Alkaline earth metal catalyzed cascade, one-pot, solvent-free, and scalable synthesis of pyranocoumarins and benzo[b]pyrans

Article abstract of DOI:10.1016/j.tetlet.2015.02.023

Ca(OTf)₂ catalyzed, cascade, one-pot, conjugate addition and annulation of various nucleophiles to the α,β-unsaturated carbonyl compounds for the synthesis of biologically potent pyranocoumarins and benzo[b]pyrans has been illustrated. This one

Full text of DOI:10.1016/j.tetlet.2015.02.023

Accepted Manuscript
Alkaline earth metal catalyzed cascade, one-pot, solvent-free and scalable syn-
thesis of pyranocoumarins and benzo[b]pyrans
Srinivasarao Yaragorla, Pyare Lal Saini, Garima Singh
PII:
S0040-4039(15)00282-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.02.023
TETL 45891
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
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5 February 2015
7 February 2015
Please cite this article as: Yaragorla, S., Saini, P.L., Singh, G., Alkaline earth metal catalyzed cascade, one-pot,
solvent-free and scalable synthesis of pyranocoumarins and benzo[b]pyrans, Tetrahedron Letters (2015), doi: http://
dx.doi.org/10.1016/j.tetlet.2015.02.023
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Graphical Abstract
Alkaline earth metal catalyzed cascade, one-pot,
solvent-free and scalable synthesis of pyranocoumarins
and benzo[b]pyrans
Leave this area blank for abstract info.
Srinivasarao Yaragorla,* Pyare Lal Saini and Garima Singh

1
Tetrahedron Letters
journal homepage: www.els evier.com
Alkaline earth metal catalyzed cascade, one-pot, solvent-free and scalable synthesis of
pyranocoumarins and benzo[b]pyrans
Srinivasarao Yaragorla,∗ Pyare Lal Saini and Garima Singh
aDepartment of Chemistry, Central University of Rajasthan, Bandarsindri, NH-8, Ajmer-distt., Rajasthan, 305817, India.
ARTICLE INFO
ABSTRACT
Article history:
Received
Ca(OTf)2 catalyzed, cascade, one-pot, conjugate addition and annulation of various nucleophiles
to the α,β-unsaturated carbonyl compounds for the synthesis of biologically potent
pyranocoumarins and benzo[b]pyrans has been illustrated. This one-pot, solvent-free, green-
synthesis employs simple reaction conditions, with high yields and endures the substrate
diversity.
Received in revised form
Accepted
Available online
Keywords:
2009 Elsevier Ltd. All rights reserved.
Pyranocoumarins
Benzopyrans
Ca(II) catalysis
Solvent-free synthesis
Cascade synthesis
electrophilic partners for the synthesis of pyranocoumarins and
cyclohexane-1,3-diones as the nucleophile in case of
benzopyran’s synthesis.8 Literature reports for the synthesis of
pyranocoumarins are known with strong acid conditions like I2-
H2SO4,5a I2-AcOH,5b expensive transition metal catalysts like,
Ru,5c AuCl3-AgOTf5d in toluene, Bi(OTf)3,5e Cu(OTf)2,5f
Pyran fused heterocyclic compounds such as pyranocoumarins
and benzo[b]pyrans (Figure 1) are important class of structural
motifs present in many natural products1 and synthetic
compounds,2 which shows a spectrum of biological activities. For
example pyrano[3,2-c]coumarins exhibit antifungal, insecticidal,
anticancer, anti-HIV, anti-inflammatory, and antibacterial
activities,3 currently they are also focused for the treatment of
skin disorders.1g Similarly, benzo[b]pyrans or 2,4-diaryl-4H-
chromen-5-ones are known to possess anticancer activity,
antijuvenilehormone activity, antiallergic, antianaphylactic
activity in trachea and anti-inflammatory activities.4
stoichiometric loading of DDQ,5g toxic reagents,5h, 5i, 5j use of less
5m
available catalysts,5k substrates5l,
and with poor yields.5n
Similarly, benzopyrans or 2,4-diaryldihydropyrans were
synthesized by using acetic acid and phosphorouspentaoxide at
o
120 C,8a triethylamine at reflux in methanol,8b anhydrous ZnCl2
in toluene n-heptane,8c anhydrous ZnCl2 in benzene n-heptane
and at reflux temperature for 28-30 h,8d BF3 etherate,8e InBr3,8f
ZnCl2/montmorillonite-K-10,8g FeCl3.6H2O in ionic liquid,8h
hydrated Ytterbium triflate,8i sulfated tin oxides,8j InCl3.4H2O.8k
Though some of these methods are efficient, most of them are
suffering from the several limitations such as longer reaction
time, strong acidic conditions, use of expensive catalysts, toxic
conditions, regioselectivity and poor yields. In view of their
biological importance, interesting structural features and also due
to the above mentioned drawbacks of some of the existing
methods, it is highly desired to develop a simple, economic and
environmentally benign methodology for the synthesis of both
pyranocoumarins and benzo[b]pyrans.
R1
O
R1
O
R2
R
O
(b)
R2
O
O
R
(a)
Figure 1. representative structures of (a) pyrano[3,2-c]coumarin
and (b) benzo[b]pyran; R1, R2 = aryl or alkyl, R= H/CH3
Owing to the broad spectrum of biological properties and
natural occurrence, pyrano[3,2-c]coumarins and benzo[b]pyrans
have become attractive synthetic targets for medicinal chemistry
point of view. The common synthetic strategy for these
compounds have been commenced with the cyclization of readily
available 4-hydroxy coumarin as nucleophile and α, β-
unsaturated carbonyl compounds (generally chalcones),5 1,3-
Recent year’s alkaline earth metal catalysts have demonstrated
as an alternative to transition metal and lanthanide based
catalysts, 10, 11 owing to their wide natural abundance, inexpensive
and non-toxic natures. Among these alkaline earth metal
catalysts, calcium salts bearing a hard conjugate base such as
diarylallylic compounds,6 propargylic alcohols,7 as the
———
∗ Corresponding author. Tel.: +91-463-238535; fax: +91-463-238722; e-mail: Srinivasarao@curaj.ac.in

2
Tetrahedron
triflate ion is highly stable to moisture and air, nevertheless
very few reports have been available about the applications of
92% yield (Table 1, entry 10). After having the optimum
conditions in hand our next goal is to check the generality of this
reaction with various α,β-unsaturated ketones. Hence under the
similar reaction conditions, 4-hydroxycoumarin 1, was treated
with various chalcones bearing halogen substitutions (Table 2,
entries, 3b, 3c, 3e, 3g, 3j, 3l), electron withdrawing groups (3d,
3h), electron donating group (3i), both halogen substituent and
electron donating groups (3f, 3k) and found that all of them are
taking part in the reaction to produce corresponding
pyranocoumarins. We may notice the difference in the yields and
may be in the reaction time also (but generally all these reactions
take place in 3-4 h) this could be due to the steric and electronic
factors around the reacting partners, but overall all of them were
12
Ca(OTf)2 in organic synthesis10a, including our recent Ritter
reaction for the green synthesis of amides.9a In continuation of
our research aimed towards the development of sustainable
synthetic methodologies,9 herewith we report Ca(II) catalyzed
one-pot, solvent-free and cascade synthesis of pyrano[3,2-
c]coumarins and banzo[b]pyrans.
Table 1. Optimization of reaction conditions for the cascade
synthesis of pyrano[3,2-c]coumarins.
giving good to excellent yields (Table 2, entries 3a-
3l).
Entr Ca(OTf)2 Bu4NPF6
y
Conditions
Yield
(mol %)
(mol %)
1
10
10
CH2Cl2, reflux,
24 h
No
reaction
2
3
4
5
10
10
10
10
10
10
10
10
Ethanol, reflux,
24 h
No
reaction
As we know that the large scale reactions are very important
towards industrial processes and also for the wide screening of
biological activities, we need to have environmentally benign and
economically favored synthetic process that can produce large
amount of the compounds. Hence to check the scalability of our
methodology, we performed a gram scale reaction of 4-
hydroxycoumarin 1 (1.0 g, 6.17 mol) and chalcone 2a (1.28 g,
6.17 mol) with 5 mol% of Ca(OTf)2 and Bu4NPF6 (5 mol%)
THF, reflux, 24 h No
reaction
Toluene, reflux,
24 h
25%
o
under solvent-free conditions at 120 C (Scheme 1). After 3 h,
we isolated 2.03 g (92%) of 2,4-diphenylpyrano[3,2-c]chromen-
5(4H)-one (3a).
water, reflux,
24 h
No
reaction
6
7
10
-
10
10
-
Neat, 120 oC, 2 h 92%
Neat, 120 oC,48 h 55%
Neat, 120 oC,30 h 90%
Neat, 120 oC,48 h 52%
Neat, 120 oC, 3 h 92%
Neat, 90 oC, 24 h 55%
8
10
-
9
-
10
11
5
5
5
5
Our first investigation was started with refluxing 4-
hydroxycoumarin 1 (100 mg, 0.61 mol) and chalcone 2a (128
mg, 0.61 mol) in presence of 10 mol% of calciumtriflate and
tetrabutyl hexafloroammoniumphosphate (10 mol %) in
dichloromethane as solvent. Even after 24 h, no product
formation was observed (Table 1, entry 1), several other solvents
like ethanol, THF, toluene and water were tried under reflux
conditions but none of them were found useful (Table 1, entries
1-5). Then an attempt was made under solvent free conditions at
o
120 C, using same catalyst combination and we were glad to
isolate the 2,4-diphenylpyrano[3,2-c]chromen-5(4H)-one (3a,
pyranocoumarin), in 92% yield after 2 h (Table 1, entry 6).
Encouraged by this result several experiments were performed
for the optimization of the reaction conditions and finally we
found that the reaction is working well with 5 mol% of Ca(OTf)2
As shown in the Scheme 2, oxygen atom of the enone (2a)
chelates with Ca-salt and thus facilitates the conjugate
addition (Michael type) of 4-hydroxycoumarin (1) to form the
intermediate I. In the second step, intramolecular nucleophilic
addition by the oxyanion of Ca-enolate takes place to result-
o
and 5 mol% of Bu4NPF6 at 120 C in a time period of 3 h with

3

4
Tetrahedron
-in the annulated intermediate (II). In the last step, calcium
catalyzed, thermodynamic elimination (possibly via E1) takes
place to yield the final product 3a.
2.45 g of 2,4-diphenyl-7,8-dihydro-4H-chromen-5(6H)-one (5a)
1
in 91% yield. All compounds were fully characterized by H
NMR, 13C NMR, Mass, IR and melting point data.13
After successful synthesis of pyranocoumarins, we planned to
extend our synthetic protocol to the similar pyran-fused
heterocyclic compounds benzo[b]pyrans which are also known as
2,4-diaryl-7,8-dihydro-4H-chromen-5(6H)-ones. In this case we
simply have to use cyclohexane-1, 3-diones as the nucleophiles
instead of 4-hydroxycoumarin, which follows the same
mechanism as shown in the Scheme 2. Our first reaction for the
synthesis of benzo[b]pyrans has been commenced with the
treatment of chalcone (2a) with simple cyclohexane-1,3-dione
In summary, we have developed an environmentally benign
Ca(II) catalyzed, efficient and scalable synthesis of biologically
important pyranocoumarins and benzopyrans. This one-pot,
solvent-free, cascade procedure endures the substrate diversity in
terms of chalcones, 4-hydroxy coumarins, and 1,3-diones. Due to
simple reaction and workup procedures, high yields, scalability,
inexpensive and occurrence of catalyst we believe that our
methodology will be superior to some of the existing methods
and serves for the synthesis of related privileged pyran molecules
as New Chemical Entities.
o
(4a) at 120 C for 3 h under solvent free conditions and we are
happy to isolate 2,4-diphenyl-7,8-dihydro-4H-chromen-5(6H)-
one (5a) in 91% yield (Scheme 3).
Acknowledgments
Two of the authors PLS and GS thank UGC and Central
University of Rajasthan for the fellowships respectively. SY
thank DST for the financial support under Fast Track Young
Scientist Scheme.
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13. General Experimental Procedure: a mixture of chalcone (1.0
equiv.), nucleophile (4-hydroxcoumarin/1,3-diones) (1.0 equiv),
Ca(OTf)2 (5 mol%) and Bu4NPF6 (5 mol%) were weighed into a
round bottom flask/test tube and heated at 120 oC in an oil bath for
3-5 h. After the completion of the reaction (monitored by TLC),
reaction mixture was diluted with minimum amount of water and
extracted into ethylacetate. Combined organic layers were dried
over anhydrous Na2SO4, solvent was removed under reduced
pressure and the crude reaction mass was purified by column
chromatography on silica gel using ethylacetate and hexanes as
the eluents to yield the Pyran-fused heterocyclic compounds.
Spectral data of all the compounds were in agreement with the
reported data.5,8. Spectral data of representive compounds: 3a.
1H NMR (500 MHz, CDCl3): δ 8.00 (dd, J = 1.5, 8 Hz, 1H),
7.77 (d, J = 7.5 Hz, 2 H), 7.53 (t, J = 8.5 Hz, 1H), 7.42-7.23
(m, 9H), 7.19-7.16 (m, 1H) 5.79 (d, J = 5 Hz, 1H), 4.66 (d, J =
5 Hz, 1H); 13C NMR (125 MHz, CDCl3): δ 161.5, 155.8,
152.8, 146.9, 143.6, 132.6, 132.0, 129.3, 128.7, 128.6, 128.5,
127.2, 124.7, 124.2, 122.7, 116.8, 114.5, 103.8, 36.6; IR
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(m/z): 352.03 (M+); mp: 170-172 oC; 3d. 1H NMR (500 MHz,
CDCl3): δ 8.19 (d, J = 8.5 Hz, 2H), 8.06 (d, J = 8 Hz, 1H),
7.76 (d, J = 8 Hz, 2H), 7.64-7.59 (m, 3H), 7.50-7.41 (m, 4H),
7.38 (d, J = 8 Hz, 1H), 5.81 (d, J = 4.5 Hz, 1H), 4.86 (d, J =
4.5 Hz, 1H); 13C NMR (125 MHz, CDCl3): δ 161.4, 156.4,
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124.8, 124.5, 123.9, 122.8, 117.0, 114.2, 102.4, 102.1, 36.7;
IR (KBr): νmax 2924, 2852, 1722, 1515, 1347, 1017 cm-1;
EIMS (m/z): 395.24 (M+-2); mp: 217-219 oC; 5a. 1H NMR
(500 MHz, CDCl3): δ 7.52 (d, J = 7 Hz, 2H), 7.30-7.17 (m,
7H), 7.1(t, J = 7.5 Hz, 1H), 5.64 (d, J = 5 Hz, 1H), 4.45 (d, J =
5 Hz, 1H), 2.66-2.53 (m, 2H), 2.36-2.25 (m, 2H), 2.01-1.90
(m, 2H); 13C NMR (125 MHz, CDCl3): δ 197.4, 166.3, 146.8,
145.2, 132.9, 128.7, 128.46, 128.43, 128.2, 126.5, 124.5,
113.8, 104.5, 37.1, 35.3, 27.8, 20.4; IR (KBr): νmax 2959,
2869, 1658, 1623, 1454, 1382 cm-1; EIMS (m/z):301.76 (M+-
1); mp: 172-174 oC; 5l. 1H NMR (500 MHz, CDCl3): δ 7.58
(d, J = 7.5 Hz, 2H), 7.37-7.25 (m, 7H), 7.16 (t, J = 7.5 Hz,
1H), 5.71 (d, J = 5 Hz, 1H), 4.98 (d, J = 5 Hz, 1H), 2.55 (s,
2H), 2.28-2.19 (m, 2H), 1.12 (s, 3H), 1.05 (s, 3H); 13C NMR
(125 MHz, CDCl3): δ 197.2, 164.6, 146.7, 145.2, 132.9, 128.7,
128.44, 128.43, 128.2, 126.6, 124.5, 112.5, 104.5, 50.9, 41.5,
35.4, 32.2, 29.2, 27.7; IR (KBr): νmax 2960, 2923, 1680, 1662,
1629, 1381, 1218 cm-1; EIMS (m/z):330 (M+); mp: 136-138
oC.
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