Imidazolone-activated donor-acceptor cyclopropanes with a peripheral stereocenter. A study on stereoselectivity of cycloaddition with aldehydes

Article abstract of DOI:10.1007/s10593-020-02778-2

[Figure not available: see fulltext.] Nucleophilic cyclopropanation of arylideneimidazolones possessing a peripheral chiral center and the subsequent fractional crystallization of diastereomers allows access to the compounds with an enantiomerically uniform configuration of the spirocyclic donoracceptor cyclopropane fragment. They were used to study the mechanism of the cycloaddition reaction with aldehydes; it was demonstrated that stereochemical information from the cyclopropane fragment is lost during the reaction.

Full text of DOI:10.1007/s10593-020-02778-2

DOI 10.1007/s10593-020-02778-2
Chemistry of Heterocyclic Compounds 2020, 56(8), 1092–1096
SHORT COMMUNICATIONS
Imidazolone-activated donor-acceptor cyclopropanes with
a peripheral stereocenter. A study on stereoselectivity
of cycloaddition with aldehydes
Andrey А. Mikhaylov1*, Pavel N. Solyev2, Andrei V. Kuleshov1,3, Vadim S. Kublitskii1,
Aleksander А. Korlyukov4, Vladislav А. Lushpa1, Mikhail S. Baranov1*
1 Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences,
16/10 Miklukho-Maklaya St., Moscow 117997, Russia; e-mail: mikhaylov_andrey@yahoo.com, baranovmikes@gmail.com
2 Engelhardt Institute of Molecular Biology, Russian Academy of Sciences,
32 Vavilova St., Moscow 119991, Russia
3 Moscow Lomonosov State University,
1, Build. 3 Leninskie Gory, Moscow 119991, Russia
4 A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 Vavilova St., Moscow 119991, Russia
Submitted July 3, 2020
Accepted after revision August 25, 2020
Translated from Khimiya Geterotsiklicheskikh Soedinenii,
2020, 56(8), 1092–1096
Nucleophilic cyclopropanation of arylideneimidazolones possessing a peripheral chiral center and the subsequent fractional
crystallization of diastereomers allows access to the compounds with an enantiomerically uniform configuration of the spirocyclic donor-
acceptor cyclopropane fragment. They were used to study the mechanism of the cycloaddition reaction with aldehydes; it was
demonstrated that stereochemical information from the cyclopropane fragment is lost during the reaction.
Keywords: donor-acceptor cyclopropanes, imidazole, spirocyclic compounds, chirality, diastereoselectivity.
Scheme 1. Cycloaddition reactions of spirocyclic DACs
The strain energy embodied in small rings provides rich
opportunities for a variety of chemical transformations.1
Substitution of vicinal positions by both acceptor and
donor substituents allows using this potential in
cycloaddition and nucleophilic opening reactions.2 The
research on the so-called donor-acceptor cyclopropanes
(DAC), in particular 1,1-cyclopropanedicarboxylates, has
been advanced greatly.2b Such cyclopropanes can be used
in (n+3) cycloaddition reactions for the synthesis of various
heterocycles: pyrroles,3 tetrahydrofurans,4 pyrrolidines,5
and many others.6
In our recent work, a new class of spirocyclic DACs
containing a heterocyclic imidazolone fragment was
proposed (Scheme 1, previous work).7 We were able to
show that, upon the promotion with protic acids, such as
p-TsOH, their activation and participation in the
cycloaddition reaction with aldehydes, resulting in the
formation of a tetrahydrofuran ring, are possible. The
reaction conditions were extended to a wide range of
containing a heterocyclic imidazolone fragment
Previous work (racemic):
Ar
Ar
R2
O
Ar
R2
R1CHO
N
O
N
R3
O
O
N
p-TsOH
N
R3
N
+
R3
N
R1
R1
R2
O
This work (chiral):
Ar
Ar
Me
N
Me
N
R1CHO
Me
O
N
stereoselectivity?
N
Me
Ph
R1
p-TsOH
O
Ph
O
control of
the reaction
stereoselection
peripheral
chiral center
enantiomerically pure
compounds
0009-3122/20/56(8)-1092©2020 Springer Science+Business Media, LLC
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Chemistry of Heterocyclic Compounds 2020, 56(8), 1092–1096
Scheme 3. Cyclopropanation of compound 2
substrates and the major regularities were revealed,
including those determining the stereoselectivity of the
process for racemic substrates.
One of the unresolved issues is the stereoselectivity of
the initial stages of this reaction. It is known that, for
1,1-cyclopropanedicarboxylates, the reaction with aldehydes
proceeds with the inversion of the configuration of the
stereocenter.4b In view of the fact that the spirocyclic DACs
proposed by us have a fundamentally different activation
mechanism, they require a separate investigation of this
step, which is presented in this work.
For such a study it prompted to obtain enantiomerically
pure imidazolone-substituted DACs (Scheme 1, this work).
Since no enantioselective methods for the synthesis of such
compounds are known at the moment,4b,8 we synthesized
compounds containing a chiral substituent at the peripheral
position in the imidazole ring and separated the
diastereomers formed after the cyclopropanation reaction.
We have previously shown that the substitution of the
imidazole ring has no noticeable effect on the course of the
cycloaddition reaction with aldehydes.7 However, the
presence of an additional stereocenter would make it
possible to reliably monitor the stereoselectivity of the
reaction using NMR, and the enantiomeric purity of the
chiral substituent would determine the configuration of the
stereocenters formed and transformed during the reaction.
The commercially available (S)-α-phenylethylamine was
chosen as the source of chirality (Scheme 2). At the first
step, it was condensed with p-anisaldehyde to form Schiff
base 1.9 The resulting imine was introduced into the
cascade reaction of the formation of an imidazolone ring
with ethyl N-(1-methoxyethylideneamino)acetate. Imidazo-
lone 2 was obtained in 77% yield after 21 days.
structural analysis (Fig. 1). The second diastereomer 3b
was obtained with a purity of 90% by recrystallization of
the evaporated mother liquor from a hexane–EtOAc
mixture.
Figure 1. Molecular structure of compound 3а with atoms
represented as thermal vibration ellipsoids of 50% probability.
We found that the reaction of isomer 3a (carried out in
an NMR tube to guarantee the purity of the starting isomer)
with 4-bromobenzaldehyde leads to the formation of a
mixture of 4 isomers of spirocycles 4 in a ratio of
1.7:1.6:1:1. The products were separated into isomer pairs
4a,b and 4с,d using column chromatography (Scheme 4).
Their structures were determined using a combination of
COSY, 1H–13C HSQC, and 1H–13C HMBC 2D NMR
spectra and correlation with previously obtained data.7 The
formation of two pairs of equimolar mixtures of
stereoisomers indicates the loss of stereochemical
information at the first stages of cycloaddition, and,
consequently, the intermediate formation of a carbocation.
Since stereochemical information is lost in the course of
the reaction, the development of enantioselective
approaches to the synthesis of chiral DACs containing an
imidazolone fragment as well as the use of such
compounds in cycloaddition reactions with aldehydes seem
to be impractical, and chirality must be induced from the
α-carbon atom of the aldehyde or using a chiral promoter.
Thus, in this study we have developed an approach to
the preparation of chiral donor-acceptor cyclopropanes
spiro-fused with an imidazolone fragment and also found
the conditions for the separation of the resulting
diastereomers using fractional recrystallization. The
presence of a chiral fragment of (S)-phenylethylamine at
the periphery of the molecule allows the obtained
Scheme 2. Synthesis of arylidenimidazolone 2
At the next step, compound 2 was subjected to a reaction
with dimethylsulfoxonium methylide in DMSO (Scheme 3).
The reaction led to the formation of an equimolar mixture
of diastereomers 3a and 3b in 86% yield. The
nonselectivity of the transformation further confirms the
absence of the influence of the introduced chiral
substituent, and, consequently, the correct choice of the
position for its introduction. The downside is the
practically identical chromatographic characteristics of the
diastereomers. In this regard, the separation of isomers was
carried out using fractional crystallization. Double
recrystallization from PhH yielded pure stereoisomer 3a.
Its structure was unambiguously established by X-ray
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Chemistry of Heterocyclic Compounds 2020, 56(8), 1092–1096
Scheme 4. Utilization of compound 3a in the reaction with 4-bromobenzaldehyde
compounds to be used as tools for studying the stereo-
selectivity of cycloaddition and nucleophilic opening
reactions with no use of chiral chromatography. For the
Cyclopropanation, preparation of compounds 3a,b.7
NaH (60% dispersion in oil, 0.11 g, 2.8 mmol, 1.15 equiv)
was added in one portion to a solution of trimethyl-
sulfoxonium iodide (0.70 g, 3.2 mmol, 1.30 equiv) in
anhydrous DMSO (5 ml) at room temperature under a flow
of argon. The reaction mixture was stirred for 20 min until
a clear solution formed and gas evolution ceased. Then,
imidazolone 2 (0.79 g, 2.5 mmol, 1 equiv) was added in
one portion. The resulting reaction mixture was stirred at
room temperature for 1 h and then partitioned between
EtOAc (30 ml) and H2O (30 ml). The organic layer was
separated, the aqueous layer was extracted with EtOAc
(10 ml). The combined organic layers were washed with
saturated aqueous NaCl (30 ml), dried over Na2SO4, and
concentrated under reduced pressure. The residue was
applied to a silica gel column and chromatographed using
hexane–EtOAc, 1:1 as eluent. Yield of an equimolar
mixture of diastereomeric cyclopropanes 3a,b 0.71 g
(2.1 mmol, 86%), Rf 0.21 (hexane–EtOAc, 1:1). Pure
diastereomers were obtained by fractional recrystallization.
(1R,3R)-1-(4-Methoxyphenyl)-5-methyl-6-((1S)-1-phenyl-
cycloaddition
reaction
of
imidazolone-substituted
cyclopropanes with aldehydes under the conditions of
activation with a protic acid, it was shown that the
stereochemical information of the cyclopropane part of the
molecule is not preserved.
Experimental
1H and 13C NMR spectra were acquired on a Bruker
Avance II 700 spectrometer (700 and 175 MHz,
respectively) at 303 K. Residual signals of the solvent
1
CDCl3 were used as internal standard: 7.26 ppm for H
nuclei, 77.16 ppm for 13C nuclei. High-resolution mass
spectra were recorded on a Bruker micrOTOF-Q II mass
spectrometer, electrospray ionization (ESI-TOF); 5 ppm
accuracy of given values. The angles of rotation were
measured on a PerkinElmer 341 automatic polarimeter.
Melting points were determined on a Kofler bench and are
uncorrected. Thin-layer chromatography was performed on
Merck 60 plates with the F254 fluorescent indicator.
Column chromatography was performed on silica gel 60
(40–63 μm) as the stationary phase.
ethyl)-4,6-diazaspiro[2.4]hept-4-en-7-one
(3a)
was
obtained by double recrystallization of the mixture of
diastereomers 3a,b from PhH (heating to reflux and
cooling to room temperature; ~5 ml for the first
crystallization and ~2 ml for the second). Yield of pure
compound 3a 123 mg, mp 171–172°C (PhH, single
(5Z)-5-(4-Methoxybenzylidene)-2-methyl-3-((1S)-1-phe-
nylethyl)-3,5-dihydro-4H-imidazol-4-one (2) was obtained
analogously to a literature method.7 Schiff base 19 (0.90 g,
3.8 mmol, 1 equiv) was mixed with ethyl N-(1-methoxy-
ethylideneamino)acetate10 (0.60 g, 3.8 mmol, 1.0 equiv).
The reaction mixture was kept for 21 days at room
temperature, during which it became brown. Volatile
reaction products were distilled off under reduced pressure,
and the residue was purified by column chromatography on
silica gel, eluent hexane–EtOAc, 3:1. Yield 0.92 g
(2.9 mmol, 77%), yellow oil which crystallized upon
standing, mp 59–60°C (hexane–EtOAc), Rf 0.26 (hexane–
EtOAc, 3:1, visualization by UV light / vanillin reagent).
20
1
crystal). [α]D +351.8 (c 1.3, CHCl3). H NMR spectrum,
δ, ppm (J, Hz): 7.34 (2H, t, J = 7.6, H Ar); 7.30–7.21 (5H,
m, H Ar); 6.85 (2H, d, J = 8.7, H Ar); 5.45 (1H, q, J = 7.3,
NCH); 3.77 (3H, s, OCH3); 3.07 (1H, t, J = 9.1, CCH);
2.13 (1H, dd, J = 8.5, J = 4.8, CH2); 2.08 (1H, dd, J = 9.7,
J = 4.8, CH2); 1.93 (3H, s, CH3); 1.81 (3H, d, J = 7.3,
CH3). 13C NMR spectrum, δ, ppm: 181.2 (C=O); 160.4 (С);
158.7 (C); 140.1 (C); 129.6 (2CH); 128.8 (2CH); 128.2
(C); 127.8 (CH); 126.6 (2CH); 113.8 (2CH); 56.3 (C); 55.3
(OCH3); 50.5 (CH); 35.8 (CH); 23.9 (CH2); 18.4 (CH3);
17.7 (СH3). Found, m/z: 335.1743 [M+H]+. C21H23N2O2.
Calculated, m/z: 335.1754.
20
1
[α]D –30.1 (c 2.1, CHCl3). H NMR spectrum, δ, ppm
(J, Hz): 8.19 (2H, d, J = 8.8, H Ar); 7.42 (2H, t, J = 7.5,
H Ar); 7.39–7.24 (3H, m, H Ar); 7.19 (1H, s, =CH); 7.01
(2H, d, J = 8.8, H Ar); 5.63 (1H, q, J = 7.2, NCH); 3.92
(3H, s, OCH3); 2.16 (3H, s, CH3); 1.90 (3H, d, J = 7.2,
CH3). 13C NMR spectrum, δ, ppm: 171.0 (C=O); 161.4
(2С); 140.1 (C); 136.7 (C); 134.2 (2CH); 128.9 (2CH);
127.8 (CH); 127.7 (CH); 127.4 (C); 126.7 (2CH); 114.5
(2CH); 55.5 (OCH3); 50.0 (CH); 18.4 (CH3); 17.6 (СH3).
Found, m/z: 321.1600 [M+H]+. C20H21N2O2. Calculated,
m/z: 321.1598.
(1S,3S)-1-(4-Methoxyphenyl)-5-methyl-6-((1S)-1-phenyl-
ethyl)-4,6-diazaspiro[2.4]hept-4-en-7-one
(3b).
The
combined mother liquors from the crystallization of isomer
3a were dissolved in PhH by heating under reflux.
Crystallization in the refrigerator (~6°C) led to the
precipitation of most of isomer 3a. The resulting mother
liquor was concentrated and the residue was recrystallized
twice from a hexane–EtOAc mixture, 4:1. Yield of
compound 3b with purity of 90% 172 mg, mp 131–132°C
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Chemistry of Heterocyclic Compounds 2020, 56(8), 1092–1096
20
(hexane–EtOAc, single crystal). [α]D +252.0 (c 0.4,
(5R,6S,8S)- and (5S,6R,8R)-6-(4-bromophenyl)-8-(4-meth-
oxyphenyl)-2-methyl-3-((1S)-1-phenylethyl)-7-oxa-1,3-di-
azaspiro[4.4]non-1-en-4-ones 4c,d, mixture of isomers in
1:1 ratio. Rf 0.05 (hexane–EtOAc, 1:1, visualization by UV
light / vanillin reagent). 1H NMR spectrum, δ, ppm (J, Hz):
one isomer: 7.65 (2H, d, J = 8.6, H Ar); 7.49 (2H, d,
J = 8.4, H Ar); 7.32 (2H, d, J = 8.4, H Ar), 7.27–7.22 (3H,
m, H Ar), 6.96 (2H, d, J = 8.6, H Ar); 6.54 (2H, dd, J = 6.5,
J = 1.8, H Ar), 5.25 (1H, t, J = 8.1, 8-CH); 5.10 (1H, s,
6-CH); 5.06 (1H, q, J = 7.3, NCH); 3.83 (3H, s, OCH3);
2.75 (1H, dd, J = 13.2, J = 7.7, CH2); 2.48 (1H, dd,
J = 13.3, J = 8.4, CH2); 1.84 (3H, s, CH3); 1.48 (3H, d,
J = 7.3, CH3); other isomer: 7.63 (2H, d, J = 8.6, H Ar);
7.39 (2H, d, J = 8.4, H Ar); 7.27–7.22 (5H, m, H Ar); 6.96
(2H, d, J = 8.6, H Ar); 6.94 (2H, dd, J = 7.2, J = 1.9, H Ar);
5.25 (1H, t, J = 8.1, 8-CH); 5.09 (1H, s, 6-CH); 4.97 (1H,
q, J = 7.2, NCH); 3.83 (3H, s, OCH3); 2.72 (1H, dd,
J = 13.2, J = 7.4, CH2); 2.50 (1H, dd, J = 13.2, J = 8.8,
CH2); 1.92 (3H, s, CH3); 1.30 (3H, d, J = 7.2, CH3).
13C NMR spectrum, δ, ppm (both isomers): 182.4 (2C=O);
162.4 and 162.3 (C); 159.8 (2C(4)–O Ar); 139.5 and 139.0
(C); 134.9 and 134.7 (C); 132.9 and 132.7 (C); 131.4 and
131.2 (2CH); 128.9 (C); 128.8 (CH); 128.7 (CH); 128.6
(CH); 128.1 (2CH); 127.8 (2 4-CH Ph); 126.5 and 126.3
(2CH); 122.4 and 122.3 (C); 114.1 (2CH); 85.9 and 85.8
(OCH); 80.81 (2OCH); 79.2 (2C); 55.5 (OCH3); 50.4 and
49.2 (NCH); 44.8 and 44.6 (CH2); 17.7 (2CH3); 17.5 (CH3);
17.3 (CH3). Found, m/z: 521.1249 [M+H]+. C28H2881BrN2O3.
Calculated, m/z: 521.1262.
X-ray structural analysis of compound 3a was
performed on an APEX DUO diffractometer (MoKα
radiation, graphite monochromator, ω-scanning). Crystals
suitable for X-ray structural analysis were grown by slow
cooling of a saturated solution in PhH. The structure was
solved using the dual-space algorithm and refined against
F2 by the least-squares technique in the full-matrix
anhisklotropic approximation using the Bruker SHELXTL
program set. Hydrogen atom positions were refined
geometrically. All calculations were performed using the
SHELXT, SHELXL, and OLEX2 software packages.11–13
The full set of X-ray structural data for compound 3a was
deposited at the Cambridge Crystallographic Data Center
(deposit CCDC 2012447).
1
CHCl3). H NMR spectrum, δ, ppm (J, Hz): 7.36 (2H, t,
J = 7.6, H Ar); 7.31–7.27 (3H, m, H Ar); 7.24 (2H, d,
J = 8.5, H Ar); 6.85 (2H, d, J = 8.7, H Ar); 5.46 (1H, q,
J = 7.2, NCH); 3.76 (3H, s, OCH3); 3.04 (1H, t, J = 9.1,
CCH); 2.15 (1H, dd, J = 8.4, J = 4.8, CH2); 2.11 (1H, dd,
J = 9.7, J = 4.8, CH2); 1.93 (3H, s, CH3); 1.80 (3H, d,
J = 7.2, CH3). 13C NMR spectrum, δ, ppm: 181.2 (C=O);
160.5 (С); 158.8 (C); 140.2 (C); 129.6 (2CH); 128.9
(2CH); 128.3 (C); 127.8 (CH); 126.7 (2CH); 113.9 (2CH);
56.4 (C); 55.4 (OCH3); 50.5 (CH); 35.9 (CH); 24.0 (CH2);
18.4 (CH3); 17.7 (СH3). Found, m/z: 335.1750 [M+H]+.
C21H23N2O2. Calculated, m/z: 335.1754.
Reaction of cyclopropane 3a with 4-bromobenz-
aldehyde. Anhydrous p-TsOH (28 mg, 0.16 mmol,
1.1 equiv) was added to a solution of cyclopropane 3a
(49 mg, 0.15 mmol, 1 equiv) and 4-bromobenzaldehyde
(81 mg, 0.44 mmol, 3.0 equiv) in anhydrous CDCl3
(0.7 ml) in an NMR tube at room temperature. The tube
was gently shaken several times until the content was
homogeneous. The color of the solution instantly changed
to a pale-yellow. The reaction mixture was kept for 1 h,
after which N,N,N',N'-tetramethylguanidine (28 μl, 25 mg,
0.22 mmol, 1.5 equiv) was added. The reaction mixture
was homogenized and kept for 10 min. At this point, the
NMR spectrum of the reaction mixture was recorded. Then,
by means of column chromatography on silica gel (eluent
hexane–EtOAc, 1:1), the products were separated into pairs
of isomers (5R,6R,8R)/(5S,6S,8S)-4a,b (less polar, 18 mg,
35 μmol, 25%) and ((5R,6S,8S)/(5S,6R,8R)-4c,d (more
polar, 31 mg, 60 μmol, 41%).
(5R,6R,8R)- and (5S,6S,8S)-6-(4-bromophenyl)-8-(4-meth-
oxyphenyl)-2-methyl-3-((1S)-1-phenylethyl)-7-oxa-1,3-diaza-
spiro[4.4]non-1-en-4-ones 4a,b, mixture of isomers in 1:1
ratio. Rf 0.21 (hexane–EtOAc, 1:1, visualization by UV
light / vanillin reagent). 1H NMR spectrum, δ, ppm (J, Hz):
one isomer: 7.55 (2H, d, J = 8.6, H Ar); 7.35–7.24 (5H, m,
H Ar); 7.18 (2H, dd, J = 8.0, J = 1.5, H Ar); 7.09 (2H, d,
J = 8.4, H Ar); 6.92 (2H, d, J = 8.6, H Ar); 5.31 (1H, q,
J = 7.3, NCH); 5.27 (1H, t, J = 7.9, 8-CH); 5.07 (1H, s,
6-CH); 3.82 (3H, s, OCH3); 2.94 (1H, dd, J = 13.2, J = 8.6,
CH2); 2.28 (1H, dd, J = 13.2, J = 7.2, CH2); 1.70 (3H, s,
CH3); 1.66 (3H, d, J = 7.3, CH3); other isomer: 7.57 (2H, d,
J = 8.6, H Ar); 7.42 (2H, d, J = 8.4, H Ar); 7.27–7.22 (5H,
m, H Ar); 6.92 (2H, d, J = 8.6, H Ar); 6.91–6.88 (2H, m,
H Ar); 5.26 (1H, q, J = 7.3, NCH); 5.28 (1H, t, J = 7.7,
8-CH); 5.11 (1H, s, 6-CH); 3.82 (3H, s, OCH3); 2.94 (1H,
dd, J = 13.2, J = 8.7, CH2); 2.26 (1H, dd, J = 13.2, J = 6.8,
CH2); 1.69 (3H, d, J = 7.3, CH3); 1.65 (3H, s, CH3).
13C NMR spectrum, δ, ppm (both isomers): 182.8 and
182.5 (C=O); 159.8 (2C); 159.6 (C(4)–O Ar); 139.5 and
139.5 (C); 134.8 and 134.4 (C); 133.6 and 133.5 (C); 130.9
(4CH); 128.9 and 128.8 (2CH); 128.6 (2CH); 128.5 (2CH);
127.9 (4-CH Ph); 126.6 and 126.3 (2CH); 122.3 and 121.8
(C); 114.1 (2CH); 88.7 and 88.5 (OCH); 80.7 and 80.6
(OCH); 78.7 and 78.6 (C); 55.5 (OCH3); 50.6 and 50.0
(NCH); 45.1 (CH2); 18.6 (CH3); 17.9 (CH3); 17.3 (CH3);
16.9 (CH3). Found, m/z: 521.1267 [M+H]+. C28H2881BrN2O3.
Calculated, m/z: 521.1262.
Supplementary information file containing details of the
1
assignment of stereoisomers, Н and 13С NMR spectra for
1
1
all synthesized compounds, COSY, H–13C HSQC, H–13C
HMBC spectra of mixtures of compounds 4a,b and 4c,d, as
well as the main crystallographic data and parameters of
the X-ray structural analysis of compound 3а, is available
at the journal website at http://link.springer.com/
journal/10593.
This work was supported by the Russian Science
Foundation (grant 19-73-00319).
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