DBU-Catalyzed Rearrangement of Secondary Propargylic Alcohols: An Efficient and Cost-Effective Route to Chalcone Derivatives

Article abstract of DOI:10.1055/s-0040-1707909

A 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-catalyzed rearrangement of diarylated secondary propargylic alcohols to give α,β-unsaturated carbonyl compounds has been developed. The typical 1,3-transposition of oxy functionality, characteristic of Mayer-Schuster rearrangements, is not observed in this case. A broad substrate scope, functional-group tolerance, operational simplicity, complete atom economy, and excellent yields are among the prominent features of the reaction. Additionally, the photophysical properties and crystal-structure-packing behavior of selected compounds were investigated and found to be of interest.

Full text of DOI:10.1055/s-0040-1707909

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© Georg Thieme Verlag Stuttgart · New York
2020, 31, A–F
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R. De et al.
Letter
Synlett
DBU-Catalyzed Rearrangement of Secondary Propargylic Alcohols:
An Efficient and Cost-Effective Route to Chalcone Derivatives
Rimpa Dea
O
OH
Antony Savarimuthub
Tamal Ballava
10 mol% DBU
CH3CN, 80 °C
R1
R1
R2
Pijush Singha
R1 = aryl, polyaryl, hetaryl; R2 = H, Me
R2
Jayanta Nandaa
Avantika Hasijac
Deepak Choprac
Broad substrate scope
Mild reaction conditions
Metal free
Lewis acid free
60–90% yield
21 examples
Mrinal K. Bera*a
0
0
0
0-0
0
0
3-
4
3
8
2-6
8
4
1
a Department of Chemistry, Indian Institute of Engineering Science and
Technology (IIEST), Shibpur P.O.-Botanic Garden, Howrah-711 103 (WB),
India
b Department of Chemistry, St. Xavier’s College (Autonomous), Palayam-
kottai, Tamil Nadu-627 002, India
c Department of Chemistry, Indian Institute of Science Education and
Research (IISER) Bhopal, Bhauri, Bhopal-462066, India
mrinalkbera26@gmail.com
Received: 06.05.2020
Accepted after revision: 22.06.2020
Published online: 24.07.2020
pounds are among the most sought-after synthetic inter-
mediate in the area of synthetic organic chemistry, as they
are extensively employed in syntheses of a variety of het-
erocyclic compounds, including pyridines,4 pyrimidines,5
imidazoles,6 pyrazoles,7 triazoles,8 pyrazolines,9 isooxaz-
oles,10 and many more.
DOI: 10.1055/s-0040-1707909; Art ID: st-2020-v0272-l
Abstract A 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-catalyzed rear-
rangement of diarylated secondary propargylic alcohols to give ,-un-
saturated carbonyl compounds has been developed. The typical 1,3-
transposition of oxy functionality, characteristic of Mayer–Schuster re-
arrangements, is not observed in this case. A broad substrate scope,
functional-group tolerance, operational simplicity, complete atom
economy, and excellent yields are among the prominent features of the
reaction. Additionally, the photophysical properties and crystal-struc-
ture-packing behavior of selected compounds were investigated and
found to be of interest.
OH
O
O
HO
OMe
OH
Xanthohumol
HO
OH
OH CO2H
O
Isoliquiritigenin
O
Key words rearrangement, propargylic alcohols, DBU, chalcones,
allenols, organocatalysis
OMe O
O
O
Sofalcone
Chalcone compounds with a characteristic 1,3-diaryl-
prop-2-en-1-one chemical scaffold are frequently found in
naturally occurring substances, and have a widespread dis-
tribution in various plants and herbs.1 Many of these natu-
rally available compounds show numerous promising bio-
logical activities, including anticancer activity,2a cancer-
preventive effects,2b antibacterial,2c antimalarial,2d antiin-
flammatory,2e antiviral,2f anti-HIV,2g antileishmanial,2h and
neuroprotective effects2i, among others. Even, a single chal-
cone derivative can demonstrate multiple types of bioactiv-
ity.3 Some representative examples of bioactive chalcones
(isoliquiritigenin and xanthohumol) and clinically approved
chalcone-based drugs (metochalcone and sofalcone) are
shown in Figure 1. Apart from their biological significance,
chalcones and other related ,-unsaturated carbonyl com-
MeO
OMe
Metochalcone
Figure 1 Bioactive chalcones and clinically approved chalcone-based
marketed drugs
Because of their therapeutic potential and their versatil-
ity as organic synthons, chalcones and other related ,-un-
saturated carbonyl compounds have long been considered
to be privileged structural units, and considerable efforts
have been devoted to developing efficient methods for their
synthesis. Conventionally, chalcones and related ,-unsat-
urated carbonyl compounds are synthesized through
Claisen–Schmidt condensations,11 Wittig reactions,12 Julia–
Kocienski olefinations,13 Friedel–Crafts acylations,14 and
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
R. De et al.
Letter
Synlett
various C–C cross-coupling reactions, such as Suzuki–
Miyaura15 and Heck couplings.16 Chalcone derivatives can also
be prepared from propargylic alcohols, which can undergo
molecular rearrangements under basic conditions to pro-
duce chalcones.17 Probably, the most extensively used
method for synthesizing chalcone derivatives is the Meyer–
Schuster (M–S) rearrangement (Scheme 1),18 an acid/tran-
sition-metal-catalyzed rearrangement of propargylic alco-
hols to chalcone derivatives. Although the M–S rearrange-
ment was originally catalyzed by protic acids, several tran-
sition-metal-catalyzed variants have recently been
developed (Scheme 1).19 Even propargylic acetates have
been shown to be excellent substrates for transition-metal-
catalyzed M–S rearrangements20 to produce chalcone and
-halochalcone derivatives (Scheme 1).
tive (entry 9). Because DBU was found to be an efficient
base for promoting the rearrangement reaction, we next
examined the reaction with reduced amounts of DBU. We
therefore examined the use of 50 and 20 mol% of the base
under similar reaction conditions; in both cases, we ob-
tained comparable yields, but the reaction time increased
to 12 hours (entries 10 and 11). The amount of base could
be reduced to less than 10 mol% without compromising the
yield (entry 12), whereas increasing the amount of base did
not have any beneficial effect (entry 13). Although the reac-
tion time is increased, a reduction in the amount of organic
base to 10 mol% is highly advantageous, as it reduces chem-
ical waste considerably, making the method more compati-
ble with environmental issues. We therefore consider the
conditions shown in entry 12 as the optimal conditions for
the reaction.
R1
O
R1, R2, R3 = H, alkyl, aryl
R3 R4 = H
2
R2
Table 1 Optimization of the Reaction Conditions
1
3
conventional M–S reaction
Ref. 18a
O
OH
H
base, solvent
OR4
1
R1
R2
R1
O
R1
O
2
80 °C, 6–18 h
sealed tube
Ru, Au, Ag
Au
2
1a
2a
3
2
R2
R3
3
1
R2
R3
Pd
Ref. 19
Ref. 20
1
3
R3
X
R1, R2 = alkyl, aryl
R3 = H, alkyl, aryl
R1, R2, R3 = alkyl, aryl
R4 = Ac; X = H, I
Entry
Base
Et3N
mol%
Solvent
Yield (%)
DBU (cat.)
O
Our approach
1
2
100
100
100
100
100
100
100
100
100
50
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
THF
NRa
NR
NR
NR
NR
85
50
NR
NR
86
84
85
85
R1, R3 = aryl, polyaryl, hetaryl
R2, R4 = H
2
i-Pr2NH
DIPEA
pyridine
imidazole
DBU
R1
R3
1
3
3
Scheme 1 Synthesis of (E)-chalcones from propargylic alcohols and ac-
4
etates
5
6
However, transition-metal catalysts are expensive, and
the related methods have their issues. Therefore, the devel-
opment of a simple and cost-effective method for the syn-
thesis of chalcone derivatives is required. Here, we report a
simple DBU-catalyzed metal- and acid-free approach lead-
ing to chalcone derivatives from secondary propargylic al-
cohols. Unlike the M–S rearrangement, a 1,3-transposition
of oxy functionality is not associated with this protocol. The
newly developed method might be successfully employed
to access many a range of substituted chalcones potentially
useful for various purposes. For our preliminary studies, we
selected 1,3-diphenylprop-2-yn-1-ol (1a) as our model
substrate. Initially, a variety of organic bases (triethylamine,
diisopropylamine, N,N-diisopropylethylamine, pyridine,
and imidazole) were tested as promoters for the reaction in
CH3CN as the solvent. None of the desired product 2a was
detected, even after six hours of heating at 80 °C in a sealed
tube (Table 1, entries 1–5). Next, we used DBU as a base,
and we were delighted to find that all the starting material
1a was consumed within six hours and that the desired
chalcone 2a was obtained in 85% yield (entry 6). DBU was
further screened as the base in various solvent systems un-
der heating conditions, but poorer results were obtained
(entries 7 and 8). Again, Et3N in THF proved totally ineffec-
7
DBU
8
DBU
9
Et3N
THF
10
11
12
13
DBU
CH3CN
CH3CN
CH3CN
CH3CN
DBU
20
DBU
10
DBU
125
a NR = no reaction.
Having established the optimal reaction conditions, we
turned our focus on exploring the substrate scope of the re-
action. For this purpose, we synthesized a series of second-
ary propargylic alcohols 1a–w from various aromatic alde-
hydes and lithiated phenylacetylene by employing slightly
modified version of the reported procedure.21 Propargylic
alcohols 1b–g, prepared from alkyl-substituted benzalde-
hydes, when subjected to the optimal reaction conditions,
gave the corresponding chalcone 2b–g in excellent yields
(Scheme 2). Moreover, propargylic alcohols 1h–k, prepared
from various methoxylated benzaldehydes proved to be ex-
cellent substrates for the present reaction, giving the corre-
sponding chalcone derivatives 2h–k. Chloro- and bromo-
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F

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R. De et al.
Letter
Synlett
substituted propargylic alcohols 1l and 1m, respectively,
were smoothly transformed into the corresponding chal-
cones 2l and 2m, albeit with slightly inferior yields. Hetero-
cycle-containing propargylic alcohols 1n and 1o readily re-
acted under the optimized conditions to afford chalcones
2n and 2o, both in 88% yield. As expected, propargylic alco-
hols with polycyclic aryl or benzyloxy substituents 1p–s af-
forded the corresponding chalcones 2p–s in good to excel-
lent yields. Propargylic alcohols 1t and 1u prepared from 1-
ethynyl-4-methylbenzene were found to be excellent sub-
strates, affording the correspond chalcones 2t and 2u in
yields of 86 and 67%, respectively. Unfortunately, propargyl-
ic alcohols 1v and 1w, prepared from isobutyraldehyde and
acetaldehyde, respectively, were found to be unresponsive
under the standard reaction conditions, and chalcones 2v
and 2w were not detected.
ineffectiveness of 1v and 1w as potential substrates indi-
rectly supports the mechanistic proposal shown in Scheme
2.
O
Ph
O
Ph
OH
N
N
2a
Ph
H
Ph
1a
Ph
•
Ph
OH
C
H
Ph
DBU-H
Ph
A
OH
Ph
•
DBU-H
Ph
B
Scheme 3 A plausible mechanism of the rearrangement reaction
Most of the chalcone derivatives were solid, and we at-
tempted to obtain crystal structures of some of the prod-
ucts for structural confirmation and to obtain mechanistic
insight. Compound 2s crystallized from 20% ethyl acetate–
hexane as white crystals suitable for X-ray analysis (Figure
2).22 Crystal-structure determination of 2s not only con-
firmed its structure unambiguously, but also confirmed
that the reaction did not follow the usual M–S pathway, as
the typical 1,3-transposition of oxy- functionality was not
observed.
Among all the various products, 2r and 2u contain
large--surface-area aromatic pyrene groups. Pyrene-based
compounds often self-assemble through strong – stack-
ing interactions.24 It was therefore interesting to examine
the self-assembly of these chalcone derivatives in the solid
and solution states. The self-assembly behavior of 2u in the
solid state was examined by single-crystal X-ray analysis by
using a suitable crystal obtained by slow evaporation in a
pentanol medium.22 Detailed crystallographic information
is presented in Table S2 of the Supporting Information (SI).
Chalcone 2u crystallizes in a triclinic crystal system with a
P1 space group. In single packing (Figure S1, SI), two mono-
mers are present, and the pyrene rings interact through ar-
omatic–aromatic interactions in antiparallel fashion. In a
higher-order assembly (Figure 3), we also noticed an aro-
matic–aromatic interaction of the phenyl rings, which is re-
sponsible for the antiparallel arrangement. Here, the
pyrene–pyrene and phenyl–phenyl ring distances are 4.321
and 4.272 Å, respectively. Moreover, the CH– interaction
(3.178 Å) between the pyrene ring surface and the H-22 hy-
drogen of the phenyl ring brings the two aromatic surfaces
close to one another. Additional two hydrogen-bonding in-
teractions (C–H···O) were observed involving a pyrene hy-
drogen [H8···O1] and a phenyl-ring hydrogen [H25···O1]
with distances of 2.372 Å and 2.499 Å, respectively. Here
the aromatic–aromatic interaction mainly helps growth in
one dimension, whereas the hydrogen-bonding interaction
helps growth in a plane.
O
O
2s
Figure 2 ORTEP diagram of compound 2s (CCDC 1998081)22
Mechanistically the current isomerization/rearrange-
ment reaction is quite interesting. The propargylic alcohol–
enone isomerization reaction proceeds through three ele-
mentary steps.23a Slow deprotonation of propargylic alco-
hol 1a produces propargylic carbanion A which is trans-
formed into the allenol intermediate C via intermediate B
(Scheme 3). Finally, a keto–enol tautomerism of C produces
enone 2a, and DBU is regenerated to initiate another cata-
lytic cycle (Scheme 2). If the proposed mechanism is ac-
ceptable, an allenol derivative might be expected to form as
an end-product from an appropriately protected propargyl-
ic alcohol. In fact, a silyloxyallene derivative has been iden-
tified in a similar base-catalyzed reaction,23b further vali-
dating our mechanistic proposal. Aliphatic propargylic al-
cohols such as 1v and 1w were found to be incompatible
under the standard reaction condition. There are two possi-
ble reasons for this: either a corresponding propargylic car-
banion similar to A is not generated from 1v or 1w, or the
carbanion is unstable due to the absence of a resonance ef-
fect from the aromatic ring (as present in A). Therefore, the
We had then studied the photophysical properties of
the large--surface-containing pyrene-based chalcone 2u
by means of UV-visible absorption spectroscopy and fluo-
rescence spectroscopy. The absorption and emission spec-
tra of 2u were investigated in two common organic solvents
(MeOH and DMSO). In the absorption spectra (Figure 4a),
shorter-wavelength (250–300 nm) and longer-wavelength
(350–440 nm) peaks are attributed to –* transitions of
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F

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R. De et al.
Letter
Synlett
O
OH
10 mol% DBU
CH3CN
80 °C, 18 h
sealed tube
2a
1a
R2
R1
R1
R2
60–90%
Substrate
Product
Substrate
Product
OH
O
OH
O
Ph
Ph
Ph
Ph
1a
2a, 85%
1b
2b, 86%
OH
O
O
O
OH
O
Ph
Ph
Ph
Ph
Ph
Ph
1c
2c, 78%
1d
2d, 82%
OH
OH
O
O
O
Ph
Ph
Ph
Ph
Ph
Ph
Et
Et
iPr
iPr
1e
2e, 87%
1f
2f, 86%
OH
OH
Ph
Ph
tBu
tBu
MeO
MeO
MeO
MeO
1g
2g, 85%
1h
2h, 88%
OH
OH
O
O
MeO
MeO
Ph
Ph
Ph
Ph
1i
2i, 85%
1j
2j, 87%
OMe
OMe
OH
OH
O
Ph
Ph
1k
OMe
2k, 89%
1l
2l, 71 %
MeO
MeO
Cl
Cl
OMe
OH
O
OH
O
Ph
Ph
Ph
O
Ph
Ph
O
1m
2m, 75 %
1n
2n, 88%
Br
Br
OH
O
O
OH
O
O
Ph
Ph
1o
2o 88%
1p
2p, 82%
S
Ph
Ph
S
OH
OH
Ph
Ph
Ph
1q
2q, 86%
1r
2r, 60%
OH
O
O
Ph
Ph
OH
O
1s
2s, 91%
BnO
BnO
OH
1t
2t, 86%
OH
O
1u
2u 67%
Ph
Ph
1v
OH
O
2v, 0%
Ph
1w
2w, 0%
Ph
Scheme 2 Substrate scope of the reaction
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F

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R. De et al.
Letter
Synlett
the phenyl ring and pyrene ring, respectively.25 In the emis-
sion spectra, a strong emission band of 2u was observed in
both solvents.
fully atom-economical process can be used to prepare
many chalcone derivatives with complex molecular archi-
tectures. The photophysical properties and molecular ar-
rangement in the crystal state of a few selected compounds
were examined successfully. Moreover, the complete atom
economy, mild reaction conditions, operational simplicity,
and broad functional-group tolerance make this method at-
tractive.
Funding Information
Financial support from CSIR New Delhi [Grant No: 02(0270)/16/EMR-
II] is most gratefully acknowledged.
C
S
I
R
N
e
w
D
e
l
h
i (02(0
2
7
0)/16/E
M
R-I
)
Acknowledgment
We sincerely thank the Sophisticated Analytical Instruments Facility
(SAIF), IIEST Shibpur for single-crystal X-ray analysis. J.N. gratefully
acknowledges a DST-INSPIRE Faculty Grant (IFA16-CH246).
Supporting Information
Supporting information for this article is available online at
Figure 3 (a) ORTEP diagram of compound 2u (CCDC1998036),22 (b)
aromatic–aromatic pyrene–pyrene and phenyl–phenyl interactions,
and (c) higher-order packing of 2u.
https://doi.org/10.1055/s-0040-1707909.
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References and Notes
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4b). It is interesting to note that the emission of 2u is de-
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the more-polar solvent.
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(27) Chalcones 2a–u; General Procedure
DBU (10 mol%) was added to a solution of the appropriate prop-
argylic alcohol 1 (1.0 equiv) in dry MeCN (0.2 M) in a sealed
tube, and the solution was mixed well by manual shaking. N2
gas was flashed into the tube, and the cap was quickly closed.
The sealed tube was placed in an oil bath at 80 °C, and the
mixture was stirred for 18 h until the substrate was completely
consumed (TLC). The mixture was then allowed to cool to r.t.
and the reaction was quenched with H2O (10 mL). The product
was extracted with Et2O (3 × 20 mL), and the combined organic
layers were washed with brine (10 mL), dried (Na2SO4), and
concentrated under reduced pressure. The crude product was
purified by column chromatography [silica gel (100–200 mesh),
PE–EtOAc (10:1)].
(2E)-1,3-Diphenylprop-2-en-1-one (2a)
White solid; yield: 43.2 mg (86%); mp 54–56 °C. IR (ATR): 3059
(=CH), 1658 (C=O), 1598 (C=C) cm–1. 1H NMR (400 MHz, CDCl3):
 =8.02 (d, J = 8.0 Hz, 2 H), 7.81 (d, J = 16.0 Hz, 1 H), 7.63 (dd, J =
8.0, 4.0 Hz, 2 H), 7.59–7.48 (m, 4 H), 7.42–7.40 (m, 3 H). 13C
NMR (100 MHz, CDCl3):  = 190.6, 144.9, 138.2, 134.9, 132.9,
130.6, 129.0, 128.7, 128.6, 128.5, 122.2.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–F