Enantioselective synthesis of dihydropyridinones via NHC-Catalyzed Aza-Claisen reaction

Article abstract of DOI:10.1021/ol202272t

N-Heterocyclic carbene catalyzed aza-Claisen annulations of enals or their α-hydroxyenone surrogates with vinylogous amides afford dihydropyridinones. The reaction proceeds with a broad range of substrates, and no nitrogen protecting group is required.

Full text of DOI:10.1021/ol202272t

ORGANIC
LETTERS
2011
Vol. 13, No. 19
5378–5381
Enantioselective Synthesis of
Dihydropyridinones via NHC-Catalyzed
Aza-Claisen Reaction
Benedikt Wanner, Jessada Mahatthananchai, and Jeffrey W. Bode*
Laboratorium fu€r Organische Chemie, ETH-Zu€rich, Zu€rich 8093, Switzerland
bode@org.chem.ethz.ch
Received August 22, 2011
ABSTRACT
N-Heterocyclic carbene catalyzed aza-Claisen annulations of enals or their R0-hydroxyenone surrogates with vinylogous amides afford
dihydropyridinones. The reaction proceeds with a broad range of substrates, and no nitrogen protecting group is required.
NHC-catalyzed annulations of R,β-unsaturated alde-
hydes and various imine derivatives provide facile entry
to a variety of N-heterocycles, often with exceptional
enantioselectivity (Scheme 1).1 The rapid generation of
structural and stereochemical diversity from a single class
of starting materials and essentially one chiral N-hetero-
cyclic carbene precursor, N-mesityl substituted chiral tria-
zolium salt 1,2 make these reactions attractive approaches
to scaffolds for further elaboration into bioactive com-
pounds. The synthetic utility of the immediate annulation
products, however, is diminished by the use of difficult to
remove protecting groups. Furthermore, while some of the
N-heterocycle forming annulations proceed with aliphatic
and heteroaromatic substituents, the majority require aryl
derivatives.
In this report we document NHC-catalyzed annula-
tions of enals or their R0-hydroxyenone surrogates and
readily prepared primary vinylogous amides. No nitro-
gen protecting group is employed,3 and the annula-
tion delivers synthetically and medicinally important
dihydropyridinone products4 suitable for direct biolog-
ical assay or further elaboration into compounds of con-
temporary interest. The use of chiral N-heterocyclic car-
bene precursor 1 delivers the annulation products with
good yields and enantioselectivies. The underlying
mechanism of this cascade reaction features a novel
NHC-catalyzed aza-Claisen rearrangement5 that evolved
from our recent studies of NHC-catalyzedCoatesꢀClaisen
rearrangements.
(4) (a) Nantermet, P. G.; Barrow, J. C.; Selnick, H. G.; Homnick,
C. F.; Freidinger, R. M.; Chang, R. S. L.; O’Malley, S. S.; Reiss, D. R.;
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(1) For example: (a) He, M.; Struble, J. R.; Bode, J. W. J. Am. Chem.
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(2) For a review of the chemistry of N-mesityl catalysts, see: Chiang,
P.-C.; Bode, J. W. TCI MAIL. 2011, 149, 2–17.
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(3) Recent examples of cascade reactions of N-protected enamine
and R,β-unsaturated aldehydes include: (a) Noole, A.; Borissova, M.;
Lopp, M.; Kanger, T. J. Org. Chem. 2011, 76, 1538–1545. (b) Buchanan,
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r
10.1021/ol202272t
Published on Web 09/09/2011
2011 American Chemical Society

The requisite enamines are either commercially available
or prepared from 1,3-ketoesters.
Scheme 1. NHC-Catalyzed Syntheses of N-Heterocycles
Table 1. Optimization of Enantioselective Annulation of Enala
base or additive
ee %
base or additive
ee %
no base (3 days)
K2CO3
66
68
73
79
i-Pr2NEt
80
80
80
86
i-Pr2NEt þ NaBF4
˚
t-BuOK
i-Pr2NEt þ 4 A MS
NMM
i-Pr2NEt at 23 °C
a Reaction conditions: 0.1 M PhCH3, 12 h at 40 °C unless noted.
Although the key R,β-unsaturated acyl azolium can be
generated via an internal redox reaction of ynals and an
NHC,13 we choose to catalytically generate this species via
an oxidation of the Breslow intermediate formed from the
combination of enals and an NHC. Our studies began by
examining the NHC-catalyzed annulation of enal 3 and
enamine 4 in the presence of N-mesityl catalyst 1 (Table 1).
Oxidant 2, reported by Kharasch14 and utilized by
Studer,8,12g was found to be the most effective for this
reaction. The choice of base had a subtle effect. Amine
bases such as DBU, NMM, and i-Pr2NEt were found to
give higher enantioselectivity than inorganic bases
(K2CO3, t-BuOK, or Clꢀ). Additives such as molecular
sieves9b or NaBF410 did not improve the selectivity. Simply
lowering the reaction temperature to 23 °C (room
temperature) led to an agreeable increase in enantioselec-
tivity. However, further decreasing the temperature to 0 °C
prolonged the reaction time without significant enhance-
ment of the selectivity.
With the optimized conditions, we explored the scope of
this variant of the aza-Claisen reaction. A number of
stable, unprotected enamines containing cyano, ester, or
nitro groups cleanly afforded the corresponding dihydro-
pyridinones in good yields and enantioselectivities
(Scheme 2). Similarly, a broad range of R,β-unsaturated
aldehydes (aliphatic, alkenyl, or aromatic) proved to be
suitable reaction partners. The best result (96% ee, entry 12)
was obtained from coupling (E)-4-methylpent-2-enal and
(Z)-3-aminobut-2-enenitrile. Less sterically hindered enals
led to slightly diminished selectivity (entries 9 to 11).
Aromatic groups either on the enals (entries 6 to 8) or
the enamines (entries 16 to 17) afforded the expected
dihydropyridinones in greater yield than the aliphatic
counterparts. It is worth noting that the geometry of the
In2010we reportedtheenantioselective, NHC catalyzed
synthesis of dihydropyranones6 from ynals and stable
enols via catalytically generated R,β-unsaturated acyl azo-
lium intermediates. A conceptually related reaction start-
ing from R,β-unsaturated acyl fluorides and silyl enol
ethers to deliver racemic dihydropyranones was reported
just prior to our study by Lupton;7 related annulations
were reported by Studer,8 Xiao,9 and You10 shortly after
our work. As part of mechanistic studies establishing that
the key CꢀC bond forming event occurs via an NHC-
catalyzed variant of the CoatesꢀClaisen rearrangement,11
we found that the formation of a metastable hemiacetal
from the R,β-unsaturated acyl azolium and the enol was
key to the success of this reaction. Simultaneously, acyl
azoliums have been established to be surprisingly reluctant
to effect amidation reactions.12 Taken together, these
observations suggested that stable, unsubstituted enam-
ines would be suitable substrates for an aza-Claisen re-
arrangement without competing amide-bond formation.
(7) (a) Ryan, S. J.; Candish, L.; Lupton, D. W. J. Am. Chem. Soc.
2009, 131, 14176–14177. (b) Candish, L.; Lupton, D. W. Org. Lett. 2010,
12, 4836–4839.
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9266–9269.
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2458. (b) Zhu, Z.-Q.; Zheng, X.-L.; Jiang, N.-F.; Wan, X.; Xiao, J.-C.
Chem. Commun. 2011, 47, 8670–8672.
(10) Rong, Z.-Q.; Jia, M.-Q.; You, S.-L. Org. Lett. 2011, 13,
4080–4083.
(11) Mahatthananchai, J.; Zheng, P.; Bode, J. W. Angew. Chem., Int.
Ed. 2011, 50, 1673–1677.
(12) (a) Daigo, K.; Reed, L. J. J. Am. Chem. Soc. 1962, 84, 659–662.
(b) Owen, T. C.; Richards, A. J. Am. Chem. Soc. 1987, 109, 2520–2521.
(c) Owen, T. C.; Harris, J. N. J. Am. Chem. Soc. 1990, 112, 6136–6137.
(d) Movassaghi, M.; Schmidt, M. A. Org. Lett. 2005, 7, 2453–2456. (e)
Bode, J. W.; Sohn, S. S. J. Am. Chem. Soc. 2007, 129, 13798–13799. (f)
Vora, H. U.; Rovis, T. J. Am. Chem. Soc. 2007, 129, 13796–13797. (g) De
Sarkar, S.; Grimme, S.; Studer, A. J. Am. Chem. Soc. 2010, 132, 1190–
1191.
(13) Zeitler, K. Org. Lett. 2006, 8, 637–640.
(14) Kharasch, M. S.; Joshi, B. S. J. Org. Chem. 1957, 22, 1439–1443.
Org. Lett., Vol. 13, No. 19, 2011
5379

enamine (eitherE orZ suchasfor entries13and 14) didnot
play a role in either reactivity or selectivity. In a prelimin-
ary attempt, R-methyl cinnamaldehyde did not participate
in this reaction.
conditions for annulations of vinylogous amides and
R0-hydroxyenones with achiral N-mesityl triazolium salt
18. Consistent with our previous study,16 triazolium cata-
lysts lacking an ortho-substitution were found to be in-
effective for this reaction (see Supporting Information for
screening). The reaction proceeded cleanly and efficiently
with these substrates and gave uniformly high yields from
R0-hydroxyenones bearing electron-rich, electron-defi-
cient, and hetero aromatic groups (Scheme 3). Again, a
range of stable, unprotected enamines containing cyano,
Scheme 2. Enantioselective Annulation of Enals and Enaminesa,b
Scheme 3. Annulation of R0-Hydroxyenone and Enaminesa,b
a Reaction conditions: 0.1 M PhCH3, 12 h, rt. b(Z)-Enamines were
used, unless noted. cYield refers to isolated yield after chromatography.
dThe absolute configuration of (R)-8 was established by X-ray analysis;
others were assigned by analogy. eAn (E)-enamine was used.
We have previously acknowledged the relative difficulty
of preparing cinnamaldehyde derivatives and introduced
R0-hydroxyenones15 as easily prepared (one step from
commercial materials) and stored surrogates. The in-
creased sterics of these substrates preclude the use of chiral
NHCs but smaller, achiral variants effect most of the
known NHC-catalyzed annulations to give racemic pro-
ducts. In many cases, the synthesis of the racemic hetero-
cycles is desirable for testing or reaction screening, and the
enantioselective variant can be accomplished by preparing,
when necessary, the corresponding enals. In order to test
this new NHC-catalyzed annulation with a greater variety
of aromatic and heteroaromatic substituents we developed
a Reaction conditions: 0.1 M PhCH3, 12 h at 40 °C, b(Z)-Enamines
were used. bYield refers to isolated yield after chromatography.
(16) Mahatthananchai, J.; Bode, J. W. Chem. Sci. 2011, DOI:
10.1039/c1sc00397f.
(17) (a) Hargreaves, J. R.; Hickmott, P. W.; Hopkins, B. J. J. Chem.
Soc. C 1968, 2599–2603. (b) Hickmott, P. W.; Sheppard, G. J. Chem.
Soc. C 1971, 2112–2115.
(18) The issue of selective N-acylation vs C-alkylation of enamines is
a subject of Mayr’s recent review on ambident reactivity: Maji, B.;
Breugst, M.; Mayr, H. Angew. Chem., Int. Ed. 2011, 50, 6915–6919.
(19) Stork, G.; Kretchmer, R. A.; Schlessinger, R. H. J. Am. Chem.
Soc. 1968, 90, 1647–1648.
(15) (a) Chiang, P.-C.; Rommel, M.; Bode, J. W. J. Am. Chem. Soc.
2009, 131, 8714–8718. (b) Chiang, P.-C.; Kim, Y.; Bode, J. W. Chem.
Commun. 2009, 4566–4568.
5380
Org. Lett., Vol. 13, No. 19, 2011

ester, ketone (acyclic and cyclic), or nitro groups all
participated in this reaction.
The metastable hemiaminal III is poised for an aza-
Claisen rearrangement, the enantiodetermining step, in
which the sense of asymmetric induction is identical
to that observed in our dihydropyranone synthesis.6
This rearrangement is followed by lactam formation
and catalyst turnover to afford dihydropyridinone
products (Figure 1).
Two examples from Schemes 2 and 3 are worth noting.
Compound 15 (Scheme 2), prepared in quantitative yield
and high enantiomeric excess from the corresponding enal
and a vinylogous amide, constitutes an asymmetric synthesis
of a precursor for an R1a adrenergic receptor antagonist with
low nano- to picomolar potency, in which no other method
for catalytic enantioselective synthesis is available.4a Simi-
larly, compound 22 (Scheme 3) represents a facile catalytic
synthesis of a precursor for selective Rho-kinase inhibitors4b
from a readily prepared R0-hydroxyenone.15
Scheme 4. N-Acylation of Enamines and Their Rearrangements
Mechanistically, we propose that the N-heterocyclic car-
bene, generated by deprotonation of the corresponding
triazolium salt, adds irreversibly16 to the enal to generate
the Breslow intermediate I. This species is oxidized by 2 to
form the key R,β-unsaturated acyl azolium II. Hickmott17
has demonstrated that the reaction of enamines such as 34
with R,β-unsaturated acid chlorides 35 proceeded first via
N-acylation over C-alkylation,18 which gave an intermediate
that underwent a sigmatropic rearrangement, possibly via
the intermediacy of a ketene (Scheme 4). Building on these
results and those of Stork19 and Stille,20 Mahalanabis21 has
invoked this mechanistic rationale in the synthesis of 33 from
acyl chloride 38 and enamine 39 (Scheme 4). In a slight
departure from these precedents, we favor a reversible 1,2
addition of the enamine to II to form hemiaminal III, an
argument that parallels our previous studies6,11 and the well-
documented properties of acyl azoliums.12aꢀc
Figure 1. Proposed mechanism of an NHC-catalyzed aza-Clai-
sen annulation cascade.
In summary, we have reported the annulation of enals or
their R0-hydroxyenones surrogates with readily prepared viny-
logous amides via an NHC-catalyzed aza-Claisen rearrange-
ment. The reaction proceeds with a broad range of substrates
to afford synthetically useful dihydropyridinones in good
yields and enantioselectivities. This reaction also constitutes
an NHC-catalyzed N-heterocycle formation, in which neither
imines nor nitrogen protecting groups are required.
Acknowledgment. This work was supported by ETH-
(20) (a) Paulvannan, K.; Stille, J. R. J. Org. Chem. 1992, 57, 5319–
5328. (b) Benovsky, P.; Stephenson, G. A.; Stille, J. R. J. Am. Chem. Soc.
1998, 120, 2493–2500. The authors thank a referee for pointing out these
reports.
(21) Chowdhury, S. K. D.; Sarkar, M.; Chowdhury, S. R.; Mahalanabis,
K. K. Synth. Commun. 1996, 26, 4233–4251.
€
Zurich and a predoctoral fellowship from Novartis to J.M.
The authors thank Pei-Chen Chiang (UPenn) for the
preparation of R0-hydroxyenones.
Supporting Information Available. Experimental pro-
cedures and characterization data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(22) Other examples of NHC-catalyzed lactam formation include ref
1 and (a) Zhang, Y.-R.;He, L.; Wu, X.;Shao, P.-L.;Ye, S. Org. Lett. 2008,
10, 277–280. (b) Duguet, N.; Campbell, C. D.; Slawin, A. M. Z.; Smith,
A. D. Org. Biomol. Chem. 2008, 6, 1108–1113. (c) Thai, K.; Wang, L.;
Dudding, T.; Bilodeau, F.; Gravel, M. Org. Lett. 2010, 12, 5708–5711.
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