Br?nsted acid-catalyzed chemodivergent reactions of: Ortho -mercaptobenzyl alcohols with 3-alkyl-2-vinylindoles and styrenes: [5+2] cyclization versus hydroxysulfenylation

Article abstract of DOI:10.1039/c6cc00920d

Chemodivergent [5+2] cyclization and hydroxysulfenylation reactions of ortho-mercaptobenzyl alcohols with 3-alkyl-2-vinylindoles and styrenes were established in the presence of Br?nsted acid, which provide efficient methods for constructing benzoxathiepi

Full text of DOI:10.1039/c6cc00920d

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DOI: 10.1039/C6CC00920D
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Brønsted Acid‐Catalyzed Chemodivergent Reactions of ortho‐
Mercaptobenzyl Alcohols with 3‐Alkyl‐2‐vinylindoles and Styrenes:
Received 00th January 20xx,
Accepted 00th January 20xx
[5+2] Cyclization versus Hydroxysulfenylation
*
Jia‐Jia Zhao, Man Tang, Hong‐Hao Zhang, Meng‐Meng Xu and Feng Shi
DOI: 10.1039/x0xx00000x
www.rsc.org/
Chemodivergent [5+2] cyclization and hydroxysulfenylation
reactions of ortho‐mercaptobenzyl alcohols with 3‐alkyl‐2‐
vinylindoles and styrenes have been established in the presence of
Brønsted acid, which provide efficient methods for constructing
benzoxathiepine scaffold and synthesizing ‐hydroxy sulfides at
good yields and excellent diastereoselectivities.
In our previous work, we established an enantioselective [4+2]
cycloaddition of ortho‐hydroxybenzyl alcohols with 3‐alkyl‐2‐
vinylindoles catalyzed by chiral Brønsted acid (B*‐H), which
constructed the chiral chroman framework in high yields and
3
e
excellent stereoselectivities (eq. 3). As a continuation of this work
5
as well as our interests in synthesizing heterocycles, and in order to
develop ortho‐mercaptobenzyl alcohol‐involved reactions for the
construction of sulfurous heterocyclic skeletons, we tentatively
employed ortho‐mercaptobenzyl alcohol to replace ortho‐
hydroxybenzyl alcohol to react with 3‐alkyl‐2‐vinylindoles in the
presence of Brønsted acid (B‐H). Very surprisingly, this reaction
turned out to be an unexpected [5+2] cyclization instead of the
anticipated [4+2] cyclization, which not only constructed a seven‐
membered benzoxathiepine scaffold, but also realized the
Oxygen and sulfur‐containing heterocyclic frameworks exist in
1
lots of natural products and biologically important compounds. So,
the construction of such scaffolds has absorbed much attention
2
from the chemists. In this context, ortho‐hydroxybenzyl alcohol has
proven to be a versatile reactant in building up oxygen‐containing
heterocycles because it can easily transform into ortho‐quinone
methide (o‐QM) intermediate in the presence of an acid, which is
readily to perform conjugate additions with nucleophiles (Nu) or
oxysulfenylation of 3‐alkyl‐2‐vinylindoles (eq. 4).
Previous work:
3
carry out cyclizations with active methylene compounds or olefins.
R
R2
R
R2
As a result, ortho‐hydroxybenzyl alcohol‐involved reactions have
developed rapidly in recent years and many oxygenous heterocyclic
skeletons have been assembled by using this reactant as a building
[
4+2] cycloaddition
R1 (3)
OH
+
R1
O
B*-H, in air
OH
HN
HN
3
block (eq. 1). However, in sharp contrast, the reactions involving
chroman scaffold
ortho‐mercaptobenzyl alcohol, a reactant with similar structure to
ortho‐hydroxybenzyl alcohol, have scarcely been investigated in
spite of the possibility that this reagent might be utilized as a
building block for the construction of sufur‐containing heterocyclic
This work:
R2
R3
R1
R3
R3
R3
O
R1
[5+2] cyclization
OH
(4)
+
N
SH
HN
B-H, in air
S
H
4
R2
frameworks (eq. 2). Therefore, exploring the ortho‐mercaptobenzyl
benzoxathiepine scaffold
alcohol‐involved reactions, especially cyclizations, is very important
for the synthesis of sulfur‐containing compounds and the
R3 OH
R3
R3
R5
R3
S
R6
OH
Hydroxysulfenylation
(5)
construction of sulfur‐containing heterocyclic scaffolds.
R4
R4
+
o-Hydroxybenzyl alcohol-involved reactions: well-developed
SH
R6
B-H, in air
HO
R5
R1
R
2
R
1
R
1
R2
R1
R2
Nu
-hydroxy sulfides
acid
OH
OH
R
Nu
(
1)
-H
2
O
or
A survey of the literature reveals that accomplishing a [5+2]
cyclization is highly desirable for constructing seven‐membered
rings, but rather challenging due to the high degree of ring strain.
or
OH
O
R
O
R
oxygenous
heterocycles
o-QM
6
o-Mercaptobenzyl alcohol-involved reactions: underdeveloped
Besides, most of the established [5+2] cyclization employed
oxidopyrylium ylides as reactants, and ortho‐mercaptobenzyl
alcohol has never been utilized as a synthon in [5+2] cyclizations. So,
this unexpected reaction established an unprecedented [5+2]
cyclization of ortho‐mercaptobenzyl alcohol. More importantly, this
2
1
R2
R1
R2
R1
R
R
7
Nu
OH
Nu
(
2)
or
?
SH
or
SH
S
R
R
sulfurous
heterocycles
[
5+2] cyclization provided an easy access to benzoxathiepine
School of Chemistry and Chemical Engineering, Jiangsu Normal University,
Xuzhou, 221116, China; Tel: +86(516) 83403183; E‐mail: fshi@jsnu.edu.cn.
Electronic Supplementary Information (ESI) available: Experimental details,
scaffold, which constitutes the core structures of many biologically
important compounds. Moreover, this reaction also offered a
unique strategy for oxysulfenylation of 3‐alkyl‐2‐vinylindoles, which
8
†
optimization of conditions, discussion on the reaction mechanisms,
characterization, original NMR spectra of all products, single crystal data of
compounds 3aa, 5aa and 7. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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started from easily available thiophenol and operated in open air. and catalyst loading were carried out in order to impVrioewveArttihclee Oynielinlde
This is very rare in oxysulfenylation of olefins because previous of product 3aa (see ESI for details). FinDalOlyI,: 1u0.n1d03e9r/Cth6eCCo0p0t9i2m0aDl
o
approaches always employed sulfenyl halides, sulfenamides, conditions (at 25 C in toluene catalyzed by 10 mol% 6g), the yield
sulfenate esters, disulfides, sulfonyl hydrazides and so on as of product 3aa was enhanced to a moderate level of 56% with an
9
sulfenylating agents.
excellent diastereoselectivity of >95:5 dr. However, the yield could
hardly be further improved because the disulfide 7 was always
generated as a by‐product in a large amount. Furthermore, the
reaction of ortho‐mercaptobenzyl alcohol 1a with styrene 4a was
utilized to optimize the conditions for the synthesis of ‐hydroxy
O
4e
O
N
O
N
O
N
O
NH
O
Ph
OMe
S
O
N
N
S
S
S
receptor binding
affinity
cytotoxic
anticancer
antiinflammatory
Figure 1. Bioactive compounds containing benzoxathiepine scaffold. sulfide 5aa under the promotion of acids 6 (Scheme 1, eq. 7).
More interestingly, when we employed styrenes instead of 3‐ Similarly, a large number of experiments were performed to find
alkyl‐2‐vinylindoles to the reaction with ortho‐mercaptobenzyl the optimal conditions (see ESI for details). Gratifyingly, this
alcohol under the similar reaction conditions (promoted by the reaction finally afforded the product 5aa in a high yield of 84%
same Brønsted acid in open air), a hydroxysulfenylation reaction although a small amount of by‐product 7 could still be observed in
occurred rather than a [5+2] cyclization (eq. 5), which offered ‐ the reaction system.
hydroxy sulfides in high yields. This reaction is also very valuable
because ‐hydroxy sulfides not only exist in biologically active the substrate scope of the chemodivergent reactions of ortho‐
molecules but also are important synthetic intermediates with wide mercaptobenzyl alcohols with 3‐alkyl‐2‐vinylindoles and
(Tables 1‐2). As shown in Table 1, the [5+2]
olefins is still very limited, and most of the transformations need to cyclization is amenable to a variety of 3‐alkyl‐2‐vinylindoles
With the optimal conditions known, we then investigated
1
2
1
0
applications. More importantly, the hydroxysulfenylation of styrenes
4
2
2
3
4
be performed in pure oxygen atmosphere and require the addition substituted with different R /R /R groups, delivering the
1
1
of peroxides and/or transition metals. So, developing more benzoxathiepine products
3 in excellent diastereoselectivities
efficient methods and/or mild conditions for hydroxysulfenylation (all >95:5 dr) and acceptable yields (41%‐56%). Notably, not
of olefins is still in a great demand. In this sense, our discovered only did a wide range of electronically different phenyl groups
2
reaction of ortho‐mercaptobenzyl alcohol with styrenes will supply serve as suitable terminal R groups (entries 1‐8), but also an
2
an efficient approach featured by simple operation and mild alkyl group (Et) could be used as a competent terminal R
3
conditions for hydroxysulfenylation of olefins.
group (entry 9). In addition, the R group of vinylindoles
2
Herein, we report the details of our new findings on Brønsted could be elongated to an ethyl group (entry 10), and C6‐
acid‐catalyzed chemodivergent [5+2] cyclization
and chloro‐substituted vinylindole 2k could also participate in the
hydroxysulfenylation reactions of ortho‐mercaptobenzyl alcohols [5+2] cyclization (entry 11). Moreover, the R group of ortho‐
with 3‐alkyl‐2‐vinylindoles and styrenes, which provide efficient mercaptobenzyl alcohols could be altered from a methyl
methods for constructing benzoxathiepine scaffold and synthesizing group to an ethyl group (entry 12). Notably, primary ortho‐
1
1

‐hydroxy
sulfides
at
good
yields
and
excellent mercaptobenzyl alcohol 1c could smoothly take part in the
diastereoselectivities.
reaction to give the product 3ca in a moderate yield (entry 13).
a
Table 1. Substrate scope of the [5+2] cyclization
1
2
3
4
b
c
Entry
3
R
R /R /R
/Me/H (2a)
/Me/H (2b)
/Me/H (2c)
/Me/H (2d)
/Me/H (2e)
/Me/H (2f)
/Me/H (2g)
/Me/H (2h)
Et/Me/H (2i)
/Et/H (2j)
/Me/6‐Cl (2k)
dr
yield (%)
56
1
2
3
4
5
6
7
8
9
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Et (1b)
C
6
H
5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
>95:5
o‐MeC
o‐FC
m‐MeC
m‐FC
p‐MeC
p‐FC
p‐ClC
6
H
4
54
6
H
4
46
6
H
4
50
6
H
4
42
6
H
4
50
6
H
4
49
6
H
4
51
52
10
11
12
13
3aj
C
6
H
5
44
3ak
3ba
3ca
C
6
H
5
41
C
6
H
5
/Me/H (2a)
/Me/H (2a)
45
H (1c)
C
6
H
5
51
Scheme 1. Model reactions employed to optimize the conditions.
Initially, the reaction of ortho‐mercaptobenzyl alcohol 1a with
a
Unless indicated otherwise, the reaction was carried out in 0.1 mmol scale
o
catalyzed by 10 mol% 6g in toluene (1 mL) at 25 C for 24 h, and the mole ratio of
b
1
c
3‐methyl‐2‐vinylindole 2a was employed to optimize the conditions 1:2 was 2:1. The dr value was determined by H NMR. Isolated yield.
for the synthesis of benzoxathiepine 3aa in the presence of acidic
As listed in Table 2, the hydroxysulfenylation reaction was
catalysts 6 (Scheme 1, eq. 6). Screening of catalysts found that only applicable to a wide scope of styrenes 4 bearing electron‐donating
or electron‐withdrawing groups at different positions of the phenyl
ring (entries 1‐8), affording the ‐hydroxy sulfide products 5 in
generally high yields (70%‐84%). Besides, 2‐naphthyl group proved
to be a suitable aromatic substituent for the hydroxysulfenylation
reaction (entry 9). Moreover, disubstituted styrene 4j also smoothly
benzoic acid 6b and racemic phosphoric acid 6g could catalyze the
5+2] cyclization (see ESI for details). In the presence of the optimal
catalyst 6g, a series of experiments such as screening solvents,
[
changing reaction temperature, molar ratio, reaction concentration
2
| J. Name., 2012, 00, 1‐3
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took part in the reaction to give the corresponding product 5aj in a conditions but in the absence of the catalyst 6gVie(ewqA.rtic1le2)O.nliAnes
good yield of 78% (entry 10). Similar to the [5+2] cyclization, ortho‐
mercaptobenzyl alcohols 1b and 1c served as suitable substrates for
the hydroxysulfenylation reaction (entries 11‐12). The successful
application of primary ortho‐mercaptobenzyl alcohol 1c in both the
expected, only trace amount of producDtOs I:31a0a.103a9n/dC65CaCa00w92e0rDe
detected, which indicated that the acidic catalyst 6g played an
important role in the two reactions. In addition, the same reactions
were conducted still under the standard conditions but in an argon
atmosphere (eq. 13). Again, no reaction (even the formation of
disulfide 7) occurred, which verified that the oxygen gas in air was
essentially important for generating the arenethiyl radicals and the
proceeding of the subsequent reactions. Thirdly, N‐methyl
protected 2‐vinylindole 2l was utilized as a substrate to react with
[
5+2] cyclization and the hydroxysulfenylation reaction indicated
that the two gem dimethyl groups in substrates 1 were not
essentially important for the two types of reactions.
a
Table 2. Substrate scope of the hydroxysulfenylation reaction
1
a under the standard conditions (eq. 14), but no reaction took
place apart from the generation of the disulfide 7. This outcome
indicated that the free N‐H group of 3‐alkyl‐2‐vinylindoles 2 was
important for the desired [5+2] cyclization. Finally, we carried out
the reaction of ortho‐mercaptobenzyl alcohol 1d bearing a
protected OH group (methoxyl group) with styrene 4a under the
standard reaction conditions (eq. 15). However, no reaction
occurred, which implied that the protected OH group was
detrimental to the reactivity of substrates 1.
1
2
b
Entry
5
R
Ar/R (4)
yield (%)
84
1
2
3
4
5
6
7
8
9
5aa
5ab
5ac
5ad
5ae
5af
5ag
5ah
5ai
Me(1a)
Me(1a)
Me(1a)
Me(1a)
Me(1a)
Me(1a)
Me(1a)
Me(1a)
Me(1a)
Me(1a)
Et(1b)
C
6
H
5
/H(4a)
/H (4b)
/H (4c)
/H (4d)
/H (4e)
/H (4f)
/H (4g)
/H (4h)
2‐naphthyl/H (4i)
/Me (4j)
o‐MeC
o‐ClC
m‐MeC
m‐ClC
p‐MeC
p‐FC
p‐ClC
6
H
4
79
6
H
4
75
6
H
4
70
6
H
4
79
6
H
4
71
6
H
4
75
6
H
4
79
69
10
11
12
5aj
C
6
H
5
78
5ba
5ca
C
C
6
H
H
5
/H (4a)
/H (4a)
89
H (1c)
6
5
58
a
Unless indicated otherwise, the reaction was carried out in 0.1 mmol scale
o
promoted by 100 mol% 6g in chloroform (1 mL) at 35 C for 12 h, and the mole
ratio of 1:4 was 2:1. Isolated yield.
b
In addition, cyclic styrene analogues as exemplified by 1H‐indene
k could be employed to the hydroxysulfenylation reaction, offering
4
the product 5ak in a considerable yield of 63% (eq. 8).
The structures of all products 3 and 5 were unambiguously
1
13
determined by H and C NMR, IR and HRMS. Besides, the
structures of products 3aa and 5aa along with by‐product 7 were
confirmed by single crystal X‐ray diffraction analysis (see ESI for
details), and the relative configuration of products 3aa was
identified to be trans also by single crystal X‐ray diffraction analysis.
So, the relative configurations of other products 3 were tentatively
assigned to be trans by analogy.
In order to gain some insight into the possible reaction
mechanism of the two chemodivergent reactions, we performed
some control experiments (Scheme 2). Firstly, TEMPO (2,2,6,6‐
tetramethylpiperidine 1‐oxyl) as a radical trapping reagent was
added to the reactions of 1a with 2a and 1a with 4a under the
standard conditions (eq. 9‐10). The results revealed no desired
products 3aa and 5aa were generated, which indicated that the two
reactions might involve an arenethiyl radical pathway. Because
disulfide 7 was a by‐product of the two reactions, and it was formed
by the coupling of two arenethiyl radicals and might re‐cleavage
into arenethiyl radical to react with 2 or 4, we tried the reactions of hydroxysulfenylation of olefins,
Scheme 2. Control experiments.
Moreover, in order to explore the use of thiophenols as a more
general class of substrates, several thiophenols 8 were subjected to
the reactions with styrene 4a under the optimal reaction conditions
1
2
(
Schemee 3). Very interestingly, instead of the expected β‐hydroxy
sulfides 5, these reactions afforded the β‐hydroxy sulfoxides 9 in
generally good yields (see ESI for details). These results indicated
that the OH group at the ortho‐benzylic position of substrates 1
played an important role in directing the reactions with styrenes 4
toward the formation of the β‐hydroxy sulfide products 5.
Scheme 3. Using thiophenols 8 as substrates.
Based on the control experiments and previous reports on
1
1
we suggested possible
disulfide 7 with 2a and 4a under the standard conditions (eq. 11). reaction mechanisms to explain the formation of products 3aa
However, no products 3aa and 5aa were formed, which implied and 5aa (Scheme 4, see ESI for detailed discussion).
that disulfide 7 was not an intermediate toward the formation of
In summary, we have discovered Brønsted acid‐catalyzed
products 3 and 5. To make matters worse, the generation of chemodivergent [5+2] cyclization and hydroxysulfenylation
disulfide 7 consumed a large amount of arenethiyl radicals, which reactions of ortho‐mercaptobenzyl alcohols with 3‐alkyl‐2‐
was detrimental to the formation of the desired products 3 and 5. vinylindoles and styrenes, which provide efficient methods for
Secondly, two model reactions were carried out under the standard
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constructing benzoxathiepine scaffold and synthesizing β‐
hydroxy sulfides at good yields and excellent
J. Am. Chem. Soc. 1986, 108, 3206; c) H. Abe, DV.ieTwsAurctichleidOan,linSe.
Kashino, Y. Takeuchi, T. Harayama, C l. 999,
D
h
O
emI: 1
.
0
P
.1
h
0
a
3
r
9
m
/C
.
6
B
C
u
C
l
0
0
1
9
20
D
diastereoselectivities. These new findings have not only
explored ortho‐mercaptobenzyl alcohol‐involved reactions for
the construction of sulfurous heterocyclic skeletons, but also
established the first [5+2] cyclization involving ortho‐
mercaptobenzyl alcohol. In addition, the [5+2] cyclization
offered a unique strategy for oxysulfenylation of 3‐alkyl‐2‐
vinylindoles, which started from easily available thiophenol
and operated in open air. More importantly, the reaction of
ortho‐mercaptobenzyl alcohols with styrenes also supplied an
efficient approach for hydroxysulfenylation of olefins in the
presence of Brønsted acid under mild conditions.
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Scheme 4. Suggested reaction mechanisms.
We are grateful for financial support from NSFC (21372002
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