Synthesis of thiochromans via [3+3] annulation of aminocyclopropanes with thiophenols

Article abstract of DOI:10.1021/acs.orglett.0c03528

We report the one-pot synthesis of 4-amino thiochromans using simple aminocyclopropanes and thiophenols through a formal [3+3] annulation reaction. This reaction proceeds under mild conditions with good functional group tolerance. The thiochroman core was

Full text of DOI:10.1021/acs.orglett.0c03528

pubs.acs.org/OrgLett
Letter
Synthesis of Thiochromans via [3+3] Annulation of
Aminocyclopropanes with Thiophenols
́
̂
Ming-Ming Wang, Seongmin Jeon, and Jerome Waser*
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ABSTRACT: We report the one-pot synthesis of 4-amino thiochro-
mans using simple aminocyclopropanes and thiophenols through a
formal [3+3] annulation reaction. This reaction proceeds under mild
conditions with good functional group tolerance. The thiochroman
core was formed with complete regioselectivity, and modification of
complex drug molecules containing an aminocyclopropane was also
realized.
Scheme 1. Oxidative Difunctionalization of
Aminocyclopropanes and [3+3] Annulations
minocyclopropanes are important building blocks in
1
Aorganic chemistry. While donor−acceptor cyclopropanes
with amino groups as donors (D−A aminocyclopropanes)
have attracted an enormous amount of attention in recent
years from organic chemists and much progress has been
achieved,2 simple aminocyclopropanes lacking electron-with-
drawing groups at the β position are relatively less studied for
ring-opening transformations. Current methodologies mainly
focused on transition metal catalysis3 or photoredox/electro-
catalysis4 to activate either the C−C bond or the amino group.
̈
In 2019, inspired by recent progress in the Hofmann−Loffler−
Freytag (HLF) reaction,5 our group reported a metal-free
method for the activation of simple aminocyclopropanes 1 via
halogenated intermediate I.6 The synthetic equivalent of a 1,3-
biscationic synthon was obtained in the form of 3-halogenated
N,O-acetals 2 after reaction with methanol (Scheme 1A).
Linear 1,3-difunctionalized amines were synthesized by adding
two nucleophiles sequentially to crude 2. Inspired by recent
examples of [3+3] annulations using donor−acceptor cyclo-
propanes as 1,3-zwitterionic synthons for annulation with 1,3-
dipoles (Scheme 1B),7 we wondered if the biscationic synthons
developed by our group could be exploited for annulation
reactions with bis-nucleophiles. During our investigation of the
reaction of these synthons with nucleophiles, we observed the
formation of thiochroman products with thiophenol (Scheme
1C). On the basis of the favored reaction of nucleophiles at the
acetal position, we expected the formation of 2-amino
thiochroman 4 via N,S-acetal 3. However, two-dimensional
NMR spectra indicated that 4-amino thiochroman 5 had been
formed. This nonclassical regioselectivity attracted our interest
and motivated us to investigate the scope of this new type of
[3+3] annulation as well as the reaction mechanism.
Herein, we report the result of these studies leading to a
highly regioselective [3+3] annulation reaction to yield 4-
amino thiochromans. Although thiochroman and its derivatives
have exhibited biological activities,8 efficient methods for the
construction of the thiochroman core remain rare.9 To the best
of our knowledge, there are only two reports disclosing [3+3]
annulations for the efficient synthesis of thiochromans from
Received: October 23, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03528
© XXXX American Chemical Society
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A

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a
thiophenols. In 2018, Pullarkat and co-workers realized a triflic
acid-catalyzed tandem allylic substitution−cyclization reaction
for the synthesis of 1,4-disubstituted thiochromans.10 A high
reaction temperature was necessary to force ring closure. More
recently, Namboothiri and co-workers reported the synthesis
of thiochromans by using indoline-2-thione as the starting
material.11 It is therefore of high interest to develop a [3+3]
formal annulation between simple aminocyclopropanes and
thiophenols for accessing 4-amino thiochromans.
Scheme 2. Scope of Thiophenols
On the basis of our preliminary result, we chose N-
cyclopropylbenzamide 1a as the starting material for the
preparation of biscationic synthon intermediate 2a by reaction
with NIS under acidic conditions in chloroform (Table 1).
a
Table 1. Optimization of the Reaction Conditions
a
b
Reactions were performed at a 0.4 mmol scale for the indicated time
entry
deviation from the standard condition
yield (%)
b
unless otherwise noted. The reaction mixture was stirred for 20 h in
the second step.
1
2
3
4
5
6
none
DCM as solvent
MeCN as solvent
without (PhO)2PO2H
adding thiophenol at the beginning
NBS instead of NIS
79
78
48
70
10
a
Scheme 3. Scope of Aminocyclopropanes
c
38
a
Reactions were performed at room temperature under air for the
b
indicated time. The yield was determined by 1H NMR using CH2Br2
as the internal standard. The reaction mixture was stirred for 3 h in
c
the first step.
After formation of 2a, 4-methylbenzenethiol was added to the
reaction mixture and a good yield of 5a was obtained (entry 1).
Running the reaction in other solvents like dichloromethane or
acetonitrile was also possible (entry 2 or 3, respectively). The
reaction took place even in the absence of diphenyl phosphate,
giving 5a in 70% yield after the same reaction time (entry 4).
However, adding 4-methylbenzenethiol at the beginning of the
reaction gave a low yield, probably due to decomposition of
thiophenol mediated by NIS (entry 5). With NBS as the
electrophile instead of NIS, the first step was efficient, but only
38% of 5a was formed during the second step (entry 6).
With the optimal conditions in hand, we then examined the
scope of thiophenols (Scheme 2). A series of thiophenols with
a methyl substituent at the para, meta, and ortho positions gave
the corresponding thiochromans 5a−d with a methyl group at
positions 6, 7(5), and 8, respectively, in good yields. With 4-
tert-butyl- or 2,4-dimethyl-substituted thiophenols, 5e and 5f
were obtained in 71% yield. A slightly lower yield was observed
for 5g when using unsubstituted thiophenol as the reaction
partner. With a phenyl- or electron-donating substituent like a
methoxy or a methylthio group at the para position, products
5h−j were obtained in yields ranging from 35% to 46%.
Electron poor thiophenols like 3-fluorothiophenol or methyl 2-
mercaptobenzoate were also tolerated, affording products 5k/
5l and 5m in 41% and 44% yields, respectively. Naphthalene-1-
thiol and naphthalene-2-thiol gave the corresponding products
5n and 5o in 80% and 78% yields, respectively.
a
Reactions were performed at a 0.4 mmol scale for the indicated time.
The second step was carried out at −20 °C.
b
the benzene ring, product 6a was isolated in a low yield,
possibly due to polymerization of the intermediate. With
electron-withdrawing groups on the benzene ring like 4-fluoro
or 4-nitro, products 6b and 6c were formed in 67% and 79%
yields, respectively. The structure of 6c was confirmed by X-ray
diffraction.12 A 2-furoyl group on the aminocyclopropane was
well tolerated, giving product 6d in 67% yield. With a pivalate
protecting group, product 6e was isolated in 76% yield.
Carbamate-substituted cyclopropanes can also undergo this
transformation, giving desired products 6f and 6g in 58% and
52% yields, respectively. The synthesis of 6f was scaled up to
10 mmol, and 1.61 g (51% yield) of 6f was obtained.
The amide-substituted cyclopropanes needed for the
annulation can be easily accessed by standard amide bond
formation between a carboxylic acid and commercial amino-
cyclopropane. Therefore, the [3+3] annulation can be used for
We then explored the reaction scope for aminocyclopro-
panes with different protecting groups (Scheme 3). With
electron-donating substituents such as a 4-methoxy group on
B
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the late-stage modification of drugs containing carboxylic acids.
This strategy was applied to the lipid-lowering agent
bezafibrate and the anti-inflammatory drug isoxepac: thiochro-
man products 7 and 8 were formed in 68% and 67% yields,
respectively (Scheme 4A).
Scheme 5. Experiments for the Elucidation of the Reaction
Mechanism
Scheme 4. Late-Stage Modification and Functionalization of
the Products
To highlight the synthetic utility of our 4-amino thiochro-
man products, a series of transformations were performed
(Scheme 4B). With Boc as the protecting group, free amine 9
was easily obtained from 6g in 76% yield. The thiochroman
product can also be oxidized to sulfoxide 10 (97% yield from
5a) or sulfone 11 (quantitative yield from 6f). It is important
to note that 4-amino thiochroman derivatives were reported to
be potent aldose reductase inhibitors13 or sirtuin 5 (SIRT5)
inhibitors.14 They were also reported to have excellent
selective antagonistic activity on an α1D adrenergic receptor.15
The unexpected formation of 4-amino thiochromans
motivated us to perform a series of experiments to gain
some insight into the mechanism. By performing the standard
reaction in CDCl3 and following the progress of the reaction
reaction took place efficiently only under basic conditions (eq
4) and conversion of isolated product 13 into thiochroman
product 5a was not very efficient even in the presence of 1.0
equiv of TFA for 16 h, as it was accompanied by the formation
of imine intermediate 14 (eq 5). When 2a was mixed with
methyl p-tolyl sulfide, no product 15 resulting from an
intermolecular Friedel−Crafts reaction was observed (eq 6).
On the basis of these experiments, we propose a speculative
mechanism (Scheme 6). Once 4-methylbenzenethiol was
added, N,S-acetal 3a was formed as the first intermediate.
Due to the nucleophilicity of the sulfur atom, an intramolecular
Scheme 6. Proposed Speculative Reaction Mechanism
1
by H NMR, we observed the conversion of 2a into a first
intermediate, which was isolated in 61% yield and was
identified as N,S-acetal 3a after purification by column
chromatography (Scheme 5, eq 1). The transformation from
3a to thiochroman 5a was spontaneous in CDCl3 and was
complete in 2 h, indicating the formation of N,S-acetal 3a is
not off-cycle (eq 2). An unknown intermediate, which can be
stabilized by adding excess K2CO3 to the solution, was
observed by 1H NMR during the transformation from 3a to 5a.
However, attempts to isolate this intermediate failed as it was
formed only in a small ratio (see the Supporting Information
for details). 2a was then reduced to give alkyl iodide 12. No
reaction was observed between 4-methylbenzenethiol and 12
under neutral or acidic conditions (eq 3). Indeed, the SN2
C
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SN2 reaction would take place to form sulfonium salt I, which
may be the unknown intermediate observed from H NMR
Complete contact information is available at:
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1
experiments (path A). The following steps involving ring
opening of the four-membered ring, intramolecular Friedel−
Crafts reaction, and deprotonation of II should be fast, as no
other intermediates were observed from 1H NMR experiments.
A mechanism involving a 1,3-shift can be considered, as imine
14 was not observed. Another way to explain the migration of
the sulfur atom from the N,S-acetal carbon atom to the
terminal carbon atom would involve an aza-Petasis Ferrier
rearrangement16 via ion pair III to give IV, followed by an
intramolecular SN2 reaction (path B). Nevertheless, path A
seems to be more reasonable, considering the stability of the
unknown intermediate in the presence of K2CO3, which would
be expected to accelerate an SN2 process.
In summary, we have developed an efficient and
regioselective [3+3] annulation for the synthesis of 4-amino
thiochromans. Good functional group tolerance was observed
for electron-donating and electron-withdrawing substituents on
both thiophenols and aminocyclopropanes. Late-stage mod-
ification of drug derivatives containing an aminocyclopropane
was successful, and deprotection of the amino group was
demonstrated. An enantioselective synthesis as well as other
annulations involving dielectrophilic synthons derived from
aminocyclopropanes is under investigation in our group and
will be reported in due time.
Notes
The authors declare no competing financial interest.
Raw NMR, MS, and IR data are available at zenodo.org (DOI:
10.5281/zenodo.4194211).
ACKNOWLEDGMENTS
■
The authors thank the Swiss National Science Foundation
(SNSF, Grants 200021_165788 and 200020_182798) and
EPFL for financial support. The authors thank Dr. R. Scopelliti
from ISIC at EPFL for X-ray crystallographic analysis.
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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.0c03528.
́
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Accession Codes
CCDC 2034409 contains the supplementary 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
́
̂
Jerome Waser − Laboratory of Catalysis and Organic
Synthesis, Institut des Sciences et Ingénierie Chimique, Ecole
Polytechnique Fédérale de Lausanne, CH-1015 Lausanne,
Switzerland; orcid.org/0000-0002-4570-914X;
Email: jerome.waser@epfl.ch
Authors
́
(5) O’Broin, C. Q.; Fernandez, P.; Martínez, C.; Muniz, K. N-
̃
Ming-Ming Wang − Laboratory of Catalysis and Organic
Synthesis, Institut des Sciences et Ingénierie Chimique, Ecole
Polytechnique Fédérale de Lausanne, CH-1015 Lausanne,
Switzerland; orcid.org/0000-0003-3401-5633
Seongmin Jeon − Laboratory of Catalysis and Organic
Synthesis, Institut des Sciences et Ingénierie Chimique, Ecole
Polytechnique Fédérale de Lausanne, CH-1015 Lausanne,
Switzerland
̈
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