Asymmetric Synthesis of α-Quaternary γ-Lactams through Palladium-Catalyzed Asymmetric Allylic Alkylation

Article abstract of DOI:10.1021/acs.orglett.8b03613

The synthesis of chiral unsaturated γ-lactams is reported featuring a highly enantioselective palladium-catalyzed asymmetric allylic alkylation of α, γ-disubstituted 2-silyloxypyrroles. This method allows a straightforward access to optically active γ-lactams bearing an α-quaternary stereogenic center in high yields (up to 93%), high regioselectivities (up to >20:1), and excellent enantioselectivities (up to 95% ee). To further demonstrate the synthetic utility of the method, the resulting allylated products were converted to various versatile chiral building blocks, such as pyrrolidines and pyrrolidinones.

Full text of DOI:10.1021/acs.orglett.8b03613

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Asymmetric Synthesis of α‑Quaternary γ‑Lactams through
Palladium-Catalyzed Asymmetric Allylic Alkylation
Tao Song,† Stellios Arseniyadis,*,†,‡ and Janine Cossy*,†
†Laboratoire de Chimie Organique, Institute of Chemistry, Biology and Innovation (CBI), ESPCI Paris/CNRS/PSL Research
University, 10 rue Vauquelin, 75231 Paris Cedex 05, France
‡Queen Mary University of London, School of Biological and Chemical Sciences, Mile End Road, London, E1 4NS, U.K.
S
* Supporting Information
ABSTRACT: The synthesis of chiral unsaturated γ-lactams is reported
featuring a highly enantioselective palladium-catalyzed asymmetric allylic
alkylation of α,γ-disubstituted 2-silyloxypyrroles. This method allows a
straightforward access to optically active γ-lactams bearing an
α‑quaternary stereogenic center in high yields (up to 93%), high
regioselectivities (up to >20:1), and excellent enantioselectivities (up to
95% ee). To further demonstrate the synthetic utility of the method, the
resulting allylated products were converted to various versatile chiral building blocks, such as pyrrolidines and pyrrolidinones.
γ‑butyrolactams4 or 2-silyloxypyrroles5,6 as precursors. In view
itrogen-containing heterocycles are very important
Nstructural units in nature. Among them, γ-lactams are
of the literature in the field, a number of methods have been
arguably among the most prevalent, as they are ubiquitous in
many biologically active natural products and pharmaceuticals1,2
such as (−)-pramanicin,2a,b azaspirene,2c−f (+)-lactacystin,2g−k
and (−)-salinosporamide2l just to name a few (Figure 1).
developed to access optically active lactams bearing a tertiary
stereogenic center at the α- or γ-position; these include
asymmetric vinylogous Michael additions,7 Mukaiyama Man-
nich-type additions,8 or aldol reactions.9 In contrast, asymmetric
construction of γ-lactams bearing a quaternary stereogenic
center is rather scarce.10−12 Moreover, there are only a few
asymmetric methods which allow a straightforward and highly
enantioselective access to γ-lactams bearing a quaternary
stereogenic center at either the α- or γ-position. One of them
is the direct enantioselective vinylogous Michael addition on
α,β-unsaturated γ-lactam using phase-transfer catalysis, which
allows γ,γ′-disubstituted γ-lactams to be accessed in high yields
and excellent enantioselectivities (Scheme 1A).13
Another approach involves the use of dimeric cinchona
alkaloids to catalyze the enantioselective sulfenylation of
β,γ‑unsaturated γ‑lactams, which affords highly functionalized
γ‑lactams bearing a quaternary stereogenic center in high yields
and excellent enantioselectivities (Scheme 1B).14
Optically active γ-lactams can also be prepared through a
Ni‑catalyzed enantioselective C-acylation of an α-substituted
γ‑lactam. The process, which is formally a three-component
coupling reaction involving a lithium enolate, a benzonitrile, and
an aryl halide, allows the formation of the corresponding β‑keto
γ-lactams in high yields and excellent enantioselectivities (up to
94% ee) (Scheme 1C).15
Finally, a decarboxylative Pd-AAA strategy was reported by
Stoltz et al. for the synthesis of not only five- but also six- and
seven-membered ring lactams bearing a quaternary stereogenic
center (Scheme 1D).16
Figure 1. Typical examples of γ-lactam-containing natural products and
pharmaceuticals.
In view of the plethora of compounds that bear this intriguing
chiral motif and the interesting biological activities displayed, a
tremendous amount of effort has been dedicated to the
development of efficient and highly selective methods to
construct diversely substituted γ-lactams.1−3 These methods
include for example the cyclization of chiral γ-amino acids, the
ring expansion of substituted β-lactams, and the transition-
metal-catalyzed intramolecular carbenoid C−H insertion.
The functionalization of γ-butyrolactams is another important
strategy to access chiral lactam derivatives. This approach
usually involves the use of N-protected α,β-unsaturated
Received: November 12, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03613
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Selected Strategies for the Asymmetric Synthesis
Table 1. Systematic Study
of Chiral γ-Lactams Bearing a Quaternary Stereogenic Center
Based on our previous results on the asymmetric synthesis of
optically active heterocyclic scaffolds through palladium-
catalyzed asymmetric allylic alkylation (Pd-AAA),17 and
precedents in the field,18 we envisioned that optically active
γ‑lactams could potentially be accessed using a similar strategy
starting from α,γ-disubstituted 2-silyloxypyrroles (Scheme 1E).
Indeed, if this approach was successful, it would allow a
straightforward and potentially highly enantioselective access to
this important scaffold.
a
b
1
All reactions were run on a 0.2 mmol scale. Determined by H
c
d
NMR on the crude reaction mixture. Isolated yield. Determined by
supercritical fluid chromatography (SFC) analysis. Reaction run with
1.0 equiv of NaOAc. Reaction run with [Pd(allyl)Cl]2 instead of
Pd2(dba)3. Reaction run for 45 h. Reaction run for 65 h. Reaction
e
f
g
h
i
j
run with 2.5 mol % of Pd2(dba)3 and 5 mol % of (R,R)-L1. Reaction
We chose to initiate our study using N-benzyl-3,5-diphenyl-2-
silyloxypyrrole 1a as a model substrate. The latter was prepared
in four steps and 50% overall yield starting from chalcone (see
Supporting Information for more details). With this compound
in hand, we started by exploring the influence of each reaction
parameter on the reactivity, regioselectivity, and enantioselec-
tivity of the reaction; the results are depicted in Table 1.
The first parameter to be evaluated was the leaving group on
the allyl donor, as it had previously been shown to influence both
the reactivity and enantioselectivity in Pd-18 and Ir-catalyzed19
asymmetric allylic alkylations. The original reaction was run in
THF at rt using cinnamyl acetate, and the catalytic system was
comprised of Pd2(dba)3 (5 mol %) and Trost’s ligand (R,R)-L1
(10 mol %), which gave the best results in all of the Pd-AAA
reactions we attempted.16 Unfortunately, under these con-
ditions, no product was observed even after 36 h (Table 1, entry
1). In order to improve the reactivity, we decided to run the
reaction at 60 °C under otherwise identical conditions, and
luckily we were able to isolate 3a albeit in only 17% yield and
33% ee (Table 1, entry 2). To improve the selectivity, we ran the
same reaction using 1.0 equiv of NaOAc. After 22 h, all of the
starting material had been converted to the corresponding α-
and γ-allylated products 3a and 4a in a 3.8:1 ratio in favor of 3a.
Unfortunately, the α-allylated product was obtained in only 20%
ee (Table 1, entry 3). This erosion of the selectivity can
potentially be attributed to a higher concentration of the lactam
enolate. Four additional allyl donor derivatives including the
methyl carbonate 2b (Table 1, entry 4), the diethylphosphonate
run with 1 mol % of Pd2(dba)3 and 3 mol % of (R,R)-L1.
2c (Table 1, entry 5), the tert-butyl carbonate 2d (Table 1, entry
6), and the benzoate 2e (Table 1, entry 7) were tried under the
aforementioned conditions. The latter afforded the desired
α‑allylated product 3a in an improved 76% yield and 58% ee.
Most importantly, the reaction could be run at rt with roughly
the same outcome in terms of both yield and enantioselectivity
(Table 1, entry 8). Interestingly, the reaction with [Pd(allyl)Cl]2
instead of Pd2(dba)3 was more sluggish and far less selective
(Table 1, entry 9).
The influence of the solvent was next evaluated (results not
shown in Table 1). As a general trend, ethers such as THF
B
DOI: 10.1021/acs.orglett.8b03613
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Organic Letters
Letter
Scheme 2. Scope of the Reaction: Variation of the N-
Protecting Group
Scheme 3. Scope of the Reaction: Variation of the Aryl
a
a
Substituents
a
b
All reactions were run on a 0.2 mmol scale. Isolated yield.
c
Determined by 1H NMR on the crude reaction mixture.
d
Determined by supercritical fluid chromatography (SFC) analysis.
(>99%, 3a/4a = 3.4:1, 58% ee), Et2O (81%, 3a/4a = 4.3:1, 59%
ee), and 2-Me-THF (85%, 3a/4a = 11.6:1, 51% ee) led to similar
enantioselectivities, with 3a obtained with ee’s ranging from 51%
to 59%. CH3CN (89%, 3a/4a = 3.3:1, 54% ee) afforded roughly
the same selectivity, while the reactions run in CH2Cl2 (38%,
3a/4a = 2.6:1, 47% ee), toluene (23%, 3a/4a = 3.8:1, 43% ee), or
a more polar solvent such as DMF (89%, 3a/4a = 2.3:1, 35% ee)
resulted in lower yields and lower enantioselectivities (see
Supporting Information for more details).
a
b
All reactions were run on a 0.2 mmol scale. Isolated yield.
c
Determined by supercritical fluid chromatography (SFC) analysis.
d
1
Determined by H NMR on the crude reaction mixture.
substrate scope by applying these conditions to various
α,γ‑diphenyl 2-silyloxypyrrole derivatives 1a−e bearing different
N-protecting groups (Scheme 2). As depicted in Scheme 2, the
best results were obtained with 1b bearing a para-(trifluoro-
methyl)benzyl N-protecting group. Substrates 1c and 1d,
respectively bearing a para- and an ortho-(methoxy)benzyl
group, were converted to the corresponding allylated products
3c and 3d in good yields (60−69%), good enantioselectivities
(81−82% ee), and an interesting 7:1 regioselectivity in favor of
the α-allylated product. In contrast, when the N-benzyl
protecting group was replaced by a N‑phenyl group, the
allylated product 3e was obtained with a similar enantiose-
lectivity (80% ee) and an excellent regioselectivity (>20:1) but
with a low yield of 20%.
As the N-para(trifluoromethyl)benzyl derivative induced the
highest enantioselectivity, a variety of 2-silyloxypyrroles
incorporating this moiety (1f−l) were synthesized and
evaluated under our optimized conditions. The results are
summarized in Scheme 3.
We first started by evaluating the influence on the selectivity
of the aromatic substituent at the C2 position of the pyrrole ring
(Ar1). The first results showed that a higher enantioselectivity
was obtained with 1f, bearing an ortho-methyl-substituted
phenyl ring (3f/4f = 5:1, 95% ee), than with 1g and 1h, which
have a meta- (3g/4g = 10:1, 81% ee) and a para-methyl-
substituted (3h/4h = 7:1, 68% ee) aromatic ring, respectively,
indicating that the steric hindrance may be an important
parameter that accounts for the enantioselectivity.
Encouraged by these results, we next evaluated the influence
of the ligand. The investigation revealed that C2-symmetric
diphosphine ligands such as L1 afforded a higher level of
selectivity (58% ee) (Table 1, entry 8) than the axially chiral
diphosphines L2 and L3 (17% and 19% ee) (Table 1, entries 10
and 11), the phosphine oxazoline L4 (16% ee) (Table 1, entry
12), the atropisomeric Phanephos L5 (3% ee) (Table 1, entry
13), the ferrocenephosphine L6 (19% ee) (Table 1, entry 14),
the DIPAMP-type ligands L7 and L8 (26% and 6% ee) (Table 1,
entries 15 and 16), and the C2-symmetric diphosphine ligand
developed by Kagan and co-workers (5% ee) (Table 1, entry 17).
The higher reactivity of cinnamyl benzoate prompted us to
run the reactions at lower temperatures. Luckily, we were able to
improve the enantioselectivity without losing any reactivity by
simply running the reaction at 0 °C (Table 1, entry 18). This
result could be further improved by running the reaction at −20
°C (73% yield, 82% ee) (Table 1, entry 19). The best result was
however obtained when running the reaction in THF at −30 °C,
conditions under which the desired α‑allylated product 3a was
obtained in high yield (89%), with great regioselectivity (12:1)
and excellent enantioselectivity (ee up to 84%) (Table 1, entry
20). Various additives such as tetrabutylammonium benzoate
and sodium benzoate were also tested, but no improvement was
observed in terms of enantioselectivity despite an improved
regioselectivity (3a/4a > 20:1; results not shown in the Table 1).
Before evaluating the substrate scope, we decided to lower the
catalyst loading to verify whether it had an impact on the
selectivity. The amount of Pd2(dba)3 was therefore reduced
from 5 mol % (Table 1, entry 19) to 2.5 mol % (Table 1,
entry 21). Interestingly, this did not change the enantioselec-
tivity (84% ee); however, it had a detrimental effect on both the
yield and regioselectivity, which went from 89% to 59% and
from 12:1 down to 6:1, respectively. Logically, further lowering
the catalyst loading to 1 mol % had a detrimental effect on the
overall process (27%, 3a/4a = 6:1, 74% ee) (Table 1, entry 22).
With our optimized conditions in hand [Pd2(dba)3 (5 mol %),
(R,R)-L1 (10 mol %), THF, −30 °C], we next examined the
Replacing the para-methyl group by an electron-withdrawing
para-trifluoromethyl gave the α-allylated product 3i in 87% yield
however with a slightly lower 70% ee.
Finally, the substrates bearing a naphthyl (1j) or biphenyl
(1k) also appeared to be suitable precursors, as the
corresponding α-allylated products 3j and 3k were both
obtained in roughly 81% ee. Finally, we evaluated the influence
of the aromatic substituent at the C4 position of the pyrrole ring
(Ar2) on the selectivity. Substrate 1l bearing a methyl
substituent at the ortho-position of the phenyl ring was thus
synthesized and subjected to our optimized Pd-AAA conditions.
C
DOI: 10.1021/acs.orglett.8b03613
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Organic Letters
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Scheme 4. Scope of the Reaction: Variation of the Allyl
Donor
Scheme 5. Post-functionalization: Synthesis of Chiral
Pyrrolidinones and Pyrrolidines
a
The absolute configuration of the newly formed quaternary
center resulting from the Pd-AAA could not be confirmed by
single crystal X-ray analysis. However, by analogy with the
selectivities previously observed on similar systems17,18 and in
accordance with the prediction model proposed by Lloyd-Jones,
Norrby and co-workers,20 where the amide proton of the
cationic [allyl-Pd-DACH] complex directs the enolate carbon to
the allyl through H-bonding, we believe the enolate approaches
the palladium π-allyl-(R,R)-L1 complex by its Re-face; this avoids
the steric clash between the bulky aryl group Ar1 and the ligand
framework (Scheme 6).
a
b
All reactions were run on a 0.2 mmol scale. Isolated yield.
c
Determined by supercitical fluid chromatography (SFC) analysis.
d
e
1
Determined by H NMR on the crude reaction mixture. The two
regioisomers 3q + 4q could not be separated; overall yield.
Scheme 6. Proposed Stereochemical Pathway
Pleasingly, the regioselectivity improved from 11:1 (3b) to
>20:1 in favor of the α-allylated product 3l, but the
enantioselectivity dropped dramatically from 89% (3b) to
69% ee (3l).
After evaluating the substrate scope, we decided to vary the
allyl donor in order to gain structural diversity. A variety of 3-
substituted allyl benzoates (2e−n) were thus prepared and
engaged in the Pd-AAA of 2-silyloxypyrroles 1b and 1m; the
results are depicted in Scheme 4.
We first evaluated various para-substituted cinnamyl benzoate
derivatives (2f−j), which all afforded good to high levels of
enantioselectivity ranging from 62% to 88% ee. The cinnamyl
benzoate bearing either a methoxy substituent at the meta
position or a naphthalene in place of the phenyl ring also
afforded high enantioselectivities ranging from 76% ee (3s) to
84% ee (3r). Most importantly, the 3-substituted allylbenzoates
bearing a heterocylic substituent such as a thiophene (3t, 87%
ee) or a furan (3u, 94% ee) afforded the best selectivities.
Unfortunately, when applying our optimized Pd-AAA con-
ditions to the monosubstituted 2-silyloxypyrrole 1m, the
corresponding α-allylated product 3v was obtained in a
moderate 66% ee albeit with excellent regioselectivity (>20:1).
To further demonstrate the synthetic utility of our method,
the reaction was run on a 1 mmol scale using 2-silyloxypyrrole
1b and 3-substituted allyl benzoate 2n. The corresponding
allylated product 3u was obtained in 84% yield and 94% ee. In
addition, the allylated product 3a was quantitatively converted
to the corresponding optically active pyrrolidinones 5a and 5b
through a hydrogenation over Pd/C (Scheme 5). Compound
3d, on the other hand, was easily converted to the corresponding
pyrrolidine 6, which was obtained in 76% yield as a single
diastereoisomer through a DIBAL-H-mediated reduction.
In summary, we have successfully developed a highly regio-
and enantioselective palladium-catalyzed allylic alkylation of
α,γ-disubstituted 2-silyloxypyrroles to access optically active
γ‑lactam derivatives bearing a quaternary stereogenic center.
Overall, the process is operationally trivial and scalable and
allows the corresponding α-allylated β,γ-unsaturated γ-lactams
to be obtained in usually high yields, good to excellent
regioselectivities ranging between 3:1 and >20:1, and up to
95% ee. Most importantly, the allylated γ-lactams could be easily
converted to various useful building blocks, including chiral
pyrrolidinones and pyrrolidines, through simple synthetic
transformations.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b03613.
1
Details of experimental procedures, H and 13C NMR
spectra, Supercritical Fluid Chromatography (SFC)
chromatograms (PDF)
D
DOI: 10.1021/acs.orglett.8b03613
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Organic Letters
Letter
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: janine.cossy@espci.fr.
*E-mail: s.arseniyadis@qmul.ac.uk.
ORCID
Stellios Arseniyadis: 0000-0001-6831-2631
Janine Cossy: 0000-0001-8746-9239
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the CSC foundation (China Scholarship
Council) is gratefully acknowledged.
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DOI: 10.1021/acs.orglett.8b03613
Org. Lett. XXXX, XXX, XXX−XXX