Gram-scale synthesis of an armed colitose thioglycoside

Article abstract of DOI:10.1021/jo501472v

A concise gram-scale synthesis of protected colitose thioglycosides for use in bacterial carbohydrate antigen synthesis is described. The synthesis proceeds in six steps and 59-70% overall yield from commercially available L-fucose, making it the most eff

Full text of DOI:10.1021/jo501472v

Note
pubs.acs.org/joc
Gram-Scale Synthesis of an Armed Colitose Thioglycoside
Dina Lloyd and Clay S. Bennett*
Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States
S
* Supporting Information
ABSTRACT: A concise gram-scale synthesis of protected colitose
thioglycosides for use in bacterial carbohydrate antigen synthesis is
described. The synthesis proceeds in six steps and 59−70% overall
yield from commercially available L-fucose, making it the most
efficient route reported to date. Key steps include regioselective
installation of a thiocarbonate using catalytic dioctyltin dichloride (10 mol%) and a tris(trimethylsilyl)silane-mediated radical
deoxygenation.
he unique carbohydrate structures found on many
mixture of this compound and thiophenol in dichloromethane
Tpathogenic bacteria has led to increased interest in was subjected to slow addition of BF3·Et2O to afford
thioglycoside 619 in excellent yield and selectivity (98%, two
developing carbohydrate-based antimicrobial vaccines.1,2 Such
vaccine approaches are viewed as particularly attractive because
many bacterial polysaccharides not only possess glycosidic
linkages that are not found in mammalian systems but also
contain unusual monosaccharides, such as deoxy sugars, that
are not found in eukaryotic species.3 This leads to a challenge
in vaccine development, however, as many of these sugars are
not readily available and must be constructed through multistep
synthesis. An example of such a sugar is the 3,6-dideoxyhexose
colitose, which is found in a number of pathogens, including
Escherichia coli, Salmonella adelaide, Salmonella greenside,
Yersinia pseudotuberculosis, and Vibrio cholera.3−7 As part of a
program directed at applying our recently developed reagent
controlled 1,2-cis-α-selective thioglycoside activation to the
construction of the E. coli O111 antigen minimum repeat unit,8
we had need of a large quantity of protected colitose
thioglycoside 1. Approaches to various colitose-based glycosyl
donors had been reported in the past; however, they all suffer
from low overall yields, lengthy synthetic routes, and/or the
requirement of stoichiometric amounts of toxic reagents.6,9−15
Here we report a short and highly efficient approach to
protected colitose thioglycosides from commercially available L-
fucose 2.
steps from 2, 12:1 β:α). Finally, deacetylation under Zemplen
́
conditions20 afforded 4, which would be used for regioselective
installation of the deoxygenation handle, in near-quantitative
yield.
With 4 in hand, we next explored its transformation to the
corresponding C-3 thiocarbonate 3, (Table 1). To this end, we
chose to examine regioselective thiocarbonylation of C-3 using
dioctyltin chloride as described by Muramatsu.17,18 Impor-
tantly, the Muramatsu conditions18 are catalytic in tin, which
provides a distinct advantage over previously reported
approaches that required the use of stoichiometric tin
reagents.6,10−13 In order to assess the most efficient method
for obtaining the desired target structure, we chose to examine
three different thiocarbonylating agents: O-phenyl chlorothio-
noformate, 1,1′-thiocarbonyldiimidazole (TCDI), and phenyl
isothiocyanate (PITC) (Table 1). Under these conditions, we
found that O-phenyl chlorothionoformate was superior to both
PITC and TDCI, providing the product 3b in good yield and
regioselectivity on both small (Table 1, entry 2) and gram scale
(Table 1, entry 3). Due to the ease of preparation of 3b on
gram scale, this compound was carried on to the critical
deoxygenation step.
In the deoxygenation step, we chose to first examine the use
of tris(trimethylsilyl)silane (TTMSS) as a nontoxic alternative
to the use of stoichiometric toxic trialkylstannane deriva-
tives.16,17,21−24 This reagent proved to be effective with 3b
(Table 2, entry 1), providing the product in excellent yield.
Notably, TTMSS-mediated deoxygenation provided the desired
product in much higher yield than tributyltin hydride (Table 2,
entry 2). Based on these results, we next examined the ability of
TTMSS to deoxygenate 3b on gram scale. Pleasingly, increasing
the scale of the reaction did not have a significant impact on the
efficiency of the reaction and we were able to obtain
thioglycoside 7 in 90% yield (Table 1, entry 3).
Retrosynthetically, we envisioned that 1 would arise from
fucose derivative 3, which possesses a handle for radical
deoxygenation (Scheme 1). While the use of stoichiometric
tributyltin hydride-mediated deoxygenation has been reported
in the past, we were interested in avoiding such toxic
reagents.6,10,12,13 Accordingly, we envisioned that this trans-
formation could be achieved using the tris(trimethylsilyl)silane-
mediated radical deoxygenation reported by Nishiyama.16,17
Returning to the retrosynthesis, 3 was envisioned to arise from
thioglycoside 4 through Muramatsu’s catalytic tin-mediated
regioselective installation of the thiocarbonate.18 Finally,
compound 4 could arise from fucose 1 using straightforward
chemistry.
In the synthetic direction, 2 was peracetylated using acetic
anhydride in pyridine to afford tetraacetate 5 (Scheme 2). A
Received: July 2, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/jo501472v | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
Scheme 1. Retrosynthesis of Colitose Thioglycoside
Scheme 2. Synthesis of Fucose Thioglycoside 4
a
Table 1. Thiocarbonylation Reactions
Scheme 3. Synthesis of Armed Colitose Thioglycoside 1
In summary, we have developed a highly selective and
efficient route to armed colitose thioglycoside 1. Starting from
L-fucose, the synthesis was completed on gram scale in six steps
and 59% overall yield. This represents the most concise route
to a colitose donor reported to date. Based on its efficiency, we
anticipate that this approach will be extremely valuable for the
construction of bacterial O-antigens, and find broad application
in the construction of antimicrobial vaccines.
EXPERIMENTAL SECTION
■
General Experimental Methods. All reactions were performed
under inert argon atmosphere, unless otherwise noted. Flash column
chromatography was performed on P-60 silica gel, 230−400 mesh.
Analytical and preparative thin-layer chromatography was carried out
on silica gel 60 F-254 plates. Products were visualized using UV or by
staining with 5% aqueous sulfuric acid or ceric ammonium molybdate.
a
Reactions conditions: 4 (0.39 mmol), Oc2SnCl2 (0.039 mmol),
substrate (0.5 mmol), PEMP (0.58 mmol), and TBAI (0.039 mmol) in
acetone (8 mL) at 25 °C. PEMP = 1,2,2,6,6-pentamethylpiperidine;
Oc2SnCl2 = dioctyltin dichloride; TBAI = tetrabutylammonium
iodide; PITC = phenyl isothiocyanate; PhO(S)Cl = O-phenyl
1
b
NMR spectra were acquired at 500 MHz for H NMR and 125 MHz
chlorothionoformate; TCDI = 1,1′-thiocarbonyldiimidazole. 1 g
for 13C NMR. Chemical shifts are reported in ppm relative to TMS
(for 1H NMR in CDCl3) or CDCl3 (for 13C NMR in CDCl3). For 1H
NMR spectra, data are reported as follows: δ shift, multiplicity (s =
singlet, m = multiplet, t = triplet, d = doublet, dd = doublet of
doublets, td = triplet of doublets, q = quartet, brs = broad singlet),
coupling constants are reported in Hz. Proton assignments were made
scale reaction in acetone (80 mL).
Table 2. Barton−McCombie Deoxygenation Reaction
Comparison of Tris(trimethylsilyl)silane and Tributyltin
a
Hydride
1
using H−1H homonuclear correlation spectroscopy (COSY) at 500
MHz. Low-resolution mass spectra (LRMS) were recorded using ESI-
MS. High-resolution mass spectra (HRMS) were obtained on a
Fourier transform ion cyclotron resonance mass spectrometer. Optical
rotations were measured at 589 nm in a 5 cm cell at 24 °C.
Materials. Solvents for reactions were dried immediately prior to
use. All other chemicals were purchased at the highest possible quality
and used as received. Compounds 4−6 were prepared according to
literature methods.19,20
entry
substrate
reagent
yield (%)
1
2
3b
3b
3b
TTMSS
Bu3SnH
TTMSS
98
84
90
b
3
Phenyl 3-O-(N-Phenylthiocarbamoyl)-1-thiol-β-L-fucopyra-
noside (3a). In a foil-covered flask phenyl 1-thio β-L-fucopyranoside
(4) (100 mg, 0.39 mmol) and dioctyltin dichloride (16 mg, 0.039
mmol) were suspended in dry acetone (8 mL), and stirred for 10 min.
To this suspension were added tetrabutylammonium iodide (14 mg,
0.039 mmol), phenyl isothiocyanate (0.05 mL, 0.5 mmol), and
1,2,2,6,6-pentamethylpiperidine (0.06 mL, 0.5 mmol). The reaction
was stirred for 57.5 h, at which point it was quenched with saturated
aqueous NH4Cl and extracted three times with ethyl acetate. The
organic layer was washed with water and brine, dried over Na2SO4,
filtrated, and concentrated in vacuo. The crude product was purified
with column chromatography (25% ethyl acetate in hexanes) to give
a
Reactions conditions: TTMSS or Bu3SnH (2 equiv), AIBN (20 mol
b
%), refluxed at 80 °C in benzene (5 mL) for 2 h. 1 g scale reaction in
benzene (100 mL). AIBN = 2,2′-azobis(2-methylpropionitrile);
TTMSS = tris(trimethylsilyl)silane Bu3SnH = tributyltin hydride
With the dideoxy thioglycoside 7 in hand, we next completed
the synthesis of 1. To this end, benzylation of 7 with benzyl
bromide in the presence of tetrabutylammonium iodide
(TBAI) afforded 1 in 90% yield (Scheme 3). Depending on
the scale, the overall synthesis proceeded in 59 to 70% yield
from commercially available L-fucose.
3a (65 mg, 43%): [α]24 = −0.075° (c 1.10, CH2Cl2); 1H NMR
D
(CDCl3, 500 MHz) δ 8.79 (brs, 1H), 8.58 (brs, 1H), 7.55−7.53 (m,
B
dx.doi.org/10.1021/jo501472v | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
3H), 7.34−7.26 (m, 6H), 7.17 (brs, 1H), 5.63 (dd, J = 6.5, 3, 1H, H3),
4.65 (d, J = 9.5, 1H, H1), 4.12 (m, 1H, H4), 3.98 (brs, 1H, H2), 3.79
(q, J = 6, 1H, H5), 2.97 (brs, 1H), 1.33 (d, J = 5, 3H, H6); 13C NMR
(CDCl3, 125 MHz) δ 187.3, 137.4, 132.5, 129.1, 128.9, 128.2, 126.2,
123.7, 121.9, 89.3, 84.6, 81.3, 74.6, 70.4, 70.0, 67.7, 16.6; LRMS (ESI,
pos. ion) m/z calcd for C19H21NO4S2 (M + Na) 414.08, found 414.18;
HRMS (ESI, pos ion) m/z calcd for C19H21NO4S2 (M + Na)
414.0810, found 414.0811.
(4 mg, 0.02 mmol), and benzene (5 mL) were used. The product was
obtained as a white powder 7 (24 mg, 82%).
1-Phenyl 2,4-Di-O-benzyl-3,6-dideoxy-1-thio-β-L-xylo-hexo-
pyranoside (1). Sodium hydride (95%, 460 mg, 18 mmol) and
tetrabutylammonium iodide (10 mg, 0.04 mmol) were suspended in
DMF (15 mL). The suspension was cooled to 0 °C and treated with a
solution of 7 (1 g, 4.1 mmol) in DMF (2 mL), while additional DMF
(20 mL) was simultaneously added. After the mixture was stirred for
10 min at 0 °C, benzyl bromide (2.1 mL, 18 mmol) was added
dropwise to the mixture and the reaction was then allowed to warm to
room temperature. After being stirred for 5 h, the reaction mixture was
quenched with saturated aqueous NH4Cl and extracted three times
with CH2Cl2. The organic layer was washed with saturated NaHCO3,
water, and brine, dried over Na2SO4, filtered, and concentrated in
vacuo. The residue was purified by column chromatography on silica
gel (5% ethyl acetate in hexanes) to give 1-phenyl 2,4-di-O-benzyl-3,6-
Phenyl 3-O-Phenoxythiocarbonyl-1-thiol-β-L-fucopyrano-
side (3b). In a foil-covered flask, phenyl 1-thio β-L-fucopyranoside
(4) (100 mg, 0.39 mmol) and dioctyltin dichloride (16 mg, 0.039
mmol) were suspended in dry acetone (8 mL) at 25 °C for 10 min.
Then, reaction was treated with tetrabutylammonium iodide (14 mg,
0.039 mmol), O-phenyl chlorothionoformate (0.07 mL, 0.5 mmol),
and 1,2,2,6,6-pentamethylpiperidine (0.06 mL, 0.5 mmol). After 3 h,
the reaction mixture was quenched with saturated aqueous NH4Cl and
extracted three times with ethyl acetate. The organic layer was washed
with water and brine, dried over Na2SO4, filtrated, and concentrated in
vacuo. The crude sample was purified by column chromatography on
silica gel (20% ethyl acetate in hexanes) to give 3 (138 mg, 90%).
Phenyl 3-O-Phenoxythiocarbonyl-1-thiol-β-L-fucopyrano-
side 3b (1 g scale). The title compound was prepared according
to the above procedure using phenyl 1-thio β-L-fucopyranoside (4)
(1.0 g, 3.9 mmol), dioctyltin dichloride (162 mg, 0.39 mmol),
tetrabutylammonium iodide (144 mg, 0.39 mmol), O-phenyl
chlorothionoformate (0.7 mL, 5 mmol), 1,2,2,6,6-pentamethylpiper-
idine (0.6 mL, 5 mmol), and acetone (80 mL). After 6 h, the crude
product was purified with column chromatography (20% ethyl acetate
dideoxy-1-thio-β-L-xylo-hexopyranoside 1 (1.5 g, 90%): [α]24
=
D
+0.283° (c 1.00, CH2Cl2); 1H NMR (CDCl3, 500 MHz) δ 7.58 (d, J =
6.5 Hz, 2H), 7.31−7.23 (m, 14H), 4.68 (t, J = 14 Hz, 2H, H1, OCH2),
4.49 (t, J = 14 Hz, 2H, OCH2), 4.37 (d, J = 12 Hz, 1H, OCH2), 3.69
(t, J = 10 Hz, 1H, H2), 3.61 (brs, 1H, H5), 3.40 (s, 1H, H4), 2.42 (d, J
= 12 Hz, 1H, H3), 1.49 (t, J = 12.5 Hz, 1H, H3), 1.27 (d, J = 5 Hz, 3H,
H6); 13C NMR (CDCl3, 125 MHz) δ 138.6, 138.3, 134.7, 131.7,
128.8, 128.5, 128.4, 128.3, 127.9, 127.8, 127.7, 127.0, 89.4, 76.7, 75.2,
72.7, 71.6, 71.2, 34.5, 17.2; LRMS (ESI, pos ion) m/z calcd for
C26H28O3S (M + Na) 443.21, found 443.27; HRMS (ESI, pos ion) m/
z calcd for C26H28O3S (M + NH4) 438.2103, found 438.2075.
in hexanes) to give 3b (1.25 g, 83%): [α]24 = +0.033° (c 1.06,
ASSOCIATED CONTENT
* Supporting Information
NMR spectra for 1, 3a, 3b, and 7. This material is available free
D
■
1
CH2Cl2); H NMR (CDCl3, 500 MHz) δ 7.59 (m, 2H), 7.43−7.28
S
(m, 6H), 7.13 (d, J = 7.5 Hz, 2H), 5.39 (d, J = 9.5 Hz, 1H, H1), 4.62
(d, J = 10 Hz, 1H, H3), 4.20 (s, 1H, H2), 4.00 (t, J = 9.7 Hz, 1H, H4),
3.82 (q, J = 6.5 Hz, 1H, H5), 2.56 (brs, 1H), 1.96 (brs, 1H), 1.39 (d, J
= 6.5 Hz, 3H, H6); 13C NMR (CDCl3, 125 MHz) δ 194.4, 153.3,
133.1, 133.1, 131.4, 129.6, 129.2, 128.5, 126.8, 121.9, 88.7, 86.0, 74.7,
69.2, 67.0, 16.6; LRMS (ESI, pos ion) m/z calcd for C19H20O5S2 (M +
Na) 414.06, found 415.18; HRMS (ESI, pos ion) m/z calcd for
C19H20O5S2 (M + NH4) 410.1096, found 410.1072.
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: clay.bennett@tufts.edu.
■
Notes
General Procedure for the Synthesis of Phenyl 3,6-Dideoxy-
1-thio-β-L-xylo-hexopyranoside (7). Thiocarbonylate 3 (0.25
mmol), tris(trimethylsilyl)silane (TTMSS) (method A) or tributyltin
hydride (Bu3SnH) (method B) (2 equiv), and 2,2′-azobis-
(isobutyronitrile) (20 mol %) were refluxed at 82 °C for 2 h in
benzene (5 mL). When the reaction was complete according to TLC,
it was concentrated in vacuo. The crude sample was purified by
column chromatography on silica gel (40% ethyl acetate in hexanes
followed by 70% ethyl acetate in hexanes) to afford pure 1-phenyl 3,6-
dideoxy-1-thio-β-L-xylo-hexopyranoside 7.
Method A. The title compound was prepared according to the
general procedure using tris(trimethylsilyl)silane (0.07 mL, 0.25
mmol), phenyl 3-O-phenoxythiocarbonyl-1-thio-β-L-fucopyranoside
3b (47 mg, 0.12 mmol) and 2,2′-azobis(isobutyronitrile) (4 mg,
0.02 mmol), and benzene (5 mL) were used. The product was
obtained as a white powder 7 (27 mg, 98%): [α]24D = +0.438° (c 1.04,
CH2Cl2); 1H NMR (CDCl3, 500 MHz) δ 7.56 (dd, J = 2.5, 2 Hz, 2H),
7.35−7.30 (m, 3H), 4.52 (d, J = 10 Hz, 1H, H1), 3.79−3.69 (m, 3H,
H2, H4, H5), 2.45−2.39 (m, 2H, H3), 1.78 (d, J = 8 Hz, 1H), 1.65 (td,
J = 8.5, 3 Hz, 2H, H3), 1.30 (d, J = 6.5 Hz, 3H, H6); 13C NMR
(CDCl3, 125 MHz) δ 132.6, 132.5, 129.1, 128,1, 92.1, 77.2, 69.3, 64.2,
39.0, 17.0; LRMS (ESI, pos ion) m/z calcd for C12H16O3S (M + Na)
263.07, found 263.09; HRMS (ESI, pos ion) m/z calcd for C12H16O3S
(M + NH4) 258.1164, found 258.1139.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Tufts University and the National Science
Foundation division of Chemistry (NSF 1300334) for generous
financial support.
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tris(trimethylsilyl)silane (1.5 mL, 5 mmol), phenyl 3-O-phenoxythio-
carbonyl-1-thio-β-L-fucopyranoside 3b (1 g, 2.5 mmol), 2,2′- azobis-
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