The unexpected product of Diels-Alder reaction between “indanocyclon” and maleimide

Article abstract of DOI:10.1016/j.molstruc.2016.10.067

A heterocyclic compound commonly known as “indanocyclon” undergoes an unexpected Diels-Alder addition with maleimide. The resulting product has been isolated and characterized in order to get an information about its structure and possible mechanism of th

Full text of DOI:10.1016/j.molstruc.2016.10.067

Journal of Molecular Structure 1130 (2017) 573e578
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Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
The unexpected product of Diels-Alder reaction between
“indanocyclon” and maleimide
,
Michał A. Dobrowolski a *, Piotr Roszkowski a, Marta Struga b, Daniel Szulczyk b
a Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
b Department of Pharmacogenomics, Faculty of Pharmacy, Medical University, 02-097 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A heterocyclic compound commonly known as “indanocyclon” undergoes an unexpected Diels-Alder
addition with maleimide. The resulting product has been isolated and characterized in order to get an
information about its structure and possible mechanism of the reaction. Extensive comparison of single
crystal properties of 3-(2,8-dioxo-1,3-diphenyl-2,8-dihydrocyclopenta[a]inden-8a(1H)-yl)pyrrolidine-
2,5-dione and favorable product of the reaction has been also performed.
Received 9 September 2016
Received in revised form
19 October 2016
Accepted 20 October 2016
Available online 21 October 2016
© 2016 Elsevier B.V. All rights reserved.
Keywords:
Indanocyclon
Maleimide
Diels-Alder
X-ray diffraction
1. Introduction
The Diels-Alder reaction is very often conducted under specific
conditions, in which all the participating reagents are neither ionic,
The Diels-Alder reaction has become one of the most efficient
and practical methods for the synthesis of six-membered carbo-
cyclic and heterocyclic rings since its discovery in 1928 [1]. The
cycloaddition is a very important process and has been extensively
studied over last few decades [2]. Importantly, heteroatoms, such as
e.g. oxygen and/or nitrogen, can be included either in the diene or
the dienophile, what increases the value of this synthetic approach
[3]. The reaction may be executed under relatively mild conditions
by heating together two components, diene and dienophile, in non-
polar solvents, followed by evaporation which usually leads to high
yields of the products. The reaction is disciplined by the
Woodward-Hoffmann rules as a [p4s þ p2s] cycloaddition occurring
in a concerted but probably not symmetrically synchronous
fashion, thus leading to highly predictable product structures in
which two new carbon-carbon sigma bonds are formed in a ste-
reospecific manner with the creation of up to four new stereogenic
centres [4]. It should be pointed out that [2 þ 2], [4 þ 4] and [6 þ 6]
cycloadditions are thermally disallowed, but there are some
recognized modifications such as radical cation Diels-Alder re-
actions or photochemical cycloadditions [5].
nor highly polar. Therefore, the role of the solvent might be sig-
nificant. It is not a decisive factor in designing Diels-Alder reaction
synthetic pathways, although combined with selected specific
dien-dienophile pair reactants may be the reason of possible side
reactions and arising of unexpected products. Here we present such
a case, where an interesting product in typical Diels-Alder reaction
was obtained.
There have been investigations on large molecules in order to
corroborate affinity to serotonin receptors (5-HT1A, 5-HT2A, 5-HT7)
[6]. Unexpected product aroused our interest for further work on
synthesizing analogues of buspirone. Maleimide moiety in this
structure may fulfill the three-point pharmacophore model for 5-
HT1A receptor ligands [7] to a larger extent, due to the unusual
single bond. Adding a piperazine fragment might result in a series
of new compounds showing strong affinity to serotonin receptors.
Here we propose a mechanism of obtaining a product (unex-
pected), that take place during well-known Diels-Alder reaction.
Optimization of reaction conditions to enhance its yield was of key
importance at this stage. Further research is planned on a series of
compounds based on the resulting imide.
2. Results and discussion
* Corresponding author.
E-mail address: miked@chem.uw.edu.pl (M.A. Dobrowolski).
The 1,3-phenylcyclopenta[a]indene-2,8-dione, also well known
http://dx.doi.org/10.1016/j.molstruc.2016.10.067
0022-2860/© 2016 Elsevier B.V. All rights reserved.

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M.A. Dobrowolski et al. / Journal of Molecular Structure 1130 (2017) 573e578
as “indanocyclon”, is a widely used heterocyclic compound for
many organic transformations [8]. Our synthetic strategy was to
use this skeleton and combine its properties with common motif in
many biologically active compounds, maleimide, in order to receive
a series of new complex butylarylpiperazin-1-yl derivatives [6]. The
synthesis started from obtaining “indanocyclon” with the applica-
tion of widely used procedure [9], where the starting material is
ninhydrin (Scheme 1). The reaction was conducted in suitable
medium, which in this case was absolute ethanol.
Received dien is characterized by quite complex structure in
case of cycloaddition reactions. However, there are no literature
reports of any limitations from the regard on possible steric hin-
drance, because the reactivity of unsaturated fragment and its
relation to dienophile is decisive. Both conditions were theoreti-
cally met. Dien-dienophile pair (“indanocyclon”- maleimide) was
subjected to standard conditions of the Diels-Alder reaction for the
purpose of obtaining desirable product (Scheme 2). Synthesized
with good yield 4,10-diphenylindeno[1,2-f]isoindole-1,3,9(2H)-tri-
one (2) reveals characteristic arrangement of coupled benzene
rings, what was documented by X-ray structural analysis. The
product is described by recognized mechanism, typical for the
majority of pericyclic reactions, so it will not be the object for
further deliberation.
(Scheme 5) is completely different from already recognized Diels-
Alder reactions, however we can draw out several presumptions.
Firstly, at least two transition states arise, so we are able to receive
two different products. Favored transition state in which 10 and 12
carbon atoms of “indanocyclon” are engaged will lead to the
product of [4 þ 2] cycloaddition. The alternative mechanism would
probably lead to the transition state in which 8 and 9 carbon atoms
will be engaged. Regrouping of electrons shall occur in the next
stage. The structure of the product shows that it is more favorable
to form the bond with 8 carbon atom of dien. We assumed that this
is linked with the smaller steric hindrance of the product and the
presence of the carbonyl group in the closest area. The carbonyl
group can additionally have the influence on creation of the
product, because of the possible hydrogen bond formation with
maleimide. It is hard to explain why the bond between maleimide
and 9 carbon atom of “indanocyclon” is not formed. We presume
that it is related to the tension generated by bond between 8 carbon
atom of “indanocyclon” and maleimide. Probably this strain is the
main reason why double bond between 9 and 12 carbon atoms is
reproduced. We suggest more privileged position of bond forma-
tion near carbonyl group because of maleimide and carbonyl group
interactions. Furthermore, two hydrogen ions are generated in re-
action environment due to formation of the favored product, which
eliminates two ions during the aromatization of the newly formed
six-membered ring. This explains why unfavored product has two
more H atoms compared with the starting materials “indanocy-
clon” and maleimide.
However, under the same reaction conditions an unusual
additional product was formed (Scheme 3). That proves the pres-
ence of competitive mechanism, different from well-known [4 þ 2]
addition reactions.
At that point we decided to study the influence of reaction
solvent, since this was the only factor that could be responsible for
the formation of alternative product (Table 1).
To understand the reasons of 3-(2,8-dioxo-1,3-diphenyl-2,8-
dihydrocyclopenta[a]inden-8a(1H)-yl)pyrrolidine-2,5-dione for-
mation we have isolated single crystals of both reaction products
and transfer them to intensive structural studies.
Changing the solvent gives poorer yields of 3-(2,8-dioxo-1,3-
diphenyl-2,8-dihydrocyclopenta[a]inden-8a(1H)-yl)pyrrolidine-
2,5-dione (3). Reaction was conducted in boiling temperatures of
chosen solvents and in the same time period. It was found that
benzene and chlorobenzene provides the largest amount of side
product. At this stage no further tests were conducted, since the
reaction was performed using only solvent and reactants.
It is supposed that in the same reaction conditions alternative
reaction is achievable. Less possible, although commonly seen
mechanism comes into mind at first glance. The [2 þ 2] cycload-
dition, but this one is reserved for photochemical reactions. There
are some exceptions from this rule, for example some cycloaddi-
tions of ethylene derivatives. It should be mentioned that the
transition state of [2 þ 2] addition products depend on radical
mechanism. However, this one has not been recognized in this case.
Product that should be isolated (Scheme 4) according to that
mechanism doesn't agree with the one we have obtain.
2.1. Crystal structures of 4,10-diphenylindeno[1,2-f]isoindole-
1,3,9(2H)-trione and 3-(2,8-dioxo-1,3-diphenyl-2,8-
dihydrocyclopenta[a]inden-8a(1H)-yl)pyrrolidine-2,5-dione
4,10-diphenylindeno[1,2-f]isoindole-1,3,9(2H)-trione (2) crys-
tallizes in the P21/c space group (Fig. 1, Table 3), the asymmetric
unit contains one molecule of (2) and acetonitrile, which was used
as a solvent. The main part of the molecule (4 rings) is almost planar
with deviation of 5.5ꢀ. Two substituted phenyl rings are tilted with
respect to benzene ring by 48.5ꢀ and 70.8ꢀ, respectively. The
structure is governed by strong NeH/O hydrogen bonding be-
tween adjacent molecules forming dimers (NeH/O distance
2.877 Å, see Table 2). Consequently, the layers built of dimeric
chains are formed (Fig. 2). Additionally, there are CeH/O in-
teractions of 3.328 Å stabilizing the structure. The crystal is
somehow “bound” together by two acetonitrile molecules lying in
We take into consideration that the mechanism of this reaction
Scheme 1. Synthesis of 1,3-phenylcyclopenta[a]indene-2,8-dione (1) from ninhydrin as a starting material.

M.A. Dobrowolski et al. / Journal of Molecular Structure 1130 (2017) 573e578
575
Scheme 2. Designed reaction of 1,3-phenylcyclopenta[a]indene-2,8-dione (1) and maleimide.
Scheme 3. Synthesis of 3-(2,8-dioxo-1,3-diphenyl-2,8-dihydrocyclopenta[a]inden-8a(1H)-yl)pyrrolidine-2,5-dione.
Table 1
Solvent - formation of additional product dependency study results.
3-(2,8-dioxo-1,3-diphenyl-2,8-dihydrocyclopenta[a]inden-
8a(1H)-yl)pyrrolidine-2,5-dione (3) crystallizes without solvent in
the Pca21 space group (Fig. 3), and the asymmetric unit contains a
single molecule. The main part of the molecule resembles a but-
terfly with the angle between two five-membered rings of 29.5ꢀ.
Two substituted phenyl rings are tilted with respect to 5-
membered ring by 47.8ꢀ and 88.9ꢀ, respectively. The structure is
stabilized by strong NeH/O hydrogen bonding between nitrogen
from maleimidyl ring and 5-membered ring oxygen (NeH/O
distance 2.843 Å, see Table 2). Additionally, there are CeH/O in-
teractions of 3.090, 3.261, 3.3 Å (Fig. 4).
No.
Solvent [reflux]
Reaction time [h]
Yield [%]
1
2
3
4
5
6
7
8
Benzene
Toluene
8
8
8
8
8
8
8
8
19
11
*
*
*
15
7
*
Acetonitrile
1,2-Dichloroethane
Cyclohexanone
Chlorobenzene
o-Xylene
Chloroform
*Additional product (3) not found (TLC).
the middle of the unit cell related by an inversion center.
Scheme 4. Expected products of [2 þ 2] cycloaddition for 1,3-phenylcyclopenta[a]indene-2,8-dione [1].

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M.A. Dobrowolski et al. / Journal of Molecular Structure 1130 (2017) 573e578
Scheme 5. Proposed mechanism of 3-(2,8-dioxo-1,3-diphenyl-2,8-dihydrocyclopenta[a]inden-8a(1H)-yl)pyrrolidine-2,5-dione formation.
3. Conclusions
Bruker AVANCE DMX400 spectrometer, operating at 300 MHz (1H
NMR) and 75 MHz (13C NMR). The chemical shift values are
New unexpected compound was synthesized. Unique product
was characterized by structural and spectrochemical analysis. 3-
(2,8-dioxo-1,3-diphenyl-2,8-dihydrocyclopenta[a]inden-8a(1H)-
yl)pyrrolidine-2,5-dione (3) was intensively studied and mecha-
nism of the reaction, in which it was obtained, was proposed. Two
types of mechanisms were excluded, because of small influence of
polar solvents (ion mechanism) and the absence of inhibitors
(radical mechanism). Presumptions considering possible mecha-
nism of reaction were made. There is definitely a significant influ-
ence of unfavorable transition state during formation of
unexpected product. The single bond formation may be attributed
to a strong NeH/O hydrogen bond between nitrogen from mal-
eimidyl ring and 5-membered ring oxygen.
expressed in ppm relative to TMS as an internal standard. Mass
spectral ESI (Electrospray Ionization) measurements were carried
out on a Mariner Perspective e Biosystem instrument with TOF
detector. The spectra were obtained in the positive ion mod with a
declustering potential 140e300 V. Flash chromatography was
performed on Merck silica gel 60 (200e400 mesh) using chloro-
form/methanol (19:1 vol) mixture as eluent. Analytical TLC was
carried out on silica gel F254 (Merck) plates (0.25 mm thickness).
4.1.1. 1,3-phenylcyclopenta[a]indene-2,8-dione (1)
1,3-diphenylpropan-2-one (4.62 g, 0.022 mol) was added to a
suspension of ninhydrin (4.02 g, 0.023 mol) in absolute ethanol
(50 ml). Subsequently, the MeOH/KOH solution (5 ml) was instilled
in portions (5e6 drops) every 15 min of reaction. The stirred re-
action mixture turned red after adding first portion, and finally
after last portion it turned violet. The reaction ended after 1 h
heating in 80 ꢀC. Obtained violet crystals (5.87 g) was washed with
cold methanol (120 ml) and then recrystallized from acetonitrile
(yield 72%). The structure of final product is already known [9],
therefore full spectrochemical analysis was not performed. Mp.
205e206 ꢀC. C24H14O2, M ¼ 334.37 gmol-1. 1H NMR (CDCl3)
Investigations on series of arylpiperazine derivatives of (3) and
their affinity to serotonin receptors are in progress.
4. Experimental
4.1. Synthesis
Melting points were determined in a Kofler's apparatus and are
uncorrected. All reactions were run under atmosphere of argon.
Solvents used in reactions were freshly dried by passing through an
activated alumina column. The NMR spectra were recorded on a
d
(ppm): 7.42e7.56 (m, 6H, CHarom); 7.70e7.77 (m, 4H, CHarom);
8.04e8.10 (m, 1H, CHarom); 8.18e8.24 (m, 1H, CHarom); 8.59e8.64
(m, 2H, CHarom). ESI MS: m/z ¼ 357.1 [M þ Na]þ (100%).

M.A. Dobrowolski et al. / Journal of Molecular Structure 1130 (2017) 573e578
577
Fig. 3. The molecular structure of (3) showing displacement ellipsoids at the 50%
probability level.
4.1.2. (a) 4,10-diphenylindeno[1,2-f]isoindole-1,3,9(2H)-trione (2)
and (b) 3-(2,8-dioxo-1,3-diphenyl-2,8-dihydrocyclopenta[a]inden-
8a(1H)-yl)pyrrolidine-2,5-dione (3)
(a) 0.55 g of maleimide was added to a solution of 1,3-
phenylcyclopenta[a]indene-2,8-dione (2.06 g, 0.0061 mol) in dry
benzene (50 ml) and then refluxed for 8 h. Subsequently, the re-
action mixture was cooled to a room temperature and then evap-
orated. Purification by column chromatography with chloroform/
methanol 19:1 as eluent resulted in 1.71 g (0.0043 mol) of yellow
precipitate (yield 76%). Recrystallization from hot toluene/meth-
anol 1:1 solution gave colorless crystals [Rf ¼ 0.68 (chloroform/
methanol 9.8:0.2)]. C27H15NO3, M ¼ 401.41 gmolꢁ1. 1H NMR (CDCl3)
Fig. 1. The molecular structure of (2) showing displacement ellipsoids at the 50%
probability level, acetonitrile molecule omitted for clarity.
Table 2
Intermolecular hydrogen bond lengths (Å) and bond angles (ꢀ) for (2) and (3).
d
(ppm): 7.40e7.44 (m, 4H, CHarom); 7.45e7.46 (d, 2H, CHarom
,
,
,
DeH/A
d (DeH)
d (H/A)
d (D/A)
:DHA
J ¼ 2.1 Hz); 7.49e7.51 (m, 2H, CHarom); 7.52e7.53 (d, 2H, CHarom
Compound (2)
N1eH1/O1
Compound (3)
N1eH51/O1
J ¼ 2.1 Hz); 7.58e7.59 (m, 2H, CHarom); 7.60e7.61 (d, 2H, CHarom
0.92
1.985
1.959
2.877
2.843
169.77
169.07
J ¼ 2.7 Hz); 11.29 (s, 1H, NH). 13C NMR (CDCl3)
d (ppm): 124.54,
124.59, 127.91 (2C), 128.59 (2C), 129.09 (3C), 129.28 (2C), 129.34,
129.87, 130.54, 130.57, 131.95, 134.24, 134.81, 135.14, 135.52, 135.58,
139.37, 141.97, 149.69, 165.73, 165.89, 190.91. ESI MS: m/z ¼ 424.1
[M þ Na]þ (100%).
0.895
Fig. 2. Crystal packing showing the dimeric chains of (2) and two acetonitrile molecules. Hydrogen bonds and short contacts showed with dashed lines.

578
M.A. Dobrowolski et al. / Journal of Molecular Structure 1130 (2017) 573e578
Table 3
Crystal data and structure refinement for (2) and (3).
Compound
(2)
(3)
Empirical formula
Formula weight
Space group
Unit cell dimensions
a [Å]
C27H15NO3, CH3CN
442.45
P21/c
C28H19NO4
433.44
Pca21
7.1918 (3)
15.3921 (8)
19.6560 (11)
95.723 (4)
2165.01 (19)
4
17.8729 (9)
7.4691 (4)
15.7345 (8)
90.00
2100.47 (19)
4
b [Å]
c [Å]
b
[ꢀ]
Volume V [Å3]
Z [molecules/cell]
Dcalculated [Mg mꢁ3
]
1.357
0.089
1.371
0.092
Absorption coefficient
range for data collection [ꢀ]
Limiting indices
m
/mmꢁ1
q
2.08e25.00
ꢁ8 < ¼ h ¼ > 8
ꢁ18 < ¼ k ¼ > 16
ꢁ21 < ¼ l ¼ > 23
7391/3827
3827/312
2.96e25.00
ꢁ21 < ¼ h ¼ > 21
ꢁ8 < ¼ k ¼ > 8
ꢁ18 < ¼ l ¼ > 18
15425/1922
1922/302
0.857
Reflections collected/unique
Data/parameters
Goodness of Fit
1.071
Final R index (I > 2
s
)
0.0583
0.1415
0.304 and ꢁ0.387
0.0263
0.0432
0.156 and ꢁ0.154
Fig. 4. Crystal packing of (3). Hydrogen bonds and short contacts showed with dashed
lines.
wR2
Largest diff. Peak and hole [Åꢁ3
]
(b) 0.55 g of maleimide was added to a solution of 1,3-
phenylcyclopenta[a]indene-2,8-dione (2.06 g, 0.0061 mol) in dry
benzene (50 ml) and then refluxed for 8 h. Subsequently, the re-
action mixture was cooled to a room temperature and then evap-
orated. Purification by column chromatography with chloroform/
methanol 19:1 as eluent resulted in 0.41 g (0.0043 mol) of yellow
precipitate (yield 19%). Recrystallization from hot ethanol gave
atoms were located from a difference map and were refined iso-
tropically. The atomic scattering factors were taken from the In-
ternational Tables [13]. Selected crystal data are given in Table 3.
CCDC 1503579-1503580 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.
colorless crystals [Rf
C
¼
0.59 (chloroform/methanol 9.8:0.2)].
28H19NO4, M ¼ 433.45 gmolꢁ1
. d (ppm):
1H NMR (DMSO-d6)
2.50e2.61 (q, 1H, CH, J ¼ 5.4 Hz); 2.70e2.79 (q, 1H, CH, J ¼ 9.0 Hz);
3.38e3.43 (dd, 1H, CH, J1 ¼ 5.7 Hz, J2 ¼ 5.4 Hz); 4.24 (s, 1H, CH);
6.92e6.94 (d, 2H, CHarom J ¼ 7.8 Hz); 7.15e7.23 (m, 3H, CHarom);
7.52e7.60 (m, 5H, CHarom); 7.75e7.77 (d, 2H, CHarom J ¼ 3.9 Hz);
7.80e7.85 (m, 1H, CHarom); 7.91e7.93 (d, 1H, CHarom J ¼ 7.2 Hz);
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