Enantioselective Cyclopropanation/[1,5]-Hydrogen Shift to Access Rauhut-Currier Product

Article abstract of DOI:10.1021/acs.orglett.0c03937

A Michael addition initiated cyclopropanation/[1,5]-hydrogen shift has been developed for the enantioselective synthesis of Rauhut-Currier products. The reaction of α-alkyl diazoesters and in situ generated o-quinone methides proceeds in the presence of c

Full text of DOI:10.1021/acs.orglett.0c03937

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Letter
Enantioselective Cyclopropanation/[1,5]-Hydrogen Shift to Access
Rauhut−Currier Product
Seung Tae Kim,† Rameshwar Prasad Pandit,† Jaesook Yun, and Do Hyun Ryu*
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ABSTRACT: A Michael addition initiated cyclopropanation/
[1,5]-hydrogen shift has been developed for the enantioselective
synthesis of Rauhut−Currier products. The reaction of α-alkyl
diazoesters and in situ generated o-quinone methides proceeds in
the presence of chiral oxazaborolidinium ion, providing Z-
stereocontrolled Rauhut−Currier products in high yields (up to
96%) with excellent Z/E selectivities (>20:1) and enantioselectiv-
ities (up to >99% ee). The synthetic utility was illustrated by
conversion of the product to 3,4-dihydrocoumarins with two
adjacent chiral stereocenters.
Table 1. Optimization of the Asymmetric Synthesis of
Rauhut−Currier Products
nantioenriched α-methylene carbonyl derivatives possess-
Eing a chiral center at the β′-position, Rauhut−Currier
(RC) products, are valuable building blocks for the synthesis of
biologically active molecules and natural products due to their
multifunctional composition.1 These derivatives can be
prepared by an asymmetric intermolecular RC reaction
(Scheme 1A). Despite recent advances in this area,2
asymmetric synthesis of β-substituted RC products such as
β-substituted RC ketones or esters has not been successful by
general RC catalysis, and to the best of our knowledge, only
one example of highly enantioselective synthesis of (E)-β-
a
entry
1
2
catalyst
4
R
yield (%)
ee (%)
Scheme 1. Synthesis of β-Substituted Rauhut−Currier
Products and [1,5]-Hydrogen Shift of
Acylalkylcyclopropane
5a
5b
5c
5b
5b
5b
4a
4a
4a
4b
4c
4a
t-butyl
t-butyl
t-butyl
Me
79
95
87
63
75
97
>99
97
95
95
c
3
4
5
Et
b
6
t-butyl
a
Reaction of 2-(1-hydroxymethyl)phenols 1a (0.2 mmol) with α-
benzyl diazoester 2 (0.4 mmol) were performed in the presence of
catalyst 5 and 3 Å molecular sieves (100 mg) in 2.0 mL of solvent at
−50 °C for 30 min. All yields refer to isolated products. The ee values
b
were determined by chiral HPLC. The reaction was performed in the
c
absence of molecular sieves. 1 mmol scale reaction was also
performed to give 4a in 90% yield with >99% ee. DCM =
dichloromethane.
Received: November 28, 2020
https://dx.doi.org/10.1021/acs.orglett.0c03937
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Letter
Table 2. Enantioselective Formation of Rauhut−Currier
a
Products with Various α-Alkyl Diazoesters
entry
4
R
yield (%)
ee (%)
1
2
3
4
5
6
7
8
4d
4e
4f
4g
4h
4i
4j
4k
4l
4-MePh
2-MePh
4-OMePh
4-BrPh
4-CF3Ph
2-Nap
1-Nap
Me
73
91
91
86
93
85
88
76
88
93
91
>99
98
>99
99
98
>99
98
>99
>99
>99
>99
Figure 1. ORTEP view of 4a. Displacement ellipsoids are drawn at
the 50% probability level.
Scheme 3. Synthetic Transformation of Rauhut−Currier
Products
9
10
11
Pen
i-Pr
vinyl
4m
4n
a
Reaction of 2-(1-hydroxymethyl)phenols 1a (0.2 mmol) with α-
benzyl diazoester 2 (0.4 mmol) was performed in the presence of
catalyst 5b and 3 Å molecular sieves (100 mg) in 2.0 mL of solvent at
−50 °C for 30 min. All yields refer to isolated products. The ee values
were determined by chiral HPLC. DCM = dichloromethane.
Scheme 2. Enantioselective Formation of Rauhut−Currier
a
Products with Various 2-(1-Hydroxyalkyl)phenols.
Z-stereocontrolled methods3b to access β-substituted RC
products is highly desirable.
Homodienyl [1,5]-hydrogen shift4 is a hydrogen transfer
reaction through a cyclopropane with an appended vinyl,5
carbonyl, or imine group4b instead of the diene system in the
general [1,5]-hydrogen shift (Scheme 1B). Although it has
been known for decades, an asymmetric version of this
reaction has not been reported to date. Recently, our group has
developed highly enantioselective catalytic Michael-initiated
cyclopropanations and tandem reactions6 with α-diazoesters
using chiral oxazaborolidinium ion (COBI)7 as a Lewis acid
catalyst. We envisioned that the reaction of o-hydroxyphenyl
alcohol 1 as a precursor of o-quinone methides8 (o-QM) (R1 =
alkyl) and α-alkyl diazoesters 2 would generate chiral
cyclopropane 3, which is ideally substituted for subsequent
[1,5]-hydrogen shift to provide highly functionalized RC
products 4 (Scheme 1C). Herein, we report the first example
of a catalytic enantioselective synthesis of RC products through
tandem Michael-initiated cyclopropanation/[1,5]-hydrogen
shift starting from 2-(1-hydroxyalkyl)phenols 1 and α-alkyl
diazoesters 2.
In connection with our hypothesis, o-hydroxyphenyl ethanol
1a and tert-butyl-2-diazo-3-phenylpropanoate 2a were consid-
ered as model substrates for optimization of the asymmetric
cyclopropanation/[1,5]-hydrogen shift (Table 1). Study of the
reaction between tert-butyl diazoester 2a having β-hydrogens
and o-QM generated from 1a in the presence of chiral
oxazaborolidiniumion catalysts 5a as Lewis acid afforded only
(Z)-β-phenyl-substituted RC product 4a in 79% yield with
high enantioselectivity (97%) (entry 1). While studying the
substituents effect of catalyst 5, we sought to change the aryl
groups (Ar1 and Ar2) of the catalyst and discovered that 5b
a
Reaction of 2-(1-hydroxyalkyl)phenols 1 (0.2 mmol) with α-benzyl
diazoester 2a (0.4 mmol) were performed in the presence of catalyst
5b and 3 Å molecular sieves (100 mg) in 2.0 mL of solvent at −50 °C
for 30 min. All yields refer to isolated products. The ee values were
determined by chiral HPLC. DCM = dichloromethane.
substituted RC products has been reported.3a A regioselective
conjugate addition of silyl-dienol ethers and subsequent
isomerization with a chiral bifunctional organocatalyst afforded
highly enantioenriched (E)-β-substituted RC products. Con-
sequently, the development of new highly enantioselective E/
B
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riched RC products 4i and 4j in high yields. Remarkably,
reactions of various β-alkyl-substituted diazo esters, such as
methyl, n-pentyl, i-propyl, and vinyl groups, resulted in high
yields and excellent enantioselectivities (>99%) (4k−4n).
Next, we turned our attention to deploying various o-QMs
with 2a for enantioselective synthesis of RC products. As
illustrated in Scheme 2, reactions of in situ generated o-QMs
possessing electron-donating (−Me, −OMe) or electron-
withdrawing (−F, −Cl) groups proceeded well under the
optimized reaction conditions to afford the desired RC
products 4o−4t in good to high yields with excellent
enantioselectivity (up to >99%). The effect of para-halogen
substituents on the phenolic −OH was significant in the
reaction leading to decreased yields of the desired RC products
(4r and 4t). This anomaly may be associated with weak
coordination of phenol 1 to COBI 5b from an electron-
withdrawing effect of para-halogens. For different alkyl
substituents at R1,9 the reactions proceeded in high yield
with high stereoselectivities (4u−4w). For the aryl substituents
at R1, 2-aryl-2,3-dihydrobenzofurans are formed via an
intramolecular rearrangement of donor−acceptor cyclopro-
pane intermediate (Scheme 1C).6e The absolute configuration
of 4a was unambiguously determined by X-ray crystallographic
analysis (Figure 1); the configurations of all other products 4
were assigned accordingly.
Scheme 4. (a) Model Experiment. (b) Deuterium
Experiment. (c) Kinetic Isotope Effect Experiment. (d)
Pretransition State and Plausible Mechanism for
Asymmetric Synthesis of Rauhut−Currier Products
Since the resulting RC products are composed of multi-
functional groups, we demonstrated their synthetic utility by
several modifications as illustrated in Scheme 3. Acid-mediated
intramolecular transesterification10 of 4a afforded coumarin
derivative 6. The exocyclic double bond in 6 was selectively
hydrogenated on a solid support11 to create an additional
chiral center affording cis-3,4-dihydrocoumarin 7 as a single
diastereomer (>99% ee). Base-promoted C2-epimerization12
furnished trans-3,4-dihydrocoumarin 8 in 81% yield without a
loss of optical purity (>99% ee). Since dihydrocoumarin
scaffolds are found in a wide range of natural products and
synthetic compounds that show diverse biological and
pharmacological activities,13 the development of stereoselective
synthetic methods for their preparation has attracted
considerable attention.11,12,14
Electrophilic aromatic substitution enabled installation of
iodine at the para position of phenol 4a to give 9 in 95% yield.
Protection of the phenolic −OH with methyl iodide furnished
10, which was then subjected to coupling reactions with
arylboronic acid15 and phenylacetylene.16 Palladium-catalyzed
cross-coupling resulted in addition of 4-methoxyphenyl (PMP)
and phenylacetylene groups to afford 11 and 12 in 85% and
91%, respectively. All these transformations proceeded well
without any loss of optical purity (>99%).
To acquire a better understanding of this [1,5]-hydrogen
shift, cis-cyclopropanes17 13 and 15, which have a similar
structure to intermediate 3, were prepared according to the
known procedure18 and subjected to thermal rearrangement
conditions in chlorobenzene (Scheme 4a). The [1,5]-hydrogen
shift of cis-cyclopropanes was corroborated by the obtained RC
products 14 and 16 in 67% and 93% yield, respectively.
For further mechanistic insight, deuterium labeling and
kinetic isotope experiments (Scheme 4b,c) were conducted,
which provided the following two facts: (a) conversion of the
β-sp3-carbon of diazoester 2a to β-sp2-carbon of product 4a
occurs during the reaction and (b) the kH/kD of 1.08 illustrates
the [1,5]-hydrogen shift is not a rate-determining step. Based
on these results and the absolute configuration of 4a, we
gave the best yield (95%) with excellent enantioselectivity
(>99%) (entries 2 and 3). Modifications of ester functionality
of 2a (−CO2Me, −CO2Et instead of −CO2t-Bu) gave
decreased yields and enantioselectivities of RC products 4b
and 4c (entries 4 and 5). Failure of the reaction to afford 4a in
the absence of molecular sieves is indicative of deactivation of
COBI catalyst by eliminated water from 1a (entry 6).6e
Initially, we evaluated the scope of various diazo compounds
using the optimized conditions for this catalytic enantiose-
lective cyclopropanation/[1,5]-hydrogen shift for the synthesis
of RC products (Table 2). Electronic variations on the β-aryl
ring of the α-alkyl diazoesters had no significant impact on
their reactivity with o-QM to deliver enantioenriched RC
products (entries 1−5). For example, reactions of diazo
compounds with electron-donating (−Me, −OMe) or -with-
drawing groups (−Br, −CF3) on the β-aryl ring proceeded
smoothly with o-QM from 1a to afford (Z)-olefinic products
(4d−4h). α-Methyl diazoesters substituted with a 1- or 2-
naphthyl group in the β-position afforded highly enantioen-
C
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propose a plausible mechanism as follows. In the pretransition
state (Scheme 4d),6e α-alkyl diazoester approached the β-
methide carbon atom of the o-QM with the ester group
positioned away from the carbonyl group of the quinone
moiety because of dipole−dipole interaction between the two
carbonyl groups. Release of catalyst followed by intramolecular
ring-closure forms the highly ordered asymmetric cyclo-
propane 3. We assume hydrogen at C1 migrates to carbonyl
oxygen in a concerted manner via six-membered ring transition
state 18. In order for the olefinic product to be (Z)-selective,
the transition state requires a cis relation between the ester
group (CO2R3) at C2 and R2. Substituent R2 at C1 is situated in
an equatorial position where one of two methylene hydrogens
can effectively overlap with the reacting C4O π-bond and
breaking C2−C3 σ-bond.4c This conformational preference in
the TS leaves substituent R2 and esters on the same side of the
CC double bond in the final products 4. The driving force
for such a [1,5]-hydrogen shift is restoration of aromaticity.
In summary, the first example of highly enantiocontrolled
catalytic formation of (Z)-β-substituted Rauhut−Currier
products has been developed. This tandem cyclopropana-
tion/[1,5]-hydrogen shift provides highly functionalized
Rauhut−Currier products in a single step with excellent
enantioselectivity. The synthetic utility of this product was
demonstrated by transformation to 3,4-dihydrocoumarins with
two chiral stereocenters. We believe that the developed
method provides an alternative route to access chiral
Rauhut−Currier products. Detailed mechanistic studies and
applications to bioactive molecules are currently under
investigation in our laboratory.
https://pubs.acs.org/10.1021/acs.orglett.0c03937
Author Contributions
†S.T.K. and R.P.P. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by National Research Foundation of
Korea (NRF) grants funded by the Korean government
(MSIT) (nos. N RF-2019R1A2C2087018 and
2019R1A4A2001440).
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ASSOCIATED CONTENT
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crystal data analysis, characterization of products, and
full analytical data with spectra (PDF)
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data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
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bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
́
́
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AUTHOR INFORMATION
Corresponding Author
■
Do Hyun Ryu − Department of Chemistry, Sungkyunkwan
University, Suwon 16419, Korea; orcid.org/0000-0001-
7615-4661; Email: dhryu@skku.edu
Authors
Seung Tae Kim − Department of Chemistry, Sungkyunkwan
University, Suwon 16419, Korea
Rameshwar Prasad Pandit − Department of Chemistry,
Sungkyunkwan University, Suwon 16419, Korea
Jaesook Yun − Department of Chemistry, Sungkyunkwan
University, Suwon 16419, Korea; orcid.org/0000-0003-
4380-7878
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(16) Senapati, B. K.; Hwang, G.-S.; Lee, S.; Ryu, D. H.
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(17) The cis geometry means the carbonyl and benzyl group are
directed cis to each other in the cyclopropane.
(18) For the preparation of cyclopropane 13 and 15, see ref 6b and
supporting materials.
(8) For selected recent papers of o-quinone methides, see:
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11764. (b) Wang, Z.; Sun, J. Recent Advances in Catalytic
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3629−3644. (c) Nielsen, C. D.-T.; Abas, H.; Spivey, A. C.
Stereoselective Reactions of ortho-Quinone Methide and ortho-
Quinone Methide Imines and Their Utility in Natural Product
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ortho-quinone methides and tandem transformations. Nat. Chem.
2020, 12, 353−362. (e) Suneja, A.; Loui, H. J.; Schneider, C.
Cooperative Catalysis for the Highly Diastereo- and Enantioselective
[4 + 3]-Cycloannulation of ortho-Quinone Methides and Carbonyl
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(9) See ref 6e and: Schneider, T. F.; Werz, D. B. Ring-Enlargement
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(10) Lee, S. I.; Jang, J. H.; Hwang, G.-S.; Ryu, D. H. Asymmetric
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lective Tandem Michale-Aldol Reaction. J. Org. Chem. 2013, 78, 770−
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Org. Lett. XXXX, XXX, XXX−XXX