Synthesis and thermal transformations of spiro-fused N- phthalimidoaziridines Dedicated to Professor Armin de Meijere on the occasion of his 75th birthday

Article abstract of DOI:10.1016/j.tetlet.2014.03.014

Oxidation of N-aminophthalimide in the presence of 2-arylideneinden-1,3- diones with electron-withdrawing substituents gives the corresponding 3-aryl-1-phthalimidospiro[aziridine-2,2′-indene]-1′,3′-diones in good yields. Heating these aziridines with stan

Full text of DOI:10.1016/j.tetlet.2014.03.014

Tetrahedron Letters 55 (2014) 2499–2503
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Tetrahedron Letters
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Synthesis and thermal transformations of spiro-fused
N-phthalimidoaziridines
⇑
Alena S. Pankova, Mikhail A. Kuznetsov
Department of Chemistry, Saint Petersburg State University, Universitetsky pr. 26, 198504 Saint Petersburg, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Oxidation of N-aminophthalimide in the presence of 2-arylideneinden-1,3-diones with electron-with-
drawing substituents gives the corresponding 3-aryl-1-phthalimidospiro[aziridine-2,20-indene]-10,30-
diones in good yields. Heating these aziridines with standard dipolarophiles (N-phenylmaleimide,
dimethyl acetylenedicarboxylate, maleate, and fumarate) leads, in most cases, to spiro[inden-2,20-pyr-
role] derivatives as products of 1,3-dipolar cycloaddition of the intermediate azomethine ylides with
up to 70–95% yields in the case of N-phenylmaleimide. As is typical for 2-acylaziridines, the competing
rearrangement into 2-aryl-4H-indeno[2,1-d][1,3]oxazol-4-ones prevails for less active dipolarophiles.
Increasing the electron-releasing properties of the 3-aryl ring allows the observation of the push–pull
effect of electron-donating and electron-withdrawing substituents on the ease of the three-membered
ring-opening.
Received 30 October 2013
Revised 10 February 2014
Accepted 4 March 2014
Available online 12 March 2014
Dedicated to Professor Armin de Meijere on
the occasion of his 75th birthday
Keywords:
Aziridine
Ó 2014 Elsevier Ltd. All rights reserved.
Azomethine ylide
1,3-Dipolar cycloaddition
Indene-1,3-dione
Spiro compounds
Functionalized aziridines serve as versatile building blocks for
the synthesis of various nitrogen heterocycles via cleavage of the
C–N or C–C bond.1–3 N-Phthalimidoaziridines have the advantage
of containing a masked N-amino group and their conversion into
pyrrole and pyrrolidine derivatives was successfully demonstrated
in our previous studies.4–9 The underlying process for such transfor-
mations is the 1,3-dipolar cycloaddition of thermally generated
azomethine ylides that can be realized in both inter- and intramo-
lecular manners affording monocyclic or polycyclic condensed
heterocycles, respectively. Obviously the involvement of spiro-
fused aziridines in such a process can result in novel spiro-fused
polycyclic heterocycles. Compounds of similar framework are of
particular interest because they often form the central core of natu-
ral products and medicinal lead compounds. For example, the spiro-
pyrrolidine moiety is found in numerous alkaloids10–15 and
antimycobacterial compounds as well as others featuring pharma-
cological properties.16–18
via addition of phthalimidonitrene to a >C@C< bond.22 We chose
2-arylideneinden-1,3-diones 3 as precursors as they would provide
a facile access to the desired spiroaziridines.23 Additionally, the
para-substituted aromatic ring allows evaluation of the substituent
effect on the stability and activity of the corresponding intermedi-
ate azomethine ylides (vide infra).
The starting arylideneindendiones 3a–c were obtained from
diethyl phthalate (1) according to the literature method
(Scheme 1).24 This strategy includes in situ generation of inden-
1,3-dione and serves as a convenient alternative to known methods
for arylideneindendione preparation.25–29 A slightly modified stan-
dard procedure4,30 for the oxidation of N-aminophthalimide with
Pb(OAc)4 in the presence of unsaturated substrates 3a–c gave azir-
idines 4a–c (Scheme 1). Compounds 4a,b gradually decomposed in
air at room temperature, but they could be stored in a refrigerator
for several months without decomposition. Our attempts to isolate
the aziridine 4c were unsuccessful, but its formation was invoked
from subsequent transformations (see below). According to their
1H NMR spectra, aziridines 4a,b exist as single invertomers with
anti-orientation of the aryl and phthalimide moieties, similar to
other trisubstituted aziridines described previously.4,5 Slow rota-
tion of the phthalimide group around the N–N bond in sterically
crowded molecules 4a,b (as well as in most of the compounds re-
ported below) led to broadening of its signals allowing the proton
signals of two similar moieties (PhthN and the indendione
We therefore investigated a hitherto unknown approach to spi-
roheterocycles via spiroaziridines, as only a few examples of spiro-
N-phthalimidoaziridines have been reported to date, and they have
never been explored in 1,3-dipolar cycloaddition.8,19–21 The con-
ventional route for the preparation of N-phthalimidoaziridines is
⇑
Corresponding author. Tel.: +7 812 428 6779; fax: +7 812 428 6939.
E-mail address: m.kuznetsov@spbu.ru (M.A. Kuznetsov).
http://dx.doi.org/10.1016/j.tetlet.2014.03.014
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

2500
A. S. Pankova, M. A. Kuznetsov / Tetrahedron Letters 55 (2014) 2499–2503
ONa
CO2Et
CO2Et
4-RC6H4CHO
AcOH, H2SO4, 50 oC, 1 h
Na, EtOAc
xylene, 140 oC, 4.5 h
CO2Et
O
a
b
c
R = NO2 ( ), Cl ( ), Me ( )
1
2
, 60%
NPhth
O
O
O
PhthNNH2, Pb(OAc)4, K2CO3
CH2Cl2, 0 oC, 0.5 h
N
O
R
O
b
R
c
a
3a, 73%,
b, 71%,
c, 71%
4a, 82%,
b, 78%,
N
PhthN =
c, used in situ
O
Scheme 1. Synthesis of aziridines 4a–c.
fragment) to be distinguished in the 1H NMR spectra of these aziri-
dines. In line with this, the broadening or disappearance of CON and
Ca signals was clearly evident in the 13C NMR spectra.
To evaluate the activity of spiroaziridines 4a–c in 1,3-dipolar
cycloaddition reactions, conventional dipolarophiles such as N-
phenylmaleimide (5), dimethyl acetylenedicarboxylate (DMAD)
(6), maleate 7, and fumarate 8 were chosen (Table 1). Optimal con-
ditions were found using 1H NMR spectroscopy and TLC monitor-
ing of the reaction mixtures with gradual increases in the
temperature from 25 °C in ꢀ15 °C/30 min increments.
The best results were achieved in the reactions with N-phenyl-
maleimide (5) in which all the expected cycloaddition products
9a–c were isolated (Table 1, entries 1, 5 and 9). In many cases tri-
cyclic oxazoles 12a,b were obtained along with the corresponding
Table 1
Reactions of aziridines 4a–c with dipolarophiles 5–8
NPhth
NPhth
O
O
O
O
O
EWG
Ar
N
N
C6H6
+
+
N
Ar
EWG
EWG
EWG
O
Ar
4a-c
5-8
9a-c, 10a,b, 11
12a,b
a
b
c
Ar = 4-O2NC6H4 ( ), 4-ClC6H4 ( ), 4-MeC6H4 ( )
O
CO2Me
CO2Me
MeO2C
N Ph
CO2Me
CO2Me
CO2Me
O
5
6
7
8
NPhth
N
NPhth
NPhth
O
O
O
O
O
O
NO2
Ar
Ar
N
N
CO2Me
CO2Me
CO2Me
CO2Me
O
N
O
Ph
9a-c
10a,b
Time (h)
11
Entry
Aziridine
Dipolarophile
T (°C)
Adduct
Yieldb (%)
Oxazole
Yieldb (%)
1
2
3
4
5
6
7
8
9
4a
4a
4a
4a
4b
4b
4b
4b
4ca
4ca
5
6
7
8
5
6
7
8
75
75
75
75
55
55
55
55
25
25
8
8
8
8
9
9
9
9
36
36
9a
10a
11
—
9b
10b
—
—
9c
—
87
66
25
—
95
77
—
—
34
—
—
—
5
12a
12a
12a
—
12b
12b
12b
—
30
60
—
14
28
37
—
5
6–8
10
—
—
a
Used in situ.
Isolated yield.
b

A. S. Pankova, M. A. Kuznetsov / Tetrahedron Letters 55 (2014) 2499–2503
2501
cycloadducts. Moreover, they were the only products in reactions
with the least active dipolarophile – dimethyl fumarate (8) (Table 1,
entries 4 and 8). Compounds 12a,b are orange solids, which were
poorly soluble in common organic solvents, and they usually
precipitated from benzene during the course of the reaction.
As mentioned above, all our attempts to isolate aziridine 4c
were unsuccessful and therefore we decided to add dipolarophiles
to the filtered reaction mixture immediately after oxidative amin-
oaziridination of 2-(4-methylbenzylidene)-1H-indene-1,3(2H)-
dione (3c). As a result, with N-phenylmaleimide (5) (2 equiv) after
1.5 days at room temperature, a precipitate formed which after
purification was identified as adduct 9c (Table 1). For other dipol-
arophiles 6–8, TLC monitoring of the reaction mixtures showed
disappearance of the spot that we tentatively assigned to the initial
aziridine 4c and formation of one new compound (orange spot
with Rf = 0.25 (SiO2, CH2Cl2)). However, our attempts to isolate this
product by crystallization or column chromatography failed. It
does not appear to be a cycloaddition product or an oxazole, since
such instability would not be expected by replacement of NO2 or Cl
substituents in stable analogues by a methyl group. We speculate
that decomposition of the three-membered ring occurs. The aziri-
dine fragment in compounds 4a–c is a part of a spirosystem where
two substituents on one carbon atom are strongly bonded with
each other. The ‘spiroactivation’ effect that appeared, for example,
in apparently much easier ring cleavage of spiro-1,1-diacylcyclo-
propanes than that of their monocyclic analogues,31,32 may have
an analogous influence on the stability of aziridines 4a–c as well.
Cleavage of the C–N bond followed by destruction of the whole
cycle may begin to dominate over concerted C–C cleavage in the
sequence 4a ? b ? c that may cause a decrease in yield, and even
disappearance of cycloaddition products (Table 1, entry 10).
The pyrrolidine protons in adducts 9a–c and 11 form simple
ABX systems, but it is well known that the values of their vicinal
coupling constants do not serve as unequivocal evidence of their
spatial relations.33 Therefore we used 2D 1H NOESY to confirm
the trans-orientation of the former aziridine proton and the pro-
tons of dipolarophile in the pyrrolidine ring of these compounds
(Table 1), which corresponds to the less sterically strained config-
uration of these products. In the 1H NMR spectra of compounds
10a,b the signals of the ring protons were observed as singlets at
6.97 and 6.86 ppm, respectively, which is consistent with the
structure of a 3-pyrroline, but not a 2-pyrroline.
atom of the 1,3-dipole; this is a known process for 2-acyl-substi-
tuted N-phthalimidoaziridines.4,34 The decrease in dipolarophile
activity in the sequence 5 ? 8 disfavors 1,3-dipolar cycloaddition
and gives oxazoles preferentially (Table 1). This is in agreement
with the fact that the activation barrier for the cycloaddition de-
pends on both the ylide and dipolarophile structures. In the pres-
ence of less active ylide ‘traps’, various intramolecular processes
come into play and their rates depend primarily on the aziridine
substituents.35 Among them, 1,5-electrocyclization is obviously
favorable because the required s-cis-orientation of an acyl substitu-
ent and terminal ylide carbon atom is inherent to ylides 13
(Scheme 2).
Less electron-withdrawing groups at the para-position in aziri-
dines 4a–c not only lead to milder reaction conditions for the
cycloaddition (75 °C for NO2 group, 55 °C—Cl, 25 °C—CH3), but also
to increased instability of the aziridine ring (aziridine 4c, Table 1,
entry 10). A similar dependence was also observed for 2,3-disubsti-
tuted N-phthalimidoaziridines36 so it proves in general that push–
pull aziridines with substituents of opposite electronic character
on neighboring carbon atoms form azomethine ylides easier than
those possessing only electron-withdrawing groups. While this
phenomenon has already been explored to increase the reactivity
of donor-acceptor substituted cyclopropane rings toward cycload-
dition,37,38 to the best of our knowledge, this is the first demonstra-
tion of analogous effects in aziridine scaffolds (compare6,7).
Since more stable spiroaziridines were obtained with electron-
withdrawing substituents, for practical reasons, we decided to fo-
cus on these when extending the substrate scope. Thus, we pre-
pared four additional N-phthalimidoaziridines 15a–d possessing
diverse electron-withdrawing groups on the aryl ring, including a
3-pyridyl moiety (Scheme 3). The starting arylideneindendiones
14a–d were synthesized in 69–73% yields from salt 2 in the same
manner as for the aforementioned substrates (Scheme 1), with
the exception of 3-pyridyl-substituted compound 14d where basic
conditions were more suitable (see the Experimental section for
details). Oxidative addition of N-aminophthalimide readily gave
target aziridines 15a–d.
NPhth
O
O
O
O
PhthNNH2, Pb(OAc)4, K2CO3
CH2Cl2, 0 oC, 0.5 h
N
Thus, the structures of all adducts 9–11 are in accordance with a
concerted mechanism of 1,3-dipolar cycloaddition of azomethine
ylides 13, generated thermally from aziridines 4a–c, to the dipol-
arophile (Scheme 2). These results are in agreement with our pre-
vious findings on the chemistry of N-phthalimidoaziridines.4,6,7
Oxazoles 12a,b originate from 1,5-dipolar electrocyclization of
the intermediate azomethine ylide 13 facilitated by the unavoid-
able proximity of any of the two keto groups to the terminal carbon
Ar
Ar
14a-d
15a, 81%,
b
, 80%,
Ar = 4-MeO2CC6H4 (a), 4-NCC6H4 (b),
4-F3CC6H4 (c), 3-pyridyl (d)
c
, 89%,
d, 71%
Scheme 3. Preparation of aziridines 15a–d.
EWG
NPhth
O
O
N
O
NPhth
Ar
Ar
EWG
N
EWG
EWG
NPhth
O
O
O
O
O
O
NPhth
Ar
Δ
N
N
9-11
12
Ar
O
N
O
NPhth
Ar
N
4a-c
- PhthNH
O
Ar
13
Scheme 2. Plausible mechanisms for the formation of adducts 9–11 and oxazoles 12.

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A. S. Pankova, M. A. Kuznetsov / Tetrahedron Letters 55 (2014) 2499–2503
NPhth
NPhth
Ar
O
O
O
O
O
NPhth
O
Ar
N
N
N
MW or Δ
75 oC, C6H6
N
Ph
+
+
O
O
Ar
O
O
N
N
O
O
Ph
Ph
15a-d
5
16
17
16a 17a
+
, 1:0.03, 71%,
a
b
16b + 17b, 1:0.04, 78%,
16c, 88%,
Ar = 4-MeO2CC6H4 ( ), 4-NCC6H4 ( ),
c
d
4-F3CC6H4 ( ), 3-pyridyl ( )
16d, 75%
Scheme 4. Reaction of aziridines 15a–d with N-phenylmaleimide 5 (isolated yields and ratios are given for reactions under MW irradiation for 1.5 h).
As N-phenylmaleimide (5) provided the highest product yields
among the dipolarophiles tested, we focused on it here as well.
In order to convert aziridines 15a–d into the expected cycload-
ducts, we used the same temperature as for the nitro-substituted
compound 4a (75 °C), but the use of microwave irradiation allowed
for reduced reaction time (1.5 h, Scheme 4). This shorter micro-
wave heating improved slightly the isolated yields. Formation of
the corresponding oxazoles of type 12 was not observed, similar
to the cases presented above (Table 1, entries 1, 5 and 9).
Table 2
Characteristic proton signals in the 1H NMR spectra of isomeric adducts 16 and 17
(CDCl3, 400 MHz)
0
0
0
Isomer
d (H3 ), ppm
d (H3a ), ppm
d (H6a ), ppm
16
6.16–6.22 d
(J = 7.3–7.5 Hz)
5.57–5.61 d
3.80–3.88 dd
(J = 10.2, 7.3–7.5 Hz)
3.92–3.95 dd
4.32–4.35 d
(J = 10.2 Hz)
3.67–3.68 d
(J = 7.8–7.9 Hz)
17
(J = 7.6–7.9 Hz)
(J = 7.6–7.9 Hz)
For aziridines 15a,b, mixtures of two isomers 16a/17a and 16b/
17b were obtained in the ratio ca. 1:0.035 (the reaction of aziridine
15c under conventional heating also gave two isomers 16c/17c in
the ratio ca. 1:0.12), but all our attempts to separate these isomeric
mixtures and to isolate the minor product using crystallization and
column chromatography were not successful. The pyrrolidine
proton signals of the ABX system in the major isomers 16a–d have
similar chemical shifts and coupling constant values as those for
adducts 9a–c, which proves the same spatial configuration
(Scheme 4). In addition, the structure of compound 16d was unam-
biguously confirmed by X-ray analysis (Fig. 1).39 The trans-orienta-
tion of the former dipolarophile fragment and pyridine-substituent
is evident from the value of the dihedral angle C(29)–C(6)–C(2)–
C(3) 101.9(2)°. In the central pyrrolidine ring the bond lengths
C(5)–C(1), C(6)–C(2) and N(2)–C(5), N(2)–C(6) are equal in pairs,
while the valence angles are different. The smallest is the angle
N(2)–C(5)–C(1) (98.14(17)°) that corresponds to the central posi-
tion in the spiro-system. The central pyrrolidine ring has a slightly
deflected ‘envelope’ conformation with the dihedral angle C(5)–
C(1)–C(2)–C(6) of ꢁ7.1(2)°.
Since we failed to isolate the minor isomers 17, their structures
are speculative. A comparison of the characteristic proton signals
of the two isomers in the area 3.5–6.5 ppm shows that the minor
species also have a pyrrolidine ring (minor proton signals in the
aromatic area of the spectrum are barely separated from those of
the main isomer) (Table 2). Obviously, the two protons of the
former N-phenylmaleimide cannot be in trans-orientation and
therefore the minor adducts appear to have all-cis-configuration
(Scheme 4). A comparison of the magnitudes of the NOE in the
2D 1H NOESY spectrum of the mixture of two isomers 16c/17c con-
firms this assumption. The all-cis-structure is significantly more
strained, which explains the very small amounts of minor isomers
and high stereoselectivity of the process as a whole.
In conclusion, we have synthesized a series of novel highly
functionalized spiro-fused N-phthalimidoaziridines. Depending
on the substituents, these compounds feature different stability
and reactivity as a result of strain in the system and the electronic
properties for push–pull aziridines. The introduction of electron-
withdrawing groups provides increased stability to the aziridines
and enables their isolation and subsequent transformations. Azo-
methine ylides generated thermally from spiroaziridines in the
presence of N-phenylmaleimide readily furnish products of 1,3-
dipolar cycloaddition with very high stereoselectivity. Such spiro-
fused heterocycles are difficult to obtain by other methods and
therefore the described transformations may serve as a convenient
method for their synthesis.
Acknowledgments
We are indebted to Prof. Khidmet S. Shikhaliev (Voronezh State
University) for providing samples of compounds 3a–c for our pre-
liminary experiments and for his valuable advice on their synthe-
sis. The authors thank Saint Petersburg State University for
financial support (research grant number 12.38.16.2011) and the
X-ray Diffraction Centre, Center for chemical analysis and materi-
als research and Center for Magnetic Resonance of Saint Petersburg
State University for XRD, MS and NMR studies, respectively.
Supplementary data
Supplementary data (Representative experimental details, char-
acterization data for compounds, copies of the 1H and 13C NMR
Figure 1. X-ray structure of adduct 16d.

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