Stereoconvergent Conjugate Addition of Arylboronic Acids to α-Angelica Lactone Derivatives: Synthesis of Stereochemically Complex γ-Butyrolactones

Article abstract of DOI:10.1021/acscatal.9b04405

Catalyzed stereoconvergent 1,4-additions to unsaturated carbonyls are rare but of high potential value. This letter details the development of enantioselective arylation reactions of boronic acids and β,γ-butenolides. These reactions are catalyzed by commercially available hydroxy[(S)-BINAP]-rhodium(I) dimer to afford stereochemically complex γ-butyrolactone derivatives. The reaction products provide functionality amenable to further manipulation and can lead to products with up to three contiguous stereocenters. The reaction proceeds under a dynamic kinetic resolution manifold by isomerizing the achiral starting material into an interconverting mixture of enantiomeric conjugate acceptors, followed by catalyst-controlled, enantiomer-selective 1,4-addition. Base-promoted racemization of the intermediate α,β-butenolide is possible due to the high kinetic and thermodynamic acidity of the γ-proton.

Full text of DOI:10.1021/acscatal.9b04405

Letter
pubs.acs.org/acscatalysis
Cite This: ACS Catal. 2019, 9, 11614−11618
Stereoconvergent Conjugate Addition of Arylboronic Acids to
α‑Angelica Lactone Derivatives: Synthesis of Stereochemically
Complex γ‑Butyrolactones
Jessica A. Griswold and Jeffrey S. Johnson*
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
S
* Supporting Information
ABSTRACT: Catalyzed stereoconvergent 1,4-additions to
unsaturated carbonyls are rare but of high potential value.
This letter details the development of enantioselective arylation
reactions of boronic acids and β,γ-butenolides. These reactions
are catalyzed by commercially available hydroxy[(S)-BINAP]-
rhodium(I) dimer to afford stereochemically complex γ-
butyrolactone derivatives. The reaction products provide
functionality amenable to further manipulation and can lead
to products with up to three contiguous stereocenters. The
reaction proceeds under a dynamic kinetic resolution manifold by isomerizing the achiral starting material into an
interconverting mixture of enantiomeric conjugate acceptors, followed by catalyst-controlled, enantiomer-selective 1,4-addition.
Base-promoted racemization of the intermediate α,β-butenolide is possible due to the high kinetic and thermodynamic acidity
of the γ-proton.
KEYWORDS: asymmetric catalysis, conjugate addition, lactones, rhodium, dynamic kinetic resolution
Scheme 1. Enantioconvergent Additions of π-Functional
Groups
he need for enantioenriched drug molecules has provided
a significant impetus for breakthroughs in the field of
T
dynamic kinetic resolution (DKR). DKR has proven to be a
powerful tool for the synthesis of enantiomerically pure
products from racemic starting materials.1 DKR reactions
commonly take advantage of a stereogenic center adjacent to
an activating group to promote racemization. For example,
DKR transformations of β-keto esters and α-keto esters have
been widely explored, as these types of substrates can be easily
racemized via the achiral enol or enolate, followed by catalyst-
controlled enantio- and diastereoselective 1,2-addition
(Scheme 1A).2,3 One could imagine a complementary
scenario, in which an α,β-unsaturated substrate containing a
stereocenter at the γ-position could be racemized via the dienol
or dienolate (Scheme 1B). One enantiomer of the starting
material could then be trapped via an asymmetric 1,4-addition,
to afford useful products with vicinal stereocenters at the β-
and γ-positions. This reaction manifold remains unknown to
the best of our knowledge, despite its direct analogy to the vast
body of enantioconvergent 1,2-additions.4
It may be worth considering possible reasons for this gulf,
especially in the context of attempting to identify an
electrophile that could potentially be appropriate for such a
transformation. Successful DKR reactions of this type would
require the γ-proton of the substrate to be of sufficient kinetic
acidity to racemize at a rate faster than the irreversible π-bond
addition. We were interested in testing the notion that α-
angelica lactone (A) and related structures might serve as
useful proelectrophiles for the transformation outlined in
Scheme 1B. Enantioconvergent conjugate additions to α-
angelica lactone are especially enticing because of that reagent's
lignocellulosic origin5 and the prevalence of the core scaffold in
bioactive structures: nearly 10% of all natural products contain
a γ-butyrolactone.6
Catalyzed enantioconvergent reactions are complex because
of the need to engineer both a competent racemization
pathway and an enantiomer-selective addition to a prochiral
functional group. Scheme 2 illustrates the higher level of
Received: October 14, 2019
Revised: November 12, 2019
© XXXX American Chemical Society
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ACS Catalysis
Letter
Scheme 2. Kinetic Issues Involved in the Stereoconvergent
Table 1. Reaction Optimization
Alkylation of α-Angelica Lactone (Gray Arrow)
catal.
org. base
(equiv)
inorg. base
(equiv)
conversion, %
a
b
entry
cd
(mol %)
(3a/B-B/LA)
er
,
1
2
3
4
5
6
7
8
9
D (2.5) Et3N (6)
E (2.5) Et3N (6)
F (2.0) Et3N (1)
F (2.0) Et3N (1)
F (2.0) Et3N (1)
F (2.0) Et3N (1)
F (2.0) NMP (1) K2CO3 (1)
F (2.0) NMP (1) K2CO3 (1)
F (2.0) NMP (6) K2CO3 (1)
CsF (6)
CsF (6)
CsF (2)
none
K2CO3 (1)
K2CO3 (1)
100 (1.7:1:0)
100 (11:1:0)
15 (1.5:1:0)
7 (1:0:0)
100 (2.5:0:1)
20 (1:0:0)
100 (6.7:0:1)
56 (3:1:0)
80 (5.0:0:1)
100 (10:0:1)
24:76
44:56
92:8
97:3
85:15
i
96:4
97:3
96:4
95:5
cd
,
e
f
g
potential complication associated with the projected applica-
tion. The desired path is A → C traced by the background gray
arrow, while other mechanistically validated paths radiate from
the center. The substrate A is a proelectrophile: the olefin must
migrate into conjugation for it to be armed in its active form as
a conjugate acceptor (A ⇆ B (≡ β-angelica lactone)). Prior
reports indicate γ-substituted α,β-unsaturated butenolides
racemize (B ⇆ ent-B) at reaction rates and under conditions
that vary widely.7 Enantioenriched β,γ-substituted γ-butyr-
olactones are commonly accessed via diastereoselective 1,4-
additions to enantioenriched γ-substituted α,β-unsaturated
butenolide, allaying some concern about diastereocontrol (epi-
C); however, there are no reports of accessing enantioenriched
products from racemic starting materials.6a,7d,e,8 In its
“unarmed” form, α-angelica lactone is known to hydrolyze to
levulinic acid (A → LA)9 and polymerize (nA → poly(A)),10
while the conjugated isomer B dimerizes (2 B → B-B).11
Critically, identification of an appropriate catalyst that can
differentiate between the two enantiomers of starting material
is an inherent challenge of any stereoconvergent trans-
formation (A → C vs A → ent-C).
Because the asymmetric rhodium-catalyzed 1,4-addition of
arylboronic acids to butenolide substrates is well-precedente-
d,6a and requires conditions known to be tolerant of amine
bases,3b we believed this system would be appropriate for an
enantioconvergent conjugate addition to α-angelica lactone.
Herein, we report the stereoconvergent arylation of γ-
substituted α,β-unsaturated butenolides via isomerization of
α-angelica lactone derivatives and concomitant rhodium-
catalyzed 1,4-addition of arylboronic acids.
e
g
g
g
10
F (2.0) NMP
K2CO3 (1)
(10)
h
g
11
F (2.0) NMP
K2CO3 (1)
90 (1:0:0)
95:5
(10)
a
b
All reactions were run on a 0.20 mmol scale. Determined by HPLC
c
d
using a chiral stationary phase. Reaction solvent CH2Cl2. Reaction
run at room temperature. Reaction T = 40 °C. Reaction solvent 1,4-
dioxane. NMP = N-methylpyrrolidine. Reaction time = 1 h. Not
e
f
g
h
i
determined.
catalyst F,13 we obtained poor conversion to the desired
product 3a but observed excellent enantio- and diastereose-
lectivity (Table 1, entry 3). Further investigation revealed that
CsF rapidly induces dimerization of the α-angelica lactone, and
when it was omitted, dimerization did not occur (Table 1,
entry 4). Given the inorganic base’s role in facilitating
transmetalation, we considered alternative additives in order
to increase conversion.14 The addition of K2CO3 successfully
improved conversion, but also accelerated the undesired
hydrolysis of α-angelica lactone to levulinic acid, LA (Table
1, entry 5). Water is necessary to achieve high conversion to
the desired product (Table 1, entry 6). Additionally K2CO3
decreases the enantioselectivity of the reaction, a fact that we
attribute to a relatively fast rate of arylation relative to the rate
of racemization of the substrate. Our group previously reported
that the rate of racemization of configurationally labile
substrates in the context of DKR reactions can be accelerated
by smaller organic bases.3b Accordingly, use of N-methyl-
pyrrolidine (NMP), a less sterically encumbered organic base,
restored excellent levels of enantioselectivity (Table 1, entry
7). Although hydrolysis is suppressed at lower temperatures,
arylation is slowed significantly, leading to increased
dimerization (Table 1, entry 8). Increasing the amount of
organic base had a dual effect of accelerating isomerization to
the requisite butenolide isomer, effectively inhibiting hydrol-
ysis, and promoting faster racemization relative to the arylation
to afford the desired product in good enantioselectivity (Table
1, entry 9). By use of 10 equiv of N-methylpyrrolidine, the
We began our studies with reaction of lactone 1a,
phenylboronic acid, and a bicyclo[2.2.2]octadiene rhodium
dimer D.12 While the reaction proceeded with high conversion
and diastereoselectivity to the desired product 3a, the reaction
displayed only modest enantioselectivity and was accompanied
by an appreciable amount of the undesired dimer B-B (Table
1, entry 1). By switching the aryl group on the complex E,
dimerization could be greatly suppressed; however, both
diastereoselectivity and enantioselectivity of the transformation
were compromised (Table 1, entry 2). Employing the
commercially available (S)-BINAP hydroxy-rhodium dimer
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Letter
b
desired product was prepared in excellent conversion,
diastereo- and enantioselectivity in 1 h (Table 1, entry 11).
Additional studies confirmed that dioxane, K2CO3, and
arylboronic acid are the best solvent, inorganic base, and
organoboron source, respectively (see Supporting Informa-
tion).
Table 3. Scope of Arylboronic Acid
With optimized conditions in hand, we sought to explore
their applicability to other reaction partners, beginning with
unsaturated lactones bearing other substituents at the γ-
position (Table 2). Substrates containing both short and long
Table 2. Scope of γ-Substituted β,γ-Unsaturated Butenolides
a
Excess boronic acid is needed because of background hydrolysis (to
benzene, see ref 16) and competitive angelica lactone dimerization.
alkyl chains afforded the desired products, 3a, 3b, and 3c, in
good yields and excellent stereoselectivities. Homologated
carbocycles at the γ-position gave desired product 3d in good
yield and enantioselectivity. The benzyl group is also tolerated
at the γ-position and gives lactone 3e in excellent yield and
stereoselectivity. A branched alkyl group worked well in the
transformation to afford 3f, albeit in slightly diminished yield,
presumably due to the steric bulk at the γ-position. All
products were obtained as a single diastereomer.
The scope of boronic acids tolerated in this reaction was
then explored (Table 3). We found a wide variety of para-
substituted arylboronic acids could be employed to generate
products containing electron-donating groups, such as p-tolyl
3g and p-anisyl 3h. We could also obtain products containing
para-substituted halogens, 3i−k, and electron-withdrawing
para-ester 3l in modest to good yields and good
enantioselectivities. Electron-rich arylboronic acids highlight
the delicate balance between the rates of arylation and
racemization. Reactions employing electron-rich arylboronic
acids require catalytic quantities of K2CO3 to achieve desirable
levels of enantioselectivity; a full equivalent of inorganic base
results in loss of enantioselectivity. We hypothesize that the
erosion is due to the accelerated arylation relative to
a
b
Reaction run with 5 mol % K2CO3. Results in brackets are for
reactions run with 4.0 equiv of ArB(OH)2.
racemization. This trend continues for meta-substituted
arylboronic acids. Products with electron-releasing substituents
at the meta-position, 3m and 3n, can be obtained in good
yields and enantioselectivities when substoichiometric K2CO3
is used. Halogen substitution at the meta-position (3o) is
tolerated as well. Pleasingly, a sterically bulky ortho-substituted
arylboronic acid provided product 3p with excellent
enantioselectivity, though greatly decreased yield. Naphthyl-
boronic acid coupled product 3q can be attained. Electron-rich
heteroaromatic boronic acids work to afford heteroaromatic-
containing lactones 3r and 3s. X-ray diffraction studies
revealed the stereochemistry of 3g, 3j, and 3q to be (4S,5S)
(See Supporting Information).15 The absolute stereochemistry
agrees with the results of Hayashi and co-workers.13,14,16,17 On
a 1 mmol scale using 1 mol % catalyst C, α-angelica lactone
and phenylboronic acid reacted to provide the derived
lactone 3a in 89% yield, >20:1 diastereoselection, and 93:7 er.
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Current limitations of the substrate scope include electron-
poor heteroaromatic and styrenyl boronic acids.3b Higher
yields can generally be achieved by employing more
arylboronic acid, but in cases of electron-rich arylboronic
acids, enantioselectivity typically suffers, presumably due to the
rate of arylation relative to the rate of racemization. We
hypothesize that under the optimized conditions, protodebor-
ylation of the arylboronic acids occurs rapidly, accounting for
moderate yields with some boronic acids. In most cases, longer
reaction times do not improve yields.14
reaction is one of the first dynamic kinetic resolutions that
takes advantage of racemization of a γ-stereocenter through
formation of a dienolate. A wide range of β-aryl, γ-substituted
γ-butyrolactones can be accessed in good yields, excellent
enantioselectivities, and as a single diastereomer. These
lactones can be further manipulated to generate useful organic
building blocks, including 1,4-diols and 1,4-amino alcohols.
Additionally, the products can be diastereoselectively alkylated
to afford stereochemically complex trisubstituted lactones.
Extension of this work to other classes of nucleophiles, as well
as application of the DKR conjugate addition manifold to
additional substrates is currently underway in our laboratory.
Further chemical transformations of the enantioenriched
β,γ-substituted γ-butyrolactones were performed in order to
access synthetically useful organic intermediates (Scheme 3).
ASSOCIATED CONTENT
■
a
Scheme 3. Synthetic Utility of Lactone Products
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.9b04405.
Experimental procedures and spectral and analytical data
(PDF)
Crystallographic data for 3j (CIF)
Crystallographic data for 3g (CIF)
Crystallographic data for 3q (CIF)
AUTHOR INFORMATION
Corresponding Author
*Email: jsj@unc.edu.
■
ORCID
Jeffrey S. Johnson: 0000-0001-8882-9881
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The project described was supported by Award R35
GM118055 from the National Institute of General Medical
Sciences. X-ray crystallography was performed by Dr. Josh
Chen (UNC Chemistry X-ray Core Laboratory). We thank Dr.
Brandie Ehrmann and Diane Weatherspoon (UNC Chemistry
Mass Spectrometry Core Laboratory) for their assistance with
mass spectrometry analysis using instrumentation acquired
under National Science Foundation Grant CHE-1726291.
a
Reagents and conditions: (a) LiAlH4, THF, 23 °C, 2 h; (b) aq.
NH4OH, 23 °C, 5 h; (c) LDA, allyl iodide, THF, −78 to 0 °C, 3 h.
The disubstituted lactone 3a can be reduced to 1,4-diol 4a
with a primary and secondary alcohol. Lactone amidolysis of
3a with ammonium hydroxide afforded enantioenriched amide
5a, which can be further reduced to afford its derived 1,4-
amino alcohol (see the Supporting Information). Finally, the
β,γ-substituted γ-butyrolactone 3a can be alkylated at the α-
position in excellent diastereoselectivity to afford γ-butyr-
olactone 6a with three contiguous stereocenters. The relative
stereochemistry of this transformation was confirmed by a 1D
NOE NMR study (see the Supporting Information).18
When the catalyzed addition of 2l to 1a was carried out in
dioxane/D2O (10:1), partial deuterium incorporation was
observed at both the α- and γ-positions of 3l (Scheme 3B).
The fact that the product is obtained with less than 100% of
1H/2H exchange carries ramifications for the mechanism of the
obligatory 1,3-prototropy. The results in part implicate [R3N−
H]+, rather than D2O, as the partner that reacts with the
dienolate7b and concurrently confirm the importance of the
organic base in the process.
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DOI: 10.1021/acscatal.9b04405
ACS Catal. 2019, 9, 11614−11618