Superacid-Mediated Intramolecular Cyclization/Condensation: Facile One-Pot Synthesis of Spirotetracyclic Indanones and Indenes

Article abstract of DOI:10.1055/s-0036-1590816

A facile, superacid-promoted, domino, one-pot synthesis of novel spirotetracyclic indanones through intramolecular Friedel-Crafts acylation/alkylation of α,β-unsaturated cinnamic acid esters is presented. Interestingly, when the β-aryl group contained ele

Full text of DOI:10.1055/s-0036-1590816

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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–E
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A
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B. V. Ramulu et al.
Letter
Synlett
Superacid-Mediated Intramolecular Cyclization/Condensation:
Facile One-Pot Synthesis of Spirotetracyclic Indanones and
Indenes
Bokka Venkat Ramulu
O
COOEt
R1
Devarapalli Ravi Kumar
R1
TfOH, DCE
50 °C, 12 h
TfOH, DCE
50 °C,36 h
R1
R2
Gedu Satyanarayana*
R2
R2
Indian Institute of Technology Hyderabad, Kandi-502 285,
Sangareddy District, Telangana, India
gvsatya@iith.ac.in
6 examples
9 examples
up to 78% yield
up to 90% yield
R
1 = F, Cl & Br
R
1 = H, Me, Et, iPr & OMe
R
2 = H & Me
R
2 = H & Me
Received: 03.05.2017
Accepted after revision: 05.06.2017
Published online: 11.07.2017
DOI: 10.1055/s-0036-1590816; Art ID: st-2017-d0323-l
N
O
N
Abstract A facile, superacid-promoted, domino, one-pot synthesis of
novel spirotetracyclic indanones through intramolecular Friedel–Crafts
acylation/alkylation of α,β-unsaturated cinnamic acid esters is present-
ed. Interestingly, when the β-aryl group contained electron-withdraw-
ing substituents such as fluoro, chloro, or bromo, the reaction took a
different mechanistic path and afforded arylindenes as the end prod-
ucts.
Me
Me
Me
HO
N
1
2
OH
Key words superacids, Friedel–Craft reaction, spiro compounds, in-
Figure 1 A biologically important indene and spirocyclic indene
danones, indenes, domino reactions
procedure.5–7 However, few methods have been developed
for the synthesis of spirotetracyclic indanones or of in-
denes. In this context, Brewster and Prudence8 reported the
synthesis of optically active 1,1′-spirobiindane and related
compounds.
Various researchers have developed a rhodium(I)-cata-
lyzed spirocyclization reaction that includes a 1,4-rhodium
migration and provides a route to spirocyclic 1-indanones.9
In 2001, Hashimoto and co-workers10 described a catalytic
asymmetric synthesis of 1,1′-spirobi[indene]-3,3′(2H,2′H)-
dione through a double intramolecular C–H insertion pro-
cess, whereas Boblak and Klumpp11 reported superacid-
promoted cyclodehydrations leading to functionalized in-
denes. Some notable acid-catalyzed reactions for the syn-
thesis of spirocyclic compounds have also been reported from
the research groups of Wang,12a Chan,12b Stark,13 Padwa,14a
Daisley,14b Reddy,15 and Liang.16
Both indenes and spirotetracyclic indanones are privi-
leged motifs in pharmaceutically active compounds, and
these motifs are also present in naturally occurring com-
pounds. Spirocyclic compounds have an sp3 carbon atom
common to two rings and are considered important scaf-
folds for ready access to a variety of cyclic products.1 The
development of new synthetic approaches for the construc-
tion of spirocyclic compounds is an interesting and chal-
lenging task in organic synthesis. For example, a spirotetra-
cyclic indene constitutes the major cyclic core of the biolog-
ically active tertiary amine 1,2 a new framework for
estrogen receptor ligands, in which it is assumed that a se-
ries of related compounds might have unprecedented bio-
logical properties due to their novel structural characteris-
tics. In addition, the simple indene compound 2 [dimetin-
dene (Fenistil)] is a well-known antihistamine drug (Figure
1).3
In a continuation of our research into domino one-pot
processes, we have developed a superacid-promoted syn-
thesis of indanones, tetracyclic indoles, and indenones,
starting from simple cinnamic acid esters,17 and an efficient
method for the synthesis of spirotetracyclic dihydrocouma-
rins in the presence of Lewis acids.18 A mild and practical
The Friedel–Crafts reaction remains a predominant
method for C–C bond formation by aromatic electrophilic
substitution.4 Over the last few decades, developments have
continued to be made in this Brønsted/Lewis acid catalyzed
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

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B. V. Ramulu et al.
Letter
Synlett
method for the synthesis of (1,1-dimethyl-3-phenylbut-3-
enyl)benzenes and indanes has also been established.19
Herein, we report a superacid-mediated, efficient, domino
one-pot synthesis of novel spirotetracyclic indanones from
pendent β-aryl α,β-unsaturated ester derivatives. Unex-
pectedly, when the β-aryl group contained an electron-
withdrawing group such as fluoro, chloro, or bromo, the re-
action took a different mechanistic path and gave arylin-
denes20 as the final products. The key bond formations in
this strategy, particularly for the synthesis of spirotetracy-
clic indanones, are identical to those reported by Matsuda
et al.9e
more stable than the endo-substituted indenone17c because
the cyclopentadienone subunit of the endo-substituted in-
denone has antiaromatic character. The yield of spirotetra-
cyclic 1-indanone 9a increased on increasing the amount of
acid (entry 7) and on increasing the temperature (entry 8).
With less acidic methanesulfonic acid the reaction did not
proceed and the starting material 3a was recovered (entry
9). The reaction of 3a with the Lewis acid AlCl3 gave neither
the desired product 9a the recovered starting material (en-
try 10). The use of CHCl3 as a medium gave the product in
moderate yield (entry 11).
We began our study with the preparation of the pen-
dent β-aryl α,β-unsaturated esters 3 from benzaldehydes 4.
Thus, addition of vinylmagnesium bromide to the benzal-
dehydes 4 gave the corresponding allylic alcohols 5 in excel-
lent yields. Jeffery–Heck coupling21 of the allylic alcohols 5
with iodoarenes 7 furnished the corresponding 1,3-diaryl-
propan-1-ones 6 in good to very good yields. Finally, the
pendent β-aryl α,β-unsaturated esters 3 were synthesized
in excellent yields from the corresponding ketones 6 by us-
ing standard Horner–Wadsworth–Emmons conditions
(Scheme 1). Note that in most cases the cinnamic acid es-
ters 3 were obtained as inseparable mixtures of E- and Z-
isomers (see Supporting Information).
Table 1 Optimization of Conditions for the Synthesis of Spirocyclic
1-Indanone 9aa
COOEt
O
O
acid, solvent
heat
+
3a
8a
9a
Entry
Acid (equiv)
Solvent
Temp (°C) Time (h) Yieldb (%)
8a 9a
0c
1
2
PTSA (2)
TiCl4 (2)
H2SO4 (3)
TfOH (3)
TfOH (3)
TfOH (3)
TfOH (6)
TfOH (6)
MsOH (6)
AlCl3 (3)
TfOH (6)
DCE
r.t.
12
24
12
12
12
24
24
30
30
30
30
0c
0c
CH2Cl2
CH2Cl2
DCE
r.t.
0c
I
e
e
3
r.t.
–
–
R2
7
OH
CHO
4
0 to r.t.
r.t.
0c
0c
MgBr
Pd(OAc)2/TEA
R1
R1
5
DCE
60
20
0d
0d
0c
20
THF, 0 °C to r.t.
dry MeCN, 80 °C
6
DCE
50
40
60
76
4
5
7
DCE
50
COOEt
O
8
DCE
50
TEPA, NaH
THF, reflux
R1
R2
R1
9
DCE
50
0c
e
e
R2
10
11
DCE
50
–
–
3
6
CHCl3
50
0d
50
TEPA = triethyl phosponoacetate
a All reactions were carried out on a 0.25 mmol scale of (E)-3a in the appro-
priate solvent (2 mL).
Scheme 1 Synthesis of ethyl cinnamates 3
b Isolated yield of the chromatographically pure product.
c Only the starting material 3a was recovered.
d No 8a was formed.
With the pendent β-aryl α,β-unsaturated esters 3 in
hand, we explored their cyclization reaction with various
acids to identify the optimal conditions. The E-isomer of es-
ter 3a (obtained by careful separation through column
chromatography) was chosen as the model compounds for
this study. Initial attempts using PTSA or TiCl4 were unsuc-
cessful, leading to the recovery of the starting material (Ta-
ble 1, entries 1–4). Interestingly, the reaction with the
Brønsted superacid TfOH at room temperature gave the ex-
pected spirotetracyclic ketone 9a along with indenone 8a
(entries 5 and 6). Note that, in the case of the indene 8a, the
double bond isomerized to the exo-position. This might be
because the exo-cyclic indenone 8a is thermodynamically
e No products 8a/9a were obtained and none of the starting material 3a
was recovered.
Among the screened conditions, the conditions show in
Table 1, entry 8 were optimal. Therefore, these conditions
were applied to the other substrates 3b–i (as E-isomers or
as mixtures of E- and Z-isomers) to examine the scope and
limitations of the procedure. Gratifyingly, the method was
amenable to various substrates possessing a range of func-
tional groups on either aromatic ring, and it gave the corre-
sponding spirotetracyclic indanones 9a–i in very good to
excellent yields (Scheme 2).
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

C
B. V. Ramulu et al.
Letter
Synlett
O
COOEt
COOEt
R1
R1
TfOH (6 equiv)
TfOH (6 equiv)
R1
R2
R2
R1
R2
R2
DCE, 50 °C, 36 h
DCE, 50 °C, 12 h
3j–o
10j–o
3a–i
9a–i
F
F
Cl
O
O
O
Me
Me
Me
9a (76%)
9b (87%)
9c (78%)
10j (68%)
10k (70%)
10l (76%)
O
O
O
Cl
Cl
Br
Me
Et
Et
Me
Me
9d (86%)
9e (90%)
9f (84%)
Me
Me
Me
O
10m (78%)
10n (75%)
10o (77%)
O
O
Me
Me
Scheme 3 Synthesis of indenes 10j–o through intramolecular acyla-
tion of the pendent β-unsaturated esters 3j–o. All reactions were carried
out on a 0.25 mmol scale of 3j–o, and yields of chromatographically
pure products 10j–o are reported.
Me
MeO
9g (86%)
9h (85%)
9i (77%)
Scheme 2 Synthesis of spirotetracyclic indanones 9a–i through acid-
tation through path d to give the carbocation intermediate
E through liberation of ethyl acetate from the protonated
species D. Finally, removal of a proton from E leads to the
formation of indene 10.
promoted acylation/alkylation of 3a–i. All reactions were carried out on
0.25 mmol scale, and yields of the chromatographically pure products
9a–i are reported.
Surprisingly, however, when the β-aryl group contained
an electron-withdrawing group such as fluoro, chloro, or
bromo, the reaction took a different mechanistic path and
gave the corresponding arylindenes 10 as final products
(Scheme 3). This might be due to the lower reactivity of the
β-aryl moiety, such that it does not undergo acylation but
instead eliminates ethyl acetate to give the aryl indenes
10j–o.
In conclusion, we have described a superacid-promoted
intramolecular Friedel–Crafts acylation/alkylation of pen-
dent β-aryl α,β-unsaturated cinnamic acid esters for the
one-pot synthesis of novel spirotetracyclic indanones.
When the β-aryl group contains electron-withdrawing
groups such as fluoro, chloro, or bromo substituents, the re-
action gives arylindenes.
A plausible reaction mechanism for the formation of
spirocyclic indanones 9 and indenes 10 is shown in Scheme
4. Initially, the protic acid activates and isomerizes the car-
bonyl group of ester 3 to yield structure A. Double-bond
isomerization of A results in the formation of structure B
(path a). Intramolecular Friedel–Crafts acylation of B and
rearomatization leads to the indenone 8. Subsequent intra-
molecular Friedel–Crafts alkylation of indenone 8 with the
second arene affords the spirocyclic indanone 9. Alterna-
tively, the pendent arene in A might react through intramo-
lecular Friedel–Crafts alkylation to give indane ester C (path
b). If the β-aryl group of C does not possess electron-with-
drawing halo groups, it might undergo intramolecular acy-
lation through path c to furnish the spirocyclic indanone 9.
However, if the β-aryl group does possess an electron-with-
drawing halo group, it undergoes acid-mediated fragmen-
Funding Information
We are grateful to the Council of Scientific and Industrial Research
(CSIR), New Delhi, for financial support [No. 02(0262)/16/EMR-II].
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Acknowledgment
B.V.R. and R.K. thank the CSIR and UGC, New Delhi, for the award of
research fellowships. We also thank the reviewers for their useful
suggestions.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1590816.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

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B. V. Ramulu et al.
Letter
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O
O
OEt
a
OEt
H
H
R1
H
R2
R1
R2
b
3
A
path b
c
path a
O
H
O
O
EtO
R1
R1
path d
OEt
R2
OEt
R2
R1
H
R2
D
C
B
path c
CH3COOEt
O
O
R1
H
R1
R1
R1
R2
H
H
R2
R2
R2
9
10
E
8
Scheme 4 Plausible mechanism for the formation of the spirocyclic indanones 9 and indenes 10
References and Notes
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(22) Spirotetracyclic indanones 9; General Procedure
An oven-dried Schlenk tube was charged with the appropriate
β-aryl α,β-unsaturated ester 3a–i (0.25 mmol) and DCE (2 mL)
under N2. TfOH (0.1 mL, 1.5 mmol) was then added and the
mixture was stirred at 50 °C for 30–36 h until the reaction was
complete (TLC). The reaction was quenched with aq. NaHCO3,
and the mixture was extracted with CH2Cl2 (3 × 20 mL). The
organic layers were combined, washed with sat. brine, dried
(Na2SO4), filtered, and concentrated under reduced pressure.
The residue was purified by column chromatography (silica gel,
PE–EtOAc) to give the appropriate spirotetracyclic indanone 9a–
i (76–90%).
Hz, 1 H, ArH), 7.27 (d, J = 7.8 Hz, 1 H, ArH), 7.21 (d, J = 7.8 Hz, 1
H, ArH), 7.08 (d, J = 7.8 Hz, 1 H, ArH), 6.57 (s, 1 H, ArH), 3.19–
3.00 (m, 2 H, CH2), 2.94 (d, J = 18.6 Hz, 1 H, CHaHb), 2.87 (d,
J = 18.6 Hz, 1 H, CHaHb), 2.55–2.44 (m, 1 H, CH2), 2.40–2.30 (m, 1
H, CH2), 2.22 (s, 3 H, ArCH3). 13C NMR (100 MHz, CDCl3):
δ = 205.9 (s, C=O), 161.6 (s, ArC), 149.0 (s, ArC), 140.2 (s, ArC),
136.9 (d, ArC), 136.0 (s, ArC), 135.4 (d, ArCH), 128.1 (d, ArCH),
127.8 (d, ArCH), 125.1 (d, ArCH), 124.3 (d, ArCH), 123.3 (d,
ArCH), 123.0 (d, ArCH), 54.4 [s, C(CH2)2], 52.4 (t, CH2), 42.9 (t,
CH2), 30.8 (t, CH2), 21.2 (q, ArCH3). HRMS (APCI+): m/z [M + H]+
calcd for C18H17O+: 249.1274; found: 249.1279.
(23) Indenes 10; General Procedure
An oven-dried Schlenk tube was charged with the appropriate
β-aryl α,β-unsaturated ester 3j–o (0.25 mmol) and DCE (2 mL)
under N2. TfOH (0.1 mL, 1.5 mmol) was then added and the
mixture was stirred at 50 °C for 12 h until the reaction was
complete (TLC). The reaction was quenched with aq. NaHCO3
and the mixture was extracted with CH2Cl2 (3 × 20 mL). The
organic layers were combined, washed with sat. brine, dried
(Na2SO4), filtered, and concentrated under reduced pressure.
The residue was purified by column chromatography (silica gel,
PE–EtOAc) to give the appropriate indene 10j–o (68–78%).
3-(4-Chlorophenyl)-5-methyl-1H-indene (10n)
Brown viscous liquid; yield: 47 mg (78%). TLC: Rf (3n) = 0.45,
(10n) = 0.60 (PE–EtOAc, 97:3, UV detection). IR (MIR-ATR):
2920, 1726, 1613, 1487, 1393, 1288, 1093, 1014, 885, 803, 731
cm–1. 1H NMR (400 MHz, CDCl3): δ = 7.57 (dd, J = 8.3 and 2.0 Hz,
2 H, ArH), 7.47 (dd, J = 8.3 and 2.0 Hz, 3 H, ArH), 7.38 (s, 1 H,
ArH), 7.14 (d, J = 8.3 Hz, 1 H, ArH), 6.58 (t, J = 2.0 Hz, 1 H, CH=C),
3.49 (d, J = 2.0 Hz, 2 H, CH2), 2.45 (s, 3 H, ArCH3). 13C NMR (100
MHz, CDCl3): δ = 144.0 (s, ArC), 143.7 (s, ArC), 141.7 (s, ArC),
135.8 (d, ArC), 134.6 (s, ArC), 133.3 (s, ArC), 131.7 (d, ArCH),
129.0 (d, 2 C, ArCH), 128.7 (d, 2 C, ArCH), 125.9 (d, ArCH), 123.9
(d, ArCH), 120.7 (d, CH=C), 37.8 (t, CH2), 21.6 (q, ArCH3). HRMS
(APCI+): m/z [M + H]+ calcd for C16H14Cl+: 241.0779; found:
241.0787.
6′-Methyl-2′,3′-dihydro-1,1′-spirobi[inden]-3(2H)-one (9b)
Brown viscous liquid; yield: 54 mg (87%); TLC: Rf (3b) = 0.50,
(9b) = 0.60 (PE–EtOAc, 96:4, UV detection). IR (MIR-ATR): 2922,
2851, 1710, 1601, 1461, 1287, 1236, 816, 763 cm–1 1H NMR
.
(400 MHz, CDCl3): δ = 7.79 (d, J = 7.8 Hz, 1 H, ArH), 7.57 (ddd,
J = 7.3, 7.3, and 1.0 Hz, 1 H, ArH), 7.41 (ddd, J = 7.3, 7.3, and 1.0
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E