Phosphoric Acid Catalyzed [4 + 1]-Cycloannulation Reaction of ortho-Quinone Methides and Diazoketones: Catalytic, Enantioselective Access toward cis-2,3-Dihydrobenzofurans

Article abstract of DOI:10.1021/acs.orglett.8b03311

A highly straightforward route to enantiomerically highly enriched cis-2,3-dihydrobenzofurans has been achieved via addition of α-diazocarbonyl compounds to in situ generated o-QMs catalyzed by a chiral Br?nsted acid. This catalytic strategy provides a direct access to 2,3-dihydrobenzofurans in high yields and with up to 91:9 dr and 99:1 er at ambient temperature. Moreover, a unique phenonium-type rearrangement accounts for product formation with an inverted 2,3-substitution pattern.

Full text of DOI:10.1021/acs.orglett.8b03311

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Phosphoric Acid Catalyzed [4 + 1]-Cycloannulation Reaction of
ortho-Quinone Methides and Diazoketones: Catalytic,
Enantioselective Access toward cis-2,3-Dihydrobenzofurans
Arun Suneja and Christoph Schneider*
Institut fur Organische Chemie, Universitat Leipzig, Johannisallee 29, 04103 Leipzig, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A highly straightforward route to enantiomerically highly enriched cis-2,3-dihydrobenzofurans has been achieved
via addition of α-diazocarbonyl compounds to in situ generated o-QMs catalyzed by a chiral Brønsted acid. This catalytic
strategy provides a direct access to 2,3-dihydrobenzofurans in high yields and with up to 91:9 dr and 99:1 er at ambient
temperature. Moreover, a unique phenonium-type rearrangement accounts for product formation with an inverted 2,3-
substitution pattern.
2,3-Dihydrobenzofuran scaffolds are ubiquitously found in
numerous complex natural products and pharmaceuticals.1
Among various bioactive derivatives,2,3 (+)-conocarpan (1a) is
reported as an insecticidal,4 antifungal,5 and antitrypanosomal
agent.6 The neolignan callislignan A (1c) and (2R,3S)-3,4′-di-
O-methylcedrusin (1d) are natural products containing this
subunit and display antibacterial7 and antitumor8 activity,
respectively (Figure 1).
afford trans-2,3-dihydrobenzofurans with excellent diastereo-
control.13 The Waser group employed chiral, enantiomerically
pure nitrogen ylides in reactions with o-QMs to assemble this
subunit with both excellent diastereo- and enantiocontrol.14
Moreover, the Yang group established a bisurea-catalyzed [4 +
1]-annulation of sulfur ylides and o-QMs to access
monosubstituted 2,3-dihydrobenzofurans with moderate to
good enantiocontrol.15 Finally, Han et al. developed a phase-
transfer-catalyzed process for the addition of bromomalonates
to stable o-QMs that proceeds with excellent enantiocon-
trol.16,17
Despite these advances in the synthesis of mainly trans-2,3-
dihydrobenzofurans, a catalytic, enantioselective access toward
2,3-cis-dihydrobenzofurans is still very limited and has not yet
been described using o-QM chemistry.18 We envisioned a
direct, Brønsted acid catalyzed [4 + 1]-cycloannulation of o-
QMs with α-diazocarbonyl compounds to afford the desired
heterocycles (Scheme 1). We have recently studied phosphoric
acid catalyzed reactions of o-QMs with a broad range of π-
nucleophiles in great detail and obtained various benzannu-
lated oxygen heterocycles with excellent enantiocontrol.19,20
Figure 1. Natural products containing 2,3-dihydrobenzofurans.
Due to their intriguing biological activities, considerable
attention has been focused recently toward their stereoselective
synthesis.9 Prominent approaches toward these scaffolds
include resolution of racemates,10a,b diastereoselective oxida-
tion,10c reduction reactions,10d intramolecular C−H inser-
tions,10e catalytic asymmetric hydrogenations,10f and acid
promoted cycloaddition reactions.10g Another obvious strategy
which has been developed only recently is the [4 + 1]-
cycloannulation11 of carbenoid compounds toward ortho-
quinone methides (o-QMs).12 In this context Zhou and co-
workers reported the addition of sulfur ylides to o-QMs to
Scheme 1. Design Plan
Received: October 16, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03311
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Organic Letters
Letter
We started our investigations with the reaction of ortho-
hydroxy benzhydryl alcohol 2a as a direct o-QM precursor and
α-diazoacetophenone (3a) in the presence of chiral Brønsted
acid PA1 (10 mol %) in chloroform (CHCl3) at 23 °C. After a
30 h reaction time, cis-2,3-dihydrobenzofuran 4a was obtained
in moderate yield and stereoselectivity (Table 1, entry 1).
the optimal choice. Further improvement in terms of both
yield and stereoselectivity was achieved with the use of MgSO4
as a dehydrating agent which furnished 4a with 82% yield and
97:3 er (entry 14). Lowering the catalyst loading to just 5 mol
% resulted in a diminished yield and enantioselectivity (entry
15). Optimal results were eventually obtained with a reduced
substrate concentration of just 0.05 M in toluene, and 10 mol
% of catalyst PA5 in combination with MgSO4 (30 mg) at
room temperature (entry 16).
a k
−
Table 1. Optimization Studies
The substrate scope was investigated next. Initially, a variety
of α-diazocarbonyl compounds 3 were submitted to reactions
with benzhydryl alcohol 2a under the optimized reaction
conditions and each furnished 2,3-dihydrobenzofurans 4a−m
in synthetically useful yields (Figure 2). Interestingly, α-
bc
,
d
e
entry PA
solvent
time (h) 4a (%)
er (major)
dr
f
1
1
1
2
3
4
5
6
5
5
5
5
5
5
5
5
5
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CH2Cl2
PhMe
m-xylene
Et2O
PhMe
PhMe
PhMe
PhMe
PhMe
30
24
24
24
24
18
20
20
16
18
96
16
16
12
24
12
50
57
63
48
58
70
68
64
72
69
21
70
77
82
65
82
71:29
76:24
88:12
77:23
83:17
96:4
93:7
95:5
97:3
95:5
86:14
95:5
96:4
97:3
93:7
56:44
62:38
66:34
70:30
62:38
82:18
76:24
77:23
82:18
83:17
67:33
81:19
81:19
83:17
80:20
86:14
2
3
4
5
6
7
8
9
10
11
12
13
14
15 ,
ik
16 ,
g
h
i
ij
98:2
Figure 2. Scope of the reaction with respect to the α-diazocarbonyl
component.
a
Reactions were carried out with 0.1 mmol of 2a and 0.11 mmol of 3a
b
in the presence of catalyst PA (10 mol %) in PhMe (0.1 M). Isolated
yield of both diastereomers after column purification. Decomposition
of the rest of the mass balance. Enantiomeric ratios (er) were
determined by chiral HPLC. Diastereomeric ratios (dr) were
determined from H NMR of crude reaction mixture. No 4 Å MS
used. With 3 Å MS (30 mg). With Na2SO4 (30 mg). With MgSO4
(30 mg). 5 mol % catalyst loading. PhMe (0.05 M) was used as
solvent.
c
d
diazoaryl ketones 3b−e substituted with an electron-donating
group reacted smoothly and afforded products 4b−e in good
yields, moderate to good diastereocontrol, and excellent
enantioselectivities of up to 98:2 er. Notably, steric hindrance
had no significant effect on the enantioselectivity of product
formation. Moreover, substrates 3f−i bearing electron-with-
drawing groups (such as halogen and CN-substituents)
afforded 4f−i in synthetically viable yields with enantiomeric
ratios of up to 98:2 er.
Heteroaromatic substrates such as 3j, having a thiophene
moiety, afforded product 4j in 67% yield and 90:10 er. The
process also worked fine for substrates 3k and 3l carrying a
bulky naphthyl group and afforded 4k and 4l, respectively, in
moderate yields and with excellent enantioselectivities of up to
97:3 er. Ethyl α-diazoacetate failed to provide the desired
product 4m even at elevated temperature.
The substrate scope was further examined with diversely
substituted o-hydroxy benzhydryl alcohols 2 and α-diazoaryl
ketones 3 as shown in Figure 3. To our delight, a variety of
different electron-rich substituents were well tolerated in both
the quinone component and methide substituents and afforded
dihydrobenzofuran products with good chemical yields and
good to very good stereocontrol. Some observations deserve
e
f
1
g
h
i
j
k
Surprisingly, the product displayed an inverted substitution
pattern which was unambiguously proven only through a
crystallographic analysis of benzofuran 4g eventually (Figure
4). Thus, the aryl ketone moiety did not appear as the 2-
substituent, but rather at the 3-position of the benzofuran.
Likewise, the PMP group was located at the 2-position and not
the 3-position of the heterocycle as originally expected.
Interestingly, the reaction could be expedited in slightly better
yield by using 4 Å molecular sieves, affording 4a in 57% yield
with 76:24 er (entry 2).
Encouraged by this initial finding a series of chiral Brønsted
acids PA2−PA6 were screened in this model reaction (entries
3−7). Among them, PA5 was found to be the optimal
phosphoric acid21 catalyst furnishing product 4a with good
chemical yield and enantioselectivity (entry 6). Subsequently
various solvents were examined, with toluene turning out to be
B
DOI: 10.1021/acs.orglett.8b03311
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Organic Letters
Letter
Scheme 2. Plausible Mechanistic Pathway
to afford phenol A. Instead of the expected O-alkylation, this
electron-rich phenol acts as a C-nucleophile and upon N2-
displacement forms a spirocyclic cyclopropane phenonium ion
B.22 The cyclopropane then ring-opens to form a resonance-
stabilized carbocation C which is trapped by the free phenol to
afford the product 4 with inverted 2- and 3-substituents. This
mechanistic scenario is further supported through the
substituent effects shown in Figure 3 which require a highly
electron-rich β-methide substituent (e.g., PMP) in the o-QM
for a successful reaction.
To shed some further light into the process, two control
experiments were performed. Phenol-protected benzhydryl
alcohol 2m failed to react with 3a under otherwise identical
conditions further supporting the essential role of the o-QM
structure for reactivity. Furthermore, a preformed Mg(PA5)2
catalyst was not able to catalyze the reaction which rules out
the possibility that the added MgSO4 also serves to form an
active metal catalyst (Scheme 3).
Figure 3. Scope of the reaction with respect to the o-hydroxy
benzhydryl alcohol component.
mentioning though: only alkoxy-substituted aryl groups and
other electron-rich heteroaryl groups were well tolerated as β-
methide substituents and gave rise to dihydrobenzofurans with
good chemical yields (e.g., 5a−b, 5e−l). Less electron-rich β-
methide substituents provided products with only low to
moderate yields (e.g., 5c−d). An alkyl-substituted methide
substrate failed altogether to furnish dihydrobenzofuran 5m.
A range of substrates differing in the quinone moiety were
studied as well and furnished products with good chemical
yields and good to excellent enantioselectivity. For example,
the 5- and 6-substituted benzhydryl alcohols 2f−k with alkyl
and halogen substituents afforded benzofurans 5e−l in
generally good yields and with excellent enantioselectivity of
up to 99:1 er. The diastereoselectivity varied here from 3 to
10:1 cis/trans.
Scheme 3. Control Experiments
The crystal structure analysis of the major cis-diastereomer
of 2,3-dihydrobenzofuran 4g (CCDC 1871281) confirmed
both the relative and absolute configuration of the products
and corroborated the structural assignment at the same time
(Figure 4). In order to showcase the practical utility of our
Finally, the products were easily converted into the
corresponding 2,3-dihydrobenzofuran-3-ols23 upon Baeyer−
Villiger oxidation24 and hydrolysis (Scheme 4). There appears
Scheme 4. Synthetic Elaboration
Figure 4. X-ray crystal structure of dihydrobenzofuran 4g.
process, a scale-up reaction was performed on a 1 mmol scale
using catalyst PA5 (10 mol %) which afforded benzofuran 4a
with high yield and stereoselectivity (79%, 86:14 dr, 98:2 er)
which could be enhanced to >99:1 er by a single
recrystallization (see the SI for details).
A reasonable mechanism for the unexpected product
formation with inverted 2- and 3-substituents is shown in
Scheme 2. Initial acid-catalyzed dehydration of 2a and o-QM
formation is followed by conjugate addition of α-diazoketone 3
to be a pronounced migratory aptitude of the benzylic carbon
over the aryl group which results in the exclusive incorporation
of the oxygen atom next to the benzofuran ring. This two-step
process proceeds with full retention of configuration and
furnishes the product in good overall yield.
In conclusion, we have developed a highly stereoselective,
phosphoric acid catalyzed [4 + 1]-cycloannulation of α-
diazocarbonyl compounds with in situ generated o-QMs. This
C
DOI: 10.1021/acs.orglett.8b03311
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
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Accession Codes
CCDC 1871281 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: schneider@chemie.uni-leipzig.de.
ORCID
(15) Yang, Q.-Q.; Xiao, W.-J. Eur. J. Org. Chem. 2017, 2017, 233.
(16) Lian, X.-L.; Adili, A.; Liu, B.; Tao, Z.-L.; Han, Z.-Y. Org. Biomol.
Chem. 2017, 15, 3670.
Christoph Schneider: 0000-0001-7392-9556
Notes
(17) For further [4 + 1]-cycloannulation reactions with o-QMs, see:
(a) Osyanin, V. A.; Osipov, D. V.; Klimochkin, Y. N. J. Org. Chem.
2013, 78, 5505. (b) Wu, B.; Chen, M.-W.; Ye, Z.-S.; Yu, C.-B.; Zhou,
Y.-G. Adv. Synth. Catal. 2014, 356, 383. (c) Lei, X.; Jiang, C.-H.; Wen,
X.; Xu, Q.-L.; Sun, H. RSC Adv. 2015, 5, 14953. (d) Shaikh, A. K.;
Varvounis, G. RSC Adv. 2015, 5, 14892. (e) Rodriguez, K. X.; Vail, J.
D.; Ashfeld, B. L. Org. Lett. 2016, 18, 4514. (f) Jiang, X.-L.; Liu, S.-J.;
Gu, Y.-Q.; Mei, G.-J.; Shi, F. Adv. Synth. Catal. 2017, 359, 3341.
(g) Jiang, F.; Luo, G.-Z.; Zhu, Z.-Q.; Wang, C.-S.; Mei, G.-J.; Shi, F. J.
Org. Chem. 2018, 83, 10060.
(18) (a) Natori, Y.; Tsutsui, H.; Sato, N.; Nakamura, S.; Nambu, H.;
Shiro, M.; Hashimoto, S. J. Org. Chem. 2009, 74, 4418.
(b) Belmessieri, D.; Morrill, L. C.; Simal, C.; Slawin, A. M. Z.;
Smith, A. D. J. Am. Chem. Soc. 2011, 133, 2714. (c) Wang, H.; Li, G.;
Engle, K. M.; Yu, J.-Q.; Davies, H. M. L. J. Am. Chem. Soc. 2013, 135,
6774.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Deutsche Forschungsgemein-
schaft (SCHN 441/11-2) and through gifts of chemicals from
BASF and Evonik. We thank Till Friedmann for support in the
̈
preparation of starting materials and Dr. Peter Lonnecke (both
University of Leipzig) for obtaining the X-ray crystal structure.
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