β,γ-regioselective inverse-electron-demand aza-Diels?Alder reactions with α,β-unsaturated aldehydes via dienamine catalysis

Article abstract of DOI:10.1021/ol501814p

A stereoselective inverse-electron-demand aza-Diels-Alder cycloaddition process of cyclic 1-aza-1,3-butadienes and α,β-unsaturated aldehydes has been developed via dienamine catalysis. This reaction exhibits excellent β,γ- regioselectivity for enal substr

Full text of DOI:10.1021/ol501814p

Letter
pubs.acs.org/OrgLett
β,γ-Regioselective Inverse-Electron-Demand Aza-Diels−Alder
Reactions with α,β-Unsaturated Aldehydes via Dienamine Catalysis
Jing Gu,† Chao Ma,† Qing-Zhu Li,† Wei Du,† and Ying-Chun Chen*,†,‡
†Key Laboratory of Drug-Targeting and Drug Delivery System of the Ministry of Education, West China School of Pharmacy, and
State Key Laboratory of Biotherapy, West China Hospital, Sichuan University, Chengdu, Sichuan 610041, China
‡College of Pharmacy, Third Military Medical University, Shapingba, Chongqing 400038, China
S
* Supporting Information
ABSTRACT: A stereoselective inverse-electron-demand aza-
Diels−Alder cycloaddition process of cyclic 1-aza-1,3-buta-
dienes and α,β-unsaturated aldehydes has been developed via
dienamine catalysis. This reaction exhibits excellent β,γ-
regioselectivity for enal substrates with substantial structural
diversity and broad functionalities, readily producing highly
enantioenriched fused piperidine derivatives and enabling
efficient sequential construction of complex polycyclic frameworks.
Scheme 1. Dienamine Catalysis in Inverse-Electron-Demand
Aza-Diels−Alder Reactions
he [4 + 2] cycloaddition between an electron-poor diene
Tand an electron-rich dienophile, namely, the inverse-
electron-demand Diels−Alder reaction (IED DA), has been
extensively explored since being disclosed.1 This type of
cycloaddition is extremely useful for the construction of O-,
N-, and S-centered heterocycles, such as dihydropyrans and
piperidines that are privileged frameworks in both organic and
medicinal fields,2 and a diversity of catalytic stereoselective
processes have been exploited.3
The HOMO-raised enamine4,5 or dienamine6,7 species
derived from an amine catalyst and aliphatic aldehydes or
α,β-unsaturated aldehydes, respectively, have performed as
good dienophiles or dienes in various DA reactions with
electron-deficient partners. As far as dienamine intermediates
are concerned, different regioselectivity was observed in aza-DA
cycloadditions with acyclic N-Ts 1-azadienes, and the reactions
commonly occurred at the proximal ipso,α-double bond. In
contrast, only one example of remoter β,γ-regioselective aza-DA
reaction with simple crotonaldehyde was obtained but with
poor enantioselectivity (Scheme 1).7 Although the Jørgensen
group developed highly β,γ-regioselective and stereoselective
IED oxo-DA reactions of enals through H-bond-directing
dienamine catalysis,8 the corresponding asymmetric aza-
version, occurring at the distal CC bond of dienamines
with broader structural diversity, has not been well developed
yet. Recently, we reported that cyclic 3-vinyl-1,2-benzoisothia-
zole-1,1-dioxides9 could regio- and chemoselectively act as
either 4π-electron-participation dienes10 or unlettered 2π-
electron-participation dienophiles11 with properly structured
trienamine species, or even biselectrophilic C3 partners in
formal [5 + 3] cycloadditions via cascade dienamine-dienamine
catalysis,12 we are encouraged to investigate their asymmetric
IED aza-DA cycloadditions with α,β-unsaturated aldehydes.
Such cyclic 1-azadienes might be more applicable substrates
favoring the distal β,γ-regioselectivity owing to the better rigid
structures in comparison with the previously applied acyclic 1-
azadienes.13
The initial study was performed with 3-styryl-1,2-benzoiso-
thiazole-1,1-dioxide 1a and crotonaldehyde 2a, catalyzed by
chiral secondary amine α,α-diphenylprolinol O-TMS ether C1
(20 mol %) and benzoic acid (BA, 20 mol %) in CHCl3.14 To
our delight, the cycloaddition proceeded smoothly with
excellent β,γ-regioselectivity at room temperature, and the
corresponding alcohol 3a was isolated in a high yield after
sequential in situ reduction with NaBH4. Pleasingly, the
enantioselectivity was quite promising (Table 1, entry 1, 81%
ee). Subsequently, a few solvents were screened, none of which
Received: June 23, 2014
Published: July 21, 2014
© 2014 American Chemical Society
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Table 1. Screening Studies of Aza-Diels−Alder
Table 2. Substrate Scope and Limitations in Reactions of
a
Cycloaddition with 1a and Crotonaldehyde 2a
Electron-Deficient 1-Azadienes and α,β-Unsaturated
a
Aldehydes 2
b
c
entry
C
acid
BA
solvent
t (h)
yield (%)
ee (%)
1
C1
C1
C1
C1
C1
C1
C1
C2
C3
C4
C1
C1
C1
C1
C1
C5
C6
CHCl3
CCl4
12
18
12
12
24
24
24
12
12
12
12
12
12
18
18
72
72
81
78
78
82
78
78
−
83
81
79
79
80
79
79
80
45
−
81
82
79
71
61
59
−
33
59
51
39
65
81
84
88
55
−
2
BA
3
BA
DCM
4
BA
DCE
5
BA
toluene
THF
6
BA
7
BA
MeCN
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
8
BA
9
BA
10
11
12
13
BA
OFBA
AcOH
SA
d
14
BA
e
15
f
BA
16
BA
17
BA
a
Unless noted otherwise, reactions were performed with 1-azadiene 1a
(0.05 mmol), crotonaldehyde 2a (0.075 mmol), amine C (0.01 mmol)
b
and acid (0.01 mmol) in solvent (0.5 mL) at 25 °C. Isolated yield for
c
d
two steps. Determined by chiral HPLC analysis; dr > 19:1. At 0 °C.
At −10 °C. Adding DEA (diethylacetamide, 1 equiv).8
e
f
provided better results unfortunately (entries 2−5), while no
reaction occurred in acetonitrile (entry 6). The attempts to
improve the enantioselectivity by using more bulky amine
catalysts15 C2−C4 were not successful, and significantly
decreased ee values were observed (entries 8−10). Using o-
fluorobenzoic (OFBA) and acetic acid also resulted in much
poorer enantiocontrol (entries 11 and 12), while the similar
good enantioselectivity was obtained in the presence of salicylic
acid (SA) (entry 13). Pleasingly, the reaction was carried out
efficiently at lower temperature, and the enantioselectivity
could be slightly improved without effect on the yield (entries
14 and 15). In contrast to what observed in IED oxo-DA
cycloadditions with H-bond-directing aminocatalysis,8 poor
reactivity and fair enantioselectivity was attained by using
bifunctional catalyst C5 (entry 16). Even thiourea C6 exhibited
no catalytic activity (entry 17).
With the optimal catalytic conditions in hand, we
consequently investigated the substrate scope and limitations
of 3-vinyl-1,2-benzoisothiazole-1,1-dioxides 1 and α,β-unsatu-
rated aldehydes 2. The resulting aldehyde cycloadducts were
directly reduced with NaBH4 to give the corresponding
alcohols 3. The results are summarized in Table 2. At first, a
variety of α,β-unsaturated aldehydes were explored in reactions
with 1-azadiene 1a. In comparison with the results of
crotonaldehyde 2a (Table 2, entry 1), excellent enantioselec-
a
Unless noted otherwise, reactions were performed with 1-azadiene 1
(0.1 mmol), enal 2 (0.12 mmol), amine C1 (0.02 mmol) and benzoic
acid (0.02 mmol) in CHCl3 (1.0 mL) at 25 °C. Isolated yield for two
b
c
steps. ee was determined by chiral HPLC analysis; unless noted
d
e
otherwise, dr > 19:1 by 1H NMR analysis. At −10 °C. C4 was used.
f
g
h
dr (6:1) in DA step. dr (7:1) in DA step. The aldehyde product was
i
obtained. The absolute configuration of 3j was determined by X-ray
analysis after derivation. The other products were assigned by analogy.
j
1-Azadiene 4 was used.
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tivity with moderate diastereoselectivity (6:1) could be
obtained for linear 2-pentenal 2b in the presence of a bulky
amine C4 (entry 2), while using amine C1 still produced better
data for the reaction of 2-hexenal 2c (entry 3). A modest ee
value with excellent diastereoselectivity was attained for 3-
phenylbut-2-enal 2d (entry 4). Importantly, γ,γ-disubstituted
enals 2e and 2f also exhibited high β,γ-regioselectivity though a
quaternary center must be generated, and remarkable stereo-
selectivity was gained even at room temperature (entries 5 and
6). In addition, α,β-unsaturated aldehydes 2g and 2h bearing
functional groups could be well tolerated, providing the desired
products in high stereoselectivity and with moderate yields
(entries 7 and 8). On the other hand, an array of 1-azadienes 1
bearing diverse aryl and heteroaryl groups were explored in
reactions with 4-methyl-2-pentenal 2e, and the corresponding
products were efficiently furnished in moderate to high yields
and with excellent stereoselectivity (entries 9−21). 1-Azadienes
bearing branched alkyl groups were compatible in reactions
with crotonaldehyde 2a catalyzed by amine C1, producing
products 3t and 3u in modest data (entries 22 and 23).
Furthermore, the analogous 1-azadienes 4 containing a 1,2,3-
benzoxathiazine-2,2-dioxide motif16 also were effectively
applied in reactions with enal 2e under the same aminocatalytic
conditions, affording cycloadducts 5a−5d in good yields and
stereocontrol (entries 24−27).
sequential intramolecular aldol and dehydration process was
conducted to deliver a polyhydrocyclopenta[b]pyridine20 7 in a
modest yield but with excellent stereoselectivity. Moreover, a
different transformation strategy could be developed when
substrate 2k with an acetyl group was used. Interestingly, the
attempt to conduct intramolecular benzoin condensation with
aldehyde precursor under N-heterocyclic carbene catalysis was
not successful, but furnished a tetracyclic product 8 after
reducing the unexpected regioselective adol product with
NaBH(OAc)3,21 albeit the overall yield is fair. Such drug-like
materials might find application in medicinal chemistry.
As outlined in Scheme 3, the reduction of the enamide group
of adducts 3e with Et3SiH and BF3·OEt2 proceeded
Scheme 3. Reduction of Enamide Group
uneventfully,5a and piperidine derivative 9 was efficiently
obtained in a high yield and with exclusive diastereoselectivity.
In conclusion, we have investigated the asymmetric inverse-
electron-demand aza-Diels−Alder cycloadditions of cyclic 1-
azadienes containing a 1,2-benzoisothiazole-1,1-dioxide or
1,2,3-benzoxathiazine-2,2-dioxide motif with α,β-unsaturated
aldehydes. Excellent distal β,γ-regioselectivity and high stereo-
selectivity were obtained by employing dienamine catalysis of a
chiral secondary amine. The substrate scope for both partners is
substantial, and some sequential transformations could be
effectively carried out with the multifunctional cycloadducts to
furnish frameworks with higher degrees of structural complex-
ity. Further studies would find more valuable applications, and
the results will be reported in due course.
As the current β,γ-regioselective aza-DA reaction via
dienamine catalysis could tolerate diverse functional group,
we are inspired to utilize some functionalized α,β-unsaturated
aldehydes, which might enable the sequential assembly with
aldehyde group to construct complex polycyclic frameworks. As
illustrated in Scheme 2, α,β-unsaturated aldehydes 2i bearing a
Scheme 2. Employing Functionalized Enal Substrates to
Construct Heterocycles with Higher Molecular Complexity
ASSOCIATED CONTENT
* Supporting Information
■
S
Complete experimental procedures and characterization of new
products, CIF file of enantiopure derivative of 3j, NMR spectra
and HPLC chromatograms. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: ycchen@scu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the NSFC
(21122056, 21372160 and 21321061), 973 Program
(2010CB833300), and Program for Changjiang Scholars and
Innovative Research Team in University (IRT13031).
sulfonamide group17 smoothly reacted with 1-azadiene 1a
under the established catalytic conditions, and a fused
polyhydro-1,6-naphthyridine derivative18 6 was produced in a
good yield and stereoselectivity after subsequent elimination
process with methanesulfonyl chloride (MsCl) and triethyl-
amine (TEA). In addition, enal 2j having a benzoyl group19 also
was successfully employed in the aza-DA reaction, and a
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