Enantioselective One-Pot Synthesis of α,β-Epoxy Ketones via Aerobic Oxidation of Cyclopropanols

Article abstract of DOI:10.1021/acs.orglett.7b01519

An efficient, mild, and environmentally benign method was developed for the asymmetric synthesis of 2-oxyranyl ketones from easily available tertiary cyclopropanols. The one-pot protocol includes the aerobic oxidation of cyclopropanol derivatives catalyzed by Mn(III) complexes followed by the poly-l-leucine-assisted stereoselective elimination of water from the intermediate peroxides with DBU to afford the corresponding epoxy ketones in high yields and good-to-excellent enantioselectivities (up to 97%).

Full text of DOI:10.1021/acs.orglett.7b01519

Letter
pubs.acs.org/OrgLett
Enantioselective One-Pot Synthesis of α,β-Epoxy Ketones via Aerobic
Oxidation of Cyclopropanols
Gabor Zoltan Elek, Victor Borovkov, Margus Lopp, and Dzmitry G. Kananovich*
́
́
Department of Chemistry and Biotechnology, Tallinn University of Technology, Akadeemia tee 15, 12618 Tallinn, Estonia
S
* Supporting Information
ABSTRACT: An efficient, mild, and environmentally benign
method was developed for the asymmetric synthesis of 2-
oxyranyl ketones from easily available tertiary cyclopropanols.
The one-pot protocol includes the aerobic oxidation of
cyclopropanol derivatives catalyzed by Mn(III) complexes
followed by the poly-L-leucine-assisted stereoselective elimi-
nation of water from the intermediate peroxides with DBU to
afford the corresponding epoxy ketones in high yields and
good-to-excellent enantioselectivities (up to 97%).
Due to growing interest to the cyclopropanol chemistry,6,7
nantiomerically enriched α,β-epoxy ketones are highly
we envisioned filling the existing gap by using easily available
Evaluable starting materials for the production of numerous
cyclopropanols 2 as starting compounds8 for the synthesis of
chiral compounds (e.g., pharmaceuticals, agrochemicals,
fragrances, etc.).1 Moreover, the epoxy ketone structural
chiral epoxy ketones 4. Tertiary cyclopropanols 2 are readily
oxidized under the aerobic conditions by using transition-metal
motif is responsible for the high bioactivity of several natural
catalysts9 to afford β-peroxyketones 3-I, which are in
equilibrium with 1,2-dioxolane form 3-II. We assumed that
the peroxides 3 can be further stereoselectively transformed
into chiral 2-oxyranyl ketones 4 by means of asymmetric
organocatalytic methods. Here, we report a general approach
for the synthesis of chiral 2-oxyranyl ketones 4 by using the
aerobic oxidation of tertiary cyclopropanols 2 to peroxyketone
intermediates 3 followed by stereoselective elimination of
water.
products.2 Since the initial report by Wynberg in 1976,3 the
majority of methods to obtain enantiomerically enriched epoxy
ketones are based on the epoxidation of α,β-unsaturated
ketones by using the Weitz−Scheffer reaction and its
modifications.4 Despite extensive studies in this field, the
developed methods are mostly limited to the reactive aromatic
chalcone-type ketones, while the epoxidation of other enone
substrates remained an arduous task due to long reaction times,
low conversions, and poor enantioselectivities.5 These short-
comings arise from the insufficient reactivity of the enone
double bond toward the addition of hydroperoxide anion. For
this reason, alkyl vinyl ketones 1 continue to be a challenging
family of substrates for this type of epoxidation (Scheme 1).5a,c
To achieve this goal, the following major steps must be
implemented: (1) development of the effective aerobic
oxidation protocol yielding β-peroxyketones 3; (2) finding
the conditions and organocatalyst suitable for the enantiose-
lective conversion of 3 into 4; and (3) combination of these
two processes into the one-pot procedure. The first oxidation
step was studied on a model compound 1-(2-phenylethyl)-
cyclopropanol (2a) (Scheme 2). Among different transition-
metal complexes used to catalyze the aerobic oxidation of
cyclopropanols,9 manganese salts were found to be the most
effective, affording high yields of β-peroxyketones.9d,e,g Short-
comings of the previously used procedure9d were the use of
benzene as a solvent and molecular oxygen as an oxidant. We
found that the aerobic oxidation of cyclopropanol 2a can be
easily achieved in less toxic solvents by simple stirring of a
solution of 2a and a manganese catalyst under air in open flask,
affording peroxide 3a in high yield (up to 95% according to
NMR analysis). The best results were achieved in environ-
mentally benign dimethyl carbonate (DMC),10 THF, and 2-
Scheme 1. Alternative Methodology for the Preparation of
Chiral 2-Oxyranyl Ketones 4 from Tertiary Cyclopropanols
2 via β-Peroxyketone Intermediates 3
Received: May 18, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01519
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Aerobic Oxidation of Cyclopropanol 2a
Table 1. Asymmetric Transformation of Peroxide 3a into 2-
Oxyranyl Ketone 4a
a
b
temp
(°C)
time
(h)
ee
entry cat. (mol %)
solvent
base
(%)
1
I−III (20%) DMC
20
20
50
1
c
86
34
88
74
68
64
58
48
32
16
70
78
66
94
d
a
2
IV (7.5%)
IV (2.5%)
IV (7.5%)
IV (7.5%)
IV (7.5%)
IV (7.5%)
IV (7.5%)
IV (7.5%)
IV (7.5%)
IV (7.5%)
IV (7.5%)
IV (7.5%)
IV (7.5%)
THF
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DIPEA
Reaction conditions: 2a (0.5 mmol) and Mn catalyst were dissolved
3
THF
20
1
in 0.5 mL of a solvent and stirred under air (open vial) until full
b
4
MeTHF
toluene
DME
20
1
conversion of starting material (TLC monitoring). OEP =
2,3,7,8,12,13,17,18-octaethylporphyrin.
5
20
1
6
20
1
7
MTBE
CH2Cl2
DMC
dioxane
CH3CN
THF
20
1
methyltetrahydrofuran (MeTHF) with 0.5 mol % of manganese
acetate and acetylacetonate catalysts (see Table S1, entries 2, 3,
and 6−8). The latter was especially effective, allowing the rate
of aerobic oxidation to increase and affording the full
conversion of starting material within 3 h even at 0.05 mol %
loading. The formation of minor amounts of identified
byproducts can be further suppressed by performing the
oxidation at 0 °C (see Table S1, entry 11).
8
20
1
9
20
1
10
11
12
13
14
15
20
1
20
1
20
12
12
1
e
THF
TMP
20
THF
DBU
40
f
IV (7.5%)
THF
DBU
−25
24
It has been demonstrated that metalloporphyrins that are
easily soluble in many organic solvents may also be effectively
used as oxidation catalysts.11 In our hands, 0.5 mol % of
MnCl(OEP) catalyst (OEP = 2,3,7,8,12,13,17,18-octaethylpor-
phyrin) afforded peroxide 3a in nearly quantitative yield in
dimethyl carbonate as a solvent (see Table S1, entry 12).
However, oxidation proceeded considerably slower compared
with the other manganese salts, presumably as a result of
additional steric hindrance caused by the porphyrin ligand.
Other potential oxygen carriers like iron and cobalt porphyrin
complexes were inefficient in catalyzing the aerobic oxidation of
2a.
a
Yield of 2-oxyranyl ketone 4a for all reactions was considered as
b
quantitative according to NMR. Enantiomeric excess was determined
c
by HPLC analysis using an AD-H column. No epoxide 4a formed.
d
Calculated according to average molecular weight of PLL polymer.
e
f
TMP = 2,2,6,6-tetramethylpiperidine. The reaction mixture was
stirred in a freezer at −25 °C for 24 h.
The main task of the current study was to perform the
enantioselective transformation of peroxide 3a into epoxy
ketone 4a. To find the suitable conditions, several well-known
organocatalysts were first tested. Our initial attempts with
cinchona-alkaloid-derived primary amine I (as TFA salt),5a,12
thiourea II, and squaramide III failed to afford the desired
epoxy ketone 4a (Table 1, entry 1). However, commercially
available poly-L-leucine (PLL on silica gel, catalyst IV)13 in the
presence of 1 equiv of DBU as a base resulted in the fast and
quantitative conversion of peroxide 3a into epoxy ketone 4a
with 86% ee (Table 1, entry 2). Although application of PLL
for the asymmetric epoxidation of chalcones was commenced
by Julia
́
and Colonna in the early 1980s14 and the initial
the amount of catalyst inevitably leads to a drastic decrease of
the enantioselectivity (Table 1, entry 3). In addition, the used
solvent strongly affects the enantiomeric purity of the product
(Table 1, entries 4−11) with the highest ee values achieved in
THF and MeTHF (86% and 88% ee, respectively).18 Weaker
bases than DBU also reduced the reaction rate and
enantioselectivity of the product (Table 1, entries 12 and
13). Temperature is another important factor in the stereo-
selective epoxide formation: the ee of product 4a dropped from
86% to 66% when the temperature was raised from 20 to 40 °C
(Table 1, entry 2 vs 14). Lowering the temperature to −25 °C
led to a noticeable improvement of the selectivity: 4a was
obtained with high 94% ee (Table 1, entry 15).
protocol was improved in the following intensive studies,5c,15
we report here for the first time the enantioselective
transformation of prochiral aliphatic peroxyketones using PLL
catalyst. The composition of PLL used in our experiments was
analyzed by MALDI MS.16 Both catalysts (commercially
purchased and prepared in our laboratory by polymerization
of L-leucine N-carboxyanhydride with 1,3-diaminopropane
initiator)17 had a number-average degree of polymerization
(DPn) of about 17−18 and average molecular weight close to
2000−2100 (see the Supporting Information). These catalysts
were used in the following screening of suitable reaction
condition to transform peroxide 3a into epoxy ketone 4a
(Table 1).
On the basis of the results of optimization studies, an
operationally simple one-pot protocol for the conversion of
cyclopropanols 2 into enantiomerically enriched 2-oxyranyl
ketones 4 was attained under the following conditions: aerobic
Starting from 3a, in the presence of 7.5 mol % of PLL the
reaction was completed within 1 h and 2-oxyranyl ketone 4a
was obtained in 86% ee at 20 °C (Table 1, entry 2). Lowering
B
DOI: 10.1021/acs.orglett.7b01519
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
oxidation at 0 °C; THF as a solvent; 0.5 mol % of Mn(acac)3 as
an oxidation catalyst. The subsequent epoxide formation by
using silica gel supported PLL and DBU at −25 °C. The
products, epoxy ketones 4, are isolated after filtration of the
catalyst and evaporation of the solvent followed by purification
with flash silica gel chromatography. Using these conditions at a
1.5 mmol scale, the isolated yield of epoxy ketone 4a from
cyclopropanol 2a was 77% and the ee was 94% (Scheme 3).
also observed the inhibition of aerobic oxidation of 2a by
MnCl(OEP) catalyst in the presence of pyridine and N-
methylimidazole. A commonly accepted mechanism of the
aerobic oxidation of cyclopropanols implies the single-electron
transfer (SET) from a cyclopropanol substrate yielding the
oxoalkyl radical, which is rapidly intercepted by molecular
oxygen.9 Strong coordination of amino ligands to the
manganese catalytic center prevents a weaker binder, such as
cyclopropanol, to interact with the metal ion, hence making the
SET process less probable.
A one-pot protocol for cyclopropanols 2n and 2m failed: the
first aerobic oxidation step proceeded smoothly (within 3 h),
but the subsequent epoxide forming step was very slow and
resulted in the tar formation for compound 2m. The
preparation of benzyl-substituted epoxy ketone 4i under
standard conditions was problematic because of decomposition
caused by enolization in the presence of DBU. This process was
suppressed by addition of acetic acid at −25 °C after the
completion of epoxidation, giving 58% isolated yield of 4i. The
nonenolizable benzyl ketones 4j and 4k were obtained in good
yields and excellent enantioselectivities (96−97% ee).
Scheme 3. Asymmetric Synthesis of (R)-2-Oxyranyl Ketones
a,b
4 by Aerobic Oxidation of Tertiary Cyclopropanols 2
To define the stereochemistry of products 4, the value of
optical rotation of 4c and 4i (see the Supporting Information)
was compared with the literature data for the corresponding
R-19 and S-isomers,20 thus confirming the R-configuration of
compounds obtained. The same stereoconfiguration is expected
from the model of Kelly and Roberts.15a−c,21 The α-helical
structure of PLL catalyst and N-terminal amino groups are
essential for the supramolecular binding of prochiral substrate 3
to provide an oxy-anion hole for the hydrogen bonding of
peroxyenolate intermediates, thus favoring a re transition state
leading to the R-enantiomer.
In conclusion, we have developed a practical and efficient
one-pot approach for the synthesis of enantiomerically enriched
2-oxyranyl ketones 4 by aerobic oxidation of easily available
cyclopropanols 2 via intermediate formation of peroxyketone
intermediates 3, followed by enantioselective epoxide formation
in the presence of poly-L-leucine catalyst and DBU. The
experimental protocol is operationally simple, utilizing
atmospheric oxygen as an eco-friendly reactant and poly-L-
leucine as a “green” catalyst. The method affords high yields
and good-to-excellent enantiomeric purity of the epoxy ketone
products. This strategy can be effectively applied as a
straightforward approach to chiral 2-oxyranyl ketones 4,
which are highly demanded synthetic intermediates.
a
Typical reaction conditions: cyclopropanol 2 (1.5 mmol), THF (1.5
mL), Mn(acac)3 (0.5 mol %); then PLL (240 mg) on SiO2, DBU (1.5
mmol). Isolated yields; enantiomeric excess was determined by
b
c
HPLC analysis using an AD-H column. Using 2 mol % of Mn(acac)3.
d
e
Yield after quenching of the reaction mixture with 1 M AcOH. No
ASSOCIATED CONTENT
■
f
reaction. TBDMS = tert-butyldimethylsilyl.
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01519.
The following examples of oxidation of different function-
alized tertiary cyclopropanols 2 to epoxy ketone 4 demonstrate
a wide scope of this method (Scheme 3). The reaction
proceeds smoothly and affords the corresponding epoxy
ketones 4 in good isolated yields (68−84%) and in high
enantioselectivities (84−97% ee), giving primary and secondary
alkyl- (4d, 4f), phenyl- (4c), and alkenyl-substituted (4e) epoxy
ketones. Even the functionalized compounds bearing 1,3-
dioxolane 4b, tetrahydropyranyl 4g, and Boc-protected
piperidine 4h moieties were obtained flawlessly.
Experimental details and characterization data for new
compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dzmitry.kananovich@ttu.ee.
ORCID
To test some mechanistic aspects of the aerobic oxidation, a
piperidine derivative 2l was also tested in this reaction. As
expected, the reaction did not occur due to the inactivation of
manganese catalyst with the amino function of substrate. We
Dzmitry G. Kananovich: 0000-0002-2664-4491
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.7b01519
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ACKNOWLEDGMENTS
■
Funding from the European Union’s Seventh Framework
Program for research, technological development and demon-
stration (Grant No. 621364 (TUTIC-Green)), from the Centre
of Excellence in Molecular Cell Engineering (Grant No. 2014-
2020.4.01.15-0013), from the Estonian Ministry of Education
and Research (Grant No. IUT 19-32) and from Tallinn
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