Exo-Selective construction of spiro-[butyrolactone-pyrrolidine] via 1,3-dipolar cycloaddition of azomethine ylides with α-methylene-γ- butyrolactone catalyzed by Cu(I)/DTBM-BIPHEP

Article abstract of DOI:10.1039/c3cc45493b

An expedient access to optically active spiro-[butyrolactone-pyrrolidine] was successfully developed via an unprecedented Cu(i)-catalyzed exo-selective 1,3-DC of azomethine ylides with α-methylene-γ-butyrolactone, which exhibited high diastereoselectivity (>98:2), excellent enantioselectivity (96->99% ee) and a broad substrate scope under mild conditions. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc45493b

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Communication
exo-Selective Construction of Spiro-[butyrolactone-pyrrolidine] via 1,3-
Dipolar Cycloaddition of Azomethine Ylides with α-Methylene-γ-
butyrolactone Catalyzed by Cu(I)/DTBM-BIPHEP†
Qing-Hua Li,a Tang-Lin Liu,a Liang Wei,a Xiang Zhou,a Hai-Yan Tao,a and Chun-Jiang Wang*a,b
5
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
applied in these reactions are 2-oxoindolin-3-ylidene,6a-e
An expedient access to optically active spiro-[butyrolactone-
cyclopropyliden acetates,7a and 2-alkylidene cycloketones.7b In
pyrrolidine] was successfully developed via an unprecedented
contrast, α-methylene-γ-butyrolactone have been seldom studied
Cu(I)-catalyzed exo-selective 1,3-DC of azomethine ylides
45 in the catalytic asymmetric 1,3-dipolar cycloaddition although the
lactone moiety possesses highly biological activity profile.8 To
the best of our knowledge, only limited racemic examples have
been reported involving α-methylene-γ-butyrolactone as the
dipolarophile so far.9 and we believe this represents a major
50 challenge.
10 with α-methylene-γ-butyrolactone, which exhibited high
diastereoselectivity (>98:2), excellent enantioselectivity (96-
>99% ee) and broad substrate scope under mild conditions.
The privileged spirocyclic [butyrolactone-pyrrolidine] skeletons
with multiple contiguous stereogenic centers are the core
15 structural elements prevalent in a large number of natural
alkaloids and unnatural compounds exhibiting important
biological activities and building blocks in organic synthesis,1
typical examples are shown in Figure 1. Spirocycle A and B are
the key intermediates for the synthesis of complex marine
20 alkaloid (-)-sarain A2a and Cephalotaxus alkaloid cephalotaxine,2b
and spirocyclic C exhibits anticancer activity.3 Inspired by the
varied and significant biological activities, different synthetic
approaches have been developed in pursuit of the structurally
diversified and stereocontrolled [butyrolactone-pyrrolidine]
Herein, we reported an unprecedented Cu(I)-catalyzed
25 skeletons in the past decades. The catalytic asymmetric 1,3- 55 asymmetric 1,3-dipolar cycloaddition of various azomethine
dipolar cycloaddition4,5 of azomethine ylides to electron-deficient
alkenes provides a powerful and atom-economical route to
synthesize a variety of structurally and stereochemically rich
spirocyclic pyrrolidines bearing spiro quaternary stereogenic
30 center.6,7 The first catalytic asymmetric synthesis of spiro
pyrrolidine-oxindole derivatives was realized by Gong via an
elegant organocatalyzed asymmetric 1,3-dipolar cycloaddition
reaction between N-Ac 2-oxoindolin-3-ylidenes and in situ
formed azomethine ylides.6a Subsequently, Cu(I)-catalyzed
35 asymmetric 1,3-dipolar cycloaddition for constructing such
structures was reported by Waldmann and us, respectively.6b,6c
Recently, asymmetric approach to spiro pyrrolidine-oxindoles
was also fulfilled by Arai and co-workers6d employing Ni(II)-
complex and Wang6e employing amine-thiourea as the
40 organocatalyst. Despite excellent results achieved for asymmetric
synthesis of spirocyclic pyrrolidines, most of dipolarophiles
ylides with α-methylene-γ-butyrolactone, providing the efficient
and facile access to the bioactive exo-selective10 spiro-
[butyrolactone-pyrrolidine] with excellent levels of diastereo-
selectivity and enantioselectivity.
60
Initially, we began our investigation by using α-methylene-γ-
butyrolactone 2 and N-(4-chloro-benzylidene)-glycine methyl
65 ester 1a as the dipolar in the presence of AgOAc/(S)-BINAP L1
(3 mol%) and Et3N (15 mol%) in DCM at room temperature.
Gratifyingly, the 1,3-dipolar addition reaction proceeded
smoothly affording exo-adduct 3a as the major diastereomer
(exo/endo = 66:34) with moderate enantioselectivity (Table 1,
70 entry 1). Encouraged by this result, different metal salts and
chiral bisphosphine ligands were subsequently examined to
establish optimal reaction conditions, and the representative
aCollege of Chemistry and Molecular Sciences, Wuhan University, Wuhan
430072, China. E-mail: cjwang@whu.edu.cn Fax: 86-27-68754067
bState Key Laboratory of Elemento-organic Chemistry, Nankai University
Tianjin, 300071, China
†Electronic supplementary information (ESI) available: Experimental
section. CCDC 935824. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc00000x.
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DOI: 10.1039/C3CC45493B
Table 1. Screening studies of the catalytic asymmetric 1,3-DC of imino
ester 1a with α-methylene-γ-butyrolactone 2a
Table 2. Substrate scope of Cu(I)-catalyzed exo-selective 1,3-DC of
various imino esters 1 with α-methylene-γ-butyrolactone 2a
CO2Me
O
8
NH
6
5
2
O
CO2Me
N
p-Cl-C6H4
O
exo-3a (5,6-trans)
[M]/L (3 mol %)
+
Entry
1
2
3
4
5
6
7
8
R
3
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
Yield (%)b Ee (%)c
+
O
CO2Me
Et3N (15 mol %)
DCM, rt
p-Cl-C6H4 (1a)
o-Cl-C6H4 (1b)
m-Cl-C6H4 (1c)
p-Br-C6H4 (1d)
Ph (1e)
p-Me-C6H4 (1f)
o-Me-C6H4 (1g)
m-Me-C6H4 (1h)
p-MeO-C6H4 (1i)
2-furyl (1j)
2-thienyl (1k)
1-Napthyl (1l)
2-Napthyl (1m)
PhCH2CH2 (1n)
Cy (1o)
88
80
72
85
83
78
70
75
80
74
84
83
85
62
65
>99
98
99
99
99
99
99
97
99
98
96
97
98
99
96
p-Cl-C6H4
O
2
8
NH
6
1a
5
2
O
p-Cl-C6H4
endo-3a (5,6-cis)
Entry
[M]
AgOAc
L
exo/endo Yield (%)b
Ee (%)c
55
74
50
83
1
2
3
4
5
6
L1
L1
L2
L3
L4
L5
66:34
60:40
83:17
83:17
>98:2
>98:2
75
80
79
72
85
88
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
Cu(CH3CN)4BF4
9
10
11
12
13
14d
15d
>99
>99
a All reactions were carried out with 0.35 mmol of 1a and 0.23 mmol of 2
in 2 mL solvent for 1-2 h. b Isolated yields of both exo-3a and endo-3a.
c
Ee of exo-3a was determined by HPLC analysis.
a All reactions were carried out with 0.35 mmol of 1 and 0.23 mmol of 2
in 2 mL DCM for 1-2 h. b Isolated yield. c Determined by HPLC analysis.
d Inorganic base CsCO3 was used, 10 h.
ring to give the corresponding exo-selective spiro-[butyrolactone-
pyrrolidines] 3 in good yields and with excellent enantio-
25 selectivities (96->99% ee) as single diastereomers (entries 1-9). It
is noteworthy that comparable results were still achieved for the
sterically hindered ortho-chloro and ortho-methyl-substituted
imino esters 1b and 1g in terms of diastereo-/enantioselectivity
(entries 2 and 7). Additionally, the heteroaryl substituted imino
30 esters 1j and 1k were also tolerated in this transformation leading
to 98% and 96% ee, respectively (entries 10 and 11). Similarly,
high enantioselectivity was obtained in the reaction of imine 1l
and 1m, derived from 1-naphthylaldehyde, 2-naphthylaldehyde
with 2 to afford the adducts 3l and 3m with 97% and 98% ee,
35 respectively (entries 12 and 13). Remarkably, less reactive alkyl
substituted imino esters worked well in this transformation
affording the similarly high level of diastereo-/enantioselectivity
(entries 14 and 15).
Having successfully achieved the excellent stereoselectivity in
40 the 1,3-dipolar cycloaddition of α-methylene-γ-butyrolactone
with glycine derived imino esters, we next turned our attention to
the more challenging homoserine lactone derived cyclic imines,
from which tricyclic skeleton bearing two quaternery stereogenic
centers and two butylactones would be generated simultaneously
45 in this transformation. A wide range of cyclic imino esters 4a-
4m, derived from various aromatic or aliphatic aldehydes, reacted
smoothly with α-methylene-γ-butyrolactone 2 to afford the
corresponding tricyclic adducts 5a-5m in good yields and
excellent stereoselectivities (Table 3). Both electron-donating and
50 electron-withdrawing substituents at different positions on the
aromatic ring were tolerated (entries 1-9). Interestingly, ortho-
chloro and ortho-methyl-substituted imino esters 4b and 4g gave
ee values up to 99% without losing both yields and stereoselect-
ivities (entries 2 and 7). Cyclic imino esters 4k derived from 2-
55 naphthylaldehyde also worked well in this transformation to
provide 5k with satisfactory result (entry 11). It is noteworthy
that better result was achieved for heteroaromatic aldehyde
derived imino ester 4j (entry 10). Comparable outcomes were
results were summarized in Table 1. Both AgOAc/(S)-BINAP
and Cu(MeCN)4BF4/(S)-BINAP complexes exhibited high exo-
selectivity, but copper complex gave better asymmetric induction
and higher catalytic activity, which was chosen as the metal
precursor in the further ligand survey (Table 1, entries 1 and 2).
The exo-selectivity was improved to 83:17 when another two
commercially-available chiral axially bisphosphine ligand (S)-
Tol-BINAP (L2) or (S)-MeO-BIPHEP (L3) was employed in this
transformation, but the latter exhibited better enantioselective
5
10 control (entries 3 and 4). Further ligand screening chiral axially
biphenyl bisphosphine skeleton revealed that the bulky and
electron-donating biphenyl ligands (R)-DTBM-segphos and (R)-
DTBM-BIPHEP
((2,2'-bis[di(3,5-di-t-butyl-4-methoxyphenyl)
phosphino]-6,6'-dimeth-oxy-1,1'-biphenyl)) not only remarkably
15 improved the diastereoselectivity (>98:2) but also gave almost
perfect enantioselectivity (Table 1, entries 5 and 6).
To explore the scope of the reaction, the Cu(I)/(R)-DTBM-
BIPHEP catalyzed 1,3-dipolar cycloaddition of various imino
esters derived from glycinate with α-methylene butyrolactone
20 were carried out under the optimized reaction conditions. The
results are summarized in Table 2, all reactions proceeded
smoothly regardless of the substitution pattern on the aromatic
2
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DOI: 10.1039/C3CC45493B
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also achieved for the less reactive imino ester derived from
aliphatic cyclohexanecarbaldehyde and 3-phenylpropanaldehyde,
respectively (entries 12 and 13). No cycloaddition reaction
occurred when trisubstituted (E)-3-benzylidenedihydrofuran-
2(3H)-one was tested as the dipolarophile probably due to the
unfavored steric hindrance and the less reactivity.
40
45
2
5
Table 3. Substrate scope of Cu(I)-catalyzed exo-selective 1,3-DC of
various cyclic imino esters 4 with α-methylene-γ-butyrolactone 2a
3
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Entry
1d
2
3
4
5
6
7
8
R
5
5a
5b
5c
5d
5e
5f
5g
5h
5i
Yield (%)b
Ee (%)c
>99
>99
99
p-Cl-C6H4 (4a)
o-Cl-C6H4 (4b)
m-Cl-C6H4 (4c)
p-Br-C6H4 (4d)
Ph (4e)
p-Me-C6H4 (4f)
o-Me-C6H4 (4g)
m-Me-C6H4 (4h)
p-MeO-C6H4 (4i)
2-thienyl (4j)
2-Napthyl (4k)
PhCH2CH2 (4l)
Cy (4m)
82
80
81
84
83
75
68
75
80
84
82
60
67
55
99
>99
>99
>99
>99
>99
>99
>99
99
5
60
9
10
11
12e
13e
5j
5k
5l
65
5m
97
a
All reactions were carried out with 0.35 mmol of 4 and 0.23 mmol of 2
in 2 mL solvent for 1-2 h. b Isolated yield. c Ee was determined by HPLC
70
d
analysis.
The absolute configuration of 5a was determined as
(7R,9R,13S) by X-ray diffraction analysis. e Inorganic base CsCO3 was
used, 10 h.
In summary, we have successfully developed a Cu(I)-catalyzed
exo-selective 1,3-dipolar cycloaddition reaction of azomethine
ylides with α-methylene-γ-butyrolactone to provide a series of
75
10 spirocyclic-[butyrolactone-pyrrolidine] bearing one to two
quaternary stereogenic centers for the first time (See ESI for the
proposed transition states for the exo-selectivity). The readily
available precursors were emlpoyed under mild conditions for the
straightforward construction of highly functionalized bicyclic or
15 tricyclic skeletons with excellent levels of stereocontrol, and the
great importance of the enantiomerically enriched spiro-
[butyrolactone-pyrrolidine] make the current methodology
particularly interesting in synthetic chemistry. Efforts are
currently underway to elucidate the mechanistic details and the
20 scope and limitations of this reaction, and the results will be
reported in due course.
80
6
85
90
7
8
This work is supported by the 973 program (2011CB808600),
NSFC (20972117, 21172176), NCET-10-0649, IRT1030, and the
Fundamental Research Funds for the Central Universities.
95
25 Notes and references
100
105
110
‡ Crystal data for (7R,9R,13S)-5a: C16H16ClNO4, Mr = 321.75, T = 293 K,
Orthorhombic, space group P212121, a = 6.4177(9), b = 12.2541(18), c =
19.540(3) Å, V = 1536.7(4) Å3, Z = 4, 2619 unique reflections, final R1 =
0.0805 and wR2 = 0.2547 for 3006 observed [I>2σ(I)] reflections, Flack
30 = 0.2(2). CCDC 935824.
9
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χ
1
For very recent review, see: (a) L. Hong and R. Wang, Adv. Synth.
10 For the early examples on the catalytic asymmetric exo-selective 1,3-
dioplar cycloaddition of azomethine ylides, see: (a) Y. Oderaotoshi,
W. Cheng, S. Fujitomi, Y. Kasano, S. Minakata and M. Komatsu,
Org. Lett., 2003, 5, 5043. (b) W. Gao, X. Zhang, and M. Raghunath,
Org. Lett., 2005, 7, 4241.
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35
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