Amine-catalyzed asymmetric (3 + 3) annulations of β′-acetoxy allenoates: Enantioselective synthesis of 4H-pyrans

Article abstract of DOI:10.1021/jacs.6b04935

The asymmetric (3 + 3) annulations of β′-acetoxy allenoates with either 3-oxo-nitriles or pyrazolones have been realized by using 6′-deoxy-6′-[(l)-N,N-(2,2′-oxidiethyl)-valine amido]quinine (6h) as the catalyst. The three functions of catalyst 6h, including Lewis base (quinuclidine N), H-bond donor (amide NH), and Br?nsted base (morpholine N), cooperatively take crucial roles on the chemo- and enantioselectivity, allowing for the construction of 4H-pyran and 4H-pyrano[2,3-c]pyrazole in high yields and enantioselectivity.

Full text of DOI:10.1021/jacs.6b04935

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Communication
Amine-Catalyzed Asymmetric (3+3) Annulations of #’-
Acetoxy Allenoates: Enantioselective Synthesis of 4H-Pyrans
Chunjie Ni, and Xiaofeng Tong
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b04935 • Publication Date (Web): 16 Jun 2016
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Amine-Catalyzed Asymmetric (3+3) Annulations of β’-Acetoxy
Allenoates: Enantioselective Synthesis of 4H-Pyrans
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Chunjie Ni, and Xiaofeng Tong*
Jiangsu Key Laboratory of Advanced Catalytic Materials & Technology, School of Petrochemical Engineering,
Changzhou University, 1 Gehu Road, Changzhou 213164, China
Supporting Information Placeholder
congested chiral amine catalyst.9 To overcome the intrinsic
ABSTRACT: The asymmetric (3+3) annulations of β’-acetoxy
challenge and accomplish high enantioselectivity, a novel tri-
functional cinchona alkaloid-based amine catalyst has been
developed.10 Thus, we saw an opportunity to demonstrate the
utility of our (3+3) annulations toward the advancement of
the amine-catalyzed asymmetric allenoate annulation and
the biologically relevant 4H-pyran11.
allenoates with either 3-oxo-nitriles or pyrazolones have
been realized by using 6’-deoxy-6’-[(L)-N,N-(2,2'-oxydiethyl)-
valine amido]quinine (6h) as the catalyst. The three
functions of catalyst 6h, including Lewis base (quinuclidine
N), H-bond donor (amide NH), and Brønsted base
(morpholine N), cooperatively take crucial roles on the
chemo- and enantio-selectivity, allowing for the construction
of 4H-pyran and 4H-pyrano[2,3-c]pyrazole in high yields and
enantioselectivity.
Me
Ph
H
AcO
O
cat (10 mol %)
NC
Ph
E
NC
Ph
E
Ph
+
CN
+
Ph
base
EtOAc, 0 oC, t
CO2Me
2a
O
Me
O
Ph
1a (1.3 equiv)
(E = CO2Me)
3aa
4
3aa
4
entry cat
base
t (h)
(yield / ee) (yield)
The cinchona alkaloids have been recognized as privileged
chiral scaffolds in asymmetric catalysis.1 In this context, due
to the strong nucleophilicity of their quinuclidine nitrogen,
cinchona alkaloids are extremely valuable tertiary amine
catalysts2 for a wide range of asymmetric reactions.3 However,
their potential toward the amine-catalyzed asymmetric an-
nulation of allenoate remains largely underexplored. It was
not until 2011 that the first asymmetric version of cinchona
alkaloid-based amine-catalyzed (2+2) annulations of alleno-
ates with imines was realized by Masson and Zhu.4 Subse-
quently, similar asymmetric (2+4) annulations of allenoates
with various oxo-dienes were developed by the groups of
Tong, Bohan, Shi, Cheng, and Xu.5 Nevertheless, the amine-
catalyzed asymmetric annulations of allenoates are sporadi-
cally reported, in sharp contrast to the well-developed phos-
phine-catalyzed analogues.6
OBn
1.1 equiv Na2CO3 120
1
2
5a
5b
63% / 32% ND
80% / 84% ND
108
N
1.1 equiv Na2CO3
21
36
3
4
5b 1.1 equiv Cs2CO3
5b 1.1 equiv NEt3
51% / ND
43%
N
5a: X = OMe
5b: X = HNAc
87% / 73% 10%
88% / 72% 6%
X
0.1 equiv NEt3
5b
36
5
1.1 equiv Na2CO3
Scheme 1. Preliminary Attempts at Asymmetric (3+3)
Annulation of 1a and 2a
Our investigation commenced from the screening of
cinchona alkaloid-based catalysts for the model reaction of
1a and 2a (Scheme 1). After several attempts, we found that,
o
with the help of Na2CO3 in EtOAc at 0 C, catalyst 5a was
able to delivere product (S)-3aa in 63% yield and 32% ee
(entry 1).12 Catalyst 5b gave much better results, affording 3aa
in 80% yield and 84% ee, albeit with a long reaction time
(entry 2). Obviuosly, the improved performances were
attributed to the additional amide NH of 5b as an H-bond
donor. It was surprise that the use of stronger base Cs2CO3
produced 51% yield of 3aa along with 43% yield of side
product 4 (entry 3). The isolation of 4 arose from direct
reaction of 1a and 2a without the involvement of amine
catalyst.13 The use of Et3N was found to be beneficial, not
only diminishing 4 to 10% but shortening the desired reation
time to 36 hours albeit only with 73% ee (entry 4). The fact
that the reaction performances strongly depended on the
base additive led us to realize that, likely due to the less
catalytic activity of 5b,9 a proper rate of nucleophile
generation would be requisite: slow rate further retarded the
desired reaction while fast one triggered the side reaction.
Here, we report the asymmetric (3+3) annulations of β’-
acetoxy allenoates 1 with 3-oxo-nitriles 2 by using cinchona
alkaloid-based tertiary amine as catalyst for enantioselective
synthesis of 4H-pyran (Scheme 1). Due to the installment of
the β’-acetoxy group, allenoates 1 are liable to form an
inherently electrophilic 1,3-dien-2-aminium intermediate via
the addition-elimination reaction with amie catalyst, thus
accommodating a pronucleophile as the other reaction
partner with the help of a base additive.7 This route is
completely different from the well-known nucleophilic
zwitterion mechanism in the field of the Lewis base catalysis
of allenoates.8 However, this feature, in turn, would bring
about a new challenge associated with the competitive
addition-elimination reaction between allenoate
nucleophilic substrate, especially in the case of the sterically
1 and
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Table 1. Identification of Catalysts 6a
Indeed, the combination of 0.1 equiv Et3N and 1.1 equiv
1
2
3
4
5
6
7
8
Na2CO3 further suppressed the side reaction and, more
importantly, imposed no negative effect on the desired
reaction (entry 5). In this case, Et3N was regenerated via the
reaction of [Et3NH]+ and Na2CO3, thus requiring only a
catalytic amount of Et3N with stoichiometric amount of
Na2CO3. Unfortunately, further optimization of reaction
conditions failed to improve the reaction efficiency and se-
lectivity.13
entry
cat
6a
%yieldb/%eec entry cat
%yield/%ee
1
2
3
4
5
92 / 70
96 / 65
85 / 68
94 / 75
96 / 67
6
7
6f
6g
6h
6i
96/ 82
96 / 87
96 / 92
96 / 90
81 / 83
6b
6c
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6d
6e
9
10
6j
aFor reaction conditions, see the Supporting Information.
bIsolated yield. cDetermined by HPLC analysis.
the substructure of morpholine Brønsted base, the amino
acid moiety was further modulated, which finally disclosed
that L-valine-derived catalyst 6h worked best, delivering 3aa
in 96% yield and 92% ee (entries 7-10).
As comparison, catalyst 6k, a derivative of 6h via
methylation of the amide NH group, was also tested for the
reaction of 1a and 2a, which gave 3aa only with 57% ee
(Scheme 2). Moreover, catalyst 6l, a variant of 6h via
replacing the tertiary amine with amide, gave 3aa only in 32%
yield. These results further demonstrated the crucial roles of
the amide NH and tertiary amine in catalyst 6h on the
asymmetric induction and reaction efficiency, respectively.
Figure 1. The structure of catalyst 6 and a postulated
working model
Despite these aforementioned challenges, the preliminary
results inspired us to focus on catalysts 6 (Figure 1), which
was accessed from the incorporation of the scaffold of 5b and
a tertiary amino acid unit.14 Compared with the individual
functions of 5b and Et3N, we envisioned that the three
different active centers of catalysts 6, including Lewis base
quinuclidine nitrogen, H-bond donor amide NH, and
Brønsted base amine, might cooperatively take effect, thus
not only enabling synchronous generation of the related 1,3-
diene-2-aminium intermediate and nucleophile partner but
also enforcing their reaction in a psuedo-intramolecular
manner (Figure 1). The cooperative effect, if workable, would
facilitate the desired reaction and make no redundant
nucleophile available for the side reaction. Meanwhile, the
newly-introduced chiral scaffold of the amino acid unit was
capable of subtly diversifying the stereodescriminating
micro-enviroment along with the robust quinine parent
skeleton. Moreover, struturally modificable catalysts 6 could
be easily prepared via the Pd(0)-catalyzed coupling reaction
of readily available 6’-OTf-quinine and the corresponding
amino amides.15
Hence, we gauged the potential of catalyst 6 toward the
chemo- and enantio-selectivity issues (Table 1). Delightfully,
catalyst 6a bearing a pyrrolidine-modified glycine unit
delivered 3aa in 92% yield (entry 1). Side product 4 wasn’t
observed, demonstrating a crucial role of the attached
Brønsted base amine on the chemoselevtivity. Catalyst 6b
with a piperidine-modified glycine unit gave somewhat lower
ee (entry 2). Catalyst 6c derived from D-alanine gave inferior
enantioselectivity than that of L-alanine-derived 6d (entries
3 and 4), indicating that the L-amino acid unit would be
favorable for enantioselectivity. While catalyst 6e bearing the
piperidine-modified L-alanine unit was not a good choice,
catalyst 6f with the morpholine-modified one delivred 3aa in
96% yield and 82% ee (entries 5 and 6). Then, on the basis of
OBn
OBn
10 mol% 6k or 6l
1.1 equiv Na2CO3
EtOAc, 0 oC, 32 h
N
N
1a
N
N
3aa
+
2a
Me
O
N
HN
with 6k: 76% yield, 57% ee
with 6l: 32% yield, 78% ee
N
N
O
O
O
6k
6l
Scheme 2. 6k or 6l-Catalyzed Reaction of 1a and 2a
With the optimal catalyst 6h in hand, we turned our
attentions to explore the reaction scope. First, a range of
allenoates 1 were examined by reacting with substrate 2a
(Scheme 3). Various aryl groups at β’C position of allenoates 1
[4-FC6H4, 4-ClC6H4, 4-BrC6H4, 4-MeC6H4, 4-MeOC6H4, 3-
BrC6H4, 2-BrC6H4, 2-MeOC6H4, 4-iPrC6H4, 2,4-(Me)2C6H3]
were found to be tolerated, and the corresponding products
3ba-3ka were isolated in 80%-95% yields and with 87%-99%
ee. Allenoate 1l with a 1-naphthyl substituent was also a
suitable substrate, delivering 3la in 88% yield and 93% ee.
Notably, N-acetyl-indole and thiophene were also compatible,
giving the corresponding products 3ma and 3na in excellent
yields and somewhat lower ee values. In contrast, the
n
reaction of allenoate 1o with an alkyl Pr substituent gave
product 3oa only in moderate yield and enatioselectivity. In
addition to methyl esters 1a-1o, benzyl esters 1p and 1q were
also examined, which exhibited very similar reactivity. For
the reactions of allenoates 1n, 1o and 1q, the somewhat lower
enantioselectivity would be due to their relatively sterically
smaller β’-substituents (R group), which might result in the
lower stereoselectivity of the newly-formed C=C bond in the
corresponding 1,3-diene-2-aminium intermediate.
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Scheme 5. 6h-Catalyzed (3+3) Annulations of 1 and 7
which reacted well with 1a to give the corresponding 4H-
pyrano[3,2-d]isoxazoles 8af and 8ag with high
enenatioselectivity abeit in relatively lower yields.
To demonstrate the synthetic potential of this (3+3) annu-
lation, an enantioselective synthesis of pyranopyrazole 13 was
studied (Scheme 6). This class of compound is known to
have fungicide activity.17 As illustrated in Scheme 6, upon
treatments of LiAlH4 reduction and catalytic hydrogenation,
the conversion of 8ab into cis-10 was achieved with good
diastereoselectivity albeit in low overall yield. Using classic
manipulations of one-carbon elongation and Friedel–Crafts
reaction18, compound 13 was finally obtained with high level
of enantioselectivity and diastereoselectivity.
Scheme 3. Substrate Scope of Allenoate 1
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Next, we moved on to investigate the substrate scope of 2
(Scheme 4). It was found that various β-aryl nitriles 2,
including phenyl substituents with either electron-
withdrawing (4-F, 4-Cl, 4-Br and 4-I) or electron-donating
groups (4-Me and 4-MeO), 1-naphthalene as well as 2-
thiophene, were well tolerated, affording products 3ab-3ai in
good yields and excellent enantioselectivity. For the cases of
β-alkyl nitriles 2j-2l, high enantioselectivity was also
obtained although the corresponding yields were moderate.
Scheme 6. Synthesis of Compound 13
In summary, we have realized the asymmetric (3+3) annu-
lations of β’-acetoxy allenoates 1 with either 3-oxo-nitriles 2
or pyrazolones
7 by using 6’-deoxy-6’ [(L)-N,N-(2,2'-
Scheme 4. Substrate Scope of 3-Oxo-nitrile 2
oxydiethyl)-valine amido]quinine 6h as the catalyst. Alt-
hough the precise working model is not clear at this stage,
the three functions of catalyst 6h, including Lewis base (qui-
nuclidine nitrogen), H-bond donor (amide N-H) as well as
Brønsted base (morpholine nitrogen), are believed to coop-
eratively take effect to enhance enantioselectivity and over-
come the side reaction, allowing for the isolation of 4H-
pyrans 3 and pyrano[2,3-c]pyrazoles 8 in high chemical yields
and enantioselectivity. Further study of the application of
catalysts 6 is currently underway.
In considering other possible pronucleophile partners for
(3+3) annulations with allenaotes 1, we elected to pursue the
use of pyrazolones 7, which would result in the biologically
interesting pyrano[2,3-c]pyrazoles16 (Scheme 5). Delightedly,
the reactions of allenoates 1 and pyrazolones 7 smoothly
occured, delivering pyrano[2,3-c]pyrazoles 8 in good yields
and with high enantioselectivity even at room temperature.
Isoxazolones 7f and 7g were also good nucleophile partners,
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O. Chem. Commun. 2013, 49, 11588. (g) Voituriez, A.; Marinetti, A.;
ASSOCIATED CONTENT
Supporting Information
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Wang, Y.; Lu, Y. Synlett 2011, 2766. (i) Zhao, Q.-Y.; Lian, Z.; Wei, Y.;
Shi, M. Chem. Commun. 2012, 48, 1724. (j) Wang, Z.; Xu, X.; Kwon, O.
Chem. Soc. Rev. 2014, 43, 2927.
(9) Due to their sterically congested scaffold, catalysts 5 and 6 are
less active than DABCO. The reduced catalytic activity would largely
retard the following two steps: i) their addition to allenoate 1a; ii) the
attack of 2a to 1,3-dien-2-aminium intermediate, which was
supported by the results of the following two control experiments.
For the detailed discussions, see the Supporting Information.
1
2
3
4
5
6
7
8
Detailed experimental procedures, characterization of new
compounds, crystallographic data (3aa and 8ae). This mate-
rial is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
9
Corresponding Author
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txf@cczu.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work is supported by NSFC (No. 21272066 and
21472042). We are grateful to Changzhou University for ex-
perimental assistance and financial support.
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