A highly site-, regio-, and stereoselective lewis acid catalyzed formal [3+3] cycloaddition of methylenecyclopropane-1,1-diesters with C,N-diarylnitrones

Article abstract of DOI:10.1002/chem.200801990

A highly site, regio, and stereoselective Lewis acid catalyzed formal [3+3] cycloaddition of methylenecyclopropane (MCP)-1,1-diesters with C,N-doarylnitrones, was conducted. It is found that the geminal installation of two electron-withdrawing groups (EWG

Full text of DOI:10.1002/chem.200801990

DOI: 10.1002/chem.200801990
A Highly Site-, Regio-, and Stereoselective Lewis Acid Catalyzed Formal
[3+3] Cycloaddition of Methylenecyclopropane-1,1-Diesters with C,N-
Diarylnitrones
Bao Hu, Jianlin Zhu, Siyang Xing, Jie Fang, Ding Du, and Zhongwen Wang*[a]
Dedicated to Professor Zhengming Li
À
Cycloaddition is one of the most powerful tools for the
construction of carbo- and heterocyclic skeletons, due to its
high efficiency and atom economy.[1] Methylenecyclopro-
panes (MCPs) are useful building blocks in organic synthesis
because they are readily available[2] and display good reac-
tivity.[3] Cycloadditions of MCPs have shown much utility in
a variety of synthetic applications.[3] There are three typical
modes of cycloaddition of MCPs (Scheme 1): a) the exocy-
c) ring-opening at the C C bond that is distal to the alkene
(mode C),[6] wherein a trimethylenemethane (TMM)[1a,7] or
a metallacyclobutane[8] transition state may be involved.
Most of these cycloadditions proceed under thermal condi-
tions or by catalysis using transition metals. Lewis acids,
such as lanthanide triflates,[9] are usually effective in promot-
ing the carbocation-related reactions under mild conditions.
Recently, Lewis acid-promoted proximal[10] and distal[11]
cycloACTHNURTGNEUaGN dditions of MCPs have been studied and have attract-
ed considerable attention.
Nitrone is one of the most useful dipoles in 1,3-dipolar
cycloadditions.[1a] The groups of Brandi and de Meijere have
studied the 1,3-dipolar cycloaddition of MCPs with nitrones
for many years.[3] However, all of these cycloadditions are of
the exocyclic [3+2] variety [Scheme 2, Eq. (2)]. In our inves-
tigation of cycloadditions of functionalized MCPs, we have
Scheme 1. Typical cycloaddition modes of methylenecyclopropanes.
clic mode (mode A)[4] in which the exocyclic carbon–carbon
double bond formally serves as a dienophile, b) proximal
ring opening of a cyclopropane bond (mode B),[5] and
[a] Dr. B. Hu, J. Zhu, S. Xing, J. Fang, D. Du, Prof. Z. Wang
State Key Laboratory of Elemento-Organic Chemistry
Institute of Elemento-Organic Chemistry, Nankai University, Tianjin
300071 (China)
Fax : (+86)22-23498191
E-mail: wzwrj@nankai.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801990.
Scheme 2. Typical cycloaddition modes of functionalized cyclopropanes
with nitrones.
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Chem. Eur. J. 2009, 15, 324 – 327

COMMUNICATION
introduced electron-withdrawing groups (EWGs) at C2- or
C3-positions of the cyclopropane ring, in an effort to change
the electronic character of the TMM transition state and
thus achieve fine-tuning of its reactivity and regioselec-
8, and 9). Decreasing the amount of YbACTHNGURETNNU(G OTf)3 resulted in
decreased conversion of MCP 1a even with prolonged reac-
tion time or at higher temperature (Table 1, entries 6 and 7).
The structure of 3a (exclusively as the Z isomer) was
established by 1H and 13C NMR spectroscopy and single-
crystal X-ray diffraction (Figure 1).[13] The resulting tetrahy-
dro-1,2-oxazine skeleton is known in many bioactive natural
products, and is a valuable intermediate for target mole-
cules.[14]
ACHTUNGTRENNUNG
tivity.[11] The groups of Lautens[11a,b] and Monti[11c] have re-
ported Lewis acid-catalyzed distal [3+2] cycloaddition of
systems substituted with one EWG. Young and Kerr[12] have
reported a [3+3] cycloaddition of donor–acceptor (D–A)
cyclopropane diesters with nitrones, catalyzed by YbACTHNUTRGNEUNG(OTf)3
[Scheme 2, Eq. (1)]. Inspired by these results, we envisioned
that the geminal installation of two EWGs at the C2 posi-
À
tion of the cyclopropane ring might further activate the C2
C3 bond toward cycloaddition. By formation of a complex
between the two carbonyl groups and the Lewis acid, a dis-
crete dipolar TMM transition state may be generated, in
which the carbanion can be well-stabilized by the two
EWGs and the carbocation can be stabilized by conjugation
with the neighboring carbon–carbon double bond. Thus, the
mode of cycloaddition of MCP-1,1-diesters with nitrones
might be changed from exocyclic [3+2] cycloaddition to the
distal [3+3] variety. A high regioselectivity might also be an-
ticipated. To our delight, in the YbACTHNUTRGNEUNG(OTf)3-promoted reac-
tions of MCP-diesters 1 with C,N-diarylnitrones 2, exclusive-
ly distal [3+3] cycloaddition products 3 were obtained
[Scheme 2, Eq. (3)]. To our knowledge, this is the first Lewis
acid-promoted distal [3+3] cycloaddition of MCPs with 1,3-
dipoles (Scheme 1, mode C).
Figure 1. X-ray crystal structure of 3a.
The first experiment was carried out using MCP-1,1-die-
ster 1a and C,N-diarylnitrone 2a in CH2Cl2 at 408C, cata-
lyzed by YbACHTUNGTRENNUNG(OTf)3 (20 mol%). After 41.5 h, 3a was ob-
tained, together with 15% of recovered 1a (Table 1,
entry 1). Solvent screening showed that both the conversion
of substrate 1a and the reaction rate were highest for the re-
action in THF (Table 1, entry 4). Several Lewis acids were
A wide range of electronically and structurally diverse
MCPs 1 can also be employed in this reaction (Table 2). The
reactions of C,N-diarylnitrone 2a with MCP-1,1-diesters 1 f–
i (as a mixture of two isomers) all gave exclusively the Z-
also tested and, of these, Yb
ACHTUNGRTEN(NUNG OTf)3 afforded better results
Table 2. Reactions of C,N-diarylnitrone 2a with various MCP-1,1-die-
sters 1.[a]
than either Sc(OTf)3 or Mg(ClO4)2·6H2O (Table 1, entries 4,
A
ACHTUNGTRENNUNG
Table 1. Optimization of conditions for the cycloaddition of MCP-1,1-
diester 1a with C,N-diarylnitrone 2a in the presence of Lewis acids.[a]
Entry
1
R1
R2
t [h]
Product 3
Yield [%][b]
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
C6H5
9.5
21
5
3a
3b
3c
3d
3e
3 f
3g
3h
3i
98
83
93
51
p-MeOC6H4
p-CH3C6H4
p-BrC6H4
p-ClC6H4
Bn
Entry Lewis acid
Solvent T [8C] t [h] Conv. of 1a [%][b]
1
2
3
4
5
6
7
8
9
Yb
Yb
Yb
Yb
Yb
Yb
Yb
N
CH2Cl2
DCE
40
41.5
85
69
70
40–50 29
1e
21.5
45.5
39.5
33
51
36
78
toluene 40–50 20
83
1 f[c]
1g[c]
1h[c]
1i[c]
1j
55[d]
94[d]
89[d]
64
THF
CHCl3
THF
THF
THF
40–50
40–50 28.5
40–50 32.5
66
40-50
9.5
>96
69
Ph
Ph
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
68[c]
n-C7H15
1-naphthyl
Ph
10.5
16.5
83[c]
64
10
11
3j
3k
66
98
Sc
N
1k
14
N
40–50 23.5
29
[a] Reaction conditions, unless otherwise stated: 2a (0.60 mmol), 1a
(0.20 mmol), Yb(OTf)3 (20 mol%), THF (7.0 mL) at 40-508C, N2 atmos-
phere. [b] Yields isolated by column chromatography on silica gel.
[a] Reaction conditions, unless otherwise stated: 2a (0.60 mmol), 1a
(0.20 mmol), Lewis acid (20 mol%), solvent (7.0 mL), N2 atmosphere.
[b] Determined by 1H NMR spectroscopy after purification by column
ACHTUNGTRENNUNG
[c] Mixture of two isomers; for E/Z ratios, see the Supporting Informa-
chromatography. [c] Yb
G
tion. [d] YbACTHGUNTERN(NUG OTf)3 (30 mol%) was used.
Chem. Eur. J. 2009, 15, 324 – 327
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www.chemeurj.org
325

Z. Wang et al.
isomers of 3 f–i, in moderate-to-excellent yields (Table 2,
entries 6–9).
dition was also developed. Further efforts on extensive
study and application of this cycloaddition are in progress.
Further studies revealed that the cycloadditions of a varie-
ty of C,N-diarylnitrones 2 with MCP 1a also proceeded
smoothly to give the corresponding products 3l–t in 61–
98% yields as single isomers (Table 3, entries 1–9). It should
be mentioned that reactions of C,N-dialkyl nitrones with
MCP 1a did not give positive results, probably due to the in-
trinsic instability of such nitrones to Lewis acids.
Experimental Section
General procedure for the [3+3] Cycloaddition of methylenecyclopro-
panes (MCPs) with C,N-diarylnitrones: YbACTHNUTRGENUG(N OTf)3 (25 mg, 0.04 mmol,
20 mol%), MCP 1 (0.20 mmol), C,N-diarylnitrone 2 (0.60 mmol) and dry
THF (7 mL) were stirred at 40–508C under a nitrogen atmosphere for
the time indicated in Tables 2 and 3. After completion of the reaction (as
monitored by TLC), the solvent was removed under reduced pressure
and the residue was purified by column chromatography on silica gel
(eluent: ethyl acetate/petroleum ether) to afford 3. 3a: Yield 98%, single
Z-isomer, white solid; m.p. 137–1388C; 1H NMR (400 MHz, CDCl3): d=
7.43 (t, J=7.2 Hz, 2H), 7.38–7.25 (m, 5H), 7.23–7.10 (m, 5H), 7.05 (s,
1H), 6.70–6.83 (m, 3H), 5.67 (s, 1H), 5.18 (d, J=13.2 Hz, 1H), 4.91 (d,
J=13.2 Hz, 1H), 3.93 (s, 3H), 3.55 ppm (s, 3H); 13C NMR (75 MHz,
CDCl3): d=170.0, 167.3, 148.6, 136.1, 135.1, 131.6, 130.5, 130.2, 129.1,
128.5, 128.1, 127.6, 127.4, 122.5, 116.8, 71.7 ,70.1 ,64.5, 53.6, 52.5 ppm; IR
(KBr): n˜ =3062, 3028, 2952, 2934, 1752, 1727, 1599, 1489, 1433, 1255,
Table 3. Reactions of MCP-1,1-diester 1a with several C,N-diarylnitrones
2.[a]
R3
R4
t [h] Product 3 Yield [%][b]
1221, 1025, 759, 695 cmÀ1
; HRMS (ESI) calcd for C27H25NO5Na
Entry
2
[M+Na]+: 466.1625; Found: 466.1624.
1
2
2b C6H5
2c C6H5
m-ClC6H4
p-ClC6H4
11
13
3l
3m
85
90
Typical Procedure for the Three-Component Reaction: YbACHTUNGTRENNUNG(OTf)3
3
4
5
6
7
8
9
10
2d p-MeOC6H4 p-ClC6H4
2e m-MeOC6H4 p-ClC6H4
2 f p-MeOC6H4 p-BrC6H4
5.5 3n
94
98
93
(30 mol%) was added to a solution of the benzaldehyde 4 (1.32 mmol)
and hydroxylamine 5 (1.20 mmol) in dry toluene (5 mL) containing acti-
vated 4 ꢁ molecular sieves. The solution was stirred under a nitrogen at-
mosphere for 3.5 h at room temperature, after which the MCP 1a
(0.4 mmol) dissolved in dry THF (5 mL) was added, and the mixture was
heated to 50–608C for 40 h. Upon disappearance of the MCP 1a, the re-
action mixture was filtered and the filtrate was evaporated in vacuo. Pure
product 3a (133 mg, 75%) was obtained after column chromatography
on silica gel (eluent: EtOAc in petroleum ether).
15
3o
5.5 3p
3q
2g p-CH3C6H4
2h p-IC6H4
2i C6H5
p-CH3C6H4 23
p-ClC6H4
CH3
68[c,d]
66[c]
61[d]
81
45.5 3r
72
3s
2j 2-furyl
2i C6H5
m-ClC6H4
CH3
25.5 3t
53[e] 3u
56[c]
[a] The reaction conditions are as outlined in Table 2. For details of pro-
cedure and analysis, see the Supporting Information. [b] Yields isolated
by column chromatography on silica gel. [c] YbACTHNURGTNEUNG(OTf)3 (30 mol%) was
used. [d] Reaction was carried out at 668C. [e] Scaled-up reaction with
1c (27 mmol).
Acknowledgements
We thank the National Natural Science Foundation of China (Nos.
20572045 and 20421202) and the Ministry of Education of China
(RFDF20070055022 and RSFACTHNUTRGENU[GN 2005]383) for financial support.
A three-component one-pot adaptation of this cycloaddi-
tion, using aldehyde 4, hydroxylamine 5, and MCP-1,1-die-
ster 1a was also developed, and afforded 3a in 75% yield
(Scheme 3).
Keywords: cycloaddition
·
Lewis acids
·
nitrones
·
selectivity · small ring systems
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À
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Cycloaddition of Methylenecyclopropane-1,1-Diesters with C,N-Diarylnitrones
COMMUNICATION
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À
[15] Two experiments were carried out to investigate N O bond cleav-
age of 3a: a) Reaction of 3a with Zn/AcOH at room temperature
for 8 h led to the decomposition of 3a; b) With SmI2/THF, at room
temperature overnight, no reaction occurred; heating the mixture at
reflux for 5 h led to the decomposition of 3a. Similar difficulties
have also been reported by Kerr and co-workers.[14j,14l] Seemingly,
À
substituents on the tetrahydro-1,2-oxazine skeleton affect N O
bond cleavage.
Received: September 26, 2008
Published online: November 25, 2008
Chem. Eur. J. 2009, 15, 324 – 327
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327