T3P mediated domino C(sp2)-H sulfenylation/annulation of enaminones and methylsulfinyls for the synthesis of chromone thioether derivatives

Article abstract of DOI:10.1039/c8nj05554h

A new regioselective method for the synthesis of 3-(methylthio)-4H-chromen-4-one and 3-(phenylthio)-4H-chromen-4-one derivatives has been developed. The reaction between o-hydroxy-phenyl-functionalized enaminones and methylsulfinyl derivatives using T3P gave good yields of chromone thioether derivatives. The reaction proceeds via domino chromone ring construction and C(sp²)-H bond sulfenylation under transition-metal-free conditions.

Full text of DOI:10.1039/c8nj05554h

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T3Ps mediated domino C(sp2)–H sulfenylation/
annulation of enaminones and methylsulfinyls for
the synthesis of chromone thioether derivatives†
Cite this:DOI: 10.1039/c8nj05554h
C. Balakrishna,ab Ramakrishna Gudipati,a Venu Kandula,a Satyanarayana Yennam,a
a
P. Uma Devibc and Manoranjan Behera
*
A new regioselective method for the synthesis of 3-(methylthio)-4H-chromen-4-one and 3-(phenylthio)-4H-
chromen-4-one derivatives has been developed. The reaction between o-hydroxy-phenyl-functionalized
enaminones and methylsulfinyl derivatives using T3Ps gave good yields of chromone thioether derivatives.
The reaction proceeds via domino chromone ring construction and C(sp2)–H bond sulfenylation under
transition-metal-free conditions.
Received 1st November 2018,
Accepted 17th December 2018
DOI: 10.1039/c8nj05554h
rsc.li/njc
considered as a privileged structure.11 3-Sulfenylated chromones, a
class of the chromone family having alkyl or aryl thioether linkage
Introduction
Propylphosphonic anhydride (T3Ps) has been commonly used at the 3-position of the chromone moiety, have found extensive
as a water scavenger and a coupling reagent for the synthesis of application in synthetic chemistry.12 Recently, 3-sulfenylated
amides.1 It has useful properties such as broad functional chromone derivatives were tested for their antitumor activity
group tolerance, low toxicity and easy work-up procedures.2 against human solid tumor cell lines including HCCLM-7, Hep-2,
Because of these reasons, new applications have been recently MDA-MB-435S and SW-480.13
developed for this reagent.3 For instance, T3Ps has been used in
The reactions involving C–S bond formation are important
dehydration chemistry that involves the conversion of carboxylic routes to install sulphur fragments into organic compounds.14
acids and amides into nitriles as in the synthesis of alkenes, The C(sp2)–S bonds such as C–S (sulfenyl) bonds play a key role
isonitriles, and substituted heterocycles.4 More recently, a con- in controlling the physical properties and biological activities
venient microwave assisted T3Ps mediated one pot pyrazolone of sulphur containing compounds.15 Development of novel and
synthesis has been reported.5 Microwave-assisted organic efficient methodologies to generate C(sp2)–S bonds has attracted
synthesis (MAOS) is a rapidly growing area in synthetic organic great attention of many organic chemists.16 Several methods are
chemistry.6 Many organic reactions proceed much faster and available for generating thioether molecules with different
with higher yields under microwave irradiation when compared sulfenylating reagents such as thiols, sulfonylchlorides, silver(I)
to conventional heating.7 For this reason, this technique has trifluoromethanethiolate, disulfides, sodium sulfinates, sulphur
been widely used in organic synthesis.8 For example, microwave powder and sulfonylhydrazides.17–19 In this connection, wide-
assisted synthesis of 3-sulfenylindoles has been reported using spread methodologies are available in the literature for the
sulfonyl hydrazides.9
construction of C(sp2)–S bonds viz. the transition-metal-catalysed
Chromone or 1,4-benzopyrone is an important scaffold in a Ullman reaction,20 the Chan–Lam cross-coupling reaction along
large number of natural products with a wide range of biological with the transition-metal-catalysed C–H bond activation.21,22 In
activities.10 The vast range of biological effects associated with recent times, as a new development, the transition-metal-free
this scaffold has resulted in the chromone ring system being oxidative coupling has emerged as a powerful complementary
approach in C–S bond forming reactions.23 The transition metal-
free cross coupling reaction has several advantages. These metal-
free24 cross coupling reactions are often practical, as they are
generally less sensitive to air and moisture. Furthermore, transition
metals are associated with drawbacks such as cost, toxicity,
need for non-commercial ligands and threshold values in
pharmaceutical products. The metal free cross-coupling trans-
formations have been successfully employed in the synthesis of
many useful organic compounds elaborated with sulfenyl groups.
a Chemistry Services, GVK Biosciences Pvt Ltd, Survey No. 125 (part) & 126,
IDA Mallapur, Hyderabad-500076, Telangana, India.
E-mail: Manoranjan.behera@gvkbio.com; Tel: +91 40 66281805
b Department of Chemistry, GITAM(Deemed to be University), Visakhapatnam,
Rushikonda, Andhra Pradesh State, 530045, India
c Department of Chemistry Gayatri Vidya Parishad College for U. G and
P. G Rushikonda, Vishakhapatnam, Andhra Pradesh State, 530045, India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8nj05554h
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Table 1 Screening optimization conditionsa
Entry T3Ps [equiv.] Solvent T [1C] DMSO [equiv.] Yieldb [%]
1
2
3
4
5
6
7
8
1.0
1.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
3.0
2.0
—
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
DMAC
DMF
RT
60
60
60
3.0
3.0
3.0
1.0
2.0
5.0
5.0
5.0
5.0
8.0
8.0
8.0
8.0
20.0
8.0
—
10
25
40
5
100
100
100
100
100
100
100
100
130
100
100
20
59
56
52
63
91
83
73
47
61
57
60c
0
9
THF
THF
10
11
12
13
14
15
16
17
EtOAc
DMAC
EtOAc
EtOAc
EtOAc
Me2SO4 100
EtOAc 100
Fig. 1 Synthetic approach of 3-sulfenylated chromone.
8.0
Wan and co-workers have reported iodine mediated radical
C(sp2)–H sulfonylation and C(sp2)–N bond oxygenation enabling the
chemoselective synthesis of 3-sulfonylated chromones by using
enaminones and sulfonylhydrazines12d (Fig. 1A). On the other hand,
the same group introduces the similar methodologies for making
3-sulfenylated chromones from enaminone using sulfonylhydrazines
and thiophenols via a KIO3-catalyzed domino reaction12h,k (Fig. 1B).
The only difference in the methodology is the use of KIO3 instead of
iodine for radical C(sp2)–H sulfenylation. However, the Yokoe group
in 1994 first reported the preparation of 3-methylthiochromone by
the reaction of dimethyl(methylthio)sulfoniumtetrafluoroborate with
enaminone12j (Fig. 1C). Later on in 2014, a facile and efficient
synthetic strategy has been reported by the Yang group using AgSCF3
and trichloroisocyanuric acid12a (Fig. 1D).
Most of the methods reported to date for the preparation of
3-sulfenylated chromones used NH4I, KIO3 and I2 as oxidants.
However to the best of our knowledge, T3Ps has not been used for
the preparation of 3-sulfenylated chromones using enaminones. In
our continuous effort to prepare natural product hybrids,25 we
developed interest in preparing the chromone thioether derivatives.
Earlier, we have reported the preparation of chromone using
T3Ps.26 Herein, we report our work on this interesting new
methodology for the preparation of 3-sulfenylated using T3Ps.
a
General conditions: enaminone 1a (1.0 equiv.), DMSO (8.0 equiv.), T3Ps
(2.0 equiv.), THF (1 mL), 100 1C. All reactions were run for 10 h. b Yields of
isolated products from enaminone. DMAC = N,N-dimethylacetamide.
c
Chromone [1b].
different reaction media and THF (Table 1, entry 10) was the most
favourable reaction medium. Further investigation revealed that
increasing the temperature to 60 1C gave the product in only 25%
yield (Table 1, entry 2). Further increasing the temperature to
100 1C does increase the yield of the reaction. When the amount
of DMSO was varied, it was found that 8.0 equivalents of DMSO are
optimal. But the use of excess DMSO and T3Ps for the sulfenyla-
tion reaction under the same reaction conditions did not increase
the yield much (Table 1, entries 14 and 15) which indicated that
it was sufficient to have 8.0 equivalents of DMSO. We have
also attempted the cyclisation reaction at elevated temperatures
however we did not have better result (Table 1, entry 13). Finally, to
validate that the sulfenylation reaction was mediated by T3Ps, a
control experiment was conducted. As expected, treating com-
pound 1a and DMSO (8.0 equiv.) in EtOAc at 100 1C without
T3Ps for 12 h gave only zero conversion (Table 1, entry 17).
Also, when the reaction was performed taking dimethyl sulphate
instead of DMSO we did not observe any product formation
(Table 1, entry 16) which showed that DMSO acts as a thiomethyl
source in the reaction.
Results and discussion
To prove the generality of this method, various substituted
During the course of our development of a synthetic methodology aryl enaminones were examined and the results are summarized in
for the preparation of chromones using enaminone, it was found Table 2. More importantly previously inaccessible naphthalene
that when enamino ketone 1 was reacted with T3Ps in the analogues and pyrazole analogues were synthesized in very good
presence of DMSO, 3-suflenylated chromone was formed. Surprised yield (compounds 3j and 3n). Also, there is no effect on the
by this result, we planned to investigate the sulfenylation reaction yield of the sulfenylation reaction upon aromatic ring substitu-
further to develop a new synthetic methodology. At the beginning, tion (ortho as well as para substituted enaminones gave similar
the reaction of enaminone with DMSO was tentatively run in the yields). It can be found that substrates having electron-
presence of T3P at room temperature and the 3-sulfenylated donating groups (CH3 and OMe) gave little higher yields than
product 3a was obtained in 10% yield (Table 1, entry 1). Inspired electron-deficient groups (NO2 and Cl). The advantages of using
by this preliminary result, we screened the cyclization reaction in T3Ps in the sulfenylation reaction compared to other available
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Table 2 Scope of 3-sulfenylated chromones using T3Ps and DMSOa,b
Table
3
Scope of 3-sulfenylated chromones using T3Ps and aryl
methylsulfoxidea,b
a
General conditions: enaminone 1a (1.0 equiv.), DMSO (8.0 equiv.),
T3Ps (2.0 equiv.), THF (1 mL) 100 1C. All reactions were run for 12 h.
a
General conditions: enaminone 1a (1.0 equiv.), aryl methyl sulfoxide
b
Yields of isolated products from enaminone 1. T3Ps (50% in EtOAc
(2.0 equiv.), T3Ps (2.0 equiv.), DMAC (0.3 mL), 90 1C. All reactions were
solution).
b
run for 10 h. Yields of isolated products from enaminone 1. DMAC =
N,N-dimethylacetamide. T3Ps (50% in EtOAc solution).
methods are low toxicity, non-allergenic, water soluble by
products and mild reaction conditions. This is the reason for
T3Ps being classified as a green reagent. Based on the above results,
we want to explore other commercially available sulfoxide for the
sulfenylation reaction with enaminone 1. When di-phenyl sulfoxide
was used, we did not observe any sulfenylation reaction instead only
chromone was isolated. But taking methyl phenylsulfoxide we could
get 3-thioaryl-chromone in good yield. After screening the optimal
conditions, various substituted enaminones were subjected to the
sulfenylation reaction using methyl phenylsulfoxide and the results
are summarised in Table 3. All the 3-thioaryl-chromones were well
characterized by 1H-NMR, 13C-NMR and LC-MS data. The yields
obtained in the sulfenylation reaction with methyl phenylsulfoxide
were moderate to good.
Fig. 2 Control experiments.
To investigate the possible process of the reaction, few
control experiments were designed and conducted as outlined presence of one methyl group in the sulfoxide reagent is must for
in Fig. 2. Thus, when the chromone 1b was treated with T3Ps the success of the sulfenylation reaction. Furthermore, we
in the presence of DMSO at 100 1C for 12 h, no product was performed a reaction in PPA instead of T3Ps under the same
formed. The result indicate that annulation to chromone is not conditions to obtain the product in only 33% yield.
the initial step in the reaction. The next step was the sulfenyla-
A plausible reaction mechanism of the deaminative sulfenylation
tion reaction of enaminone with T3Ps and di-phenylsulfoxide. was proposed in Fig. 3. The first step of this mechanism is the nucle-
Chromone 1b was formed instead of 3a when di-phenylsulfoxide ophilic attack of oxygen atoms of DMSO to form T3Ps-complex III.
was considered as the sulfenylation source. We conclude that the The 2nd nucleophilic attack of enaminone on T3Ps methyl
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of 50% T3Ps in EtOAc (343 mg, 1.04 mmol) and methyl phenyl
sulfoxide (145 mg, 1.04 mmol) at 0 1C. After 15 min, the
reaction mixture was heated to 90 1C for 10 h. The reaction
progress was monitored by TLC. Then the reaction mixture was
cooled to room temperature, diluted with ethyl acetate, washed
with water and brine, dried with anhydrous Na2SO4, and
concentrated under vacuum. The crude residue was purified
by flash chromatography (silica gel, hexane/EtOAc) to give
desired product 4a (81%) as a colourless solid. The same
procedure was used to prepare compounds 4b–4p.
Conflicts of interest
Fig. 3 Plausible mechanism of 3-sulfenylated chromone.
There are no conflicts of interest.
sulfoxide complex III to neutralize sulfonium ions generated
complex IV which undergoes cyclization to form cyclic intermediate V.
Intermediate V undergoes the E1cb mechanism to eliminate
dimethyl amine and form 3-sulfinyl-chromone derivative VI. The
sulfinyl complex VI undergoes intramolecular methyl transfer to
form the final compound 3a. This explains the basic requirement
of one methyl group in the sulfinyl complex VI for the success of
the T3Ps catalyzed sulfenylation reaction using methyl sulfinyls.
Acknowledgements
The authors are grateful to GVK Biosciences Pvt Ltd, for the
financial support and encouragement. Help from the analytical
department for the analytical data is appreciated. We thank
Dr Sudhir Kumar Singh for his invaluable support and motivation.
Notes and references
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In summary, a new methodology has been established by the
reaction between o-hydroxyphenyl-functionalized enaminones and
methylsulfinyl derivatives using T3Ps via domino chromone ring
construction and C(sp2)–H bond sulfenylation under transition-
metal-free conditions. This is the first report of T3Ps mediated
preparation of 3-sulfenylated chromone. We believe that this method
will find wide spread application in the synthesis of sulfenylated
chromone derivatives.
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