Enantioselective Reactions of 2-Sulfonylalkyl Phenols with Allenic Esters: Dynamic Kinetic Resolution and [4+2] Cycloaddition Involving ortho-Quinone Methide Intermediates

Article abstract of DOI:10.1002/anie.201700250

We report herein a dynamic kinetic resolution (DKR) involving ortho-quinone methide (o-QM) intermediates. In the presence of Et₃N and the cinchonine-derived nucleophilic catalyst D, the DKR of 2-sulfonylalkyl phenols with allenic esters afforded chiral benzylic sulfones in 57–79 % yield with good to excellent enantioselectivity (85–95 % ee). Furthermore, with 2-(tosylmethyl)sesamols or 2-(tosylmethyl)naphthols, from which stable o-QM substrates can be generated, a formal [4+2] cycloaddition delivered 4-aryl- or alkyl-substituted chromans with excellent enantioselectivity (88–97 % ee).

Full text of DOI:10.1002/anie.201700250

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International Edition: DOI: 10.1002/anie.201700250
German Edition: DOI: 10.1002/ange.201700250
Organocatalysis
Enantioselective Reactions of 2-Sulfonylalkyl Phenols with Allenic
Esters: Dynamic Kinetic Resolution and [4+2] Cycloaddition Involving
ortho-Quinone Methide Intermediates
Ping Chen, Kai Wang, Wengang Guo, Xianghui Liu, Yan Liu,* and Can Li*
Abstract: We report herein a dynamic kinetic resolution
(DKR) involving ortho-quinone methide (o-QM) intermedi-
ates. In the presence of Et3N and the cinchonine-derived
nucleophilic catalyst D, the DKR of 2-sulfonylalkyl phenols
with allenic esters afforded chiral benzylic sulfones in 57–79%
yield with good to excellent enantioselectivity (85–95% ee).
Furthermore, with 2-(tosylmethyl)sesamols or 2-(tosylmethyl)-
naphthols, from which stable o-QM substrates can be gener-
ated, a formal [4+2] cycloaddition delivered 4-aryl- or alkyl-
substituted chromans with excellent enantioselectivity (88–
97% ee).
and cycloaddition reactions,[8] have been developed, to the
best of our knowledge, there has been no report of a DKR
involving o-QMs.
Herein, we report the results of our investigation.
Through the racemization of 2-sulfonylalkyl phenols 1 via o-
QM intermediates and subsequent asymmetric addition
catalyzed by a nucleophilic amine, the developed DKR
afforded benzylic sulfones with good to excellent enantiose-
lectivity (85–95% ee; Scheme 1, left). Furthermore, a highly
enantioselective [4+2] cycloaddition process was developed
by the use of 2-(tosylmethyl)sesamol or 2-(tosylmethyl)naph-
thol, which could generate stable o-QMs as the substrates to
deliver 4-substituted chromans with excellent enantioselec-
tivity (88–97% ee; Scheme 1, right).
Sulfones have found numerous applications in agricultural
science and medicinal chemistry, and have been widely used
in organic synthesis as versatile synthons.[1] Over the past
decades, a number of methods have been reported for the
synthesis of achiral and/or racemic sulfones.[2] However, the
development of asymmetric catalytic methodologies for
direct access to enantiomerically enriched sulfones is much
less advanced.[3] In particular, the efficient catalytic prepara-
tion of both aryl- and alkyl-substituted benzylic sulfones
remains challenging.
In 2015, we developed the first organic-base-catalyzed
asymmetric reaction with o-QMs generated in situ.[4] Notably,
2-(tosylmethyl)phenol, a precursor to o-QMs in the presence
of a base, bears a benzylic sulfone group, which provides an
excellent platform for the design of a new DKR of racemic 2-
(tosylmethyl)phenol (1). It was envisioned that the DKR of
racemic 1 would be possible by reaction with a suitable
electrophilic partner in the presence of a chiral catalyst and
a base, whereby the chiral catalyst might preferentially
promote the reaction of one enantiomer of 1, while the base
could effectively mediate racemization via an o-QM inter-
mediate, thus leading to the full conversion of 1 into a chiral
benzylic sulfone. Although several asymmetric reactions
involving o-QM intermediates, such as dialkoxylation, car-
boalkoxylation,[5] 1,4-addition,[6] transfer hydrogenation,[7]
Scheme 1. Different reaction pathways of 2-sulfonylalkyl phenols with
allenic esters via o-QM intermediates: DKR versus [4+2] cycloaddition.
To test the viability of the DKR, we began our studies by
examining the racemization of enantiomerically enriched 2-
(tosylmethyl)phenol 1a (80% ee) in the presence of bases
(see the Supporting Information). Et3N was found to be most
effective for the racemization, affording nearly racemic 1a
(4% ee) in 2 h at room temperature.
Allenic ester 2a was selected as the reaction partner of 1a
for its high electrophilicity.[9] Since Et3N may provide racemic
products as a result of a possible background reaction, control
experiments were first performed to examine the effect of
different organic bases in promoting this model reaction of 1a
with 2a (Table 1, entries 1 and 2). Gratifyingly, Et3N was
found to be ineffective for the transformation even at
100 mol% loading (Table 1, entry 1). On the other hand,
the stronger nucleophilic base DABCO could induce the
reaction to give the corresponding product 3a with high
reactivity (Table 1, entry 2). Encouraged by these results, we
proceeded to examine the catalytic potential of some
cinchona-derived nucleophilic catalysts A–D. The reaction
of 1a and 2a with catalyst A[10] proceeded smoothly to give
product 3a with 43% ee and 53% conversion in 12 h (Table 1,
entry 3). This result suggested that catalyst A alone could be
[*] P. Chen, K. Wang, W. Guo, X. Liu, Prof. Y. Liu, Prof. C. Li
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences
457 Zhongshan Road, Dalian 116023 (China)
E-mail: yanliu503@dicp.ac.cn
canli@dicp.ac.cn
P. Chen, K. Wang, X. Liu
Graduate University of Chinese Academy of Sciences
Beijing 100049 (China)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201700250.
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Table 1: Optimization of the reaction conditions for DKR.[a]
Entry
Catalyst
Solvent
Additive
Conv. [%][b]
ee [%][c]
1[d]
2
3
4
5
Et3N
DABCO
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
none
none
none
0
>90
53
>90
>90
>90
trace
>90
>90
A
A
A
B
C
D
D
43
43
54
21
n.d.
75
91
Et3N[d]
Et3N[d]
Et3N[d]
Et3N[d]
Et3N[d]
Et3N[d]
6
7
8
9[e]
Scheme 2. Scope of the DKR. Reaction conditions: 1 (0.2 mmol), 2
(0.4 mmol), D (0.04 mmol), Et3N (0.2 mmol), CHCl3 (2.0 mL), À208C,
5 days and then 08C, 1 day. Yields are for the isolated product after
column chromatography. The ee values were determined by HPLC
analysis on a chiral stationary phase. Bn=benzyl.
[a] Unless otherwise specified, reactions were conducted with 1a
(0.1 mmol), 2a (2.0 equiv, 0.2 mmol), and the catalyst (0.02 mmol,
20 mol%) in the solvent (1.0 mL) at room temperature for 12 h.
[b] Conversion was determined by 1H NMR spectroscopy of the crude
mixture. [c] The ee value was determined by HPLC analysis on a chiral
stationary phase. [d] Et3N (1.0 equiv). [e] Reaction temperature: À208C
for 5 days and then 08C for another day. Boc=tert-butoxycarbonyl,
Ts =p-toluenesulfonyl, n.d.=not determined.
affording the corresponding products 3n,o with 85 and
86% ee. When the tolyl group of 2-(tosylmethyl)phenol 1a
was replaced with a phenyl group, the DKR reaction
proceeded smoothly and gave the corresponding product 3p
in 69% yield with 85% ee. The absolute configuration of the
DKR product 3a was unambiguously assigned as S by single-
crystal X-ray diffraction analysis (see the Supporting Infor-
mation), and that of others was assigned by analogy.
During our studies on the scope of the DKR, the reaction
of 1o with 2a under the standard conditions gave an isomeric
mixture of the 4-substituted chromans 4a and 5a, instead of
the expected DKR product 3q (Scheme 3). Similar results
used to mediate the kinetic resolution of 1a with 2a. To
improve the efficiency of the reaction, we performed the
DKR with 1.0 equivalent of Et3N as an additive. Under
otherwise identical conditions, the reaction proceeded to
completion in 6 h with similar enantioselectivity (Table 1,
entry 4). After solvent screening (see the Supporting Infor-
mation), chloroform was identified as the solvent of choice for
subsequent optimization of the reaction (Table 1, entry 5).
The quinidine-derived catalysts B[11] and C[12] did not afford
satisfactory enantioselectivity (Table 1, entries 6 and 7). To
our delight, the use of cinchonine-derived catalyst D[13]
significantly improved the enantioselectivity (75% ee) with-
out loss of reactivity (Table 1, entry 8). By lowering the
reaction temperature to À208C, the enantioselectivity of the
catalysis was improved to 91% ee albeit with the need for
a prolonged reaction time for full conversion of 1a (Table 1,
entry 9).
The scope of the DKR procedure was studied with D as
the catalyst under the optimized reaction conditions
(Scheme 2). A broad range of 2-(tosylmethyl)phenols with
various aryl substituents reacted smoothly with 2a to afford
the desired products 3a–h with good to high enantioselectiv-
ity (85–91% ee). However, no reaction was observed for
substrates bearing an ortho-substituted phenyl ring (with an
ortho substituent either on the R group or on the quinone
methide fragment), probably owing to the severe steric
congestion. Notably, alkyl-substituted 2-(tosylmethyl)phenols
were also suitable substrates, giving the corresponding
products 3i–m with even higher enantioselectivity (up to
95% ee). The effects of the ester group (COOR’) on the
allenic ester were investigated. It was found that methyl ester
2b and benzyl ester 2c were amenable to the reaction,
Scheme 3. Reactions between 2-(tosylmethyl)naphthol 1o and allenic
ester 2a.
were obtained in reactions of 2-(tosylmethyl)sesamol and
allenic esters. These results suggested that substrates 1, which
can generate more stable o-QMs, reacted with allenic esters
preferentially through a [4+2] cycloaddition pathway to give
the corresponding 4-substituted chromans. In view of the
observed bioactivity of natural products and pharmaceuticals
containing a chiral chroman structure,[14] we hoped to
establish this new methodology with high efficiency. Gratify-
ingly, the screening of various chiral nucleophilic amine
catalysts[15] (see the Supporting Information) led to the
identification of cinchona alkaloid catalyst E,[16] the use of
2
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which in the presence of K2CO3 gave 4a with 93% ee in 86%
yield (Scheme 3).
Thus, the scope of the [4+2] cycloaddition was inves-
tigated (Scheme 4). 2-(Tosylmethyl)naphthols with both elec-
tron-donating and electron-withdrawing substituents at dif-
ferent positions on the phenyl ring (R) reacted with allenic
Scheme 5. Synthetic transformations of products 3 and 4. DIBAL-
H=diisobutylaluminum hydride.
The observation that the product was formed with the same
enantioselectivity (91% ee) as in the reaction of the corre-
sponding 2-(tosylmethyl)sesamol confirms the [4+2] cyclo-
addition pathway via o-QM intermediates.
On the basis of the above control experiment and the
absolute configuration of the products, we propose the
following reaction mechanism to explain the different path-
ways from the key o-QM intermediates (Figure 1): 2-(Tosyl-
methyl)phenols 1 may undergo rapid racemization in the
presence of Et3N via o-QM intermediates (K2 @ K1). At the
same time, catalyst D reacts with the allenic ester 2 to
generate a chiral zwitterionic intermediate 9, which could
deprotonate the phenol group in 2-(tosylmethyl)phenol 1 to
Scheme 4. Scope of the [4+2] cycloaddition. Reaction conditions:
1 (0.2 mmol), 2 (0.5 mmol, 2.5 equiv), K2CO3 (0.4 mmol, 2.0 equiv), E
(0.04 mmol, 20 mol%), chlorobenzene (4.0 mL), 24 h. Yields are for
the isolated product after column chromatography. The ee values were
determined by HPLC analysis on a chiral stationary phase. [a] The
reaction was carried out with a 40 mol% catalyst loading for 5 days.
esters (2a or 2c) to afford the corresponding products 4a–h
with excellent enantioselectivity (90–96% ee). A substrate
bearing an alkyl group (R) was also tolerated with a higher
catalyst loading and longer reaction time (product 4i,
90% ee). Different substituents on the quinone methide
fragment were tolerated as well, and chroman products 4j–
l were obtained with 88–97% ee. The absolute configuration
of 4a was established as R on the basis of X-ray crystallo-
graphic analysis (see the Supporting Information).
To evaluate the practical utility of the DKR and [4+2]
cycloaddition, we conducted several further synthetic trans-
formations (Scheme 5). The enantiomerically enriched ben-
zylic sulfone 3a, which bears an a,b-unsaturated ester group,
could be further reduced to the corresponding allylic alcohol 6
in good yield. Meanwhile, a gram-scale [4+2] cycloaddition of
1o and 2c under the standard conditions provided the desired
product 4e in 68% yield with 94% ee. Reduction of the olefin
and removal of the benzyl group through hydrogenation with
Pd/C proceeded readily in a one-pot process to form 8 in good
yield with high diastereoselectivity. Notably, the treatment of
4e with a large amount of LiAlH4 provided chiral alcohol 7
with 93% ee.
To verify the proposed [4+2] cycloaddition pathway via
stable o-QM intermediates, the reaction of an isolated o-QM
as a diene with allenic ester 2c in the presence of catalyst E to
form 4l was investigated (see the Supporting Information).
Figure 1. Catalytic cycles of the DKR and [4+2] cycloaddition.
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give intermediates 10 and 11. Subsequently, the nucleophilic
addition of 10 to 11 would produce 3 and regenerate the
catalyst. In this step, the reaction of (S)-10 with 11 would be
much faster than that of (R)-10 (KS @ KR, according to
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In conclusion, we have developed a highly efficient
protocol for the DKR of racemic 2-sulfonylalkyl phenols on
the basis of asymmetric nucleophilic addition to allenic esters
and racemization via an o-QM intermediate, to afford
a variety of optically active benzylic sulfones with high
enantioselectivity (up to 95% ee). With 2-(tosylmethyl)sesa-
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Acknowledgements
This research was supported financially by the Natural
Science Foundation of China (21322202, 21472187,
21532006) and the Strategic Priority Research Program of
the Chinese Academy of Sciences (Grant No.
XDB17020200).
Conflict of interest
The authors declare no conflict of interest.
Keywords: benzylic sulfones · chromans · cycloaddition ·
dynamic kinetic resolution · ortho-quinone methides
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Manuscript received: January 10, 2017
Final Article published: && &&, &&&&
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Angewandte
Communications
Chemie
Communications
Organocatalysis
P. Chen, K. Wang, W. Guo, X. Liu, Y. Liu,*
C. Li*
&&&& — &&&&
Enantioselective Reactions of 2-
Sulfonylalkyl Phenols with Allenic Esters:
Dynamic Kinetic Resolution and [4+2]
Cycloaddition Involving ortho-Quinone
Methide Intermediates
Resolutely following the chosen path: 2-
Sulfonylalkyl phenols were transformed
into chiral benzylic sulfones with Et3N
and a cinchonine-derived catalyst I by
dynamic kinetic resolution (DKR) via an
ortho-quinone methide (o-QM; see
scheme). In contrast, 2-(tosylmethyl)-
sesamols and -naphthols, which form
stable o-QMs, underwent formal [4+2]
cycloaddition under similar conditions
with another cinchona alkaloid catalyst II
to give 4-substituted chromans.
6
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Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!