Chemoenzymatic approaches to obtain chiral-centered selenium compounds

Article abstract of DOI:10.1016/j.tet.2012.09.087

The synthesis of chiral-centered selenium compounds is presented. Enantioselective oxidations of these organoselenium compounds were performed using a wide range of biocatalysts, including Baeyer-Villiger monooxygenases, oxidoreductases-containing Aspergillus terreus and lipase (Cal-B) in the presence of oxidants. Finally, efficient synthesis of enantiopure organoselenium compounds using a kinetic resolution approach mediated by Cal-B was achieved.

Full text of DOI:10.1016/j.tet.2012.09.087

Tetrahedron 68 (2012) 10431e10436
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Chemoenzymatic approaches to obtain chiral-centered selenium compounds
Patrícia B. Brondani a, Nathalie M.A.F. Guilmoto a, Hanna M. Dudek b, Marco W. Fraaije b,
,
Leandro H. Andrade a
*
a
~
~
Instituto de Química, Universidade de Sao Paulo, Av. Prof. Lineu Prestes 748, SP 05508-900, Sao Paulo, Brazil
b Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 June 2012
Received in revised form 17 September 2012
Accepted 19 September 2012
Available online 26 September 2012
The synthesis of chiral-centered selenium compounds is presented. Enantioselective oxidations of these
organoselenium compounds were performed using
a wide range of biocatalysts, including
BaeyereVilliger monooxygenases, oxidoreductases-containing Aspergillus terreus and lipase (Cal-B) in
the presence of oxidants. Finally, efficient synthesis of enantiopure organoselenium compounds using
a kinetic resolution approach mediated by Cal-B was achieved.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Selenium
Monooxygenase
Lipase
Selenium oxidation
1. Introduction
of enamines and ketones, enantioselective addition of dio-
rganozinc to aldehydes and enantioselective conjugate addition of
Organoselenium compounds are very useful in organic synthe-
sis, and they are regularly employed in the preparation of many
organic compounds including natural products.1 They can be used
in nucleophilic, electrophilic, and radical reactions due to their
chemical properties and can be incorporated into a variety of sub-
strates under mild conditions.2 Additionally, organoselenium
compounds have shown biological activities like antioxidant, anti-
tumor, antimicrobial, and antiviral activities.3 Including these
activities, we can mention that chiral and achiral organoselenanes
can inactivate protein tyrosine phosphatases (PTPs)4 and cysteine
proteases5 (cathepsins S and V) thereby acting as enzyme inhibitors.
organometallic to enones.2 Indeed, asymmetric synthesis using
organochalcogen compounds in general is of current interest in the
field of organometallic chemistry.7 Despite the great usefulness of
chiral selenides, the asymmetric synthesis of optically active
organoselenium compounds is still
a challenge in selenium
chemistry.8
Very few studies on the enzymatic reactions of selenium com-
pounds for synthetic purposes have been published to date.9 En-
zymatic processes, due to their wide applications under mild
conditions, are environmentally friendly reactions. From the syn-
thetic organic point of view the enzyme-catalyzed reactions pres-
ent a vast opportunity to selenium chemistry regarding selectivity,
including regio-, enantio-, and chemoselectivity.
In the search for novel chiral-centered selenium compounds to
be used in our studies for the development of protease inhibitors,4,5
we encountered a lack of synthetic methodologies to obtain such
compounds. We here describe a study about the application of
different chemoenzymatic approaches to obtain chiral-centered
selenium compounds, see Fig. 1. An oxidative approach was applied
to racemic organoselenium compounds in order to perform their
kinetic resolution in enzyme-mediated reactions (Fig. 1a). The
second approach involved kinetic resolution of precursor 1-
phenylethanols by lipases, and their transformation to the desired
chiral organoselenium compounds using selenium chemistry
(Fig. 1b).
The
compounds, which involves selenoxides with a
This reaction has been used as an efficient synthetic tool to prepare
alkenes and
-unsaturated ketones.6 Selenoxides are generally
b
-elimination reaction is the best-known reaction of selenium
b-hydrogen atom.
a,b
prepared from the corresponding selenides using common oxidants
like hydrogen peroxide and sodium metaperiodate.6
Besides being interesting synthetic intermediates, chiral orga-
noselenium compounds have been satisfactorily used as catalysts
or chiral ligands in organic transformations such as hydrosilylation
* Corresponding author. Tel.: þ55 11 3091 2287; fax: þ55 11 3815 5579; e-mail
address: leandroh@iq.usp.br (L.H. Andrade).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.09.087

10432
P.B. Brondani et al. / Tetrahedron 68 (2012) 10431e10436
Fig. 1. Chemoenzymatic approaches to obtain chiral-centered selenium compounds.
2. Results and discussion
regenerating system with phosphite dehydrogenase (PTDH) and
a sacrificial substrate (sodium phosphite). In this test we observed
that the oxidation reaction occurred with good enantioselectivity
(84% ee, Table 1, entry 1). Next, we increased the amount of
phosphite in the reaction medium leading to higher enantiose-
lectivity (98% ee, Table 1, entry 2). By applying the same reaction
conditions to compound 4 (with the benzyl group attached to the
selenium atom and no substituent in the aromatic ring) lower
enantioselectivity was observed (14% ee, Table 1, entries 6 and 7).
As hydrogen peroxide can be produced by BVMOs, we also eval-
2.1. Synthesis of chiral-centered selenium compounds: kinetic
resolution using a selenium oxidation approach
Initially, we synthesized phenylselenium derivatives (1e3) in
which the selenium atom is attached to a phenyl and a 1-
phenylethyl group. The protocol was based on the inversion of
configuration of the phenylethanol derivatives by their reactions
with phenylselenocyanide and tributylphosphine (Scheme 1).10
Scheme 1. Synthesis of selenides 1e6: (a) PhSeCN, Bu3P, toluene, 20 h, rt (b) DMAP, CH2Cl2, MsCl, 16 h, rt (c) NaBH4, EtOH/DMF, 3 h, rt.
Next, we performed the synthesis of benzylselenides derivatives
(4e6). Initially, mesylated phenylethanol derivatives were pre-
pared in situ from their corresponding alcohols and mesyl chloride
(MsCl). Then, benzyl selenoate was prepared from benzyl bromide
(BnBr), elemental selenium, and NaBH4, which was then trans-
ferred to the mesylated derivatives to give the benzylselenides
(4e6) (Scheme 1).11 It is noteworthy that the addition of DMF is
relevant for the success of the reaction. In fact, DMF improves the
yield by solubilizing the selenolate anion, which is otherwise
known to exist as boron complex possessing diminished
nucleophilicity.11
uated the use of catalase, an enzyme that degrades hydrogen
peroxide into dioxygen and water. However, we observed several
by-products even with no PAMO in the reaction medium (control
reactions). These results could be due to the low purity of com-
mercial catalase that can contain small quantities of other
oxidoreductases.
In an effort to investigate other BVMOs, we performed reactions
using M446G PAMO, CHMO, and HAPMO as biocatalysts. When
M446G PAMO mediated the reactions, good conversions but low
enantioselectivity were observed (Table 1, entries 3 and 8). When
CHMO or HAPMO was employed, no reaction was observed (Table 1,
entries 4, 5, 9, and 10).
2.1.1. Oxidative kinetic resolution of chiral selenides mediated by
BVMOs. BaeyereVilliger monooxygenases (BVMOs) are flavin-
containing and NAD(P)H-dependent enzymes that catalyze the
incorporation of one oxygen atom into organic substrates.12 BVMOs
usually mediate reactions with high enantio- and/or regiose-
lectivity13 and they are known to be excellent alternatives for
performing the oxidation of aldehydes and ketones to their corre-
sponding esters, the oxygenation of heteroatoms (sulfur, nitrogen,
phosphorus, boron, and selenium) and even epoxidation
reactions.9c,14
For the selenium oxidation approach using BVMOs, we first
selected the phenylselenide derivative 1 as model substrate and
PAMO as biocatalyst. This compound was selected to evaluate the
influence of a phenyl group directly attached to the selenium
atom on the oxidation reaction. We also employed a cofactor
By using appropriate reaction condition with PAMO, we de-
cided to explore the oxidative kinetic resolution of several
other selenides (2, 3, 5, and 6, Table 1, entries 11e14). In all cases,
the selenium oxidation was observed but with low or no
enantioselectivity.
In an ideal oxidative kinetic resolution of selenides, a
b-elimi-
nation reaction should be observed with one enantiomer, which
would produce RSeOH and styrene derivative (RCH]CH2), and the
other enantiomer would be untouched. After assigning the absolute
configuration we concluded that, when the selenium oxidation
occurs, the (R)-selenide undergoes the oxidation faster than the (S)-
selenide.
2.1.2. Oxidative kinetic resolution of (RS)-phenylselenide 1 mediated
by Aspergillus terreus. We have recently described that A. terreus

P.B. Brondani et al. / Tetrahedron 68 (2012) 10431e10436
10433
Table 1
Oxidative kinetic resolution of selenium-containing aromatic compounds 1e6 mediated by BVMOsa
Entry
Substrate
BVMO
Phosphite (mM)
Time (h)
Conv.b (%)
eec (%)
1
2
3
4
5
6
7
8
1
1
1
1
1
4
4
4
4
4
2
3
5
6
PAMO
PAMO
M446G
HAPMO
CHMO
PAMO
PAMO
M446G
HAPMO
CHMO
PAMO
PAMO
PAMO
PAMO
20
50
50
50
50
20
50
50
50
50
50
50
50
50
5
5
5
24
24
5
5
5
24
24
8
52
49
45
84
98
10
(d)d
(d)d
53
(d)e
(d)e
(d)e
14
56
51
10
9
(d)d
(d)d
51
(d)e
(d)e
9
10
11
12
13
14
8
5
5
54
49
47
(d)e
(d)e
49
a
Tris buffer 50 mM pH 7.5, DMSO (1%), substrate (10 mM), phosphite (20 or 50 mM), PTDH (1
Conv.¼conversion based on selenide consumption using HPLC analysis.
Enantiomeric excess measured by HPLC.
No reaction was observed.
No ee was observed.
m
M), NADPH (0.2 mM), and BVMO (8 mM).
b
c
d
e
strains (URM 3571 and CCT 3320) produce oxidoreductases,
Table 2
Oxidative kinetic resolution of compound (RS)-1 mediated by CAL-Ba
which can mediate BaeyereViliger reactions, deracemization of
several phenylethanol derivatives, and selenium oxidations.9d,15
We tested these fungal strains for the oxidation of the phe-
nylselenide derivative 1, which was the best substrate in the pre-
vious reactions with BVMOs. The reactions were carried out using
buffer, substrate, and fungal cells, which were incubated at 32 ꢀC
for 1, 3, and 5 days. All bio-oxidations were followed by a control
reaction in the absence of fungal cells. However, even in 5 days, the
phenylselenide 1 remained in the racemic form. This lack of re-
activity can be attributed to the presence of a steric hindrance near
to the selenium atom. As (RS)-phenylselenide 1 has two bulky
moieties surrounding the selenium atom, it can be hindering the
oxidation mediated by enzymes from A. terreus. In a previous study
with fungi and organoselenium alcohols, we observed a similar
result, in which selenium oxidation is very slow or does not occur
at all.9d
Entry Oxidant Solvent
Temperature Time Conv.b eec
(ꢀC)
(h)
(%)
(S)-1 (%)
1
2
3
4
5
6
7
H2O2
H2O2
Hexane
Hexane
Hexane
Hexane/MeOH (9:1) 25
Hexane/MeOH (1:1) 25
Hexane/EtOAc (9:1) 25
Hexane/MeOH (9:1) 25
0
25
0
2
90
(d)e
(d)e
(d)e
7
f
0.5 100
2
2
2
2
15
2
2
2
2
NaIO4
NaIO4
NaIO4
NaIO4
NaIO4
(d)d
5
3
(d)e
(d)e
7
(d)d
52
8
m-CPBA Hexane
25
46
100
90
45
9
10
11
m-CPBA Hexane/MeOH (9:1) 25
m-CPBA Hexane/EtOAc (9:1) 25
m-CPBA Toluene
(d)e
5
25
100
(d)e
2.1.3. Oxidative kinetic resolution of (RS)-phenylselenide 1 mediated
by Candida antarctica lipase B (CAL-B) in the presence of oxi-
dants. Lipases are also known to catalyze oxidations when in the
presence of oxidizing species such as H2O2 or peracids, which
constitutes an alternative method for bio-oxidations. They can be
used in epoxidation reactions or, by lipase-mediated in situ for-
mation of peroxy acids, even in BaeyereVilliger reactions.16 Among
the lipases studied so far, Novozym 435, a commercial preparation
of lipase from C. antarctica, has been shown the best results in
oxidative transformations becoming the most popular lipase for
this purpose.16
In this context, we decided to explore the use of lipase in the
presence of oxidants (H2O2, NaIO4, m-CPBA) in order to perform
enantioselective oxidation of organoselenium compounds. There-
fore, the lipase-induced enantioselective oxidation reactions were
carried out at 0 ꢀC and room temperature in organic solvent. Con-
trol reactions in the absence of lipase and/or oxidant were also
studied (Table 2).
a
Substrate (0.25 mM), solvent (1 mL), CAL-B (20 mg), oxidant (3 equiv), 0 or
25 ꢀC, 2e15 h.
b
c
d
e
f
Conv.¼conversion based on selenide consumption using HPLC analysis.
Enantiomeric excess measured by HPLC.
No reaction was observed.
No ee was observed.
2 equiv.
In most cases, no enantioselectivity (Table 2, entries 1, 2, 7, 9, 10,
and 11) or no reaction was observed (Table 2, entries 3 and 6). In
control reactions, we observed oxidation reaction without lipase,
which can be the reason for the low enantioselectivity. However,
the best system used was m-CPBA in n-hexane at room tempera-
ture (Table 2, entry 8), which resulted in enantioselective oxidation
of (R)-1. In fact, these results demonstrate the potential of peracids
and lipase for enantioselective selenium oxidation, without pre-
cedent until now.

10434
P.B. Brondani et al. / Tetrahedron 68 (2012) 10431e10436
2.2. Synthesis of chiral-centered selenium compounds: ki-
netic resolution using a transesterification approach
phenone monooxygenase (HAPMO) from Pseudomonas fluo-
rescens, and cyclohexanone monooxygenase (CHMO) from Aci-
netobacter sp. were overexpressed and purified as previously
described.18 Phosphite dehydrogenase was obtained as previously
described.19 Both sodium phosphite and NADPH were purchased
from SigmaeAldrich. Gas chromatography (GC) analyses were
performed on a Shimadzu GC-17A instrument equipped with
a flame ionization detector. High performance liquid chroma-
tography (HPLC) analyses were performed on a Shimadzu LC-
Another approach was evaluated to obtain both enantiomers of
chiral selenides 1e6. Kinetic resolution of phenylethanol derivatives
through a lipase-catalyzed transesterification reaction was the pro-
tocol of choice. This process was performed with vinyl acetate as acyl
donor and Cal-B as biocatalyst (Scheme 2).17 We obtained the corre-
sponding (R)-acetate and the untouched (S)-alcohol in high ee, >99%.
Scheme 2. Synthesis of enantiopure chiral selenides 1e6: (a) Cal-B, vinyl acetate, n-hexane, 32 ꢀC, 160 rpm, 24 h; (b) PhSeCN, Bu3P, THF, 16 h, rt; (c) DMAP, MsCl, CH2Cl2, 16 h, rt; (d)
[BnSeꢁ] prepared from elemental selenium, NaBH4, benzyl bromide, EtOH/DMF, 3 h rt; (e) NaOH 1 M, MeOH, 1 h.
After column chromatography and ester hydrolysis, both enan-
tiomerically pure phenylethanol derivatives were achieved. Then,
these alcohols were transformed into the chiral selenides 1e6 using
the same protocol described in the Section 2.1. As the insertion of
the selenium moiety involved a Mitsunobu reaction or a nucleo-
philic substitution reaction with a selenolate species, we noticed
that an (S)-alcohol will provide an (R)-selenide while an (R)-alcohol
will provide an (S)-selenide.
10AD or Shimadzu SPD-M1OA liquid chromatograph equipped
with variable wavelength UV detector (deuterium lamp
190e600 nm). Sterile materials were used to perform the ex-
periments involving microorganisms. A laminar flow cabinet
Fisher Hamilton (Class II Biological Safety Cabinet) was used to
handle microorganisms.
a
4.2. General procedure for synthesis of chiral organoselenium
compounds (1e3)
3. Conclusion
The enantiomerically pure forms of 1-phenylethanols de-
rivatives were prepared by lipase-catalyzed transesterification re-
actions of their racemic forms with vinyl acetate according to the
methodology described in literature.17 Each enantiomer of organo-
selenium compounds (1e3) was synthesized from 1-phenylethanol
derivatives by their reactions with phenylselenocyanide and tribu-
tylphosphine as described in literature.10
Among the BVMOs tested, PAMO showed the best results for ox-
idation of chiral selenides. PAMO was able to oxidize
phenylselenium-containing compound with no substituent in the
aromatic ring with great enantioselectivity (98% ee). The mutant
M446Gshowedasimilarsubstrateacceptance,butitdisplayedlower
enantioselectivity. The BVMOs, HAPMO, and CHMO were not able to
oxidize any of the tested substrates. When whole cells of A. terreus
were used, no significant selenium oxidation was observed. By using
another approach, Cal-B in the presence of m-CPBA as oxidant spe-
cies, enantioselective conversion was observed (45% ee). By applying
a different approach, lipase-catalyzed transesterification reactions
provided us the enantiomerically pure alcohols, which were trans-
formed into the desired chiral benzyl and phenylselenides.
4.2.1. Phenyl(1-phenylethyl)selane (1). Yellow oil (82%), 1H NMR
(200 MHz, CDCl3):
7.20e7.60 (m, 10H).
d
1.73 (d, J¼7.0 Hz, 3H), 4.43 (q, J¼7.0 Hz),
13C NMR (50 MHz, CDCl3):
d
22.1, 42.4, 127.1,
127.7, 128.3, 128.8, 129.1, 131.4, 135.4, 141.6, 143.5. LRMS (EI) m/z (%):
262 (Mþ, 5), 156 (8), 105 (100), 79 (10), 78 (9), 77 (14), 51 (7). FT-IR
(film, cmꢁ1
) nmax: 3057, 2961, 2918, 1576, 1474, 1436, 1021, 762, 691,
22
467. [
a
]
22 ꢁ70 (c 1.0, CH2Cl2) for (S)-enantiomer (ee>99%) and [
a]
D
D
4. Experimental
4.1. General
þ87 (c 1.0, CH2Cl2) for (R)-enantiomer (ee>99%).
The enantiomeric excess was analyzed by HPLC equipped with
a ChiracelÒ OJ-H column. Eluent¼mixture of n-heptane and iso-
propanol or n-hexane and iso-propanol (99:1). Flow rate¼0.5 mL/
min; UV detector¼254 nm; retention times: tR¼21.55 min (S) and
31.88 min (R) or tR¼16.01 min (S) and 31.13 min (R).
Recombinant phenylacetone monooxygenase (PAMO) from
Thermobifida fusca, M446G PAMO mutant, 4-hydroxyaceto-

P.B. Brondani et al. / Tetrahedron 68 (2012) 10431e10436
10435
4.2.2. Phenyl-[1-(4-tolylethyl)]selane (2). Yellow oil (87%), 1H NMR
4.3.2. Benzyl-[1-(4-tolylethyl)]selane (5). Yellow oil (50%), 1H NMR
(200 MHz, CDCl3):
d
1.73 (d, J¼7.0 Hz, 3H), 2.30 (s, 3H), 4.44 (q,
(200 MHz, CDCl3):
d
1.69 (d, J¼7.0 Hz, 3H), 2.31 (s, 3H), 3.80 (s, 2H),
J¼7 Hz, 1H), 7.04e7.63 (m, 9H). 13C NMR (50 MHz, CDCl3):
d
21.1,
3.99 (q, J¼7.0 Hz, 1H), 7.12e7.25 (m, 9H). 13C NMR (50 MHz, CDCl3):
22.3, 42.2, 127.0, 127.7, 129.1, 131.4, 135.2, 136.6, 140.5. LRMS (EI) m/z
d 21.1, 32.5, 41.0, 52.9, 127.1, 128.3, 128.9, 128.9, 137.0, 139.1, 140.0,
(%): 276 (Mþ, 4),156 (6), 119 (100), 91 (20), 77 (12), 65 (5), 51 (4). FT-
144.8. LRMS (EI) m/z (%): 290 (Mþ, 3), 207 (4), 182 (3), 120 (12), 119
IR (film, cmꢁ1
)
nmax: 3054, 2961, 2919, 1576, 1511, 1474, 1437, 1020,
(100), 118 (13), 117 (14), 91 (55), 77 (8), 65 (10), 51 (6). FT-IR
22
816, 736, 689, 532, 466. [
mer (ee>99%) and [
(ee>99%).
The enantiomeric excess was analyzed by HPLC equipped with
a ChiracelÒ OJ-H column. Eluent¼n-heptane. Flow rate¼0.5 mL/
min; UV detector¼220 nm; retention times: tR¼37.74 min (S) and
41.54 min (R).
a]
ꢁ151 (c 1.0, CH2Cl2) for (S)-enantio-
(film, cmꢁ1
) nmax: 3024, 2917, 1655, 1510, 1440, 1029, 816, 757,
D
22
a
]
þ130 (c 1.0, CH2Cl2) for (R)-enantiomer
695, 591, 492. [
a
]
22 þ63 (c 1.0, CH2Cl2) for (S)-enantiomer (ee>99%)
D
D
22
and [
a]
ꢁ76 (c 1.0, CH2Cl2) for (R)-enantiomer (ee>99%).
D
The enantiomeric excess was analyzed by HPLC equipped with
a ChiracelÒ OJ-H column. Eluent¼n-heptane. Flow rate¼0.5 mL/
min; UV detector¼220 nm; retention times: tR¼50.65 min (S) and
54.90 min (R).
4.2.3. 1-(4-Fluorophenyl)ethyl-phenylselane (3). Yellow oil (79%),
4.3.3. Benzyl-[1-(4-fluorophenyl)ethyl]selane (6). Yellow oil (43%),
1H NMR (200 MHz, CDCl3):
d
1.72 (d, J¼7.2 Hz, 3H), 4.43 (q,
J¼7.2 Hz, 1H), 6.80e6.96 (m, 2H), 7.14e7.28 (m, 5H), 7.38e7.43 (m,
2H). 13C NMR (50 MHz, CDCl3):
22.2, 41.6, 114.8, 115.2, 127.9,
1H NMR (200 MHz, CDCl3):
d
1.64 (d, J¼7.2 Hz, 3H), 3.69 (s, 2H), 3.80
(q, J¼7.2 Hz, 1H), 6.9e7.2 (m, 9H). 13C NMR (50 MHz, CDCl3):
d 22.9,
d
37.6, 52.87, 114.9, 115.3, 115.3, 128.4, 128.4, 128.5, 128.7, 161.4 (d,
128.6, 128.8, 128.8, 135.6, 161.6 (d, J¼244 Hz). LRMS (EI) m/z (%):
J¼243 Hz). LRMS (EI) m/z (%): 294 (Mþ, 4), 124 (7), 123 (100), 122
280 (Mþ, 6), 262 (4), 105 (100), 91 (41), 77 (9), 65 (7), 51 (4). FT-IR
(7), 103 (20), 91 (22), 77 (5), 65 (6), 51 (3). FT-IR (film, cmꢁ1
) nmax:
(film, cmꢁ1
692, 533, 469. [
(ee>99%) and [
(ee>99%).
)
nmax: 3070, 2965, 2864, 1602, 1508, 1223, 834, 741,
2965, 2919, 1655, 1603, 1508, 1222, 1028, 834. [
CH2Cl2) for (S)-enantiomer (ee>99%) and [
CH2Cl2) for (R)-enantiomer (ee>99%).
a
]
þ84 (c 1.0,
22
D
22
22
a
]
ꢁ180 (c 1.0, CH2Cl2) for (S)-enantiomer
a]
ꢁ100 (c¼1.0,
D
D
22
a
]
þ168 (c 1.0, CH2Cl2) for (R)-enantiomer
D
The enantiomeric excess was analyzed by HPLC equipped with
a ChiracelÒ OJ-H column. Eluent¼mixture of n-heptane and iso-
propanol (99:1). Flow rate¼0.5 mL/min; UV detector¼220 nm; re-
tention times: tR¼27.28 min (S) and 31.62 min (R).
The enantiomeric excess was analyzed by HPLC equipped with
a ChiracelÒ OJ-H column. Eluent¼n-heptane. Flow rate¼0.5 mL/
min; UV detector¼254 nm; retention times: tR¼27.88 min (S) and
32.61 min (R).
4.4. General procedure for selenium oxidation reactions me-
diated by BVMOs
4.3. General procedure for synthesis of chiral organoselenium
compounds (4e6)11
To a flask (2 mL, EppendorfÒ tube) containing the solution of the
To a 50 mL round-bottomed flask (I) were added the enantio-
merically pure forms of phenylethanol derivative (1 mmol, pre-
pared as described in Section 4.2), CH2Cl2 (50 mL), and DMAP
(2 mmol, 0.244 g). After 15 min, MsCl (1.5 mmol, 0.12 mL) was
added slowly. The reaction mixture was stirred overnight at room
temperature.
racemic selenide compounds 1e6 (1 M in DMSO, 5
Tris/HCl buffer at pH 7.5 (50 mM, 440 L), phosphite solution
(500 mM, 20 or 50 L), NADPH (100 mM, 10 L), PTDH (100 M,
L), and BVMO (100 M, 20 or 40 L). Reaction mixture was
mL) were added
m
m
m
m
5
m
m
m
shaken at 200 rpm and 30 ꢀC (PAMO and M446G PAMO mutant) or
150 rpm and 25 ꢀC (HAPMO and CHMO) for the appropriate time
(Table 1). The reaction mixture was extracted with EtOAc
(3ꢂ0.5 mL) and the organic extract was dried over Na2SO4 and
analyzed by HPLC. Control experiments in absence of enzyme were
performed for all substrates, and no selenium oxidation was
observed.
In a second 50 mL round-bottomed flask (II) were added sele-
nium (1 mmol, 0.080 g), EtOH (3 mL), DMF (2 mL), and NaBH4
(1 mmol, 0.037 g). The mixture was stirred for 15 min and then
BnBr (1 mmol, 0.119 mL) was added. After stirring 3 h at room
temperature, additional amount of NaBH4 (1.5 mmol, 0.055 g) was
added. The reaction mixture was stirred over more 20 min. The
content of flask (I) was added slowly to the flask (II) and the
resulting mixture was stirred at room temperature for 3 h.
After that, the ethanol/DMF was removed in vacuum, and
H2O (5 mL) was added into the crude material. The resulting
aqueous mixture was extracted with Et2O (2ꢂ5 mL) and the
organic extract was dried over MgSO4. The solvent was removed
in vacuum and the mixture was purified by flash column chro-
matography (pentane as eluent) providing the pure product
(50e75% yield).
4.5. General procedure for selenium oxidation mediated by A.
terreus URM 3571 and CCT 3320
Erlenmeyer flasks (250 mL) containing 100 mL of culture me-
dium (malt extract, 20 g/L) were inoculated with Aspergillus spores.
Growth was carried out in an orbital shaker (160 rpm) at 32 ꢀC for 5
days. The fungal cells were harvested by filtration.
The biotransformation was performed by re-suspending the
fungal cells (1 g) with Tris/HCl (50 mM, pH 7.5, 50 mL) in an
Erlenmeyer flask (125 mL). To this cell suspension, selenide 1
(0.1 mmol) was added and incubated in an orbital shaker (160 rpm)
at 32 ꢀC for 1e5 days. Then reaction mixture was filtered and the
aqueous layer was extracted with ethyl acetate (2ꢂ20 mL). The
organic extract was dried over Na2SO4 and analyzed by chiral HPLC.
Control experiments in absence of fungi were performed.
4.3.1. Benzyl-(1-phenylethyl)selane (4). Yellow oil (65%), 1H NMR
(200 MHz, CDCl3):
d
1.69 (d, J¼7.0 Hz, 3H), 3.70 (s, 2H), 4.07 (q,
J¼7.0 Hz, 1H), 7.19e7.28 (m, 10H). 13C NMR (50 MHz, CDCl3):
d 22.5,
37.7, 53.0, 126.6, 127.3, 128.3, 128.8, 139.0, 144.1. LRMS (EI) m/z (%):
276 (Mþ, 6), 262 (4), 105 (100), 91 (41), 77 (9), 65 (7), 51 (4). FT-IR
(film, cmꢁ1
) nmax: 3026, 2920, 1493, 1452, 1177, 760, 695, 608, 478.
[
a
]
22 þ30 (c 1.0, CH2Cl2) for (S)-enantiomer (ee>99%) and [
a]
22 ꢁ45
4.6. General procedure for selenium oxidation reactions me-
diated by CAL-B
D
D
(c 1.0, CH2Cl2) for (R)-enantiomer (ee>99%).
The enantiomeric excess was analyzed by HPLC equipped with
a ChiracelÒ OJ-H column. Eluent¼mixture of n-heptane and iso-
propanol (99:1). Flow rate¼0.8 mL/min; UV detector¼254 nm; re-
tention times: tR¼22.75 min (S) and 26.88 min (R).
To a flask (2 mL, EppendorfÒ tube) containing the selenide 1
(0.25 mmol, 65 mg) were added the organic solvent (1 mL) and
CAL-B (20 mg). After 5 min, the oxidant (2 or 3 equiv, 0.5 or

10436
P.B. Brondani et al. / Tetrahedron 68 (2012) 10431e10436
9. (a) Holland, H. I.; Carter, I. M. Bioorg. Chem. 1983, 12, 1e7; (b) Latham, J. A.;
Branchaud, B. P.; Chen, Y. C. J.; Walsh, C. J. Chem. Soc., Chem. Commun. 1986,
528e530; (c) Branchaud, B. P.; Walsh, C. T. J. Am. Chem. Soc. 1985, 107,
2153e2161; (d) Da Costa, C. E.; Comasseto, J. V.; Crusius, I. H. S.; Andrade, L. H.;
Porto, A. L. M. J. Mol. Catal. B: Enzym. 2007, 45, 135e139; (e) Andrade, L. H.;
Pedrozo, E. C.; Leite, H. G.; Brondani, P. B. J. Mol. Catal. B: Enzym. 2011, 73,
63e66; (f) Comasseto, J. V.; Gariani, R. A. Tetrahedron 2009, 65, 8447e8459.
10. Clive, D. I. J.; Chittattu, G. J.; Farina, V.; Kiel, W. A.; Menchen, S. M.; Russel, C. G.;
Singh, A.; Wong, C. K.; Curtis, N. J. J. Am. Chem. Soc. 1980, 102, 4438e4447.
11. Wirth, T.; Cox, M. Phosphorus, Sulfur Silicon Relat. Elem. 2007, 180, 659e666.
0.75 mmol) was added. Reaction mixture was shaken at room
temperature or 0 ꢀC for the appropriate time (Table 2). The enzyme
was filtered and the solution was washed with distilled water and
the aqueous layer was extracted with n-hexane (2ꢂ1 mL). The or-
ganic extract was dried over Na2SO4 and analyzed by HPLC. Control
experiments in absence of enzyme and/or oxidant were performed.
Supplementary data
~
12. (a) Pazmino, D. E. T.; Dudek, H.; Fraaije, M. W. Curr. Opin. Chem. Biol. 2010, 14,
138e144; (b) Nolan, L. C.; O’Connor, K. E. Biotechnol. Lett. 2008, 30, 1879e1891;
(c) Van Berkel, W. J. H.; Kamerbeek, N. M.; Fraaije, M. W. J. Biotechnol. 2006, 124,
670e689.
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2012.09.087.
13. (a) De Gonzalo, G.; Mihovilovic, M. D.; Fraaije, M. W. ChemBioChem 2010, 11,
2208e2231; (b) Kayser, M. M. Tetrahedron 2009, 65, 947e974; (c) Mihovilovic,
M. D. Curr. Org. Chem. 2006, 10, 1265e1287; (d) Kamerbeek, N. M.; Janssen, D.
B.; van Berkel, W. J. H.; Fraaije, M. W. Adv. Synth. Catal. 2003, 345, 667e678.
14. (a) Colonna, S.; Gaggero, N.; Carrea, G.; Ottolina, G.; Pasta, P.; Zambianchi, F.
Tetrahedron Lett. 2002, 43, 1797e1799.
References and notes
1. Uemura, S. Phosphorus, Sulfur Silicon Relat. Elem. 2001, 171, 13e30.
~
2. Godoi, M.; Paixao, M. W.; Braga, A. L. Dalton Trans. 2011, 40, 11347e11355 and
references therein.
15. (a) Keppler, A. F.; Porto, A. L. M.; Crusius, I. H. S.; Comasseto, J. V.; Andrade, L. H.
Enzyme Microb. Technol. 2005, 36, 967e975; (b) Assis, L. F.; Kagohara, E.; Omori,
A. T.; Comasseto, J. V.; Andrade, L. H.; Porto, A. L. M. Food Technol. Biotechnol.
2007, 45, 415e419.
16. (a) Xu, Y.; Khaw, N. R. B. J.; Li, Z. Green Chem. 2009, 11, 2047e2051; (b)
Orellana-Coca, C.; Camacho, S.; Adlercreutz, D.; Mattiasson, B.; Hatti-Kaul, R.
Eur. J. Lipid Sci. Technol. 2005, 107, 864e870; (c) Moreira, A. M.; Nascimento, M.
G. Catal. Commun. 2007, 8, 2043e2047.
3. (a) Mugesh, G.; Singh, H. Chem. Soc. Rev. 2000, 29, 347e357; (b) Nogueira, C. W.;
Zeni, G.; Rocha, J. B. T. Chem. Rev. 2004, 104, 6255e6286.
4. (a) Abdo, M.; Liu, S.; Zhou, B.; Walls, C. D.; Wu, L.; Knapp, S.; Zhang, Z.-Y. J. Am.
Chem. Soc. 2008, 130, 13196e13197; (b) Piovan, L.; Alves, M. F. M.; Juliano, L.;
Bromme, D.; Cunha, R. L. O. R.; Andrade, L. H. J. Braz. Chem. Soc. 2010, 21,
2108e2118; (c) Piovan, L.; Wu, Li.; Zhang, Z.; Andrade, L. H. Org. Biomol. Chem.
2011, 9, 1347e1351.
5. Piovan, L.; Alves, M. F. M.; Juliano, L.; Bromme, D.; Cunha, R. L. O. R.; Andrade, L.
H. Bioorg. Med. Chem. 2011, 19, 2009e2014.
17. (a) Cheng, Y.; Xu, G.; Wu, J.; Zhang, C.; Yang, L. Tetrahedron Lett. 2010, 51,
2366e2369 and references therein; (b) Andrade, L. H.; Omori, A. T.; Porto, A. L.
M.; Comasseto, J. V. J. Mol. Catal. B: Enzym. 2004, 29, 47e54.
6. Ogawa, A. Selenium and Tellurium in Organic Synthesis In Main Group
Metal in Organic Synthesis; Yamamoto, H., Oshima, K., Eds.; Wiley-VCH:
Weinheim, 2005.
7. Mugesh, C.; Singh, H. B. Acc. Chem. Res. 2002, 35, 226e236.
8. (a) Zhu, C.; Huang, Y. Curr. Org. Chem. 2006, 10, 1905e1920 and references
therein; (b) Ruano, J. L. G.; Torrente, E.; Alonso, I.; Rodriguez, M.; Martín-Castro,
A. M.; Degl’Innocenti, A.; Frateschi, L.; Capperucci, A. J. Org. Chem. 2012, 77,
1974e1982.
18. (a) Fraaije, M. W.; Wu, J.; Heuts, D. P. H. M.; van Hellemond, E. W.; Spelberg, J.
H. L.; Janssen, D. B. Appl. Microbiol. Biotechnol. 2005, 66, 393e400; (b) Ka-
merbeek, N. M.; Moonen, M. J. H.; van der Ven, J. G. M.; van Berkel, W. J. H.;
Fraaije, M. W.; Janssen, D. B. Eur. J. Biochem. 2001, 268, 2547e2557; (c) Do-
noghue, N. A.; Norris, D. B.; Trudgill, P. W. Eur. J. Biochem. 1976, 63, 175e192.
~
19. Dudek, H. M.; Gonzalo, G.; Pazmino, D. E. T.; Stepniak, P.; Wyrwicz, L. S.;
Rychlewski, L.; Fraaije, M. W. Appl. Environ. Microbiol. 2011, 77, 5730e5738.