Enantioselective annulations for dihydroquinolones by in situ generation of azolium enolates

Article abstract of DOI:10.1021/ja505880r

A convergent, catalytic asymmetric formal [4 + 2] annulation for the synthesis of dihydroquinolones has been developed. Carboxylic acids can be employed as precursors to NHC enolates through an in situ activation strategy. Simultaneous generation of a reactive aza-o-quinone methide under the basic conditions employed for NHC generation leads to a dual activation approach.

Full text of DOI:10.1021/ja505880r

Communication
pubs.acs.org/JACS
Open Access on 07/13/2015
Enantioselective Annulations for Dihydroquinolones by in Situ
Generation of Azolium Enolates
Anna Lee, Ashkaan Younai, Christopher K. Price, Javier Izquierdo, Rama K. Mishra, and Karl A. Scheidt*
Department of Chemistry, Center for Molecular Innovation and Drug Discovery, Chemistry of Life Processes Institute, Northwestern
University, Silverman Hall, Evanston, Illinois 60208, United States
S
* Supporting Information
QM).15 This species could behave as an electrophile with an
NHC-enolate to provide the desired dihydroquinolone in a
convergent process (Figure 1A). However, given the potential
ABSTRACT: A convergent, catalytic asymmetric formal
[4 + 2] annulation for the synthesis of dihydroquinolones
has been developed. Carboxylic acids can be employed as
precursors to NHC enolates through an in situ activation
strategy. Simultaneous generation of a reactive aza-o-
quinone methide under the basic conditions employed for
NHC generation leads to a dual activation approach.
ihydroquinolones are found in numerous natural products
1
Dand therapeutically relevant compounds.
Due to the
prevalence of this structural motif in bioactive compounds and
approved pharmaceuticals such as cilostazol, cartelolol, and
pinolinone, as well as its utility as a synthetic intermediate for the
synthesis of other bioactive compounds such as tetrahydroquino-
lines, the development of new asymmetric methods to fashion
dihydroquinolone is a high value goal.2 Previous approaches for
the preparation of enantioenriched quinolone derivatives mainly
utilize transition metal catalysis.3 Within the area of organo-
́
catalysis, the Cordova group reported an asymmetric synthesis of
1,2-dihydroquinolidines using chiral amine catalysts,4 and the Lu
group reported an asymmetric synthesis of 2-aryl-2,3-dihydro-4-
quinolones using bifunctional thiourea catalysts.5 More recently,
the groups of Akiyama6 and Gong7 independently reported a
selective approach for the preparation of tetrahydroquinoline
derivatives using chiral phosphoric acids.8 However, a convergent
and efficient asymmetric approach for the synthesis of the
dihydroquinolin-2-one scaffold is currently underdeveloped, and
such a transformation could facilitate access to this privileged
scaffold with various substitution patterns and high levels of
enantioselectivity.
Figure 1. Dual activation strategy.
Organocatalysis has emerged as a powerful strategy to
construct various hetero- and carbocyclic systems. N-Hetero-
cyclic carbenes (NHCs) in particular demonstrate great
versatility and selectivity to facilitate many transformations,
including enolate processes and nontraditional umpolung
reactivity.9 We have been engaged in enhancing NHC reactivity
and selectivity through the integration of this Lewis base catalysis
mode with other strategies, including Lewis acids,10,11 Lewis
bases such as fluoride,12 and Brønsted acids.13,14 On the basis of
this cooperative catalysis/activation concept, we envisioned that
a “dual activation” approach could lead to the formation of
dihydroquinolone derivatives. The dual activation classification
would apply if a compatible Brønsted base could be leveraged to
both provide NHC formation from the azolium salt precatalysts
and promote in situ creation of an aza-o-quinone methide (aza-o-
high reactivity of the aza-o-QM, it was also entirely possible at the
onset of these investigations that a nucleophilic NHC and/or
base would simply undergo an unproductive addition reaction.
At the onset, cinnamaldehyde and tert-butyl carbamate (Boc)
protected 2-aminobenzyl chloride were employed as the
nulcleophile and electrophile precursors, respectively (not
shown). With cinnamaldehyde as the enolate progenitor, a
mixture of several products was observed, with dimers formed
through the enolate (not shown) and homoenolate (shown,
Figure 1B) pathways predominating. This dimerization with
enals as substrates for NHC-enolate or NHC-homoenolate
Received: June 11, 2014
Published: July 13, 2014
© 2014 American Chemical Society
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Journal of the American Chemical Society
Communication
a
reactions continues to be a significant challenge in reaction
design in the area of carbene catalysis.16 We hypothesized that
carboxylic acids could be activated in situ through the
appropriate reagent combinations to react with a NHC and
thus generate the desired NHC-enolate/acyl azolium. Once the
carboxylic acid is activated, the acylation of the NHC and
subsequent enolization could form the necessary NHC enolate,
thereby eliminating the possibility of dimerization observed with
enals. Carboxylic acids present two advantages: (1) they are
readily available and more stable compared to enals, α-aryloxy
acetalaldehydes,17 and α-functionalized alkyl aldehydes used
previously in carbene-catalyzed enolate reactions,18 and (2) they
are less prone to undergo enolate/aldol chemistry compared to
ketones and activated esters. In situ activation of acids to access
enolate reactivity has proven successful in organocatalysis,19 but
this strategy has not seen successful implementation in carbene
catalysis.20 A potential complication in this context with carbene
catalysis would be the direct addition of the nucleophilic C2
position of the NHC to any in situ activation reagent. Recently,
the Chi group has demonstrated the ability to access NHC
enolates through the acylation of carbenes with saturated highly
activated aryl esters.21a,11b,21b However, the use of more stable
acid substrates directly in NHC catalysis has not been reported.
Herein, we report an asymmetric annulation reaction for the
synthesis of dihydroquinolone derivatives with carboxylic acids
via in situ generated azolium enolates (Figure 1).22
Table 1. Optimization of Asymmetric Reaction Conditions
To test our hypothesis, we screened several acid activating
reagents (isobutyl chloroformate, SOCl2, 2,4,6-trichlorobenzoyl
chloride, etc.) with hydrocinnamic acid and achiral NHC
catalysts. Carbonyldiimidazole (CDI) in combination with an
achiral 5,5-triazolium catalyst provided the desired dihydroqui-
nolone in high isolated yield with minimal side products. After
further screening with chiral triazolium catalysts (Table 1), we
found that isobutyl-substituted catalyst D provided the desired
product in moderate yield and enantioselectivity (entry 4). We
hypothesized that the main reason for the depressed yield and
enantioselectivity stemmed from decomposition of our in situ
generated acyl imidazole and/or aza-o-quinone methide. By
lowering the reaction temperature (4 °C), we were able to
slightly improve both the yield and enantioselectivity (entry 5).
Increasing the amount of CDI also improved the reaction yield
(entry 6). The most drastic increase in yield was obtained when
we added 60 mol % of imidazole, which presumably acts as a
homogeneous scavenger of HCl to prevent the inactivation of
our starting materials and catalyst in the reaction (entry 7). We
turned our attention toward improving the enantioselectivity and
screened various amino acid derived 5,5-triazolium catalysts
(entries 8−16). The best results were obtained with benzyl-
substituted catalyst G in terms of reactivity and enantioselectiv-
ity, with the desired product in 80% yield and 94:6 er (entry 10).
With catalyst H, the desired product was obtained in slightly
improved er, but decreased yield (entry 14).
With the optimized reaction conditions in hand, the scope of
this formal [4 + 2] annulation was explored (Table 2). The
reaction turned out to be tolerant of various starting acids. The
desired dihydroquinolone derivatives were obtained in good
yields (52−84%) with high enantioselectivities (up to 98:2 er)
with alkyl (3l and 3m), benzyl (3a−3i), aryl (3j), and heteroalkyl
(3k) substituted acids. The products derived from meta-
substituted acids (3d and 3e) were obtained with slightly higher
enantioselectivity; however, a noticeable electronic effect of the
substituents was not observed. Gratifyingly, aliphatic carboxylic
acids were suitable substrates under our reaction conditions. The
a
Conditions: 1a (0.05 mmol, 1.0 equiv), 2a (2.0 equiv), triazolium
b
(0.2 equiv), Cs2CO3 (2.5 equiv) at 23 °C for 15 h. Determined by
NMR analysis with 1,3,5-trimethoxybenzene as an internal standard.
c
d
e
Determined by HPLC analysis. Reaction conducted at 4 °C. 60 mol
% of imidazole was added.
isopropyl-substituted hydroquinolone was obtained in high yield
(81%) and excellent enantioselectivity (98:2 er) by employing
isovaleric acid (3l). Interestingly, this catalytic system could also
provide high yield and enantioselectivity for acids of minimal size
such as propanoic acid (3m). The tolerance of functionality and
substitution on the aminobenzyl chloride 2 was also investigated.
Both electron-withdrawing and electron-rich groups were
tolerated at the C-4, C-5, and C-6 positions affording the desired
products 3n−3t in good to high yields and high enantioselectiv-
ities. However, no product was isolated when using C-3
substituted aminobenzyl chlorides because many of these
potential substrates were not stable under the reaction
conditions.
Two different mechanisms are hypothesized for the formation
of dihydroquinolone 3 from enol I-A (Scheme 1): (1) a
concerted [4 + 2] process and (2) a formal [4 + 2] process
involving a stepwise Michael addition and annulation. The
addition of catalyst G to acyl imidazole 1 generated in situ and
subsequent formal 1,2-H migration generates NHC-enolate I-A.
The reaction between the intermediate I-A and Boc-amino-
benzyl chloride 2 could proceed through either a concerted [4 +
2] pathway or a stepwise NHC-acylation process. In the [4 + 2]
pathway, concerted addition of in situ generated aza-o-quinone
methide 2 and NHC enolate I-A via II (via either endo or exo
transition states) could provide the desired product 3.23 In the
Michael addition/acylation pathway, carbon−carbon bond
formation through the less sterically demanding transition state
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Journal of the American Chemical Society
Communication
a
possibility of I−B based on DFT calculations of these two
Table 2. Substrate Scope
rotamers (Figure 2).24−26
Figure 2. DFT model for asymmetric induction.
The enantioenriched dihydroquinolone product 3a was
converted into enantioenriched tetrahydroquinoline derivative
4 and thioamide 5 to demonstrate the utility of this new
transformation (Scheme 2). After removal of the Boc protecting
a
Scheme 2. Synthetic Transformations
a
Reactions conducted on 0.3 mmol scale. Yields are of isolated
a
Reagents: (a) TFA, CH2Cl2; (b) Lawesson’s reagent, CH2Cl2; (c)
product after column chromatography. Enantiomeric ratio was
b
TFA, CH2Cl2; (d) LiAlH4, AlCl3, Et2O/DCM
determined by HPLC analysis. Reaction conducted on 5 mmol
c
scale. Absolute configuration was determined by X-ray crystallog-
raphy.
group, the resulting N−H lactam was reduced using LiAlH4/
AlCl3 to afford the desired tetrahydroquinoline derivative (4),
which is another important scaffold due to its bioactive
properties as antihypertensives, oxytocin antagonists, vaso-
pressin antagonists, and calcium antagonists.27 The amide can
also be converted into the corresponding thioamide (5) without
racemization.28 This heteroatom conversion provides a platform
for further known transformations (e.g., annulations and amidine
formation) to access potentially bioactive architectures.29
In conclusion, the first highly efficient enantioselective NHC-
catalyzed annulation reaction for the synthesis of dihydroquino-
lones has been developed. This transformation features a dual
activation system focusing on combining a Lewis base (NHC)
and in situ generated active nucleophiles and electrophiles. In
addition, the utilization of unactivated carboxylic acids directly as
substrates provides the desired dihydroquinolone derivatives and
importantly avoids generation of dimerization side products
observed when employing enals. The demonstration that
activating strategies and highly reactive aza-o-quinone methides
are compatible with nucleophilic carbene catalysis presents new
opportunities to pursue in the context of asymmetric heterocyclic
synthesis.
Scheme 1. Proposed Reaction Pathway
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and spectral data for new compounds,
geometries, and energies of all structures discussed. This material
is available free of charge via the Internet at http://pubs.acs.org.
(III) and subsequent tautomerization forms acyl azolium
intermediate IV, which then undergoes N-acylation to afford 3
and the NHC, now free to re-enter the catalytic cycle. The
current model for asymmetric induction is based on the more
favored NHC-enolate conformation I-A versus the higher energy
■
S
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Journal of the American Chemical Society
Communication
(d) Yang, Q.-Q.; Wang, Q.; An, J.; Chen, J.-R.; Lu, L.-Q.; Xiao, W.-J.
Chem.Eur. J. 2013, 19, 8401−8404.
AUTHOR INFORMATION
■
Corresponding Author
scheidt@northwestern.edu
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was generously provided by the NIH NIGMS
(GM073072). We thank Michael Wang (NU) for assistance with
X-ray crystallography.
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