Construction of All-Carbon Quaternary Stereocenters via Sequential Photoactivation/Isothiourea Catalysis

Article abstract of DOI:10.1021/acs.orglett.9b02892

Highly enantioselective [4 + 2] cyclizations of azadienes with in situ generated ketenes were developed through sequential visible-light photoactivation/isothiourea catalysis, which offers a novel approach for the creation of all-carbon quaternary stereoc

Full text of DOI:10.1021/acs.orglett.9b02892

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Construction of All-Carbon Quaternary Stereocenters via Sequential
Photoactivation/Isothiourea Catalysis
Tao Fan, Zi-Jing Zhang, Yu-Chen Zhang, and Jin Song*
Department of Chemistry, University of Science and Technology of China, Hefei, 230026, China
S
* Supporting Information
ABSTRACT: Highly enantioselective [4 + 2] cyclizations of azadienes with in situ generated ketenes were developed through
sequential visible-light photoactivation/isothiourea catalysis, which offers a novel approach for the creation of all-carbon
quaternary stereocenters through disubstituted C1-ammonium enolates. The visible-light-induced sustained release of reactive
ketene species through Wolff rearrangement of α-diazoketones is crucial for achieving high levels of chemical efficiency and
stereoinduction.
he chiral isothioureas (ITUs)1 have emerged as one of
the most powerful organocatalysts for the discovery of
photoactivation and chiral amine catalysis in the mechanistic
study of the asymmetric [2 + 2] annulation reaction of ketenes
and imines.16 Recently, the Xiao group successfully developed
the photoinduced Wolff rearrangement/Pd-catalyzed [2 + n]
cycloaddition sequence, which provides rapid access to a wide
variety of functionalized heterocyclic compounds.17 Intrigued
by their work, and our research interests18 in Lewis base
catalysis, we developed a novel approach to efficient access of
disubstituted C1-ammonium enolates from photogenerated
ketenes and ITUs. With this purpose in mind, aurone-derived
α,β-unsaturated imines19 were carefully selected as suitable
electrophilic partners owing to their driving force of
aromatization (Scheme 1c). This class of compounds has
been widely employed as a privileged four-atom synthon in
asymmetric [4 + n] annulation reactions.20 Here we report the
sequential photoinduced Wolff rearrangement/chiral isothiour-
ea-catalyzed transformation of α-diazoketones and azadienes
for the construction of benzofuran-fused heterocyclic com-
pounds bearing all-carbon quaternary stereocenters.
Initially, the model reaction of α-diazoketone 1a and
auronederived α,β-unsaturated imine 2a was investigated via
sequential visible-light photoactivation17 and Lewis base
catalysis (Table 1). Screening of Lewis base catalysts (LB)
including NHC precatalysts 4, benzoylquinine 5, and ITU
catalyst 6a revealed the superiority of the ITU catalyst 6a with
regard to both catalytic activity and stereoinduction (Table 1,
entries 1−3 vs 4). Encouraged by this, further optimization was
done with a survey of the ITU catalysts 6 (entries 4−9), and
T
novel asymmetric synthetic transformations, allowing access to
a wide array of heterocycles and bioactive compounds. Since
the first demonstration in 2008,2 numerous chiral isothiourea
catalyzed enantioselective transformations that proceed via an
ammonium enolate intermediate3 have been developed4 and
have shown promising applications in the field of catalytic
organic synthesis. A wide range of carboxylic acids, esters,
anhydrides, and acyl imidazoles5 can be activated with ITUs to
form C1-ammonium enolates (R2 = H, Scheme 1a, left side)3b
and can then react with a broad chemical class of electrophilic
species, especially with transition-metal-mediated electrophiles
developed recently.6 However, despite remarkable progress
achieved in the past decade, many challenges remain. The
generation of disubstituted C1-ammonium enolates (R2 ≠ H)
from readily available starting materials for the enantioselective
construction of all-carbon quaternary stereocenters7 represents
one of the most challenging tasks (Scheme 1a, right side).
Ketenes are a family of privileged C2-synthons for [2 + n]
annulations in organic synthesis.8 Within this area, disub-
stituted ketenes (isolated or generated in situ) have proven to
be versatile building blocks for the construction of all-carbon
quaternary stereocenters catalyzed by nucleophilic Lewis
bases,9 such as cinchona alkaloid derivatives,10 planar-chiral
ferrocene catalysts,11 N-heterocyclic carbenes (NHCs),12 and
chiral phosphine catalysts.13 Notably, photochemistry14 has
proved to be an efficient and green synthetic method for the
generation of ketenes through Wolff rearrangement15 of α-
diazoketones, which opens up new opportunities for sequential
annulation reactions (Scheme 1b). The pioneering work
reported by the Lectka group introduced the sequential
Received: August 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02892
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Construction of All-Carbon Quaternary
Stereocenters via Disubstituted C1-Ammonium Enolate
Catalysis
Table 1. Condition Optimization
b
c
d
entry
LB
solvent
yield (%)
d.r.
ee (%)
e
1
4a
4b
5
6a
6b
6c
6d
6e
6f
6d
6d
6d
−
THF
THF
THF
THF
THF
THF
THF
THF
53
33
35
69
<5
75
69
69
54
88
96
96
n.d.
n.d.
62:38
67:33
55:45
89:11
11(2)
2(2)
8(5)
82
e
2
3
4
5
6
7
8
9
10
11
f
82:18
>95:5
89:11
77:23
>95:5
>95:5
>95:5
71
97
75
41
97
98
99
THF
CDCl3
i-Pr2O
i-Pr2O
i-Pr2O
i-Pr2O
g
12
13
14
the newly designed catalyst 6d was proven to be the optimal
one (entry 7). The results of the solvent screen (entries 10 and
11; see Table S1 in Supporting Information for details)
showed that the use of diisopropyl ether gave the best results
(entry 11). The enantioselectivity was further improved at
lower reactant concentrations (entry 12). In the absence of 6d
or light, no desired product was detected, thus indicating that
the ITU catalyst and light irradiation were both crucial to
promote the transformation (entries 13 and 14).
With the optimal conditions in hand, the generality of this
protocol was evaluated initially by the reaction of α-
diazoketone 1a with various azadienes 2 (Scheme 2). A
variety of N-substituents, including Ts, Ms, and Ns, were
tolerated to give 3aa−3ac in yields ranging from 49% to 95%
with high levels of stereoselectivity (up to >95:5 d.r. and 99%
ee). A range of aurone-derived α,β-unsaturated imines
containing both electron-donating and electron-withdrawing
substituents at the meta or para positions of the benzene ring
were well-tolerated, and the desired benzofuran derivatives
(3ad−3ao) were obtained in 70−99% yields with excellent
stereoselectivities (94:6−>95:5 d.r., 96−99% ee). Similarly,
azadienes with other aromatic ring substituents, such as 2-
naphthalene, 2-thiophene, and 2-furan, delivered the corre-
sponding products (3ap−3ar) with satisfactory results. The
structure and stereochemistry of the corresponding products
were confirmed unambiguously by single-crystal X-ray
diffraction analysis of 3aa (CCDC 1941477).
g
g h
,
6d
a
Reaction conditions: 1a (0.2 mmol), 2a (0.1 mmol), and Lewis base
catalyst (20 mol %) in solvent (1.0 mL) at rt under the irradiation of 5
b
c
W × 2 m blue LED strips for 24 h. NMR yields. The diastereomeric
ratios were determined by H NMR spectroscopic analysis. The ee
values were determined by HPLC. The ee of the minor diastereomer
is shown in parentheses. With Cs2CO3 (20 mol %). The opposite
enantiomer was obtained. Diisopropyl ether (i-Pr2O, 2.0 mL) was
d
1
e
f
g
h
used. In the dark. Ts: tosyl. n.d. = not detected.
tron-donating or electron-withdrawing substituents at the para
or meta positions on the benzene ring, the reaction led to the
corresponding products (3ba−3ka) in good yields (87−99%)
with excellent stereoselectivities (93:7−>95:5 d.r., 93−99%
ee). In addition, aryl-substituted 1-diazopropan-2-ones bearing
naphthyl and heteroaryl moieties were also good partners for
this transformation and delivered products (3la and 3ma)
smoothly. Moreover, replacement of the methyl group of α-
diazoketone 1a with other linear alkyl groups, such as ethyl and
n-butyl groups, afforded the desired products (3na, 3oa, 3nf,
and 3pa) in satisfactory results, albeit a prolonged reaction
time and increased catalyst usage were necessary. However,
simple monosubstituted α-diazoketones and aliphatic α-
diazoketones were unreactive substrates for this reaction.
Further investigations were performed to gain a deeper
mechanistic understanding of the sequential Wolff rearrange-
ment/ITU-catalyzed [4 + 2] annulation reaction (Figure 1; see
the Supporting Information for details). Initially, a one-pot
two-step reaction was performed, involving the in situ
formation of ketene 1a′ under the irradiation of blue light,
followed by the addition of azadiene 2a and chiral isothiourea
catalyst 6d (Figure 1a). Significant decreases in the activity and
product selectivity were obtained (3aa, 23% yield, 91:9 d.r.,
We next turned our attention to the scope of the α-
diazoketone component of the reaction. As indicated in
Scheme 3, a diverse set of α-diazoketones 1 underwent the
sequential Wolff rearrangement/ITU-catalyzed [4 + 2]
annulation reaction smoothly, affording target benzofuran-
fused dihydropyridinone derivatives 3 in high yields (up to
99%) with excellent stereocontrol (up to >95:5 d.r., up to 99%
ee). When aryl-substituted 1-diazopropan-2-ones bear elec-
B
DOI: 10.1021/acs.orglett.9b02892
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Scheme 2. Substrate Scope for Azadienes
Scheme 3. Substrate Scope for α-Diazoketones
a
Reaction conditions: 1a (0.2 mmol), 2 (0.1 mmol), and chiral
isothiourea catalyst 6d (20 mol %) in diisopropyl ether (2.0 mL) at rt
under the irradiation of 5 W × 2 m blue LED strips for 24 h. Yields of
isolated products 3. The diastereomeric ratios were determined by 1H
NMR spectroscopic analysis. The ee values were determined by
a
Reaction conditions: 1 (0.2 mmol), 2 (0.1 mmol), and chiral
isothiourea catalyst 6d (20 mol %) in diisopropyl ether (2.0 mL) at rt
under the irradiation of 5 W × 2 m blue LED strips for 24 h. Yields of
isolated product 3. The diastereomeric ratios were determined by 1H
NMR spectroscopic analysis. The ee values were determined by
b
c
d
HPLC. For 120 h. For 72 h. For 48 h.
b
c
HPLC. 30 mol % of 6d, 96 h. 40 mol % of 6d, 120 h.
and 65% ee). Next, we conducted several controlled experi-
ments (Figure 1b). Use of an isolated ketene 1a′ or ketene
precursor 7a as a substrate would lead to the formation of 3aa
in low yield with poor stereocontrol. Carboxylic esters (7b and
7c) and carboxylic acids 7d (with TsCl as activator) were
found to be unsuccessful reactants. Comparing these results
(obtained in Figure 1a and 1b) with those achieved under
standard conditions (Table 1, entry 12, 3aa obtained in 95%
yield, >95:5 d.r., and 99% ee), it is found that the in situ
generation of ketenes through photoactivation offers many
advantages and benefits over the previous situation. Finally, the
reaction profile for the photoactivated Wolff rearrangement of
1
1a was monitored by analyzing the H NMR spectra of the
reaction mixture, and almost quantitative conversion was
completed after 10 h (Figure 1c). The diagram in Figure 1d
shows the reaction profile for the model reaction. The ketene
intermediate 1a′ was not detected until the sequential reaction
was nearly complete. These results implied that the ITU-
catalyzed [4 + 2] annulation reaction of ketene 1a′ is relatively
faster than the photogeneration of 1a′. All these results
suggested the visible-light-induced sustained release of ketenes
is identified as a vital factor for achieving high levels of reaction
efficiency and stereocontrol in the sequential photoactivation/
isothiourea catalysis.
The reaction of α-diazoketone 1a with azadiene 2a can be
scaled-up to a 2.0 mmol scale successfully (Scheme 4, reaction
a). The resulting dihydropyridinones are structurally interest-
ing and ready for further chemical transformations. The N-
tosyl group can be removed with magnesium powder in
methanol (MeOH) at 60 °C to give the unprotected
dihydropyridinone 8 in 86% yield, with >95:5 d.r., and 97%
ee (Scheme 4, reaction b). The reduction of 3aa using
Figure 1. Further investigations on the reaction mechanism.
diisobutylaluminum hydride (DIBAL-H) and Et3SiH/BF3·
Et2O provided the chiral benzofuran-fused tetrahydropyridine
C
DOI: 10.1021/acs.orglett.9b02892
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Organic Letters
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Scheme 4. Preparation of 3aa in 2.0 mmol Scale and Further
Derivatizations
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02892.
■
S
Experimental procedures and spectroscopic data (PDF)
Accession Codes
CCDC 1941477 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
*E-mail: jill@ustc.edu.cn.
ORCID
Jin Song: 0000-0003-0449-1727
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from NSFC (Grants
21702199) and the Anhui Provincial Natural Science
Foundation (Grant No. 1808085QB30).
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DOI: 10.1021/acs.orglett.9b02892
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