Asymmetric synthesis of 1,3-oxathiolan-5-one derivatives through dynamic covalent kinetic resolution

Article abstract of DOI:10.1016/j.tet.2014.03.059

The asymmetric synthesis of 1,3-oxathiolan-5-one derivatives through an enzyme-catalyzed, dynamic covalent kinetic resolution strategy is presented. Dynamic hemithioacetal formation combined with intramolecular, lipase-catalyzed lactonization resulted in

Full text of DOI:10.1016/j.tet.2014.03.059

Accepted Manuscript
Asymmetric Synthesis of 1,3-Oxathiolan-5-one Derivatives through Dynamic Covalent
Kinetic Resolution
Yan Zhang, Fredrik Schaufelberger, Morakot Sakulsombat, Chelsea Liu, Olof
Ramström
PII:
S0040-4020(14)00407-4
10.1016/j.tet.2014.03.059
TET 25400
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 16 January 2014
Revised Date: 3 March 2014
Accepted Date: 17 March 2014
Please cite this article as: Zhang Y, Schaufelberger F, Sakulsombat M, Liu C, Ramström O, Asymmetric
Synthesis of 1,3-Oxathiolan-5-one Derivatives through Dynamic Covalent Kinetic Resolution,
Tetrahedron (2014), doi: 10.1016/j.tet.2014.03.059.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
Graphical Abstract
Asymmetric Synthesis of 1,3-Oxathiolan-5-
one Derivatives through Dynamic Covalent
Kinetic Resolution
Leave this area blank for abstract info.
Yan Zhanga, Fredrik Schaufelbergera, Morakot Sakulsombata, Chelsea Liua and Olof Ramströma,*
aDepartment of Chemistry KTH - Royal Institute of Technology Teknikringen 30, Stockholm 10044, Sweden

1
ACCEPTED MANUSCRIPT
Tetrahedron
journal homepage: www.elsevier.com
Asymmetric Synthesis of 1,3-Oxathiolan-5-one Derivatives through Dynamic
Covalent Kinetic Resolution
Yan Zhanga, Fredrik Schaufelbergera, Morakot Sakulsombata, Chelsea Liua and Olof Ramströma,*
aDepartment of Chemistry KTH - Royal Institute of Technology Teknikringen 30, Stockholm 10044, Sweden
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The asymmetric synthesis of 1,3-oxathiolan-5-one derivatives through an enzyme-
catalyzed, dynamic covalent kinetic resolution strategy is presented. Dynamic
hemithioacetal formation combined with intramolecular, lipase-catalyzed lactonization
resulted in good conversions with moderate to good enantiomeric excess (ee) for the final
products. The process was evaluated for different lipase preparations, solvents, bases, and
reaction temperatures, where lipase B from Candida antarctica (CAL-B) proved most
efficient. The substrate scope was furthermore explored for a range of aldehyde structures,
together with the potential access to nucleoside analog inhibitor core structures.
Available online
Keywords:
dynamic chemistry
dynamic kinetic resolution
enzyme catalysis
hemithioacetal formation
lactonization
2009 Elsevier Ltd. All rights reserved.
nucleoside analogs
1. Introduction
Although classical kinetic resolution processes are often very
efficient, the maximum theoretical yield is limited to 50% while
Asymmetric synthesis of chiral core structures remains an
important field in organic chemistry, particularly for the
production of active pharmaceutical ingredients. One such
interesting motif is constituted by the 1,3-oxathiolan core. This
structure type, together with the related 1,3-oxathiolan-5-ones,
are attractive targets, not only for their existence in natural
products and broad biological activities, but also due to their
importance as intermediates for a range of highly successful and
useful pharmaceuticals. For example, they possess inhibitory
activities toward human type-II (non-pancreatic) secretory
phospholipase A2 (PLA2);1 the oxathionyl-nucleosides
emtricitabine (Coviracil) and lamivudine (3TC) remain two of
the most potent antiviral drugs as nucleoside reverse transcriptase
inhibitors (NRTIs) for the treatment of diseases such as human
immunodeficiency virus (HIV) or hepatitis B.2-3 Since the initial
discovery of the antiviral activity of this motif, the synthesis of
enantiomerically pure 1,3-oxathiolan-5-one derivatives has
received significant attention.4-6 To induce a chiral element into
the highly sensitive oxathiolane skeleton, biocatalytic
transformations have long been a method of choice owing to its
high degree of stereoselectivity, high efficiency, mild reaction
conditions and advantageous environmental properties. For
example, classical kinetic resolution protocols were used in the
preparation of a lamivudine intermediate through hydrolysis of
the undesired enantiomer.7-11
maintaining the highest possible enantiomeric excess (ee) of the
respective transformation. This inherent limitation of the kinetic
resolution concept can however be circumvented through the
introduction of dynamics in the system. Continuous racemization
of the starting material in conjunction with the resolution process,
results in dynamic kinetic resolution (DKR), where the maximum
theoretical yield can reach 100% without reducing the
enantiomeric purity.12-17 The value of DKR protocols in enzyme-
catalyzed asymmetric synthesis has been successfully illustrated
by for example transition metal-catalyzed racemization of the
substrates.18-19 Another approach involves dynamic covalent
reactions, where the racemic substrate is continuously reformed
through the reversible nature of participating chemical bonds
20-21
(Scheme 1).16,
Both these approaches can be efficiently
coupled to enzyme catalysis, leading to kinetic resolution of the
optimal product. The dynamic covalent process can furthermore
be extended to the resolution of complex systems.22-27
O
ꢀ
OH
O
fast
O
O
R
R
S
S
S
R
R
O
O
O
O
HS
O
+
R
H
O
OH
O
slow
S
Scheme 1. Asymmetric synthesis of 1,3-oxathiolan-5-ones through
Corresponding author. Fax: (+46) 8 7912333; e-mail address:
ramstrom@kth.se (O. Ramström)
dynamic covalent kinetic resolution (DCKR).

2
Tetrahedron
ACCEPTED MANUSCRIPT
Recently, we reported the in situ formation of 1,3-
Lipase catalysis is highly solvent-dependent, and this aspect
oxathiolan-5-one derivatives through lipase-catalyzed γ-
lactonization from a complex dynamic hemithioacetal system.26
Due to the reversible hemithioacetal transformation, the
unselected structures were instantly recycled back to the starting
materials, and the final products after the enzymatic
transformation were obtained with both high chemo- and
was next addressed for the current system using the model
reaction between compounds 1a and 2. Four enzyme-compatible
solvents were thus tested: toluene, tert-butyl methyl ether
(TBME), diethyl ether, and tetrahydrofuran (THF). The cyclized
product 4a was detected in all of the four solvents, of which
toluene, the reaction media with lowest polarity, resulted in better
ee while the reaction rate was similar to the others. This can be
partly ascribed to its low miscibility with water, thus not
interrupting the bound water layer on the surface of lipase.
stereoselectivities in
a one-pot process. This protocol is
particularly useful for enzyme classification and substrate
identification, but further exploration of the scope and synthetic
utility of this reaction is warranted. Herein we present an
optimized asymmetric synthesis of 1,3-oxathiolan-5-one through
dynamic covalent kinetic resolution, using hemithioacetal
chemistry coupled with a lipase-catalyzed cyclization, as well as
its further usage for the construction of oxathiolane nucleoside
skeletons.
The temperature, another crucial parameter for both the
equilibrium distribution and the stereoselectivity of the
enzymatic transformation, was subsequently studied. The model
reaction was therefore performed at temperatures ranging from
40 °C to -25 °C. In consistency with previous results, the
enantioselectivities were improved at lower temperatures, a result
that can be explained by a more rigid enzyme structure at lower
temperatures, resulting in a better fitting of the optimal
2. Results/Discussion
enantiomer. The larger activation-energy differences (ꢀꢀG ꢀ
)
The kinetic resolution function of the overall protocol was
based on enzyme-catalyzed transformation of transient,
intermediate hemithioacetals. Lipases, which belong to the
hydrolase class of enzymes, have in this context proven very
efficient and selective for transesterification of secondary
alcohols in organic solvents, and together with other advantages
such as commercial availability, absence of cofactor
requirements and broad substrate scope, they were chosen for the
resolution process. However, lipases from different sources may
behave differently toward the same transformation, and in order
to optimize the reaction conditions, screening of a range of
lipases for the kinetically controlled lactonization step was
initially performed. The lipases from Pseudomonas fluorescens
(PFL), Burkholderia (Pseudomonas) cepacia (PS, PS-CI and PS-
IM), Candida antarctica (CAL-B), and Candida rugosa (CRL),
were thus probed for the cyclization using a model reaction
involving isobutyraldehyde (1a) and methyl 2-sulfanylacetate (2).
The results clearly demonstrated the lipase differentiation (Table
1), and outside of the fact that PS and CRL did not catalyze the
reaction under the tested conditions, the use of PS-IM, PS-CI and
PFL led to slow reaction progress and very low ee of the product.
The activities are in this case likely challenged by the double role
of methyl 2-sulfanylacetate; serving both as substrate precursor
and acyl donor. On the other hand, CAL-B provided reasonable
conversion and good stereoselectivity, with an enantiomeric ratio
(E-value) of 12. Thus, CAL-B was chosen for further studies of
the reaction.
with decreased temperatures can also contribute to the higher
enantioselectivity.28-29 The best ees were obtained at -25 °C, at
which point the reaction rate was still reasonably high.
Reversible hemithioacetal formation has proven useful for
efficient formation of dynamic systems in both aqueous and
organic media.26, 30 In organic solution, base catalysis has been
shown to accelerate the reversibility of the reaction, and displace
the equilibrium of the system toward the hemithioacetals. Thus,
the effect of organic bases on the conversion and stereoselectivity
was also investigated in the current study. Besides triethylamine
(Et3N), which was used in the aforementioned dynamic system,
other bases such as morpholine, 4-methylmorpholine (NMM), 4-
methylimidazole and 4-dimethylaminopyridine (DMAP) were
tested (Table 2). Among these, the reactions with 4-
methylmorpholine resulted in the best ee, but showed slightly
lower conversion than using Et3N within the same timeframe.
Incidentally, 4-methylmorpholine was also the weakest base with
a pKa around 7.4. It was also found that increasing the amount of
base led to faster reactions, but at the cost of diminished
enantioselectivity. This can be explained by the facilitation of the
non-enzymatic background cyclization upon higher degree of
base, although also resulting in an increase in reversibility rates.
In the case of single reactions compared to complex dynamic
systems, smaller amounts of base were sufficient for attaining
high reversibility rates.
Table 2. Enzyme-catalyzed DKR with different bases.a
Table 1. Enzyme-catalyzed dynamic covalent kinetic resolution with
O
different lipases.a
OH
O
O
O
base
O
CAL-B
O
HS
+
S
H
S
OH
O
O
O
O
Et3N
Lipase
O
O
HS
+
S
H
S
1a
2
3a
4a
O
O
1a
Base
amount
Conversion
(%)b
ee
(%)c
2
3a
4a
Entry
Base
Entry
Enzyme
Conversion (%)b
ee (%)c
1
2
3
4
5
6
7
triethylamine
morpholine
0.1 eq
0.1 eq
0.1 eq
0.1 eq
0.1 eq
0.5 eq
1.0 eq
48
12
39
37
43
42
46
80
-
1
2
3
4
5
6
CAL-B
PS-IM
PS-CI
PS
35
11
8
77
14
17
-
4-methylmorpholine
4-methylimidazole
95
80
86
83
83
0
4-dimethylaminopyridine
4-methylmorpholine
4-methylmorpholine
CRL
0
-
PFL
13
20
a Reactions were carried out with 1a (0.1 mmol), 2 (1.2 equiv.), Et3N
a Reactions were carried out with 1a (0.1 mmol), 2 (1.2 equiv.), base, 4 Å MS
(20 mg), and CAL-B (5 mg) in toluene (0.6 mL) at -25 °C for 4 d.
b Determined by 1H NMR spectroscopy.
c The ees of 4a were determined by HPLC analysis using Daicel Chiralpak OJ
column, 99:1 Hex:iPrOH.
(0.5 equiv.), 4 Å MS (20 mg), and lipase preparation (50 mg; 5 mg for CAL-
B) in toluene (0.6 mL) at -25 °C for 3 d.
b Determined by 1H NMR spectroscopy.
c The ees of 4a were determined by HPLC analysis using Daicel Chiralpak OJ
column, 99:1 Hex:iPrOH.

3
Other efforts to optimize the reaction conditions were also
carried out, including lipase loading, substrate concentration, and
water activity of the solution. However, it was generally found
that the enantioselectivity decreased at higher conversion. At this
point it was suspected that methanol, formed as a side product
from the cyclization step, could potentially affect the reaction. It
is known that short linear alcohols can deactivate lipases through
interference with the hydration layer of the enzyme.31-34
Therefore, two equivalents of methanol were added to the DKR
system at the beginning of the reaction in order to test this effect.
A dramatically decreased conversion was indeed observed,
supporting the hypothesis of the inhibitory effects of methanol
for CAL-B. For the purpose of removing the methanol produced
during the reaction, two different methods were adopted for the
system. The first one involved addition of anhydrous calcium
chloride salt (CaCl2) into the solution. CaCl2 is not only capable
of interacting with methanol to form a stable crystalline complex,
but also serves as a salt-hydrate pair in the solution to control the
water activity.35-36 After the addition of CaCl2, the conversion
increased from 51% to 73%, while the ee remained at a high
level of 88%. However, there appeared to be a limitation of
methanol binding ability and stability, as the result varied
significantly between different aldehydes and also upon changes
of reaction conditions. The other method for methanol removal
was the addition of a large excess of molecular sieves (MS),
which would be able to trap the produced methanol. By addition
of 300 mg 4 Å MS per reaction, the reaction went smoothly with
a conversion of 73% over a reasonable timeframe, along with a
good ee of 81%. More importantly, the methanol trapping effect
of MS proved more robust than CaCl2 when applied to other
substrates or in other conditions.
conversions. For substrates with more branched aliphatic
ACCEPTED MANUSCRIPT
substituents, such as 3-methylbutanal (1b), 2-ethylbutanal (1c),
and cyclohexanecarbaldehyde (1d), the enantioselectivities were
high, while for the long chain aldehyde octanal (1e), much lower
ee was obtained. These differences are likely due to how well the
structures can be accommodated into the active site of CAL-B.
Large substituents without other binding potentials, such as the
heptyl group, may have difficulty to fit into the active site of
CAL-B.37-38
When an aromatic aldehyde was applied to this system (Entry
6, Table 3), the reaction was very slow even at the higher
temperature of 0 °C, whereas the ee remained at more than 80%.
An aldehyde with a protected alpha-oxygen was also tested
(Entry 7, Table 3), as this could in principle provide a direct route
to interesting chiral NRTI’s such as lamivudine or emtricitabine.
Surprisingly, despite the bulky nature of the protecting group,
very clean reactions with a high conversion from starting
material to the O-protected 1,3-oxathiolan-5-one 4g were
achieved, albeit with slightly lower enantioselectivity. This may
also be ascribed to the non-optimal fit with the CAL-B
stereospecificity pocket.
In order to demonstrate that the oxathiolan-5-ones,
synthesized with the dynamic covalent kinetic resolution protocol,
in principle could be used for the construction of oxathiolane
nucleosides, we performed a brief synthesis starting from
racemic compound 4g (Scheme 2). The lactone functionality was
first reduced with diisobutyl aluminium hydride to the
corresponding lactol, which was subsequently acetylated using
standard conditions to obtain oxathiolane product 5 in two
steps.39 This intermediate could then be transformed to the
corresponding O-protected nucleoside under Vorbrüggen
Table 3. Dynamic covalent kinetic resolution of aldehydes.a
conditions,39-40 using pre-silylated N4-acetylcytosine in
a
O
OH
O
trimethylsilyl iodide-catalyzed N-glycosylation procedure. The
obtained nucleoside 6 could in principle be further functionalized
or deprotected to obtain the pharmaceutical product 3TC.41
O
base
O
CAL-B
O
HS
+
R
S
R
H
O
S
R
O
1a-g
2
3a-g
4a-g
Conversion
(%)b
NHAc
Entry
R group
Product
ee (%)c
OTMS
N
N
N
O
1.) DIBAL-H,
toluene, -78°C
2.) MeOH
OAc
O
O
N
,
TMSI
MeCN, 0°C
AcHN
1
4a
73
81
O
S
S
3.) Ac2O, DMAP, NEt3
CH2Cl2, 0°C to r.t.
O
S
OBn
OBn
78%, dr 1.4:1
OBn
anti-diastereomer
52%
2
3
4
5
6
7
4b
4c
4d
4e
4f
55
79
67
75
16
93
85
90
92
71
83
71
4g
6
5
+
Scheme 2. Synthesis to nucleoside 6, an important intermediate for
3TC.
3. Conclusions
In summary, a dynamic covalent kinetic resolution protocol
based on reversible hemithioacetal formation was successfully
applied to the synthesis of 1,3-oxathiolan-5-one derivatives.
Using lipase-catalyzed resolution, selective lactonization of the
substrate structures were achieved in a one-pot process. After a
series of optimizations of the reaction conditions by use of wild-
type CAL-B, good yields and enantioselectivities for a range of
substrates were obtained. Furthermore, some of the synthesized
1,3-oxathiolan-5-one derivatives showed potential for a simple
access to the core structure of active pharmaceutical nucleoside
analogs.
4g
a Reactions were carried out with 1a-g (0.1 mmol), 2 (1.2 equiv.), NMM
(0.1 equiv.), 4 Å MS (300 mg), and CAL-B (5 mg) in toluene (0.6 mL) at -
25 °C for 4 d.
b Determined by 1H NMR spectroscopy.
c The ees of 4a were determined by HPLC analysis using Daicel Chiralpak OJ
column.
4. Experimental Section
After having identified optimized conditions for the model
reaction, a range of other aldehydes (1b-g) were further evaluated,
most of which (4b-g) gave moderate to good ees with reasonable
4.1. General

4
Tetrahedron
Reagents were obtained from commercial suppliers and used
Conversion: 55%, enantiomeric excess (ee): 85%, determined
ACCEPTED MANUSCRIPT
as received. Lipase PS “Amano” IM (EC 3.1.1.3) was purchased
from Amano Enzyme Inc. All other lipases: lipases from
Pseudomonas fluorescens (PFL), Burkholderia (Pseudomonas)
cepacia (PS and PS-CI), Candida antarctica (CAL-B), and
by HPLC analysis (Daicel Chiralpak OJ column 99:1 Hex:iPrOH,
1
0.5 mL/min; tR 27.7 min; tR 30.0 min. H NMR (500 MHz,
CDCl3, 25 °C) δ 0.95 (d, J = 6.7, 3H, CH3), 0.97 (d, J = 6.7, 3H,
CH3), 1.61-1.68 (m, H, CH), 1.78-1.88 (m, 1H, CH2), 1.91-2.00
(m, 1H, CH2), 3.60 (d, J = 16.5, 1H, CH2), 3.69 (d, J = 16.5, 1H,
CH2), 5.54 (t, J = 6.6, 1H, CH); 13C NMR (125 MHz, CDCl3,
25 °C) δ 22.4, 22.5, 25.4, 31.8, 45.6, 81.3, 172.9. HRMS: found
161.0640, calc. for C7H13O2S [M+H+] 161.0631.
1
Candida rugosa (CRL) were purchased from Sigma-Aldrich. H
and 13C NMR data were recorded on a Bruker Avance 400 (100)
MHz and/or a Bruker Avance 500 (125) MHz, respectively.
Chemical shifts are reported as δ values (ppm) with CDCl3 (1H
NMR δ 7.26, 13C NMR δ 77.0) or DMSO-d6 (1H NMR δ 2.50, 13
C
4.3.3. 2-(Pentan-3-yl)-1,3-oxathiolan-5-one (4c)42
NMR δ 39.5) as internal standard. J values are given in Hertz
(Hz). Analytical high performance liquid chromatography
(HPLC) with chiral stationary phase was performed on an HP-
Agilent 1110 Series controller and a UV detector, using a Daicel
Chiralpak OJ column (4.6 × 250 mm, 10 µm). Solvents for
HPLC use were of spectrometric grade. ATR-IR spectroscopy
was performed on a Thermo Scientific Nicolet iS10 spectrometer.
High resolution mass spectroscopy was performed by the
Instrument station of the National Center for Genetic Engineering
and Biotechnology (BIOTEC), Thailand, and at the Institute of
Chemistry at University of Tartu, Estonia. Thin layer
chromatography (TLC) was performed on precoated Polygram®
SIL G/UV 254 silica plates (0.20 mm, Macherey-Nagel),
visualized with UV-detection. Flash column chromatography was
performed on silica gel 60, 0.040-0.063 mm (SDS).
Conversion: 79%, enantiomeric excess (ee): 90%, determined
by HPLC analysis (Daicel Chiralpak OJ column 99:1 Hex:iPrOH,
1
0.5 mL/min; tR 19.7 min; tR 22.5 min. H NMR (500 MHz,
CDCl3, 25 °C) δ 0.92 (t, J = 7.4, 6H, (CH3)2), 1.30-1.40 (m, 1H,
CH), 1.46-1.56 (m, 3H, (CH2)2), 1.66-1.74 (m, 1H, (CH2)2), 3.56
(d, J = 16.7, 1H, CH2), 3.66 (d, J = 16.7, 1H, CH2), 5.50 (d, J
=6.4, 1H, CH); 13C NMR (125 MHz, CDCl3, 25 °C) δ 11.1, 21.5,
21.9, 31.8, 46.4, 85.6, 173.1. HRMS: found 175.0787, calc. for
C8H15O2S [M+H+] 175.0787.
4.3.4. 2-Cyclohexyl-1,3-oxathiolan-5-one (4d)11
Conversion: 67%, enantiomeric excess (ee): 92%, determined
by HPLC analysis (Daicel Chiralpak OJ column 99:1 Hex:iPrOH,
1
0.5 mL/min; tR 28.0 min; tR 32.1 min. H NMR (500 MHz,
CDCl3, 25 °C) δ 1.02-1.33 (m, 6H, (CH2)3), 1.67-1.83 (m, 4H,
(CH2)2), 1. 96 (d, J = 12.5, 1H, CH), 3.57 (d, J = 16.8, 1H, CH2),
3.66 (d, J = 16.8, 1H, CH2), 5.29 (s, 1H, CH); 13C NMR
(125 MHz, CDCl3, 25 °C) δ 25.6, 26.2, 27.7, 28.9, 31.5, 43.9,
87.0, 173.1.
4.2. General procedure for dynamic covalent kinetic resolution
Aldehydes 1a-1g (0.1 mmol each), methyl 2-sulfanylacetate
(0.12 mmol), 4-methylmorpholine (0.01 mmol) and dry toluene
(0.75 mL) were added into a 1.5 mL sealed-cap vial containing
CAL-B (5 mg) and CaCl2 (20 mg) or 4Å molecular sieves (300
mg). CAL-B was dried under vacuum for two days before use.
4.3.5. 2-Heptyl-1,3-oxathiolan-5-one (4e)26
1
The vial was kept at -25 °C without stirring, and H NMR was
Conversion: 75%, enantiomeric excess (ee): 71%, determined
by HPLC analysis (Daicel Chiralpak OJ column 99:1 Hex:iPrOH,
used to monitor the reaction progress. After four days, the
reaction mixture was filtered through a cotton-stoppered pipette.
Then, saturated NH4Cl was added, and the resulting mixture was
kept stirring at r.t. for 30 min. CH2Cl2 was subsequently added,
and the aqueous layer was extracted three times (3 mL CH2Cl2
each). The combined organic layer was dried over MgSO4 and
removed in vacuo. The crude products were purified using
column chromatography (Hexane:EtOAc = 20:1 or 15:1).
1
0.5 mL/min; tR 42.0 min; tR 48.5 min. H NMR (500 MHz,
CDCl3, 25 °C) δ 0.88 (t, J = 6.8, 3H, CH3), 1.20-1.37 (m, 10H,
(CH2)5), 1.78-1.86 (m, 1H, CH2), 1.96-2.06 (m, 1H, CH2), 3.62
(d, J = 16.6, 1H, CH2), 3.67 (d, J = 16.6, 1H, CH2), 5.48 (t, J =
6.7, 2H, CH2); 13C NMR (125 MHz, CDCl3, 25 °C) δ 14.2, 22.7,
25.1, 29.1, 31.8, 36.8, 82.6, 173.0.
4.3.6. 2-(Pyridin-2-yl)-1,3-oxathiolan-5-one (4f)26
4.3. General procedure for the synthesis of racemic compounds
Conversion: 16%, enantiomeric excess (ee): 83%, determined
by HPLC analysis (Daicel Chiralpak OJ column 80:20
Aldehydes 1a-1g (3 mmol), methyl 2-sulfanylacetate (3.6
mmol), and Et3N (0.3 mmol), were dissolved in toluene (1 mL),
to which CAL-B (5 mg), and 4Å molecular sieves (10 mg) were
1
Hex:iPrOH, 0.5 mL/min; tR 51.8 min; tR 59.2 min. H NMR
(500 MHz, CDCl3, 25 °C) δ 3.79 (d, J = 16.4, 1H, CH2), 3.87 (d,
J = 16.4, 1H, CH2), 6.50 (s, 1H, CH), 7.27-7.32 (m, 1H, CH),
7.44 (d, J = 7.8, 1H, CH), 7.76 (t, J = 7.8, 1H, CH), 8.61 (d, J =
4.8, 1H, CH); 13C NMR (125 MHz, CDCl3, 25 °C) δ 31.7, 81.4,
119.8, 124.1, 137.3, 149.9, 157.0, 173.0.
1
added. The mixture was stirred at 60 °C for six days, and H
NMR was used to monitor the reaction progress. After
completion of the reaction, CAL-B was removed by filtration,
and the solution was washed by saturated NH4Cl, while stirring
at r.t. for 30 min. The mixture was extracted with CH2Cl2 (3 mL
× 3), and the organic layer was dried over MgSO4 and evaporated
in vacuo. The crude products were finally purified using column
chromatography (Hexane:EtOAc = 20:1 or 15:1).
4.3.7. 2-(Benzyloxymethyl)-1,3-oxathiolan-5-one (4g)41
Conversion: 93%, enantiomeric excess (ee): 71%, determined
by HPLC analysis (Daicel Chiralpak OD-H column 95:5
1
Hex:iPrOH, 0.5 mL/min; tR 74.3 min; tR 80.3 min. H NMR
4.3.1. 2-Isopropyl-1,3-oxathiolan-5-one (4a)1
(500 MHz, CDCl3, 25 °C) δ; 3.51 (d, J = 16.3, 1H, CH2), 3.66
(dd, J1 = 4.3, J2 = 10.9, 1H, CH2), 3.68 (d, J = 16.3, 1H, CH2),
3.72 (dd, J1 = 4.3, J2 = 10.9, 1H, CH2), 4.55 (s, 2H, CH2), 5.52 (t,
J = 3.9, 1H, CH), 7.21-7.31 (m, 5H, CH), 7.44 (d, J = 7.8, 1H,
CH), 7.76 (t, J = 7.8, 1H, CH), 8.61 (d, J =4.8, 1H, CH); 13C-
NMR (125 MHz, CDCl3, 25 °C) δ 31.1, 73.1, 73.8, 79.9, 127.7,
128.0, 128.6, 137.3, 172.9.
Conversion: 73%, enantiomeric excess (ee): 81%, determined
by HPLC analysis (Daicel Chiralpak OJ column 99:1 Hex:iPrOH,
1
0.5 mL/min; tR 26.8 min; tR 30.7 min. H NMR (500 MHz,
CDCl3, 25 °C) δ 1.01 (d, J = 6.7 Hz, 3H, CH3), 1.06 (d, J =
6.7 Hz, 3H, CH3), 2.06-2.16 (m, 1H, CH), 3.58 (d, J = 16.3 Hz,
1H, CH2), 3.68 (d, J = 16.3 Hz ,1H, CH2), 5.31 (d, J = 2.9 Hz,
1H, CH); 13C NMR (125 MHz, CDCl3, 25 °C) δ 17.2, 18.1, 31.7,
34.5, 87.7, 173.3.
4.4. 5-Acetoxy-2-benzyloxymethyl-1,3-oxathiolane (5)41
4.3.2. 2-Isobutyl-1,3-oxathiolan-5-one (4b)

5
To a solution of 2-benzyloxymethyl-1,3-oxathiolan-5-one
(384 mg, 1.67 mmol) in anhydrous toluene (16 mL) was added a
solution of diisobutylaluminium hydride in hexanes (1 M, 2.5 mL,
2.50 mmol) dropwise over 30 minutes at -78 °C. The mixture
was stirred for 80 minutes under N2 atmosphere, keeping the
temperature below -70 °C at all times. The reaction was
quenched by dropwise addition of cold, anhydrous methanol (5
mL) over 20 minutes and the reaction mixture was stirred for a
further 15 minutes at -78 °C followed by 2 h at r.t. Washing with
saturated aqueous Rochelle salt solution (12 mL), distilled H2O
(2 × 12 mL) and brine (2 × 12 mL), followed by drying with
MgSO4, filtering and concentration in vacuo yielded a crude oil
that was used in the next step without further purification. The
crude product mixture was dissolved in anhydrous CH2Cl2 (12
mL) in a round bottom flask and the solution was cooled to 0°C
and evacuated two times with N2. Et3N (0.23 mL, 1.67 mmol)
was added dropwise under vigorous stirring and the solution was
stirred for 10 minutes, after which acetic anhydride (0.20 mL,
2.12 mmol) and DMAP (65 mg, 0.53 mmol, dissolved in 0.5 mL
CH2Cl2) were added via syringe. The solution was slowly
warmed to r.t. and stirred under N2 for 14 h. Quenching by slow
addition of saturated aqueous NaHCO3 solution (12 mL) was
followed by extraction of the aqueous phase with CH2Cl2 (3 × 12
mL). The combined organic phases were dried with MgSO4,
yellow solid. Purification by column chromatography (EtOAc)
ACCEPTED MANUSCRIPT
allowed for separation of the diastereoisomers to yield the trans
(28.7 mg, 0.127 mmol, 35%) and cis (34.4 mg, 0.157 mmol,
43%) isomers as white solids. cis-6: Rf 0.09 (EtOAc); H-NMR
1
(500 MHz, CDCl3) δ 9.43 (s, br, 1H), 8.39 (d, 1H, J = 7.5 Hz),
7.35-7.41 (m, 5H), 7.23 (d, 1H, J = 7.5 Hz), 6.34 (dd, 1H, J =
2.3, 5.2 Hz), 5.36 (t, 1H, J = 3.2 Hz), 4.65 (s, 2H), 4.04 (dd, 1H,
J = 2.8, 11.2 Hz), 3.83 (dd, 1H, J = 3.6, 11.2 Hz), 3.58 (dd, 1H, J
= 5.4, 12.7 Hz), 3.22 (dd, 1H, J = 2.2, 12.7 Hz), 2.25 (s, 3H); 13C-
NMR (125 MHz, CDCl3) δ 170.6, 162.9, 154.9, 145.6, 137.1,
128.6, 128.2, 127.7, 96.0, 87.5, 73.9, 69.5, 39.5, 29.7, 24.9; 1D-
NOE NMR pulse at 6.34 ppm, δ 8.39, 5.36, 3.58, 3.22; 1D-NOE
NMR pulse at 5.36 ppm, δ 6.34, 4.04, 3.83, 3.58; trans-6: Rf 0.12
(EtOAc); 1H-NMR (500 MHz, CDCl3) δ 9.15 (s, br, 1H), 7.74 (d,
1H, J = 7.5 Hz), 7.41 (d, 1H, J = 7.5 Hz), 7.28-7.36 (m, 5H), 7.47
(dd, 1H, J = 1.6, 5.1 Hz), 5.68 (t, 1H, J = 4.9 Hz), 4.60 (s, 2H),
3.61-3.65 (m, 3H), 3.21 (dd, 1H, J = 1.7, 12.5 Hz), 2.25 (s, 3H);
HRMS: found 362.1169, calc. for C17H20N3O4S [M+H+]
362.1169.
Acknowledgments
This work was in part supported by the Swedish Research
Council and the Royal Institute of Technology. YZ thanks the
China Scholarship Council for a special scholarship award.
filtered
and
concentrated.
Purification
by
column
chromatography (19:1-15:1 hexanes/EtOAc) yielded an
inseparable mixture of the product diastereomers as a slightly
yellow, viscous oil (232 mg, 0.87 mmol, 52%). trans-5: Rf 0.24
Supplementary data
1
(Hexanes/EtOAc 8:1); H-NMR (500 MHz, CDCl3) δ 7.29-7.36
Supplementary data associated with this article can be found
in the online version, at http://dx.doi.org/10.1016/j.tet.
20XX .XX.XXX.
(m, 5H), 6.69 (d, 1H, J = 4.3 Hz), 5.53 (dd, 1H, J = 4.5, 5.6 Hz),
4.62 (s, 2H), 3.73 (dd, 1H, J = 5.9, 10.6 Hz), 3.66 (dd, 1H, J =
4.2, 10.6 Hz), 3.31 (dd, 1H, J = 4.3, 11.5 Hz), 3.11 (d, 1H, J =
1
11.5 Hz); 2.09 (s, 3H); cis5: Rf 0.24 (Hexanes/EtOAc, 8:1); H-
References and notes
NMR (500 MHz, solvent) δ 7.29-7.36 (m, 5H), 6.59 (d, 1H, J =
4.1 Hz), 5.51 (dd, 1H, J = 2.3, 4.9 Hz), 4.61 (s, 2H), 3.78 (dd, 1H,
J = 7.0, 10.5 Hz), 3.73 (dd, 1H, J = 5.9, 10.6 Hz), 3.28 (dd, 1H, J
= 4.2, 11.8 Hz), 3.15 (d, 1H, J = 11.8 Hz); 1.99 (s, 3H); 13CNMR
(125 MHz, CDCl3, mixture of diastereomers) δ 169.8, 169.7,
137.8, 137.7, 128.5, 128.0, 127.82, 127.77, 127.75, 127.68, 99.3,
99.1, 86.1, 84.6, 74.2, 73.6, 73.4, 72.4, 38.0, 37.4, 21.2, 21.1.
1. Higashiya, S.; Narizuka, S.; Konno, A.; Maeda, T.; Momota, K.;
Fuchigami, T. J. Org. Chem. 1998, 64, 133-137.
2. Beach, J. W.; Jeong, L. S.; Alves, A. J.; Pohl, D.; Kim, H. O.;
Chang, C. N.; Doong, S. L.; Schinazi, R. F.; Cheng, Y. C.; Chu, C.
K. J. Org. Chem. 1992, 57, 2217-2219.
3. Jeong, L. S.; Alves, A. J.; Carrigan, S. W.; Kim, H. O.; Beach, J.
W.; Chu, C. K. Tetrahedron Lett. 1992, 33, 595-598.
4. Chu, C. K.; Beach, J. W.; Jeong, L. S.; Choi, B. G.; Comer, F. I.;
Alves, A. J.; Schinazi, R. F. J. Org. Chem. 1991, 56, 6503-6505.
5. Humber, D. C.; Jones, M. F.; Payne, J. J.; Ramsay, M. V. J.;
Zacharie, B.; Jin, H.; Siddiqui, A.; Evans, C. A.; Tse, H. L. A.;
Mansour, T. S. Tetrahedron Lett. 1992, 33, 4625-4628.
6. Roy, B. N.; Singh, G. P.; Srivastava, D.; Jadhav, H. S.; Saini, M.
B.; Aher, U. P. Org. Process Res. Dev. 2009, 13, 450-455.
7. Milton, J.; Brand, S.; Jones, M. F.; Rayner, C. M. Tetrahedron
Lett. 1995, 36, 6961-6964.
4.5. 2-Benzyloxymethyl-1,3-oxathiolan-5-yl-N4-acetylcytosine
(6)
Silylated N4-acetylcytosine was generated in situ by stirring
N-acetylcytosine43
(55.4
mg,
0.362
mmol)
with
hexamethyldisilazane (1.0 mL, 4.8 mmol) and a crystal of
(NH4)2SO4 at reflux (135 °C) under argon until all solid had
dissolved (approximately 3 h). The solution was slowly cooled to
r.t., and the hexamethyldisilazane was removed under anhydrous
conditions. The system was evacuated five times with argon, and
dried under vacuum for 30 minutes. Meanwhile, 5-acetoxy-2-
benzyloxymethyl-1,3-oxathiolane (60.0 mg, 0.224 mmol) was
dissolved in anhydrous acetonitrile (6 mL) and stirred with pre-
activated 3 Å molecular sieves for 30 minutes under N2. The
solution was transferred via syringe to the dried silylated N4-
acetylcytosine, and the resulting solution was cooled to 0 °C.
TMSI (70.0 ꢁL, 0.504 mmol) was added dropwise over
15 minutes and the resulting yellow solution was stirred under
argon at 0 °C for 1 h. The reaction was quenched by addition of a
mixture of ethyl acetate (10 mL) and aqueous NaHCO3 solution
(5 wt%, 6 mL) and the resulting two-phase system was stirred at
r.t. for 15 minutes. The mixture was further diluted with ethyl
acetate (10 mL) and the organic phase was washed with saturated
aqueous NaHCO3 solution (15 mL), distilled H2O (2 × 15 mL)
and brine (2 × 15 mL). Drying with MgSO4, filtration and
concentration in vacuo yielded the crude product mixture as a
8. Mahmoudian, M.; Baines, B. S.; Drake, C. S.; Hale, R. S.; Jones,
P.; Piercey, J. E.; Montgomery, D. S.; Purvis, I. J.; Storer, R.;
Dawson, M. J.; Lawrence, G. C. Enzyme Microb. Technol. 1993,
15, 749-755.
9. Cousins, R. P. C.; Mahmoudian, M.; Youds, P. M. Tetrahedron:
Asymmetry 1995, 6, 393-396.
10. Gaede, B. J.; Nardelli, C. A. Org. Process Res. Dev. 2004, 9, 23-
29.
11. Popp, A.; Gilch, A.; Mersier, A.-L.; Petersen, H.; Rockinger-
Mechlem, J.; Stohrer, J. Adv. Synth. Catal. 2004, 346, 682-690.
12. Pellissier, H. Tetrahedron 2011, 67, 3769-3802.
13. Pellissier, H. Adv. Synth. Catal. 2011, 353, 659-676.
14. Kim, Y.; Park, J.; Kim, M.-J. ChemCatChem 2011, 3, 271-277.
15. Lee, J. H.; Han, K.; Kim, M.-J.; Park, J. Eur. J. Org. Chem. 2010,
2010, 999-1015.
16. Vongvilai, P.; Larsson, R.; Ramström, O. Adv. Synth. Catal. 2008,
350, 448-452.
17. Paetzold, J.; Bäckvall, J. E. J. Am. Chem. Soc. 2005, 127, 17620-
17621.
18. Kim, C.; Lee, J.; Cho, J.; Oh, Y.; Choi, Y. K.; Choi, E.; Park, J.;
Kim, M.-J. J. Org. Chem. 2013, 78, 2571-2578.

6
Tetrahedron
19. Sato, Y.; Kayaki, Y.; Ikariya, T. Chem. Commun. 2012, 48,
32. Gandhi, N. N.; Patil, N. S.; Sawant, S. B.; Joshi, J. B.; Wangikar,
ACCEPTED MANUSCRIPT
3635-3637.
P. P.; Mukesh, D. Catal. Rev. 2000, 42, 439-480.
33. Gandhi, N. N.; Sawant, S. B.; Joshi, J. B.; Mukesh, D. Chem. Eng.
J. 1997, 65, 159-164.
34. Maceiras, R.; Vega, M.; Costa, C.; Ramos, P.; Márquez, M. C.
Chem. Eng. J. 2011, 166, 358-361.
20. Lutje Spelberg, J. H.; Tang, L.; Kellogg, R. M.; Janssen, D. B.
Tetrahedron: Asymmetry 2004, 15, 1095-1102.
21. Hu, L.; Schaufelberger, F.; Zhang, Y.; Ramström, O. Chem.
Commun. 2013, 49, 10376-10378.
22. Vongvilai, P.; Ramström, O. J. Am. Chem. Soc. 2009, 131, 14419-
14425.
23. Sakulsombat, M.; Vongvilai, P.; Ramström, O. Org. Biomol.
Chem. 2012, 9, 1112-1117.
24. Vongvilai, P.; Angelin, M.; Larsson, R.; Ramström, O. Angew.
Chem. Int. Ed. 2007, 46, 948-950.
35. Halling, P. Biotechnol. Tech. 1992, 6, 271-276.
36. Léonard-Nevers, V.; Marton, Z.; Lamare, S.; Hult, K.; Graber, M.
J. Mol. Catal. B: Enzym. 2009, 59, 90-95.
37. Uppenberg, J.; Oehrner, N.; Norin, M.; Hult, K.; Kleywegt, G. J.;
Patkar, S.; Waagen, V.; Anthonsen, T.; Jones, T. A. Biochemistry
1995, 34, 16838-16851.
25. Zhang, Y.; Hu, L.; Ramström, O. Chem. Commun. 2013, 49,
1805-1807.
38. Kazlauskas, R. J.; Weissfloch, A. N. E.; Rappaport, A. T.; Cuccia,
L. A. J. Org. Chem. 1991, 56, 2656-2665.
26. Sakulsombat, M.; Zhang, Y.; Ramström, O. Chem. Eur. J. 2012,
18, 6129-6132.
27. Sakulsombat, M.; Zhang, Y.; Ramström, O. in Constitutional
Dynamic Chemistry, Vol. 322 (Ed.: M. Barboiu), Springer: Berlin
Heidelberg, 2012; pp. 55-86.
39. Barral, K.; Hider, R. C.; Balzarini, J.; Neyts, J.; De Clercq, E.;
Camplo, M. Bioorg. Med. Chem. Lett. 2003, 13, 4371–4374.
40. Beach, J. W.; Jeong, L. S.; Alves, A. J.; Pohl, D.; Kim, H. O.;
Chang, C. N.; Doong, S. L.; Schinazi, R. F.; Cheng, Y. C.; Chu, C.
K. J. Org. Chem. 1992, 57, 2217–2219.
28. Overbeeke, P. L. A.; Ottosson, J.; Hult, K.; Jongejan, J. A.; Duine,
J. A. Biocatal. Biotransform. 1999, 17, 61.
41. Huang, J. J.; Rideout, J. L.; Martin, G. E. Nucleosides Nucleotides
1995, 14, 195–207.
29. Sakai, T.; Kawabata, I.; Kishimoto, T.; Ema, T.; Utaka, M. J. Org.
Chem. 1997, 62, 4906.
42. Miller, E. H.; Hechenbleikner, I.; Homberg, O. A. Patent
FR1386914, 1965.
30. Caraballo, R.; Dong, H.; Ribeiro, J. P.; Jiménez-Barbero, J.;
Ramström, O. Angew. Chem. Int. Ed. 2010, 49, 589-593.
31. Faber, K. Biotransformations in organic chemistry, 4th ed.,
Springer, 2000.
43. Okabe, M.; Sun, R. C.; Zenchoff, G. B. J. Org. Chem. 1991, 56,
4392–4397.

ACCEPTED MANUSCRIPT
Supporting Information
Asymmetric Synthesis of 1,3-Oxathiolan-5-one Derivatives through
Dynamic Covalent Kinetic Resolution
Yan Zhang, Fredrik Schaufelberger, Morakot Sakulsombat, Chelsea Liu and Olof Ramström*
KTH - Royal Institute of Technology, Department of Chemistry, Teknikringen 30, Stockholm, Sweden
Fax: (+46) 8 7912333; E-mail: ramstrom@kth.se
Figure S1. 1H NMR and 13C NMR spectra of compound 4a
Figure S2. HPLC chromatogram of enantioenriched and racemic compound 4a
Figure S3.1H NMR and 13C NMR spectra of compound 4b
Figure S4. HPLC chromatogram of enantioenriched and racemic compound 4b.
Figure S5. ATR-IR spectrum of compound 4b
S2
S2
S3
S3
S4
S4
S5
S5
S5
S6
S6
S7
S7
S8
S8
S9
S9
S10
S11
Figure S6. 1H NMR and 13C NMR spectra of compound 4c
Figure S7. HPLC chromatogram of enantioenriched and racemic compound 4c
Figure S8. ATR-IR spectrum of compound 4c
Figure S9. HPLC chromatogram of enantioenriched and racemic compound 4d
Figure S10. 1H NMR and 13C NMR spectra of compound 4d
Figure S11. HPLC chromatogram of enantioenriched and racemic compound 4e
Figure S12. 1H NMR and 13C NMR spectra of compound 4e
Figure S13. HPLC chromatogram of enantioenriched and racemic compound 4f
Figure S14. 1H NMR and 13C NMR spectra of compound 4f.
Figure S15. HPLC chromatogram of enantioenriched and racemic compound 4g
Figure S16. 1H NMR and 13C NMR spectra of compound 4g
Figure S17. ATR-IR spectra of compound 5
Figure S18. 1H NMR and 13C NMR spectra of compound 6
Figure S19. ATR-IR spectrum of compound 6
S1

ACCEPTED MANUSCRIPT
Figure S1. 1H NMR and 13C NMR spectra of compound 4a.
Figure S2. HPLC chromatogram of enantioenriched and racemic compound 4a.
S2

ACCEPTED MANUSCRIPT
Figure S3.1H NMR and 13C NMR spectra of compound 4b.
Figure S4. HPLC chromatogram of enantioenriched and racemic compound 4b.
S3

ACCEPTED MANUSCRIPT
Figure S5. ATR-IR spectra of compound 4b (neat, ATR corrected).
Figure S6. 1H NMR and 13C NMR spectra of compound 4c.
S4

ACCEPTED MANUSCRIPT
Figure S7. HPLC chromatogram of enantioenriched and racemic compound 4c.
Figure S8. ATR-IR spectra of compound 4c (neat, ATR corrected).
Figure S9. HPLC chromatogram of enantioenriched and racemic compound 4d.
S5

ACCEPTED MANUSCRIPT
Figure S10. 1H NMR and 13C NMR spectra of compound 4d.
Figure S11. HPLC chromatogram of enantioenriched and racemic compound 4e.
S6

ACCEPTED MANUSCRIPT
Figure S12. 1H NMR and 13C NMR spectra of compound 4e.
Figure S13. HPLC chromatogram of enantioenriched and racemic compound 4f.
S7

ACCEPTED MANUSCRIPT
Figure S14. 1H NMR and 13C NMR spectra of compound 4f.
Figure S15. HPLC chromatogram of enantioenriched and racemic compound 4g.
S8

ACCEPTED MANUSCRIPT
Figure S16. 1H NMR and 13C NMR spectra of compound 4g.
Figure S17. ATR-IR spectra of compound 5 (neat, ATR corrected).
S9

ACCEPTED MANUSCRIPT
Figure S18. 1H NMR and 13C NMR spectra of compound 6, including 1D NOE NMR.
S10

ACCEPTED MANUSCRIPT
Figure S19. ATR-IR spectra of compound 6 (neat, ATR corrected).
S11