Asymmetric Michael reaction between aldehydes and nitroalkanes promoted by pyrrolidine-containing C2-symmetric organocatalysts

Article abstract of DOI:10.1007/s11172-019-2568-2

Bifunctional C₂-symmetric organocatalysts derived from chiral 1,2-diaminoethanes and (S)-2-aminomethylpyrrolidine were fi rst used for promoting the asymmetric Michael addition of aliphatic aldehydes to nitroalkenes. The synthesized enantioenri

Full text of DOI:10.1007/s11172-019-2568-2

Russian Chemical Bulletin, International Edition, Vol. 68, No. 7, pp. 1402—1406, July, 2019
1402
Asymmetric Michael reaction between aldehydes and nitroalkanes
promoted by pyrrolidine-containing C2-symmetric organocatalysts
K. A. Bykova,a,b A. A. Kostenko,a A. S. Kucherenko,a and S. G. Zlotina
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
E-mail: alexkostenko93@yandex.ru
bDmitry Mendeleev University of Chemical Technology of Russia,
9 Miusskaya pl., 125047 Moscow, Russian Federation.
E-mail: bykova_ksyusha@mail.ru
Bifunctional C2-symmetric organocatalysts derived from chiral 1,2-diaminoethanes and
(S)-2-aminomethylpyrrolidine were first used for promoting the asymmetric Michael addition
of aliphatic aldehydes to nitroalkenes. The synthesized enantioenriched (up to 82% ee) products
can be transformed into various biologically active -aminobutyric acid derivatives.
Key words: asymmetric catalysis, organocatalysis, squaramides, Michael reaction, aldehydes,
nitroalkanes.
Asymmetric Michael addition of aldehydes and ketones
Results and Discussion
to nitroalkenes promoted by chiral organocatalysts is an
efficient enantioselective approach for carbon—carbon
bond formation.1,2 For instance, this reaction is widely
used for synthesizing the prodrugs of pregabalin, phenibut,
baclofen,3 nakinadine B,4 and -aminobutyric acid (GABA)
derivatives and their analogs. The common organocata-
lysts for these reactions are poorly available C1-symmet-
ric pyrrolidine derivatives bearing either thiourea5 or
squaramide6 moieties. These moieties provide additional
activation and necessary spatial orientation of the start-
ing reactants in the activated complex.7 Besides, the use
of high catalyst loading (up to 20 mol.%) and high-boil-
ing aprotic solvents (DMSO, DMF) and ionic liquids is
necessary to achieve high product yields and enantio-
selectivity.8 All these requirements hamper the prod-
uct separation and regeneration of valuable catalysts.
Earlier,9 we have shown that C2-symmetric organocata-
lysts bearing squaramide moieties lack the above-men-
tioned drawbacks. The majority of C2-symmetric organo-
catalysts are poorly soluble in organic solvents that simpli-
fies their isolation from the reaction mixtures. To date,
C2-symmetric pyrrolidine-derived squaramides were ap-
plied only for catalyzing the reactions between ketones
(mainly, cyclic ones) and nitroalkenes.10 To the best of
our knowledge, no organocatalysts of this type have been
used in asymmetric Michael reactions involving aliphatic
aldehydes. In the present work, we describe addition of
alkanals to nitroalkenes promoted by C2-symmetric or-
ganocatalysts.
Earlier, we have synthesized new C2-symmetric N,N´-bis-
[(pyrrolidin-2-ylmethyl)squaramides] 1—3 from chiral
(R,R)- and (S,S)-1,2-diaminoethanes and revealed that
these compounds efficiently catalyze the addition of ke-
tones to different nitro olefins.10 In the present work, we
widened the scope of the substrates suitable for the reac-
tions promoted by catalysts 1—3 and performed the more
complex asymmetric addition reactions between aliphatic
aldehydes and nitro olefins.
The optimal organocatalyst was identified using the
model reaction of isobutyric aldehyde 4a (2 equiv.) with
-nitrostyrene 5a (1 equiv.). The reaction was carried out
in the presence of catalysts 1—3 (10 mol.%) in a THF
solution at room temperature (Table 1, entries 1—6).
It was found that under the model reaction conditions
squaramide S,S-3 bearing (1S,2S)-diaminocyclohexane
and (S)-(pyrrolidin-2-yl)methylamine moieties exhibited
the highest catalytic performance. Next, we studied the
effect of different solvents on the reaction between 4a and
5a promoted by organocatalyst S,S-3. The highest stereo-
induction (76% ee) was achieved in toluene (see Table 1,
entry 7). Other solvents (EtOAc, CH2Cl2, EtOH) gave
unsatisfactory results (entries 8—10). To our surprise, the
reaction occurs even in water to produce the target prod-
uct in very high yield (86%) but with poor enantioselec-
tivity (23% ee) (entry 11).
Catalyst S,S-3 (10 mol.%, in toluene) providing the
highest yield and enantioselectivity in the model reaction
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1402—1406, July, 2019.
1066-5285/19/6807-1402 © 2019 Springer Science+Business Media, Inc.

Asymmetric Michael addition
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
1403
Scheme 1
in 74% yield and 79% ee. It is interesting to note that
nitro diene 5i reacts with isobutyric aldehyde 4a regiospe-
cifically at the position 2 to afford product 6i with moder-
ate enantioselectivity (68% ee). The reaction of acetalde-
hyde 4b (5 M solution in THF) and nitrostyrene 5a leads
to product 6j in high yield (99%) and enantioselectivity
(82% ee). In the case of propanal 4c, the reaction proceeds
with very high diastereoselectivity (dr (2S,3R) : (2R,3R) =
= 95 : 5) and 63% ee of the major diastereomer 2S,3R-6k
(1H NMR and HPLC data).
Organocatalyst S,S-3 can be easily recovered and re-
used (Table 2). After the reaction completion, the reaction
mixture was concentrated in vacuo, the products were
extracted with diethyl ether, and the organic layer
was decanted. Then the new portions of the starting com-
pounds and the solvent were added to the residue and
the reaction was repeated under the same conditions. After
five runs, high resolution mass spectrometry reveals
no changes in the structure of catalyst S,S-3. Some reduc-
tion of catalytic activity in the fifth run can be reasonably
was used for evaluation of the substrate scope (Scheme 2).
Thus, isobutyric aldehyde 4a efficiently reacts with aro-
matic nitro olefins 5, including ones bearing the electron-
donating (5b—f) and electron-withdrawing (5g) substitu-
ents at the aromatic ring, to give the target products in
66—79% yields and 70—77% ee. Furyl-substituted nitro-
alkene 5h also reacts readily with 4a to give product 6h

1404 Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
Bykova et al.
Table 1. Selection of the catalyst and optimization of the reaction
conditions for the synthesis of compound 6a by the reaction of
isobutyric aldehyde 4a with -nitrostyrene 5aa
Table 2. Recovery of the catalyst S,S-3 used to catalyzed
the reaction between compounds 4a and 5aa
Cycle
Yield of 6a (%)
ee of 6a (%)
Entry
Catalyst
Solvent
Yieldb of
eec of 6a
1
70
68
66
63
60
76
76
74
72
70
6a (%)
(%)
2
1
2
3
4
5
6
7
8
S,R-1
S,S-1
S,S-2
S,R-2
S,S-3
S,R-3
S,S-3
S,S-3
S,S-3
S,S-3
S,S-3
THF
THF
16
25
55
16
56
52
70
53
45
77
86
44
65
50
60
74
66
76
71
66
71
23
3
4
THF
5b
THF
a Reaction conditions: catalyst S,S-3 (5 mg, 0.005 mmol,
10 mol.%), aldehyde 4a (7.2 mg, 0.1 mmol), -nitro-
styrene 5a (7.5 mg, 0.05 mmol), toluene (0.1 mL),
~20 C, stirring, 48 h.
THF
THF
PhMe
EtOAc
CH2Cl2
EtOH
H2O
b Reaction time was 56 h.
9
10
11
lysts 1—3 in the Michael reaction of aliphatic aldehydes
with nitroalkenes. We succeeded to achieve high yields
(up to 99%) and enantiomeric excesses (up to 82% ee) of
the products. It was found that nitroalka-1,3-diene 5i
reacts with isobutyric aldehyde 4a regiospecifically at the
position 2. Catalyst S,S-3 can be recovered and reused
almost without loss of catalytic activity and selectivity.
A series of synthesized products 6a—e seem to be valuable
chiral precursors of the GABA analogs (phenibut, baclofen,
rolipram, and pregabalin) and some other biologically
active compounds.12
a
Suspension of the catalyst 1—3 (0.005 mmol, 10 mol.%),
aldehyde 4a (7.2 mg, 0.1 mmol), -nitrostyrene 5a (7.5 mg,
0.05 mmol), and the solvent (0.1 mL), ~20 C, 48 h.
b Isolated yield of 6a (silica gel flash chromatography).
c Enantiomeric excess was determined by HPLC on chiral phase
Chiralpak AD-H; an absolute (R)-configuration of compound
6a was confirmed by a comparison of its rotating angle with
published data.11
Scheme 2
Experimental
Commercially available reagents were purchased from Sigma-
Aldrich Co. The solvents were purified following the standard
procedures. Catalysts 1—3 and the starting nitro olefins 5 were
synthesized as described earlier.10,13 Column chromatography
was performed with Aldrich silica gel (0.060—0.200 m) with
gradient elution with hexane—ethyl acetate (2 : 11 : 1). 1H and
13C NMR spectra were run with a Bruker AM300 instrument
(working frequencies of 300 (1H) and 75 MHz (13C)) at 25 C.
High resolution mass spectrometry was performed with a Bruker
microTOF II instrument. Enantiomeric compositions of products
6 were analyzed using a Staier HPLC system (Akvilon, Russia)
equipped with a UV detector (220—254 nm) and 25-cm chiral
columns Chiralpak AD-H, AS-H, and OD-H (Chiral Technol-
ogies). The racemic samples of compounds 6 used as the standards
in determining the enantiomeric excesses (ee) by HPLC were
synthesized from the corresponding reactants in the presence of
pyrrolidine (10—20 mol.% in toluene).
Catalytic Michael reaction between compounds 4 and 5 (gen-
eral procedure). To a suspension of organocatalyst 1—3 (5.0 mg,
0.005 mmol, 10 mol.%) and nitro olefin 5 (0.05 mmol) in toluene
(0.1 mL), aldehyde 4a—c (0.1 mmol) was added. Acetaldehyde
4b was used as a THF solution (0.1 mmol of 4b correspond to
20 L of 5 M solution). The reaction mixture was stirred at room
temperature for 48 h and concentrated in vacuo. Products 6 were
extracted with diethyl ether (34 mL) and then purified by silica
gel flash chromatography (gradient elution with hexane—ethyl
acetate, 2 : 11 : 1). Spectral properties of compounds 6 coincide
with those reported earlier.12
4: R1 = R2 = Me (a), R1 = R2 = H (b), R1 = Me, R2 = H (c)
5: R3 = Ph (a), 4-MeOC6H4 (b), 2-MeOC6H4 (c), 4-ClC6H4 (d),
4-BrC6H4 (e), 4-FC6H4 (f), 4-O2NC6H4 (g), 2-furyl (h),
PhCHCH (i)
Compound 6
R1
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
R2
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
R3
Ph
Yield (%) ee (%)
a
b
c
d
e
f
g
h
i
70
73
75
79
66
70
81
74
55
99
99*
76
4-MeOC6H4
2-MeOC6H4
4-ClC6H4
4-BrC6H4
4-FC6H4
4-O2NC6H4
2-furyl
77
70
75
72
73
80
79
68
82
PhCHCH
Ph
j
k
Me
H
Ph
63 (2S,3R)
22 (2R,3R)
* Diastereomeric ratio (2S,3R) : (2R,3R) = 95 : 5.
explained by the inevitable losses of the catalyst during
recovery.
In summary, in the present work we pioneered in the
use of a series of C2-symmetric squaramide organocata-

Asymmetric Michael addition
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
1405
(R)-2,2-Dimethyl-4-nitro-3-phenylbutanal (6a). Yield 6.08 mg
CH2); 7.01—7.05 (m, 2 H, Ar); 7.17—7.21 (m, 2 H, Ar); 9.51
(70%), yellow oil. []D + 6.2 (c 0.5, CHCl3). 1H NMR
(s, 1 H, CHO). Found: m/z 240.1033 [M + H]+. C12H15FNO3.
Calculated: M = 240.1031. Enantiomeric excess was determined
by HPLC on a stationary chiral phase Chiralpak OD-H using
20
(CDCl3), : 0.86 (s, 3 H, CH3); 0.99 (s, 3 H, CH3); 3.67—3.71
(m, 1 H, CH), 4.59 (m, 1 H, CH2), 4.73—4.79 (m, 1 H, CH2);
7.09—7.21 (m, 5 H, ArH); 9.39 (s, 1 H, CHO). 13C NMR
(CDCl3), : 18.5, 21.2, 48.2, 48.1, 76.3, 127.6, 128.5, 128.9,
135.3, 204.2. Found: m/z 222.1128 [M + H]+. C12H16NO3.
Calculated: M = 222.1125. Enantiomeric excess was determined
using HPLC on a stationary chiral phase Chiralpak AD-H, detec-
detection at 254 nm (hexane—propan-2-ol (80 : 20), 0.7 mL min–1
;
t(R) = 16.2 min, t(S) = 30.5 min).
(R)-2,2-Dimethyl-4-nitro-3-(4-nitrophenyl)butanal (6g).
20
Yield 9.31 mg (81%), yellow oil. []D +8.0 (c 1.0, CHCl3).
1H NMR (CDCl3), : 1.06 (s, 3 H, CH3); 1.17 (s, 3 H, CH3);
3.94 (dd, 1 H, CH, J = 13.5 Hz, J = 3.9 Hz); 4.77 (dd, 1 H, CH2,
J = 13.5 Hz, J = 3.9 Hz); 4.90 (t, 1 H, CH2, J = 13.5 Hz); 7.42
(d, 2 H, Ar, J = 8.7 Hz); 8.21 (d, 2 H, Ar, J = 8.7 Hz); 9.48
(s, 1 H, CHO). Found: m/z 267.0973 [M + H]+. C12H15O5N2.
Calculated: M = 267.0975. Enantiomeric excess was determined
by HPLC on a stationary chiral phase Chiralpak OD-H using
tion at 220 nm (hexane—propan-2-ol (98 : 2), 0.5 mL min–1
;
t(R) = 24.7 min, t(S) = 25.8 min).
(R)-3-(4-Methoxyphenyl)-2,2-dimethyl-4-nitrobutanal (6b).
20
Yield 7.66 mg (73%), yellow oil. []D –5.0 (c 1.0, CHCl3).
1H NMR (CDCl3), : 1.01 (s, 3 H, CH3); 1.13 (s, 3 H, CH3);
3.74 (dd, 1 H, CH, J = 11.4 Hz, J = 4.5 Hz); 3.79 (s, 3 H, OCH3);
4.66 (dd, 1 H, CH2, J = 12.6 Hz, J = 4.2 Hz); 4.81 (dd, 1 H,
CH2, J = 12.6 Hz, J = 11.4 Hz); 6.84 (d, 2 H, Ar, J = 8.7 Hz);
7.11 (d, 2 H, Ar, J = 8.7 Hz); 9.51 (s, 1 H, CHO). Found: m/z
252.1228 [M + H]+. C13H18NO4. Calculated: M = 252.1231.
Enantiomeric excess was determined using HPLC on a station-
ary chiral phase Chiralpak OD-H, detection at 220 nm (hex-
ane—propan-2-ol (75 : 25), 0.8 mL min–1; t(R) = 15.2 min,
t(S) = 21.3 min).
detection at 220 nm (hexane—propan-2-ol (75 : 25), 0.8 mL min–1
;
t(R) = 25.5 min, t(S) = 39.6 min).
(R)-3-(2-Furyl)-2,2-dimethyl-4-nitrobutanal (6h). Yield 6.33 mg
(74%), yellow oil. []D –19.0 (c 1.0, CHCl3). 1H NMR
20
(CDCl3), : 1.05 (s, 3 H, CH3); 1.18 (s, 3 H, CH3); 3.92 (dd, 1 H,
CH, J = 11.1 Hz, J = 3.6 Hz); 4.57 (dd, 1 H, CH2, J = 12.6 Hz,
J = 3.6 Hz); 4.75 (dd, 1 H, CH2, J = 12.6 Hz, J = 11.1 Hz); 6.21
(d, 1 H, Ar, J = 3.0 Hz); 6.30 (s, 1 H, Ar); 7.36 (s, 1 H, Ar); 9.50
(s, 1 H, CHO). Found: m/z 212.0920 [M + H]+. C10H13O4N.
Calculated: M = 212.0918. Enantiomeric excess was determined
by HPLC on a stationary chiral phase Chiralpak OD-H using
(R)-3-(2-Methoxyphenyl)-2,2-dimethyl-4-nitrobutanal (6c).
20
Yield 7.38 mg (75%), yellow oil. []D +14.4 (c 0.5, CHCl3).
1H NMR (CDCl3), : 1.05 (s, 3 H, CH3); 1.1 (s, 3 H, CH3); 3.82
(dd, 1 H, CH, J = 11.1 Hz, J = 3.6 Hz); 4.57 (dd, 1 H, CH2,
J = 12.6 Hz, J = 3.6 Hz); 4.75 (dd, 1 H, CH2, J = 12.6 Hz,
J = 11.1 Hz); 6.21 (d, 1 H, Ar, J = 3.0 Hz); 6.95 (s, 1 H, Ar);
7.12 (s, 1 H, Ar); 9.51 (s, 1 H, CHO). Found: m/z 251.2784
[M + H]+. C13H17NO4. Enantiomeric excess was determined
using HPLC on a stationary chiral phase Chiralpak OD-H,
detection at 220 nm (hexane—propan-2-ol (75 : 25), 0.8 mL min–1
;
t(R) = 9.5 min, t(S) = 14.0 min).
(R)-2,2-Dimethyl-3-nitromethyl-5-phenylpent-4-enal (6i).
20
Yield 5.28 mg (55%), yellow oil. []D –1.9 (c 1.0, CHCl3).
1H NMR (CDCl3), : 1.17 (s, 3 H, CH3); 1.18 (s, 3 H); 3.30
(m, 1 H); 4.51 (m, 2 H), 6.03 (dd, 1 H, J = 15.7 Hz, J = 9.8 Hz);
6.54 (d, 1 H, J = 15.7 Hz); 7.25—7.37 (m, 5 H, Ar); 9.53 (s, 1 H,
CHO). Found: m/z 265.1546 [M + NH4]+. C14H21N2O3.
Calculated: M = 265.1552. Enantiomeric excess was determined
by HPLC on a stationary chiral phase Chiralpak OD-H using
detection at 254 nm (hexane—propan-2-ol (80 : 20), 0.7 mL min–1
;
t(R) =12.7 min, t(S) = 21.3 min).
(R)-3-(4-Chlorophenyl)-2,2-dimethyl-4-nitrobutanal (6d).
20
Yield 9.05 mg (79%), yellow oil. []D +3.0 (c 1.0, CHCl3).
1H NMR (CDCl3), : 1.01 (s, 3 H, CH3); 1.13 (s, 3 H, CH3);
3.77 (dd, 1 H, CH, J = 11.4 Hz, J = 4.2 Hz); 4.68 (dd, 1 H, CH2,
J = 13.2 Hz, J = 4.2 Hz); 4.82 (dd, 1 H, CH2, J = 13.2 Hz,
J = 11.4 Hz); 7.12 (t, 1 H, Ar, J = 2.4 Hz); 7.15 (t, 1 H, Ar,
J = 2.4 Hz); 7.29 (t, 1 H, Ar, J = 2.4 Hz); 7.32 (t, 1 H, Ar,
J = 2.4 Hz); 9.48 (s, 1 H, CHO). Found: m/z 256.0732 [M + H]+.
detection at 220 nm (hexane—propan-2-ol (80 : 20), 0.8 mL min–1
;
t(R) = 15.1 min, t(S) = 16.4 min).
(R)-4-Nitro-3-phenylbutanal (6j). Yield 9.65 mg (99%), yel-
low oil. []D20 –7.1 (c 1.0, CHCl3). 1H NMR (CDCl3), : 2.95
(d, 2 H, CH2, J = 7.1 Hz); 4.03—4.16 (m, 1 H, CH); 4.58—4.72
(m, 2 H, CH2); 7.20—7.40 (m, 5 H, Ar); 9.71 (t, 1 H, CHO,
J = 1.9 Hz). Found: m/z 194.0815 [M + H]+. C10H12NO3.
Calculated: M = 194.0812. Enantiomeric excess was determined
by HPLC on a stationary chiral phase Chiralpak AS-H using
C
12H15ClNO3. Enantiomeric excess was determined using HPLC
on a stationary chiral phase Chiralpak OD-H, detection at 220 nm
(hexane—propan-2-ol (75 : 25), 0.8 mL min–1; t(R) = 13.7 min,
t
(S) = 20.1 min).
detection at 220 nm (hexane—propan-2-ol (70 : 30), 1.0 mL min–1
;
(R)-(4-Bromophenyl)-2,2-dimethyl-4-nitrobutanal (6e). Yield
t(R) = 14.5 min, t(S) = 19.2 min).
7.92 mg (66%), yellow oil. []D20 +2.0 (c 0.5, CHCl3). 1H NMR
(CDCl3), : 1.00 (s, 3 H, CH3); 1.12 (s, 3 H, CH3); 3.77 (dd, 1 H,
CH, J = 4.0 Hz, J = 11.2 Hz); 4.68 (dd, 1 H, CH2, J = 4.0 Hz,
J = 13.2 Hz); 4.83 (dd, 1 H, CH2, J = 11.2 Hz, J = 13.2 Hz);
7.10 (d, 2 H, Ar, J = 8.4 Hz); 7.47 (d, 2 H, Ar, J = 8.4 Hz); 9.49
(s, 1 H, CHO). Found: m/z 300.0225 [M + H]+. C12H15BrNO3.
Calculated: M = 300.0230. Enantiomeric excess was determined
by HPLC on a stationary chiral phase Chiralpak OD-H using
(3R)-2-Methyl-4-nitro-3-phenylbutanal (6k). Yield 10.25 mg
(99%), yellow oil. []D +8.3 (c 1.0, CHCl3). 1H NMR of
20
a diastereomeric mixture (CDCl3), : 0.89 (d, 3 H, CH3,
J = 7.3 Hz); 2.66—2.72 (m, 1 H, CH); 3.70—3.76 (m, 1 H, CH);
4.57—4.62 (m, 1 H, CH2); 4.69—4.74 (m, 1 H, CH2); 7.07—7.27
(m, 5 H, Ar); 9.54 (s, 0.05 H, CHO, (2R,3R)-isomer); 9.71
(s, 0.95 H, CHO, (2S,3R)-isomer). Found: m/z 208.0972 [M + H]+.
C11H14NO3. Calculated: M = 208.0969. Isomeric composition
was determined by HPLC on a stationary chiral phase Chiralpak
OD-H using detection at 220 nm (hexane—propan-2-ol (80 : 20),
detection at 254 nm (hexane—propan-2-ol (80 : 20), 0.7 mL min–1
;
t(R) = 22.0 min, t(S) = 34.5 min).
(R)-(4-Fluorophenyl)-2,2-dimethyl-4-nitrobutanal (6f). Yield
7.65 mg (70%), yellow oil. []D20 +1.2 (c 0.5, CHCl3). 1H NMR
(CDCl3), : 1.01 (s, 3 H, CH3); 1.13 (s, 3 H, CH3); 3.77—3.81
(m, 1 H, CH); 4.67—4.72 (m, 1 H, CH2); 4.80—4.86 (m, 1 H,
0.8 mL min–1; t(2S,3R) = 25.3 min, t(2S,3S) = 19.2 min; t(2R,3R)
= 28.6 min, t(2S,3R) = 22.8 min).
=
Recovery of organocatalyst S,S-3 (general procedure). After
completion of the reaction, the mixture was concentrated

1406 Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
Bykova et al.
in vacuo. Product 6a and unreacted starting compounds were
extracted with diethyl ether and the organic layer was decanted.
The remaining catalyst S,S-3 was dried in vacuo (1.0 Torr, 50 C,
1 h) and then a new portions of the starting compounds 4a
(7.2 mg, 0.1 mmol) and 5a (7.5 mg, 0.05 mmol) and toluene
(0.1 mL) were added. The reactions were carried out as de-
scribed above.
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This work was financially supported by the Russian
Foundation for Basic Research (Project No. 18-33-00930).
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Received March 21, 2019;
in revised form April 16, 2019;
accepted April 26, 2019