Iodine-DMSO catalyzed chemoselective oxidative aromatization and deallylation, nondeallylation of aryl allyl ether of tetrahydro-β-carboline

Article abstract of DOI:10.1002/jhet.4265

We have developed a simple method for the chemoselective aromatization of tetrahydro-β-carboline with selective nondeallylation O-allyl groups in the presence of iodine (100 mol %) in dimethyl sulfoxide/H₂O₂. A convergent approach toward the oxidative aromatization with selective deallylation (deprotection) of O-allyl-tetrahydro-β-carboline using iodine in dimethyl sulfoxide/HCl has been described. The present protocol contains cheap catalyst, easy work up, normal reaction conditions, and high selectivity.

Full text of DOI:10.1002/jhet.4265

Received: 28 November 2020
DOI: 10.1002/jhet.4265
Revised: 16 February 2021
Accepted: 10 March 2021
A R T I C L E
Iodine-DMSO catalyzed chemoselective oxidative
aromatization and deallylation, nondeallylation of aryl allyl
ether of tetrahydro-β-carboline
1
2
1
Sunil V. Gaikwad
| Milind V. Gaikwad | Pradeep D. Lokhande
1
Department of Chemistry, Centre for
Abstract
Advanced Research, Savitribai Phule Pune
University, Pune, India
We have developed a simple method for the chemoselective aromatization of
tetrahydro-β-carboline with selective nondeallylation O-allyl groups in the
presence of iodine (100 mol %) in dimethyl sulfoxide/H O . A convergent
2
Department of Chemistry, Dr. D. Y. Patil
A.C.S. College Pimpri, Savitribai Phule
Pune University, Pune, India
2
2
approach toward the oxidative aromatization with selective deallylation
deprotection) of O-allyl-tetrahydro-β-carboline using iodine in dimethyl sulf-
Correspondence
(
Sunil V. Gaikwad, Department of
Chemistry, Centre for Advanced
Research, Savitribai Phule Pune
University, Pune 411007, India.
Email: sunilunipune2012@gmail.com
oxide/HCl has been described. The present protocol contains cheap catalyst,
easy work up, normal reaction conditions, and high selectivity.
1
| INTRODUCTION
heterocyclic compounds with a wide range of biological
activity (Figure 1).
[
15]
In organic synthesis, functional group transformation, pro-
tection, and selective deprotection have their significant
Since the last decade, our research group explores the
iodine chemistry in the novel organic transformation, always
useful to develop a cost-effective, safe, greener new synthesis
.[1]
importance For the protection of phenols, alcohols, acids,
and amine functional groups, the allyl group is commonly
used as a protecting group. The stability of allyl group in
the acidic and basic conditions is an important factor during
.[16]
approach for organic synthesis
The milder acidic nature
of iodine facilitates its usage in organic synthesis. Previously,
we have reported the iodine-mediated useful chemoselective
transformation, such as oxidative chemoselective dehydroge-
nation, aromatization, and their synthesis utility for natural
.[2]
organic transformation As per literature, various methods
are found for the selective deallylation and nondeallylation
[3,4]
[17–18]
0
of the O-allyl group in the organic transformation.
The
product synthesis.
chalcones is followed by oxidative cyclizations to flavones.
The substoichiometric amount of I in DMSO has been used
O-allyl-deallylation for a 2 -allyloxy
[19]
protection and deprotection strategies must be performed in
[5]
the vicinity of a variety of other functional groups. The
deprotection of allyl has been achieved by using various
reagents such as metal salts (Pd, Rd, Ir, Ru, Cu, Zn) or their
2
,
[20]
for the cascade synthesis of flavones. The iodine in DMSO
at 130 C, it prefers deprotection of allyl group carboxylic
and direct C- deallylation of allyl group from the
active methylene position along with chemoselective oxi-
ꢀ
[
6]
[7]
[8]
[9]
[10]
[21]
expensive complexes DDQ, ZnCl2, NBS, OsO4,
ester
[11]
[12]
[13]
p-TSA,
SmI2,
Pd(PPh3)4 mol %) with K CO3 and
The copper boryl reagents enable the
2
[13]
molecular iodine.
dative dehydrogenation of allyloxy substituted β-anilino-
[22]
selective cleavage of aryl allyl ethers and selective non-
deprotection of allyl ester, but such transformation needed
dihydro-chalcones to β-anilino-chalcones
was preferred
reaction over deallylation (Figure 2). The Significant biologi-
[14]
reaction activator.
cal properties of β- carboline makes it synthetically impor-
[23]
Therefore, it is desirable to develop new methodologies
that afford high yielding with nondeallylation and oxida-
tive aromatization/dehydrogenation will occur under
mild reaction conditions. β-carbolines are important
tant Unit.
Considering the difficulties in the coinciding
deallylation or nondeallylation accompanied by dehydro-
genation/aromatization of O-allyl THβC with low
J Heterocyclic Chem. 2021;1–7.
wileyonlinelibrary.com/journal/jhet
© 2021 Wiley Periodicals LLC.
1

2
GAIKWAD ET AL.
FIGURE 1 Bioactive β-carboline
A
B
C
D
E
FIGURE 2 Synthesis of O-allyl-
β-carboline and β-carboline phenol;
(
deallyation occurs with the addition
+
of H )
B
A
temperature, the development of an efficient and envi-
ronmentally friendly method is highly desirable.
the consecutive aromatization with nondeallylation of
O-allyl aryl ether (THβC).
Herein, we wish to report a chemoselective nonde-
allylation followed by aromatization of O-allyl THβC
using iodine in DMSO and external oxidant H O . Also, 2 | RESULT AND DISCUSSION
2
2
we have disclosed the consecutive deallylation followed
by aromatization of O-allyl-THβC using molecular iodine
in DMSO/HCl. As per the literature search, to the best of
our knowledge, there are no such reports available for
The O-allyl tetrahydro-β-carboline 3a was conveniently
prepared by the treatment of a variety of O-allyl aldehyde
2 with tryptophan/tryptamine/tryptophan methyl ester

GAIKWAD ET AL.
3
in acetic acid by Pictet Spengler condensation to achieve
the required substrate (Scheme 1).
entry 1, 2, 3). While on increasing the temperature from
ꢀ
ꢀ
60 C to 80 C, the aromatic product was isolated with
the O-allyl group intact with the OH group with a 50%
yield (Table 1, entry 4, 5). The aromatization with nonde-
allylation formation was confirmed by spectral analysis.
To study the comparative effectiveness and to over-
come the limitation of our previously reported protocol,
the development of a new methodology for aromatization
followed by nondeallylation O-allyl is essential. Recently,
we have been reported I -/DMSO-/H O -mediated aroma-
1
In the H NMR spectra, the peak at 11.94 (s, 1H)
corresponding to N-H proton. The allyl group shows the
characteristic peak pattern 6.24–6.00 (m, 1H), 5.47 (d,
J = 15.8 Hz, 1H), 5.32 (d, J = 9.3 Hz, 1H), 4.71 (d,
J = 5.1 Hz, 2H) and singlet at 3.95 for OMe group. In
2
2 2
17,18]
[
tization of THβC to β-carboline.
Herein, we have
proposed to apply a similar reaction condition on O-allyl-
substituted THβC.
13
Therefore, we have explored the amount of iodine–
CNMR, spectra showed the 166.36 corresponding to the
DMSO/H O2 required to catalyze the reaction with
carbonyl of the COOMe group. From the spectral analy-
sis, it conforms to the product methyl 1-(4-[allyloxy] phe-
nyl)-9H-pyrido [3,4–b] indole-3-carboxylate 6d formed
with nondeallylation. Next, we moved to increase the
yield of the product; therefore, we have used 50 mol%
2
methyl
1-(4-[allyloxy]phenyl)-2,3,4,9-tetrahydro-1H-
pyrido[3,4–b]indole-3-carboxylate 3g as a model sub-
strate. Initially, we have applied 10 mol%, 20 mol% and
2
0 mol% iodine in DMSO and external oxidant H O on
2 2
ꢀ
O-allyl tetrahydro-β-carboline methyl ester; no significant
and 100 mol% of the iodine in DMSO/H O at 90 C,
2
2
changes were observed at room temperature (Table 1,
with H O ; it was observed that the aromatization of O-
2 2
allyl-tetrahydo-β-carboline methyl ester obtained with
very good yield and nondeallylation(Table 1, entry 9).
The time required for completion of the reaction was
longer when 25 mol% of iodine was used. However,
50 and 100 mol% iodine were found as adequate quanti-
ties for the oxidative aromatization of O-allyl-THβC with
nondeallylation with less reaction time (Table 1, entry
7, 9). By this result, we have decided to use 100 mol%
iodine in DMSO/H O as the best reaction condition for
2
2
aromatization of O-allyl-THβC. To understand the role of
DMSO solvent and H O external oxidant, we performed
2
2
SCHEME 1 Preparation of O-allyl tetrahydro-β-carboline
the reaction on the same substrate in DMSO solvent, and
TABLE 1 Optimization of reaction
condition
Sr/no
Iodine Mol%
Oxidant
Temperature
Time/h
Yield 6
no
Yield 5
1
2
3
4
5
6
7
8
9
10
20
20
25
25
30
50
75
100
25
50
-
RT
RT
60
80
90
90
90
90
90
90
100
24
24
24
12
8
-
-
-
-
-
-
-
-
-
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
no
no
45
65
8
65
8
75
5
77
4
85
[
[
19]
19]
1
0
1
HCl
HCl
3
75
78
1
3
Note: Use of DMSO in HCl at higher temperature reaction get decomposed, and dellylation occurs, and
without the use of HCl, no good result observed.

4
GAIKWAD ET AL.
TABLE 2 Synthesis of O-allyl-β-carboline
TABLE 2 (Continued)
1
2
84%
77%
9
64%
50%
10
3
4
70%
80%
11
78%
80%
12
5
6
78%
81%
a drop of hydrochloric acid (HCl) gives corresponding
β-carboline with deprotection of O-allyl groups (Table 1,
entry 10, 11).
Therefore, 100 mol% iodine, DMSO, and external oxi-
dant H O (1 equiv.) give the best result. With the opti-
2
2
mized conditions in hand, with this result, we turned our
attention toward testing the scope of the protocol with a
variety of tetrahydro-β-carboline. The series of O-allyl
substituted tetrahydro-β-carboline having H, COOH, and
COOMe groups at C-3 position was synthesized and
7
8
70%
62%
ꢀ
treated with iodine in DMSO/H O at 100 C for up to 3h
2
2
reaction time; the O-allyl substituted-β-carboline was iso-
lated with 50–84% yield. The THβC with H at C-3 posi-
tion 3a, 3b, and 3c was treated with iodine in DMSO/
ꢀ
H O at 100 C, obtaining compound 6a, 6b, and 6c with
2
2
84, 77, and 70% yield (Table 2, entry 1, 2, 3). We tested
the generality of the protocol by subjecting other sub-
strates 3b, 3d, and 3f (COOH at C-3) in the reaction and
found that, in each case, the transformation was success-
ful, affording the corresponding products 6a, 6b, and 6c.
Therefore, in the first set of reactions, several substituted

GAIKWAD ET AL.
5
THβC (3a-3f) were evaluated, and it was found that the
substrates which hold O-allyl group at meta position of
phenyl ring afforded 6c in 65–70% yields. Next, we inves-
tigated the substrates 3g, 3h, 3i, and 3j (Table 2, entry
β-carboline-3-carboxylic acid 7b, 7c with 64, 50% yield
(Table 2, entry 9, 10).
In order to evaluate the possibility of applying this
methodology on the variety of substrate and to avoid pre-
vious limitations, and to check the selectivity of the pre-
sent protocol, we attempted the synthesis of flavanone
4
, 5, 6, 7) bearing ester moiety at C-3 position of O-allyl
THβC methyl ester; we treated with iodine in DMSO/
[
19]
H O under the standardized conditions leading to for-
(Scheme 3).
When
2
2
mation of corresponding O-allyl-β-carboline methyl ester
Table 2, 6d, 6e, 6f, 6g) with 72–80% yield. Similarly, the
meta O-allyl substituted β-carboline-methyl ester gave
0% yield.
Further, we extended our investigation toward the
selective deallylation followed by dehydrogenation of O-
the
(E)-3-(4-[allyloxy]-3-methoxyphenyl)-
(
1-(2-[allyloxy]-5-methylphenyl)prop-2-en-1-one 8a was
ꢀ
treated with iodine in DMSO/H O at 100 C, it gives
2
2
7
2-(4-[allyloxy]-3-methoxyphenyl)-3,6-dimethyl-4H-
chromen-4-one 9a with 85% yield (Scheme 3).
The methods show the new approaches for the non-
O-deallylation followed by aromatization of O-allyl-
THβC. Compared to the previous studies, this method is
more superior in terms of the selectivity, temperature,
reaction time, cost, and efficiency.
[19,20]
allyl THβC; As per our previous report
, we have
found that deallylation proceed by using molecular
+
ꢀ
iodine in DMSO+H (20 mol %) at 60 C and optimized
condition in our hand (Table 1, entry 11). Thereafter, we
have applied our previously developed protocol molecu-
+
ꢀ
lar iodine in DMSO/H with 50 and 100 mol% at 90 C; it
was observed that the aromatization of O-allyl-tetrahydo-
β-carboline 3a, 3b, and 3g (Table 2, entry 11, 12) gave 4a,
3 | POSSIBLE MECHANISM
5
a with 75–80% yield with deallylation (Table 2,
entry 11, 12).
Recently, we have reported a novel methodology for
chemoselective aromatization of THβC with intact of the
On the basis of the above experiments, a tentative reac-
tion mechanism is delineated in (Scheme 4. This path-
way involves the generation of iodonium ion in acidic
medium from the reaction of DMSO and I , subsequently
2
[18]
acid group at C-3 position (Scheme 2) ; O-allyl
tetrahydro-β-carboline-3-carboxylic acid 3b was treated
with I /DMSO/H O at 70 C for 30 min to 1.5 h afforded
β-carboline-3-carboxylic acid 7a, 62% yield with retention
of acid group (Scheme 2). The O-allyl–substituted
tetrahydro-β-carboline-3-carboxylic acid 3c, 3d (Table 2,
regenerating the molecular iodine (I ) by further removal
2
.
[24]
of H O and DMS (Scheme 4)
2
ꢀ
In the acidic medium, the O-allyl oxygen protonated
molecular iodine, which activates the C C bond and
forms a three-member iodonium intermediate stabilized
2
2 2
[24c]
with protic DMSO solvent forming solvated complex,
entry 9, 10) treated with I2 (1equiv.) DMSO H O2
and DMSO solvent might be helping in deallylation pro-
cess. (Scheme 4b). The iodide ion is essential, which initi-
ates the reaction and forms deallylated product 4 with
molecular iodine and formation of allyl alcohol in the
reaction could not be traced.
2
ꢀ
(1 equiv.) at 70 C for 1 h afforded O-allyl–substituted
In diversely, the use of eternal oxidant H O in the
2
2
reaction medium facilitated the oxidation of iodide to
[
24a–27]
iodine,
and best yield was obtained when H O
2 2
SCHEME 2 Synthesis of 1-(4-[allyloxy] phenyl)-9H-pyrido
3,4-b]indole-3-carboxylic acid
[
SCHEME 4 Proposed mechanism for aromatization and
SCHEME 3 Synthesis of flavanone
deallylation of allyl ether using iodine in DMSO

6
GAIKWAD ET AL.
+
THβC using iodine in DMSO, H . The metal-free
milder condition, operational simplicity, and low tem-
perature reaction are some of the salient features that
render this protocol to be a better alternative to the
regular organic synthesis.
ACKNOWLEDGMENTS
Sunil V. Gaikwad is thankful to UGC-BSR. (UGC file No
F.4-3/2006[BSR] dated Dec 2011), UGC-New Delhi.
DATA AVAILABILITY STATEMENT
Additional supporting information found online in the
Supporting Information section at the end of this article.
ORCID
SCHEME 5 Proposed mechanic for aromatization of O-
allyl THβC
Sunil V. Gaikwad https://orcid.org/0000-0003-0754-
1945
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