PPh3-Catalyzed [3 + 2] Spiroannulation of 1C,3N-Bisnucleophiles Derived from Secondary β-Ketoamides with δ-Acetoxy Allenoate: A Route to Functionalized Spiro N-Heterocyclic Derivatives

Article abstract of DOI:10.1021/acs.orglett.7b00910

A [3 + 2] annulation of α-substituted secondary β-ketoamides with δ-acetoxy-modified allenoate has been developed in the presence of phosphine catalyst. In this spiroannulation reaction, β-ketoamides were used as the bis-nucleophilic partner while the γ,δ-carbon of 5-acetoxypenta-2,3-dienoate participated as a C2 synthon, affording the desired functionalized five-membered N-heterocyclic derivatives in moderate to excellent yields and diastereoselectivities under mild conditions. Preliminary attempts on the asymmetric variant of this reaction have been also examined, giving the corresponding products with moderate ee values.

Full text of DOI:10.1021/acs.orglett.7b00910

Letter
pubs.acs.org/OrgLett
PPh3‑Catalyzed [3 + 2] Spiroannulation of 1C,3N‑Bisnucleophiles
Derived from Secondary β‑Ketoamides with δ‑Acetoxy Allenoate: A
Route to Functionalized Spiro N‑Heterocyclic Derivatives
Jiaojiao Xing,† Yu Lei,† Yu-Ning Gao,† and Min Shi*,†,‡,§
†Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China
University of Science and Technology, 130 Mei Long Road, Shanghai 200237, People’s Republic of China
‡State Key Laboratory and Institute of Elemento-organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
§State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354
Fenglin Lu, Shanghai 200032, People’s Republic of China
S
* Supporting Information
ABSTRACT: A [3 + 2] annulation of α-substituted secondary β-ketoamides with δ-acetoxy-modified allenoate has been
developed in the presence of phosphine catalyst. In this spiroannulation reaction, β-ketoamides were used as the bis-nucleophilic
partner while the γ,δ-carbon of 5-acetoxypenta-2,3-dienoate participated as a C2 synthon, affording the desired functionalized
five-membered N-heterocyclic derivatives in moderate to excellent yields and diastereoselectivities under mild conditions.
Preliminary attempts on the asymmetric variant of this reaction have been also examined, giving the corresponding products with
moderate ee values.
ive-membered N-heterocycles with quaternary stereo-
(Scheme 1), the acidities of which can be regulated by
Fcenters are valuable intermediates in organic synthesis, modification of the electronic properties of the R substituent
and they are also fundamental structural motifs in natural
products and pharmaceuticals.1 As its subunits, spirocyclic
Scheme 1. Characteristics of β-Ketoamides
frameworks containing five-membered cyclic amides are
frequently found in several biologically active molecules (Figure
1). Thus, the exploration of new synthetic approaches to directly
construct this skeleton is highly desirable in the area of synthetic
organic chemistry and medicinal chemistry.2
on amide. Previous related reports revealed that the exceptional
reactivity of these pronucleophiles is the consequence of self-
activation, an intramolecular hydrogen bond between the acidic
N−H proton and the ketone carbonyl group, and this hydrogen
bond will increase the acidity of the α-proton in which the N−H
proton is crucial to obtain high yields of nucleophilic addition
products.4
Figure 1. Selected examples of natural products with spiro-fused
heterocyclic motifs.
During the past decades, the nucleophilic catalysis using
allenoates as electrophilic reagents has emerged as a powerful
synthetic tool to construct diversified carbo- and heterocycles.8
Since the first example of phosphine-catalyzed annulation of
The secondary β-ketoamide is one kind of 1,3-dicarbonyl
compound and has been proven to be a promising class of
pronucleophile for organocatalytic transformations such as
unsubstituted butadienoate with electron-deficient olefins
reported by Lu in 1995,9 research on the potential reaction
Michael addition,3 α-hydroxyamination,4 α-arylation,5 α-photo-
models based on different kinds of allenoates has been well-
alkylation,6 and spiroannulation.7 The characteristic of secondary
followed (Scheme 2). Typically, unsubstituted allenoates can
β-ketoamides is that they possess two distinct acidic protons, one
is the C−H proton at α-position of dicarbonyl groups and
another is the N−H proton at the secondary amide moiety
Received: March 27, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00910
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Organic Letters
Letter
Scheme 2. Phosphine-Catalyzed Annulations Involved with
Allenoates
Table 1. Optimization of Reaction Conditions for the [3 + 2]
Spiroannulation
a
b
entry
cat. (mol %)
solvent
additive
temp (°C) yield (%)
1
PPh3
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CH3CN
THF
50
50
50
50
50
50
50
50
50
50
50
50
50
50
20
37
28
65
65
81
97
33
52
82
78
0
2
PPh3
Cs2CO3
3
PPh3
AcOH
4
PPh3
H2O
5
PPh3
MeOH
6
PPh3
PhCOOH
PhCOOH
PhCOOH
PhCOOH
PhCOOH
PhCOOH
PhCOOH
PhCOOH
PhCOOH
PhCOOH
7
DABCO
DMAP
PMePh2
PMe2Ph
8
9
10
11
12
13
14
15
PPh3
PPh3
PPh3
PPh3
71
53
66
63
DCE
toluene
a
All reactions were carried out with 1a (0.1 mmol), 2 (0.2 mmol),
offer their α,β,γ-olefinic carbons and function as one-, two-, or
three-carbon synthons in the reactions with a variety of
electrophilic coupling partners, undergoing [1 + 4],10 [2 +
3],11 and [3 + 2]12 annulations (Scheme 2, a). Other types of
substituted allenoates and their reactiveness have also been
extensively investigated. For example, the α,β,γ- and β′-carbons
of α-substituted allenoates have been disclosed by Kwon and co-
workers to play the role of two-13 or three-13c,14 or four-
carbon13c,15 synthons in the corresponding annulation cases
(Scheme 2, b). Moreover, in 2010, Tong’s group revealed that
the installation of an acetate group at the β′-position of 2,3-
butadienoate16 could generate a novel [4 + 1]16a annulation
mode (Scheme 2, c). Successively, they also reported that δ-
acetate modified allenoates could work as two- or three-carbon17
units in producing the corresponding carbocycles (Scheme 2, d).
Despite these significant achievements on the utilization of the
internal CC bond of allenoate in annulations, to the best of our
knowledge, there are only scarce reports on phosphine-catalyzed
annulations applying the δ-carbon to the formed cyclic motifs. In
2015, Huang’s group reported a new strategy for which the δ-
carbon atom of a γ-substituted allenoate18 (Scheme 2, e)
incorporated into a cyclic product as one of the endocyclic atoms
through a reversible proton-transfer process between the
zwitterionic intermediates.18d Herein, we wish to report the
successful implementation of these tactics to provide a new
pathway of phosphine-catalyzed [3 + 2] spiroannulation, using α-
substituted secondary β-ketoamides 1 and the γ and δ sites of 5-
acetoxypenta-2,3-dienoate 2 to selectively afford functionalized
five-membered N-heterocyclic derivatives (Scheme 2, f).
catalyst (20 mol %), and PhCOOH (0.1 mmol) in solvent (3.0 mL) at
50 °C for 12 h. Isolated yield by column chromatography; the
diastereoisomers could not be separated by column chromatography.
b
examined, but no better result was obtained (Table 1, entries 7−
10). The screening of solvent indicated that toluene was the
optimal choice (Table 1, entries 12−14). Carrying out the
reaction at room temperature did not favor the formation of 3a
(Table 1, entry 15). It should be noted that no reaction occurred
in the absence of phosphine catalyst (Table 1, entry 11). Based
on these experimental results, the best reaction condition was
using PPh3 (20 mol %) as catalyst, PhCOOH (1.0 equiv) as
additive, and carrying out the reaction at 50 °C in toluene (3 mL)
for 12 h.
With the optimized reaction conditions in hand, we then
turned our attention toward the reaction scope and limitations,
and the results are summarized in Scheme 3. All of the reactions
proceeded smoothly under the optimal conditions, giving the
desired products in moderate to high diastereoselectivities and
excellent yields. Substrates 1b−d having different ring sizes had
little impact on the yield, but a larger cyclic ring resulted in lower
dr value, giving the annulation products 3b, 3c, and 3d in 94%,
95%, 99% yields along with 81:19, 70:30, and 60:40 dr values,
respectively. As for substrates 1e−i with different functional
groups on the benzene ring of indanone and naphthalenone, the
reactions also proceeded efficiently to afford the corresponding
products 3e−i in 87−99% yields along with 60:40−87:13 dr
values. Gratifyingly, the reaction also tolerated straight chain β-
ketoamide and heterocyclic aromatic ring. The use of 1j and 1k as
substrates produced the desired products 3j and 3k in 99% yield
along with 82:18 dr value and in 99% yield along with 88:12 dr
value, respectively. The relative configuration of the major
diastereoisomer of (E)-3a has been assigned by X-ray diffraction.
Their ORTEP drawings and the CIF data are presented in the
Supporting Information.
On the basis of our previous work on nucleophilic phosphine
catalysis, we initiated the investigation on the reaction of β-
ketoamide 1a and δ-acetoxy allenoate 2 in the presence of 20 mol
% of PPh3. A [3 + 2] annulation took place smoothly in toluene at
50 °C for 12 h, affording spiro-fused five-membered heterocyclic
product 3a in 37% isolated yield (Table 1, entry 1). Adding 1.0
equiv of Cs2CO3 did not improve the yield of 3a (Table 1, entry
2). The examination of proton sources such as AcOH, H2O,
MeOH, and PhCOOH revealed that PhCOOH is the best
additive, giving the desired product 3a in 97% yield (Table 1,
entries 3−6). Subsequently, various nucleophilic catalysts were
Using δ-acetoxy allenoate 2 as the substrate, we next examined
its reaction with β-ketoamides bearing a distinct substituent on
the N atom to elucidate the effect of the protecting group R
(Scheme 4). Whether R was an alkyl- or an arylsulfonyl group,
the reaction proceeded smoothly to give the corresponding
B
DOI: 10.1021/acs.orglett.7b00910
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catalyzes, we extended our attention to explore the asymmetric
variant of this [3 + 2] annulation of β-ketoamides with allenoate
2. We found that the annulation catalyzed by chiral multifunc-
tional phosphine derived from glycine produced 3a in 24% yield
and 51% ee of the major diastereoisomer along with 71% de. In
addition, the use of chiral Binap in this annulation afforded 3e in
77% yield and 35% ee of the major diastereoisomer along with
82% de (Scheme 5, see Schemes SI-I and SI-II for the more
details).
Scheme 3. Substrate Scope of the [3 + 2] Spiroannulation
Scheme 5. Chiral Phosphine-Catalyzed Asymmetric [3 + 2]
Annulation of β-Ketoamides with Allenoate 2
a
Isolated yield by column chromatography; the diastereoisomers could
b
not be separated by column chromatography. Determined by 1H
NMR spectroscopy of the isolated product. The reaction was
c
According to our experimental results and the previously
reported literature, a plausible mechanism for this [3 + 2]
annulation has been outlined in Scheme 6. The reaction started
performed at 80 °C.
Scheme 4. Substrate Scope of β-Ketoamides with Different
Functional Group at the N Atom
Scheme 6. Plausible Mechanism for the Formation of 3
a
Isolated yield by column chromatography; the diastereoisomers could
b
not be separated by column chromatography. Determined by 1H
NMR spectroscopy of the isolated product. The reaction was
with the formation of a zwitterionic intermediate I, generated by
β-addition of phosphine into the allene moiety. Meanwhile, the
detached acetate anion captured the α-proton of amide 1,
producing intermediate II, which underwent a nucleophilic
attack onto the δ site of intermediate I to afford intermediate III.
This intermediate could be transformed to intermediate IV via a
proton shift from amide group. Then an intramolecular Michael
addition (IMMA) took place to give another zwitterionic
intermediate V, which underwent a [1,2]-proton transfer to
afford intermediate VI. The elimination of phosphine furnished
the final [3 + 2] annulation product 3. Thus, we believe that the
addition of PhCOOH may be beneficial to the two H-transfer
processes.
c
performed at 80 °C.
annulation products 3l−o in moderate to good yields (54−99%)
with moderate to excellent dr values (83:17 to >20:1),
respectively. Only in the case of 1o was the adduct 3o obtained
in 54% yield along with 89:11 dr value, perhaps due to the
strongly electron-withdrawing −NO2 group. Substrate 1p having
a benzoyl amide group afforded the desired product 3p in 89%
yield and >20:1 dr value. To our delight, the reactions also
tolerated substrates 1 with different aryl groups on the N atom,
delivering the corresponding products 3q−w in moderate to
excellent yields (56−99%) and dr values (80:20 to >20:1). The
structure of (E)-3e has also been assigned by X-ray diffraction.
In view of the fact that chiral phosphines have been applied
successfully in plenty of excellent asymmetric phosphine
In summary, we have disclosed a novel protocol of PPh3-
catalyzed [3 + 2] spiroannulation of 1C,3N-bisnucleophiles
derived from α-substituted secondary β-ketoamides with δ-
acetoxy-modified allenoate, giving the highly functionalized five-
C
DOI: 10.1021/acs.orglett.7b00910
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b00910.
Experimental procedure and characterization data for all
compounds (PDF)
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X-ray crystallographic data for 3a (CIF)
X-ray crystallographic data for 3e (CIF)
AUTHOR INFORMATION
Corresponding Author
■
*Fax: (+86)-21-6416-6128. E-mail: mshi@mail.sioc.ac.cn.
ORCID
Min Shi: 0000-0003-0016-5211
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Basic
Research Program of China [(973)-2015CB856603], the
Strategic Priority Research Program of the Chinese Academy
of Sciences (Grant No. XDB20000000), the National Natural
Science Foundation of China (20472096, 21372241, 21572052,
20672127, 21421091, 21372250, 21121062, 21302203, and
20732008), and the Fundamental Research Funds for the
Central Universities 222201717003.
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