B(C6F5)3 catalyzed synthesis of dihydropyrano[3,2-b]chromenediones under solvent-free conditions

Article abstract of DOI:10.1080/00397911.2019.1585542

A simple, efficient, solventless, and one step, B(C₆F₅)₃ catalyzed, synthesis of dihydropyrano[3,2-b]chromenediones from dimedone, aldehyde and kojic acid is described. This protocol proceeds smoothly, accommodates aromati

Full text of DOI:10.1080/00397911.2019.1585542

Synthetic Communications
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B(C6F5)3 catalyzed synthesis of dihydropyrano[3,2-
b]chromenediones under solvent-free conditions
Yuvaraj P. Sarnikar, Yogesh D. Mane, Dhanraj O. Biradar & Bhimrao C. Khade
To cite this article: Yuvaraj P. Sarnikar, Yogesh D. Mane, Dhanraj O. Biradar & Bhimrao C. Khade
(2019): B(C6F5)3 catalyzed synthesis of dihydropyrano[3,2-b]chromenediones under solvent-free
conditions, Synthetic Communications, DOI: 10.1080/00397911.2019.1585542
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https://doi.org/10.1080/00397911.2019.1585542
B(C6F5)3 catalyzed synthesis of dihydropyrano[3,2-b]
chromenediones under solvent-free conditions
Yuvaraj P. Sarnikara, Yogesh D. Maneb, Dhanraj O. Biradarc, and
Bhimrao C. Khaded
b
aDepartment of Chemistry, Dayanand Science College, Latur, MH, India; Department of Chemistry,
c
Shri Chhatrapati Shivaji College, Omerga, MH, India; Department of Organic Chemistry Division, Indian
d
Institute of Chemical Technology, Hyderabad, TS, India; Department of Chemistry, College of Arts,
Commerce & Science, Parbhani, MH, India
ABSTRACT
ARTICLE HISTORY
Received 16 August 2018
A simple, efficient, solventless, and one step, B(C6F5)3 catalyzed, syn-
thesis of dihydropyrano[3,2-b]chromenediones from dimedone, alde-
hyde and kojic acid is described. This protocol proceeds smoothly,
accommodates aromatic as well as heteroaromatic aldehydes and
gives dihydropyrano[3,2-b]chromenediones in excellent yield.
KEYWORDS
Aldehyde; chromenedione;
dimedone; kojic acid;
tris(pentafluoro-
GRAPHICAL ABSTRACT
phenyl)borane
Introduction
Diversity-oriented, one-pot multi-component synthesis, especially of compounds having
molecular skeleta found in both natural products and drug-like molecules, have
attracted the attention of both academic and industrial researchers.[1] Being efficient
and highly convergent, multi-component reactions (MCRs) produces structurally and
stereochemically complex molecules in a single step.[2] Chromenes are known for their
several biological activities such as anticonvulsant,[3] anti-anaphylactic,[4] anticancer,[5]
antibacterial[6] and are used for the treatment of Huntington’s, Parkinson’s and
Alzheimer’s diseases.[7,8] kojic acid and its derivatives also exhibit many therapeutic
applications. It inhibits melanin formation by suppressing tyrosinase and acts as a good
skin lightener.[9] Moreover, it also acts as an antioxidant[10] in foodstuff, insecticide,[11]
pesticide, herbicide,[12] antimicrobial agent[13] and bidentate ligand to form biologically
CONTACT Yogesh D. Mane
maneyd@gmail.com
Department of Chemistry, Shri Chhatrapati Shivaji College,
NH-9, Omerga, MH 413606, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article is available online at on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

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Y. P. SARNIKAR ET AL.
potent metal complexes.[14] Heterocyclic compounds harboring chromene and kojic
acid nucleus exhibit even more biological activities involving antifungal,[15] antineoplas-
tic,[16] antiproliferative,[17] anti-HIV,[18] antiviral,[19] anticonvulsant,[20] anti-inflamma-
tory,[21] anti-oxidative,[22] antibacterial,[23] antimicrobial agent,[24] tyrosinsae inhibitor[9]
or skin whitening agent[9] and antimelanogenic.[25] Chromenes[26] and kojic acid deriva-
tives[25] exhibit diverse biological potencies.
Because of easy availability, lability and biological potency of inexpensive kojic acid,
several kojic acid derivatives have been synthesized.[27] In spite of the applications of
chromene analogs, little attention has been paid to the synthesis of dihydropyrano[3,2-
b] chromenediones and only a few methods are accessible, which deal with the synthesis
of the essential heterocycles. In recent years, InCl3, Alum, CeCl3ꢀ7H2O/SiO2, FeCl3/SiO2,
Bi(OTf)3, b-cyclodextrin, Yb(PFO)3 and chloroaluminate ionic liquid-modified
silica-coated magnetic nanoparticles have been used as a catalyst for the synthesis of
dihydropyrano[3,2-b]chromenediones.[28–35] However, many of these reported synthetic
methods suffer either from high temperature, longer reaction times, low yield, use of
toxic solvents and a significant amount of moisture sensitivity and expensive catalysts.
Although the reported methods are useful, the development of additional efficient syn-
thetic methods with efficient catalysts is still in demand. Therefore, owing to the
pharmaceutical and synthetic importance of dihydropyrano[3,2-b] chromenediones,
development of a direct and efficient synthetic method is highly desirable.
Moderate Lewis acidic nature of BCF facilitates its usage in organic synthesis. In
recent times, Tris(pentafluorophenyl)Borane (BCF) has evolved as a mild, non-toxic,
environmentally benign, moisture-tolerant, air-stable, heat-stable, inherently
electrophilic, moderate and versatile Lewis acid imparting high chemo-, region- and
stereoselectivity in many organic transformations.[36] Low catalytic loading and mois-
ture-tolerance, makes BCF a superior acid catalyst than traditional Lewis acids. In this
paper, we report one step and convenient synthetic method for the synthesis of a series
of dihydropyrano[3,2-b]chromenedione derivatives from Dimedone, aldehydes and kojic
acid under solvent-free conditions in the presence of B(C6F5)3 as an efficient solid
Lewis acid catalyst.
Results and discussion
To demonstrate our methodology, we purchased kojic acid, substituted benzenoid as
well as non benzenoid aromatic/heteroaromatic aldehydes and dimedone commercially
and used without any purification. Synthesis of dihydropyrano[3,2-b]chromenediones
(4a–l) has been achieved from dimedone (1), aldehyde (2a–l), and kojic acid (3) in the
presence of B(C6F5)3 as a solid acid catalyst at 110 ꢁC. The structure of products (4a–l)
was confirmed by comparing spectroscopic data with literature values. To optimize
reaction conditions, we initially examined the role of catalyst and screened several solid
acid catalysts. Dimedone (1), benzaldehyde (2a) and kojic acid (3) were selected as
model starting materials and the reaction was carried out under solvent-free condition
at 110 ꢁC using 10 mol% of B(C6F5)3 as a catalyst. The progress of the reaction was
checked by TLC and after 2 h the product 4a was obtained in 55% yield (Table 1, entry

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Table 1. Screening of catalyst for synthesis of 4a.
Entry
Catalyst (mol%)
Temp (ꢁC)
Time (min)
Yielda (%)
1
2
3
4
5
6
7
8
–
110
110
110
110
110
110
110
110
110
110
110
110
200
120
120
90
90
90
90
90
90
90
00
55
67
75
83
84
32
41
47
54
69
30
B(C6F5)3 (1)
B(C6F5)3 (2)
B(C6F5)3 (5)
B(C6F5)3 (10)
B(C6F5)3 (15)
p-TsOH (10)
Fe3O4 (10)
ZnCl2 (10)
BiCl3 (10)
9
10
11
12
FeCl3 (10)
DABCO
90
90
aYield refers to an isolated yield after column chromatography.
1
1). The formation of compound 4a was confirmed by comparing H, 13C-NMR and
mass spectroscopic data with literature data.
In order to enhance the yield of compound 4a, we repeated the same reaction with 2,
5, 10 and 15 mol% of B(C6F5)3 and we got product 4a in 83% yield within 1.5 h when
10 mol% of BCF was used as catalyst (Table 1, entry 5). To realize the efficiency of
B(C6F5)3, same reaction was repeated with different solid acid catalysts such as p-TsOH,
Fe3O4, ZnCl2, FeCl3, BiCl3, B(C6F5)3 and basic catalyst DABCO. Among these tested
solid acid catalysts, BCF was found as excellent catalysts in terms of reaction time and
chemical yield (Table 1, entry 5). To know the effect of solvent on chemical yield of
this reaction, we performed same reaction with different solvents such as tetrahydro-
furan, toluene[28], dichloromethane,[28] diethyl ether, acetonitrile[28], ethanol[28] and
water and found that solvent-free condition provides the best results (Table 2, entry 1).
Therefore, the use of 10 mol% of B(C6F5)3 under the neat condition is superior for
this conversion.
Under the optimized reaction conditions, we synthesized several dihydropyrano[3,2-
b]chromenediones (4a–l) using aromatic/heteroaromatic aldehydes in 72–90% yield. All
mentioned reactions proceeded well to give the corresponding product in excellent
yields and accommodated multifunctional aldehydes also. However, sterically unhin-
dered and more electrophilic aldehydes (Table 3, entries 4g–j) gave excellent yields in
shorter reaction times than those of hindered and less electrophilic aldehydes (Table 3,
entry 4d and 4e).
Based on the above results and literature reports,[24] a plausible mechanism for the
synthesis of dihydropyrano[3,2-b]chromenediones is illustrated in Scheme 1. B(C6F5)3
coordinates with the carbonyl oxygen of aldehyde and makes it more susceptible
towards the nucleophilic attack by enol form of dimedone giving enone. Enone cyclises

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Y. P. SARNIKAR ET AL.
Table 2. Screening of solvent for synthesis of 4a.
Entry
Solvent
Temp (ꢁC)
Time (min)
Yielda (%)
1
2
3
4
5
6
7
8
9
–
THF
110
66
110
40
82
35
50
80
100
90
180
180
180
180
180
180
180
180
83
42
24
26
45
32
35
22
n.d.
Toluene
DCM
CH3CN
Et2O
CHCl3
EtOH
H2O
aYield refers to an isolated yield after column chromatography.
with kojic acid by [4 þ 2] cycloaddition pericyclic reaction to give the final
adduct (4a–l).
Experimental
General procedure for the synthesis of 2-(hydroxymethyl)-7,7-dimethyl-10-phenyl-
6,7,8,10-tetrahydropyrano[3,2-b]chromene-4,9-dione (4a)
Dimedone 1 (50 mg, 0.357 mmol), Benzaldehyde 2a (38 mg, 0.358 mmol), kojic acid 3
(50 mg, 0.352 mmol) and Tris(pentafluorophenyl)Borane (10 mol%) was charged in a
round bottom flask. The resulting reaction mass was stirred at 110 ꢁC for 1.5 h. After
complete conversion, as indicated by TLC, the reaction was quenched with water (5 ml)
and extracted with ethyl acetate (3 ꢂ 5 mL). Organic layer was washed with brine and
dried over anhydrous Na2SO4, filtered and concentrated in vacuum. The crude product
was purified by column chromatography over silica gel using ethyl acetate–hexane (6:4)
as eluent to afford the pure product 4a in 83% yield. White solid (104 mg (83%) yield),
mp 184–186 ꢁC (lit.1 mp: 186–188 ꢁC); TLC Rf = 0.49 (EtOAc: Hexane = 6:4); IR ꢀmax
(KBr, cmꢃ1): 3364, 3076, 2948, 1664, 1619, 1521, 1374, 1210, 1191, 1073, 993, 952, 716;
1H NMR (400 MHz, CDCl3 & DMSO-d6): d 1.04 (s, 3H), 1.13 (s, 3H), 2.28–2.20 (m,
2H), 2.70–2.60 (m, 2H), 4.45–4.38 (m, 2H), 4.88 (s, 1H), 6.54 (s, 1H), 7.38–7.21 (m,
5H); 13C NMR (100 MHz, DMSO-d6): d 27.4, 28.9, 32.2, 38.3, 40.8, 50.3, 60.4, 112.0,
112.2, 127.7, 128.0, 128.7, 137.4, 140.4, 151.7, 163.9, 167.9, 171.4, 196.3; HRMS (ESI)
m/z [M þ H]þ Calculated for C21H21O5: 353.1311, found: 353.1383.

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Table 3. Synthesis of dihydropyrano[3,2-b]chromenediones (4a–l) under B(C6F5)3 catalysis.
Entry
Aldehyde
Product
Time (min)
90
Yielda (%)
83
4a
4b
4c
4d
95
95
95
80
78
75
4e
4f
100
80
72
88
4g
85
84
(continued)

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Y. P. SARNIKAR ET AL.
4h
85
75
82
87
4i
4j
4k
4l
70
80
90
90
85
78
aYield refers to an isolated yield after column chromatography.
Scheme 1. Plausible mechanism for formation of 4a–l.

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10 -(2,6-difluorophenyl)-2-(hydroxymethyl)-7,7-dimethyl-6,7,8,10-
tetrahydropyrano[3,2-b]chromene-4,9-dione (4j)
White solid (122 mg (90%) yield), mp 153–155 ꢁC; TLC Rf = 0.37 (EtOAc: Hexane =
6:4); IR ꢀmax (KBr, cmꢃ1): 3388, 2959, 1728, 1635, 1586, 1467, 1377, 1228, 1148, 1003,
1
790; H NMR (400 MHz, CDCl3 & DMSO-d6): d 1.07 (s, 3H), 1.14 (s, 3H), 2.29–2.17
(m, 2H), 2.60–2.55 (m, 2H), 4.13 (s, 1H), 4.49–4.44 (m, 2H), 5.26 (s, 1H), 6.46 (s, 1H),
7.27–7.21 (m, 3H); 13C NMR (100 MHz, DMSO-d6): d 27.4, 29.0, 32.3, 37.9, 40.8, 50.4,
60.4, 66.4, 112.0, 129.0, 129.5, 133.7, 137.5, 139.0, 151.2, 164.2, 168.4, 171.4, 196.4;
HRMS (ESI) m/z [M þ H]þ Calculated for C21H19F2O5: 389.1201, found: 389.1174.
Conclusion
In conclusion, we have developed a BCF catalyzed direct one pot, simple, efficient, solv-
ent-free synthetic protocol for the exclusive synthesis of dihydropyrano[3,2-b]chromene-
diones from dimedone, aldehyde and kojic acid in good to excellent yields. High yields,
shorter reaction times, easy workup, high atom-economy, solvent-free conditions, low
cost and easy handling of the catalyst are important highlights of this protocol.
Acknowledgments
The authors are grateful to the Department of Postgraduate Studies and Research in Chemistry,
Dnyanopasak College, Parbhani and Shri Chhatrapati Shivaji College, Omerga, India, for provid-
ing laboratory facilities and Indian Institute of Chemical Technology, Hyderabad for spec-
tral analysis.
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