C. A. Lewis
Baran Group Meeting
1/23/2010
Samarium (II) Iodide
0) Additives for SmI2 cont'd
Water/Amine Mixtures
O
Reduction using water
Me
O
SmI2 (4 equiv.)
Me
OH
Me
Me
Me
Me
H2O or amine - 7 days < 20%
H2O and amine - < 10s > 99%
Aromatic only
OH
OH
SmI2
H2O (56 equiv.)
3 min, 90%
Amine = NEt3, TMEDA, PMDTA
Dahlen, A.; Hilmersson, G. Tetrahedron Lett. 2002, 43, 7197-7200.
O
OH
Ph
Ph
Ph
99%
Ph
O
O
100%
Ph
EtO
O
Ph
S
Ph
Me
D
SmI2 (3 equiv.)
D2O (excess)
30 min, 75%
> 98% ds
OH
Alcohols
Me
Product
Alkene
Product
Ph
Curran. J. Org. Chem. 1993, 58, 5008.
O
> 99%
> 99%
SmI2 2.5 equiv per olefin
Et3N 5 equiv. or TMEDA 2.5 equiv. or PMDTA 1.7 equiv.
H2O 6.25 equiv.
Alkene
S
Ph
99%
5 min
SmI2 (2.2 equiv.)
H2O (33 equiv.)
60 min
Ph
EtO
20 s
5 min., > 99%
5 min., > 99%
10 s, > 99%
No reaction
O
Me
OH
D
Concellon, Chem. Eur. J. 2003, 9, 1775.
O
OH
Me
Me
20%
R
Relative Rate
O
R
HO
Me
~0.3
R'
OH
1
R'
OH
O
O
5
Me
HO
O
Me
OH
Me
12%
Me
Me
70
Me
Me
Me
Me
Me
Me
12%
Me
Me
Dahlen, A.; Hilmersson, G. Tetrahedron Lett. 2001, 42, 5565-5569.
Me
56%
Dahlen, A.; Hilmersson, G. Tetrahedron Lett. 2003, 44, 2661-2664.
C. A. Lewis
Baran Group Meeting
1/23/2010
Samarium (II) Iodide
1) Barbier
Acyl Substitution
Intermolecular
O
+
Me
O
1) SmI2 (2 equiv.)
2) 0.1 N HCl
BnBr
30 min.
69%
Me
Me
Me
O
OH
O
Me
SmI2 (2 equiv.)
THF, Fe(III) cat.
OH
91%
Me
Ph
I
Girard, P.; Namy, J. L.; Kagan, H. B. J. Am. Chem. Soc. 1980, 102, 2693.
O
X
+
Molander, J. Org. Chem. 1993, 58, 7216.
I
OH
1) SmI2 (2 equiv.)
2) H2O
3
OH
O
SmI2 (4 equiv.)
THF, HMPA
O
X = Br 6 min. 54%
X = I 2 min. 85%
H
OH
67%
H
3
H
Cl
Souppe, J.; Namy, J. L.; Kagan, H. B. Tetrahedron Lett. 1982, 23, 3497.
Molander, J. Org. Chem. 1991, 56, 1439.
Intramolecular
O
Applications to Total Synthesis
O
O
Me
HO Me O
SmI2 (2 equiv.)
-78 °C to rt
Me
O
100%, 92% dr
Me
I
Molander, J. Am. Chem. Soc. 1987, 109, 453.
H
Br
Br
O
O
H H H
SmI2 (9 equiv.)
HMPA
68%
HO
OH
Me
I
NMe2
H
SmI2, THF
O
H
Me
O
60%
OMe
Me
Me
Me
Single Diastereomer
(Unknown Identity)
OMe
OH
H
OMe
TBSO
Me
O
TBS
Me
HO
H
Molander, J. Org. Chem. 1998, 63, 9031.
O
Br
O
TBS
Me
Me
OMe
O
82%
O
O
Me
Me
TBSO
H
Cook, J. Org. Chem. 1988, 53, 2327.
COCl SmI (2 equiv.)
2
THF, -78 °C
H
Me
Me
H
O
Me
SmI2 (2 equiv.)
-78 °C to rt
aq. NaHCO3
74%
HO
Me
CONMe2
+
HO
CONMe2
Me
11:1
Molander, J. Org. Chem. 1987, 109, 453.
Me
Me
Me
Me
H
OH
O
O
H
TBSO
Me
HO
kendomycin
Me
O
Panek, Org. Lett. 2008, 10, 3813.
C. A. Lewis
2) Reformatsky / Aldol Type
5-Membered Rings
O
O
X=
halogen
or sulfone
R"
SmI2
R"
R
O
SmI2
O
SmI2
X
R
R'
H H
R"
R
R'
H
R'
O
SmI2
X
R
R'
SmI2
X
R
R"
O
R'
Skrydstrup, J. Org. Chem. 2002, 67, 2411.
R'
O
Br
N
O
O
Br
O
O
SmI2
O
Ph
O
t-Bu
Me
t-Bu
71%
O
O
O
Br BnO Me
O
SmI2
Me
Me OBn
O
O
O
O
Me
Molander, J. Am. Chem. Soc. 1991, 113, 3889.
Cl
NC
O
Me
-45 °C, 93%
HO
Me
5:1 dr
OH
Me
PMBO
O
Me
7 equiv. SmI2
OTIPS
-78 °C, 70%
~5:1 dr
OBn
O
O
O
CN
H
TBSO
O
Mukaiyama, Chem. Lett. 1997, 1139.
Mukaiyama, Chem. Eur. J. 1999, 5, 121.
4-Membered Rings
O
Me
O
OTBS OPMB
5 equiv. SmI2
Me
Me
O
OH OTBS
O
OH
65%
> 99% dr
NH
Matsuda, J. Org. Chem. 1997, 62, 1368.
BnO
t-Bu
Me
Br
N
Taxol Synthesis - Part 1
Molander, J. Am. Chem. Soc. 1987, 109, 6556. Me
O
O
-78 °C, 85%
OH
Ph
SmIII
Me
O
2 equiv. SmI2
3'-keto-nucleosides unstable
H
Ph
NH
O
OTBS
1,3-Asymmetric Induction
O
O
O
O
O
28%
6-Membered Rings
R"
R"
EtO2C
55%
SmI2
R
HN
OH
CO2Et
EtO2C
SmI2
+
N
O
OBz
O
O
SmI2 (2.5 equiv.)
H O
N
X=
SmI2
carbonyl etc
O
Baran Group Meeting
1/23/2010
Samarium (II) Iodide
BzO
O
O
Me
OTIPS
R
O
Me
O
O
Me
R = !-Glu OH
paeoniflorigenin
Corey, J. Am. Chem. Soc. 1993, 115, 8871.
O
Me
H
OMe
Me
SmI2, THF
O
Me
O
O
Aldol or Pinacol?
Me
0 °C, 74%
HO
Me
HO
O
O
Me
O
H
OMe
Me
O
Me
Arseniyadis, Eur. J. Org. Chem. 2005, 4082.
C. A. Lewis
3) Carbonyl Alkene/Alkyne Coupling
O
!-unsaturated aldehydes/ketones dominate
Ring sizes of 3 and up accessible
SmIII
SmIII
CO2Me
O
O
R
R
SmI2 (2 equiv.)
THF, t-BuOH
0 °C to rt
CHO
Me
Z = 60%
E = 87%
CO2Bn
SmIII
OH
O
CO2Bn
Guibe, Tetrahedron Lett. 2002, 43, 9517.
CHO
Me
SmI2 (2 equiv.)
THF, MeOH, 0 °C
Me
CO2Bn
SmI2 (2 equiv.)
THF, MeOH, -78 °C
CO2Et
Me
Me
Possible T.S.
O
TBSO
O
O
Me
Me
Me
O
Me
HO
Me
H
OO
O
CO2Me
69%, >99% dr
Me
O
TBS
Me
H
H
O
Me
O
O
O
TBS
O
O
H
Me
Me
Minor
O
O
CO2Me
TBSO
HO
H
O
H
CO2Me
Carbohydrates
MeO2C
Me
Major
O
Me
O
O
Me
Molander, J. Am. Chem. Soc. 1989, 111, 8236.
SmI2 (2 equiv.)
THF, MeOH, -78 °C
TBSO
H
O
O
CO2Me
CO2Me
CO2Me
H
CO2Me
HO Me O
HO Me O
Me
HO
Me
H
OO
Barbier 100%
Me
Me
Me
OH
Procter, J. Chem. Soc., Perkin Trans. 1 2000, 681.
Syn Coupled Products
O O
SmI2 (2 equiv.)
t-BuOH, -78 °C
O
Me
87%
OTBS
O
O
Me
Me
CO2Et
O
TBSO
73%, >99% dr
O
CO2Me
Me
Me
Me
65%
O
Me
Me
HO
O
CO2Bn
O
O
CO2Me
TBSO
SmIII
O
Me
TBSO
Me
O
Me
CO2Me
64% major 4:1 dr
O
O
Me
Me
Me
HO
SmI2 (2 equiv.)
THF, MeOH, -78 °C
TBSO
Anti Coupled Products
Me
Baran Group Meeting
1/23/2010
Samarium (II) Iodide
CO2Me
TBSO
Me
OO
O
H
HO
Me
Sm
CO2Me
H
H
TBSO
O
O
Me
"It is particularly noteworthy that a single stereochemical result was obtained from four possible products."
Me
O
OMe
O
O
Me
Me
Only Diastereomer
Enholm, J. Am. Chem. Soc. 1989, 111, 6463.
C. A. Lewis
Baran Group Meeting
1/23/2010
Samarium (II) Iodide
3) Carbonyl Alkene/Alkyne Coupling
En route to Isocarbacyclin
!-Lacones
SmI2
i-PrOH/THF/HMPA
1 min. 0 °C, 95%
O
+
CO2Me
SmI2, 2 equiv.
t-BuOH, THF
-70 °C, 30 min.
OHC
O
O
(no HMPA, 4h, 82%)
71%, 9:1 dr
HO
H
H
Bu
Inanaga, Tetrahedron Lett. 1986, 27, 5763.
O
TBSO
OTBS
H
Cyclooctanes
Me
Bu
OTBS
TBSO
SmI2, 2 equiv.
t-BuOH, HMPA
rt, 1.5 h
Me
OH
Me
Me
OSmI2
R
Me
60%, > 30:1 dr
Me
TBSO
H
Koruzumi, Tetrahedron 1990, 46, 6689.
Molander, J. Org. Chem. 1994, 59, 3186.
MeO2C
R = Bu
8-endo-trig
OMe
Me
MeO2C
MeO
SmI2, 2 equiv.
t-BuOH, HMPA
rt, 1.5 h
Me
Me HO
SiMe3
Bu 76%
OMe
O
MeO
Me
MeO
O
Me
MeO2C
R
OMe
R = TMS
Reissig, Tetrahedron 2000, 56, 4267.
Me
MeO
R = Bu or TMS
52%
Reissig, Synlett 2004, 422.
Me
H
OH
MeO
OH
SmI2, 2 equiv.
t-BuOH, HMPA
MeO
85%, > 18:1 dr
MeO
Me
Me
MeO
OMe
isoschizandrin
Me
Applications to Total Synthesis
Me
O
SmI2, 2 equiv.
Me t-BuOH, HMPA
55%
(32% dimer)
N
Boc
OTBS
Me
HO
H
CO2H
H
O
N
Boc
OTBS
N
H
OH
"-kainic acid
Cossy, Synlett. 1998, 507.
Molander, J. Org. Chem.
2003, 68, 9533.
C. A. Lewis
3) Carbonyl Alkene/Alkyne Coupling
A Successful Alkyne
O
Me
O
Polyethers
Me
O
H
Me
O
H
SmI2, 2.2 equiv.
MeOH, THF
0 °C, 1 h
O
H
H
Me
O
OH
Me
91%
CO2Et
O
Me
O
H
H
H
For brevetoxin B
Nakata, Tetrahedron Lett. 1999, 40, 2811.
O
H
O
H
H
O
O
CO2Et
Me
H
SmI2, THF
MeOH, rt
Ph
O
H
H
H
Me
O
H
O
O
Me
O
O
EtO2C
O
O
O
O
Me
39%
O
OBn
O
En route to mucocin
67%
Ar
Nicolaou, Angew. Chem. Int. Ed. 2006, 45, 7086.
Enone
Cl
Me
SmI2, LiCl
t-BuOH, THF
-78 °C
O
H
CbzHN
SmI2, t-BuOH
THF, -78 °C
CO2Me
O
Me
Me
Ar
75%
NCO
Me
Me
Me
OH
Me
Nicolaou, Angew. Chem. Int. Ed. 2007, 46, 3942.
Me
S(4-py)
Me
TFA
87%
O
Thioester-acrylate
CbzHN
O
Me
CO2Et
CO2Me
O
SmI2, HFIP
HMPA, THF
-78 °C
Nakata, Angew. Chem. Int. Ed. 2002, 41, 4751.
O
TFA
25%
(2 steps)
H
O
87%
O
OH
Me
O
OBn
O
Me
Me
Pancrazi, Tetrahedron Lett. 1996, 37, 5523.
SmI2, HFIP
HMPA, THF
-78 °C
Me
SmI2, 3 equiv.
MeOH, THF HO
-5 °C, 15 min.
O
Local Heroes - Platensimycin
O
Multiple Aldehydes
Me
O
H
Me
46%, 2:1 dr
For yessotoxin
Nakata, Org. Lett. 2002, 4, 3943.
O
Me
Me
O
84%
OH
Me
TMS
O
O
O
O
SmI2, 10 equiv.
TMS t-BuOH, THF
-78 °C, 1 h.
90%
H
Me O
CO2Et
O
Nakata, Tetrahedron Lett. 2000, 41, 7673.
Ph
Baran Group Meeting
1/23/2010
Samarium (II) Iodide
Me
Skrydstrup, J. Org. Chem. 2006, 71, 4766 and 8219.
After DBU of cyclic carbamate
Cl
Me
Cl
H
O
SmI2
Me
Me
N
H
O
Me
Me
O
C O
N
H
Wood, J. Am. Chem. Soc. 2008, 130, 2087.
C. A. Lewis
Baran Group Meeting
1/23/2010
Samarium (II) Iodide
4) Pinacol
Inositols
OBn
I
O
I
Sm
O
O
O
SmI2
R
BnO
R
R'
OBn
SmI2, t-BuOH
-78 °C to rt
O
BnO
BnO
OH
OBn
HO
Sm
OH
R
If alpha alkoxy is present, generally
proceeds anti
I
SmI2
R'
O
OBn
OBn
SmI2,6 equiv.
-78 °C
O
BnO
BnO
OH
I
O
Me
Me
OEt
SmI2, MeOH
-78 °C
I
Me
Sm
77%, 200:1
O
BnO
BnO
O
OH
O
Me
O
Me
H
BnO
HO
CO2Et
O
BnO
BnO
TIPSO
Me
BnO
OTIPS
CO2Et
OH
OTIPS
BnO
SmIII
BnO
BnO
TIPSO
O
H
Major
HO
OH
+
OTIPS
O
OBn
70%, > 20:1 dr
BnO
R'
OH
OBn
Mioskowski, Tetrahedron Lett. 1994, 35, 6671.
O
R
BnO
OBn
High degree of stereoselectivity
for intramolecular cyclizations
I
OH
BnO
+
60%
93:7 syn:anti
O
R'
OBn
OH
BnO
H
O
O
SmIII
Minor
d'Alarcao, Tetrahedron: Asymmetry 1998, 9, 2783.
Molander, J. Org. Chem. 1988, 53, 2132.
Me
The anti Effect
MeO2C
MeO2C
O
SmI2, t-BuOH
Me -78 °C to rt
81%
TBDPSO
MeO2C
Me
TBDPSO
OH
MeO
SmI2, DMA, THF
then
FmocValOH, EDC
HOBt, DMF
N
O
Me
TBDPSO
N
N
Cbz
+
OH
O
Me
O
O
45-50%
OH
OH
N
OBn
96:4
NH
MOM
Me
Me
Hanessian, Tetrahedron Lett. 1991, 32, 1125.
SmI2,
O t-BuOH
BnO
O
BnO
Me
OBn
OBn
64%
Me
BnO
BnO
BnO
SmIII
O
BnO
OH
Me
BnO
O
N
HN
N
Cbz
O
H
O
FmocHN
HO
O
OH
Me
N
Me
Iadonisi, Tetrahedron 1997, 53,11767.
Nicolaou, Angew. Chem. Int. Ed. 2003, 42, 1753.
OBn
NH
MOM
C. A. Lewis
Baran Group Meeting
1/23/2010
Samarium (II) Iodide
5) Heterocycles (Forming C-N or C-O Bonds)
6) Fragmentation
OAc
O
Chiral Auxiliary
O
Me
Ph
O
Me
O
O
Bu
O
Me
Me
O
Me
O
Me
OH OH
Me
OH OH
Ph
94% ee, 97:3 dr
SmI2,MeOH
HMPA, THF, 0 °C
Me
Me
O
NMe2
Me
O
Me
Me
100%
TBSO
Bu
Me
Me
Motherwell, Tetrahedron Lett. 1991, 32, 6211.
Me
Bu
SmI2,t-BuOH
-78 °C, 55-57%
58%
Me
NMe2
O
1) SmI2,THF, DMPU
2) AcCl
TBSO
O
O
O
OH
O
Me
Ph
Kuwajima, J. Am. Chem. Soc. 1998, 120, 12980.
Ph
Fukazawa, J. Am. Chem. Soc. 1997, 119, 1482.
Ring-Expansion
TBDMSO
EtO2C
Me
TBDMSO
SmI2,t-BuCOOH
HMPA, THF, 0 °C
Bu
N
H
70%
O
N
H
Me
O
Bu
O
N
H
Me
Bu
adalinine
Honda, Org. Lett. 2000, 2, 3925.
Me O
Me O
H
SmI2, THF
H
H
O
I2SmO
O
O
O
PhCHO/SmI2
THF, 0 °C
Me
O
OH
Me
Me
Me
Me
Hulme, Tetrahedron Lett. 1997, 38, 8245.
H
H
Me
H
Sm
Me
H
R
O
H
OH
H
O
O
OH
H
then H2O
O
via
Me
Me O
Me
Me
OSmI2
Intermediate Proposed by Authors
Me
Me
H
H
Evans-Tischenko
CHO
OH
H
O
cyclocitrinol
HO
O
Schmalz, Synlett 2007, 1881.
C. A. Lewis
6) Fragmentation cont'd
More Local Heroes
En route to the guanacastepenes
O
PMP
O
O
O
PMP
O
SmI2,MeOH
THF then PhSeBr
O
Me
Me
Me
Baran, J. Am. Chem. Soc. 2008, 130, 7241.
O
HO
Me
DMPU as additive
40% 1.5:1 dr
H
OSmLn
R1
R5
R3
O
paeonilactone B O
R1
R3
H
SmI2
Allyl Samarium
Me
O
O
Me
O
O O
Me
OH
Me
O
HO
Me
OH
Me
Me
Molander, J. Org. Chem. 1997, 62, 2935.
H
Me
O
OH
88%, 12:1 dr
Me
Me
OH
SmI2, HMPA
52%, > 200:1 dr
SmI2, HMPA
THF, MeOH, rt
Me
Me
OH
Up to 33%
Molander, J. Org. Chem. 1996, 61, 5885.
Mechanism Fun
Me
R4
Aurrecoechea, Tetrahedron Lett. 1995, 36, 7129.
OSmLn
DMPU
less coordinating
no clear preference
SmI2
R6
R5
R3
Generally Favored Product
E/Z ratios from 1.5:1 to exclusive
Kilburn, Chem. Commun. 1998, 1875.
O
R5
R6
R1
I2SmO
Me
Me
+
SmI2, HMPA
R6
R2 OH OH
OH
H
O
HMPA
more coordinating
favors pseudoequatorial
R2 OH R4
O
+
H
O
H
R4
O
H
O
H
Epoxides
R2
O
H
63%, 10:1 dr
H
TMSO
TMSO
Me
HO
Me
SmI2, t-BuOH
HMPA 0 °C to rt
Br
[Br+]
TMSO
O
Sorensen, Org. Lett. 2002, 4, 2063.
Sorensen, J. Am. Chem. Soc. 2006, 128, 7025.
Me
I2SmO
Br
SePh
Me
Me
Me
O
O
50%
Me
Baran Group Meeting
1/23/2010
Samarium (II) Iodide
SmI2
O
OH
Me
O
SmI2
Me
Molander, Tetrahedron 1997, 53, 8067.
O
Me
SmI2 6 equiv.
MeOH, HMPA, 0 °C
64%, 3.6:1 dr
HO
OH
Me
TMS
H
Molander, unpublished results
TMS
C. A. Lewis
7) Deoxygenation
O
Bu
Taxol again
O
SmI2,t-BuOH
THF, rt, 3 h
Bu
Bu
Bu
76%
SmI2,THF
65 °C, 3 h
O
S
Et
p-Tol
O
S
Ph
S
Ph
S
Bu
94%
O
S
Ph
Bu
99%
O
O
Ph
Me
SmI2, Ac2O
THF -78 °C
Me
Me
H
HO BzO AcO
Me
Me
Me
92%
O
OTES
H
HO BzO AcO
O
Danishefsky, J. Am. Chem. Soc. 1996, 118, 2843.
Kagan, J. Am. Chem. Soc. 1980, 102, 2693.
O
O
OTES
Et
77%
O
Me
Me
High loading of SmI2, could
be useful?
p-Tol
Baran Group Meeting
1/23/2010
Samarium (II) Iodide
Bu
93%
S
Me
O
O
P
Ph
Bu
26%
9) Cascade
Me
O
O
Ph
Ph
H
76%
O
75%
SmI2 (22 to 44 equiv.), HMPA, THF, 20 to 65 °C, usually 1 min.
Me
Me
SmI2,
HMPA, THF
O
OH
O
Inanaga, Chem. Commun. 1989, 298.
8) Elimination
Me
Me
Me
Me
Me
O
Me
OMe
MeO
OH
Me
O
OH
Me
Tatsuta, Tetrahedron Lett. 2006, 47, 5415.
N
Br
O
N OTs
N
SmI2
O
SmI2
Me
Me
Me
SmI2
N
Br
O
N
76%
NH
Br
O
OEt
OMe
O
SmI2, THF,
then MeOH
Me
O
O
O
N
Me
O
Me
Me
Br
OSmI2
O
70%
Me
O
SmI2, i-PrOH
-78 to -20 °C
Me
O
Me
Me
Me
HN
O
Lindel, Angew. Chem. Int. Ed. 2005, 44, 2295.
O
Yamada, Tetrahedron Lett. 1993, 34, 1501.
C. A. Lewis
Baran Group Meeting
1/23/2010
Samarium (II) Iodide
9) Cascade
10) Other Sm Sources and Applications
Me
O
SmI2, HMPA
THF 0 °C
then p-TsOH
O
Me
Me
OH
H
Me
O
Pinacol with Mg
Me
SmI2 (0.1 equiv.)
TMSCl (3 equiv.)
Mg (8 equiv.)
58%
O
H
H
Curran, J. Am. Chem. Soc. 1988, 110, 5064.
SmI2 (cat.), O2
THF, benzene
CO2Me 10 to 25 °C
Me
15% 3:1 dr
SmI2 MW = 404, expensive and usually at least double the
mass of the substrate of interest is required
Catalytic Samarium Processes
Me
O
CO2Me
O
Me
CHO
Ph
OH
TMSCl is used to remove the Sm from the alcohol,
Endo, J. Am. Chem. Soc. 1996, 118, 11666.
generating SmI2Cl which is easier to reduce
O 1.4 equiv.
Mes
O
SmI2 (0.1 equiv.)
LiI (5 - 6 equiv.)
TMSOTf (2 equiv.)
Zn•Hg (excess)
CO2Me
O
Me
O
OOH
epi also isolated
O
O
O
78-84%
Ph
Ph
I
Me
H
O
O
CO2Me
O
Ph
Ph
57%
O
OOH
OH
67%
82%
O
Corey, Tetrahedron Lett. 1997, 38, 2045.
CO2Me
Me
O
O
CO2Me
O
Me
O
O
Ph
Mg (8 equiv.)
THF, 65 °C
N
H
Ph
Ph
Ph
Ph
Ph
~1:1 dr
SmI2
2.0 equiv. 96%
0.2 equiv. 42%
O
NH
HN
+
NH
Ph
Ph
Ph
HN
Ph
Arrunziata, Tetrahedron Lett. 1998, 39, 3333.
Mischmetall (La 33%, Ce 50%, Nd 12%, Pr 4%, Sm and others 1%, $12/kg)
CO2Me
Me
O
O
O
I
+
Me
O
SmI2 (0.2 equiv.)
1.4 equiv MM
O
hexyl
90%
HO Me
hexyl
(ethyl, benzyl also)
Namy, J. Org. Chem. 1999, 64, 2944.
Corey, Tetrahedron Lett. 1994, 35, 539.
Namy, Eur. J. Org. Chem. 2002, 2989.