Synthesis of [1,4]Thiazino[4,3- a]indol-10-one Derivatives through Radical Anti Aza-Michael Addition of 2′-Aminochalcones

Article abstract of DOI:10.1021/acs.orglett.1c02160

An efficient method for the synthesis of [1,4]thiazino[4,3-a]indole derivatives using sodium chlorodifluoroacetate (ClCF2CO2Na) and elemental sulfur as the difluoromethylthiolating reagent system has been developed. Three-component reactions of 2′-aminochalcones, sulfur, and ClCF2CO2Na under basic conditions using TEMPO as the oxidant afforded [1,4]thiazino[4,3-a]indol-10-ones containing a difluoromethyl thioether moiety in good yields. Four bonds including one C-N, two C-S, and one C-C bonds are selectively formed in the sequential transformation process.

Full text of DOI:10.1021/acs.orglett.1c02160

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Letter
Synthesis of [1,4]Thiazino[4,3‑a]indol-10-one Derivatives through
Radical Anti Aza-Michael Addition of 2′-Aminochalcones
Pingshun Zhang, Linjie Yang, Wanzhi Chen,* Miaochang Liu,* and Huayue Wu
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ABSTRACT: An efficient method for the synthesis of [1,4]thiazino[4,3-a]indole derivatives using sodium chlorodifluoroacetate
(ClCF2CO2Na) and elemental sulfur as the difluoromethylthiolating reagent system has been developed. Three-component
reactions of 2′-aminochalcones, sulfur, and ClCF2CO2Na under basic conditions using TEMPO as the oxidant afforded
[1,4]thiazino[4,3-a]indol-10-ones containing a difluoromethyl thioether moiety in good yields. Four bonds including one C−N, two
C−S, and one C−C bonds are selectively formed in the sequential transformation process.
rganofluorine compounds exhibit unique physicochem-
thiolether as the difluoromethylthiolation agents including
PhSCF2 SiMe3 ,6 PhSCF2 Br,7 PhSO2 CF2 H,8 and
PhSO2CF2SiMe3, etc.9 Cyclic difluoromethyl thioethers have
been less studied, and only a few synthetic methods have been
reported in recent years. Basically, the synthesis of cyclic
difluoromethyl thioethers can be accomplished through
nucleophilic reactions of thiols and CF2-containing compounds
(Scheme 1, a and b). DBU-catalyzed [4 + 2] annulation
between gem-difluoroolefins and 2-mercaptobenzaldehydes
successfully gave rise to the desired products.10 The reaction
of 2-mercaptophenone and ClCF2COONa yielded 2,2-
difluoro-2,3-dihydrobenzo[b]thiophens, which was achieved
through reaction of a thiolate with difluorocarbene, followed
by intramolecular nucleophilic addition.11 Alternatively, cyclic
difluoromethyl thioethers could be obtained from SCF2-
containing compounds (Scheme 1, c and d). In this context,
2,2-difluorothiochromane derivatives were obtained via radical
addition of difluoromethyl xanthate to various terminal
alkenes12 or visible-light-induced arylthiofluoroalkylations of
unactivated heteroarenes and alkenes.13
O
ical and chemical reactivities, which have been widely
used as materials and pharmaceuticals.1 Selectively introducing
a fluorine atom or fluoroalkyl group can effectively change the
biological activity, physical activity, and reactivity of organic
molecules (Figure 1).2 Among various fluorinated moieties
Figure 1. Examples of drug molecules containing −SCF2− scaffolds.
Obviously, the combination of sulfur and ClCF2CO2Na is
employed to modify the properties of a targeted products,
difluoromethylene group CF2 was the most prevailing one.3 In
particular, incorporation of a difluoromethyl thioether moiety
SCF2 to a drug molecule often enhances its metabolic
property, lipophilicity, and oxidative stability.4 Thus, the
development of efficient approaches to construct difluoromeh-
tyl thioethers are appealing and valuable for pharmaceutical
industry.5
the most convenient and economic agent for the construction
−
of ambiphilic −SCF2 moiety. We have described that three-
component reactions of 2′-hydroxychalcones, ClCF2CO2Na,
and sulfur under basic conditions afforded HCF2S-containing
Received: June 28, 2021
Published: July 13, 2021
These difluoromehtyl thioethers are usually synthesized by
sequential sulfenylation and fluoroalkylation reactions.4b
Various compounds containing the −SCF2− moiety have
been employed in the construction of difluoromethyl
https://doi.org/10.1021/acs.orglett.1c02160
© 2021 American Chemical Society
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a
Scheme 1. Synthesis of Cyclic Difluoromethyl Thiolethers
Table 1. Optimization of Reaction Conditions
entry
base
oxidant
solvent
yield (%)
1
2
3
4
5
6
7
8
9
10
KOCH3
KOCH3
KOCH3
NaOt-Bu
Cs2CO3
KOAc
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
DTBP
DMF
NMP
HMPA
NMP
NMP
NMP
NMP
NMP
NMP
NMP
32
60
45
14
78
23
15
52
43
27
DBU
Cs2CO3
Cs2CO3
Cs2CO3
K2S2O8
1,4-BQ
a
Reaction conditions: chalcone 1a (0.2 mmol), ClCF2CO2Na 2a (0.6
mmol), S8 (1.2 mmol), base (0.6 mmol) oxidant (0.4 mmol), 3 Å
molecular sieve (100 mg), and solvent (1.5 mL) at 70 °C in air for 30
h. DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene; TEMPO = 2,2,6,6-
tetramethyl-1-piperidinyloxy; DTBP = 2-(tert-butylperoxy)-2-methyl-
propane; BQ = benzoquinone.
Compound 3a was isolated in 60% yield when NMP was the
solvent. The effect of various inorganic and organic bases was
examined (Table 1, entries 4−7). It seems that organic bases
are less efficient, and Cs2CO3 is the most suitable base in
which case 3a was isolated in 78% yield. Finally, we checked
the influence of the oxidant, and we found that TEMPO is the
best oxidant among the inorganic and organic oxidants
examined (Table 1, entries 5 and 8−10). Unexpectedly,
pyridine-N-oxide with the similar structure as TEMPO is
totally ineffective. The full optimization details are listed in the
Supporting Information (Table S2).
After optimization, we explored the substrate scope of 1. As
shown in Scheme 2, the double cyclization reaction involving
difluoromethylthiolation of a variety of 2′-amidochalcones
bearing electron-donating and electron-withdrawing substitu-
ents such as alkyl, phenyl, alkoxy, cyano, acetyl, CF3, NO2,
CH3SO2, and halogen groups at the 4-positions occurred
under the mild conditions furnishing 3a−3m in good yields
(49−85%). The results told that electron-donating groups are
preferred. Unexpectedly, the transformation of N-(2-(3-(4-
(dimethylamino)phenyl)acryloyl)phenyl)acetamide 1n failed
to give the corresponding product 3n, and most of 1n was
recovered. The reason is not clear at the present. The m-
substituted substrates having an electron-donating and
electron-withdrawing group, including of alkoxy, CF3, and
halogen were well tolerated and gave 3o−3r in good yields.
Unfortunately, the reactions of N-(2-(3-(2-methoxyphenyl)-
acryloyl)phenyl)acetamide and N-(2-(3-(2-chlorophenyl)-
acryloyl)phenyl)acetamide afforded 3s and 3t in relatively
lower yields, 41% and 38%, respectively, due to steric
hindrance. The polysubstituted substrates having alkoxy and
alkyls groups were successfully transformed to 3u−3x in 59−
73% yields. The naphthalene derivatives reacted similarly with
sulfur and ClCF2CO2Na to give 3y and 3z, respectively.
4H-chromen-4-one and 9H-thieno[3,2-b]chromen-9-one de-
rivatives in good yields.14 The transformation is initiated by
oxa-Michael addition. We inferred that in the case of 2′-
aminochalcones radical anti-Michael addition would happen to
afford difluoroalkylthiolated indole derivatives in the presence
of an oxidant. Herein, we report the reaction of 2′-
aminochalcones, sulfur, and ClCF2CO2Na. A variety of
indolins-3-one derivatives were obtained in good yields.
We began our optimization tests by using N-(2-(3-(p-
tolyl)acryloyl)phenyl)acetamide 1a as the model substrate.
Reaction of 1a, sulfur, and ClCF2CO2Na 2a was initially
performed in DMF in the presence of KOCH3 and 2,2,6,6-
tetramethylpiperidinooxy (TEMPO). As expected, an indole
derivative 3a containing a SCF2 unit was generated in 32%
yield through anti-aza Michael addition (Table 1, entry 1).
Unexpectedly, further cyclization to form a thiazine ring was
−
observed due to unusual nucleophilic addition of −SCF2 to
the amide group. The structure of 3a was identified by NMR
spectroscopy and finally determined by X-ray diffraction
analysis. Next, we examined the effect of various solvents.
Strongly polar solvents are preferred (Table 1, entries 2 and 3).
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Scheme 2. Scope of Substrates 1
Scheme 3. Reaction of Unprotected 2′-Aminochalcone
mixtures of 6a and 6b were afforded in 74% and 75% yields,
respectively.
Scheme 4. Scope of Substrates 4
When N-(2-(3-(p-tolyl)acryloyl)phenyl)ethanesulfonamide
7a was treated with sulfur and ClCF2CO2Na in DMF in the
presence of Cs2CO3 and TEMPO, the reaction did occur, but
affording the fused compound 1-(p-tolyl)-3H,9H-thiazolo[3,4-
a]indol-3,9-dione 8a as a light-yellow solid in 40% yield
(Scheme 5). Probably, the SCO moiety is originated from
hydrolysis of thiocarbonyl fluoride (SCF2) in situ generated
from sulfur and ClF2COONa, as reported.18
We also tested the reactions of 2′-acetamionchalcone
analogues containing a furyl, thiophene-yl, or 2-pyridine
groups. These heterocyclic compounds also proceeded
smoothly, and the corresponding [1,4]thiazino[4,3-a]indole
derivatives 3aa−3dd were obtained in 73−84% yields.
Unfortunately, the 3-pyridine derivative was totally unreactive,
and no desired product was isolated. It was interestingly
observed that the sequential reaction of 4-imidazolyl-2′-
acetamionchalcone afforded [1,4]thiazino[4,3-a]indol-10-one
derivative 3ee in 42% yield, and further difluoromethylth-
iolation simultaneously occurred at 2-position of imidazole
group. 2-(Difluoromethylthio)-1H-imidazoles are known com-
pounds which have been prepared through nucleophilic
substitution of S-(difluoromethyl)sulfonium salt15 or radical
coupling of Zn(SO2CF2H)16 with 1H-imidazole-2-thiols. The
three-component reaction is not compatible with the
unsaturated ketone with an alkyl substituent at 3-position,
and thus, 3ff was not obtained. Finally, 2′-acetamionchalcones
containing alkoxy groups at the 4′- and 5′-positions also gave
the desired [1,4]thiazino[4,3-a]indol-10-ones 3gg and 3hh in
moderate yields. The N-acetyl group could also be replaced by
other aliphatic acyl groups with isolated yields of 54−70%.
However, reactions of 2′-benzoamino and 2′-pivalamino
compounds yielded complicated mixtures.
Scheme 5. Reaction of Substrates 7a
The three-component reaction of 2-acylamino chalcone is
quite fascinating since four bonds including one C−N, two C−
S, and one C−C bonds have to be selectively formed in one-
pot. The synthetic utility of the reaction was demonstrated by
a scale-up experiment. Compound 3e was obtained in 78.6%
yield at a gram-scale. To gain some mechanistic insights into
the transformation, a few control experiments were performed.
The control experiments clearly showed that the cyclization
was initiated via radical anti-Michael addition (see the
Supporting Information). It was reported that the nucleophilic
aza-Michael addition of 2′-aminochalcone affords 2,3-dihy-
droquinolin-4(1H)-ones due to 6-endo-trig cyclization.17 The
present anti-Michael addition and 5-exo-trig cyclization of 2′-
aminochalcone indicate its radical nature.19 Based on the
above observations and the literature reports,20 a plausible
radical mechanism is proposed (Scheme 6). The reaction was
initiated by deprotonation of 1 giving acylamido anion A,
which was readily oxidized by TEMPO to generate the
acylamido radical B. The subsequent intraradical addition of B
It has been reported that unprotected 2′-aminochalcones
were easily converted to 2,3-dihydroquinolin-4(1H)-ones via
simple aza-Michael addition.17 Under the conditions stated in
Scheme 2, only 2′-(2-chloro-2,2-difluoroacetamido)chalcone
5a was isolated in 49% yield from the reaction of 2′-
aminochalcone under our conditions, as shown in Scheme 3.
Then we turned to 2′-alkylaminochalcones (Scheme 4). Both
sulfur and fluorine were not involved in the products, and the
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Authors
Scheme 6. Plausible Mechanism
Pingshun Zhang − Department of Chemistry, Zhejiang
University, Hangzhou 310027, China
Linjie Yang − Department of Chemistry, Zhejiang University,
Hangzhou 310027, China
Huayue Wu − College of Chemistry and Materials
Engineering, Wenzhou University, Wenzhou 325027, China;
orcid.org/0000-0003-3431-561X
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c02160
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(No. 22071214) for financial support.
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c02160.
Experimental procedures and spectra (PDF)
Accession Codes
CCDC 2079900−2079901 and 2080082 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Wanzhi Chen − Department of Chemistry, Zhejiang
University, Hangzhou 310027, China; orcid.org/0000-
0002-7076-1521; Email: chenwzz@zju.edu.cn
Miaochang Liu − College of Chemistry and Materials
Engineering, Wenzhou University, Wenzhou 325027, China;
orcid.org/0000-0002-4603-3022; Email: mcl@
wzu.edu.cn
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