Non-hydrogen bond catalyst-mediated diastereoselective conjugate additions of 5: H -oxazol-4-ones to o -hydroxyphenyl-substituted p -quinone methides

Article abstract of DOI:10.1039/d0ob01558j

An efficient DBU-catalyzed conjugate addition of 5H-oxazol-4-ones to o-hydroxyphenyl-substituted p-quinone methides has been developed, affording the valuable diarylmethanes in high yields with excellent diastereoselectivity. This strategy demonstrates a robust access to a wide range of diarylmethane derivatives possessing biologically significant o-hydroxyphenol and p-hydroxyphenol moieties under mild reaction conditions.

Full text of DOI:10.1039/d0ob01558j

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DOI: 10.1039/D0OB01558J
Journal Name
COMMUNICATION
Non-hydrogen Bond Catalyst Mediated Diastereoselective
Conjugate Additions of 5H-Oxazol-4-ones to o-Hydroxyphenyl
Substituted p-Quinone Methides
Received 00th January 20xx,
Accepted 00th January 20xx
Ziyang Wang,+,a Anqi Huang,+,a Fang Fang,b Pengfei Li,*b Guokai Liu,*c and Wenjun Li*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient DBU-catalyzed conjugate addition of 5H-oxazol-4-ones
to o-hydroxyphenyl substituted p-quinone methides has been
developed, affording the valuable diarylmethanes in high yields
with excellent diastereoselectivity. This strategy demonstrates a
robust access to a wide range of diarylmethane derivatives
possessing biological significant o-hydroxyphenol and p-
hydroxyphenol moieties under mild reaction conditions.
A) Selected bioactive compounds containing diarylmethane unit
OH
Ph
OH
O
O
O
CO2H
O
N
O
O
Ph
CO2H
OH
O
OH
tolerodine
podophyllotoxin
methylenedisalicylic acid
PTP1B inhibitors
Diarylmethane motif is significant structural unit found in
numerous natural products and synthetic compounds, which
have exhibited interesting biologically properties of
antimuscarinic,1 antineoplastic,2 PTP1B inhibitor,3 GPR40
agonist,4 dual H3R-antagonist-NET inhibitor,5 and antidiabetic
activities6 (Scheme 1A). Owing to their promising applications,
many efforts have been devoted to the efficient synthesis of
diarylmethane derivatives. In this context, the catalytic
conjugate additions of o-quinone methides (o-QMs)7 and p-
quinone methides (p-QMs)8 opened a concise and direct access
to diarylmethane derivatives. In particular, o-hydroxyphenyl
substituted p-QMs have been successfully involved in a lot of
organocatalytic asymmetric transformations in the presence of
chiral thioureas,9 squaramides,10 phosphoric acids,11 N-
heterocyclic carbines,12 phosphines,13 and phosphonium
salts,14 respectively. Despite these most prevailing methods,
developing more efficient and practical organocatalytic
reactions of o-hydroxyphenyl substituted p-QMs as well as
reaction partners for the construction of diarylmethane motif
antimuscarinic activity
antineoplastic activity
OH
O
N
HO
O
O
N
H
diarylpropanoic acid
GPR40 agonist
diaryldiamine
dual H3R-antagonist-NET inhibitor
B) This work
O
OH
t-Bu
t-Bu
t-Bu
t-Bu
O
R1
DBU (2 mol%)
+
OH
OH
O
N
CH2Cl2, 25 oC, 48 h
N
O
R2
R2
Ar
R1
O
25 examples, 82-99% yield, >20:1 dr
(+/-)
Ar
Scheme 1 Selected bioactive compounds containing diarylmethane unit and this work.
remain highly desirable.15-18
On the other hand, 5H-oxazol-4-ones have been commonly
employed as nucleophile precursor in the organocatalytic
asymmetric reactions for the synthesis of various α-alkyl-α-
hydroxy carboxylic acid derivatives.19-26 However, the reaction
between o-hydroxyphenyl substituted p-QMs and 5H-oxazol-4-
ones was not reported. Based on our previous work on the
reactions of quinone methides10a,11a,11c,11i,12a,18a,26 and 5H-
oxazol-4-ones,20e,25 we present herein a non-hydrogen bond
catalyst mediated diastereoselective conjugate addition of 5H-
oxazol-4-ones to o-hydroxyphenyl substituted p-QMs for the
construction of diarylmethane motifs (Scheme 1B).
a. Z. Wang, A. Huang, Prof. W. Li
Department of Medicinal Chemistry, School of Pharmacy, Qingdao University,
Qingdao 266021, China. E-mail: liwj@qdu.edu.cn
b. F. Fang, Prof. P. Li
Department of Chemistry and Shenzhen Key Laboratory of Marine Archaea Geo-
Omics, Southern University of Science and Technology, Shenzhen 518055, China.
E-mail: lipf@sustech.edu.cn; flyli1980@gmail.com
c. Prof. G. Liu
School of Pharmaceutical Sciences, Shenzhen University Health Science Centre,
Shenzhen University, Shenzhen 518060 China. E-mail: gkliu@szu.edu.cn
We started our investigations with the model reaction
between o-hydroxyphenyl substituted p-QM 1a and 5-ethyl-2-
phenyloxazol-4(5H)-one 2a in dichloromethane (CH2Cl2) at 25
+
These authors contributed equally.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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Table 1 Condition optimization.
Table 2 Scope of 5H-oxazol-4-ones 2.
View Article Online
DOI: 10.1039/D0OB01558J
O
OH
O
OH
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
O
O
catalyst (5 mol%)
R1
DBU (2 mol%)
CH2Cl2, 25 o
OH
+
OH
OH
+
OH
solvent, 25 o
C
O
O
N
N
N
C
O
O
N
R1
Ph
Ar
O
N
O
1a
2a
1a
2
(+/-)-3aa
(+/-)-3
Ph
Ar
NH
N
N
N
N
N
N
H
N
Entrya
R1
Et
Ar
Ph
Ph
Ph
Ph
Ph
Ph
Time [h]
Yield [%]b
drc
N
N
DBU
pyrrolidine
Et3N
TMG
DMAP
DABCO
1
48
48
72
48
48
48
48
48
48
48
48
48
48
48
48
48
(+/-)-3aa, 91
(+/-)-3ab, 99
(+/-)-3ac, 85
(+/-)-3ad, 99
(+/-)-3ae, 98
(+/-)-3af, 94
(+/-)-3ag, 88
(+/-)-3ah, 99
(+/-)-3ai, 98
(+/-)-3aj, 99
(+/-)-3ak, 98
(+/-)-3al, 99
(+/-)-3am, 99
(+/-)-3an, 99
(+/-)-3ao, 82
(+/-)-3ap, 93
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
2
Me
n-Pr
i-Pr
i-Bu
Bn
Et
Entrya Catalyst
Solvent
Time [h]
24
Yield [%]b
drc
3
1
pyrrolidine
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
(+/-)-3aa, <5
(+/-)-3aa, 40
(+/-)-3aa, 62
(+/-)-3aa, 88
(+/-)-3aa, 76
(+/-)-3aa, 99
(+/-)-3aa, 65
(+/-)-3aa, 87
(+/-)-3aa, 53
(+/-)-3aa, 68
(+/-)-3aa, 88
(+/-)-3aa, 91
(+/-)-3aa, 83
-
4
2
Et3N
24
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
5
3
DABCO
TMG
24
6
4
24
7
4-FC6H4
5
DMAP
DBU
24
8
Et
4-ClC6H4
4-BrC6H4
4-MeC6H4
3-BrC6H4
3-MeC6H4
6
24
9
Et
7
DBU
24
10
11
12
13
14
15
16
a
Et
8
DBU
THF
24
Et
9
DBU
toluene
CH3OH
(CH2Cl)2
CH2Cl2
CH2Cl2
24
Et
10
11
12
13
a
DBU
24
Et
3-MeOC6H4
2-FC6H4
DBU
24
Et
DBU (2 mol%)
DBU (1 mol%)
48
Et
2-ClC6H4
2-BrC6H4
48
Et
Unless noted, a mixture of 1a (0.05 mmol), 2a (0.06 mmol), catalyst (5 mol%)
in the solvent (0.3 mL) at 25 oC for the time given. b Isolated yield. c Determined
by 1H NMR.
Unless noted, 1a (0.05 mmol), 2 (0.06 mmol), DBU (2 mol%) in CH2Cl2 (0.3 mL) at
25 oC for 48 h. b Isolated yields. c Determined by 1H NMR.
substituent R1 in 5H-oxazol-4-ones 2 effected the yield and
selectivity slightly and furnished products (+/-)-3aa-af in 85-99%
yields with >20:1 dr (entries 1-6). Moreover, both electron-
withdrawing and electron-donating groups could be
introduced into the aromatic ring of Ar in 5H-oxazol-4-ones 2,
affording the corresponding adducts (+/-)-3ag-ap in 82-99%
yields with >20:1 dr (entries 7-16). Notably, neither significant
electronic affect nor essencial steric-hindrance affect was
observed. These results indicated that a broad scope of 5H-
oxazol-4-ones was compatible under the standard conditions
to synthesize diarylmethane derivatives.
With these encouraging data in hand, we then examined the
scope of o-hydroxyphenyl substituted p-QMs 1 by the DBU-
catalyzed reactions of 5-ethyl-2-phenyloxazol-4(5H)-one 2a
under the standard conditions. As shown in Table 3, all the
probed o-hydroxyphenyl substituted p-QMs reacted smoothly
with 2a to afford the corressponding adducts in high yields
with diastereoselectivity. Notably, either electron-withdrawing
or electron-donating groups could be introduced into the
aromatic ring of substrate 1 with a slight affect on the yield
and diastereoselectivity. Furthermore, the DBU-catalyzed
reaction between o-hydroxyphenyl substituted p-QM 1d and
5-methyl-2-phenyloxazol-4(5H)-one 2b proceeded smoothly to
furnish the desired product (+/-)-3db in 99% yield with >20:1
diatereoselectivity (entry 9). The structure of (+/-)-3db were
unambiguously confirmed by X-ray crystallography.27 Taken
altogether, the DBU-catalyzed reaction has been successfully
oC for 24 h (Table 1). Pyrrolidine was found to be feeble and
trace of product (+/-)-3aa was formed (entry 1). Using Et3N as
catalyst afforded the desired adduct (+/-)-3aa in 40% yield
with >20:1 dr (entry 2). When DABCO was employed as
catalyst, the yield of adduct (+/-)-3aa was improved to 62%
without compromising the diastereoselectivity (entry 3).
Pleasingly, product (+/-)-3aa was obtained in 88% yield
with >20:1 dr from the TMG-mediated reaction (entry 4).
Further screening of catalysts identified DBU as the suitable
catalyst, enabling the formation of adduct (+/-)-3aa in 99%
yield with >20:1 dr (entry 6). Furthermore, the screening of
reaction media revealed that solvent seriously affected the
yield but had no effect on diastereoselectivity (entries 7-11).
And dichloromethane was found to be optimal for the
transformation. Importantly, decreasing catalyst loading from
10 mol% to 2 mol% also resulted in satisfactory results,
furnishing the desired adduct (+/-)-3aa in 91% yield with >20:1
dr after a prolonged time (entry 12). Further decreasing
catalyst loading to 1 mol% led to the formation of adduct (+/-)-
3aa in 83% yield with >20:1 dr after 48 h (entry 13).
With the optimal conditions in hand, we then examined the
substrates scope (Table 2). Firstly, the scope of 5H-oxazol-4-
ones 2 was surveyed by the DBU-catalyzed reaction of o-
hydroxyphenyl substituted p-QM 1a. Generally, a series of 5H-
oxazol-4-ones was compatible to afford the corresponding
adducts in high yields with diastereoselectivities. In details,
2 | J. Name., 2012, 00, 1-3
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Table 3 Scope of o-hydroxyphenyl p-QMs 1.
View Article Online
OH
A) Control experiments
O
DOI: 10.10t3-B9u/D0OB01558J
t-Bu
O
OH
t-Bu
t-Bu
O
t-Bu
t-Bu
t-Bu
t-Bu
DBU (2 mol%)
X
O
O
+
CH2Cl2, 25 oC, 48 h
X
R1
DBU (2 mol%)
N
+
O
N
OH
1
OH
O
CH2Cl2, 25 oC, 48 h
N
O
O
Ph
6
5
Ph
R2
(+/-)-5aa 99% yield, 5:3 dr
(+/-)-5ba 92% yield, 1:1 dr
2
3
R2
4a X = H
4b X = OTBS
N
R1
2a
Ph
O
2
4
1
(+/-)-3
Ph
B) Proposed mechanism
2a
N
Entry a
R1
R2
Yield [%]b
drc
(+/-)-3aa
N
DBU
1
2
3
4
5
6
7
8
9
Et
Et
Et
Et
Et
Et
Et
Et
Me
4-F
(+/-)-3ba, 93
(+/-)-3ca, 88
(+/-)-3da, 87
(+/-)-3ea, 99
(+/-)-3fa, 88
(+/-)-3ga, 86
(+/-)-3ha, 92
(+/-)-3ia, 99
(+/-)-3db, 99
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
4-Cl
4-Br
4-Me
3-Br
[DBU-H]
OH
t-Bu
t-Bu
O
O
O
O
3-MeO
2-Me
O
N
O
N
N
O
N
O
O
H
O
2-MeO
4-Br
I-1'
Ph
Ph
t-Bu
I-2
I-1
Ph
O
1a
t-Bu
a Unless noted, 1 (0.05 mmol), 2 (0.06 mmol), DBU (2 mol%) in CH2Cl2 (0.3 mL) at 25
oC for 48 h. b Isolated yields. c Determined by 1H NMR.
Scheme 3 Control experiments and proposed reaction mechanism.
Control experiments were also surveyed to gain insight into
the reaction mechanism. When phenyl substituted p-QM 4a
was employed to react with 5-ethyl-2-phenyloxazol-4(5H)-one
2a under the standard conditions, 1,6-adduct (+/-)-5aa was
obtained in 99% yield with 5:3 dr (Scheme 3A). When the
hydroxyl group in o-hydroxyphenyl substituted p-QM 1a was
blocked by TBS group, the reaction still proceeded smoothly to
afford 1,6-adduct (+/-)-5ba in 91% yield with 1:1 dr (Scheme
3A). These data indicated that the reaction between o-
hydroxyphenyl substituted p-QMs 1 and 5H-oxazol-4-ones 2
could proceed via 1,6-conjugate addition without keeping the
diastereoselectivity at a high level. Based on these results and
according to the report,18a a proposed reaction mechanism
was shown in Scheme 3B. The o-hydroxyphenyl substituted p-
QM 1a was supposed to interact with intermediate I-1’ via π–π
stacking. Furthermore, there existed the synergistic hydrogen-
bonding effect between hydroxyl group of p-QM 1a and
intermediate I-1’ to gain high diastereoselectivity.
A) Scale up
OH
O
t-Bu
t-Bu
t-Bu
t-Bu
O
DBU (2 mol%)
OH
O
+
OH
N
CH2Cl2 (12.0 mL)
25 oC, 48 h
O
N
O
Ph
(+/-)-3aa
Ph
2a
453.6 mg, 2.4 mmol
1a
20.0 mg, 2.0 mmol
910.2 mg, 91% yield, >20:1 dr
B) Survey with chiral squaramide
OH
O
t-Bu
t-Bu
t-Bu
t-Bu
O
squaramide (10 mol%)
OH
O
N
+
OH
O
*
CH2Cl2 (0.3 mL)
5 oC, 48 h
*
N
Ph
O
1a
2a
3aa
68% yield, 0% ee, >20:1 dr
Ph
N
OMe
H
N
OMe
H
NH
HN
In conclusion, we have developed an efficient non-hydrogen
bond catalyst mediated diastereoselective conjugate additions
of 5H-oxazol-4-ones to o-hydroxyphenyl substituted p-QMs.
With the aid of DBU, the 1,6-conjugate addition furnished
structurally diverse diarylmethane derivatives in high yields
with excellent diastereoselectivity. Particularly noteworthy
was that this methodology was featured by low catalyst
loading, mild reaction conditions, broad substrate scope, high
efficiency and diastereoselectivity.
We are grateful for financial support from National Natural
Science Foundation of China (21502043), the Special Funds of
the Taishan Scholar Program of Shandong Province
(tsqn201812047), the Natural Science Foundation of Shandong
Province (ZR2017JL011), and Shenzhen Innovation of Science
and Technology Commission (ZDSYS201802081843490). The
authors acknowledge the assistance of SUSTech Core Research
Facilities.
N
squaramide
N
O
O
Scheme 2 Further investigations.
extended to a series of o-hydroxyphenyl substituted p-QMs.
Having established the substrate scope of the DBU-catalyzed
reactions, we investigated the potential of the current catalyst
system. When the reaction was scaled up to 2.0 mmol of o-
hydroxyphenyl substituted p-QM 1a under the standard
conditions, product (+/-)-3aa was obtained in 91% yield
with >20:1 dr (Scheme 2A). Furthermore, we also carried out a
preliminary investigation on the organocatalytic asymmetric
version of o-hydroxyphenyl substituted p-QM 1a and 5H-
oxazol-4-one 2a. However, only racemic product was obtained
in the presence of a chiral squaramide catalyst (Scheme 2B).
This journal is © The Royal Society of Chemistry 20xx
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Tan, H. Zhang, Z. Jiang, Y. Chen, X. Ren, C. JiangVaienwdATrti.clWe Oannlinge,
Adv. Synth. Catal., 2020, 362, 1058.
Conflicts of interest
There are no conflicts to declare.
15 For organic amine catalyzed reactions,DsOeIe::10(a.1)0Z39. /YDe0,OLB. 0B1a5i5,8YJ.
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27 CCDC 2017889 ((+/-)-3db).
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4 | J. Name., 2012, 00, 1-3
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