chromatography. As a
result, we obtained an enantiomerically pure 5-thiosugar derivative from
petrochemicals for the first time. To make the synthesis more significant and
advantageous, we derivatized the disaccharide analog into the 6-O-phosphate derivative, 5SManα1,2Man6P, as a
potential inhibitor of a golgi α-1,2-mannosidase. The
native Manα1,2Man6P structure
is contained in the biosynthetic intermediates of the N-glycans specifically expressed on lysosomal hydrolases [15]. Mannose-6-phosphates
(Man6P) at the nonreducing ends of the N-glycans,
exposed by the action of a golgi α-1,2-mannosidase
toward the Manα1,2Man6P structures
[16], are specifically recognized by the cargo proteins bound for lysosomes [17].
Thus, the nondigestive property of 5SMan would allow 5SManα1,2Man6P to be a
potential blocking agent against the distribution of lysosomal enzymes into
lysosomes, though the mannosidase is unspecific for the phosphate group.


## 2. Results and Discussion

Ethyl 1,4-di-O-acetyl-5-thiomannopyranosyluronate 3 was synthesized from 1,4-diacetocxy-1,3-butadiene 1 and ethyl thioxoacetate 2, generated in situ from the
anthracene adduct, through hetero Diels-Alder and osmium oxidation reactions in
a reported method [11, 12]. Prior to the reduction of the carboxylate moiety,
compound 3DL was subjected to
Fischer methanolysis to give methyl (methyl 5-thiomannopyranosid)uronate 4DL in 62%.
4DL: Rf =  0.23 (CHCl3-MeOH, 9 : 1), 1H NMR (270 MHz,
CDCl3) δ 4.53 (d, 1H, J  =  3.6 Hz, H-1), 4.22 (t, 1H, J  =
3.6 Hz, H-2), 4.16 (t, 1H, J = 10 Hz, H-4), 3.79 (s, 1H, CO2CH3),
3.72 (dd, 1H, J = 3.6, 10 Hz,
H-3), 3.67 (t, 1H, J  = 10 Hz, H-5), 3.49 (s, 3H, OCH3); 13C NMR (67.8 MHz, CDCl3) δ 170.6, 87.2, 71.9, 70.8, 56.5, 53.0, 43.8;
ESI (M + Na)+261.   As attempted reduction
of 4DL with LiAlH4 ended
in decomposition, 4DL was treated
with NaBH4, which has been occasionally successful in the reduction
of sugar esters [18], to give methyl 5-thiomannopyranoside 5DL in 77%. Spectral data of its tetraacetate were consistent with
those reported for D-enantiomer [19]. Acetylation
followed by acetolysis of compound 5 produced per-O-acetyl-5-thiomannopyranose 6DL in 85% yield, whose NMR data
coincided with those reported for its D-enantiomer [20].
We thus obtained the protected racemic 5-thiomannose 6DL from petrochemicals in six steps and 9% overall yield. The
synthesis of the corresponding 5-thio-D-mannose
derivative 6D has been achieved in
ten steps from D-mannose with 14% overall yield [20], showing
that the merit of this study is the fewer synthetic steps and the use of
petrochemicals for raw material. Although the low yield is a downside, the most
impractical aspect of this method resided in the production of racemic compounds.
To minimize the demerits, we next studied the use of the racemic 5-thiomannose
derivative 6DL for the synthesis of
a disaccharide analog.Per-O-acetyl-5-thiomannose 6DL was derivatized to 1-O-trichloroacetimidate 7DL in the same manner as that of D-enantiomer [21] in 80% yield. The glycosidation
reaction of 7DL toward a 3-O-benzyl-protected mannopyranoside 8 [22] was conducted with the
conditions reported for the synthesis of a similar 5-thio-D-mannose-containing
disaccharide, in which the 3-O-benzoyl
group caused the predominant production of an orthoester derivative [21]. Use
of the benzyl group for 3-O-protection
prevented the orthoester formation and methyl 2-O-(5-thio-α-D and L-mannopyranosyl) mannopyranoside
derivatives (9D : 9L  =  1 : 1) were obtained in 78% yield from the racemic glycosyl
donor 7DL. The diastereomers were
easily separated by silica gel column chromatography. The stereochemistry of
the diastereomers was confirmed by comparison of Rf values,
[α]D25,
and 1H NMR 9D:Rf =
0.43 (hexane-ethyl
acetate, 1 : 1); [α]D25 +67.7° (c 0.73, CHCl3; +66.8° for the same
product synthesized from D-enatiomer of 7); 1H NMR (270 MHz, CDCl3)δ7.50-7.26
(m, 10H, Ph), 5.68 (s, 1H,
PhCH), 5.67 (dd, 1H, J  =  2.8, 4.0 Hz, H-2′), 5.45
(t, 1H, J = 10 Hz, H-4′), 5.37
(dd, 1H, J 2.8,
10 Hz, H-3′), 5.00 (d, 1H, J1’,2’  4.0 Hz, H-1′), 4.91-4.60 (dd, 2H, J 12.2 Hz,
PhCH2), 4.66 (d, 1H, J1,2  1.3 Hz, H-1′), 4.88 (d, 1H, J = 12.2 Hz, PhCHH),
4.66 (d, 1H, J = 2.3 Hz, H-1), 4.62 (d, 1H, J =
12.2 Hz, PhCHH), 4.30 (dd, 1H, J =
5.6, 12.2 Hz, H-6a′), 4.26 (dd, 1H, J = 4.6, 10.2 Hz, H-6a), 4.17 (t, 1H, J =
9.6 Hz, H-4), 4.10 (dd, 1H, J = 2.3, 3.0 Hz, H-2), 4.09 (dd, 1H, J = 3.6, 12.2 Hz, H-6b′), 3.97 (dd, 1H, J = 3.0, 9.6 Hz, H-3), 3.91 (t, 1H, J = 10.2 Hz,
H-6b), 3.77 (ddd, 1H, J = 4.6, 9.6, 10.2, H-5), 3.45 (ddd, 1H, J = 3.6, 5.6, 10 Hz, H-5′), 3.37 (s, 3H, OCH3),
2.12, 2.08, 2.04, 2.00 (s × 4, 12H, COCH3); 13C NMR (67.8 MHz, CDCl3) δ 170.5, 169.75. 169.70, 169.5, 138.4, 137.5,
128.8, 128.2, 128.1, 127.4, 127.3, 126.1, 101.5, 101.0, 83.3,
79.5, 76.0, 75.8, 73.4, 70.6, 70.3, 69.5, 68.7, 63.8, 62.1,
54.8, 39.3, 21.0, 20.7, 20.6; HR-ESMS calcd for C35H42O14SNa
(M + Na)+ 741.2194, found 741.2282. 9L:Rf = 0.35 (hexane-ethyl
acetate, 1:1); [α]D25-72.9° (c 0.79, CHCl3); 1H NMR (270 MHz, CDCl3) δ7.52-7.26
(m, 10H, Ph), 5.69 (s, 1H, PhCH), 5.45-5.40 (m, 3H, H-2′, H-3′, H-4′), 4.84(d,
1H, J = 12.2 Hz, PhCHH), 4.70 (d,
1H, J = 12.2 Hz, PhCHH),
4.69 (d, 1H, J = 3.3 Hz, H-1′), 4.68 (d, 1H, J = 1.5 Hz, H-1), 4.29 (dd, 1H
, J = 1.5, 3.6 Hz, H-2), 4.25 (dd, 1H, J = 4.5, 10 Hz, H-6a), 4.13 (t, 1H, J =
9.6 Hz, H-4), 4.04 (dd, 1H, J = 4.6, 11.9 Hz, H-6a′), 3.96 (dd, 1H, J = 3.6,
9.6 Hz, H-3), 3.90 (t, 1H, J = 10 Hz, H-6b), 3.80-3.72 (m, 2H, H-5, H-5′),
3.59 (dd, 1H, J = 3.3,
11.9 Hz , H-6b′), 3.36 (s, 3H, OCH3),
2.18, 2.01, 2.002, 1.996 (s × 4, 12H, COCH3); 13C NMR (67.8 MHz, CDCl3) δ 170.5,
170.2, 169.7, 169.6 (C = O), 138.2, 137.6, 128.9, 128.3, 128.1,
127.6, 126.1 (Ph), 101.5 (PhCH), 98.3 (C-1),
80.4 (C-1′), 78.8 (C-3), 74.4, 74.1, 72.9 (PhCH2),
71.5, 70.2, 68.9, 68.7, 64.0, 61.3 (C-6′), 54.9 (OCH3),
38.7 (C-5′), 21.1, 20.7, 20.65, 20.6 (COCH3); HR-ESMS
calcd for C35H42O14SNa
(M + Na)+ 741.2194, found 741.2242.
with those of 9D*
synthesized from 5-thio-D-mannopyranosyl donor 7D. This is the first synthesis of an enatiomerically pure
5-thiosugar derivative from racemic petrochemicals.As stated in
the introductory section, we modified the synthesized disaccharide9D into 6-O-phosphate. The benzylidene group of compound 9D was deprotected by hydrolysis (99%) to give compound 10, which was then subjected to
phosphorylation conditions giving regioselectively the di-O-benzyl 6-O-phosphate 11 in 58% yield. Deacetylation (12 in 91%) followed by debenzylation
gave compound 13 in 83% yield. 13:Rf = 0.20 (iPrOH-H2O, 3:1); [α]D25  +45.1° (c 1.0, H2O); 1H NMR (270 MHz, D2O)
δ4.86 (d, 1H, J 1.5 Hz, H-1), 4.72 (d, 1H, J1,2  4.0 Hz, H-1′),
4.20 (dd, 1H, J= 2.8, 4.0 Hz, H-2′), 3.89-3.50 (m, 3H, H-2, H-6a,
H-6b), 3.80 (dd, 1H, J = 3.3, 11.9 Hz, H-6a′), 3.75-3.50 (m, 6H, H-3, H-4, H-5,
H-3′, H-4′, H-6b′), 3.26 (s, 3H, OCH3),
2.98 (m, 1H, H-5′);13C NMR (67.8 MHz, D2O) δ 100.2, 86.8, 77.7, 72.5, 72.34, 72.2, 72.0,
70.6, 70.1, 66.7, 63.3, 60.9, 55.2, 44.6; 31P NMR (109.4 MHz,
D2O) δ 3.75.


## 3. Conclusion

We achieved the synthesis of an
enantiomerically pure 5-thio-D-mannose-containing
disaccharide derivative from racemic raw materials for the first time. To
pursue the advantage of 5-thiosugar, we derivatized the disaccharide into 5SManα1,2Man6P, the
analog of the partial structure of an intermediate oligosaccharide of the N-glycans on lysosomal enzymes. 5-thiopyranosides
are in general glycosidase-resistant and the analog is a potential inhibitor of
a golgi α-1,2-mannosidase
that cleaves Manα1,2Man6P into
Man6P.Scheme 1Scheme 2
a. AcCl/MeOH; b. NaBH4/H2O; c. (i) Ac2O-Py, (ii)
Ac2O, H2SO4; d. (i) H2NNH2•
AcOH/DMF,
(ii) K2CO3, CCl3CN; e. 8, TESOTf/CH2Cl2;
f. 80% AcOH; g. (i) (BnO)2PN(iPr)2, 1H-tetrazole, (ii) 30%
H2O2; h. Et3N-CH3OH; i. 10%
Pd-C, H2, AcOH-EtOH.



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*Source: 784173-2008-05-25.xml*