Base-mediated alternate route for multi-functionalized allylbenzenes using ring transformation strategy

Article abstract of DOI:10.1080/00397911.2020.1782430

A simple yet innovative approach for the construction of multi-substituted allylbenzenes 14, 16 and 18 with ‘donor-acceptor’ functionalities has been delineated through base-mediated ring transformation of 6-aryl-2H-pyran-2-ones 13, 15 and 17 respectively

Full text of DOI:10.1080/00397911.2020.1782430

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Base-mediated alternate route for multi-
functionalized allylbenzenes using ring
transformation strategy
Samata E. Shetgaonkar & Fateh V. Singh
To cite this article: Samata E. Shetgaonkar & Fateh V. Singh (2020): Base-mediated alternate
route for multi-functionalized allylbenzenes using ring transformation strategy, Synthetic
Communications, DOI: 10.1080/00397911.2020.1782430
To link to this article: https://doi.org/10.1080/00397911.2020.1782430
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2020.1782430
Base-mediated alternate route for multi-functionalized
allylbenzenes using ring transformation strategy
Samata E. Shetgaonkar and Fateh V. Singh
Chemistry Division, School of Advanced Sciences, VIT University, Chennai, Tamil Nadu, India
ABSTRACT
ARTICLE HISTORY
Received 21 February 2020
A simple yet innovative approach for the construction of multi-sub-
stituted allylbenzenes 14, 16 and 18 with ‘donor-acceptor’ function-
alities has been delineated through base-mediated ring
transformation of 6-aryl-2H-pyran-2-ones 13, 15 and 17 respectively
with commercially available 5-hexen-2-one 11 as carbanion source.
The reactions were carried out at room temperature without using
any catalyst or organometallic reagents with excellent functional
group tolerance. Moreover, the present method is possible alternate
route to access allylbenzenes apart from usual metal-catalyzed cross-
coupling reactions.
KEYWORDS
Allylbenzene; 5-hexen-2-
one; 2H-pyran-2-ones; ring
transformation; second-
ary amines
GRAPHICAL ABSTRACT
Introduction
Alkenylbenzenes are prominent scaffolds in synthetic organic chemistry[1–3] and present
as key structural unit in many bioactive natural products.[4,5] For instance, phenylpropa-
noids containing allylbenzene as basic skeleton are widely found in various parts of
plants such as leaf, root and fruits exhibiting diverse biological activities (Figure 1).
Safrole 1 is a flavoring agent extracted from Sassafras, Cinnamomum and Piper plants[6]
while Myristicin 2 is a hallucinogen agent found in the fruits of nutmeg.[7] Another
derivative of phenylpropenes is dillapiole 3 isolated from piper aduncum, show anti-
inflammatory, fungicide, larvicide and insecticide activities.[8,9] Elimicin 4 obtained
CONTACT Fateh V. Singh
fatehveer.singh@vit.ac.in
Chemistry Division, School of Advanced Sciences, VIT
University, Chennai, Tamil Nadu, India.
Supplemental data for this article can be accessed on the publisher’s website
ß 2020 Taylor & Francis Group, LLC

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S. E. SHETGAONKAR AND F. V. SINGH
Figure 1. Natural products containing alkenylbenzene basic skeleton 1–9.
from Daucus carota L. and Peucedanum pastinacifolium respectively has significant anti-
bacterial and anti-viral effects.[10,11] Eugnol 5 and methyleugenol 6 are present in clove
(Syzygium aromaticum),[12] while chavibetol 7 and hydroxychavicol 8 are found in Piper
betle plant,[13–16] have been widely used as medicinal agents and building blocks for the
synthesis of bioactive compounds. Estragole 9 extracted from the fennel plant, is
famous for its flavors and fragrances in food and cosmetic industry.[17] Additionally, the
metal-catalyzed coordination–insertion copolymerization of olefins with allylbenzene as
monomers have been well studied.[18,19] Very recently, Ali et al[20] successfully utilized
naturally occurring safrole 1 and eugenol 5 to prepare catechol-based epoxy monomers
for the synthesis of mussel-inspired polymers. Also, alkenylbenzenes are precursors for
the synthesis of pharmaceutically important molecules such as amphetamine[21] and
CNS stimulant Prolintane.[22]
Most common and frequently employed methodology for the preparation of substi-
tuted allylarenes is metal-catalyzed cross-coupling reaction between aryl halides/organo-
metallic reagents and allylic alcohols or acetates or silanolate salts.[23–26] A ligand-free
approach comprising iron catalyzed cross-coupling reaction of allyl acetates with aryl
Grignard reagents has been developed by Wangelin’s research group.[27] In 2017, Bao
et al.[28] reported Pd-catalyzed coupling of chloromethylarenes with allyl pinacolborate
to prepare allylbenzenes using PPh3 as ligand and CsF as base. Later, Wang et al.[29]
performed pyridyl-directed ortho-C–H allylation of arenes with allylic amines catalyzed
by ruthenium. Chiummiento et al.[30] disclosed a non-classical synthetic route toward
polyoxygenated allylbenzenes featuring Claisen rearrangement of allyl aryl ether medi-
ated by Et2AlCl. Furthermore, the selective decarbonylation of 4-phenylbutyric acid to
corresponding allylbenzene using palladium catalyst has been developed.[31] Recently,

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Scheme 1. Synthesis of allylbenzenes 12 via ring transformation of 3-cyano-4-methylsulfanyl-2H-
pyran-2-one 10 with 5-hexen-2-one 11.
Dong et al.[32] synthesized allylbenzene via Rh-catalyzed oxidative dehydroxymethylation
of 4-phenyl-1-butanol. Although, there are several classical approaches available for
multi-substituted allylbenzenes, non-classical methods remain sparse. Moreover, nearly
all synthetic routes are associated with the use of expensive metal catalysts, ligands,
organometallic reagents or extreme reaction conditions. Thus, we focused on inventing
a mild, selective and commercially viable protocol to synthesize novel polysubstituted
allylbenzene derivatives. As our interest in the chemistry of 2H-pyran-2-ones contin-
ues,[33–36] we report herein a metal-free approach for the synthesis of highly functional-
ized allylbenzenes through base-mediated ring transformation of 2H-pyran-2-ones with
5-hexen-2-one in DMF at room temperature. The versatility of the current approach
lies in its ability to introduce different functional groups into the benzene moiety of
allylbenzenes efficiently.
Result and discussion
To begin with, we prepared precursor 2H-pyran-2-ones which are
fundamental synthons for the construction of diversified aromatic[37–42] and
heteroaromatic systems[43–46] with unique photophysical and biological properties. The
synthesis of 2H-pyran-2-ones have been carried out by the reaction of ethyl 2-cyano-
3,3-dimethylsulfanylacrylate with different aryl ketones under basic conditions to give 6-
aryl-3-cyano-4-methylsulfanyl-2H-pyran-2-ones 10 followed by amination with cyclic
secondary amines to give substituted 6-aryl-4-sec. amino-2H-pyran-2-one-3-carbonitriles
13.[47–50] Thus, in order to explore the synthetic utility and study the wider scope of the
lactone chemistry, we disclosed facile synthesis of multi-functionalized allylbenzene
derivatives in high yields through ring transformation strategy.
Initially, an independent reaction of 6-aryl-3-cyano-4-methylsulfanyl-2H-pyran-2-ones
10 with 5-hexen-2-one 11 was performed in the presence of KOH (1.2 equiv) in DMF
at room temperature (Scheme 1). The desired product 12 was obtained in 40% yield
along with many undesirable side products. Low yield of product 12 could be mainly
due to the good leaving ability of methylsulfanyl group thereby making C-4 position
more electrophilic as compare to C-6 position of lactone ring. As a result, C-4 carbon
becomes more susceptible to nucleophillic attack of carbanion which ultimately
gives rise to many unwanted side products.[51,52] Hence, in order to reduce the electro-
philicity at C-4 position, the SMe group of lactone 10 was replaced with various cyclic
secondary amines such as piperidine, morpholine and N-phenylpiperazine to give
6-aryl-cyano-4-amino-2H-pyran-2-ones 13.

4
S. E. SHETGAONKAR AND F. V. SINGH
Table 1. Optimization of base and solvent for the ring transformation of 2H-pyran-2-ones 13a with
5-Hexen-2-one 11.
Entry
Base
Solvent
Reaction time (h)
14a Yields (%)
1
2
3
4
5
6
7
8
9
KOH
KOtBu
K2CO3
Pyridine
Et3N
KOH
KOH
DMF
DMF
DMF
DMF
DMF
DMSO
MeOH
THF
10
6
65
62
20
15
traces
63
–
13
14
24
10
13
15
8
KOH
KOH
–
–
DCM
So next, using amine incorporated lactone 13a we perform ring transformation reac-
tion in presence of potassium hydroxide as a base in DMF at room temperature. The
anticipated ring transformed product 14a was obtained in 65% yield (Table 1, entry 1).
Next, in order to get the best reaction condition, we thought to perform the same reac-
tion using different bases and solvents. When the strong base KOtBu was used, a slight
decreased in product yield was observed (Table 1, entry 2). From Table 1, it is evident
that the reaction proceeded sluggishly in presence of weak bases like K2CO3, pyridine
and Et3N confirming KOH as the choice of base for the ring transformation reaction
(Table 1, entries 3–5). Moreover, changing the solvent media from DMF to DMSO gave
product 14a in almost similar yield (Table 1, entry 6). However, the reaction failed to
proceed in solvents such as MeOH, THF and DCM and unreacted starting materials
were recovered (Table 1, entries 7–9).
Further to study the scope of the present synthetic route, the ring transformation
reaction was performed with different 2H-pyran-2-ones 14 by varying substituents pre-
sent at C-4, C-5 and C-6 positions of the pyranone ring and the results are outlined in
Table 2. In the beginning, scope of the reaction was examined by introducing different
functional groups on the phenyl moiety present at C-6 position of the pyranone ring
(Table 2, entries 1–11). The reaction worked smoothly with halogen containing aro-
matic substituents and the desired products 14c–g were isolated in excellent yields
(Table 2, entries 3–7).
Further phenyl ring bearing electron donating groups such as OMe and Me were
also studied and the corresponding allylbenzenes 14 h–k were obtained in good yields
(Table 2, entries 8–11). Further when we introduced methyl group at C-5 position of
lactones 13 l–m, the ring transformed products were obtained in moderate yields

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Table 2. Synthesis of functionalized allylbenzenes 14 via ring transformation of 6-aryl-2h-pyran-2-
ones 13 with 5-hexen-2-one 11.
Entry
R
R1
Secondary amine
14
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
H
Me
Me
H
H
4-F
4-Cl
4-Cl
Piperidin-1-yl
Morpholino
Piperidin-1-yl
Piperidin-1-yl
Morpholino
14a
14b
14c
14d
14e
14f
14g
14h
14i
10
9
8
8
8
11
8
9
9
8
65
70
78
85
88
84
75
74
76
73
71
60
62
4-Br
piperidin-1-yl
4-Phenylpiperazin-1-yl
Piperidin-1-yl
2,4-Cl2
4-OMe
4-OMe
3,4-OMe
Me
9
Morpholino
Piperidin-1-yl
Piperidin-1-yl
Piperidin-1-yl
10
11
12
13
14j
14k
14l
10
13
9
H
4-Cl
4-Phenylpiperazin-1-yl
14m
Table 3. Synthesis of heteroaryl-cored allylbenzenes 16 via ring transformation of 6-heteroaryl-2H-
pyran-2-ones 15 with 5-Hexen-2-one 11.
Entry
Ar
Y
16
Time (h)
Yield (%)
1
2
3
4
5
2-Thienyl
2-Thienyl
2-Furanyl
2-Furanyl
2-Pyridyl
CH2
O
CH2
O
16a
16b
16c
16d
16e
10
9
9
10
11
72
74
73
70
78
O
(Table 2, entries 12–13). This could be because of the steric or electronic properties
of the substituent thereby affecting the reaction pathway. Thus, from the obtained
results we infer that presence of electron withdrawing groups in the phenyl moiety of
lactone 13 facilitates the ring transformation reaction as it makes C-6 carbon more

6
S. E. SHETGAONKAR AND F. V. SINGH
Table 4. Synthesis of naphthyl-cored allylbenzenes 18 via ring transformation of 6-naphthyl-2H-
pyran-2-ones 17 with 5-Hexen-2-one 11.
Entry
Ar
Y
18
Time (h)
Yield (%)
1
2
3
1-Nap
2-Nap
2-Nap
CH2
CH2
O
18a
18b
18c
13
11
10
73
76
77
electrophilic and enhances its reactivity toward ketone 11. Moreover, the reaction tol-
erated a wide range of functional groups thereby providing a platform for further
synthetic manipulations.
Furthermore, to show the versatility of this approach, we synthesized 6-heteroaryl-
2H-pyran-2-ones 15 and successfully utilized them in the ring transformation reaction
to furnish corresponding heteroaryl incorporated allylbenzenes in 70–78% yields (Table
3, entries 1–5). From Table 3, it is clear that reaction worked perfectly well with both
thienyl- and furanyl-cored lactones 15a–d and anticipated products were obtained in
70–74% yields (Table 3, entries 1–4). Next, 6-pyridyl-2H-pyran-2-one 15e was also syn-
thesized and employed in the ring transformation reaction to give targeted pyridyl-fused
allylbenzenes 16e in 78% yield (Table 3, entry 5).
Further in order to synthesize sterically hindered allylbenzene derivatives, we pre-
pared bulky 6-naphthyl-2H-pyran-2-ones 17a–c by reacting parent precursor ketene
dithioacetal with 1- or 2-acetonaphthone as carbanion source under basic condition
as mentioned earlier.[33] The reaction of bulky lactones 17a–c with 5-hexen-2-one 11
gave allylbenzenes derivatives in good yields (Table 4, entries 1–3).
Based on available literature,[38,40] the proposed mechanism for the synthesis of allyl-
benzenes 14 via ring transformation of 6-aryl-2H-pyran-2-ones 13 is shown in
Scheme 2. Reaction begins with the formation of carbanion under basic condition which
further attacks C-6 position of 2H-pyran-2-one 13 to form intermediate 19. Next, inter-
mediate 19 underwent intramolecular cyclization to generate bicyclic intermediate 20.
Subsequently, intermediate 20 furnishes allylbenzenes 14 upon decarboxylation and
dehydration.
In conclusion, we have developed an alternate route for the construction of multi-
functionalized allylbenzene derivatives 14, 16 and 18 through base-mediated carbanion-
induced ring transformation of 6-aryl-2H-pyran-2-ones with 5-hexen-2-one 11 as
carbanion source. This is a unique protocol for synthesizing complex allylbenzenes
functionalized with donor-acceptor “push-pull” system embedded in the aromatic ring.

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Scheme 2. Plausible mechanism for the Synthesis of allylbenzenes 14 via ring transformation of 6-
aryl-2H-pyran-2-ones 13 with 5-Hexen-2-one 11.
Further all the reactions were performed at room temperature without using any harsh
conditions or expensive catalyst or organometallic reagents. Moreover, a wide range of
substituents are compatible under the optimized reaction condition providing a direct
platform for further synthetic manipulations. Furthermore, the application of this
approach and the utility of allylbenzenes as substrates are currently in progress.
Experimental section
Multi-functionalized allylbenzenes 14a–m, 16a–e and 18a–c; general procedure
The powdered potassium hydroxide (1.2 equiv.) was added to a mixture of 6-aryl-2H-
pyran-2-ones 13 or 15 or 17 (1.0 mmol, 1.0 equiv.) and 5-hexen-2-one 11 (1.2 mmol,
1.2 equiv.) in dry DMF. The resulting reaction mixture was stirred at room temperature
for 8–13 h. The progress of reaction was monitored by thin layer chromatography
(TLC). After the completion of the reaction, ice-cold water (10 mL) was added and the
resulting mixture was neutralized with dilute HCl. The residue obtained was extracted
with ethyl acetate (3 ꢀ 10 mL). The combined organic layer was dried over NaSO4, fil-
tered and concentrated under reduced pressure. Purification of obtained crude products
through column chromatography on neutral alumina (EtOAc/hexane, 1:4) afforded
allylbenzenes 14 or 16 and 18.
2-Allyl-3-methyl-5-(piperidin-1-yl)-[1,1’-biphenyl]-4-carbonitrile (14a)
White solid; yield: 0.205 g (0.650 mmol, 65%); mp: 83–85 ꢁC.
IR (KBr): 2213 cmꢂ1 (CN).

8
S. E. SHETGAONKAR AND F. V. SINGH
1H NMR (CDCl3, 400 MHz): d ¼ 1.44–1.57 (m, 2H, CH2), 1.64–1.76 (m, 4H, 2CH2),
2.44 (s, 3H, CH3), 2.97–3.08 (m, 4H, 2NCH2), 3.15 (d, J ¼ 5.2 Hz, 2H, CH2), 4.67 (dd,
J1¼1.2 Hz, J2¼17.2 Hz, 1H, CH), 4.96 (dd, J1¼1.2 Hz, J2¼10.4 Hz, 1H, CH), 5.68–5.82
(m, 1H, 1CH), 6.66 (s, 1H, ArH), 7.13–7.23 (m, 2H, ArH), 7.27–7.36 (m, 3H, ArH).
13C NMR (CDCl3, 100 MHz): d ¼ 18.5, 24.1, 26.2, 34.0, 53.5, 107.3, 115.7, 118.1,
118.3, 127.5, 128.1, 128.5, 129.0, 136.2, 141.5, 142.9, 147.7, 155.4.
GC-MS: 316 (Mþ).
Anal. Calcd for C22H24N2: C, 83.50; H, 7.64; N, 8.85. Found: C, 83.22; H, 7.54;
N, 8.60.
1
Supporting information: Full experimental details, H and 13C NMR spectra.
Acknowledgments
We thank the SAIF department, VIT Vellore for spectrometric data. Fateh V. Singh is thankful
to VIT Chennai for providing seed grant to do this research work.
Funding
Authors are thankful to VIT Chennai for providing financial assistance.
ORCID
Fateh V. Singh
http://orcid.org/0000-0002-6358-4408
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