Formal synthesis of cytosamine - A component of nucleoside antibiotics, the amicetin family

Article abstract of DOI:10.1081/SCC-100104831

A facile route to a synthetic precursor of the nucleoside antibiotics, amicetins, was investigated employing stable phenyl thioglycosides as key building blocks.

Full text of DOI:10.1081/SCC-100104831

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
FORMAL SYNTHESIS OF CYTOSAMINE—A
COMPONENT OF NUCLEOSIDE ANTIBIOTICS, THE
AMICETIN FAMILY
Hideyuki Sugimura & Ken-ichi Watanabe
To cite this article: Hideyuki Sugimura & Ken-ichi Watanabe (2001) FORMAL SYNTHESIS OF
CYTOSAMINE—A COMPONENT OF NUCLEOSIDE ANTIBIOTICS, THE AMICETIN FAMILY, Synthetic
Communications, 31:15, 2313-2321, DOI: 10.1081/SCC-100104831
To link to this article: http://dx.doi.org/10.1081/SCC-100104831
Published online: 09 Nov 2006.
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Date: 12 February 2016, At: 01:41

SYNTHETIC COMMUNICATIONS, 31(15), 2313–2321 (2001)
FORMAL SYNTHESIS OF CYTOSAMINE—
A COMPONENT OF NUCLEOSIDE
ANTIBIOTICS, THE AMICETIN FAMILY
Hideyuki Sugimura* and Ken-ichi Watanabe
The Noguchi Institute, Kaga 1-8-1, Itabashi-ku,
Tokyo 173, Japan
ABSTRACT
A facile route to a synthetic precursor of the nucleoside
antibiotics, amicetins, was investigatedemploying stable
phenyl thioglycosides as key building blocks.
Cytosamine is a common skeleton of the amicetin group of antibiotics
produced in the fermentation broth of various species of Streptomyces and
Aethrobacter.1 The structure features the presence of a unique deoxyhexo-
pyranosyl nucleoside in which the 40-hydroxyl group is glycosylated by an
amino sugar calledamosamine.
Amicetin was isolatedin the early 1950s by several groups, 2–5 andthe
correct structure was establishedin 1963 by Stevens et al. 6 Totally about ten
disaccharide nucleosides having closely related structures have been found
to date.7–12 The latest example is the cytosaminomycins which were isolated
as anticoccidial agents from a species of Streptomyces obtainedfrom a soil
sample.12
*Corresponding author. Current address: Faculty of Education and Human
Science, Yokohama National University, Yokohama 240-8501, Japan; E-mail:
sugimura@edhs. ynu.ac.jp
2313
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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SUGIMURA AND WATANABE
Figure 1. Structure of the amicetin family antibiotics.
The synthetic study of these antibiotics is significantly important in
order to not only elucidate their relation between the structures and biolo-
gical properties but also to design novel artificial molecules related to these
antibiotics. However, only one total synthesis of such a nucleoside anti-
biotic, plicacetin—one of the simplest disaccharide nucleosides of the
13
amicetin family, has been accomplishedby Stevens et al. in 1972.
Recently, we developed a stereoselective route to a trideoxyhexopyr-
anosyl nucleoside,14 which corresponds to the component of cytosamine, as
part of our ongoing search to establish intramolecular glycosylation for
nucleoside synthesis.15 Alternatively, the recent progress in methodologies
for the synthesis of complex oligosaccharides prompted us to develop more
facile routes to amicetin family antibiotics. In this paper, we describe a
formal synthesis of cytosamine via coupling of the above-obtainedhexo-
pyranosyl nucleoside and an amosamine equivalent using stable thioglyco-
sides as key building blocks.
The route to thioglycoside 6, a synthetic equivalent of amosamine, is
shown in Scheme 1. Starting from commercially available penta-O-acetyl-b-
D-galactopyranose, phenyl 1-thiogalactopyranoside (2) was preparedaccor-
ding to a reported procedure16 with slight modifications. Protection of the
4- and 6-hydroxyls with a benzylidene group was followed by benzylation of
the 2,3-hydroxyls to afford fully protected thioglycoside 3 in 83% overall
yield. The 4,6-O-benzylidene was removed under acidic conditions to pro-
vide diol 4, whose primary hydroxyl group underwent selective tosylation,
followed by reduction with lithium triethylborohydride,17 yielding 6-deoxy
derivative 5. Finally, substitution of the 4-hydroxyl with azide via a 4-O-
mesyl derivative was accomplished in a general manner to afford the requi-
site thioglycoside 6 in a total goodyield.

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2315
Scheme 1. Preparation of the glycosyl donor.
With the amosamine-type glycosyl donor in hand, we next investigated
the glycosylation reaction of 20,30,60-trideoxy-b-D-erythro-hexopyranosyl
nucleoside 8.15 To date, numerous methods have been reported for glycoside
synthesis.18 However, selective formation of a-gluco-type glycosides using
thioglycosides as the glycosyl donor is still limited. Fukase et al. have
reportedthat the use of an NBS-LiClO 4 combination for activation of thio-
glycosides furnished a-glycosides.19 However, when we employedthis acti-
vation system for the coupling between nucleoside 8 andthioglycoside 6, the
reaction did not take place and only the starting materials were recovered.
Alternatively, treatment of 8 and 6 with Me2S(SMe)BF4 as the activator led
to the desired nucleoside disaccharide, although the yield and the selectivity
regarding the anomeric center were not satisfactory (16%, a/b ¼ 2).
As is well-known, thioglycosides can be converted into glycosyl fluor-
ides, on treatment of NBS-DAST or NBS-HFÁPy, which serve as powerful
glycosylating agents in the presence of a silver salt andtin(II) chloride. For
instance, reaction of a benzyl-protected D-galactopyranosyl fluoride acti-
vatedby AgOTf-SnCl in Et2O gave an a-galactosylatedproudct with
2
high selectivity.20 We therefore appliedthis system to the synthesis of the
disaccharide nucleoside. Treatment of thioglycoside 6 with NBS-HFÁPy
afforded a glycosyl fluoride 7 in 95% yield( a/b ¼ ca.2). Subsequently, the
reaction of the glycosyl fluoride 7 with nucleoside 8 was promotedby
AgOTf-SnCl2 (5 equiv.) in the presence of MS 4A in Et2O-dichloroethane
(9:1) at room temperature for 40 h to furnish the desired a-anomer of the

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SUGIMURA AND WATANABE
Scheme 2. Synthesis of the disaccharide nucleoside.
disaccharide nucleoside 9 in 52% isolatedyieldalong with its b-anomer
(11%) andrecovery of unreactednucleoside (11%). Both prolongedreac-
tion andincreasedamounts of activator didnot leadto improvement in the
yield of the product or complete consumption of the starting nucleoside.
The anomeric configuration of the a-anomer 9 was confirmedby
1H NMR analysis. The anomeric proton appearedas a doublet at d 4.84
with J ¼ 3.4 Hz andindicatedan equatorial proton. By contrast, that of the
b-anomer appearedat 4.42 ppm with J ¼ 7.3 Hz, indicating an axial proton.
The disaccharide nucleoside 9 has a closely relatedstructure that has
been synthesizedby Stevens et al. andcouldbe convertedinto cytosamine
itself.13 Hence, we accomplisheda formal synthesis of cytosamine here.
EXPERIMENTAL
1H and 13C NMR spectra were obtainedon a JEOL JNM-EX400
spectrometer in CDCl3 with Me4Si as an internal standard. J Values are
given in Hz. IR spectra were recorded on a Perkin-Elmer Model 1600
spectrophotometer. TLC was performedon plates coatedwith silica gel
60 F254 (Merck). For column chromatography, Wakogel C-300 (Wako
Chemicals) was used. All solvents used in the reactions were distilled from
an appropriate drying agent and stored over molecular sieves.
Phenyl 1-thio-b-D-galactopyranoside (2). To a solution of 1,2,3,4,6-
penta-O-acetyl-D-galactose (1) (10.0 g, 25.6 mmol) andPhSH (3.2 ml,
31 mmol) in CH2Cl2 (50 mL) under Ar was added BF3ÁOEt2 (15.7 mL,

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CYTOSAMINE AND AMICETIN
2317
128 mmol) at room temperature. The reaction mixture was stirredfor 6 h
andthen pouredinto saturatedaqueous NaHCO solution. The organic
3
layer was separated, dried, and concentrated. The residue was purified by
flash chromatography (hexane-AcOEt, 2:1) to provide phenyl 2,3,4,6-tetra-
1
O-acetyl-1-thio-b-D-galactopyranoside (10.94 g, 97%). H NMR d 1.97 (s,
3H), 2.04 (s, 3H), 2.09 (s, 3H), 2.12 (s, 3H), 3.98 (dt, 1H, J ¼ 5.9, 13.2), 4.21
(dd, 1H, J ¼ 4.4, 11.2), 4.77 (dt, 1H, J ¼ 5.9, 15.6), 5.08 (dd, 1H, J ¼ 3.4,
15.6), 5.28 (dt, 1H, J ¼ 4.4, 5.9, 15.6), 5.46 (d, 1H, J ¼ 3.4), 7.11–7.36 (m,
5H). To a solution of this thiogalactoside (6.72 g, 15.3 mmol) in MeOH
(100 mL) was added 30% NaOMe solution (8.4 mL) at 0ꢀC. The reaction
mixture was stirredfor 20 min at the same temperature andthen neutralized
with DOWEX 50W (Hþ-form). After removal of the resin by filtration,
concentration of the filtrate afforded phenyl 1-thio-b-D-galactopyranoside
(2) (3.85 g, 93%). 1H NMR d 2.05–2.17 (m, 1H), 2.48–2.49 (m, 1H),
2.68–2.80 (m, 1H), 3.49 (d, 1H, J ¼ 15.6), 3.61–3.72 (m, 2H), 4.57 (d, 1H,
J ¼ 9.3), 7.25–7.36 (m, 5H). Anal. Calcd. for C12H16O5S: C, 52.93; H, 5.92.
Found: C, 52.53; H, 6.07.
Phenyl 2,3-di-O-benzyl-4,6-O-benzylidene-1-thio-b-D-galactopyranoside
(3). According to the literature,21 thiogalactopyranoside (2) (3.85 g,
14.2 mmol) was treated with benzaldehyde dimethylacetal (2.3 mL,
15.3 mmol) andtetrafluoroboric acidetherate (2.1 mL, 14.3 mmol) in
DMF (140 mL) for 7 h at room temperature. After addition of Et3N
(2.0 mL, 14.3 mmol), the mixture was evaporated under reduced pressure.
The residue was purified by flash chromatography (hexane-AcOEt, 2:1) to
provide phenyl 4,6-O-benzylidene-1-thio-b-D-galactopyranoside (4.31 g,
1
84%). H NMR d 2.46–2.56 (m, 2H), 2.88 (d, 1H, J ¼ 15.1), 2.99 (d, 1H,
J ¼ 15.1), 3.70–3.72 (m, 3H), 4.24–4.54 (m, 4H), 5.53 (d, 1H, J ¼ 15.1),
7.26–7.71 (m, 10H). To a solution of phenyl 4,6-O-benzylidene-1-thio-b-D-
galactopyranoside (4.31 g, 12.0 mmol) in DMF (120 mL) was added NaH
(60% in mineral oil, 1.43 g, 36.0 mmol). After 1 h, benzyl bromide (3.6 mL,
30 mmol) was added to the reaction mixture, and stirring was continued
overnight. The reaction was quenched by addition of MeOH. The mixture
was neutralizedwith saturatedaqueous NH 4Cl solution andthen extracted
with CHCl3. The organic layer was washed with water several times, dried,
and concentrated. The residue was purified by flash chromatography
1
(hexane/EtOAc, 2:1) to afford 3 (6.42 g, 99%). H NMR d 3.45 (d, 1H,
J ¼ 1), 3.63 (dd, 1H, J ¼ 3.4, 9.3), 3.90 (t, 1H, J ¼ 9.3), 4.01 (dd, 1H,
J ¼ 1.5, 10.7), 4.16 (d, 1H, J ¼ 2.9), 4.38 (dd, 1H, J ¼ 1.5, 10.7), 4.62 (d,
1H, J ¼ 9.8), 4.65–4.76 (m, 4H), 5.49 (s, 1H), 7.15–7.45 (m, 16H), 7.50–
7.55 (m, 2H), 7.71 (d, 1H, J ¼ 6.8); IR (KBr) 2864, 1453, 1367, 1168,
1092, 1058, 1027 cm–1. Anal. Calcdfor C 33H32O5S: C, 73.31; H, 5.97.
Found: C, 73.21; H, 5.93.

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SUGIMURA AND WATANABE
Phenyl 2,3-di-O-benzyl-1-thio-b-D-galactopyranoside (4). According to
the literature,21 3 (322 mg, 0.6 mmol) was treatedwith tetrafluoroboric acid
(48%wt. in water, 0.09 mL) in MeCN (12 mL) at room temperature over-
night. After addition of Et3N (0.2 mL), the mixture was evaporatedunder
reduced pressure. The residue was dissolved in CHCl3, which was washed
with saturatedaqueous NaHCO solution. The organic layer was dried,
3
concentrated, and purified by flash chromatography (hexane/EtOAc, 1:1)
to afford 4 (244 mg, 91%). 1H NMR d 2.85 (br, 1H), 2.96 (br, 1H), 3.43 (t, 1H,
J ¼ 5.4), 3.54 (dd, 1H, J ¼ 2.9, 8.8), 3.71–3.80 (m, 2H), 3.90 (dd, 1H, J ¼ 6.4,
11.8), 4.03 (d, 1H, J ¼ 2.9), 4.63 (d, 1H, J ¼ 9.8), 4.64 (d, 1H, J ¼ 11.7), 4.68 (d,
1H, J ¼ 11.7), 4.72 (d, 1H, J ¼ 10.3), 4.80 (d, 1H, J ¼ 10.3), 7.15–7.45
(m, 13H), 7.50–7.58 (m, 2H); IR (KBr) 3406, 2870, 1454, 1363, 1078 cm–1.
Anal. Calcdfor C 26H28O5S: C, 69.00; H, 6.24. Found: C, 68.84; H, 6.24.
Phenyl 2,3-di-O-benzyl-6-deoxy-1-thio-b-D-galactopyranoside (5). To a
solution of 4 (1.02 g, 2.25 mmol) in pyridine (20 mL) was added p-toluene-
sulfonyl chloride (0.644 g, 3.38 mmol) at 0ꢀC. The reaction mixture was
stirredat room temperature overnight andthen quenchedwith water. The
mixture was extractedwith CHCl 3 three times. The combinedorganic layer
was dried, concentrated, and purified by flash chromatography (hexane/
EtOAc, 2:1) to afford6- O-tosyl ester (1.15 g, 85%). The tosylate (0.558 g,
0.92 mmol) was dissolved in THF (2 mL) under Ar. Lithium triethylboro-
hydride (1 M in THF, 3 mL) was added, and stirring was continued for 3 h.
Hydrogen peroxide (30%, 3 mL) was added dropwise at 0ꢀC, andthe mix-
ture was then allowedto warm to room temperature andwas extractedwith
CHCl3. After drying and concentration of the organic layer, the residue was
purifiedby flash chromatography (hexane/EtOAc, 4:1) to afford 5 (0.348 g,
1
87%) as a syrup. H NMR d 1.37 (d, 3H, J ¼ 6.3), 2.28 (d, 1H, J ¼ 2.6),
3.52–3.62 (m, 2H), 3.69 (t, 1H, J ¼ 9.2), 3.81–3.86 (m, 1H), 4.60 (d, 1H,
J ¼ 9.9), 4.71 (s, 2H), 4.74 (d, 1H, J ¼ 10.2), 4.83 (d, 1H, J ¼ 10.2), 7.15–7.45
(m, 13H), 7.50–7.59 (m, 2H); IR (KBr) 3430, 1365, 1130, 1085, 1057 cm–1.
Anal. Calcdfor C 26H28O4S: C, 71.53; H, 6.46. FoundC, 71.54; H, 6.59.
Phenyl 4-C-azido-2,3-di-O-benzyl-4,6-dideoxy-1-thio-b-D-glucopyrano-
side (6). To a solution of 5 (0.254 g, 0.582 mmol) in pyridine (3 mL) was
added methanesulfonyl chloride (0.14 mL, 1.75 mmol) at 0ꢀC. The reaction
mixture was stirredat the same temperature for 2 h andthen quenchedwith
water. The mixture was extractedwith ether several times andthe combined
organic layer was washedwith dil. HCl, water, andsaturatedaqueous
NaHCO3 solution. Drying andconcentration of the organic layer gave the
crude 4-O-mesyl ester. This was dissolved in DMF (6 mL), and sodium azide
(0.189 g, 2.91 mmoꢀl) was then added to the solution. The reaction mixture
was stirredat 100 C for 7 h. After cooling, the mixture was diluted with
ether. The organic layer was washed with water, dried, and concentrated.

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CYTOSAMINE AND AMICETIN
2319
The residue was purified by TLC (hexane/EtOAc, 19:1) to afford 6 (0.208 g,
77%). 1H NMR d 1.39 (d, 1H, J ¼ 5.9), 3.15–3.27 (m, 2H), 3.44–3.58 (m, 2H),
4.62 (d, 1H, J ¼ 9.2), 4.72 (d, 1H, J ¼ 10.2), 4.83 (d, 1H, J ¼ 10.6), 4.88 (d,
1H, J ¼ 10.6), 4.92 (d, 1H, J ¼ 10.2), 7.20–7.46 (m, 13H), 7.46–7.60 (m, 2H);
IR (KBr) 2108, 1454, 1358, 1274, 1129, 1094, 1043, 1028 cmꢁ1. Anal. Calcd
for C26H27N3O3S: C, 67.66; H, 5.90. Found: C, 67.66; H, 5.96.
Disaccharide nucleoside 9. To a solution of thioglycoside 6 (73.0 mg,
0.158 mmol) in CH2Cl2 (2 mL) was added an HF-pyridine complex
(0.16 mL) followedby N-bromosuccinimide (31 mg, 0.17 mmol) at –35ꢀC.
The mixture was allowedto warm to 0 ꢀC over 1 h. The reaction mixture was
diluted with EtOAc and poured into saturated aqueous NaHCO3 solution.
The organic layer was washedwith saturatedaqueous NaHCO
solution
3
and brine, dried, and concentrated. The residue was purified by flash chro-
matography (hexane-EtOAc, 19:1) to affordglycosyl fluoride 7 (56.0 mg,
95%). The fluoride was obtained as a 2:1 (a/b) mixture of anomers and
was usedfor the following step without separation.
To a suspension of AgOTf (167 mg, 0.65 mmol), SnCl2 (123 mg,
0.65 mmol), andcrushed4-A molecular sieves (200 mg) in ether (4 mL)
was added nucleoside 815 (31.2 mg, 0.13 mmol) in ether (2 mL) and1,2-
dichloroethane (1 mL) at 0ꢀC. After stirring for 5 min, glycosyl fluoride 7
(72.6 mg, 0.195 mmol) in ether (2 mL) was added, and stirring was continued
at room temperature for 40 h. The reaction mixture was diluted with EtOAc
andfilteredthrough Celite. The filtrate was washedwith aqueous NaHCO
3
solution and brine. The organic layer was dried and concentrated. The
residue was purified by TLC (hexane-EtOAc, 1:1) to afford 9 (40.2 mg,
52%), accompaniedby its b-anomer (8.2 mg, 11%) andthe starting nucleo-
side 8 (11%). Compound 9: [a]2D5 þ 192ꢀ (c 1.26, CHCl3); H NMR d 1.26
1
(d, 3H, J ¼ 6.4), 1.32 (d, 3H, J ¼ 6.4), 1.34–1.45 (m, 1H), 1.57–1.71 (m, 1H),
2.13–2.21 (m, 1H), 2.26–2.33 (m, 1H), 3.10 (t, 1H, J ¼ 9.8), 3.30 (ddd, 1H,
J ¼ 2.9, 4.4, 10.7), 3.53–3.63 (m, 2H), 3.73–33.83 (m, 2H), 3.96 (s, 3H), 4.60
(d, 1H, J ¼ 11.7), 4.75 (d, 1H, J ¼ 11.7), 4.81 (d, 1H, J ¼ 10.3), 4.84 (d, 1H,
J ¼ 3.4), 4.95 (d, 1H, J ¼ 10.3), 5.78 (dd, 1H, J ¼ 2.0, 10.3), 5.92 (d, 1H,
J ¼ 7.3), 7.25–7.45 (m, 10H), 7.69 (d, 1H, J ¼ 7.3); 13C NMR d 18.4, 18.8,
26.5, 30.5, 54.5, 66.6, 67.9, 73.5, 73.9, 75.7, 76.8, 79.6, 89.8, 82.9, 92.9, 95.8,
127.9, 128.0, 128.2, 128.3, 128.4, 128.6, 137.7, 137.9, 142.2, 155.3, 171.6.
Anal Calcdfor C 31H37N5O7: C, 62.93; H, 6.30. Found: C, 62.92; H, 6.36.
1
b-Anomer of 9: H NMR d 1.30 (d, 3H, J ¼ 6.4), 1.33 (d, 3H, J ¼ 5.9),
1.41–1.65 (m, 1H), 1.83 (ddd, 1H, J ¼ 3.9, 11.2, 13.2), 2.16–2.34 (m, 1H),
3.08–3.31 (m, 3H), 3.39–3.51 (m, 2H), 3.60–3.69 (m, 1H), 3.96 (s, 3H),
4.42 (d, 1H, J ¼ 7.3), 4.73 (d, 1H, J ¼ 10.7), 4.79 (d, 1H, J ¼ 10.7), 4.88
(d, 1H, J ¼ 10.7), 4.90 (d, 1H, J ¼ 10.7), 5.77 (dd, 1H, J ¼ 2.0, 10.3), 5.91
(d, 1H, J ¼ 7.3), 7.22–7.40 (m, 10H), 7.66 (d, 1H, J ¼ 7.3).

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SUGIMURA AND WATANABE
ACKNOWLEDGMENT
This work was supportedby the Ajinomoto Awardin Synthetic
Organic Chemistry, Japan.
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Receivedin Japan September 4, 2000

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