Catalytic thiolysis of chemoenzymatically derived vinylepoxides. Efficient synthesis of homochiral phenylthioconduritol F

Article abstract of DOI:10.1016/j.tetasy.2006.01.024

The chemoenzymatic synthesis of enantiomerically pure phenylthioconduritol F obtained in 44% overall yield is described. The key step of the synthesis is the Yb(III) thiolysis of a vinyl epoxide, which was studied in depth. The methodology is amenable to scale up and expandable to the preparation of other thiocyclitols.

Full text of DOI:10.1016/j.tetasy.2006.01.024

Tetrahedron: Asymmetry 17 (2006) 474–478
Catalytic thiolysis of chemoenzymatically derived vinylepoxides.
Efficient synthesis of homochiral phenylthioconduritol F
Ana Bellomo and David Gonzalez*
Departamento de Quı´mica Orga´nica, Facultad de Quı´mica, Universidad de la Repu´blica, C.C. 1157, Montevideo, Uruguay
Received 27 December 2005; accepted 24 January 2006
Abstract—The chemoenzymatic synthesis of enantiomerically pure phenylthioconduritol F obtained in 44% overall yield is
described. The key step of the synthesis is the Yb(III) thiolysis of a vinyl epoxide, which was studied in depth. The methodology
is amenable to scale up and expandable to the preparation of other thiocyclitols.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
b-Hydroxysulfides are important compounds in organic
synthesis.1,2 They can be found as subunits in many bio-
logically active natural and synthetic products,3 and are
of interest in the synthesis of leukotrienes and
thiosugars.4,5
Phenylthioconduritol F 8 was prepared from bromobenz-
ene as depicted in Scheme 1. The key intermediate in
this sequence is the known epoxide 322 available in
two steps from the homochiral metabolite 1 biosynthe-
sized by whole-cell fermentation of bromobenzene with
Pseudomonas putida F39/D.23 cis-Dienediol 1 was
protected as its corresponding acetonide 2 with 2,2-
dimethoxypropane in the presence of a catalytic amount
of p-toluenesulfonic acid. Oxidation of the more reac-
tive double bond at 0 °C with m-chloroperbenzoic
acid, rendered exclusively the a-oxirane as previously
described.22,24–26
A direct route for the preparation of b-hydroxysulfides
is the ring opening of epoxides by sulfur-containing
nucleophiles.4,6,7 As special types of heterocycles, epox-
ides are susceptible to reaction with a number of nucleo-
philes (amines, alcohols, thiols, cyanides, etc.) leading
to a wide range of functionalized intermediates.8–11 It
is well documented that the nucleophilic ring opening
of the oxirane ring with thiophenol can be promoted
by Brønsted acids, Lewis acids, metal salts,12–15 alu-
mina,16,8 and under solvent-free conditions.17 However,
despite the rich literature on the chemistry of epoxide
ring opening, there are only a few reports concerning
the nucleophilic ring opening of vinyloxiranes with sul-
fur reagents.18,19
With the a-epoxide in hand, we subjected it to several
acid catalyzed ring-opening conditions with thiophenol
to improve the yield of the desired hydroxysulfide
(Scheme 1).
In order to optimize the reaction yield and stereoselec-
tivity of the ring-opening process, we performed the
reaction under a variety of conditions regarding the nat-
ure of the acid, the reaction temperature, and the solvent
used. Yb(OTf)3 and CeCl3 were chosen as representative
Lewis acids and p-toluenesulfonic acid (p-TsOH) as a
typical Brønsted catalyst. The reactions were carried
out from below room temperature, up to 60 °C over
periods ranging from 20 min to several days. We ana-
lyzed the nature and yield of the products and deter-
mined the best reaction conditions.
In connection with our ongoing effort on the prepara-
tion of chiral building blocks of chemoenzymatic
origin,20,21 we herein report a practical route to
homochiral phenylthioconduritol derivatives. We have
optimized the conditions regarding catalysts, reaction
temperature, and solvent, and applied them to the
synthesis of the previously unreported thiocyclitol 8.
*
Table 1 shows some of our initial observations. In the
absence of a catalyst, the reaction does not proceed
Corresponding author. Tel.: +598 29247881; fax: +598 29241906;
e-mail: davidg@fq.edu.uy
0957-4166/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.01.024

A. Bellomo, D. Gonzalez / Tetrahedron: Asymmetry 17 (2006) 474–478
475
Scheme 1. Synthesis and thiolysis of vinyl epoxide 3. Reagents and conditions: (a) toluene dioxygenase expressed in P. putida F39/D, 3 g/L; (b) 2,2-
dimethoxypropane, p-TsOH, acetone, rt, 30 min, 98%; (c) m-chloroperbenzoic acid, CH2Cl2, rt, overnight, 85%; (d) see Tables 1 and 2.
practically at or below room temperature (entries 1–4).
Even after 14 days most of the starting material re-
mained in the solution with only a maximum of 30%
combined yield of sulfides 4 and 5 was obtained. If the
uncatalyzed reaction was carried out at 60 °C, the start-
ing material disappeared rapidly. Nevertheless, stereo-
selectivity was totally lost and the isolated yield of
sulfides did not improve (entry 4).
We studied the effect of the reaction temperature in this
system and found that it had a large influence on the
reactivity and selectivity of the reaction (Table 1, entries
8–11). When the reaction of 3 with thiophenol was car-
ried out at room temperature, it went to completion in
an hour and hydroxysulfide 4 was afforded in 78% yield.
However, the same reaction run at ꢀ5, 0, and 4 °C pro-
duced only small amounts of the desired product. The
good catalytic effect of the Yb(III) triflate in the present
thiolysis can be attributed to the strong oxophilicity of
the lanthanide(III) in this compound, which allows the
metal to tightly coordinate to the oxirane oxygen, favor-
ing the nucleophilic ring-opening process.
We initially tested the ability of p-TsOH, a common and
economical Brønsted acid, and it showed a significant
catalytic effect. Complete consumption of the starting
material was observed in 1 h, but the yield of the desired
sulfides was prohibitively low (entry 5). Interestingly, an
epimeric mixture of tosylates 6 was formed in a signifi-
cant, 15% isolated, yield. Consequently, we decided to
use Lewis acids as ring-opening promoters. Although
the use of cerium chloride as a catalyst in various organ-
ic transformations is well documented,27 when applied
to this system, 90% of the starting material was recov-
ered unchanged (entry 6). Thus, we decided to use it in
combination with potassium iodide as a cocatalyst to
reinforce the activity of CeCl3.28 Although the reaction
was complete in only 20 min at room temperature, it
rendered the desired product in only 10% isolated yield,
with very low stereoselectivity (entries 6 and 7). Our best
results were obtained using Yb(OTf)3 (entry 11). The
epimeric sulfides were obtained in 89% combined yield
and in a good (7.5:1) anti/syn ratio.
Once the nature of the catalyst was defined, we at-
tempted to uncover the best reaction solvent and to min-
imize the amount of catalyst used in the reaction. We
concluded that the use of less than 30% molar of the cat-
alyst slowed down the reaction rate, as well as reducing
the obtained yield as described in Table 2 (entries 1–5).
The solvent effect is not remarkable but, as expected, the
use of an oxygenated solvent is detrimental to the reac-
tion results (entry 5–7).
Literature precedent,29 as well as our previous experi-
ence in the regiospecific opening of vinyl epoxides of
type 3, indicated that the regiochemistry is controlled
by stereoelectronic effects, through a 1,2-addition
process (SN2 process) to give the anti-1,2-adducts, as
Table 1. Thiolysis of epoxide 3 under various catalytic conditionsa
Entry
Catalystb
Reaction time
T (°C)
anti/syn (4/5) ratio
Isolated yield (%)
1
2
3
4
5
6
7
8
No catalyst
No catalyst
No catalyst
No catalyst
p-TsOH
CeCl3Æ7H2O
CeCl3Æ7H2O/KI
Yb(OTf)3
8 h
0
1.0
1.0
1.2
1.1
1.0
Traces
Traces
30
30
15
Traces
10
Traces
Traces
30
48 h
2 weeks
1 h
1 h
8 h
20 min
3 weeks
8 h
24 h
1 h
rt
rt
60
rt
rt
rt
ꢀ5
0
1.7
7.0
7.0
7.5
7.5
9
10
11
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
4
rt
89
a All reactions were carried out in toluene as solvent.
b 30% mol unless otherwise stated.

476
A. Bellomo, D. Gonzalez / Tetrahedron: Asymmetry 17 (2006) 474–478
Table 2. Effect of the amount of catalyst and nature of the solvent in the reaction resulta
Entry
Catalyst (mol %)
Solvent
Reaction time (h)
anti/syn (4/5) ratio
Isolated yield (%)
1
2
3
4
5
6
7
5
10
15
20
30
30
30
Toluene
Toluene
Toluene
Toluene
Toluene
CH2Cl2
THF
48
24
24
24
1
Traces
Traces
50
50
89
7.0:1
7.5:1
7.5:1
7.0:1
5.0:1
2
8
80
50
a All reactions were carried out at rt.
we observed experimentally. In the conditions used, the
syn-hydroxysulfide was always formed in addition to the
expected anti-isomer, resulting from the trans-diaxial
attack of the nucleophile.30,31 The formation of 5, under
Lewis acid catalysis could be rationalized considering
that the catalyst is able to polarize the oxirane C–O
bond until its complete rupture determining the genera-
tion of a stable allylic carbocation species. The forma-
tion of the syn-adduct was minimized when the acidic
catalyst present in the reaction medium was Yb(OTf)3
(Table 1).
4. Experimental
4.1. General method
All non-hydrolytic reactions were carried out under a
nitrogen atmosphere with standard techniques for the
exclusion of moisture. All solvents were purified and
dried prior to use. The commercially available reagents
were purchased from Aldrich and used without further
purification. Melting points were determined on a Leitz
Microscope Heating Stage, Model 350 apparatus and
are uncorrected. Nuclear magnetic resonance spectra
were recorded on Bruker Avance DPX-400 instrument
with chloroform-D as a solvent. Chemical shifts are
given in parts per million downfield from tetramethyl-
silane. Optical rotations were measured using a Zuzi
412 automatic polarimeter with a 7 mL cell (concentra-
tion c given as g/100 mL). Infrared spectra were
recorded in KBr and measured in cmꢀ1, using a
Matheson Excalibur spectrometer. Low-resolution mass
spectra were recorded on a Shimadzu GS-MS QP 1100
EX instrument using the electron impact mode. High-
resolution mass spectra were recorded in a Q-TOF mass
spectrometer (Micromass, Manchester, UK). Elemental
analyses were performed in a Fisons EA 1108 CHNS-O
analyzer. Analytical TLC was performed on silica gel
60F-254 plates and visualized with UV light (254 nm)
and/or anisaldehyde–H2SO4–AcOH as detecting agent.
Flash column chromatography was performed in silica
gel (Kieselgel 60, EM Reagents, 230–400 mesh).
In order to convert sulfide 4 into a deoxyconduritol
analogue, it was subjected to reductive dehalogenation
conditions affording 7, which was treated with a cat-
ion-exchange resin in order to remove the acetonide
group. Subsequent resin filtration, concentration, and
purification by flash chromatography rendered optically
active (+)-8 as a white solid in 44% overall yield from
metabolite 1 (Schemes 1 and 2).
The structures of all new compounds were unequivo-
1
cally established by H NMR and 13C NMR spectro-
scopy, and fully characterized. In the cases of
compounds 4 and 5 NOE experiments were performed
in order to assign the correct stereochemistry.
3. Conclusion
In conclusion, an effective chemoenzymatic route to
enantiomerically pure phenylthioconduritol F has been
developed using only a sub-stoichiometric amount of
Yb(OTf)3 to catalyze the thiolysis of vinylepoxide 3.
The strategy is of wide scope and can be expanded to
the preparation of other thioconduritol analogues. The
use of compound 7 as a building block in the synthesis
of cyclitol conjugates with sugars and terpenes, as well
as the inhibition properties of 8 toward glycosidases
are the focus of our current research.
4.2. General procedure for the thiolysis of epoxide 3
A solution of vinyl epoxide 3 (119.5 mg, 0.48 mmol) in
anhydrous toluene (3.0 mL) was treated with PhSH
(0.1 mL, 0.97 mmol) and Yb(OTf)3 (0.14 mmol,
30 mol %). The reaction mixture was stirred for 1 h at
room temperature until consumption of the starting
material monitored by TLC. The mixture was diluted
with CH2Cl2, washed with saturated aq NaHCO3, and
Scheme 2. Optimized route for the preparation of phenylthioconduritol F. Reagents and conditions: (a) PhSH, Yb(OTf)3, toluene, rt, 1 h, 78%; (b)
Bu3SnH, AIBN, THF, reflux, 4 h, 75%; (c) Dowex 50-H+, MeOH–H2O, rt, 1 h, 90%.

A. Bellomo, D. Gonzalez / Tetrahedron: Asymmetry 17 (2006) 474–478
477
aq NaCl. The organic layer was dried over anhyd
MgSO4 and removed under reduced pressure to afford
a crude oily product consisting of a mixture of phenyl-
thioderivatives 4 and 5, which was purified by flash
column chromatography (80:20 hexane/ethyl acetate)
(75), 59 (61), 55 (76); HRMS: calcd for C15H18O3S
(M++Na+): 301.0863; found: 301.0744. Dm (%): 0.004.
4.6. (1S,2R,3R,6S)-6-(Phenylthio)-cyclohex-4-ene-
1,2,3-triol 8
furnishing pure
4 (137.7 mg, 78% yield) and 5
(18.8 mg, 11% yield).
A mixture of compound 7 (60.4 mg, 0.22 mmol),
Dowex-50 (H+ form) resin (650 mg) and MeOH–H2O
(3.0–0.1 mL) was stirred at room temperature for 1 h.
After completion of the reaction, the resin was filtered
off and the solvent was removed under reduced pressure.
Purification by flash chromatography (20:80 hexane/
4.3. (1S,2R,3S,6S)-6-(Phenylthio)-2,3-isopropyliden-
dioxy-4-bromocyclohexene-1-ol 4
20
Colorless oil; ½aꢁD ¼ þ73:6 (c 3.2, CHCl3); IR (neat)
3500–3400 b, 1219.2, 1074.5, 868.1, 748.5, 692.5; 1H
NMR (CDCl3) d (ppm): 1.43 (s, 3H, CH3), 1.51
(s, 3H, CH3), 2.80 (br s, 1H, OH), 3.56 (d, 1H, J
8.4 Hz, H-6), 3.66 (t, 1H, J 8.4 Hz, J 8.4 Hz, H-1),
4.21 (t, 1H, J21 8.1 Hz, J 6.1 Hz, H-2), 4.66 (d, 1H, J
6.2 Hz, H-3), 6.30 (d, 1H, J 2.4 Hz, H-5), 7.35 (m, 3H,
Ph), 7.49 (m, 2H, Ph); 13C NMR (CDCl3) d (ppm):
26.3, 28.6, 52.2, 71.2, 77.7, 78.9, 111.1, 119.6, 128.7
(di), 129.7 (di), 132.6, 133.5 (di); EIMS (relative inten-
sity): 218 (M+ꢀBrꢀC3H6O, 44), 110 (PhS, 100), 77
(Ph, 14), 65 (70); Anal. Found C, 50.68; H, 4.88;
C15H17BrO3S requires C, 50.43; H, 4.80.
ethyl acetate) rendered optically active (+)-8 as a white
22
solid (47.2 mg, 90%). Mp: 108–111 °C; ½aꢁD ¼ þ127:2
(c 0.64, MeOH); IR (neat) 3476–3279 b, 1124.6,
1066.8, 877.7, 738.8, 688.7; 1H NMR (CDCl3) d
(ppm): 2.44 (br s, 3H, OH), 3.58 (d, 1H, J 8.6 Hz, H-
6), 3.68 (m, 1H, H-2), 3.79 (t, 1H, J 9.4 Hz, J 9.2 Hz,
H-1), 4.28 (br d, 1H, J 4.3 Hz, H-3), 5.87 (m, 1H, J
11.9 Hz, J 7.8 Hz, H-5), 5.92 (d, 1H, J 11.7 Hz, H-4),
7.32–7.35 (m, 3H, Ph), 7.51 (d, 2H, Ph); 13C NMR
(CDCl3) d (ppm): 53.0, 66.4, 70.4, 73.4, 127.3, 128.4
(di), 129.5 (di), 132.5, 132.9, 133.6; EIMS (relative inten-
sity): 238 (M+, 16), 110 (PhS, 100), 77 (Ph, 11), 65 (46),
55 (40); Anal. Found C, 60.43; H, 5.81; C12H14O3S
requires C, 60.48; H, 5.92.
4.4. (1S,2R,3S,6R)-6-(Phenylthio)-2,3-isopropyliden-
dioxy-4-bromocyclohexene-1-ol 5
20
Acknowledgments
White solid; mp: 118–120 °C; ½aꢁD ¼ ꢀ103:3 (c 0.7,
CHCl3); IR (neat) 3500–3400 b, 1221.1, 1076.4, 870.0,
750.4, 692.5; H NMR (CDCl3) d (ppm): 1.41 (s, 3H,
Financial support for this work was obtained from the
International Foundation for Science in collaboration
with the Organisation for the Prohibition of Chemical
Weapons (IFS Grant F/3078-2) and DINACYT (Pro-
ject FCE 8068). A.B. is grateful to PEDECIBA and
CONICYT for a Doctoral Fellowship. The authors
1
CH3), 1.47 (s, 3H, CH3), 4.03 (m, 1H, H-6), 4.12 (t,
1H, J 5.4 Hz, J 9.5 Hz, H-1), 4.39 (t, 1H, J 11.1 Hz, J
5.6 Hz, H-2), 4.58 (d, 1H, J 5.5 Hz, H-3), 6.16 (d, 1H,
J 3.5 Hz, H-5), 7.36 (m, 3H, Ph), 7.51 (m, 2H, Ph);
13C NMR (CDCl3) d (ppm): 26.5, 28.1, 51.4, 68.0, 76.5
(di), 110.5, 123.9, 128.7, 128.9, 129.1, 129.4, 129.8,
132.6, 133.3; EIMS (relative intensity): 218
(M+ꢀBrꢀC3H6O, 44), 110 (PhS, 100), 77 (Ph, 20), 65
(75); Anal. Found C, 50.97; H, 5.03; C15H17BrO3S
requires C, 50.43; H, 4.80.
´
acknowledge Dr. Alejandra Rodrıguez (PTP-FQ) and
Dr. Marcos Eberlin (UNICAMP) for performing the
HRMS analyses and Mr. Horacio Pezaroglo (FC) for
recording the NMR spectra.
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