Tandem aldol condensation-Diels-Alder-aromatization sequence of reactions: A new pathway for the synthesis of 2-tetralone derivatives

Article abstract of DOI:10.1139/cjc-2016-0246

A series of new polysubstituted derivatives of 2-tetralones possessing two ester groups were synthesized via a tandem aldol condensation-Diels-Alder-aromatization sequence of reactions. All the three steps took place in one pot and in the presence of amin

Full text of DOI:10.1139/cjc-2016-0246

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Tandem aldol condensation-Diels–Alder-aromatization sequence of reactions: a new
pathway for the synthesis of 2-tetralone derivatives
1
2
3
M. Saeed Abaee,a Esmail Doustkhah,a Mohaddeseh Mohammadi,a Mohammad M.
Mojtahedi,a and Klaus Harmsb
4
5
aDepartment of Organic Chemistry and Natural Products, Chemistry and Chemical
Engineering Research Center of Iran, Pajohesh Blvrd., 17th km of TehranꢀKaraj Highway,
P.O.Box 14335ꢀ186, Tehran, Iran
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Tel: (+98)21ꢀ44787749
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Fax: (+98)21ꢀ44787785
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bFachbereich Chemie der PhilippsꢀUniversitaet Marburg, HansꢀMeerweinꢀStrasse, Dꢀ35032
Marburg, Germany
Corresponding author's eꢀmail: abaee@ccerci.ac.ir
This paper is dedicated to Professor Dale E. Ward for 30 years of professorship at U of S.

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Abstract: A series of new polysubstituted derivatives of 2ꢀtetralones possessing two ester
groups were synthesized via a tandem aldol condensationꢀDiels‒Alderꢀaromatization
sequence of reactions. All the three steps took place in one pot and in the presence of
aminofunctionalized silica coated Fe3O4 nanoparticles as the catalyst. In situ formed dienes
reacted with diethyl acetylenedicarboxylate at room temperature and the process was followed
by spontaneous aromatization of the cycloadducts to produce high yields of the final tetralone
products. Further studies suggest that the process goes through an initial aldol condensationꢀ
cycloaddition sequence followed by oxidation and rearrangement steps. After completion of
the reactions, the catalyst could be recycled and reused efficiently in next reactions.
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Key words: tandem reaction, tetralones, aldol condensation, Diels‒Alder reaction,
aromatization.
Resume:

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Introduction
1
Tetralones constitute a group of organic intermediates which are used in the synthesis of
several biologically important compounds1ꢀ3 or in the preparation of other chemicals such as
lipodioxigenase,4 substituted naphthols,5 or laser pigments.6 In addition, tetralone derivatives
are used in the synthesis of sertraline, a potent drug for the treatment of depression and
anxiety related problems.7 Different methods are reported for the synthesis of tetralones such
as intramolecular cyclization of phenylbutyric acid,8ꢀ10 reductive cleavage of naphthyl
ethers,11 or the Haworth reaction.12
2
3
4
5
6
7
8
The [4+2] Diels‒Alder (DA) cycloaddition is arguably one of the most versatile reactions in
synthetic organic chemistry and after almost one century from its discovery,13 the DA reaction
still finds every day applications as the key step in the synthesis of important compounds,14,15
intermediates,16,17 natural products,18 and pharmaceuticals.19 Nowadays, a fast growing and
interesting strategy involves the use of the DA reaction in tandem protocols20,21 for rapid and
efficient access to diverse libraries of complex products22 and target molecules.23,24
In the framework of our studies on aldol condensation reactions, we have reported the
synthesis of a series of styrylcyclohexꢀ2ꢀenone dienes (e.g. 4a') in the presence of LiClO4 and
TMSNEt2 and under solventꢀfree conditions.25 Subsequently, these dienes were explored for
their DA reactivities with Nꢀphenylmaleimide,26 methyl acrylate,21,27 and maleic
anhydride.21,28 The results led us to the synthesis of several derivatives of dehydrodecaline
skletone with different substitution patterns. In continuation of using heterogeneous
catalysts,29,30 here we introduce a new pathway for efficient synthesis of tetralone derivatives
via a tandem aldol condensationꢀDAꢀoxidative aromatization sequence, as exemplified in
Figure 1 for the synthesis of 4a. The whole process takes place in one pot by the use of
magnetically separable silicaꢀcoated functionalized nanoparticles of Fe3O4 (FNꢀFe3O4). The
catalyst, which was prepared by using a known procedure,31 consists of a superparamagnetic
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21
22
23
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25

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core of iron oxide coated with silica and functionalized with 3ꢀaminopropylꢀtriethoxysilane
and was used to produce a new series of polysubstituted tetralone derivatives. Consequently,
single products are obtained in high yields and the nanocatalyst is recoverable32,33 so that it
can be recycled into the next reactions without considerable loss of its efficiency.
1
2
3
4
5
6
Figure 1. Oneꢀpot aldol condensationꢀDA reactionꢀaromatization pathway for the synthesis of
4a.
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Results and discussion
We first studied a model reaction by reacting enone 1 with 4ꢀMeC6H4CHO (2a) and
dienophile 3a under various sets of conditions (Table 1). Treatment of an equimolar mixture
of 1 and 2a with substoichiometric amounts of the catalyst followed by the addition of a
toluene solution of diethyl acetylenedicarboxylate (DEAD, 3a) led to 73% and 52% formation
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22
o
of 4a under refluxing (entry 1) or 50 C (entry 2) conditions, respectively. Repeating the
reaction without the catalyst gave no product and reactants were recovered entirely (entry 3).
To see if fully functionalized iron oxide particles are required to promote the reactions, some
amines were used, where lower quantities of 4a were obtained (entries 4ꢀ6). Search to find the
optimum solvent for the process supported that toluene can favour the highest conversion of
the reactants to 4a (entries 7ꢀ11).

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Table 1. Optimization of OneꢀPot Synthesis of 4a.
1
Entry Catalyst
Solvent Yield (%)a,b
1
FNꢀFe3O4 PhMe 73
2
FNꢀFe3O4 PhMe
52c
0
3
none
PhMe
PhMe
PhMe
4
Et3N
10
15
20
50
50
5
Et2NH
6
Et2NSiMe3 PhMe
FNꢀFe3O4 EtOH
FNꢀFe3O4 H2O
7
8
9
FNꢀFe3O4 MeCN 20
FNꢀFe3O4 MeOH 30
10
11
FNꢀFe3O4 THF
0
aIsolated yields.
2
3
4
5
6
b(i) 1 + 2a + catalyst, rt, 6 h; (ii) DEAD (in refluxing solvent), 18 h.
c50 oC.
To show the generality of the process, the optimized conditions were used to evaluate the
reactions of 1 with 3 and other aldehydes (Table 2). Therefore, benzaldehyde (entry 2) and its
derivatives bearing electron donating (entries 1 & 3ꢀ5) and electron withdrawing (entries 6ꢀ8)
substituents underwent a oneꢀpot sequence of reactions with 1 and 3 to produce the respective
products 4 in good yields. All reactions were complete within 20ꢀ24 h and gave a derivative
of 4 as the major product of the process.
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8
9
10
11
12
13
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15

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Table 2. Oneꢀpot synthesis of derivatives of 4.
1
Entry
Ar
3 (R)
Product
4a
Yield (%)a
1
2
3
4
5
6
7
8
4ꢀMeC6H4
C6H5
3a (Et)
3a (Et)
3a (Et)
3b (Me)
3a (Et)
3a (Et)
3a (Et)
3a (Et)
73
74
80
77
81
75
75
82
4b
3ꢀMeC6H4
4ꢀMeC6H4
4ꢀMeOC6H4
3ꢀMeOC6H4
4ꢀClC6H4
2ꢀthienyl
4c
4d
4e
4f
4g
4h
aIsolated yields.
2
3
4
5
6
7
8
9
The structure of the products was elucidated by HꢀNMR and 13CꢀNMR spectroscopic
methods. In the proton spectrum, the relative numbers of both aliphatic and aromatic protons
were diagnostic in determining the structure. In high field, signals consist of two ethoxy, two
equivalent geminal methyl, and two different methylene groups indicating the corporation of
reactants 3 and 1 in the structure of the product. On the other hand, besides the aromatic
protons of the aldehyde residue, the presence of an additional singlet aromatic proton at
slightly above 7.0 ppm was a strong indication for the aromatization of the final adduct. To
confirm the structure of the adducts, a single crystal of 4a was also prepared and subjected to
Xꢀray analysis. The results are depicted in Figure 2 and confirm the proposed structure.
1
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11
12
13

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1
2
Figure 2. Crystal structure of 4a. Displacement ellipsoids at 50% probability level.
3
Next, the recyclability of the catalyst was studied by using the recovered particles in the
model reaction (Figure 3, top). Thus, after completion of the process, the catalyst was
separated by using an external magnetic field (Figure 3, bottom) and reused in next reactions,
where it could be effective for at least additional 5 consecutive runs without showing
significant decrease in its performance.
4
5
6
7
8
9
75
50
25
0
10
11
12
13
14
15
16
17
18
19
1
2
3
4
5
6
Run
Figure 3. Successful reuse of the catalyst.

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It can be concluded from the results that the process initiates with an aldol condensation to
produce the respective diene 4' (Figure 4). The formation of the diene is then followed by a
DA cycloaddition with 3 so that 4'' is formed. The final stabilizing rearrangement and
aromatization steps give the ultimate product 4.
1
2
3
4
5
6
Figure 4. Suggested reaction pathway.
In order to shed light on the mechanism of the process, derivatives of 4' were prepared
separately and when treated with 3 in refluxing toluene under atmospheric air pressure, the
expected products 4 were formed within 15ꢀ18 h. Moreover, when the addition of 1 to 3b and
4ꢀMeC6H4CHO was conducted under inert atmosphere, the direct cycloaddition adduct 5 was
separated and characterized by 1HꢀNMR spectroscopy. Then, this intermediate was
independently converted to 4d in refluxing toluene to further confirm the suggested pathway
of the reaction (Figure 5).
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8
9
10
11
12
13
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15
Figure 5. Oneꢀpot formation of 5 and its stepwise conversion to 4d.

Page 9 of 17
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Conclusion
1
2
3
4
5
6
7
8
In summary, we succeeded in developing a new procedure for the synthesis of polysubstituted
tetralone derivatives by using a oneꢀpot procedure. As a result, the three reactants combine to
produce the desired products in high yields and the catalyst could be recovered and reused in
next reactions efficiently. We are planning to extend the results to the same reactions of
unsubstituted derivatives of 1 (3ꢀmethylcyclohexꢀ2ꢀenone) to evaluate the feasibility of
synthesizing polysubstituted naphthols (Figure 6). The study with other singly activated
dienophiles and heterodienophiles are also underway.
9
Figure 6. A oneꢀpot sequence of four steps to polysubstituted naphthols.
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21
22
23
Experimental
General
Melting points are uncorrected. FTꢀIR spectra were recorded using KBr disks on a Shimadzu
Prestigeꢀ21 spectrometer. NMR spectra were obtained on a FTꢀNMR Bruker Avance (300
MHz) or FTꢀNMR Bruker Ultra ShieldTM (500 MHz) as CDCl3 solutions using Me4Si as
internal standard reference. Elemental analyses were performed using a Thermo Finnigan
Flash EA 1112 instrument. MS spectra were obtained on a Fisons 8000 Trio instrument at
ionization potential of 70 eV. TLC experiments were carried out on preꢀcoated silica gel
plates using petroleum ether/EtOAc (4:1) as the eluent. Dienes 1 were synthesized using a
known method.21 All reagents and starting materials were purchased from commercial
sources. Aldehydes were redistilled or recrystallized before being used. All products are new
and were identified based on their physical and spectral properties.

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General procedure for the synthesis of products 4
1
A mixture of isophorone 1 (690 mg, 5.0 mmol), an aldehyde 2 (5.0 mmol), and FNꢀFe3O4
(100 mg) was stirred for 6 h at room temperature until TLC showed disappearance of the
reactants and formation of the intermediate dienes 2. At this point, a solution of DEAD 3a
2
3
4
(800
µL, 5.0 mmol) in toluene (5.0 mL) was added to the mixture and was refluxed for 18 h
5
under atmospheric air pressure. After completion of the reaction, the catalyst was separated by
using an external magnetic bar. The remaining solution was concentrated under reduced
pressure and the product was isolated from the residue by column chromatography
(EtOAc/hexane, 1:3). Products were characterized by NMR, IR, and mass spectroscopy.
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Characterization data of new products
Diethyl 6,6-dimethyl-8-oxo-3-(
(4a)
p-tolyl)-5,6,7,8-tetrahydronaphthalene-1,2-dicarboxylate
M.p. 137ꢀ139 ˚C. IR (KBr): 1742, 1690, 1735, 1588 cmꢀ1. 1HꢀNMR (300 MHz, CDCl3): 7.29ꢀ
7.20 (
m
, 5H); 4.43 (
q
,
J
= 7.0 Hz, 2H); 4.06 ( = 7.0 Hz, 2H); 2.90 (
q
,
J
s, 2H); 2.56 (s, 2H);
2.39 (
s
, 3H); 1.39 (
t,
J
= 7.0 Hz, 3H); 1.11 (s, 6H); 0.99 (t, J
= 7.0 Hz, 3H). 13CꢀNMR (75
MHz, CDCl3): 196.4; 167.6; 166.6; 145.5; 144.4; 137.9; 135.6; 132.9; 131.9; 129.3; 129.1;
127.8; 127.3; 61.2; 61.1; 51.8; 42.5; 33.4; 27.5; 20.7; 13.7; 13.4. MS (70 eV) m/z 408 (M+),
335, 178, 165, 29. Anal. Calcd for C25H28O5: C, 73.51; H, 6.91. Found: C, 73.66; H, 6.71.
Diethyl
(4b)
6,6-dimethyl-8-oxo-3-phenyl-5,6,7,8-tetrahydronaphthalene-1,2-dicarboxylate
M.p. 105ꢀ107 ˚C. IR (KBr): 1736, 1679, 1585 cmꢀ1. HꢀNMR (300 MHz, CDCl3): 7.41ꢀ7.26
1
(m, 6H); 4.44 (
q
,
J
= 6.5 Hz, 2H); 4.02 ( = 6.5 Hz, 2H); 2,91 (
q
,
J
s, 2H); 2.56 (s, 2H); 1.39 (t,
J
= 6.5 Hz, 3H); 1.11 (
s
, 6H); 0.94 (t, J
= 6.5 Hz, 3H). 13CꢀNMR (75 MHz, CDCl3): 196.3;
168.5; 167.1; 145.6; 144.9; 139.5; 134.1; 131.9; 130.2; 128.4; 128.2; 128.1; 128.0; 61.9; 61.5;

Page 11 of 17
11
52.6; 43.9; 33.7; 28.1; 13.9, 13.4. MS (70 eV) m/z 394 (M+), 351, 351, 322, 265; 167. Anal.
Calcd for C24H26O5: C, 73.08; H, 6.64. Found: C, 73.19; H, 6.37.
1
2
Diethyl 6,6-dimethyl-8-oxo-3-(
(4c)
m
-tolyl)-5,6,7,8-tetrahydronaphthalene-1,2-dicarboxylate
3
4
M.p. 95ꢀ97 ˚C. IR (KBr): 1743, 1689, 1590 cmꢀ1. 1HꢀNMR (300 MHz, CDCl3): 7.29ꢀ7.26 (
m,
5
2H); 7.18ꢀ7.12 (
m
, 3H); 4.43 (
q
,
J
= 7.0 Hz, 2H); 4.04 (
q
,
J
= 7.0 Hz, 2H); 2.90 (
s, 2H); 2.55
6
(s
, 2H); 2.37 ( , 3H); 1.39 (
s
t,
J
= 7.0 3H); 1.10 (
s
, 6H); 0.96 (
t,
J
= 7.0, 3H). 13CꢀNMR (75
7
MHz, CDCl3): 196.3; 168.6; 167.2; 145.7; 144.9; 139.3; 138.0; 134.0; 131.8; 128.9; 128.7;
128.3; 127.9; 126.0; 125.1; 61.9; 61.5; 52.5; 43.7; 33.7; 28.0; 21.3; 13.8; 13.5. MS (70 eV)
m/z 408 (M+), 379, 364, 304, 211. Anal. Calcd for C25H28O5: C, 73.51; H, 6.91. Found: C,
73.56; H, 6.69.
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Dimethyl
6,6-dimethyl-8-oxo-3-(
p
-tolyl)-5,6,7,8-tetrahydronaphthalene-1,2-
dicarboxylate (4d)
1
M.p. 187ꢀ189 ˚C. IR (KBr): 1745, 1689, 1585 cmꢀ1. HꢀNMR (500 MHz, CDCl3): 7.30 (
s,
1H); 7.26ꢀ7.23 (
m, 4H); 3.94 (s, 3H); 3.59 (s, 3H); 2.99 (s, 2H); 2.55 (s, 2H); 2.39 (s, 3H);
1.10 (
s
, 6H). 13CꢀNMR (125 MHz, CDCl3): 196.8; 169.7; 168.2; 146.1; 145.5; 138.8; 136.8;
134.4; 132.5; 130.3; 129.7; 128.4; 128.3; 53.3; 53.0; 52.8; 44.2; 34.1; 28.5; 21.6. MS (70 eV)
m/z 380 (M+), 349, 321, 238, 165. Anal. Calcd for C23H24O5: C, 72.61; H, 6.36. Found: C,
72.76; H, 6.51.
Diethyl
3-(4-methoxyphenyl)-6,6-dimethyl-8-oxo-5,6,7,8-tetrahydronaphthalene-1,2-
dicarboxylate (4e)
M.p. 153ꢀ155 ˚C. IR (KBr): 1739, 1694, 1608, 1588 cmꢀ1. 1HꢀNMR (300 MHz, CDCl3): 7.29
(d, J = 8.5 Hz, 2H); 7.26 (s, 1H); 6.93 (d, J = 8.5 Hz, 2H); 4.43 (q, J = 7.0 Hz, 2H); 4.07 (q, J
= 7.0 Hz, 2H); 3.84 (
s, 3H); 2.90 (s, 2H); 2.55 (s, 2H); 1.39 (t, J = 7.0 Hz, 3H); 1.10 (s, 6H);
1.02 (t, J
= 7.0 Hz, 3H). 13CꢀNMR (75 MHz, CDCl3): 196.3; 167.6; 166.7; 159.5; 145.5;

Page 12 of 17
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144.1; 132.9; 131.8; 130.8; 129.3; 129.2; 127.1; 114.1; 61.2; 61.0; 55.2; 51.8; 42.5; 33.3;
27.5; 13.7; 13.4. MS (70 eV) m/z 424 (M+), 300, 262, 295, 224. Anal. Calcd for C25H28O6: C,
70.74; H, 6.65. Found: C, 70.66; H, 6.77.
1
2
3
Diethyl
3-(3-methoxyphenyl)-6,6-dimethyl-8-oxo-5,6,7,8-tetrahydronaphthalene-1,2-
4
dicarboxylate (4f)
5
M.p. 103ꢀ105 oC. IR (KBr): 1717, 1679, 1587, 1495 cmꢀ1. 1HꢀNMR (300 MHz, CDCl3): 7.25ꢀ
6
7.20 (
m
, 2H); 6.87ꢀ6.84 (
m, 2H); 6.82ꢀ6.80 (
m, 1H); 4.36 (
q
,
J
= 7.0 Hz, 2H); 3.97 (
q
,
J
= 7.0
7
Hz, 2H); 3.74 (
s
, 3H); 2.84 (
s
, 2H); 2.49 (
s
, 2H); 1.32 (
t,
J
= 7.0 Hz, 3H); 1.04 (
s
, 6H); 0.90 (
t,
8
J
= 7.0 Hz, 3H). 13CꢀNMR (75 MHz, CDCl3): 196.3; 168.5; 167.2; 159.6; 145.5; 145.0; 140.8;
9
134.1; 131.8; 130.3; 129.5; 128.2; 120.5; 114.2; 113.4; 62.0; 61.6; 55.3; 52.6; 43.8; 33.8;
28.1; 13.9; 13.5. MS (70 eV) m/z 424 (M+), 380, 352, 295. Anal. Calcd for C25H28O6: C,
70.74; H, 6.65. Found: C, 70.59; H, 6.51.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Diethyl
3-(4-chlorophenyl)-6,6-dimethyl-8-oxo-5,6,7,8-tetrahydronaphthalene-1,2-
dicarboxylate (4g)
M.p. 139ꢀ141 ˚C. IR (KBr): 1740, 1690, 1588, 1489 cmꢀ1. 1HꢀNMR (300 MHz, CDCl3): 7.39
(d
,
J
= 6.5 Hz, 2H); 7.27 (
d
,
J
= 6.5 Hz, 2H); 7.22 (
s
, 1H); 4.43 (
q
,
J
= 6.5 Hz, 2H); 4.05 (
q, J
= 6.5 Hz, 2H); 2.91 ( , 2H); 2.56 (
s
s
, 2H); 1.39 ( = 6.5 Hz, 3H); 1.11 (
t
,
J
s
, 6H); 1.01 ( = 6.5
t, J
Hz, 3H). 13CꢀNMR (75 MHz, CDCl3): 196.2; 168.4; 166.9; 145.1; 144.3; 137.9; 134.5; 131.8;
129.4; 128.6; 128.4; 128.2; 128.0; 62.0; 61.7; 52.6; 43.8; 33.7; 28.1; 13.9; 13.6. MS (70 eV)
m/z 428 (M+), 384, 358, 299, 264, 228. Anal. Calcd for C24H25ClO5: C, 67.21; H, 5.88.
Found: C, 67.38; H, 5.55.
Diethyl
6,6-dimethyl-8-oxo-3-(thiophen-2-yl)-5,6,7,8-tetrahydronaphthalene-1,2-
dicarboxylate (4h)
M.p. 105ꢀ107 ˚C. IR (KBr): 2963, 1726, 1683, 1586, 1367 cmꢀ1. 1HꢀNMR (300 MHz, CDCl3):
7.34 (dd, J = 1.0, 4.0 Hz, 1H); 7.05 (dd, J = 2.0, 2.5 Hz, 1H); 7.01ꢀ6.99 (m, 2H); 4.35 (q, J =

Page 13 of 17
13
6.0 Hz, 2H); 4.01 (
q
,
J
= 6.0 Hz, 2H); 2.83 (
s
, 2H); 2.47 (
s
, 2H); 1.31 (
t,
J
= 6.0, 3H); 1.03 (
t,
1
J
= 6.0, 3H); 1.02 (
m
, 6H). 13CꢀNMR (75 MHz, CDCl3): 196.2; 168.3; 167.2; 144.8; 140.0;
2
137.6; 133.9; 132.0; 130.3; 128.3; 127.8; 127.4; 127.3; 62.0; 61.9; 52.6; 43.7; 33.7; 28.1;
13.9; 13.7. MS (70 eV) m/z 400 (M+), 356, 328, 271, 172, 39. Anal. Calcd for C22H24O5S: C,
65.98; H, 6.04. Found: C, 66.11; H, 6.22.
3
4
5
Dimethyl
6,6-dimethyl-8-oxo-3-(
p
-tolyl)-3,4,5,6,7,8-hexahydronaphthalene-1,2-
6
dicarboxylate (5)
7
1HꢀNMR (300 MHz, CDCl3): 7.06 (
3.91 ( , 3H); 3.64 ( , 3H); 3.93ꢀ3.59 (
3.0, 18.0 Hz, 1H); 2.38ꢀ2.28 ( , 1H); 2.28 (
2.28 ( , 3H).
s
, 4H); 4.03 (dd
, 1H); 3.02 (dd
, 3H); 2.15 (
,
J
= 2.5, 8.5 Hz, 1H); 3.95ꢀ3.92 (
= 9.0, 18.0 Hz, 1H); 2.50 (dd
= 19.0 Hz, 1H); 2.28 ( , 3H);
m
, 1H);
8
s
s
m
,
J
,
J
=
9
m
s
d
,
J
s
10
11
12
13
14
15
16
17
18
19
20
21
22
s
X-ray crystal structure analysis of 4a
Crystallographic data for 4a has been deposited with the Cambridge Crystallographic Data
Center as supplementary publication no. CCDCꢀ1455609. Copies of these data can be
obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
[Fax: int. Code +44(1223)336ꢀ033; Eꢀmail: deposit@ccdc.cam.ac.uk or via
www.ccdc.cam.ac.uk/conts/retrieving.html].
Acknowledgments
Authors would like to thank Iran National Science Foundation (INSFꢀ92024196) and the
CCERCI research council for financial support of this work.
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Graphical Abstract
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Tandem aldol condensation-Diels–Alder-aromatization sequence of reactions: a new
pathway for the synthesis of 2-tetralone derivatives
M. Saeed Abaee, Esmail Doustkhah, Mohaddeseh Mohammadi, Mohammad M. Mojtahedi,
and Klaus Harms