Asymmetric palladium-catalyzed umpolung cyclization of allylic acetate-aldehyde using formate as a reductant

Article abstract of DOI:10.1039/c5cc02176f

Palladium/chiral diphosphine-catalyzed umpolung cyclization of allylic acetate-aldehyde using formate as a terminal reductant affords cis-disubstituted pyrrolidine, tetrahydrofuran, and spiro carbocycle in high enantioselectivity. The formate does not cause allylpalladium reduction under the catalysis. The highly stereoselective cyclization would proceed through a cationic η¹-allylpalladium ligated by diphosphine.

Full text of DOI:10.1039/c5cc02176f

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Asymmetric Palladium‐Catalyzed Umpolung Cyclization of Allylic Acetate‐
Aldehyde using Formate as a Reductant
Hirokazu Tsukamoto,*a Ayumu Kawasea and Takayuki Doia
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Palladium/chiral diphosphine‐catalyzed umpolung cyclization
L*n
Pd+
Pd0 , L*n
M, - Pd0
b or c
R1
R1
R1
of allylic acetate‐aldehyde using formate as
a terminal
OAc
M
reductant affords cis‐disubstituted pyrrolidine, tetrahydrofuran,
and spiro carbocycle in high enantioselectivity. The formate
10 does not cause allylpalladium reduction under the catalysis.
The highly stereoselective cyclization would proceed through a
cationic η1‐allylpalladium ligated by the diphosphine.
a
R2
R2
R2
allyl acetate 1
!3-allylpalladium(II) 2
allylmetal 3a
M=Zn, B, et al.
R
Pd2+ , L*n
R1
R1
R1
4
O
L*n
Pd
R
*
M
*
c
Allylic acetate is a versatile electrophilic allylating agent under
Pd(0) catalysis via an η3‐allyl intermediate (Tsuji‐Trost reaction).1
15 Asymmetric Tsuji‐Trost reactions, through use of chiral ligands,
have led to the synthesis of numerous natural products.2
Moreover, allylic acetate 1 can be converted to the umpolung of
electrophilic intermediate η3‐allylpalladium 2 by reduction or
transmetalation in the presence of stoichiometric metallic
20 reagents resulting in nucleophilic allylmetal 3a (Scheme 1).3
Catalytic and asymmetric nucleophilic allylation was considered
unfeasible due to lack of direct interaction between an
electrophile and chiral Pd catalyst, until Zanoni pioneered the
process.4a Minnaard4b and Zhou4c,d built upon the method and
25 proposed that their Et2Zn‐ or Et3B‐mediated asymmetric
allylation of aldehydes would involve η1‐allylpalladium 6 bearing
an Et group and chiral monophosphorus ligand rather than allyl‐
zinc and –borane 3a, which Tamaru assumed to be
intermediates for the non‐asymmetric variant.3a Similarly,
30 Nakamura and Yamamoto propose that η1‐allylpalladium 6 with
a chiral π‐allyl ligand is the intermediate in enantioselective
imine allylations.5 In the catalytic process, η1‐allyl complex 6,
which is in equilibrium to 2, but is rarely detected, is typically
formed by transmetalation of Pd(II) catalysts containing a special
- Pd2+
R2
R2
Y
OH R2
allylmetal 3b
M=Sn, Si, B
!1-allylpalladium(II) 6
homoallyl alcohol 5
Y=Et, allyl, R2, OAc
R1
M = metal, L* = chiral ligand
n = coordination number
d
8
Pd2+ , L*n
R2 B(OH)2
R2 Pd+
a oxidative addition. b reduction.
c transmetalation.d carbopalladation.
c
organoboron 7
45
Scheme 1. Pd‐Catalyzed Umpolung Allylation of Aldehydes Leading to
Homoallyl Alcohol 5 through Allylmetal 3a and η1‐Allylpalladium 6
Previously we reported diastereo‐ and enantioselective
50 cyclization of allene‐aldehyde 9 with arylboronic acid 7 under
Pd(II) catalysis (Scheme 2).12 We proposed the highly
stereoselective reaction would result from Zimmerman‐Traxler
transition state 1016 consisting of the aldehyde and cationic η1‐
allylpalladium ligated with 5,5’‐bis(diphenylphosphino)‐4,4’‐bi‐
55 1,3‐benzodioxole (SEGPHOS, 12a), a commercially available
chiral diphosphine.17 However, the η1‐allyl complex 10 can
further react with 7 to give neutral (η1‐allyl)arylpalladium, which
Kurosawa18 and Szabo7 reported to be nucleophilic.19 To explore
the participation of the aryl ligand in the cyclization, we set out
60 to investigate the Pd(0)‐catalyzed enantioselective cyclization of
allylic acetate‐aldehyde 13, which would proceed through the
common intermediate 10. We intended to mitigate the
formation of Tsuji‐Trost type byproducts 1420 and 15.21 In
35 ligand such as bis(π‐allyl)5, 6, PCP pincer7, 8, and N‐heterocyclic
10
carbene9,
with either allylmetal 3b or combination of
organoboron reagent 7 and allene 8.11, 12 Umpolung allylations
that do not require metallic reagents have clear advantages:
neither the stoichiometric metallic byproducts nor multiple
40 allylmetal species (leading to difficult chiral control) are
generated.13, 14 Herein, we report the first asymmetric umpolung
cyclization of allylic acetate‐aldehyde without the aid of
stoichiometric metallic reagents.15
addition, achieving high enantioselectivity necessitated
a
65 nonmetallic terminal reductant for catalytic turnover that is
unable to reduce allylpalladium to give 16 or 16’ (see Scheme
2) .22
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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Page 2 of 4
Table 1. Optimization of Reaction Conditions for Transformation of (±)‐
13a‐c into 11a
View Article Online
DOI: 10.1039/C5CC02176F
R
O
Ph
RO
Ph
Catalyst (10 mol%)
Reductant (3 equiv)
AcO
4
a
b
R
X
X
TsN
TsN
P
P
O
H
X
Pd+
3
CH3CN, 50 °C
previous report
this report
OH
O
O
9
13
11a
(±)-13
R = Ac (a), EtO2C (b), HCO (c)
X = NTs (9a),
NBoc, O, CR2
R
10
Entry 13
Catalyst
Pd(dppe)2
Pd(dppe)2
Pd(PPh3)4
Pd(dppe)2
Reductant
Time (h) Yield (%)a
X
X
X
R
H
O
1b
2c
3b
4
a
a
a
a
a
a
a
a
a
a
a
a
b
c
‐
‐
‐
8
72
1
17
8
28
Trace
Trace
45
O
14
O
X
15
O
O
PAr2
PAr2
R
R
H
R
HCO2H/Et3Nd
HCO2H/Bu3Nd
HSiEt3
X
O
5
6
7
8
Pd(dppe)2
Pd(dppe)2
49
OH
11 (up to 97%ee)
16
16'
O
O
12a: Ar = phenyl
12b: Ar = 3,5-xylyl
24
46
expected byproducts
Pd2dba3‐dppee
PdCp(η3‐allyl)‐dppee
Pd(OAc)2‐dppee
Pd[P(o‐tolyl)3]2‐dppee
Pd[P(o‐tolyl)3]2
Pd[P(o‐tolyl)3]2‐12ae
Pd[P(o‐tolyl)3]2‐12ae
Pd[P(o‐tolyl)3]2‐12ae
HCO2H/Bu3Nd
HCO2H/Bu3Nd
HCO2H/Bu3Nd
HCO2H/Bu3Nd
HCO2H/Bu3Nd
HCO2H/Bu3Nd
HCO2Hf
8
5
59
60
a) R-B(OH)2 7, cat. Pd(OAc)2-(S)-SEGPHOS (12a), CH3CN, rt;
b) HCO2H-Bu3N, cat. Pd[P(o-tolyl)3]2-12a or (S)-DM-SEGPHOS (12b) , CH3CN, 50 °C.
9
8
69
10
11
12
13
14
3
48
78
0
Scheme 2. SEGPHOS‐Ligated Pd‐Catalyzed Cyclizations of Allene 9 and
Allylic Acetate 13 Leading to 11
5
4
7
72
86 (95)
70 (90)
55 (91)
‐
Allylic acetate‐aldehyde 13a tethered by p‐toluenesulfonamide
barely underwent the cyclization in acetonitrile at 50 °C to
furnish 11a cis‐selectively in the presence of stoichiometric
a
b
d
45
Enantiomeric excess of (+)‐11a is shown in parenthesis.
c
Reaction with 100 mol% of catalysts.
Reaction at 80 °C.
e
The ratio of HCO2H/R3N is 1:1. [Pd]:diphosphine = 1:1.5. dppe = 1,2‐
10 amount of achiral Pd catalyst, i.e. Pd(dppe)2 (Table 1, Entry 1). It
is noteworthy that expected byproducts 14 and 15 shown in
Scheme 2 were not obtained. Neither a catalytic amount of
Pd(dppe)2 nor stoichiometric amount of Pd(PPh3)4 gave 11a
(Entries 2 and 3). In order to improve the reaction efficiency,
15 nonmetallic reductants were screened under Pd(dppe)2 catalysis
(10 mol%) (Entries 4‐6). Among the reductants we tested, formic
acid in combination with tributylamine23 dramatically increased
the yield of 11a (Entry 5). Interestingly, reduction of acetate
leading to 16 or 16’ (Scheme 2) was not observed under the
20 reaction conditions. In situ preparation of Pd‐diphosphine
complexes was also examined (Entries 7‐10). Use of Pd(0) and
Pd(II) sources including Pd2dba3, PdCp(η3‐allyl), Pd[P(o‐tolyl)3]2,
and Pd(OAc)2 with a slight excess of dppe ligand gave better
results than Pd(dppe)2 (Entries 7‐10 vs. 5). Pd[P(o‐tolyl)3]2‐dppe
25 gave 11a in the highest yield, while the Pd source did not
promote the cyclization by itself (Entry 10 vs. 11). Instead of
achiral dppe ligand, use of chiral (S)‐SEGPHOS (12a) gave (+)‐11a
in 86% yield with excellent diastereo‐ and enantioselectivity
(Entry 12). The absolute configurations of (+)‐11a turned out to
30 be (3S, 4S) and identical to that of the product by the Pd(II)‐(S)‐
SEGPHOS catalyzed phenylative cyclization of allene‐aldehyde 9a
(Scheme 2).24 Carbonate 13b underwent the cyclization without
the aid of an amine because alkoxide is generated in situ from
the leaving group through decarboxylation (Entry 13). Similarly,
35 addition of the formic acid reductant is not necessary in the
cyclization of formate 13c because hydride is generated (Entry
14).22, 25 With the optimized reaction conditions in hand, we next
investigated substrate scope (Table 2).
bis(diphenylphosphino)ethane. f 1 equiv of HCO2H was used.
50 Substitution of the sulfonamide in the tether with Boc‐protected
nitrogen caused formation of reduced byproducts as well as
pyrrolidine 11d using Pd‐SEGPHOS (Entry 1). Gratifyingly, the
reduction was completely suppressed by using more σ‐donating
DM‐SEGPHOS (12b) instead of SEGPHOS to give 11d in 75% yield
55 and 95% ee (Entry 2). The DM‐SEGPHOS‐ligated Pd catalyst was
also effective for the enantioselective cyclization of ether 13e
(Entry 3). In contrast to the highly cis‐selective reaction leading
to heterocycles 11d and 11e, cyclization of 13f containing C3‐
tether resulted in the formation of cis‐ and trans‐substituted
60 cyclopentanols 11f and 11f’ in the ratio of 7 to 3 while
enantiomeric ratios of each diastereomer were still high (Entry
4). Mosher’s method revealed that the absolute configuration of
α‐styryl group in trans‐11f’ was opposite to that in cis isomer 11f.
To our surprise, the diastereoselectivity is much poorer than that
65 of the Pd(II)‐catalyzed cyclization of allene‐aldehyde to give 11f
(see Supporting Information). Isomeric allylic acetates (Z)‐ and
(E)‐13f’ also underwent the cyclization to give diastereomeric
mixtures of 11f and 11f’, but with reverse selectivity, i.e. 4.5:1
and 1:2.5, respectively (Entries 5 and 6). These results indicate
70 that the reaction proceeds through the corresponding η3‐
allylpalladium intermediate, but that the geometries of the
kinetically formed η3‐allyl complex considerably affect the
product distribution. The interpretation was also supported by
the exclusive transformation of cyclic allylic acetates 13g and
75 13h into spiro compounds 11g and 11h via the geometrically
fixed η3‐allyl complexes (Entries 7 and 8). This method allowed
introduction of a wide variety of substituent into the vinyl group
in the products 11i‐l, although the smaller substituent, the lower
enantioselectivity (Table 1, Entry 12 and Table 2, Entries 9‐12).
40
2 | Journal Name, [year], [vol], 00–00
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Table 2. Substrate Scopea
allylpalladiums 10 and 10’, respectively. Sterically demanding
substituent
allylpalladium
R
in the allylic moiety would favor anti‐η3‐
Entry
Substrate
Product
Time (h)
Yield (%)b
50 (90)d
75 (90)
View Article Online
syn‐complex DOtIh:r1o0u.1g0h39/C5ηCC3‐0η211‐7η63F
over
Ph
Ph
isomerization.26 Although Zimmerman‐Traxler transition states
20 for 10 and 10’ give cis‐ and trans‐substituted cyclopentanols 11
and 11’ (X=C) respectively, a ring strain in the trans‐fused
transition state 10’ biases the equilibrium toward cis‐fused 10 to
some extent and increases the ratio of cis‐11 to trans‐11’.
Particularly, the presence of heteroatom in the tether would
25 make the strain much severer owing to C‐N and C‐O bonds
shorter than C‐C bond and lead to the exclusive formation of cis‐
substituted pyrrolidine and tetrahydrofuran 11 (X=N and O). On
AcO
4
1c
2e
3e
1
10
8
BocN
BocN
3
OH
O
(3S, 4S)-11d
(±)-13d
(±)-13d
(3S, 4S)-11d
Ph
Ph
AcO
4
72 (97)
O
O
3
OH
O
(3S, 4S)-11e
(±)-13e
Ph
AcO
BnO
BnO
11f: 70 (93)
11f’: 29 (94)
4e
5e
9
2
2
the other hand, SN ‐ or SN ’‐type oxidative addition of Z‐ and E‐
Ph
O
allylic acetates 13’ to Pd(0) results in predominant formation of
30 anti‐ and syn‐η3‐allylpalladium, respectively. In addition, the
previously reported cyclization of allene‐aldehyde 9 would
proceed through (Z)‐η1‐allylpalladium 10, which is kinetically
formed by carbopalladation of terminal double bond in 9 from
the less hindered face,27 because the high cis‐selectivity was
35 retained even during the formation of cyclopentanol 11f (see
Supporting Information).
2
(±)-13f
BnO
OAc
Ph
1
BnO
(1R, 2R)-11f
OH
11f: 49 (97)
11f’: 11 (76)
BnO
10
+
Ph
BnO
2
O
BnO
BnO
(Z)-13f'
Ph
1
OH
(1R, 2S)-11f'
BnO
BnO
11f: 23 (71)
11f’: 54 (99)
6e
10
5
OAc
O
Pd
R
(E)-13f'
H
a
R
O
AcO
5
X
H
X
R
7e, f
57 (86)g
77 (91)g
9
X
P
P
O
H
X
1
Pd
O
OH
OH
R
O
(1R, 5R)-11g
(±)-13g
b
AcO
cis-11
(Z)-!1-allyl-
AcO
(Z)-13'
5
palladium 10
8e, f
5
1
O
OH
R
O
c
H
X
(1R, 5R)-11h
(±)-13h
b
b
H
H
HCOO
TsN
AcO
(±)-13
4
9e
12
48
60
1.5
62 (69)h
86 (76)
51 (88)
64 (87)
TsN
R
R
3
OH
O
(±)-13i
R
O
P
P
O
H
X
H
X
Pd
(3S, 4S)-11i
AcO
(E)-13'
X
Me
HCOO
Me
4
OH
trans-11'
(E)-!1-allyl-
10c
11e
12e
TsN
TsN
palladium 10'
3
OH
O
a carbopalladation of allene. b oxidative addition. c !3-!1-!3 isomerization.
(3S, 4S)-11j
(±)-13j
Scheme 3. Plausible Mechanism for the Cyclizations of 9, 13, (Z)‐ and (E)‐
13’ Leading to 11 and 11’
TMS
TMS
HCOO
4
TsN
TsN
3
OH
O
40 This study has uncovered the first example of the
enantioselective umpolung cyclization of allylic acetate‐aldehyde.
In contrast to the intermolecular variants reported by Zhou et.
al4, diphosphine ligands are essential and metallic reagents are
unnecessary in our case. The formate serves as the terminal
45 reductant and does not reduce allylpalladium under the catalysis.
The highly enantioselective cyclization supports the existence of
cationic η1‐allylpalladium intermediates and is suitable for the
synthesis of heterocycles but also spiro carbocycles that were
previously difficult to access. Cyclic homoallylic alcohol products
50 generated in these reactions should be useful intermediates in
synthesis of natural products and scaffolds for peptide mimics
and chiral ligands since they contain a rich array of preparatively
important functional groups.28 The scope of the reaction is
increased when compared to allene‐aldehyde cyclization, as
55 various vinyl substituents can be incorporated in the product.
Further studies on the different behavior of these cyclization
reactions are underway.
(3S, 4S)-11k
(±)-13k
Cl
Cl
4
TsN
TsN
OAc
3
OH
O
(3S, 4S)-11l
13l (E:Z=1:2.5)
a
Reaction conditions: 10 mol% Pd[P(o‐tolyl)3]2, 15 mol% ligands,
reductant (3 equiv HCO2H/Bu3N (1:1) for acetate, 1 equiv HCO2H for
b
formate), CH3CN, 50 °C. Enantiomeric excess of products is shown in
c
d
5 parenthesis. ligand=12a. Reduced products 16d and 16d’ were also
e
obtained in 23% combined yield.
ligand=(S)‐DM‐SEGPHOS (12b).
f
g
Reaction at 80 °C.
Enantiomeric excess was determined as p‐
h
nitrobenzoate of 11. Trans isomer 11i’ was also obtained in 16% yield
(60% ee).
10
The diastereoselectivities observed in these cyclization reactions
suggest how the allylpalladium intermediates are generated and
participate in the cyclization (Scheme 3). Oxidative addition of
allylic acetate 13 to Pd(0) would produce both anti‐ and syn‐η3‐
15 allylpalladiums, which are in equilibrium with (Z)‐ and (E)‐η1‐
This work was partly supported by a Grant‐in‐Aid from the Japan
This journal is © The Royal Society of Chemistry [year]
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13 Ru‐catalyzed carbonyl allylation with carbon monoxide as reductant:
(a) Y. Tsuji, T. Mukai, T. Kondo and Y. Watanabe, J. Organomet.
Society for Promotion of Sciences (No.24590004), The Research
70
75
Foundation for Pharmaceutical Sciences, and SUNTRY
FOUNDATION for LIFE SCIENCES. We also thank Dr. Matthew
Lebar for editing the wording in this manuscript.
Chem., 1989, 369, C51‐C53; (b) T. Kondo, H. Ono, N. Satake, T.
View Article Online
DOI: 10.1039/C5CC02176F
Mitsudo and Y. Watanabe, Organometallics, 1995, 14, 1945‐1953;
(c) S. E. Denmark and S. T. Nguyen, Org. Lett., 2009, 11, 781‐784; (d)
C.‐M. Yu, S. Lee, Y.‐T. Hong and S.‐K. Yoon, Tetrahedron Lett., 2004,
45, 6557‐6561.
5
Notes and references
a Aramaki‐aza aoba 6‐3, Aoba‐ku, Sendai 980‐8578, Japan. E‐mail:
hirokazu@mail.pharm.tohoku.ac.jp; Fax: +81 22 795 6867; Tel: +81 22
795 6867;
14 Ir‐catalyzed enantioselective intermolecular carbonyl allylation with
isopropanol as a reductant: (a) I. S. Kim, M.‐Y. Ngai and M. J. Krische,
J. Am. Chem. Soc., 2008, 130, 6340‐6341; (b) I. S. Kim, M.‐Y. Ngai and
M. J. Krische, J. Am. Chem. Soc., 2008, 130, 14891‐14899.
† Electronic Supplementary Information (ESI) available: Deteiled
10 experimental procedures, spectroscopic data and copies of NMR spectra.
See DOI: 10.1039/b000000x/
1
(a) P. W. Jolly, Angew. Chem. Int. Ed. Engl., 1985, 24, 283‐295; (b) J.
Tsuji in Handbook of Organopalladium Chemistry for Organic
Synthesis (Ed.: E. Negishi), WILEY, New York, 2002, vol 2, pp. 1669‐
1687; (c) L. Acemoglu and J. M. J. Williams in Handbook of
Organopalladium Chemistry for Organic Synthesis (Ed.: E. Negishi),
WILEY, New York, 2002, vol 2, pp. 1689‐1705; (d) G. Poli, G. Prestat,
F. Liron and C. Kammerer‐Pentier, Top. Organomet. Chem., 2012, 38,
1‐64.
15
16 (a) Y. Yamamoto and N. Asao, Chem. Rev., 1993, 93, 2207‐2293; (b) S.
E. Denmark and J. Fu, Chem. Rev., 2003, 103, 2763‐2793.
17 H. Shimizu, I. Nagasaki, K. Matsumura, N. Sayo and T. Saito, Acc.
Chem. Res., 2007, 40, 1385‐1393.
95 18 (a) H. Kurosawa and A. Urabe, Chem. Lett., 1985, 1839‐1840; (b) H.
Kurosawa, A. Urabe, K. Miki and N. Kasai, Organometallics, 1986, 5,
2002‐2008.
19 Dramatic solvent effect on the enantioselectivity described in ref. 12
would reflect susceptibility of the transmetalation in each solvent.
1989, 30, 617‐620.
21 (a) Y. T. Han, N.‐J. Kim, J.‐W. Jung, H. Yun, S. Lee and Y.‐G. Suh, Arch.
Pharm. Res., 2011, 34, 1437‐1442; (b) J. Agarwal, C. Commandeur, M.
Malacria and S. Thorimbert, Tetrahedron, 2013, 69, 9398‐9405.
23 (a) N. A. Cortese and R. F. Heck, J. Org. Chem., 1978, 43, 3985‐3987;
(b) S. Cacchi, E. Morera and G. Ortar, Tetrahedron Lett., 1984, 25,
4821‐4824.
24 The absolute configurations of major enantiomers were determined
by Mosher’s method. (a) I. Ohtani, T. Kusumi, Y. Kashman and H.
Kakisawa, J. Am. Chem. Soc., 1991, 113, 4092‐4096; (b) J. A. Dale, D.
L. Dull and H. S. Mosher, J. Org. Chem., 1969, 34, 2543‐2549; (c) J. A.
Dale and H. S. Mosher, J. Am. Chem. Soc., 1973, 95, 512‐519.
25 J. Tsuji and I. Minami, Acc. Chem. Res., 1987, 20, 140‐145.
110
1971, 93, 2642‐7768; (b) P. Corradini, G. Maglio, A. Musco and G.
Paiaro, J. Chem. Soc., Chem. Commun., 1966, 618‐619; (c) M. Tsutsui
and A. Courtney, Adv. Organomet. Chem., 1977, 16, 241‐282; (d) J.
W. Faller and M. T. Tully, J. Am. Chem. Soc., 1972, 94, 2676‐2679.
120 27 (a) M. Yoshida, K. Matsuda, Y. Shoji, T. Gotou, M. Ihara and K.
Shishido, Org. Lett., 2008, 10, 5183‐5186; (b) T.‐H. Huang, H.‐M.
Chang, M.‐Y. Wu and C.‐H. Chang, J. Org. Chem., 2002, 67, 99‐105.
28 (a) K. K. D. Amarasinghe and J. Montgomery, J. Am. Chem. Soc., 2002,
124, 9366‐9367; (b) C.‐M. Yu, J. Youn and J. Jung, Angew. Chem. Int.
125
Ed., 2006, 45, 1553‐1555; (c) H. Huang and C. J. Forsyth, J. Org.
Chem., 1995, 60, 2773‐2779; (d) P. Yuan, R. Plourde, M. R.
Shoemaker, C. L. Moore and D. E. Hansen, J. Org. Chem., 1995, 60,
5360‐5364; (e) M. L. Falck‐Pedersen and K. Undheim, Tetrahedron,
2005, 61, 6829‐6835.
130
60 10 W. Wang, T. Zhang and M. Shi, Organometallics, 2009, 28, 2640‐
2642.
11 (a) C. D. Hopkins and H. C. Malinakova, Org. Lett., 2004, 6, 2221‐
2224; (b) C. D. Hopkins, L. Guan and H. C. Malinakova, J. Org. Chem.,
2005, 70, 6848‐6862; (c) C. D. Hopkins and H. C. Malinakova, Org.
Lett., 2006, 8, 5971‐5974; (d) X. Yu and X. Lu, Org. Lett., 2009, 11,
4366‐4369; (e) X. Yu and X. Lu, J. Org. Chem., 2011, 76, 6350‐6355.
12 H. Tsukamoto, T. Matsumoto and Y. Kondo, Org. Lett., 2008, 10,
1047‐1050.
65
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