An efficient synthesis of medicinally important indole based triarylmethanes by using propylphosphonic anhydride (T3P)

Article abstract of DOI:10.1080/00397911.2020.1743319

We have developed an economical and efficient method for the synthesis of medicinally and synthetically important indole-based triarylmethanes by using indoles and benzhydrols in the presence of propylphosphonic anhydride (T3P). This methodolog

Full text of DOI:10.1080/00397911.2020.1743319

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
An efficient synthesis of medicinally important
indole based triarylmethanes by using
propylphosphonic anhydride (T3P®)
Srinivas Cheruku, Chaithra Nagaraju, Poornima Shetty, Swarup Hassan A,
Sandhya Nagarakere C, Kumara Manikyanally N & Mantelingu Kempegowda
To cite this article: Srinivas Cheruku, Chaithra Nagaraju, Poornima Shetty, Swarup Hassan A,
Sandhya Nagarakere C, Kumara Manikyanally N & Mantelingu Kempegowda (2020): An efficient
synthesis of medicinally important indole based triarylmethanes by using propylphosphonic
anhydride (T3P®), Synthetic Communications, DOI: 10.1080/00397911.2020.1743319
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2020.1743319
An efficient synthesis of medicinally important indole
based triarylmethanes by using propylphosphonic
R
anhydride (T3PV)
Srinivas Cherukua, Chaithra Nagarajub, Poornima Shettyc, Swarup Hassan Ab,
Sandhya Nagarakere Cd, Kumara Manikyanally Na, and Mantelingu Kempegowdab
b
aDepartment of Studies in Chemistry, Yuvaraja’s College, University of Mysore, Mysuru, India; Synthetic
c
Laboratory, DOS in Chemistry, University of Mysore, Mysuru, India; Department of Chemistry, Srinivas
d
School of Engineering, Mangalore, India; Department of Chemistry, St. Philomena’s College,
Mysuru, India
ABSTRACT
ARTICLE HISTORY
Received 13 February 2020
We have developed an economical and efficient method for the syn-
thesis of medicinally and synthetically important indole-based triaryl-
methanes by using indoles and benzhydrols in the presence of
R
propylphosphonic anhydride (T3PV). This methodology is an alter-
KEYWORDS
Alcohols; nitriles;
N-substituted amides;
nate approach for the C–C bond formation with good to excellent
yields. In this T3P-mediated dehydration approach, the by-product is
highly soluble in water, so that it can be done at larger scale also. In
addition to that this efficient protocol has several advantages such
as mild reaction conditions, short reaction time and operational sim-
plicity. We have successfully synthesized pyrrole, imidazothiadiazole
and imidazolo pyridine based triarylmethanes also.
propylphosphonic
R
anhydride (T3PV);
solvent-free condition
GRAPHICAL ABSTRACT
Introduction
Triarylmethane frameworks belong to a class of bioactive molecules, which are widely
featured in dyes,[1] material sciences,[2] medicinal chemistry,[3] bio-organic chemistry,[4]
natural products and pharmacologically important compounds.[5] For example, GPR40
modulator (I) has emerged as a promising pharmacological agent for treating dia-
betes,[6] compound II represents a potential antituberculotic activity, Letrozole,
Vorozole and compound (V) are potent non-steroidal aromatase inhibitors, compound
CONTACT Mantelingu Kempegowda
Mysore, Mysuru, Karnataka 570006, India; Kumara Manikyanahalli N
kmlingu@gmail.com
Synthetic Laboratory, DOS in Chemistry, University of
kumaramanikya@yahoo.com Department of
Studies in Chemistry, Yuvaraja’s College, University of Mysore, Mysuru 570005, India.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
S. CHERUKU ET AL.
O
N
N
O
O
O
N
N
N
N
HO
N
N
N
N
Cl
N
NC
CN
S
O
GPR40 modulator (I)
antituberculotic (II)
Letrazole (III)
Vorozole (IV)
N
N
N
N
S
N
O
N
N
HO
O
HO
Cl
HO
OH
(V)
(VI)
(VII)
(VIII)
Cl
Figure 1. Biologically active triarylmethanes.
O
F
OH
HO
N
HO
H
N
N
NH
N
O
S
OH
Br
O
O
N
H
N
H
N
N
H
(A)
(B)
(C)
(D)
Figure 2. Bioactive indole derivatives.
(VI) is a potent antitubercular, compounds (VII) and (VIII) are potent antifungal and
antiviral agents, respectively (Fig. 1).[7] Owing to their unique structural properties and
potential applications in drug discovery and functional material design, the synthesis of
triarylmethanes has received continuous attention from the chemistry community.[8]
Among heterocyclic compounds, indoles are privileged and found in many natural
products and also as pharmaceutical agents due to their vast biological activities.
Especially 3-substituted indole derivatives are building blocks of many promising thera-
peutic agents such as analgesic,[9] antiviral,[10] antibacterial,[11] potent kinase inhibi-
tors[12] and hypoglycemic. The below compounds (Fig. 2) are the bioactive indole
derivatives, compound (A) is a potent antifungal and antibacterial agent,[13] compound
(B) called Sumatriptan is used for the treatment of migraine headache,[14] compound
(C) is an aromatase inhibitor against breast cancer[15] and compound (D) functions as
HIV-1 integrase inhibitor.[16]
The bioactivity of 3-substituted indoles encouraged us to synthesize 3-(bis(aryl)-
methyl)-1H-indole and different catalytic strategies have been reported by different
groups using Lewis acids,[17] Brønsted acids[18] or transition metal complexes as cata-
lysts. Huanrong et al. reported iron catalyzed C–C bond activation with 1,3-dicarbonyl
units as leaving groups,[19] Debjit et al. reported palladium(II)-catalyzed efficient C-3
functionalization of indoles with benzylic and allylic alcohols.[20] Hidemasa et al.
reported gold-catalyzed direct substitution of benzyl alcohols with various nucleophiles
provides methodology for the formation of C–C and C–N bonds,[21] Kobayashi and
coworker reported DBSA-catalyzed dehydrativenucleophilic substitutions of alcohols in

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SYNTHETIC COMMUNICATIONSV
3
water.[22] Likewise different groups reported different catalytic strategies using Lewis
acids,[17] Brønsted acids[18] or transition metal complexes as catalysts. However, these
methods suffers from significant limitations like limited substrate scope, as contamin-
ation of metals with desired product, low yields, harsh reaction conditions and expen-
sive reagents. Hence, new methods for indole functionalization continue to attract
more attention.
However, to the best of our knowledge, no examples of T3P inter molecular dehydra-
tive alkylation of indoles with alcohols have been previously described.[23] In this con-
text, it is highly essential to develop novel approach of dehydrative reactions of alcohols
for the construction of triarylmethane scaffolds in accordance with environmental
friendliness. Besides, the dehydration of alcohols has been considered as having high
atom and step-economy. In continuation of our interest in the applications of T3P,[24]
cycloaddition reactions[25] and synthesis of heterocyclic compounds,[26] we explored a
novel approach to access triarylmethane frameworks using T3P (Scheme 1), which hav-
ing wide application in the field of synthetic organic chemistry because of its excellent
reaction selectivity, less toxicity, low epimerization character, less allergenic and broad
functional group tolerance.[27]
Results and discussion
The reaction of un-substituted indole 1a with benzhydrol 2a was selected as the model
reaction to establish the best reaction condition. Initially, the reaction was carried out
in the absence of T3P at reflux condition; the desired product did not form (Table 1,
entry 1). The feasibility of the reaction was checked by using indole 1a (1 equiv.) with
R
benzhydrol 2a (1 equiv.) using T3PV (25 mol%) in the presence of toluene as a solvent
at 90 ꢀC (Table 1). Pleasantly, required product 3a obtained with 20% yield after 14 h
(Table 1, entry 2). Inspired by this result, the amount of T3P was increased from
25 mol% to 50 mol% and 100 mol%, surprisingly complete consumption of starting
materials were observed with 100 mol% and the product 3a was isolated in 85% yield
(Table 1, entries 3 and 4). Further increasing the amount of T3P from 100 mol% to
125 mol% no significant improvement was observed (Table 1, entry 5), thus T3P loading
was optimized at 100 mol%. In order to improve the results, solvent was changed from
toluene to xylene and 1,4-dioxane (Table 1, entries 6 and 7) and could not get better
results. In addition to this, various solvents like THF and DMF were also tried, but only
trace amount of product was observed (Table 1, entries 8 and 9). Further reaction was
performed by using water as a solvent and also under solvent-free condition but there
was no product (Table 1, entries 10 and 11). With continuous interest, the effect of
temperature was also tested by a decrease and increase of temperature from 90 ꢀC to
80 ꢀC & 100 ꢀC (Table 1, entries 12 and 13), and we could not find better results.
Finally, we found that reactions of 1a with 2a in the presence of T3P (1 equiv. or
100 mol%) at 90 ꢀC for 14 h was ideal (Table 1, entry 4).
With the above optimized reaction conditions, the efficiency and versatility of this
newly developed approach was generalized using a variety of substituted indoles, hetero-
cyclic compounds and various benzhydrols, the results are summarized in Scheme 2.
Thus, the developed methodology preceded smoothly using 100 mol% of T3P with a

4
S. CHERUKU ET AL.
Previous work
O
Br
FeCl3, DCE, 50 oC, 3 h
Br
+
N
H
N
H
O
Li, Huanrong et al
PdCl2(CH3CN)2, DCE,
90 oC, 4 h
Br
Br
OH
+
N
H
N
H
Das, Debjit et al
O
OH
Br
Cat. Au(III)/TPPMS
Br
+
OH
N
H
N
H
Hikawa, Hidemasa et al
R1
R2
DBSA
(10 mol %)
R1
+
H2O
R2
Ar
Nu
+
Nu
H
Ar
OH
H2O
Kobayashi et al
Our work
Ph
N
Ph
N
R
N
Ph
Ph
R1
Ph
Ph
N
N
R
T3P, toluene, 90 oC
T3P, toluene, 90 oC
N
OH
R1
Scheme 1. Benzylation of heterocycles using benzhydrol and T3P.
variety of substituted benzhydrols, like methyl, fluoro and chloro, in all the cases reac-
tion went smoothly. Further, the influence of substituents on the heterocycles like
indoles, pyrroles, imidazothiadiazoles and imidazolo pyridines was also investigated,
both electron donating and electron withdrawing groups have found to be compatible
to give the desired products with an excellent yields.

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SYNTHETIC COMMUNICATIONSV
5
Table 1. Optimization of reaction conditions.
OH
T3P
+
N
H
N
2a
1a
H
3a
T3P
(mol%)
Entry
Solvent
Temp (ꢀC)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
Toluene
–
25%
50%
Reflux
90
90
90
90
90
90
80
90
90
90
80
100
24
16
16
14
14
14
14
14
14
14
14
14
14
–
20
44
85
82
45
57
Trace
Trace
NR
20
Toluene
Toluene
Toluene
Toluene
Xylene
1,4-Dioxane
THF
DMF
Water
100%
125%
100%
100%
100%
100%
100%
100%
100%
100%
9
10
11
12
13
No solvent
Toluene
Toluene
70
75
R
Reaction condition: 1a (1.0 equiv.) and 2a (1.0 equiv.) in the presence of T3PV (1.0 equiv., 50% in ethyl acetate) in tolu-
ene at 90 ꢀC for 14 h.
The bold values are represent the best optimized condition obtained for synthesis of compounds.
The mechanism for the benzylation of heterocycles was proposed and shown in
Scheme 3. In the first step, activation of 1a takes place when lone pair of electrons on
the oxygen atom of 1a attacks on A (T3P) to form an intermediate B and secondly,
intermediate B was attacked by indole 2a to form intermediate D by eliminating the
by-product open hydrated T3P which is C. The intermediate C abstracts hydrogen from
intermediate D followed by re-aromatization of indole takes place to give 3a as a prod-
uct with the expulsion of intermediate E.
Conclusion
In conclusion, we have developed an efficient T3P-mediated dehydrativebenzylation
method for the synthesis of medicinally and synthetically important indole-based triar-
ylmethanes by using indoles and benzhydrols. Along with indole derivatives; pyrrole,
imidazothiadiazole and imidazolo pyridine based triarylmethanes were also successfully
executed. The obtained scaffolds are of high interest due to their potential biological
and pharmaceutical activity.
Experimental procedure
Materials and methods
All work relating to analytical thin layer chromatography were performed with E.
Merck silica gel 60F254 aluminum plates and were visualized with UV light. The follow-
ing mobile phases were employed for TLC: chloroform, methanol, hexane and ethyl
acetate in different ratios. The instrumental techniques employed for the

6
S. CHERUKU ET AL.
Ph
N
Ph
N
R
N
Ph
Ph
R1
Ph
Ph
OH
N
N
R
°
°
N
T3P, toluene,
90 C
T3P, toluene, 90 C
R1
F
F
Br
N
N
N
4 (78%)
H
N
2 (79%)
3 (83%)
1( 85%)
H
H
H
F
F
F
F
Br
Br
N
N
H
N
H
N
8 (77%)
7 (75%)
5 (80%)
6 (78%)
F
F
F
N
S
N
S
N
N
N
H
N
Br
Br
H
Br
Br
N
N
9 (85%)
10 (82%)
11 (86%)
F
12 (84%)
F
F
N
N
N
N
N
N
N
14 (91%)
N
CF3
13 (80%)
15 (84%)
CF3
16 (87%)
Cl
Cl
N
N
N
N
N
N
CF3
N
N
MeO
CF3
20 (86%)
19 (88%)
18 (82%)
17 (93%)
N
N
21 (92%)
Cl
Scheme 2. Substrate scope of reactions.

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SYNTHETIC COMMUNICATIONSV
7
OH
Pr
P
O
O
P
Pr
O
O
Pr
O
OH
P
O
O
P
Pr
O
P
P
Pr
O
+
O
O
+
O
N
H
Pr
B
2a
1a
A
Pr
P
Pr
P
Pr
OH
O
O
O
Pr
Pr
P
Pr
P
+
P
OH
O
O
HO
+
P
H
O
O
O
O
O
O
N
N
H
H
E
3a
D
C
Scheme 3. Plausible mechanism.
1
characterization of the newly synthesized compounds include H NMR, 13C NMR and
1
mass spectroscopy. H and 13C NMR spectra were recorded on a Bruker-AV (500, 400
and 126, 101 MHz, respectively) and Agilent WM (400 and 100 MHz) Fourier trans-
forms spectrophotometer in CDCl3 or DMSO-d6 solution using tetramethylsilane
(TMS) as internal standard. Chemical shifts were recorded in ppm relative to TMS.
Mass and purity were recorded on an LC–MSD-Trap-XCT (Agilent Technologies Inc.).
All the reagents and chemicals used were from Sigma Aldrich chemicals.
General procedure
To a stirred solution of alcohol (1 equiv.) and hetero cyclic compound (1 equiv.) in tolu-
ene (20 vol.) was added T3P reagent (1 equiv.) at room temperature. The reaction mix-
ture was heated to 90 ꢀC for 14 h. The reaction was monitored by TLC till completion
of starting material. The reaction mixture was cooled to room temperature, diluted with
ethyl acetate and washed with sodium bicarbonate (10%) solution, water followed by
brine solution. The organic layer was dried over sodium sulfate and concentrated under
reduced pressure; the crude product was purified by column chromatography to afford
the desired compound.
Acknowledgments
We thank NMR facility, Institute of Excellence, University of Mysore, Manasagangotri, Mysore,
India for spectral data.
Funding
The author is thankful to financial support from the DBT Glue Grant [No. BT/PR23078/MED/
29/1253/2017] dated: 22/03/2018 and VGST [K-FIST (L1)] Grant [No. KSTePS/VGST/K-FIST

8
S. CHERUKU ET AL.
(L1)/2017-18/GRD-681/79/2018-2019] dated: 17/09/2018; and Minor Research Project U.O. [NO:
DV6/375/MRP/2017-18], dated: 12/02/2018 is gratefully acknowledged.
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