Regio- and enantioselective Baeyer-Villiger oxidation: Kinetic resolution of racemic 2-substituted cyclopentanones

Article abstract of DOI:10.1021/ol501737a

A kinetic resolution of racemic 2-substituted cyclopentanones via highly regio- and enantioselective Baeyer-Villiger oxidation has been successfully developed. The reaction could afford the normal 6-substituted δ-lactones in up to 98% ee and >19/1 regioselectivity. Meanwhile, the unreacted ketones were recovered in excellent ee values (up to 98%). It represents the best results of the kinetic resolution of racemic 2-substituted cyclopentanones via nonenzymic asymmetric BV oxidation.

Full text of DOI:10.1021/ol501737a

Letter
pubs.acs.org/OrgLett
Regio- and Enantioselective Baeyer−Villiger Oxidation: Kinetic
Resolution of Racemic 2‑Substituted Cyclopentanones
Lin Zhou, Xiaohua Liu, Jie Ji, Yuheng Zhang, Wangbin Wu, Yangbin Liu, Lili Lin, and Xiaoming Feng*
Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu
610064, P. R. China
S
* Supporting Information
ABSTRACT: A kinetic resolution of racemic 2-substituted
cyclopentanones via highly regio- and enantioselective
Baeyer−Villiger oxidation has been successfully developed.
The reaction could afford the normal 6-substituted δ-lactones
in up to 98% ee and >19/1 regioselectivity. Meanwhile, the
unreacted ketones were recovered in excellent ee values (up to
98%). It represents the best results of the kinetic resolution of racemic 2-substituted cyclopentanones via nonenzymic
asymmetric BV oxidation.
he Baeyer−Villiger (BV) oxidation1 plays an important
not been accomplished in these contexts, and the enantiomeric
excess of the resulted δ-lactones was no less than 58% ee.8b
Herein, we would like to report an enantioselective BV
oxidation and kinetic resolution of 2-substituted cyclo-
pentanones. The regio- and enantioselectivity of the reaction
are well addressed. In the presence of a chiral N,N′-dioxide−
Sc(III) complex catalyst10 and additives, optically active δ-
lactones generated preferentially from a normal CHR-group
migration, and the unreactive 2-substituted cyclopentanones
were recovered in high enantioselectivities (Scheme 1b).
At the beginning, the catalytic system that is efficient for the
asymmetric BV oxidation of 2-arylcyclohexanones was sub-
jected to the reaction of 2-phenylcyclopentanone 1a. Although
high enantioselectivities were achieved for both the normal δ-
lactone 2a and the abnormal δ-lactone 3a, the regioselectivity
(rs) and yield were poor (29% yield, 1/1.6 rs; Scheme 2). We
then restarted to optimize the reaction conditions using 2-
phenylcyclopentanone 1a and m-chlorobenzoperoxoic acid (m-
CPBA) as the model substrate and oxidant,11 respectively.
Performing the reaction in EtOAc at 0 °C without a catalyst
afforded the racemic lactone 2a in 25% yield as a single isomer
(Table 1, entry 1), whereas only a trace amount of the products
was observed in the presence of the chiral N,N′-dioxide L1−
Sc(OTf)3 complex catalyst (Table 1, entry 2). To improve the
reactivity, the efficiency of additives was then examined.
Delightedly, when the reaction was carried out with 1.0 equiv
of Al(OiPr)3 as the additive, the L1−Sc(OTf)3 complex could
give the δ-lactones in 17% yield. The ratio of the normal δ-
lactone 2a to the abnormal δ-lactone 3a reached 5/1, and the
ee value of the major δ-lactone 2a was excellent (99% ee, Table
1, entry 3). The addition of 4 Å molecular sieves (MS) could
further enhance the yield to 26% with the regio- and
enantioselectivity maintained (Table 1, entry 4). Compara-
T
role in organic synthesis since it provides a straightforward
access to esters or lactones from ketones. Especially, the
asymmetric BV oxidation of substituted cyclic ketones is highly
valuable for the synthesis of optically active lactones.2−7
Impressive results have been achieved for the asymmetric BV
oxidations of prochiral cyclic ketones with biocatalysts,3 chiral
metal complexes,4 or organocatalysts.5 Notably, the asymmetric
BV oxidation of racemic 2-substituted cyclic ketones results in a
kinetic resolution leading to optically active lactones and 2-
substituted cyclic ketones. The related nonenzymatic catalytic
system remains still elusive.6 In 1994, Bolm8a and Strukul8b
reported the discovery of chiral bisoxazoline-Cu(II) and
diphosphine-Pt(II) catalyzed asymmetric BV oxidation of 2-
substituted cyclic ketones, respectively. Hitherto, the progress
of this process is the chiral N,N′-dioxide−Sc(III) catalyzed BV
oxidation of 2-arylcyclohexanones by our group, and it is
interesting to note that abnormal regioselectivity occurred in
this case (Scheme 1a).9 Unfortunately, the kinetic resolution of
2-substituted cyclopentanones via asymmetric BV oxidation has
Scheme 1. Regio- and Enantioselective BV Oxidation of
Racemic 2-Substituted Cyclohexanones and 2-Substituted
Cyclopentanones
Received: June 16, 2014
Published: July 16, 2014
© 2014 American Chemical Society
3938
dx.doi.org/10.1021/ol501737a | Org. Lett. 2014, 16, 3938−3941

Organic Letters
Letter
achieved. The major δ-lactone 2a was yielded in 98% ee, and
the unreacted ketone 1a was recovered in 44% yield with 90%
ee (Table 1, entry 8).12 As for the chiral backbone moiety, the
N,N′-dioxide L2 derived from L-proline acid exhibited
superiority in enantiocontrol toward this reaction compared
with the ones derived from (S)-pipecolic acid (L3) and (S)-
ramipril acid (L4) (Table 1, entry 8 vs entries 9 and 10). The
relationship between the enantioselectivity of the recovered
ketone and the regioselectivity of the δ-lactones indicated that
the two products primarily resulted from each enantiomer of
the racemic ketone (entries 9 and 10). Therefore, the
optimized conditions were identified as 5 mol % of the chiral
N,N′-dioxide L2−Sc(OTf)3 catalyst, Al(OEt)3, and 4 Å MS as
the additives in EtOAc at −20 °C.
Scheme 2. Chiral N,N′-Dioxide−Sc(III) Complex Catalyzed
Kinetic Resolution of Racemic 2-Phenyl Cyclic Ketones via
Asymmetric BV Oxidation and N,N′-Dioxide Ligands Used
in This Study
Under the optimal conditions (Table 1, entry 8), the
asymmetric BV oxidation and kinetic resolution of a series of 2-
substituted cyclopentanones 1 was examined. As shown in
Table 2, the stereocontrol of the reaction was not sensitive to
the electronic property of substituents on the phenyl ring of 2-
arylcyclopentanones 1. Generally, the normal δ-lactones 2 as
the major products were obtained with a good to excellent ratio
of 2 to 3 (6.3/1−>19/1 rs). Good to excellent enantiomeric
excesses were achieved for the major products 2 (92−98% ee)
and unreacted ketones 1 (80−98% ee; entries 1−11). The
regioselectivity and reactivity were sensitive to the substituted
position on 2-aryl group of the substrates rather than the
electronic property. The substrates with 3-substituents on the
phenyl ring afforded the lactones with decreased regioselectivity
and were recovered partly in lower ee values compared with
those containing 4-substituted ones (entries 2, 4, and 8 vs 3, 5,
and 9). Especially, the racemic 2-([1,1′-biphenyl]-4-yl)cyclo-
pentanone 1f underwent the oxidation reaction well, affording
the major δ-lactone 2f in 98% ee with up to 13/1 ratio of 2f to
3f. And the unreacted ketone 1f was isolated in 95% ee and
44% yield (entry 6). Moreover, 2-naphthyl substituted
cyclopentanone 1k was also tolerable, and the major product
2k and unreacted ketone 1k were obtained in 94% ee and 92%
ee, respectively (entry 11). Besides, the absolute configuration
of 2h was determined to be S by X-ray analysis,13 and the
recovered ketone 1d was assigned as R by comparison of the
tively, a low yield was obtained in the presence of 4 Å MS
without Al(OiPr)3 (Table 1, entry 5).
Further optimization of the reaction conditions was then
aimed at exploring the efficiency of other aluminum alkoxides
as the additives (Table 1, entries 6−7). Fortunately, improved
reactivity, as well as regio- and enantioselectivity, was observed
when Al(OEt)3 was employed as an additive (Table 1, entry 7).
The 2-phenylcyclopentanone 1a was recovered in 60%
conversion with 40% ee, and δ-lactone 2a was given in 99%
ee. It is not possible to clearly rationalize the role of Al(OEt)3
in the catalytic process at present. The primary IR spectrum of
the mixture of Al(OEt)3 and m-CPBA gives new peaks (see
Supporting Information). We proposed that Al(OEt)3 prior
reacts with m-CPBA to generate an activate species which has
enhanced the oxidative ability so that the reactivity was
obviously improved. Next, the efficiency of other N,N′-dioxide
ligands was explored (Scheme 2). When the ligand L2
containing 2,6-diethyl-4-methylaniline was introduced, 56%
conversion of the products with a 9.6/1 ratio of 2a to 3a was
Table 1. Optimization of the Reaction Conditions for the Kinetic Resolution of Racemic 2-Phenylcyclopentanone 1a
b
c
b
conversion (%)
ee (%)
rs
a
entry
ligand
additive
MS
1a
2a + 3a
1a
2a
2a/3a
1
2
3
4
5
6
7
8
9
10
−
−
−
−
−
−
75
97
83
74
90
87
60
44
39
31
25 (2a)
trace
17
−
−
4
−
−
−
−
5/1
L1
L1
L1
L1
L1
L1
L2
L3
L4
Al(OiPr)3
Al(OiPr)3
−
99
99
99
99
99
98
97
97
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
4 Å MS
26
20
3
6.2/1
4.6/1
5/1
10
Al(OtBu)3
Al(OEt)3
Al(OEt)3
Al(OEt)3
Al(OEt)3
13
8
40
40
90
60
70
5.8/1
9.6/1
2.8/1
2.3/1
56
61
69
a
Unless otherwise noted, all reactions were performed with 1a (0.1 mmol), L (5 mol %), Sc(OTf)3 (5 mol %), Al(OR)3 (0.1 mmol), and 4 Å MS
(30.0 mg) in EtOAc (1.0 mL) at −20 °C, and then m-CPBA (0.1 mmol in 1.0 mL of EtOAc) was added at −20 °C. The reaction mixture was stirred
b
c
1
at −20 °C for 18 h. The conversion and the ratio of 2a/3a were determined by H NMR. Determined by chiral HPLC analysis.
3939
dx.doi.org/10.1021/ol501737a | Org. Lett. 2014, 16, 3938−3941

Organic Letters
Letter
Table 2. Substrate Scope for the Kinetic Resolution of Racemic 2-Substituted Cyclopentanones
b
c
d
yield (%)
ee (%)
rs
a
entry
a-l: R
a: Ph
1
2 + 3
1
2
2/3
1
42
45
46
40
43
44
46
46
41
43
44
57
42
53
55
52
58
56
54
53
53
56
54
56
41
55
90
90
85
98
94
98
94
92
98
97
9.6/1
>19/1
12/1
2
b: 4-MeC6H4
c: 3-MeC6H4
d: 4-MeOC6H4
e: 3-MeOC6H4
f: 4-PhC6H4
g: 4-FC6H4
3
e
4
98 (R)
80
15/1
5
7/1
6
95
13/1
7
95
15.6/1
11.5/1
6.3/1
11/1
f
8
h: 4-ClC6H4
i: 3-ClC6H4
j: 4-BrC6H4
k: 2-naphthyl
l: benzyl
92
97 (S)
98
9
87
10
11
12
13
86
96
92
94
9.5/1
1.33/1
13/1
g
14
87 (62)
h
d: 4-MeOC6H4
90 (R)
95
a
Unless otherwise noted, the reactions were performed with 1 (0.1 mmol), L2 (5 mol %), Sc(OTf)3 (5 mol %), Al(OEt)3 (0.1 mmol, 16.2 mg), and
4 Å MS (30.0 mg) in EtOAc (1.0 mL) at −20 °C, and then m-CPBA (0.1 mmol in 1.0 mL of EtOAc) was added at −20 °C. The reaction mixture
b
c
d
e
was stirred at −20 °C for 18 h. Isolated yield. Determined by chiral HPLC analysis. The regioselectivity of 2/3 was determined by 1H NMR. The
f
absolute configuration of 1d was confirmed to be R by comparison of the optical rotation with this reported in the literature. The absolute
configuration of 2h was determined to be S by X-ray analysis. The ee of the δ-lactone 3l was 62%. Using 3.0 mmol of 1d as the substrate.
g
h
optical rotation with the literature.14 It indicated that the (S)-
ketone preferred to be oxidized to δ-lactone 2 in the
asymmetric catalytic system and the migration proceeded
with retention of configuration at the migrating carbon atom.
Optically active 2-arylcyclopentanones did not racemize under
the asymmetric catalytic system but suffered slight racemization
by column chromatography via silica at higher temperature.12
When 2-benzylcyclopentanone 1l was subjected to the BV
oxidation, the regioselectivity and reactivity decreased ob-
viously. The corresponding mixed δ-lactones 2l and 3l were
obtained with 1.33/1 rs, as well as 87% and 62% ee,
respectively. As a result, the unreacted ketone 1l was recovered
with only 14% ee (entry 12).15 Furthermore, by treatment of
3.0 mmol of the racemic 1d, the product 2d was produced
smoothly in 55% yield with up to 95% ee and 13/1 rs, and the
unreacted ketone 1d was recovered in 42% yield with 90% ee,
which showed the synthetic utility of the catalytic system (entry
13).
the racemic 2-phenylcyclobutanone 4 gave the mixed γ-lactones
5 and 6 with only 1.33/1 rs. The normal lactone 5 and the
unreacted ketone 4 were isolated in 50% and 42% ee,
respectively. The racemic 2-phenylcyclohexanone 7 generated
the abnormal ε-lactone 9 slightly more than the normal ε-
lactone 8. The normal ε-lactone 8 and the unreacted ketone 7
were isolated in low ee values, albeit ε-lactone 9 was given in
93% ee. It was proposed that the different ring strain and
conformations of the 2-substituted cyclic ketones, and the steric
hindrance created by a chiral catalyst as well, would account for
the migratory aptitude in the circumstances.
In summary, we have successfully developed a highly regio-
and enantioselective BV oxidation of racemic 2-substituted
cyclopentanones using the highly efficient N,N′-dioxide−ScIII
complex catalyst. The asymmetric BV oxidation could afford
the desired δ-lactones in 87−98% ee and moderate to good
regioselectivities. Moreover, the kinetic resolution process gave
optically active ketones in excellent ee (up to 98%). To the best
of our knowledge, it represents the best example of the kinetic
resolution of 2-substituted cyclopentanones via nonenzymatic
asymmetric BV oxidation to date. Further study on the reaction
mechanism and extensive exploration of the methodology to
other cyclic ketones are underway.
Next, the catalytic system was explored for the BV oxidations
of the racemic 2-phenylcyclobutanone 4 and 2-phenylcyclo-
hexanone 7.16 The product distribution and enantioselectivities
changed obviously. As shown in Figure 1, the BV oxidation of
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, spectral and analytical data for the
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
Figure 1. Asymmetric BV oxidations of the racemic 2-phenyl-
cyclobutanone 4 and 2-phenylcyclohexanone 7.
*E-mail: xmfeng@scu.edu.cn.
3940
dx.doi.org/10.1021/ol501737a | Org. Lett. 2014, 16, 3938−3941

Organic Letters
Letter
Commun. (Cambridge, U. K.) 2008, 5499. (g) Reetz, M. T.; Wu, S. J.
Am. Chem. Soc. 2009, 131, 15424. (h) Stewart, J. D.; Reed, K. W.; Zhu,
J.; Chen, G.; Kayser, M. M. J. Org. Chem. 1996, 61, 7652.
(7) For selected examples for the asymmetric BV oxidation of bicyclic
cyclobutanones, see: (a) Alphand, V.; Furstoss, R. J. Org. Chem. 1992,
57, 1306. (b) Alphand, V.; Furstoss, R. J. Mol. Catal. B: Enzymatic
2000, 9, 209. (c) Schulz, F.; Leca, F.; Hollmann, F.; Reetz, M. T.
Beilstein J. Org. Chem. 2005, 1, 10. (d) Watanabe, A.; Uchida, T.; Irie,
R.; Katsuki, T. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5737. (e) Bolm,
C.; Schlingloff, G. J. Chem. Soc., Chem. Commun. 1995, 1247. (f) Lopp,
M.; Paju, A.; Kanger, T.; Pehk, T. Tetrahedron Lett. 1996, 37, 7583.
(g) Kanger, T.; Kriis, K.; Paju, A.; Pehk, T.; Lopp, M. Tetrahedron:
Asymmetry 1998, 9, 4475. (h) Bolm, C.; Beckmann, O. Chirality 2000,
12, 523. (i) Colladon, M.; Scarso, A.; Strukul, G. Synlett 2006, 3515.
(j) Xu, S.; Wang, Z.; Zhang, X.; Ding, K. Eur. J. Org. Chem. 2011, 110.
(8) (a) Bolm, C.; Schlingloff, G.; Weickhardt, K. Angew. Chem. 1994,
106, 1944; Angew. Chem., Int. Ed. Engl. 1994, 33, 1848. (b) Gusso, A.;
Baccin, C.; Pinna, F.; Strukul, G. Organometallics 1994, 13, 3442.
(c) Peng, Y. G.; Feng, X. M.; Yu, K. B.; Li, Z.; Jiang, Y. Z.; Yeung, C.-
H. J. Organomet. Chem. 2001, 619, 204.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(21321061, 21290182, and 21172151), the National Basic
Research Program of China (973 Program: 2010CB833300),
and the Ministry of Education (20110181130014) for financial
support.
REFERENCES
■
(1) (a) Baeyer, A.; Villiger, V. Ber. Dtsch. Chem. Ges. 1899, 32, 3625.
For selected reviews, see: (b) Krow, G. R. Org. React. 1993, 43, 251.
(c) Renz, M.; Meunier, B. Eur. J. Org. Chem. 1999, 737. (d) Strukul, G.
Angew. Chem. 1998, 110, 1256; Angew. Chem., Int. Ed. 1998, 37, 1198.
(e) ten Brink, G.-J; Arends, I. W. C. E.; Sheldon, R. A. Chem. Rev.
2004, 104, 4105.
(2) For selected books, see: (a) Bolm, C. In Asymmetric Synthesis−
The Essentials; Christmann, M., Brase, S., Eds.; Wiley-VCH:
(9) Zhou, L.; Liu, X. H.; Ji, J.; Zhang, Y. H.; Hu, X. L.; Lin, L. L.;
Feng, X. M. J. Am. Chem. Soc. 2012, 134, 17023.
̈
Weinheim, 2006; p 57. (b) Bolm, C.; Beckmann, O. In Comprehensive
Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.;
Springer: Berlin, 1999, Vol. 2; p 803. (c) Bolm, C. In Advances in
Catalytic Processes; Doyle, M. P., Ed.; JAI Press: Greenwich, 1997; Vol.
2, p 43.
(3) For selected examples for the asymmetric BV oxidation of
prochiral cyclic ketones catalyzed by biocatalysts, see: (a) Taschner, M.
J.; Black, D. J.; Chen, Q. Z. Tetrahedron: Asymmetry 1993, 4, 1387.
(b) Stewart, J. D.; Reed, K. W.; Martinez, C. A.; Zhu, J.; Chen, G.;
Kayser, M. M. J. Am. Chem. Soc. 1998, 120, 3541. (c) Mihovilovic, M.
D.; Chen, G.; Wang, S.; Kyte, B.; Rochon, F.; Kayser, M. M.; Stewart,
J. D. J. Org. Chem. 2001, 66, 733. (d) Rial, D. V.; Bianchi, D. A.;
Kapitanova, P.; Lengar, A.; van Beilen, J. B.; Mihovilovic, M. D. Eur. J.
Org. Chem. 2008, 1203. (e) Reetz, M. T.; Brunner, B.; Schneider, T.;
Schulz, F.; Clouthier, C. M.; Kayser, M. M. Angew. Chem. 2004, 116,
4167; Angew. Chem., Int. Ed. 2004, 43, 4075. (f) Schwarz-Linek, U.;
(10) (a) Liu, X. H.; Lin, L. L.; Feng, X. M. Acc. Chem. Res. 2011, 44,
574. (b) Shen, K.; Liu, X. H.; Lin, L. L.; Feng, X. M. Chem. Sci. 2012,
3, 327. (c) Feng, X. M.; Liu, X. H. In Scandium: Compounds,
Productions and Applications; Greene, V. A., Ed.; Nova Science, New
York, 2011; p 1. (d) Huang, S.; Ding, K. Angew. Chem. 2011, 123,
7878; Angew. Chem., Int. Ed. 2011, 50, 7734. (e) Liu, X. H.; Lin, L. L.;
Feng, X. M. Org. Chem. Front. 2014, 1, 298.
(11) Using H2O2 or TBHP as the oxidant, no BV product was
observed.
(12) The optically active 2-arylcyclopentanones suffer slow
racemization by column chromatography on silica gel at high
temperature.
(13) CCDC 998771 (2h) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Krodel, A.; Ludwig, F.-A.; Schulze, A.; Rissom, S.; Kragl, U.; Tishkov,
̈
(14) Shen, Y.-M.; Wang, B.; Shi, Y. Angew. Chem. 2006, 118, 1457;
V. I.; Vogel, M. Synthesis 2001, 6, 947. (g) Rudroff, F.; Rydz, J.; Ogink,
F. H.; Fink, M.; Mihovilovic, M. D. Adv. Synth. Catal. 2007, 349, 1436.
(4) For selected examples for the asymmetric BV oxidation of
prochiral cyclic ketones catalyzed by chiral metal complexes, see:
(a) Bolm, C.; Luong, T. K. K.; Schlingloff, G. Synlett 1997, 1151.
(b) Sugimura, T.; Fujiwara, Y.; Tai, A. Tetrahedron Lett. 1997, 38,
6019. (c) Shinohara, T.; Fujioka, S.; Kotsuki, H. Heterocycles 2001, 55,
237. (d) Uchida, T.; Katsuki, T. Tetrahedron Lett. 2001, 42, 6911.
(e) Bolm, C.; Beckmann, O.; Cosp, A.; Palazzi, C. Synlett 2001, 1461.
(f) Bolm, C.; Frison, J.-C.; Zhang, Y.; Wulff, W. D. Synlett 2004, 1619.
(g) Frison, J.-C.; Palazzi, C.; Bolm, C. Tetrahedron 2006, 62, 6700.
(h) Paneghetti, C.; Gavagnin, R.; Pinna, F.; Strukul, G. Organometallics
1999, 18, 5057. (i) Cavarzan, A.; Bianchini, G.; Sgarbossa, P.; Lefort,
L.; Gladiali, S.; Scarso, A.; Strukul, G. Chem.Eur. J. 2009, 15, 7930.
(j) Colladon, M.; Scarso, A.; Strukul, G. Synlett 2006, 3515.
Angew. Chem., Int. Ed. 2006, 45, 1429.
(15) The BV reaction of 2-methylcyclopentanone was tested, but the
method of determination of the ee value is not established.
(16) Under the optimal reaction conditions, no BV product was
observed for 2-phenylcycloheptanone.
(5) For selected examples for the asymmetric BV oxidation of
prochiral cyclic ketones catalyzed catalyzed by organocatalysts, see:
(a) Murahashi, S.-I.; Ono, S.; Imada, Y. Angew. Chem. 2002, 114, 2472;
Angew. Chem., Int. Ed. 2002, 41, 2366. (b) Xu, S.; Wang, Z.; Zhang, X.;
Zhang, X.; Ding, K. Angew. Chem. 2008, 120, 2882; Angew. Chem., Int.
Ed. 2008, 47, 2840. (c) Xu, S.; Wang, Z.; Li, Y.; Zhang, X.; Wang, H.;
Ding, K. Chem.Eur. J. 2010, 16, 3021.
(6) For selected examples for the asymmetric BV oxidation of
racemic cyclic ketones catalyzed by biocatalysts, see: (a) Alphand, V.;
Archelas, A.; Furstoss, R. J. Org. Chem. 1990, 55, 347. (b) Kayser, M.
M.; Chen, G.; Stewart, J. D. J. Org. Chem. 1998, 63, 7103. (c) Berezina,
N.; Alphand, V.; Furstoss, R. Tetrahedron: Asymmetry 2002, 13, 1953.
(d) Bocola, M.; Schulz, F.; Leca, F.; Vogel, A.; Fraaije, M. W.; Reetz,
M. T. Adv. Synth. Catal. 2005, 347, 979. (e) Hollmann, F.; Taglieber,
A.; Schulz, F.; Reetz, M. T. Angew. Chem. 2007, 119, 2961; Angew.
Chem., Int. Ed. 2007, 46, 2903. (f) Reetz, M. T.; Wu, S. Chem.
3941
dx.doi.org/10.1021/ol501737a | Org. Lett. 2014, 16, 3938−3941