Tetrahedron Letters
Facile intramolecular Diels–Alder reaction of 4-(o-propargyloxy)
styrylcoumarins leading to highly fluorescent coumarin-containing
polycyclic compounds
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Adil I. Khatri, Shriniwas D. Samant
Department of Chemistry, Institute of Chemical Technology, N.M. Parekh Marg, Matunga, Mumbai 400019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The 4-(o-propargyloxy)styrylcoumarins are prepared by the condensation of O-propargylated
salicylaldehyde with substituted coumarin-4-acetic acids. The intramolecular Diels–Alder reaction of
4-(o-propargyloxy)styrylcoumarins, without any catalyst gives fused-ring coumarins. The reaction in
boiling nitrobenzene leads to aromatization of the initial Diels–Alder adduct and these aromatized
products are highly fluorescent.
Received 22 October 2013
Revised 24 February 2014
Accepted 25 February 2014
Available online 12 March 2014
Ó 2014 Published by Elsevier Ltd.
Keywords:
4-(o-Propargyloxy)styrylcoumarins
Intramolecular Diels–Alder reaction
Polycyclic coumarins
Dehydrogenation
Fused-ring coumarins
Fluorescence
Coumarins are widely available in natural and synthetic prod-
ucts.1 In material science, they are employed in laser dyes,2 ions
and molecular sensors3 and polymers.4 Such compounds are
mostly polycyclic compounds containing a coumarin ring. Hence,
the synthesis of such polycyclic coumarins has attracted consider-
able attention.
benzo[c]coumarins which are fluorescent.8 This provided an idea
to attempt IMDA reaction of 4-styrylcoumarins containing a prop-
erly poised dienophilic moiety. The reaction is expected to provide
complex polycyclic compounds containing a coumarin moiety.
Further, such a reaction may involve interesting regioselection.
Thus, we are reporting here a carbocyclic IMDA on (E) 4-[2-[2-
(prop-2-yn-1-yloxyl)phenyl]ethenyl]-2H-1-benzopyran-2-ones
which provides novel highly fused ring coumarins. This is the first
Letter of IMDA of a coumarin with propargyl dienophile leading to
polycyclic compounds with a coumarin ring.
Coumarin-4-acetic acids (5a–h) were prepared by the Pech-
mann reaction of phenols (3a–h) and acetone dicarboxylic acid
(4) (prepared in situ by the reaction of citric acid and concd
H2SO4).9 Salicylaldehyde was O-propargylated by reacting it with
propargyl bromide in the presence of K2CO3 to obtain 6. Aromatic
aldehydes condense with coumarin-4-acetic acids, in the presence
of a base, to form 4-styrylcoumarins via Knoevenagel reaction fol-
lowed by decarboxylation.6d
Irrefutably, the Diels–Alder reaction has great potential to build
polycyclic compounds. Properly designed intramolecular Diels–Al-
der (IMDA) reaction may provide complex ring systems, which are
difficult to obtain by other routes. Moreover, IMDA reactions are
always more facile as compared to their intermolecular counter-
parts.5 Through IMDA reactions, highly functionalized fused-ring
systems containing a coumarin can be built. The Diels–Alder reac-
tions involving coumarin based dienophiles and dienes are
reported.6 Carbocyclic IMDA reaction on coumarin, where the pyr-
one ring in the coumarin moeity provides the diene component,
converts the pyrone ring into benzene ring through the expulsion
of CO2 from the adduct. Thus, even with a weaker dienophile like
propargyl group, attached to the 3-position through an ester func-
tion, that is propargyl ester of coumarin-3-carboxylic acid, under-
goes the IMDA reaction.7
The reaction of 6 and 5a–h using piperidine as a base gave (E) 4-
[2-[2-(prop-2-yn-1-yloxyl)phenyl]ethenyl]-2H-1-benzopyran-2-
ones (7a–h) (Scheme 1).14 In the 1H NMR spectrum of 7a, a triplet
at 2.59 ppm (4 J = 2.4 Hz) due to the acetylenic proton, a doublet at
4.82 ppm (4 J = 2.4 Hz) due to the methylene protons of the propar-
We have recently reported the intermolecular Diels–Alder
reaction of 4-styrylcoumarins with dienophiles leading to
gyl group, and a pair of doublets at
m = 7.66 and 7.45 ppm due to
the styryl group protons, with trans geometry (3 J = 16.2 Hz), were
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Corresponding author. Tel.: +91 22 3361 2606; fax: +91 22 2269 2102.
obtained.15 The structure was confirmed by the 1H–1H COSY
0040-4039/Ó 2014 Published by Elsevier Ltd.