Enantioselective Synthesis of Functionalized 4-Aryl Hydrocoumarins and 4-Aryl Hydroquinolin-2-ones via Intramolecular Vinylogous Rauhut-Currier Reaction of para-Quinone Methides

Article abstract of DOI:10.1021/acs.orglett.7b01331

A novel strategy for the asymmetric construction of functionalized 4-aryl-3,4-dihydrocoumarins and 4-aryl-3,4-dihydroquinolin-2-ones via an intramolecular vinylogous Rauhut-Currier reaction of para-quinone methides (p-QMs) under the bifunctional catalysis

Full text of DOI:10.1021/acs.orglett.7b01331

Letter
pubs.acs.org/OrgLett
Enantioselective Synthesis of Functionalized 4‑Aryl Hydrocoumarins
and 4‑Aryl Hydroquinolin-2-ones via Intramolecular Vinylogous
Rauhut−Currier Reaction of para-Quinone Methides
Xiang-Zhi Zhang,† Kang-Ji Gan,† Xiao-Xue Liu,† Yu-Hua Deng,† Fang-Xin Wang,† Ke-Yin Yu,†
Jing Zhang,† and Chun-An Fan*,†,‡
†State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, 222
Tianshui Nanlu, Lanzhou 730000, China
‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, China
S
* Supporting Information
ABSTRACT: A novel strategy for the asymmetric construction of functionalized
4-aryl-3,4-dihydrocoumarins and 4-aryl-3,4-dihydroquinolin-2-ones via an intra-
molecular vinylogous Rauhut−Currier reaction of para-quinone methides (p-QMs)
under the bifunctional catalysis of chiral amine-phosphine is described. This
intramolecular mode for the catalytic enantioselective 1,6-conjugate addition of p-
QMs has been explored for the first time, delivering two types of synthetically
important heterocycles in high yields and enantioselectivites.
he heterocyclic skeleton with 4-aryl-3,4-dihydrocoumarin
or 4-aryl-3,4-dihydroquinolin-2-one exists in many bio-
Scheme 1. Catalytic Enantioselective Approaches
T
logically active natural products and pharmaceuticals,1 exhibiting
anticancer1c and antibacterial1e activities (Figure 1). These
Figure 1. Selected bioactive natural products and pharmaceuticals.
structurally unique scaffolds are also common building blocks in
natural product synthesis.2 Owing to their promising biological
profiles as well as synthetic utilities, numerous protocols for the
assembly of 4-aryl-3,4-dihydrocoumarins and 4-aryl-3,4-dihdro-
quinolin-2-ones have been reported.3,4 Among them, only a
handful of approaches have been developed for the catalytic
asymmetric version. For example, rhodium-catalyzed 1,4-
addition of arylboronic acids with coumarins5 or asymmetric
hydrogenation of 4-aryl coumarins6 has provided straightforward
routes to such types of 4-aryl-3,4-dihydrocoumarins (Scheme
1a). Recently, the Hou group has realized the kinetic resolution
of 4-substituted-3,4-dihydrocoumarins via a palladium-catalyzed
allylic substitution for optically active trans-3,4-disubstituted-
dihydrocoumarin derivatives7 (Scheme 1a). In addition, these
skeletons could be also accessed by [4 + 2] annulation of o-
quinone methides with different two-carbon synthons in the
presence of chiral organocatalysts8 (Scheme 1b). Meanwhile, N-
heterocyclic carbene-catalyzed annulation of phenols with enals
or alkynals has been demonstrated as a valuable method for the
generation of 4-substituted-3,4-dihydrocoumarins9 (Scheme
1b). To our knowledge, however, only two reports have achieved
the catalytic asymmetric synthesis of 4-aryl-3,4-dihydroquinolin-
2-ones through rhodium/palladium-catalyzed reaction of
arylboronic acids with acrylamides10a and palladium-catalyzed
cyclocarbonylation of 2-vinylanilines,10b respectively (Scheme
1c). Thus, it is highly desirable to explore alternative catalytic
enantioselective methodologies to construct both 4-aryl-3,4-
dihydrocoumarins and 4-aryl-3,4-dihydroquinolin-2-ones.
para-Quinone methides (p-QMs)11,12 as vinylogous Michael
acceptors have attracted much attention in the synthetic
community. To date, 1,6-conjugate addition,13,14 [2 + 1]-
annulation,15 [3 + 2]-annulation,16 [4 + 1]-annulation,17 and [4
Received: May 3, 2017
Published: June 5, 2017
© 2017 American Chemical Society
3207
DOI: 10.1021/acs.orglett.7b01331
Org. Lett. 2017, 19, 3207−3210

Organic Letters
Letter
+ 2]-annulation18 constitute the main context of the
methodological studies on p-QMs. With our continuing interest
in the chemistry of p-QMs,14a,j,k,15c,17a the installation of an
electrophilic allyl ester or allyl amide group at the ortho position
on the aromatic ring of p-QMs might render the possibility of a
new intramolecular Rauhut−Currier reaction,19,20 which would
lead to access to the synthetically important, optically active
heterocycles (Scheme 1d). Recently, two reports involving
intermolecular Rauhut−Currier-type 1,6-conjugate addition of
p-QMs with vinyl ketones have been presented by Liu and
Zhang21a and Wu.21b Herein, we described an enantioselective
synthesis of functionalized 4-aryl-3,4-dihydrocoumarins and 4-
aryl-3,4-dihydroquinolin-2-ones through an unprecedented
intramolecular vinylogous Rauhut−Currier reaction of p-QMs
under the catalysis of nucleophilic chiral phosphines.
With the optimized conditions in hand, we then explored the
substrate scope (Scheme 2). A series of p-QM esters 1a−1m with
Scheme 2. Substrate Scope of p-QM Esters
We initially conducted the model reaction using p-QM ester
1a in the presence of various chiral phosphine catalysts 3a−3i
(Table 1). The C2-symmetric chiral catalysts (R)-BINAP 3a and
a
Table 1. Optimization of Reaction Conditions
b
c
entry
catalyst
time (h)
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
3a ((R)-BINAP)
24
12
24
24
24
10
2.5
1
NR
71
ND
−24
24
electronically different substituents on aromatic ring A were first
examined, giving the corresponding 4-aryl hydrocoumarins 2a−
2m in high yields (89−99% yield) with moderate-to-excellent
enantioselectivities (50−99% ee). Notably, while using p-QM
esters 1b and 1n with sterically bulky ortho-substituents, the
reaction proceeded smoothly to afford 2b (99% yield) and 2n
(99% yield), respectively, but with a dramatic decrease of
enantioselectivities (low to 13% ee). The absolute configuration
of 2e was unambiguously assigned by X-ray crystallographic
analysis.23 In addition, p-QM ester 1o with a less bulky isopropyl
group at the α,α′-position could afford desired product 2o in high
yield and enantioselectivity (92% yield, 97% ee). Moreover, a
stereoisomeric mixture of 1p (R1 = Me, R2 = t-Bu, E/Z = 1.5:1)
was also examined, and product 2p was readily obtained without
a negative influence on both reactivity and enantioselectivity
(95% yield, 94% ee).
3b ((R,R)-DIOP)
3c
3d
3e
3f
85
80
24
NR
85
ND
72
3g
3h
3i
98
92
98
98
24
NR
ND
a
Performed with 1a (0.1 mmol) and catalyst 3 (0.01 mmol) in
CH2Cl2 (2.0 mL) at 15 °C. Yield of isolated products. Determined
by HPLC.
b
c
To further expand the substrate scope of this methodology,
various p-QM amides 4a−4e were then probed (Scheme 3).
Compared with cases using p-QM esters (Scheme 2), the
reaction of p-QM amides with the electron withdrawing N-Ts
group could generally be completed in 15 min, showing a
significant increase in reactivity. p-QM amides containing
electron-neutral (R = H), electron-withdrawing (R = F), or
electron-donating substituents (R = Me, OMe) at the C6
position of aromatic ring A could afford the desired 4-aryl-3,4-
dihydroquinolin-2-ones 5a−5d in high yields (up to 98% yield)
and excellent enantioselectivities (up to 94% ee). The absolute
stereochemsitry of 5a was clearly determined by X-ray
crystallographic analysis.23 Moreover, when p-QM amide 4e
bearing a C7−CF3 group was exposed to the optimized
conditions, product 5e could be furnished with a satisfactory
result (91% yield, 94% ee). It should be noted that substrate 4f
with an electron-donating N-methyl group did not undergo the
expected intramolecular cyclization, mostly due to the highly
decreased electrophilicity of the corresponding N-methylated
acrylamide moiety.
(R,R)-DIOP 3b were first tested in this reaction (entries 1 and 2).
Despite no reactivity observed using atropisomeric chiral
bis(triaryl)phosphine 3a, the desired hydrocoumarin 2a could
be obtained in 71% yield, albeit with 24% ee, under the catalysis
of centrally chiral phosphine 3b. To improve the enantiose-
lectivity, we then examined a series of bifunctional amine-
phosphine catalysts 3c−3i (entries 3−9). As compared with
negative results in the cases employing 1,3-amine-phosphines
3c−3e (entries 3−5), amino acid-derived 1,2-amine-phosphine
catalyst 3f gave dramatically improved enantiocontrol with 72%
ee (entry 6). Encouraged by this promising result, two N-Boc-
1,2-amine-phosphines 3g and 3h22 were further investigated
(entries 7 and 8). To our delight, catalyst 3h with increasing size
of the substituent on the chiral center delivered the optimal result
(98% yield, 98% ee, entry 8). Notably, cyclic trans-1,2-amine-
phosphine catalyst 3i was also surveyed (entry 9), but
surprisingly, there was no reaction observed in this case.
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DOI: 10.1021/acs.orglett.7b01331
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Organic Letters
Letter
in the presence of an increased amount of AlCl3 (15 equiv) at an
enhanced temperature (50 °C), the formation of unusual
rearrangement product 6e could be improved to 72% yield, to
some extent demonstrating the positive influence of reaction
temperature on such [3.3] sigmatropic rearrangement.
Scheme 3. Substrate Scope of p-QM Amides
On the basis of the crystallographically determined absolute
configurations of products 2e and 5a, a tentative model was
proposed for the enantiocontrol observed in this reaction (Figure
2). Mechanistically, initial nucleophilic attack of 1,2-amine-
Figure 2. Proposed model for the enantioselectivity.
phosphine catalyst 3h on the acrylate moiety of p-QMs provided
the zwitterionic intermediates A stabilized by hydrogen bonding
interaction, and subsequently, a stereoselective 1,6-addition of
less hindered Re face of enolate motif to Si face of p-QM unit
could predominantly occur to give desired chiral products 2 and
5.
For the synthetic utility of this protocol to be demonstrated, a
gram-scale reaction of 1a was performed with a lower catalyst
loading of 3h (0.05 equiv) (Scheme 4, route a), and the
In conclusion, a highly catalytic enantioselective method for
the effective synthesis of chiral-functionalized coumarins and
quinolinones has been developed via an intramolecular vinyl-
ogous Rauhut−Currier reaction of p-QMs under the bifunctional
catalysis of nucleophilic chiral amine-phosphines. A series of
synthetically useful, optically active 4-aryl-3,4-dihydrocoumarins
and 4-aryl-3,4-dihydroquinolin-2-ones were achieved in high
yields (up to 99% yield) and excellent enantioselectivities (up to
99% ee). This is the first example of a catalytic enantioselective
1,6-addition reaction of p-QMs in an intramolecular fashion, the
utility of which has been manifested by the practicability of scale-
up and the diverse transformations of heterocyclic products. The
current exploration of introducing additional electrophilic sites
provides a new perspective for methodological design concern-
ing the chemistry of p-QMs.
Scheme 4. Gram-Scale Synthesis and Its Transformations
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b01331.
hydrocoumarin 2a was readily obtained (98% yield, 98% ee).
With 2a in hand, its transformations were then explored. A
facially selective hydrogenation of 2a (route b) led to the
formation of desired dihydrocoumarin 6a with vicinal stereo-
centers (90% yield, >20:1 dr, 98% ee),24 and pleasingly, AlCl3-
mediated de-tert-butylation of 6a (route c) could smoothly give
7a (92% yield, 98% ee). Moreover, a diastereoselective
cyclopropanation of 2a with 2-bromomalonic ester (route d)
proceeded to deliver the spirocycle 6b (99% yield, >20:1 dr, 98%
ee).24 Furthermore, when 2a was subjected to NaOMe-
promoted alcoholysis (route e), the resulting acyclic product
6c could be obtained in 99% yield without detectable
racemization (97% ee) at the diarylmethine stereocenter. Upon
treating 2a with 4 equiv of AlCl3 at 15 °C (route f), the expected
de-tert-butylation product 6d was formed in 44% yield and 98%
ee, but interestingly, an unexpected product 6e resulting from a
[3.3] sigmatropic rearrangement was isolated in 50% yield. The
structure of 6e was further assigned by X-ray crystallographic
analysis.23 Notably, while this de-tert-butylation was conducted
Experimental procedures and spectral data (PDF)
X-ray data for compounds 2a (CIF), 5a (CIF), and 6e
(CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: fanchunan@lzu.edu.cn.
ORCID
Chun-An Fan: 0000-0003-4837-3394
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from NSFC (21572083,
21322201, 21290180), FRFCU (lzujbky-2015-48, lzujbky-2016-
ct02, lzujbky-2016-ct07), PCSIRT (IRT_15R28), the 111
Project of MOE (111-2-17), and the Chang Jiang Scholars
3209
DOI: 10.1021/acs.orglett.7b01331
Org. Lett. 2017, 19, 3207−3210

Organic Letters
Letter
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(c) Li, W.; Zhang, J. Chem. Soc. Rev. 2016, 45, 1657 and references
therein.
(23) CCDC 1546775 (2e), 1546776 (5a), and 1546777 (6e) contain
the supplementary crystallographic data (Supporting Information).
(24) The relative configurations of 6a and 6b were determined by
NOE.
3210
DOI: 10.1021/acs.orglett.7b01331
Org. Lett. 2017, 19, 3207−3210