Enzymatic synthesis of novel chiral sulfoxides employing Baeyer-villiger monooxygenases

Article abstract of DOI:10.1002/ejoc.201000890

Optically active sulfoxides are compounds of high interest in organic chemistry. Herein, we report the preparation of a set of chiral heteroaryl alkyl, cyclohexyl alkyl, and alkyl alkyl sulfoxides by using enantioselective sulfoxidation reactions employing three Baeyer-Villiger monooxygenases (BVMOs). Careful selection of the reaction conditions, starting sulfide, and biocatalyst can be used to achieve good to excellent enantiomeric excess values. Thus, valuable chiral synthons can be obtained by performing the reactions under mild and environmentally friendly conditions. The most promising biotransformations that employ a BVMO cell-free extract preparation have been developed on a 250-mg scale to give the chiral sulfoxides in high yields in most of the reactions. Copyright

Full text of DOI:10.1002/ejoc.201000890

FULL PAPER
DOI: 10.1002/ejoc.201000890
Enzymatic Synthesis of Novel Chiral Sulfoxides Employing Baeyer–Villiger
Monooxygenases
Ana Rioz-Martínez,[a] Gonzalo de Gonzalo,[a] Daniel E. Torres Pazmiño,[b]
Marco W. Fraaije,[b] and Vicente Gotor*[a]
Keywords: Biocatalysis / Monooxygenases / Oxidation / Sulfur / Enantioselectivity
Optically active sulfoxides are compounds of high interest in
organic chemistry. Herein, we report the preparation of a set
of chiral heteroaryl alkyl, cyclohexyl alkyl, and alkyl alkyl
sulfoxides by using enantioselective sulfoxidation reactions
employing three Baeyer–Villiger monooxygenases (BVMOs).
Careful selection of the reaction conditions, starting sulfide,
and biocatalyst can be used to achieve good to excellent en-
antiomeric excess values. Thus, valuable chiral synthons can
be obtained by performing the reactions under mild and en-
vironmentally friendly conditions. The most promising bio-
transformations that employ a BVMO cell-free extract prepa-
ration have been developed on a 250-mg scale to give the
chiral sulfoxides in high yields in most of the reactions.
In the last few years, biocatalysis has emerged as a valu-
able synthetic tool for the preparation of high added-value
compounds.[6] Some of the advantages of biocatalytic meth-
ods are that enzymes can afford high efficiencies and regio-
and/or enantioselectivities working under mild reaction
conditions and an increasing applicability due to the recent
advances in biotechnology. There are two major groups
of enzymes that can be used for the oxidation of prochiral
sulfides to the corresponding optically active sulfoxides:
peroxidases[7] and monooxygenases.[8] This latter family of
enzymes catalyzes the transfer of one oxygen atom from
molecular oxygen to an organic substrate by using various
cofactors for the oxidative processes.
Baeyer–Villiger monooxygenases (BVMOs) are nicotin-
amide cofactor dependent flavoproteins able to perform the
oxidation of carbonyl compounds (the Baeyer–Villiger oxi-
dation) and the oxygenation of different heteroatoms, in-
cluding sulfur.[9] Thus, BVMOs have been described in the
enantioselective oxidation of sulfides, dithianes, sulfites, and
racemic sulfoxides.[10] In the last few years, several BVMOs
have been described and applied in biocatalytic processes.[11]
Two well-known examples are phenylacetone monooxygen-
ase (PAMO)[12] and 4-hydroxyacetophenone monooxygen-
ase (HAPMO),[13] which are primarily active on aromatic
compounds. Recently, mutagenesis studies have allowed the
preparation of PAMO mutant (M-PAMO) that has shown
a different substrate profile when compared to the wild-
type.[14]
Introduction
There are several examples of synthetic chiral sulfoxide
targets, as many molecules with high biological activity con-
tain the sulfinyl group with a defined configuration.[1] The
increasing interest in these compounds extends even further
than the molecules themselves into their use as chiral cata-
lysts in chemistry.[2] Thus, the chirality of organic sulfoxides
has been widely exploited in organic synthesis: enantiopure
sulfoxides are important auxiliaries in asymmetric synthesis
as well as chiral ligands in enantioselective catalysis. This is
mainly due to their availability, high asymmetric induction,
and the fact that chiral sulfoxides that induce optical ac-
tivity can be removed from the target compound easily un-
der mild conditions. Optically active heteroaryl alkyl sulfox-
ides offer an advantage over other sulfoxides by exhibiting
chelating properties that present a potential interest in
asymmetric synthesis.
Methods currently available to obtain the molecules of
interest include: kinetic resolution of racemic sulfoxides by
reduction or oxidation processes,[3] asymmetric oxidation of
prochiral sulfides by employing different oxidants,[4] or the
nucleophilic addition of different ligands to diastereochemi-
cally pure chiral sulfinates.[4a,5]
[a] Departamento de Química Orgánica e Inorgánica,
Instituto de Biotecnología de Asturias,
Universidad de Oviedo,
c/ Julián Clavería 8, 33006 Oviedo, Spain
Fax: +34-985-103448
In previous work, the PAMO- and HAPMO-catalyzed
oxidation of aryl alkyl sulfides gave high selectivities de-
pending on the nature of the substrate and the biocatalyst
employed.[15] Whereas PAMO was able to catalyze the oxi-
dation of benzyl alkyl sulfides, HAPMO preferred to oxid-
ize thioanisole derivatives. Herein, we describe the use of a
View this journal online at
E-mail: vgs@fq.uniovi.es
[b] Laboratory of Biochemistry,
Groningen Biomolecular Sciences and Biotechnology Institute,
University of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000890.
wileyonlinelibrary.com
Eur. J. Org. Chem. 2010, 6409–6416
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6409

V. Gotor et al.
FULL PAPER
biocatalytic methodology for the preparation of different conversion (c = 23%; Table 1, Entry 8). It seems that the
heteroaryl, cyclohexyl, alkyl, and cyclic sulfoxides by em- presence of a double bond had a positive effect on HAPMO
ploying Baeyer–Villiger monooxygenases.
activity as compared to that observed for the propyl group.
Finally, 2-(butylthio)pyridine (7a) was not oxidized by
HAPMO. The 2-(alkylthio)pyridines were also tested in
presence of PAMO and M-PAMO. Surprisingly, M-PAMO
was able to transform sulfide 4a into (R)-2-(ethylsulfinyl)-
pyridine (4b) with 13% conversion and 76%ee after 24 h
(Table 1, Entry 10). Again, this biocatalyst showed an op-
posite enantiopreference with respect to HAPMO. For all
the BVMO-catalyzed oxidations, only small amounts (less
than 8%) of sulfone formed by the over-oxidation of the
obtained sulfoxide were detected for some substrates. This
indicates that this second oxidation step could not influence
the optical purity of the final sulfoxides to a great extent
Results and Discussion
Unless otherwise stated, all enzymatic sulfoxidations
were performed with purified enzymes. A second enzymatic
system, glucose-6-phosphate/glucose-6-phosphate dehydro-
genase, was used to regenerate the nicotinamide cofactor
NADPH.[16]
BVMO-Catalyzed Oxidation of Heteroaryl Alkyl Sulfides
Initially, our experiments were devoted to the prepara- when starting from the prochiral sulfides. The BVMOs were
tion of pyridine methyl sulfoxides containing the nitrogen also tested in the biooxidation of other nitrogen-containing
atom at the 2-, 3-, or 4-position by employing the three heterocyclic sulfides such as 2-(methylthio)-1H-imidazole
available BVMOs. Reactions were performed in Tris-HCl (8a), 2-(methylthio)pyrimidine (9a), and 2-(methylthio)-1H-
50 m buffer (pH 9.0) at 30 °C for the two PAMO enzymes
(PAMO and M-PAMO) and at 20 °C for HAPMO.[17] After
24 h, it was observed that HAPMO was able to catalyze the
Table 1. BVMO-catalyzed oxidation of heteroaryl sulfides 1–14a.[a]
enzymatic sulfoxidation of the three derivatives with moder-
ate to high conversions and excellent enantiomeric excess
values. The best substrate for this enzyme was 2-(meth-
ylthio)pyridine (1a), as it was possible to obtain the corre-
sponding enantiopure (S)-sulfoxide 1b with complete con-
version (Table 1, Entry 1). The use of substrates containing
the nitrogen atom further away from the methylthio group
led to lower conversions. Thus, (S)-3-(methylsulfinyl)pyr-
idine (2b) was obtained with 95% conversion (c), whereas
4-(methylthio)pyridine (3a) was less effectively oxidized to
the (R)-sulfoxide (c = 63%; Table 1, Entries 2 and 3, respec-
tively). When wildtype PAMO and its M446G mutant were
employed in the oxidation of these substrates, only (R)-3-
(methylsulfinyl)pyridine was obtained, whereas compounds
1a and 3a showed no reactivity. The sulfoxidation per-
formed in the presence of M-PAMO led to the formation of
(R)-2b with 54% conversion and 88%ee (Table 1, Entry 4),
whereas PAMO showed a lower activity (c = 26%) and
selectivity (39%ee). It is important to highlight that both
enantiomers of sulfoxide 2b can be obtained with good to
excellent optical purities by simply modifying the biocata-
lyst employed, as HAPMO displays opposite enantioprefer-
ence to PAMO enzymes. Due to the excellent results ob-
tained in the HAPMO-catalyzed oxidation of 1a, we de-
cided to analyze the effect of the alkyl chain on the oxi-
dation of the 2-pyridine derivative. This enzyme was able to
catalyze the sulfoxidation of the ethyl, n-propyl, and allyl
sulfides with complete enantioselectivity. Enantiopure (S)-
2-(ethylsulfinyl)pyridine (4b) can be synthesized with total
conversion after 24 h (Table 1, Entry 6). When the length of
the alkyl chain was extended to a propyl group, enantiopure
(S)-5b was recovered with a strong decrease in the enzy-
matic activity (c = 11%). This trend has been described pre-
viously for this biocatalyst in the oxidation of thioanisole
derivatives.[15b] As depicted in Table 1, the oxidation of 2-
Entry Substrate
Enzyme
c [%][b]
ee [%][c]
Ն99 (S)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1a
2a
HAPMO Ն99
HAPMO
HAPMO
M-PAMO
PAMO
95
63
54
26
Ն99 (S)[d]
Ն99 (R)[d]
88 (R)[d]
39 (R)[d]
Ն99 (S)[d]
Ն99 (S)
Ն99 (S)[d]
–
3a
2a
2a
4a
HAPMO Ն99
5a
HAPMO
HAPMO
HAPMO
M-PAMO
HAPMO
PAMO
M-PAMO
HAPMO
PAMO
11
23
Յ3
13
69
92
64
71
22
7
6a
7a
4a
76 (R)[d]
Ն99 (R)[e]
81(S)[e]
11a
11a
11a
12a
12a
12a
25 (S)[e]
Ն99 (S)[e]
34 (R)[e]
–
M-PAMO
[a] For reaction conditions, see the Experimental Section. [b] Deter-
mined by using GC. [c] Determined by using HPLC, except for
sulfoxide 2b, which was determined by using GC. [d] Absolute con-
figurations determined by comparison of the retention times for
these compounds with those obtained in the asymmetric sulfoxid-
ation of 2–4a and 6a by using the Kagan methodology. [e] Absolute
configurations determined by comparison of the HPLC times with
(allylthio)pyridine (6a) led to (S)-6b with 99%ee and low those published.
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Synthesis of Chiral Sulfoxides by using Baeyer–Villiger Monooxygenases
benzo[b]imidazole (10a), but no reaction was observed after moderate optical purity (35%ee; Table 2, Entry 3), whereas
long reaction times even after modification of some of the this biocatalyst was not able to oxidize the ethyl derivative.
reaction parameters such as pH or temperature.[17,18] Fi- Reactions performed in the presence of M-PAMO led to
nally, the heteroaryl oxygenated substrate 2-furfuryl methyl sulfoxide (S)-15b with moderate conversion and enantio-
sulfide (11a) was oxidized in the presence of the three meric excess, whereas (S)-16b was obtained after 24 h with
BVMOs. Enantiopure (R)-11b was obtained by employing only 19% conversion and 31%ee. As expected from the pre-
HAPMO (Table 1, Entry 11), whereas the use of the two vious results, M-PAMO was able to catalyze the oxidation
PAMO enzymes led to lower ee values in the preparation of of 16a for which PAMO did not show activity due to the
the opposite enantiomer, especially when using M-PAMO. larger active site of the former. Cyclohexyl propyl sulfide
Compound (S)-11b can be achieved with high conversion (17a) was also subjected to BVMO-catalyzed sulfoxidation,
and 81%ee by using PAMO (Table 1, Entry 12). A sulfur- but no reaction was detected in the presence of the three
containing heteroaryl sulfide such as 2-(methylthio)thio- biocatalysts, indicating that the propyl group limits the ac-
phene (12a) was also analyzed. HAPMO was able to trans- tivity of these enzymes when oxidizing cyclohexyl deriva-
form 12a into enantiopure sulfoxide (S)-12b with good con- tives.
version after 24 h, whereas PAMO showed lower activity
The effect of pH on the enzymatic sulfoxidation of 15a
and selectivity in the preparation of the opposite enantio- catalyzed by the three BVMOs was analyzed (Figure 1).
mer, (R)-12b (c = 22% and 34%ee; Table 1, Entry 15). Its HAPMO was able to catalyze the oxidation of 15a with
M446G mutant catalyzed the sulfoxidation with a very low complete conversion and high enantiomeric excess regard-
activity (Table 1, Entry 16). Finally, none of the enzymes less of the pH values studied (even at pH 10.5). For both
catalyzed the oxidation of benzo-fused derivatives 13a and PAMO enzymes, the optimum pH for sulfoxidation activity
14a, oxygen- and sulfur-containing analogs of benzoimid- ranged from 9.0 to 10.0. Concerning the enzymatic selectiv-
azole 10a.
ity, (S)-15b can be obtained with the highest enantiomeric
excess at pH 10.5 (43%ee), whereas pH 9.0 was more ade-
quate for M-PAMO catalyzed sulfoxidations (49%ee). Fur-
ther increases in pH led to a gradual loss of enantio-
selectivity. This observation is different from that of pre-
vious work, which showed that this biocatalyst is capable
of catalyzing Baeyer–Villiger oxidations with a high conver-
sion for the synthesis of 1-isochromanone at pH 10.5.[19]
Biocatalyzed Oxidation of Cyclohexyl Alkyl Sulfides
Both PAMO and HAPMO have been described as
BVMOs that are primarily active on aromatic com-
pounds.[15] We decided to extend the substrate range by test-
ing nonaromatic sulfides. Initially, the biooxidation of dif-
ferent cyclohexyl alkyl sulfides was analyzed upon incu-
bation with HAPMO, PAMO, and its mutant. As shown in
Table 2, HAPMO was able to catalyze the sulfoxidation of
cyclohexyl methyl sulfide (15a) and cyclohexyl ethyl sulfide
(16a) to the corresponding (S)-sulfoxides with total conver-
sion and excellent enantiomeric excess values after 1 d
(Table 2, Entries 1 and 2). PAMO yielded (S)-cyclohexyl
methyl sulfoxide (15b) after 24 h with 96% conversion and
Table 2. Biocatalyzed oxidation of cyclohexyl alkyl sulfides 15–
17a.[a]
Figure 1. Effect of pH on the oxidation of cyclohexyl methyl sulfide
(15a) catalyzed by the three BVMOs.
Biooxidation of Cyclic and Linear Aliphatic Sulfides
Entry
Substrate
Enzyme
c [%][b]
ee [%][c]
Other nonaromatic sulfides were employed as substrates
for the three BVMOs. These enzymes have been employed
in the sulfoxidation of cyclic sulfides tetrahydrothiophene
(18a) and tetrahydro-2H-thiopyran (19a). No oxidation
product was detected for both PAMO and M-PAMO at
pH 9.0 and 30 °C. On the contrary, when both substrates
were oxidized in the presence of HAPMO, high percentages
(close to 80%) of final sulfoxides 18b and 19b were obtained
(Table 3, Entries 1 and 2). Furthermore, no starting sulfides
were detected, as a moderate amount of sulfone was formed
1
2
3
4
5
6
15a
16a
15a
16a
15a
16a
HAPMO
HAPMO
PAMO
PAMO
M-PAMO
M-PAMO
Ն99
Ն99
96
5
67
Ն99(S)
96(S)[d]
35(S)
–
49(S)
19
31(S)[d]
[a] For reaction conditions, see the Experimental Section. [b] Deter-
mined by using GC. [c] Determined by using HPLC. [d] Absolute
configuration was determined by comparing the retention times on
HPLC for this compound with those obtained in the asymmetric
sulfoxidation of 16a by using the Kagan methodology.
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V. Gotor et al.
FULL PAPER
in both reactions (ca. 20%). Therefore, HAPMO was able Moreover, this biocatalyst was able to catalyze the oxi-
to catalyze the first oxidation step (from the sulfide to the dation of sulfoxides to the corresponding sulfones in the
sulfoxide), and compounds 18–19b were suitable substrates kinetic resolution of some racemic sulfoxides by oxidation
for conversion to the corresponding sulfones.
processes. The favored enantiomer will be converted into
the corresponding sulfone, whereas the unreactive one will
remain as an enantioenriched compound. As described in
Table 4, the oxidation of sulfoxides is a really fast process,
requiring short reaction times to achieve high conversions,
as compared to sulfoxide formation. The oxidation of race-
mic alkyl pyridyl sulfoxides, for example, (Ϯ)-2-(methylsulf-
inyl)pyridine (1b), occurred with a 60% conversion after
0.5 h, leading to the (S)-sulfoxide with a good enantiomeric
excess (Table 4, Entry 1). When the nitrogen atom was lo-
cated further away from the sulfoxide moiety (compounds
2b and 3b), the reactions were still fast but the enzyme selec-
tivity was much lower, giving enantiomeric excess values
close to 20%. HAPMO-catalyzed oxidation of nonaromatic
sulfoxides occurred with lower activity as compared to that
of heteroaromatic analogs. It typically required 5 h to reach
conversions of around 60% for (Ϯ)-cyclohexyl methyl sulf-
oxide (15b) and (Ϯ)-methyl n-octyl sulfoxide (22b), but with
higher selectivity. Thus, (S)-15b can be recovered with
82%ee, whereas enantiopure (S)-22b was obtained.
Table 3. Biooxidation of cyclic and linear aliphatic sulfides 18–
22a.[a]
Entry Sulfide
Enzyme
Conversion
ee b [%]
a [%]
b [%]
c [%]
1
2
3
4
5
6
7
8
9
18a
19a
20a
21a
22a
20a
21a
20a
21a
HAPMO
HAPMO
HAPMO
HAPMO
HAPMO
PAMO
PAMO
M-PAMO
M-PAMO
–
–
–
76
80
Ն99
Ն99
75
40
34
45
39
24
20
–
–
12
–
–
–
–
–
–
Ն99 (S)[b]
Ն99 (S)[c]
Ն99 (S)[d]
65 (S)[b]
50 (S)[c]
53 (S)[b]
36 (S)[c]
Table 4. HAPMO-catalyzed biooxidation of racemic sulfoxides 1–
3b, 15b, and 22b.[a]
–
13
60
66
55
61
[a] For reaction conditions, see the Experimental Section. Conver-
sions and enantiomeric excess values were determined by using GC.
[b] Absolute configuration was determined by comparing the spe-
cific rotation value with published data. [c] Absolute configuration
was established in a manner analogous to that used for n-butyl
methyl sulfoxide (20b). [d] Absolute configuration determined by
comparing the retention times on HPLC.
Entry
Substrate
t [h]
c [%][b]
ee [%][c]
Finally, our efforts were devoted to develop the oxidation
of linear aliphatic sulfides. As can be observed in Table 3,
the best results were again obtained by using HAPMO as
the biocatalyst. n-Butyl methyl and n-butyl ethyl sulfoxides
(20b and 21b, respectively) were achieved in enantiopure
form and with complete conversion after 24 h (Table 3, En-
tries 3 and 4). HAPMO was then employed in the oxidation
of a linear sulfide containing a longer alkyl chain such as
methyl octyl sulfide (22a) to give 75% of enantiopure (S)-
methyl n-octyl sulfoxide (22b) and 12% of corresponding
sulfone 22c (Table 3, Entry 5). PAMO and its M446G mu-
tant were not able to transform this substrate, probably due
to its size. Nevertheless, both enzymes were able to perform
the oxidation of alkyl butyl sulfides 20a and 21a with mod-
erate conversions (34–45%) and enantiomeric excess values
(36–65%; Table 3, Entries 6–9).
1
2
3
4
5
(Ϯ)-1b
(Ϯ)-2b
(Ϯ)-3b
(Ϯ)-15b
(Ϯ)-22b
0.5
0.6
0.5
5.0
4.5
60
67
55
60
55
71 (S)
21 (S)
24 (R)
82 (S)
Ն99 (S)
[a] For reaction conditions, see the Experimental Section. [b] Deter-
mined by using GC. [c] Determined by using HPLC, except for (Ϯ)-
2b, which was determined by using GC.
Kinetic Parameters of HAPMO for the Oxidation of
Prochiral Sulfides 1a, 2a, 11a, 15a, and 20a
To have a better knowledge of HAPMO as an oxidative
biocatalyst of prochiral sulfides, the kinetic parameters for
this biocatalyst were determined when catalyzing the sulf-
oxidation of substrates presenting different structures
(Table 5).
Heteroaryl alkyl and alkyl alkyl sulfides that were tested
showed good KM values in the range 0.1–1.7 m. All the
sulfides that were tested presented similar kcat values, with
HAPMO-Catalyzed Biooxidation of Racemic Sulfoxides
In general, sulfoxidations catalyzed by HAPMO led to activities in the range of 0.3–2.9 s–1, with the highest value
the highest enantiomeric excess values and conversions. being obtained for the oxidation of cyclohexyl methyl sulf-
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Synthesis of Chiral Sulfoxides by using Baeyer–Villiger Monooxygenases
Table 5. Kinetic parameters for the HAPMO-catalyzed sulfoxid-
ation of prochiral sulfides.
Experimental Section
General Methods: Recombinant histidine-tagged phenylacetone
Entry
Substrate
kcat
KM
[mM]
kcat/KM
monooxygenase (PAMO),[20] its M446G mutant (M-PAMO),[14b]
[s–1]
[s–1 mM–1]
and
recombinant
4-hydroxyacetophenone
monooxygenase
(HAPMO)[13c] were over-expressed and purified as described pre-
viously. One unit of BVMO will oxidize 1.0 µmol of thioanisole to
methyl phenyl sulfoxide per minute at pH 9.0 and 25 °C in the pres-
ence of NADPH. Glucose-6-phosphate dehydrogenase from Leu-
conostoc mesenteroides was obtained from Fluka-Biochemika. Cell-
free extract from over-expressed HAPMO on E. coli TOP10 was
obtained by following a similar procedure as described pre-
viously.[21] Sulfides 11a and 12a were acquired from Alfa Aesar,
substrate 15a was supplied by Sigma–Aldrich–Fluka, compounds
14a and 18–21a were purchased from Acros Organics, and sulfide
22a was obtained from TCI Europe. All other reagents and solvents
were of the highest quality available and were obtained from
Sigma–Aldrich–Fluka and Acros Organics. Chemical reactions
were monitored by analytical TLC, which was performed on Merck
silica gel 60 F254 plates and visualized by UV irradiation. Flash
chromatography was carried out with silica gel 60 (230–240 mesh,
Merck). Melting points were taken in open capillary tubes by using
a Gallenkamp apparatus. IR spectra were recorded with a Perkin–
Elmer 1720-X Fourier transform infrared spectrophotometer by
1
2
3
4
5
1a
2a
11a
15a
20a
2.48Ϯ0.26
0.32Ϯ0.03
0.35Ϯ0.03
2.87Ϯ0.21
2.02Ϯ0.16
1.7Ϯ0.02
0.29Ϯ0.01
0.15Ϯ0.02
0.13Ϯ0.01
0.40Ϯ0.03
1.5
1.1
2.3
22.1
5.0
ide. These activities are 6 to 12 times lower than those ob-
tained for the preferred substrates of this enzyme, aceto-
phenones that have electron-withdrawing groups in the aro-
matic moiety.[13c] Better catalytic efficiencies were found for
substrates with alkyl substituents (compounds 15a and
20a), whereas 10 to 20 times lower efficiencies were ob-
served for the heteroaromatic-containing sulfides, as com-
pared to 15a.
Conclusions
1
using KBr pellets. H NMR, 13C NMR, and DEPT spectra were
The synthesis of a set of chiral heteroaryl alkyl, cyclo-
hexyl alkyl, and alkyl alkyl sulfoxides by enantioselective
sulfoxidation reactions catalyzed by three BVMOs is de-
scribed. This work demonstrates the potential of BVMOs
in the preparation of these high added-value compounds.
recorded with tetramethylsilane (TMS) as the internal standard
with a Bruker AC-300 DPX (1H: 300.13 MHz; 13C: 75.5 MHz)
spectrometer. EI+ mass spectra were recorded with a Hewlett Pack-
ard 5973 mass spectrometer. APCI+ and ESI+ mass spectra were
measured with a chromatograph mass detector. High-resolution
HAPMO was able to perform the enzymatic oxidation of mass spectra were obtained with a Bruker Microtof-Q-spectrome-
ter. Optical rotations were measured with a Perkin–Elmer 241 pola-
rimeter. GC analyses were performed with a Hewlett Packard 6890
Series II chromatograph. For all the analyses, the injector and FID
temperatures were 225 and 250 °C, respectively. HPLC analyses
were performed with Hewlett Packard 1100 LC liquid chromato-
graph. Kinetic determinations were performed with a Varian
Cary50Bio UV/Vis spectrophotometer.
pyridine methyl sulfides with moderate to high conversions
and excellent enantiomeric excess values, whereas PAMO
and M-PAMO were only able to oxidize 3-(methylthio)pyr-
idine. HAPMO was also able to oxidize a set of 2-(alk-
ylthio)pyridines with complete selectivity and decreasing
conversions for substrates with longer alkyl chains. Other
nitrogen-containing heterocyclic sulfides as pyrimidine,
imidazole, and benzoimidazole were not suitable substrates
for the BVMOs. Oxidation of oxygen- and sulfur-contain-
ing heteroaryl alkyl sulfides catalyzed by HAPMO also led
to good conversions and high optical purities. Cyclohexyl
methyl and cyclohexyl ethyl sulfoxide were obtained with
high and moderate conversions and enantiomeric excess
values, respectively, in the oxidations catalyzed by HAPMO
and M-PAMO, whereas no sulfoxidation was achieved with
substrates containing longer alkyl chains. The HAPMO-
catalyzed oxidation of cyclohexyl methyl sulfide was hardly
affected by pH. Meanwhile, PAMO and its mutant were
strongly influenced by pH. Cyclic and linear aliphatic sul-
fides were converted by HAPMO with excellent and good
conversions, respectively. In contrast, PAMO and M-
PAMO sulfoxidation of alkyl alkyl sulfides led to moderate
conversion and enantiomeric excess. Kinetic resolution of
racemic sulfoxides catalyzed by HAPMO was a faster pro-
cess when compared to enzymatic sulfoxidation. This bio-
catalyst showed lower activity and higher enantioselectivity
towards nonaromatic sulfoxides than aromatic ones. Scaling
up of some of the biotransformations by using a cell-free
extract afforded the chiral sulfoxides in moderate to good
yields.
Sulfides 1a, 3a, and 8–10a were synthesized by reaction of the cor-
responding thiol and potassium carbonate with iodomethane and
triethylamine in CH2Cl2 at 0 °C (yields ranged from 5 to 99%).[22]
Compound 2a was prepared according to a known procedure[23] by
treating 3-bromopyridine with isopropylmagnesium chloride and
1,2-dimethyldisulfane in dry THF at room temperature under a
nitrogen atmosphere (83% yield).
Compounds 4–7a, 16a, and 17a were synthesized by treating the
corresponding thiol (250 mg) at 0 °C with sodium and ethyl or pro-
pyl iodide or allyl bromide in dry MeOH under a nitrogen atmo-
sphere. The yields obtained ranged from 48 to 81%.[15b] Sulfide
13a was prepared by treatment of 2-(methylthio)benzoxazole with
methyl iodide by following the published procedure.[24]
Compounds 1a,[23] 2a,[23] 3a,[25a] 4a,[25b] 8a,[25c] 9a,[25d] 10a,[25e] and
13a[24] exhibit physical and spectral properties in accord with those
reported.
2-(Propylthio)pyridine (5a): Yield: 203.3 mg (59%). Pale-brown li-
1
quid. IR (KBr): ν = 3050, 2929, 1579, 1454, 1124 cm–1. H NMR
˜
3
(300.13 MHz, CD3OD, 25 °C): δ = 1.21 (t, JH,H = 7.4 Hz, 3 H,
CH3), 1.85–1.92 (m, 2 H, CH2CH3), 3.27 (t, JH,H = 7.2 Hz, 2 H,
SCH2), 7.19–7.20 (m, 1 H, pCHar), 7.40 (d, JH,H = 8.1 Hz, 1 H,
3
3
3
oCHar), 7.72–7.75 (m, 1 H, mCHar), 8.52 (d, JH,H = 4.2 Hz, 1 H,
NCHar) ppm. 13C NMR (75.5 MHz, CD3OD, 25 °C): δ = 14.0
(CH3), 24.1 (CH2), 33.4 (CH2), 120.9 (CHar), 123.4 (CHar), 138.0
Eur. J. Org. Chem. 2010, 6409–6416
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6413

V. Gotor et al.
FULL PAPER
(CHar), 150.5 (CHar), 161.4 (Car) ppm. MS (ESI+): m/z (%) = 176
(100) [M + Na]+. HRMS (ESI+): calcd. for C8H11AgNS [M +
Ag]+ 259.9658; found 259.9654.
(50 m, pH 8.0–10.5, 0.5 mL) containing 1% DMSO, glucose-6-
phosphate (2.0 equiv.), glucose-6-phosphate dehydrogenase
(5 units), NADPH (0.2 m), and the corresponding Baeyer–Vil-
liger monooxygenase (1.0 unit). The mixture was shaken at 250 rpm
at the selected temperature in a rotary shaker for 24 h. Reactions
were then stopped and extracted with ethyl acetate (2ϫ0.5 mL).
The organic layers were dried with Na2SO4 and analyzed directly
by GC and/or HPLC to determine the conversion and the enantio-
meric excess values of the final products.
2-(Allylthio)pyridine (6a): Yield: 190.0 mg (56%). Pale-brown li-
1
quid. IR (KBr): ν = 3050, 2923, 1636, 1579, 1454, 1124 cm–1. H
˜
3
NMR (300.13 MHz, CDCl3, 25 °C): δ = 3.83 (d, JH,H = 6.8 Hz, 2
H, SCH2), 5.10 (dd, JH,H = 0.9 Hz, JH,H = 10.1 Hz, 1 H,
CH=CHH), 5.32 (dd, JH,H = 0.9 Hz, JH,H = 15.0 Hz, 1 H,
CH=CHH), 5.89–6.03 (m, 1 H, CH=CH2), 6.95–6.99 (m, 1 H,
pCHar), 7.17 (d, JH,H = 8.1 Hz, 1 H, oCHar), 7.46 (t, JH,H
8.0 Hz, 1 H, mCHar), 8.42 (d, JH,H = 4.0 Hz, 1 H, NCHar) ppm.
13C NMR (75.5 MHz, CDCl3, 25 °C): δ = 32.9 (CH2), 117.5 (CH2),
119.4 (CH), 122.1 (CHar), 133.7 (CHar), 135.9 (CHar), 149.3 (CHar),
158.4 (Car) ppm. MS (EI+): m/z (%) = 151 (21) [M]+, 136 (100),
124 (15), 118 (36), 79 (35). HRMS (ESI+): calcd. for C8H9AgNS
[M + Ag]+ 257.9507; found 257.9503.
2
3
2
3
3
3
Enzymatic Preparation of Chiral Sulfoxides (S)-1b, (S)-2b, (R)-3b,
(S)-4–6b, (S)-15, (S)-20b, and (S)-21b by Employing Cell-Free Ex-
tract from HAPMO on a Multimilligram Scale: Sulfides 1–6a, 15a,
20a, and 21a (250 mg) were dissolved in Tris-HCl buffer (50 m,
pH 9.0, 40 mL) containing 1% DMSO, NADPH (0.2 m), glucose-
6-phosphate (2 equiv.), glucose-6-phosphate dehydrogenase
(1250 units), and HAPMO cell-free extract (46 mL). The mixture
was shaken at 250 rpm at 20 °C, and the reaction was monitored
by TLC. After 24 h, the reaction was stopped and extracted with
ethyl acetate (5ϫ30 mL). The organic layers were dried with
Na2SO4 and evaporated under reduced pressure. The crude residues
were purified by flash chromatography by using different concen-
trations of CH2Cl2/MeOH as eluent, (98:2) for 1b, 5b, and 6b;
(95:5) for 2b, 3b, and 4b; and (90:10) for 15b, 20b, and 21b, to
obtain completely enantiopure (S)-1b, (S)-2b, (R)-3b, (S)-4–6b, (S)-
15b, (S)-20b, (S)-21b.
=
3
2-(Butylthio)pyridine (7a): Yield: 304.5 mg (81%). Colorless liquid.
IR (KBr): ν = 3100, 2958, 1579, 1556, 1121 cm–1. 1H NMR
˜
3
(300.13 MHz, CD3OD, 25 °C): δ = 1.13 (t, JH,H = 6.0 Hz, 3 H,
CH3), 1.61–1.70 (m, 2 H, CH2CH3), 1.80–1.90 (m, 2 H, SCH2CH2),
3
3.31 (t, JH,H = 7.2 Hz, 2 H, SCH2), 7.21–7.26 (m, 1 H, pCHar),
3
3
7.43 (d, JH,H = 9.0 Hz, 1 H, oCHar), 7.77 (t, JH,H = 6.0 Hz, 1 H,
mCHar), 8.53–8.54 (m, 1 H, NCHar) ppm. 13C NMR (75.5 MHz,
CD3OD, 25 °C): δ = 14.3 (CH3), 23.3 (CH2), 31.1 (CH2), 32.9
(CH2), 120.9 (CHar), 123.5 (CHar), 138.0 (CHar), 150.5 (CHar),
168.5 (Car) ppm. MS (EI+): m/z (%) = 167 (28) [M]+, 138 (72), 125
(100), 111 (72), 78 (64). HRMS (ESI+): calcd. for C9H13AgNS [M
+ Ag]+ 273.9820; found 273.9823.
(؎)-2-(Methylsulfinyl)pyridine [(؎)-1b]: Yield: 81.2 mg (72%). Pale-
yellow oil. IR (KBr): ν = 2924, 1644, 1579, 1426, 1032 cm–1. 1H
˜
NMR (300.13 MHz, CD3OD, 25 °C): δ = 2.86 (s, 3 H, CH3), 7.35–
7.40 (m, 1 H, pCHar), 7.91–8.01 (m, 2 H, oCHar and mCHar), 8.61
Cyclohexyl Ethyl Sulfide (16a): Yield: 223.4 mg (72%). Pale-yellow
3
(d, JH,H = 4.7 Hz, 1 H, NCHar) ppm. 13C NMR (75.5 MHz,
1
liquid. IR (KBr): ν = 3010, 2928, 2852, 1448, 1262, 998 cm–1. H
˜
CD3OD, 25 °C): δ = 41.2 (CH3), 119.2 (CHar), 124.5 (CHar), 138.0
(CHar), 149.5 (CHar), 169.5 (Car) ppm. MS (EI+): m/z (%) = 141
(15) [M]+, 124 (7), 93 (32), 78 (100), 51 (46). HRMS (ESI+): calcd.
for C6H7NNaOS [M + Na]+ 164.0141; found 164.0135. (S)-1b:
Yield: 267.9 mg (95%).
NMR (300.13 MHz, CDCl3, 25 °C): δ = 1.21–1.29 (m, 8 H, CH3
and 5 CH), 1.59–1.62 (m, 1 H, CH), 1.74–1.76 (m, 2 H,
CHHCHSCHH), 1.94–1.98 (m, 2 H, CHHCHSCHH), 2.15–2.65
(m, 3 H, SCH and SCH2) ppm. 13C NMR (75.5 MHz, CDCl3,
25 °C): δ = 15.0 (CH3), 23.9 (CH2), 25.8 (2 CH2), 26.1 (2 CH2),
33.6 (CH2), 43.0 (CH) ppm. MS (EI+): m/z (%) = 144 (5) [M]+, 82
(7), 67 (7), 40 (100). HRMS (ESI+): calcd. for C8H16NaS [M +
Na]+ 167.0865; found 167.0869.
(؎)-3-(Methylsulfinyl)pyridine [(؎)-2b]: Yield: 39.4 mg (35%). Pale-
yellow oil. IR (KBr): ν = 2921, 1651, 1580, 1416, 1037 cm–1. 1H
˜
NMR (300.13 MHz, CD3OD, 25 °C): δ = 3.09 (s, 3 H, CH3), 7.82–
3
7.87 (m, 1 H, mCHar), 8.37–8.41 (m, 1 H, CHarCS), 8.92 (d, JH,H
Cyclohexyl Propyl Sulfide (17a): Yield: 136.2 mg (40%). Pale-yel-
= 3.5 Hz, 1 H, CHarN), 9.04 (s, 1 H, NCHarCS) ppm. 13C NMR
(75.5 MHz, CD3OD, 25 °C): δ = 43.7 (CH3), 126.3 (CHar), 134.1
(CHar), 143.7 (Car), 146.6 (CHar), 153.3 (CHar) ppm. MS (EI+):
m/z (%) = 141 (100) [M]+, 126 (65), 78 (81), 51 (84), 39 (32). HRMS
(ESI+): calcd. for C6H7NNaOS [M + Na]+ 164.0141; found
164.0140. (S)-2b: Yield: 253.8 mg (90%), [α]2D5 = –34.3 (c = 1.21,
acetone), Ն99%ee.
low liquid. IR (KBr): ν = 3010, 2929, 2853, 1448, 1261 cm–1. 1H
˜
3
NMR (300.13 MHz, CDCl3, 25 °C): δ = 0.94 (t, JH,H = 4.1 Hz, 3
H, CH3), 1.21–1.36 (m, 6 H, 6 CH), 1.53–1.65 (m, 2 H, 2 CH),
1.74–1.77 (m,
2 H, CHHCHSCHH), 1.93–2.03 (m, 2 H,
3
CHHCHSCHH), 2.51 (t, JH,H = 7.2 Hz, 2 H, SCH2), 2.58–2.63
(m, 1 H, SCH) ppm. 13C NMR (75.5 MHz, CDCl3, 25 °C): δ =
13.6 (CH3), 23.3 (CH2), 25.8 (CH2), 26.1 (2 CH2), 33.2 (2 CH2),
33.7 (CH2), 43.4 (CH) ppm. MS (EI+): m/z (%) = 158 (30) [M]+,
115 (19), 82 (75), 67 (77), 55 (58), 40 (100). HRMS (ESI+): calcd.
for C9H18NaS [M + Na]+ 181.1027; found 181.1022.
(؎)-4-(Methylsulfinyl)pyridine [(؎)-3b]: Yield: 22.6 mg (20%). Yel-
low oil. IR (KBr): ν = 2921, 1620, 1579, 1408, 1037 cm–1. 1H NMR
˜
(300.13 MHz, CD3OD, 25 °C): δ = 2.91 (s, 3 H, CH3), 7.78 (d,
3
3JH,H = 6.2 Hz, 2 H, CHarCSCHar), 8.80 (d, JH,H = 3.9 Hz, 2 H,
Racemic sulfoxides 1–6b, 11b, 12b, 15b, 16b, 20b, and 21b were
obtained by oxidation of the corresponding sulfides 1–6a, 11a, 12a,
15a, 16a, 20a, and 21a (100 mg) with H2O2 in MeOH at room
temperature (yields ranged from to 10 to 98%, as indicated for each
of the compounds).
Compounds (Ϯ)-11b,[26a] (Ϯ)-12b,[26b] (Ϯ)-16b,[25c] (Ϯ)-18b,[26d]
(Ϯ)-19b,[26e] (Ϯ)-20b,[26f] and (Ϯ)-22b[26g] also exhibit physical and
spectral properties in accord with those reported.
CHarNCHar) ppm. 13C NMR (75.5 MHz, CD3OD, 25 °C): δ = 43.4
(CH3), 120.0 (2 CHar), 151.5 (2 CHar), 158.1 (Car) ppm. MS (EI+):
m/z (%) = 141 (4) [M]+, 125 (100), 92 (30), 79 (20), 51 (29). HRMS
(ESI+): calcd. for C6H7NOS [M + H]+ 142.0321; found 142.0338.
(R)-3b: Yield: 141.0 mg (50%), [α]2D5 = +41.1 (c = 0.95, acetone),
Ն99%ee.
(؎)-2-(Ethylsulfinyl)pyridine [(؎)-4b]: Yield: 11.2 mg (10%). Pale-
yellow oil. IR (KBr): ν = 2963, 1650, 1580, 1449, 1012 cm–1. 1H
˜
3
General Method for the BVMO-Catalyzed Oxidation of Sulfides 1–
22a or Sulfoxides (؎)-1–3b, (؎)-15b, and (؎)-22b: In a typical ex-
periment, starting sulfides 1–22a (10 m) or corresponding sulfox-
NMR (300.13 MHz, CD3OD, 25 °C): δ = 1.35 (t, JH,H = 7.4 Hz,
3 H, CH3), 3.10–3.19 (m, 1 H, CHHS), 3.22–3.43 (m, 1 H, CHHS),
3
7.70–7.74 (m, 1 H, pCHar), 8.10 (d, JH,H = 9.0 Hz, 1 H, oCHar),
ides (Ϯ)-1–3b, (Ϯ)-15b, and (Ϯ)-22b were dissolved in Tris-HCl 8.27 (t, JH,H = 9.0 Hz, 1 H, mCHar), 8.85 (d, 3JH,H = 4.6 Hz, 1 H,
3
6414
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 6409–6416

Synthesis of Chiral Sulfoxides by using Baeyer–Villiger Monooxygenases
NCHar) ppm. 13C NMR (75.5 MHz, CD3OD, 25 °C): δ = 5.8
Determination of the Kinetic Parameters for the HAPMO Oxidation
(CH3), 48.5 (CH2), 121.8 (CHar), 126.7 (CHar), 139.9 (CHar), 151.5 of Sulfides 1a, 2a, 11a, 15a, and 20a: Unless indicated otherwise,
(CHar), 164.1 (Car) ppm. MS (ESI+): m/z (%) = 178 (100) [M +
Na]+. HRMS (ESI+): calcd. for C7H9NNaSO [M + Na]+ 178.0297;
found 178.0310. (S)-4b: Yield: 279.0 mg (85%), [α]2D5 = –56.2 (c =
1.05, acetone), Ն99%ee.
all activities measurements were performed in air-saturated 50 m
Tris-HCl buffer (pH 9.0). HAPMO concentration was measured
photometrically by using
a molar extinction coefficient of
12.4 m–1 cm–1 at 440 nm for protein bound to FAD. Enzymes ac-
tivities were determined spectrophotometrically at 30 °C by moni-
toring the decrease in absorbance at 340 nm due to the NADPH
oxidation. For compounds 1a and 2a, measurements were per-
formed at 370 nm. This wavelength was selected because these sul-
fides displayed significant absorbance at 340 nm, the λmax of
NADPH. The standard assay mixture contained stock solutions of
sulfides in DMSO (1.0 ), 10 m NADPH, and the biocatalyst
(0.2–0.8 µ).
(؎)-2-(Propylsulfinyl)pyridine [(؎)-5b]: Yield: 69.6 mg (63%). Pale-
yellow oil. IR (KBr): ν = 2966, 1650, 1577, 1453, 1022 cm–1. 1H
˜
3
NMR (300.13 MHz, CD3OD, 25 °C): δ = 1.25 (t, JH,H = 7.3 Hz,
3 H, CH3), 1.73–1.99 (m, 1 H, CHHCH3), 2.02–2.09 (m, 1 H,
CHHCH3), 3.07–3.17 (m, 1 H, CHHS), 3.26–3.35 (m, 1 H, CHHS),
7.70–7.74 (m, 1 H, pCHar), 8.08–8.14 (m, 1 H, oCHar), 8.22–8.31
3
(m, 1 H, mCHar), 8.85 (d, JH,H = 4.8 Hz, 1 H, NCHar) ppm. 13C
NMR (75.5 MHz, CD3OD, 25 °C): δ = 13.6 (CH3), 16.8 (CH2),
57.2 (CH2), 121.4 (CHar), 126.7 (CHar), 140.1 (CHar), 151.4 (CHar),
164.8 (Car) ppm. MS (ESI+): m/z (%) = 192 (100) [M + Na]+.
HRMS (ESI+): calcd. for C8H11NNaOS [M + Na]+ 192.0454;
found 192.0455. (S)-5b: Yield: 16.6 mg (6%).
Supporting Information (see footnote on the first page of this arti-
cle): Experimental data of sulfides 1–4a, 8–10a and sulfoxides 11b,
12b, 16b, 18b, 19b, and 22b; determination of absolute configura-
tions of sulfoxides 2–4b, 6b, and 16b by employing the Kagan meth-
1
odology;[27] GC and HPLC data and H and 13C-NMR spectra of
(؎)-2-(Allylsulfinyl)pyridine [(؎)-6b]: Yield: 67.2 mg (61%). Yellow
compounds 5a, 6a, 7a, 16a, 17a, 1b, 2b, 3b, 4b, 5b, 6b, 15b, and
21b.
oil. IR (KBr): ν = 2928, 1637, 1578, 1425, 1036 cm–1. 1H NMR
˜
2
3
(300.13 MHz, CDCl3, 25 °C): δ = 3.58 (dd, JH,H = 13.1 Hz, JH,H
= 7.4 Hz, 1 H, CHHS), 3.83 (dd, JH,H = 13.1 Hz, JH,H = 7.2 Hz,
1 H, CHHS), 5.15 (d, JH,H = 17.7 Hz, 1 H, CHH=CH), 5.26 (d,
2
3
3
Acknowledgments
3JH,H
= 9.0 Hz, 1 H, CHH=CH), 5.57–5.69 (m, 1 H,
A.R.-M. (FPU Program) thanks the Spanish Ministerio de Ciencia
e Innovación (MICINN) for her predoctoral fellowship, which is
financed by the European Social Fund. G.d.G. thanks MICINN
by personal funding. This work was supported by the Ministerio
de Educación y Ciencia (MEC) (Project CTQ 2007–61126). M.W.F.
and D.E.T.P. received support from the EU-FP7 “Oxygreen” pro-
ject.
CH2=CHCH2), 7.35 (t, 3JH,H = 4.3 Hz, 1 H, pCHar), 7.89–7.90 (m,
3
2 H, oCHar and mCHar), 8.60 (d, JH,H = 4.6 Hz, 1 H, NCHar)
ppm. 13C NMR (75.5 MHz, CDCl3, 25 °C): δ = 57.5 (CH2), 120.5
(CH), 123.7 (CH2), 124.4 (CHar), 125.0 (CHar), 137.7 (CHar), 149.4
(CHar), 163.5 (Car) ppm. MS (ESI+): m/z (%) = 190 (100) [M +
Na]+. HRMS (ESI+): calcd. for C8H9NNaOS [M + Na]+ 190.0303;
found 190.0306. (S)-6b: Yield: 52.8 mg (19%), [α]2D5 = –39.8 (c =
0.75, acetone), Ն99%ee.
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[5] a) J. K. Whitesell, M.-S. Wong, J. Org. Chem. 1991, 56, 4552–
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[6] a) V. Gotor, I. Alfonso, E. García-Urdiales, Asymmetric Or-
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(؎)-Cyclohexyl Methyl Sulfoxide [(؎)-15b]: Yield: 110.1 mg (98%).
Pale-yellow liquid. IR (KBr): ν = 2931, 1451, 1033 cm–1. 1H NMR
˜
(300.13 MHz, CD3OD, 25 °C): δ = 1.58–1.66 (m, 6 H, 6 CH),
1.91–1.94 (m,
2 H, CHHCHSCHH), 2.28–2.31 (m, 2 H,
CHHCHSCHH), 2.84 (s, 3 H, SCH3), 2.85–2.88 (m, 1 H, SCH)
ppm. 13C NMR (75.5 MHz, CD3OD, 25 °C): δ = 25.7 (CH2), 26.5
(CH2), 26.7 (CH2), 26.9 (CH2), 27.6 (CH2), 35.1 (CH3), 61.6 (CH)
ppm. MS (EI+): m/z (%) = 146 (7) [M]+, 83 (51), 67 (71), 55 (92),
40 (100). HRMS (ESI+): calcd. for C7H14NaOS [M + Na]+
169.0658; found 169.0649. (S)-15b: Yield: 255.5 mg (91%).
(؎)-n-Butyl Methyl Sulfoxide [(؎)-20b]: Yield: 87.5 mg (76%). Yel-
low oil. IR (KBr): ν = 2960, 2937, 1652, 1038 cm–1. 1H NMR
˜
3
(300.13 MHz, CD3OD, 25 °C): δ = 1.18 (t, JH,H = 6.6 Hz, 3 H,
butyl CH3), 1.63–1.77 (m, 2 H, CH2CH3), 1.87–1.98 (m, 2 H,
CH2CH2S), 2.82 (s, 3 H, methyl CH3), 2.90–3.13 (m, 2 H, CH2S)
ppm. 13C NMR (75.5 MHz, CD3OD, 25 °C): δ = 14.3 (CH3), 23.2
(CH2), 26.0 (CH2), 38.4 (CH3), 54.9 (CH2) ppm. MS (ESI+): m/z
(%) = 120 (9) [M]+, 64 (100), 41 (88). (S)-20b: Yield: 249.0 mg
(88%), [α]2D5 = +32.5 (c = 0.5 in ethanol), Ն99%ee.
(؎)-n-Butyl Ethyl Sulfoxide [(؎)-21b]: Yield: 72.6 mg (64%). Yel-
low oil. IR (KBr): ν = 2962, 2935, 1648, 1045 cm–1. 1H NMR
˜
3
(300.13 MHz, CD3OD, 25 °C): δ = 1.18 (t, JH,H = 7.4 Hz, 3 H,
3
butyl CH3), 1.51 (t, JH,H = 7.6 Hz, 3 H, ethyl CH3), 1.63–1.77 (m,
2 H, CH2CH3), 1.88–1.98 (m, 2 H, CH2CH2S), 2.86–3.11 (m, 4 H,
CH2SCH2) ppm. 13C NMR (75.5 MHz, CD3OD, 25 °C): δ = 7.6
(CH3), 14.5 (CH3), 23.5 (CH2), 26.3 (CH2), 46.7 (CH2), 52.4 (CH2)
ppm. MS (EI+): m/z (%) = 134 (6) [M]+, 118 (26), 78 (100), 41 (81).
HRMS (ESI+): calcd. for C6H14NaOS [M + Na]+ 157.0658; found
157.0668. (S)-21b: Yield: 252.7 mg (89%).
Eur. J. Org. Chem. 2010, 6409–6416
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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V. Gotor et al.
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Received: June 21, 2010
Published Online: October 15, 2010
6416
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 6409–6416