New tandem reactions of metal carbenoids. Intermolecular formation of azomethine ylide from methyl 2-diazo-2-phenylacetate and schiff base: Intramolecular 1,3-dipolar cycloaddition

Article abstract of DOI:10.1007/s11178-005-0267-y

Rhodium acetate-catalyzed decomposition of methyl 2-diazo-2-phenylacetate in the presence of substituted N-methylbenzylideneamines possessing an activated alkenyl fragment (dipolarophile) in the side chain gives products of intramolecular cycloaddition of

Full text of DOI:10.1007/s11178-005-0267-y

Russian Journal of Organic Chemistry, Vol. 41, No. 6, 2005, pp. 922–932. Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 6, 2005,
pp. 939–949.
Original Russian Text Copyright © 2005 by Khlebnikov, Novikov, Bespokoev, Kostikov, Kopf, Starikova, Antipin.
New Tandem Reactions of Metal Carbenoids.
Intermolecular Formation of Azomethine Ylide
from Methyl 2-Diazo-2-phenylacetate and Schiff Base:
Intramolecular 1,3-Dipolar Cycloaddition
A. F. Khlebnikov1, M. S. Novikov1, A. A. Bespokoev1, R. R. Kostikov1, J. Kopf2,
Z. A. Starikova3, and M. Yu. Antipin3
1 St. Petersburg State University, Universitetskii pr. 26, St. Petersburg, 198504 Russia
e-mail: Alexander.Khlebnikov@pobox.spbu.ru
2 Institut für Anorganische Chemie, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany
3 Nesmeyanov Institute of Organometallic Compounds, Russian Academy of Sciences, Moscow, Russia
Received October 15, 2004
Abstract—Rhodium acetate-catalyzed decomposition of methyl 2-diazo-2-phenylacetate in the presence of
substituted N-methylbenzylideneamines possessing an activated alkenyl fragment (dipolarophile) in the side
chain gives products of intramolecular cycloaddition of intermediate Z,E- and E,Z-azomethine ylides. The
cycloaddition is regioselective, and the products are hexahydrochromeno[4,3-b]pyrrole derivatives. The
stereoselectivity of the process depends on the temperature. In the temperature range from 20 to 80°C, the
major stereoisomer is that with cis junction of the tetrahydropyran and pyrrolidine rings. N-Phenylazomethine
ylides generated from methyl 2-diazo-2-phenylacetate and alkyl 4-[2-(phenyliminomethyl)phenoxy]-2-
butenoates at 40°C undergo cyclization to aziridines at a higher rate, as compared to the rate of cycloaddition to
the internal dipolarophile. N-Phenylazomethine ylides generated by thermolysis of the corresponding aziridine
or by the “deprotonation” method react with equal regio- and stereoselectivity to give intramolecular
cycloaddition products, hexahydrochromeno[4,3-b]pyrrole derivatives with trans-fused tetrahydropyran and
pyrrolidine rings. Analysis of the experimental and calculation data suggests preference of the endo transition
state in the cycloaddition of the examined azomethine ylides.
Intramolecular 1,3-dipolar cycloadditions of azo-
methine ylides underlie effective procedures for the
synthesis of various fused, bridged, and cage-like poly-
cyclic nitrogen-containing compounds [1]. An import-
ant problem is the stereoselectivity of cycloaddition of
ylide intermediates, for it determines the efficiency of
this approach as applied to a particular synthetic task.
Low stereoselectivity often results from configura-
tional heterogeneity of ylide intermediate owing to
fairly severe conditions of its generation [2–4]. The
stereoselectivity problem may be solved by develop-
ment of such methods for generation of azomethine
ylides which would allow their geometry to be con-
trolled. One of these methods is based on reactions of
carbenes or carbenoids with compounds possessing
a C=N bond and is referred to as “carbene” technique.
These reactions occur as a rule under mild conditions,
and they make it possible to generate ylides which
cannot be obtained by other methods [5].
The synthetic sequence intermolecular carbene gen-
eration of azomethine ylide–intramolecular cycloaddi-
tion almost was not studied as a method of synthesis of
nitrogen-containing polycyclic compounds. Only re-
cently, we described examples of such syntheses with
the use of difluorocarbene and dichlorocarbene [6–8].
The goal of the present study was to develop ap-
proaches to tandem synthesis of fused heterocycles
via the above reaction sequence involving carbenoids
generated from diazo compounds. As model system we
used azomethine ylides generated by addition of car-
bene-like species derived from methyl 2-diazo-2-phe-
nylacetate to N-methyl- and N-phenylbenzylidene-
amines I–IV possessing an activated alkenyl fragment
(dipolarophile) in the ortho position of the aromatic
ring. While analyzing factors determining the stereo-
selectivity of intramolecular cycloaddition of azo-
methine ylides we used for comparison so-called
“deprotonation” technique which is based on thermal
1070-4280/05/4106-0922 © 2005 Pleiades Publishing, Inc.

NEW TANDEM REACTIONS OF METAL CARBENOIDS.
923
Scheme 1.
MeO2C
Ph
MeO2C
Ph
Rh2(OAc)4
N2
RhLn
MeO2C
O
N
CO2R
O
N
CO2R
RhLn
Ph
CO2Me
Me
Me
O
Ph
I, II
VII, VIII
O
H
H
CO2R
Ph
CO2Me
Va, VIa
CO2R
+
H
H
N
N
Ph
Me
Me
CO2Me
Vb, VIb
I, Va, Vb, VII, R = Et; II, VIa, VIb, VIII, R = Me.
condensation of the corresponding aldehyde with
amine [9].
with Schiff bases II and III in which the activating
group is CO2Me rather than CO2Et. The relative posi-
tion of the phenyl group on C2 and CO2Me group on
C3 can readily be determined from the chemical shift
of the methoxy protons in the H NMR spectrum. The
structure of compound Va was unambiguously proved
The reaction of Schiff base I with methyl 2-diazo-
2-phenylacetate in boiling methylene chloride in the
presence of a catalytic amount of Rh2(OAc)4 gave two
stereoisomeric chromeno[4,3-b]pyrroles Va and Vb as
a result of intramolecular 1,3-dipolar cycloaddition of
the ylide moiety to the C=C bond in intermediate
azomethine ylide VII (Scheme 1). The major isomer
(compound Va) was obtained in 59% yield, whereas
minor isomer Vb was not isolated in the pure state.
The cis-junction of the pyran and pyrrolidine rings in
Va follows from the spin–spin coupling constant
between 3a-H and 9b-H, which is equal to 8.3 Hz, and
from the presence of two coupling constants (J = 4.4,
5.5 Hz) between the 3a-H and 4-H protons. It is known
that the cis coupling constant between 3a-H and 9b-H
in analogous 5+6-fused systems ranges from 6 to
9 Hz, while the corresponding trans constant is equal
to 11–12 Hz. Moreover, both coupling constants be-
tween 3a-H and 4-H in cis-fused systems are lesser
than 10 Hz, while in trans-fused systems one of these
exceeds 10 Hz [2–4, 10, 11].
1
by X-ray analysis (Fig. 1).
The reaction of Schiff base II with methyl 2-diazo-
2-phenylacetate in the presence of Rh2(OAc)4 led to
formation of two stereoisomeric chromeno[4,3-b]pyr-
roles VIa and VIb. By column chromatography we
isolated a mixture of compounds VIa and VIb (overall
yield 59%) at a ratio of 7:1. The pure isomers were
isolated by fractional crystallization from methanol–
hexane. The NMR spectra of compound VIa were
almost similar to those of Va, except for signals
belonging to protons in the ester fragments. Therefore,
isomer VIa was assigned the same structure as Va. It
should be noted that the chemical shift of methyl
protons in the CO2Me group on C3 is 3.7–3.9 ppm, i.e.,
it is typical of trans arrangement of the methoxycar-
bonyl group and phenyl group on C2.
The structure of compound VIb was determined by
The configuration of C3 is determined by stereo-
selectivity of 1,3-dipolar cycloaddition and trans-con-
figuration of the double C=C bond in the initial Schiff
base. The configuration of C2 in molecules Va and Vb
analysis of the NMR spectra. The coupling constant
1
between 3a-H and 9b-H in the H NMR spectrum is
equal to 11.7 Hz, and interaction between the 3a-H and
4-H protons is characterized by two constants, one of
which is large (5.4 and 11.8 Hz). These data indicate
trans junction of the pyran and pyrrolidine rings in
1
cannot be established on the basis of the H NMR
spectra; therefore, further experiments were performed
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

KHLEBNIKOV et al.
924
we examined intramolecular cycloaddition in azome-
C29
C28
thine ylide VIII generated by deprotonation [11, 16].
For this purpose, a mixture of aldehyde IX and methyl
2-methylamino-2-phenylacetate in toluene was heated
at the boiling point with simultaneous removal of
water as azeotrope. As a result, we obtained the same
products (VIa and VIb, Scheme 2) as in the reaction
with ylide VIII generated by the carbene method, but
the isomer ratio was different (2:1).
O27
C5
O6
C25
C4
C7
C8
C3
O26
C13
O22
C2
C9
C12
The ratio of stereoisomers depends on a number of
factors, one of which is the reaction temperature. We
performed a series of experiments in which ylide VIII
was generated using both carbene and deprotonation
techniques at various temperatures. The results are
collected in Table 1. It is seen that the selectivity
decreases as the temperature rises. This may be due to
leveling of the rates of reactions leading to isomers
VIa and VIb and isomerization of VIb into VIa. By
special experiments we found that isomers VIa and
VIb are stable under the given conditions. According
to the 1H NMR data, heating of compound VIa or VIb
for 4 h in benzene in the presence of 4 mol % of
Rh2(OAc)4 or for 40 h in the presence of Et3N·HCl
gives no isomerization products.
C21
N1
C11
C10
C24
O23
C14
C20
C19
C15
C16
C17
C18
Fig. 1. Structure of the molecule of ethyl (2RS,3RS,3aRS,-
9bSR)-1-methyl-2-methoxycarbonyl-2-phenyl-1,2,3,3a,4,9b-
hexahydrochromeno[4,3-b]pyrrole-3-dicarboxylate (Va)
according to the X-ray diffraction data.
isomer VIb. The configuration of C3 is determined by
stereoselectivity of 1,3-dipolar cycloaddition and trans-
configuration of the double C=C bond in the initial
Schiff base. In addition, the 3-H and 3a-H protons are
coupled through a constant (J3,3a) of 11.9 Hz, typical
of their trans arrangement [23, 25, 26, 28, 30]. The
configuration of C2 in VIb follows from the chemical
shift of methyl protons in the CO2Me group on C3
(δ 3.31 ppm). The upfield shift of the CO2Me signal
relative to its usual position (δ 3.7–3.9 ppm) is caused
by the cis arrangement of the methoxycarbonyl group
on C3 and phenyl group on C2 [12–15].
The reactions of Schiff bases III and IV with
methyl 2-diazo-2-phenylacetate in boiling methylene
chloride afforded the corresponding 1,3-cyclization
products XII and XIII (yield 40 and 60%, respec-
tively) instead of 1,3-dipolar cycloaddition products of
intermediate azomethine ylides X and XI (Scheme 3).
According to Doyle et al. [17], N-benzylideneanilines
under analogous conditions give rise to aziridines with
cis orientation of the aryl groups in positions 2 and 3.
For comparison, we synthesized methyl cis-1,2,3-tri-
phenylaziridine-2-carboxylate (XIV) whose steric
structure was determined [17] by X-ray analysis; how-
It is known that N-alkyl-substituted azomethine
ylides can also be generated by condensation of the
corresponding aldehyde and secondary amine (“depro-
tonation” technique) [9]. With a view to estimate the
efficiency of the carbene procedure for generation of
azomethine ylides and compare the selectivities in
cycloaddition of ylides generated by different methods,
1
ever, its H NMR spectrum was not reported. On the
basis of the spectral data, compounds XII and XIII
were assigned the structure with cis-oriented aromatic
substituents.
Aziridines having a strong π-acceptor group at
a ring carbon atom are known to readily undergo ring
opening at the C2–C3 bond at elevated temperature to
give azomethine ylides [1]. Aziridine XII was subject-
ed to thermolysis in order to estimate the stereoselec-
tivity of intramolecular cycloaddition in N-aryl-sub-
stituted azomethine ylides generated by the “aziridine”
technique. A solution of aziridine XII was heated in
anhydrous xylene under reflux, and we thus obtained
87% of diastereoisomeric products XVa and XVb at
Scheme 2.
CHO
NHMe·HCl
+
Ph
CO2Me
O
CO2Me
IX
Et3N, toluene, 100°C
[VIII]
VIa
+
VIb
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

NEW TANDEM REACTIONS OF METAL CARBENOIDS.
925
Table 1. Ratios of stereoisomeric cycloaddition products obtained from azomethine ylide VIII generated by the carbene and
deprotonation methods
Carbene method
Reaction
time, h
Schiff base II, mmol
PhC(N2)CO2Me, mmol
Temperature, °C Solvent
Ratio VIa:VIb
0.10
1.00
0.25
1.90
0.25
20
40
80
CH2Cl2
CH2Cl2
Benzene
8
8
4
8:1
7:1
4:1
00.125
Deprotonation method
Aldehyde IX, mmol
PhCH(NHMe)CO2Me, mmol
0.25
1.00
0.50
2.00
80
Benzene
Toluene
580
4
3:1
2:1
1100
a ratio of 3:1 (Scheme 4). The isomers were separated
by fractional crystallization from methanol–heptane.
Their structure was determined on the basis of their
NMR spectra. The trans junction of the pyran and
pyrrolidine rings in molecules XVa and XVb follows
from the coupling constant between the 3a-H and 9b-H
protons, which is equal to 11.6 Hz, and from the pres-
ence of one large coupling constant between 3a-H and
4-H (J 5.4 and 11.7 Hz) [2–4, 10, 11]. The configura-
tion at C3 is determined by stereoselectivity of
1,3-dipolar cycloaddition and trans-configuration of
the double C=C bond in the initial Schiff base; also,
a large coupling constant between 3-H and 3a-H
(J3,3a = 12.5 Hz) is typical of trans arrangement of the
3-H and 3a-H protons [2–4, 10, 11]. The configuration
at the C2 atom in isomer XVa follows from the chem-
ical shift of methyl protons in the CO2Me group on C3
(δ 3.38 ppm). The signal from these protons appears in
a stronger field (as compared to its usual position) due
to cis orientation of the methoxycarbonyl group on C3
with respect to the phenyl group on C2. By contrast,
the chemical shift of the CO2Me protons in isomer
XVb has its usual value (δ 3.87 ppm), indicating trans
arrangement of the methoxycarbonyl group on C3 and
phenyl group on C2 [12–15]. The structure of isomer
XVa was finally proved by X-ray analysis (Fig. 2).
It should be noted that thermolysis of aziridine XIII
in the presence of a small amount of water gives the
corresponding cycloadducts as minor products (overall
yield less than 15%), while the major products are
aldehyde XVI and ethyl 2-phenyl-2-phenylamino-
acetate. The latter are formed by hydrolysis of ylide XI
(Scheme 5). These data suggest that the hydrolysis of
ylide intermediate occurs at a much higher rate than
does the intramolecular cycloaddition at the activated
C=C bond.
In order to compare the selectivities in the reactions
of ylide X generated by different methods (thermolysis
of aziridine XII and deprotonation) we performed con-
densation of aldehyde IX with methyl 2-phenyl-2-
phenylaminoacetate (Scheme 6). This reaction also af-
forded compounds XVa and XVb. However, to ensure
an acceptable rate, the reaction was carried out in
boiling xylene, which should affect the diastereo-
Scheme 3.
N2
Ph
CO2Me
O
N
CO2R
O
N
CO2R
O
CO2R
Rh2(OAc)4, CH2Cl2
40°C
Ph
CO2Me
N
CO2Me
Ph
Ph
Ph
Ph
X, XI
III, IV
XII, XIII
III, X, XII, R = Me; IV, XI, XIII, R = Et.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

KHLEBNIKOV et al.
926
Scheme 4.
O
O
H
H
Δ
CO2Me
CO2Me
Ph
CO2Me
XVb
XII
[X]
+
H
H
N
N
Ph
Ph
Ph
CO2Me
XVa
Scheme 5.
O
CO2Et
NHPh
CO2Me
H2O
Δ
+
XIII
[XI]
Ph
CHO
XVI
Scheme 6.
NHPh·HCl
Et3N, 145°C, 45 h
IX
+
[X]
XVa
+
XVb
Ph
CO2Me
isomer ratio. Therefore, we performed a similar reac-
tion in toluene with incomplete conversion of the
initial compounds. In this case, the cycloaddition
stereoselectivity was almost the same as in the trans-
formation of azomethine ylide X generated by the
aziridine technique (Table 2). By special experiments
we found that isomers XVa and XVb are stable under
no isomerization of XVa and XVb occurred on heating
for 40 h in toluene in the presence of an equimolar
amount of Et3N·HCl.
Thus, all the examined azomethine ylides capable
of undergoing intramolecular 1,3-dipolar cycloaddition
at activated double C=C bond react in a regioselective
fashion, yielding only fused polycyclic compounds
(no regioisomeric bridged structures were detected; cf.
1
the given conditions: according to the H NMR data,
Scheme 7.
Carbene ylides
O
CO2R
O
N
CO2R
H
H
Ph
N
ROCO
R'
R'
CO2R
Ph
(E,Z)-Ylide
(Z,Z)-Ylide
ΔH°, kcal/mol: R' = Me
–90.7
–57.1
–90.8
–57.6
R' = Ph
Noncarbene ylides
O
CO2R
O
CO2R
H
H
N
CO2R
N
Ph
R'
R'
Ph
CO2R
(Z,E)-Ylide
(E,E)-Ylide
ΔH°, kcal/mol: R' = Me
–98.8
–62.5
–98.5
–61.9
R' = Ph
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NEW TANDEM REACTIONS OF METAL CARBENOIDS.
927
Table 2. Ratios of stereoisomeric cycloaddition products obtained from azomethine ylide X generated by the aziridine and
deprotonation methods
Deprotonation method
Aldehyde IX, mmol PhCHN(Ph)CO2Me, mmol Temperature, °C Solvent
Reaction time, h
Ratio XVa:XVb
1.50
1.50
3.00
3.00
110
145
Toluene
Xylene
50a
35a
1:3
1:3
Aziridine method
110
Aziridine XII
Toluene
04a
1:3
a
Conversion ≈ 40%.
[7]). The stereoselectivity of this process depends on
the substituent at the nitrogen atom, ylide generation
method, and temperature.
1,3-Dipolar cycloaddition can involve transition
states (TS) of two types, endo and exo; in endo-TS the
activating group and ylide nitrogen atom are oriented
at one side, and in exo-TS these fragments are oriented
at opposite sides. As shown in Schemes 8 and 9, the
four isomeric ylide could give rise to four stereo-
isomeric intramolecular cycloaddition products. It is
seen that stereoisomers Va/Vb and VIa/VIb obtained
in the reactions with N-methyl-substituted Schiff bases
could be formed only from (E,Z)- or (Z,E)-ylide and
that the major isomer could be formed from carbene
(E,Z)-ylide through endo-TS and from noncarbene
(Z,E)-ylide through exo-TS. On the other hand, the
minor isomer could be formed from carbene (E,Z)-
ylide through exo-TS and from noncarbene (Z,E)-ylide
through endo-TS (Scheme 10).
Each ylide VII, VIII, X, and XI can exist as four
stereoisomeric structures which, in keeping with the
generally accepted denotations [9], may be referred
to as (E,Z), (Z,Z), (Z,E), and (E,E) isomers (Scheme 7).
In addition, the first two isomers may be regarded as
carbene-like, for just these isomers should be formed
by addition of electrophilic carbene (carbenoid) at the
lone electron pair on the nitrogen atom of Schiff base
having E configuration. Scheme 7 also shows en-
thalpies of formation of the above ylides, calculated
by the PM3 semiempirical method with full geometry
optimization.
Taking into account that noncarbene ylides are
more stable than carbene ylides (according to the cal-
culations) and that the reaction stereoselectivity de-
creases as the temperature rises, we believe that ylide
generation by the carbene method at low temperature
gives rise mainly to the reaction channel involving
endo-TS, which leads to formation of major products
Va and VIa with cis-fused pyran and pyrrolidine rings.
Elevated temperature favors isomerization of the car-
bene ylide into noncarbene, and the fraction of minor
isomers Vb and VIb (which are also formed through
the corresponding endo-TS) with s trans-fused rings
increases. The endo-TS was also preferred over exo-TS
in many other 1,3-dipolar cycloaddition reactions of
azomethine ylides [9]. Unfortunately, it is difficult to
estimate the relative rates of formation of carbene and
noncarbene ylides generated by the deprotonation
method.
O27
C29
O28
C26
C3
C23
C5
O6
C24
C22
C4
C8
C9
C7
C21
C20
C12
C11
C25
C13
C2
N1
C30
C10
O31
O32
C14
C19
C15
C33
C18
C16
C17
Increase in steric hindrances to cycloaddition in
N-phenyl-substituted ylides X and XI generated by the
carbene procedure at 40°C is responsible for formation
of only 1,3-cyclization products, aziridines XII and
XIII. These compounds could be formed via thermally
Fig. 2. Structure of the molecule of dimethyl (2RS,3RS,-
3aRS,9bSR)-1,2-diphenyl-1,2,3,3a,4,9b-hexahydrochromeno-
[4,3-b]pyrrole-2,3-dicarboxylate (XVa) according to the
X-ray diffraction data.
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KHLEBNIKOV et al.
928
Scheme 8.
O
CO2Et
H
H
O
H
CO2R
H
O
H
H
Ph
CO2R
CO2Et
H
N
R
CO2R
N
Ph
N
R
Ph
H
R
CO2R
endo-TS
exo-TS
O
CO2R
O
CO2R
H
H
Carbene
ylide
Noncarbene
ylide
Ph
N
N
CO2R
R'
R'
CO2R
Ph
(E,Z)-Ylide
(Z,E)-Ylide
O
H
H
CO2R
O
O
H
CO2R
CO2R
H
H
H
Ph
CO2R
H
N
R
Ph
CO2R
N
Ph
N
R
H
R
CO2R
exo-TS
endo-TS
Scheme 9.
O
CO2Et
H
H
O
H
H
CO2R
CO2R
O
H
H
CO2R
CO2Et
H
N
Ph
N
Ph
N
Ph
H
R
R
R
CO2R
endo-TS
exo-TS
O
CO2R
O
CO2R
H
H
Carbene
ylide
Noncarbene
ylide
ROCO
N
N
Ph
R'
R'
Ph
CO2R
(E,E)-Ylide
(Z,Z)-Ylide
O
H
H
CO2R
Ph
CO2R
H
O
O
H
H
H
CO2R
CO2R
H
N
R
CO2R
Ph
N
Ph
N
R
H
R
CO2R
exo-TS
endo-TS
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

NEW TANDEM REACTIONS OF METAL CARBENOIDS.
929
Scheme 10.
N-Ме-Ylides
endo-TS
Carbene
(E,Z)-ylide
Noncarbene
(E,E)-ylide
Va, VIa
Major product
S
T
-
o
e
x
e
x
o
-
T
S
endo-TS
Noncarbene
(Z,E)-ylide
Carbene
(Z,Z)-ylide
Vb, VIb
Minor product
Scheme 11.
N-Ph-Ylides
XII, XIII
Δ
Carbene
Noncarbene
(E,Z)-ylide
(E,E)-ylide
exo-TS
endo-TS
exo-TS
–57.1
–61.9
XVa
XVb
Major product
Minor product
endo-TS
Noncarbene
(Z,E)-ylide
Carbene
(Z,Z)-ylide
–62.5
–57.6
Δ
XII, XIII
allowed conrotatory closure of carbene (Z,Z)-ylide or
noncarbene (E,E)-ylide. Presumably, the first channel
involving kinetically preferred carbene (Z,Z)-ylide is
operative at low temperature.
chain. The product structure indicates that the process
occurs as regioselective intramolecular cycloaddition
of only (Z,E)-/(E,Z)-ylides with formation of fused
polycyclic compounds, hexahydrochromeno[4,3-b]pyr-
role derivatives. The reaction stereoselectivity depends
on the temperature. In the temperature range from 20
to 80°C, isomers with cis-fused tetrahydropyran and
pyrrolidine rings are mainly formed. Replacement of
the methyl group on the nitrogen atom by bulky phenyl
substituent changes the reaction direction. N-Phenyl-
substituted azomethine ylides generated from methyl
2-diazo-2-phenylacetate and (2-phenyliminomethyl-
phenoxy)-2-butenoic acid esters at 40°C undergo
cyclization to the corresponding aziridine at a higher
rate, as compared to the cycloaddition at the internal
dipolarophile moiety. N-Phenyl azomethine ylides gen-
erated by thermolysis (110–145°C) of the correspond-
ing aziridine or by the deprotonation method react with
similar regio- and stereoselectivity to give intramolec-
ular cycloaddition products, hexahydrochromeno-
[4,3-b]pyrrole derivatives with trans-fused tetrahydro-
The rate of intramolecular cycloaddition in
N-phenyl-substituted ylides X and XI generated by
thermolysis (145°C) and deprotonation is lower; there-
fore, these ylides are likely to exist in equilibrium.
In this case, the observed stereoselectivity is readily
rationalized in terms of the above assumed preference
of endo-TS in the cycloaddition process: both products
are formed from the two most stable isomers of
the azomethine ylide through endo-TS: more stable
(Z,E)-ylide gives rise to major isomer XVa, and less
stable (E,E)-ylide, to minor isomer XVb (Scheme 11).
Thus we have studied intramolecular 1,3-dipolar
cycloaddition in azomethine ylides generated by the
carbene method, i.e., by rhodium acetate-catalyzed
reaction of methyl 2-diazo-2-phenylacetate with ortho-
substituted N-methylbenzylideneamines containing
an activated C=C dipolarophilic fragment in the side
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

KHLEBNIKOV et al.
930
pyran and pyrrolidine rings. Analysis of the experi-
mental and calculation data showed that the endo-
transition state is preferred in the cycloaddition of the
examined azomethine ylides. Tandem process at about
40°C, which includes carbene generation of azo-
methine ylides by reaction of methyl 2-diazo-2-phenyl-
acetate in the presence of rhodium acetate with
N-alkylbenzylideneamines containing an activated
C=C dipolarophilic fragment in the side chain, is
preferred from the viewpoint of selective synthesis of
hexahydrochromeno[4,3-b]pyrrole derivatives with
cis-fused tetrahydropyran and pyrrolidine rings.
ether). IR spectrum (CHCl3), ν, cm–1: 940, 1180, 1220,
1260, 1460, 1490, 1580, 1610, 1730, 2950, 3020,
1
3060. H NMR spectrum (CDCl3), δ, ppm: 1.27 t (3H,
SCH3, J = 7.2 Hz), 2.21 s (3H, NCH3), 3.00 d (1H,
3-H, J = 6.9 Hz), 3.31 d.d.d.d (1H, 3a-H, J3a,3 = 6.9,
J3a,9b = 8.3, J3a,4 = 4.4, 5.5 Hz), 3.85 s (3H, OCH3),
3.93 d (1H, 9b-H, J9b,3a = 8.3 Hz), 3.95 d.d (1H, 4-H,
J3a,4 = 4.5, J4,4 = 11.0 Hz), 4.14 d.d (1H, 4-H, J3a,4
=
2
5.5, J = 11.0 Hz), 6.98–7.03 m (2H, Harom), 7.20–
7.32 m (5H, Harom), 7.59–7.61 m (2H, Harom). 13C NMR
spectrum (DEPT, CDCl3), δC, ppm: 13.7 (CH3); 33.6
(NCH3); 39.5 (C3a); 50.9 (OCH3); 57.7 (C3); 59.2
(C9b); 60.5 (CO2CH2); 69.2 (ArOCH2); 76.9 (C2);
117.3, 120.5, 123.48, 127.2, 127.4, 127.7, 128.4, 130.5,
139.8, 156.7 (Carom); 169.8 (C=O); 170.9 (C=O).
Found, %: C 69.81; H 6.58; N 3.20. C23H25NO5.
Calculated, %: C 69.86; H 6.37; N 3.54. The structure
of Va was proved by the X-ray diffraction data
(Fig. 1). C23H25NO5, M 395.44. Unit cell parameters:
a = 24.449(5), b = 24.449(5), c = 7.0278(18) Å; α =
β = γ = 90°; V = 4201.0(17) Å3; dcalc = 1.250 g/cm3;
tetragonal crystal system; space group I-4 (no. 82);
Z = 8; λ = 0.71073 Å, temperature 293 K, crystal habit
0.5×0.4×0.2 mm, RAll = 0.063, wR2 = 0.1151; 6703 re-
flections, 3126 independent reflections (Rint = 0.0285).
Following the general procedure, from 0.233 g
(1 mmol) of Schiff base II in 3.5 ml of methylene
chloride, 20 mg (4.5 mol %) of Rh2(OAc)4, and
0.336 g (1.9 mmol) of methyl 2-diazo-2-phenylacetate
in 8 ml of methylene chloride we obtained 0.225 g
(59%) of a mixture of dimethyl (2RS,3RS,3aRS,9bSR)-
1-methyl-2-phenyl-1,2,3,3a,4,9b-hexahydrochromeno-
[4,3-b]pyrrole-2,3-dicarboxylate (VIa) and dimethyl
(2RS,3SR,3aSR,9bSR)-1-methyl-2-phenyl-1,2,3,3a,-
4,9b-hexahydrochromeno[4,3-b]pyrrole-2,3-dicarbox-
ylate (VIb) at a ratio of 7:1. Isomer VIa was isolated
by repeated crystallization from hexane–methanol.
mp 122–123°C. IR spectrum (CHCl3), ν, cm–1: 990,
1010, 1130, 1250, 1450, 1490, 1585, 1605, 1740, 2850,
3040. 1H NMR spectrum (CDCl3), δ, ppm: 2.22 s (3H,
NCH3), 3.04 d (1H, 3-H, J3,3a = 6.9 Hz), 3.32 d.d.d.d
(1H, 3a-H, J = 4.5, 5.4, 6.9, 8.3 Hz), 3.72 s (3H,
EXPERIMENTAL
The melting points were determined on a Boetius
melting point apparatus; uncorrected values are given.
The IR spectra were recorded on a Carl Zeiss UR-20
instrument using 400-μm cells. The NMR spectra were
obtained on a Bruker DPX 300 spectrometer at 300
1
and 75 MHz for H and 13C, respectively. The ele-
mental compositions were determined on an HP-185B
CHN analyzer. The progress of reactions was moni-
tored by thin-layer chromatography using Silufol UV-
254 plates. Silica gel Merck 60 was used for column
chromatography. Methylene chloride was dried by dis-
tillation over P2O5.
Reactions of Schiff bases I–IV and N-benzyli-
deneaniline with methyl 2-diazo-2-phenylacetate.
Schiff base I, 0.248 g (1 mmol), was mixed under
argon with 4.4 mg (1.1 mol %) of Rh2(OAc)4 and
3.5 ml of methylene chloride. The mixture was heated
to the boiling point, and a solution of 0.342 g
(1.95 mmol) of methyl 2-diazo-2-phenylacetate in
6.5 ml of methylene chloride was added dropwise
under stirring over a period of 6 h using a syringe. The
progress of the reaction was monitored by TLC using
hexane–ethyl acetate (10:1) as eluent. The isomer ratio
of the products was determined as follows. A sample
of the reaction mixture (2–3 ml) was filtered through
a thin layer of silica gel, and the solvent was washed
with 50 ml of methylene chloride. The filtrate was
combined with the washings and evaporated, and
1H NMR spectrum of the residue was recorded. The
products were isolated by column chromatography
(gradient elution with hexane–ethyl acetate, 15:1, to
ethyl acetate). The major isomer (Va, 0.260 g) was
recrystallized from hexane–diethyl ether.
OCH3), 3.85 s (3H, OCH3), 3.94 d (1H, 9b-H, J9b,3a
=
2
8.3 Hz), 3.95 d.d (1H, 4-H, J4,3a = 4.5, J = 11.1 Hz),
2
4.15 d.d (1H, 4-H, J4,3a = 5.4, J = 11.1 Hz), 6.98–
7.03 m (2H, Harom), 7.21–7.36 m (5H, Harom), 7.58–
7.61 m (2H, Harom). 13C NMR spectrum (CDCl3), δC,
ppm: 34.5 (NCH3); 40.4 (C3a); 51.9 (OCH3); 52.5
(OCH3); 58.6 (C3); 60.1 (C9b); 70.1 (CH2); 74.6 (C2);
121.4, 124.4, 128.1, 128.4, 128.5, 129.3, 131.4, 140.6,
157.7 (Carom); 170.8 (C=O); 172.3 (C=O). C22H23NO5.
Found, %: C 69.29; H 5.98; N 3.92. Calculated, %:
C 69.28; H 6.08; N 3.67.
Ethyl (2RS,3RS,3aRS,9bSR)-1-methyl-2-meth-
oxycarbonyl-2-phenyl-1,2,3,3a,4,9b-hexahydro-
chromeno[4,3-b]pyrrole-3-carboxylate (Va). Yield
0.240 g (59%). mp 115–118°C (from hexane–diethyl
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

NEW TANDEM REACTIONS OF METAL CARBENOIDS.
931
Following the general procedure, from 0.590 g
(2.0 mmol) of Schiff base III in 4 ml of methylene
chloride, 4.4 mg (3.75 mol %) of Rh2(OAc)4, and
0.220 g (1.25 mmol) of methyl 2-diazo-2-phenyl-
acetate in 6 ml of methylene chloride we isolated by
column chromatography (gradient elution with hexane–
ethyl acetate, 10:1, to ethyl acetate) 0.248 g (40%) of
methyl (2RS,3RS)-3-{2-[(E)-3-(methoxycarbonyl)-2-
propenyloxy]phenyl}-1,2-diphenylaziridine-2-carbox-
ylate (XII). mp 104–105°C (from CH2Cl2–hexane). IR
spectrum (CHCl3), ν, cm–1: 965, 1030, 1120, 1255,
1280, 1305, 1445, 1495, 1595, 1660, 1715, 2850, 3050.
1H NMR spectrum (CDCl3), δ, ppm: 3.59 s (3H,
OCH3), 3.78 s (3H, OCH3), 4.73 s (1H, 3-H), 4.68–
4.84 m (2H, CH2), 6.39 d.t (1H, =CH, J = 2.2,
15.7 Hz), 6.69–6.77 m (2H, Harom), 7.05–7.40 m (13H,
Harom, =CH). 13C NMR spectrum (CDCl3), δC, ppm:
48.6 (C3); 51.3 (CH3O); 52.2 (CH3O); 55.7 (C2); 65.7
(CH2O); 109.9, 118.8, 120.3, 120.6, 122.5, 123.4,
127.1, 127.2, 128.2, 128.3, 128.4, 128.7, 134.0, 142.2,
149.5, 155.8 (Carom); 166.3 (C=O); 168.2 (C=O).
Found, %: C 73.24; H 5.58; N 3.04. C27H25NO5. Cal-
culated, %: C 73.12; H 5.68; N 3.16.
aniline and 5 mg (4.5 mol %) of Rh2(OAc)4 in 1 ml
of methylene chloride. The product was isolated by
column chromatography (eluent hexane–ethyl acetate,
20:1), followed by recrystallization from hexane–di-
ethyl ether. Yield 35 mg (56%). H NMR spectrum
(CDCl3), δ, ppm: 3.56 s (3H, OCH3), 4.50 s (1H, 3-H),
1
7.05–7.75 m (18H, Harom).
Reactions of aldehyde IX with methyl 2-meth-
ylamino-2-phenylacetate and methyl 2-phenyl-2-
phenylaminoacetate. A 25-ml flask equipped with
a reflux condenser and a Dean–Stark trap was charged
with 0.234 g (1 mmol) of aldehyde IX, 0.431 g
(2 mmol) of methyl 2-methylamino-2-phenylacetate
hydrochloride, 15 ml of toluene, and 0.202 g (2 mmol)
of triethylamine. The mixture was heated for 4 h under
reflux, the progress of the reaction being monitored by
TLC (hexane–ethyl acetate, 7:1). When the reaction
was complete, the solvent was removed, the residue
was extracted with diethyl ether, and the extract was
concentrated and subjected to column chromatography
using hexane–ethyl acetate (10:1) as eluent to isolate
0.342 g (90%) of a mixture of diastereoisomers VIa
and VIb. Analytically pure samples of VIa and VIb
were obtained by fractional recrystallization from
methanol–hexane. The data for VIa are given above.
Following the general procedure, from 0.154 g
(0.5 mmol) of Schiff base IV in 4 ml of methylene
chloride, 4.4 mg (2.21 mol %) of Rh2(OAc)4, and
0.220 g (1.25 mmol) of methyl 2-diazo-2-phenyl-
acetate in 6 ml of methylene chloride we isolated by
column chromatography (gradient elution with hexane–
ethyl acetate, 15:1, to ethyl acetate) 0.140 g (60%) of
methyl (2RS,3RS)-1,2-diphenyl-t-3-{2-[(E)-3-(ethoxy-
carbonyl)-2-propenyloxy]phenyl}aziridine-r-2-carbox-
ylate (XIII). mp 85–87°C (from diethyl ether). IR
spectrum (CHCl3), ν, cm–1: 940, 1110, 1250, 1310,
Compound VIb. mp 172–174°C (from methanol–
hexane). IR spectrum (CHCl3), ν, cm–1: 990, 1020,
1260, 1470, 1505, 1595, 1620, 1760, 2820, 3020.
1H NMR spectrum (CDCl3), δ, ppm: 2.70 m (1H,
3a-H, J4, 3a = 5.4, J9b, 3a = 11.7, J4,3a = 11.8, J3,3a
=
11.9 Hz), 2.72 s (3H, NCH3), 3.31 s (3H, OCH3),
3.87 d (1H, 9b-H, J9b,3a = 11.7 Hz), 3.90 s (3H, OCH3),
4.06 d (1H, 3-H, J3,3a = 11.9 Hz), 4.22 d.d (1H, 4-H,
1
J4,4 = 9.6, J4,3a = 11.8 Hz), 4.40 d.d (1H, 4-H, J4,3a
=
1460, 1490, 1595, 1660, 1720, 2980, 3040. H NMR
5.4, J4,4 = 9.6 Hz), 6.90–7.02 m (2H, Harom), 7.19–
7.30 m (4H, Harom), 7.50–7.54 m (3H, Harom). 13C NMR
spectrum (CDCl3), δC, ppm: 37.1 (NCH3); 41.3 (C3a);
51.3 (OCH3); 52.0 (OCH3); 54.3 (C3); 63.0 (C9b); 69.6
(CH2); 80.4 (C2); 116.4, 120.0, 122.7, 127.1, 127.6,
127.7, 128.1, 137.6, 154.9 (Carom); 169.7 (C=O);
172.0 (C=O). Found, %: C 69.71; H 5.98; N 3.96.
C22H23NO5. Calculated, %: C 69.28; H 6.08; N 3.67.
spectrum (CDCl3), δ, ppm: 1.31 t (3H, CCH3, J =
7.1 Hz), 3.59 s (3H, OCH3), 4.24 q (2H, OCH2CH3,
J = 7.1 Hz), 4.73 s (1H, 3-H), 4.68–4.84 m (2H, CH2),
6.36 d.d.d (1H, =CH, J = 1, 2.3, 15.7 Hz), 6.69–6.77 m
(2H, Harom), 7.04–7.35 m (13H, Harom, =CH). 13C NMR
spectrum (CDCl3), δC, ppm: 13.9 (CH3); 48.6 (C2 or
C3); 52.1 (OCH3); 55.6 (C2 or C3); 60.2 (CH2O); 65.8
(CH2O); 109.9, 118.8, 120.3, 121.0, 122.5, 123.4,
127.1, 127.2, 128.1, 128.2, 128.4, 128.7, 134.0, 141.9,
149.4, 155.8 (Carom, CH=); 165.9 (C=O); 168.2 (C=O).
Found, %: C 73.49; H 6.09; N 2.97. C28H27NO5. Cal-
culated, %: C 73.51; H 5.95; N 3.06.
Methyl cis-1,2,3-triphenylaziridine-2-carbox-
ylate (XIV). A solution of 47 mg (0.27 mmol) of
methyl 2-diazo-2-phenylacetate in 1 ml of methylene
chloride was added dropwise under stirring at 40°C to
a solution of 48 mg (0.27 mmol) of N-benzylidene-
Following an analogous procedure, from 0.351 g
(1.5 mmol) of aldehyde IX, 0.431 g (3 mmol) of
methyl 2-phenyl-2-phenylaminoacetate hydrochloride,
and 0.303 g (3 mmol) of triethylamine in 15 ml of
xylene (35 h under reflux) by column chromatography
(hexane–ethyl acetate, 10:1) we isolated 0.382 g
(86%) of a mixture of dimethyl (2RS,3RS,3aRS,9bSR)-
1,2-diphenyl-1,2,3,3a,4,9b-hexahydrochromeno-
[4,3-b]pyrrole-2,3-dicarboxylate (XVa) and dimethyl
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005

KHLEBNIKOV et al.
932
(2RS,3SR,3aSR,9bSR)-1,2-diphenyl-1,2,3,3a,4,9b-
hexahydrochromeno[4,3-b]pyrrole-2,3-dicarboxylate
(XVb). Analytically pure samples of XVa and XVb
were obtained by fractional crystallization from
methanol–hexane.
resulting solution was heated for 4 h under reflux.
According to the NMR data, the major products were
cycloadducts XVa and XVb. An analytically pure
sample of XVa was isolated by fractional crystalliza-
tion from methanol–hexane.
This study was performed under financial support
by the Ministry of Education and Science of the
Russian Federation, “Universities of Russia” program
(project no. ur.05.01.316), and by the Russian Founda-
tion for Basic Research (project no. 05-03-33257).
Major isomer XVa. mp 203–234°C (decomp., from
methanol–hexane). IR spectrum, ν, cm–1: 990, 1050,
1260, 1325, 1460, 1490, 1500, 1605, 1703, 2890,
1
2850, 3045. H NMR spectrum (CDCl3), δ, ppm:
2.82 m (1H, 3a-H, J4,3a = 5.4, J9b,3a = 10.6, J4,3a = 11.7,
J3,3a = 12.5 Hz), 3.38 s (3H, OCH3), 3.52 s (3H,
OCH3), 4.00 d (1H, 3-H, J3,3a = 12.5 Hz), 4.34 d.d (1H,
4-H, J4,4 = 9.7, J4,3a = 11.7 Hz), 4.53 d.d (1H, 4-H,
REFERENCES
1. Synthetic Application of 1,3-Dipolar Cycloaddition
Chemistry toward Heterocycles and Natural Products,
Padwa, A. and Pearson, W.H., Eds., New. York: Wiley,
2002.
J4,3a = 5.4, J4,4 = 9.7 Hz), 4.85 d (1H, 9b-H, J9b,3a
10.6 Hz), 6.55–6.57 m (1H, Harom), 6.79–7.37 m (11H,
arom), 7.65–7.68 m (2H, Harom). 13C NMR spectrum
=
H
(CDCl3), δC, ppm: 39.9 (C3a); 51.6 (OCH3); 52.1
(OCH3); 56.7 (C3); 58.9 (C9b); 69.5 (CH2); 80.9 (C2);
116.1, 117.4, 119.0, 119.8, 121.0, 124.7, 126.1, 127.4,
127.8, 127.9, 128.0, 128.3, 128.6, 136.9, 146.9, 153.3
(Carom); 168.3 (C=O); 170.8 (C=O). Found, %: C 73.04;
H 5.74; N 2.96. C27H25NO5. Calculated, %: C 73.12;
H 5.68; N 3.16. The structure of XVa was proved by
the X-ray diffraction data (Fig. 2). C27H25NO5,
M 443.50. Unit cell parameters: a = 8.5648(8), b =
13.2531(13), c = 20.023(2) Å; α = β = γ = 90°; V =
2272.81(40) Å3; dcalc = 1.296 g/cm3. Orthorhombic
crystals, space group P212121 (no. 19), Z = 4; λ =
0.71073 Å, temperature 293 K, crystal habit 0.45×
0.4×0.35 mm; RAll = 0.046, wR2 = 0.0827; 27626 re-
flections; 5355 independent reflections (Rint = 0.0285).
2. Tsuge, O., Ueno, K., and Ueda, I., Heterocycles, 1981,
vol. 16, p. 1503.
3. Armstrong, P., Grigg, R., Jordan, M.W., and Ma-
lone, J.F., Tetrahedron, 1985, vol. 41, p. 3547.
4. Grigg, R., Donegan, G., Gunaratne, H.Q.N., Ken-
nedy, D.A., Malone, J.F., Sridharan, V., and Thianpata-
nagul, S., Tetrahedron, 1989, vol. 45, p. 1723.
5. Khlebnikov, A.F., Novikov, M.S., and Kostikov, R.R.,
Usp. Khim., 2005, vol. 74, p. 183.
6. Novikov, M.S., Khlebnikov, A.F., Besedina, O.V., and
Kostikov, R.R., Tetrahedron Lett., 2001, vol. 42, p. 533.
7. Voznyi, I.V., Novikov, M.S., Khlebnikov, A.F.,
Kostikov, R.R., and Kopf, J., J. Chem. Soc., Perkin
Trans. 1, 2002, p. 1628.
8. Voznyi, I.V., Novikov, M.S., Khlebnikov, A.F., and
Kostikov, R.R., Izv. Ross. Akad. Nauk, Ser. Khim., 2004,
p. 1044.
Minor isomer XVb. mp 184–185°C (from metha-
1
nol–hexane). H NMR spectrum (CDCl3), δ, ppm:
9. Tsuge, O. and Kanemasa, S., Adv. Heterocycl. Chem.,
3.09 d.d.d.d (1H, 3a-H, J4,3a = 5.5, J9b,3a = 10.6, J4,3a
=
1989, vol. 45, p. 231.
11.5, J3,3a = 12.4 Hz), 3.23 (1H, 3-H, J = 12.4 Hz),
3.64 s (3H, OCH3), 3.87 s (3H, OCH3), 4.39 d.d (1H,
4-H, J4,4 = 9.6, J4,3a = 11.5 Hz), 4.95 d.d (1H, 4-H,
10. Barr, D.A., Grigg, R., Gunaratne, H.Q.N., Kemp, J.,
McMeekin, P., and Sridharan, V., Tetrahedron, 1988,
vol. 44, p. 557.
11. Kanemasa, S., Sakamoto, K., and Tsuge, O., Bull.
Chem. Soc. Jpn., 1989, vol. 62, p. 1960.
J4,3a = 5.5, J4,4 = 9.6 Hz), 4.95 d (1H, 9b-H, J9b,3a
=
10.6 Hz), 6.70–7.25 m (14H, Harom). 13C NMR spec-
trum (CDCl3), δC, ppm: 41.9 (C3a); 51.6 (OCH3); 52.5
(OCH3); 60.4, 61.1 (C3, C9b); 70.7 (CH2); 80.0 (C2);
116.0, 119.2, 119.7, 124.7, 127.0, 127.3, 127.4, 127.5,
127.6, 128.8, 137.9, 144.7, 153.2 (Carom); 169.4 (C=O);
171.6 (C=O). Found, %: C 73.14; H 5.98; N 3.14.
C27N25HO5. Calculated, %: C 73.12; H 5.68; N 3.16.
12. Tsuge, O., Ueno, K., Kanemasa, S., and Yorozu, K.,
Bull. Chem. Soc. Jpn., 1986, vol. 59, p. 1809.
13. Achiwa, K., Sugiyama, K., and Sekiya, M., Chem.
Pharm. Bull., 1985, vol. 33, p. 1975.
14. Achiwa, K. and Sekiya, M., Tetrahedron Lett., 1982,
vol. 23, p. 2589.
Thermolysis of methyl t-3-{2-[(E)-3-(methoxy-
carbonyl)-2-propenyloxy]phenyl}-1,2-diphenylazir-
idine-r-2-carboxylate (XII). A 25-ml flask equipped
with a refluxed condenser capped with a drying tube
was charged under argon with 77 mg (0.17 mmol) of
aziridine XII in 10 ml of anhydrous toluene, and the
15. Burdisso, M., Gamba, A., Gandolfi, R., and Oberti, R.,
Tetrahedron, 1988, vol. 44, p. 3735.
16. Confalone, P.N. and Huie, E.M., J. Am. Chem. Soc.,
1984, vol. 106, p. 7175.
17. Doyle, M.P., Wenhao, H., and Timmons, D.J., Org.
Lett., 2001, vol. 3, p. 933.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 41 No. 6 2005