Enantioselective cyanosilylation of ketones catalyzed by Mn(salen)/Ph3PO

Article abstract of DOI:10.1016/j.tetasy.2006.04.011

TMSCN asymmetrically adds to a variety of ketones by catalysis with 1/Ph₃PO. This is a double activation where 1 acts as a Lewis acid and Ph₃PO as a Lewis base. Various ketones were subjected to the enantioselective addition to give

Full text of DOI:10.1016/j.tetasy.2006.04.011

Tetrahedron: Asymmetry 17 (2006) 1165–1169
Enantioselective cyanosilylation of ketones catalyzed by
Mn(salen)/Ph3PO
Sung Soo Kim,* Sang Hyuck Lee and Ju Myung Kwak
Department of Chemistry, Inha University, Incheon 402-751, South Korea
Received 27 January 2006; revised 10 April 2006; accepted 13 April 2006
Abstract—TMSCN asymmetrically adds to a variety of ketones by catalysis with 1/Ph3PO. This is a double activation where 1 acts as a
Lewis acid and Ph3PO as a Lewis base. Various ketones were subjected to the enantioselective addition to give up to 85% ee.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
of methyl ketones.18 Jacobsen has proven that chiral thio-
urea is a very effective catalyst for the enantioselective
cyanosilylation of ketones.19 Feng has succeeded in the cat-
alytic asymmetric cyanosilylation of ketones with a chiral
amino acid salt.20 The enantioselective cyanosilylation of
aldehydes and ketones employing Al(salen)/Ph3PO was
developed by us.21–23 We also investigated the similar reac-
tions for aldehydes utilizing 1/Ph3PO.24 Herein, we report
the cyanosilylation of ketones employing 1/Ph3PO as a
catalyst.
Asymmetric addition of trimethylsilyl cyanide (TMSCN)
to carbonyl compounds and subsequent hydrolysis pro-
duced chiral cyanohydrins that are useful intermediates
for the synthesis of numerous pharmaceuticals.1–3 Such
chiral compounds are useful intermediates for the synthesis
of pharmaceuticals. The two functional groups (–OH and
–CN) can be easily transformed into various homochiral
ones including a-hydroxy acids,4,5 a-hydroxy aldehydes,6
a-hydroxy ketones,6 b-hydroxy amines5,6 and a-amino acid
derivatives.7
Ph
N
Ph
N
Snapper and Hoveyda have established that a chiral pep-
tide ligand and Al(OiPr)3 can effectively promote the enan-
tioselective addition of TMSCN to ketones.8 Belokon and
North reported the asymmetric addition of TMSCN to
ketones catalyzed by a bimetallic, chiral (salen) titanium
complex.9 Shibasaki has disclosed the first general catalytic
enantioselective cyanosilylation of ketones by a novel
bifunctional catalyst containing Ti(IV).10,11 Feng has
described the enantioselective cyanosilylation of ketones
employing chiral salen-Ti(IV) complex as the Lewis acid
and achiral N-oxide as the Lewis base.12,13 Shibasaki has
carried out enantioselective cyanosilylation of ketones uti-
+
Mn
O
O
-
Cl
1
2. Results and discussion
The amount of Mn(salen) was compared for the best ee,
which indicates that 5 mol % gives a better outcome than
10 mol % (entries 1 and 2). The additive quantity was also
varied (entries 2–6) with 50 mol % (entry 5) proving to be
the optimal amount. The catalyst amount was further re-
duced to 1 mol % from 5 mol %, giving rise to negative ef-
fects in terms of reaction time, yield and ee (entries 5 and
7). Temperature decreases from rt to 0 °C greatly lengthen
the reaction time (45–96 h) with lower yields (entries 5 and
8). CH2Cl2 was better solvent than Et2O, THF and toluene
lizing the chiral ligand complexed with Gd(OiPr)3 and
14
Sm(OiPr)3,15 respectively. Deng has used chiral cinchona
alkaloid catalysts for the enantioselective cyanosilylation
of ketones.16,17 Corey has shown that a chiral oxazaboro-
lidinium salt is an excellent catalyst for the cyanosilylation
*
Corresponding author. Tel.: +82 32 864 4966; fax: +82 32 867 5604;
e-mail: sungsoo@inha.ac.kr
0957-4166/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.04.011

1166
S. S. Kim et al. / Tetrahedron: Asymmetry 17 (2006) 1165–1169
˚
(entries 9–11). Addition of CF3CH2OH, NMO and 4 A
molecular sieves, respectively, to the additive Ph3PO
showed no favourable influence for the ee. The reaction
conditions of entry 5 of Table 1 were chosen for enantios-
elective cyanosilylation of ketones. Numerous ketones have
undergone cyanosilylation under these conditions (Table
2). The role of the catalyst during the reaction is shown
in Figure 1. Both Mn of Mn(salen) and silicon atom of
TMSCN could have bonded to carbonyl oxygen. Ph3PO
may assist CN of TMSCN to make a bond with carbonyl
carbon. The possible mechanism and transition state21–24
involved in this catalytic reaction is given in Figure 1.
H3C
Ar
N
C
PO
Ph3
Si
O
CH3
Ar
OSiMe3
CN
O
Mn
N
N
O
H
H
Figure 1. Transition state involved in the enantioselective cyanosilylation
of ketones by double-activation catalysis.
Substituted acetophenones (entries 1–7) show significant
variation of reactivities in terms of reaction time and ee.
The yields varied from 82% to 93%. Moderately electron-
withdrawing groups (entries 2, 3 and 4) reduce the reaction
time (from 45 to 20–36 h) and increase the yield. p-Bromo-
acetophenone gave the highest ee of 85% among the pres-
ent reactions. Weakly electron-attracting substituents
(entry 5) increase the reaction time with similar yield and ee
(compare entries 1 and 5). Strongly electron-withdrawing
Table 1. Enantioselective cyanosilylations of ketones catalyzed by Mn(salen) under various conditionsa
Entry
Substrate
Solvent
Mn(salen) (mol %)
PO(Ph)3 (mol %)
Temperature (°C)
Time (h)
% Yield
% ee
1
2
3
4
5
6
7
8
9
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Et2O
10
5
5
5
5
5
1
5
5
5
5
10
10
20
40
50
100
50
50
50
50
50
rt
rt
rt
rt
rt
rt
rt
0
49
45
50
50
45
32
45
96
56
80
48
50
55
71
83
82
83
67
12
90
39
38
41
50
60
62
63
61
48
61
50
21
28
Acetophenone
rt
rt
rt
10
11
THF
Toluene
a Substrate concentration is 1 M.
Table 2. Catalytic asymmetric cyanosilylation of ketones with Mn(salen)/Ph3POa
OSiMe3
H3C
O
Mn(salen), Ph3PO
CH2Cl2, r.t.
+
Me3SiCN
CN
R
R
CH3
Entry
Substrateb
r
Time (h)
% Yield
% eec
Config.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Acetophenone
0
45
20
26
36
50
10
6
45
38
7
82
91
90
89
82
93
89
38
75
90
95
94
89
56
63
63
57
85
60
46
50
55
57
82
75
72
60
58
Rd
Rd
Rd
—
—
—
—
—
—
—
—
—
Rd
—
3-Chloroacetophenone
4-Chloroacetophenone
4-Bromoacetophenone
4-Fluroacetophenone
3-Nitroacetophenone
4-Nitroacetophenone
4-Methoxyacetophenone
Isobutylrophenone
4-Methoxyphenylacetone
3,4-Dichlorophenylacetone
1-Phenylbutan-2-one
Benzylacetone
0.37
0.24
0.26
0.15
0.71
0.81
ꢀ0.12
—
—
—
—
—
5
32
10
70
1-Indanone
—
a 5 mol % of 1 and 50 mol % of Ph3PO were used for the cyanosilylations.
b Substrate concentration is 1 M.
c Determined by HPLC.8,10,11,14,22
d The reported specific reactions indicate positive values with (R)-configuration. Present specific rotations also carry a positive value.8,10,11,14,22,25

S. S. Kim et al. / Tetrahedron: Asymmetry 17 (2006) 1165–1169
1167
substituents (entries 6 and 7) greatly reduce the reaction
time with comparable yield but relatively low ee (46%
and 50%). Strongly electron-donating group (entry 8) gives
rise to poor yield and ee. Isopropyl phenyl ketones (entry 9)
produce somewhat low yield and ee. 1-(4-Methoxyphe-
nyl)propan-2-one and 1-(3,4-dichlorophenyl)propan-2-one
(entries 10 and 11) reacted quite fast (7 h and 5 h) with
comparatively high yield and ee (82% and 75%). 4-Phenyl-
butan-2-one (entry 13) reacts much faster than 1-phenyl-
butan-2-one (entry 12), but the latter shows a higher
yield and ee. 2,3-Dihydroinden-1-one reacts very slowly
to give a low yield (entry 14). Al(III) appears to have effec-
tive bonding with chiral peptide8 and chiral salen21–23 for
the formation of catalytic site. Ti(salen) complexes9,12,13
are also used for the catalytic enantioselective cyanosilyla-
tion of ketones. Here Mn site forms partial bond with
carbonyl oxygen for the enantioselective cyanosilylations
of ketones. Accordingly, the addition of TMSCN to
ketones occurs on the plane depicted in Figure 1.
2.35 mmol) were refluxed in absolute ethanol (10 mL) for
3 h. A small amount of water was added to the reaction
mixture, then it was allowed cool to 2 °C and kept at that
temperature for 2 h. The product was collected by suction
filtration to afford a yellow powder, yield 0.53 g (70%) mp
197–198 °C. IR (KBr): 3087, 3062, 3030, 2958, 2909, 2869,
1626, 1598, 1469, 1454, 1442, 1362, 1250, 1174, 876, 776,
1
700 cmꢀ1. H NMR: d 1.22 (18H, s, t-Bu), 1.42 (18H, s,
t-Bu), 4.72 (2H, s, CH–N), 6.98 (2H, d, J = 2.4 Hz, Ar–
H), 7.18 (10H, s, Ph), 7.31 (2H, d, J = 2.4 Hz, Ar–H),
8.40 (2H, s, CH@N), 13.59 (2H, s, OH). 13C NMR: d
29.4, 31.4, 34.0, 34.9, 80.0, 117.7, 126.2, 127.0,
127.3, 127.9, 128.1, 136.2, 139.7, 139.9, 157.8, 167.1.
HRMS (EI) m/z: calcd for C44H56N2O2 644.4342, found
644.4329.
4.2. Synthesis of [(S,S)-N,N0-bis(3,5-di-tert-butylsalicyli-
dene)-1,2-diphenylethylenediamine]chloromanganese(III)
Ph
N
Ph
N
Ph
N
Ph
N
Mn
Cl
t-Bu
O
O
t-Bu
t-Bu
OH HO
t-Bu t-Bu
t-Bu
t-Bu
t-Bu
3. Conclusion
Solid Mn(OAc)2Æ4H2O (0.40 g, 1.63 mmol) was added to
solution of (S,S)-N,N0-bis(3,5-di-tert-butyl-salicylid-
a
An effective double-activation process for cyanosilylation
of ketones has been developed utilizing chiral Mn(salen)/
Ph3PO as catalyst and additive. Moderately electron-with-
drawing substituents give rise to a relatively better out-
come. The electronic nature of the substituent has no
consistent effect on ee or yield. The presence of a methylene
group between benzene ring and carbonyl gives good ee
and yield. The mechanism of cyanosilylation is depicted.
We are continuing to explore for other optimal catalysts
for the enantioselective reactions.
ene)-1,2-diphenylethylenediamine (0.51 g, 0.79 mmol) in
absolute ethanol (10 mL), and the dark brown mixture
was refluxed for 2 h under air. Solid LiCl (0.11 g,
2.60 mmol) was then added and the mixture was refluxed
for an additional 2 h and then stirred at 70 °C overnight.
The reaction mixture was cooled, and then water was
added resulting in the precipitation of a brown powder,
which was collected by suction filtration. The powder
was redissolved in CH2Cl2 and extracted with water
and brine. The organic phase was dried over anhydrous
Na2SO4, and the solvent evaporated to afford a brown
powder, yield 0.54 g (93%), mp > 300 °C. IR (KBr):
3063, 3027, 2956, 2904, 2867, 1610, 1534, 1455, 1429,
1317, 1252, 1174, 857, 700, 579 cmꢀ1. MS(FAB) m/z
697.6 (MꢀCl)+. Anal. Calcd for C44H54ClMnN2O2Æ1/
4. Experimental
4.1. Synthesis of (S,S)-N,N0-bis(3,5-di-tert-butylsalicyli-
dene)-1,2-diphenylethylenediamine
Ph
N
Ph
N
CHO
Ph
Ph
+
t-Bu
OH
t-Bu
t-Bu
OH HO
t-Bu t-Bu
t-Bu
H2N
NH2
(S,S)-1,2-Diphenylethylenediamine (0.25 g, 1.18 mmol)
and 3,5-di-tert-butyl-2-hydroxybenzaldehyde (0.55 g,
2H2O: C, 71.19; H, 7.47; N, 3.77. Found: C, 71.36; H,
7.47; N, 3.66.

1168
S. S. Kim et al. / Tetrahedron: Asymmetry 17 (2006) 1165–1169
4.3. Silylcyanation of the ketones catalyzed by Mn(salen)
and Ph3PO
CEL OB-H, iPrOH/hexane = 0.5/99.5, flow = 0.25 mL/
min) 17.0 and 18.4 min.
To a stirred CH2Cl2 solution of Mn(salen) (5 mol %) and
POPh3 (50 mol %) was added a ketone (1mmol) and stirred
for 30 min at rt TMSCN (1.5 mmol) was added to the reac-
tion mixture using a syringe pump and the mixture reacted
at the same temperature for 5–50 h. The solvent was then
evaporated. The crude product was further purified by
flash chromatography (hexane/ethyl acetate = 9:1) to give
cyanohydrin in more than 75% yield. The sample was iden-
4.3.5. 2-Trimethylsilyloxy-2-(4-fluorophenyl)propanenitrile.
1H NMR (CDCl3) d 0.18 (s, 9H), 1.84 (s, 3H), 7.08 (m,
2H), 7.52 (m, 2H) 13C NMR (CDCl3) d 1.0, 33.5, 71.0,
22
115.6, 121.4, 126.5, 138.0, 162.2 ½aꢁD ¼ þ15:3 (c 1.4,
CHCl3, 60% ee) HPLC (DAICEL CHIRALCEL OB-H,
iPrOH/hexen = 1/99, flow = 0.25 mL/min) 16.4 and
17.8 min.
1
tified by H, 13C NMR, HRMS and ee % was determined
4.3.6. 2-Trimethylsilyloxy-2-(30-nitrophenyl)propanenitrile.
1H NMR (CDCl3) d 0.22 (s, 9H), 1.86 (s, 3H), 7.34–7.55
(m, 4H aromatic H); HRMS(M+) calcd for C12H16N2O3Si:
264.0930; found: 264.0935 HPLC (DAICEL CHIRAL-
by chiral HPLC column (DAICEL CHIRALCEL OJ-H,
DAICEL CHIRALCEL OD and DAICEL CHIRALCEL
OB-H). 1H and 13C NMR were taken utilizing Varian
Jemini 2000 (200 MHz) or Varian Unity Inova 400
(400 MHz) NMR spectrometer. Hewlett–Packard 5890A
Gas Chromatograph/Jeol JMS-DX303 Mass Spectrometer
was used for HRMS data. Analytical high performance
liquid chromatography (HPLC) was performed on Gilson
305 series HPLC using the indicated chiral column. All
data were in accordance with literature values. Absolute
configurations were determined by optical rotations.26
i
CEL OJ-H, PrOH/hexane = 0.5/99.5, flow = 1 mL/min)
32.6 and 34.4 min.
4.3.7. 2-Trimethylsilyloxy-2-(40-nitrophenyl)propanenitrile.
1H NMR (CDCl3) d 0.26 (s, 9H), 1.89 (s, 3H), 7.75 (d,
2H), 8.30 (d, 2H) 13C NMR (CDCl3) d 1.0, 33.5,
22
71.0, 115.6, 121.4, 126.5, 138.0, 162.2 ½aꢁD ¼ þ8:1 (c 1.62,
CHCl3, 50% ee) HRMS(M+) calcd for C12H16N2O3Si:
264.0930; found: 264.0933 HPLC (DAICEL CHIRAL-
4.3.1. 2-Trimethylsilyloxy-2-phenylpropanenitrile. 1H NMR
(CDCl3) d 0.19 (s, 9H), 1.87 (s, 3H), 7.38–7.58 (m, 5H,
CEL OJ-H, PrOH/hexane = 1/99, flow = 0.25 mL/min)
i
51.38 and 54.61 min.
aromatic H); 13C NMR (CDCl3) d 1.03, 33.5, 71.6, 121.6,
24
128.6, 142.0; ½aꢁD ¼ þ10:4 (c 1.14, CHCl3, 63% ee) {lit.
4.3.8.
2-Trimethylsilyloxy-2-(4-methoxylphenyl)propane-
20
½aꢁD ¼ þ21:9 (c 1.18, CHCl3, for the (R)-enantiomer with
nitrile. 1H NMR (CDCl3) d 0.1 (s, 9H), 1.73 (s, 1H),
93% ee)} HRMS(M+) calcd for C12H17NOSi: 219.1079;
found: 219.1082 HPLC (DAICEL CHIRALCEL OB-H,
iPrOH/hexane = 0.5/99.5, flow = 0.25 mL/min) 19.6 and
20.7 min.
2.71–2.93 (m, 2H), 3.75 (s, 3H), 6.76–7.23 (m, 4H);
20
½aꢁD ¼ þ18:2 (c 1.5, CHCl3, 82% ee) HRMS(M+) calcd
for C14H21NO2Si: 263.1342; found: 263.1345 HPLC (DAI-
CEL CHIRALCEL OJ-H, iPrOH/hexane = 0.25/99.75,
flow = 0.5 mL/min) 28.5 and 29.9 min.
4.3.2. 2-Trimethylsilyloxy-2-(3-chlorophenyl)propanenitrile.
1H NMR (CDCl3) d 0.22 (s, 9H), 1.86 (s, 3H), 7.34–7.55
(m, 4H aromatic H); 13C NMR (CDCl3) d 1.0, 33.4, 70.9,
121.0, 122.7, 124.8, 128.8, 129.9, 134.6, 144.0;
½aꢁD ¼ þ11:2 (c 1.3, CHCl3, 63% ee) {lit. ½aꢁD ¼ þ7:1 (c
0.34, CHCl3, for the (R)-enantiomer with 33% ee)}
HRMS(M+) calcd for C12H16ClNOSi: 253.0690; found:
253.0692 HPLC (DAICEL CHIRALCEL OB-H, iPrOH/
hexane = 0.25/99.75, flow = 0.25 mL/min) 29.8 and
32.8 min.
4.3.9. 2-Trimethylsilyloxy-2-phenyl-3-methyl-butanenitrile.
1H NMR (CDCl3) d 0.12 (s, 9H), d 1.03 (q, J = 7.4Hz
20
6H), 1.97 (m, 1H), 7.38 (m, 3H), 7.50 (m, 2H); ½aꢁD
¼
24
26
þ23:1 (c 2.1, CHCl3, 57% ee) HRMS(M+) calcd for
C14H21NOSi: 247.1392; found: 247.1395 HPLC (DAICEL
CHIRALCEL OD, iPrOH/hexane = 0.25/99.75, flow =
0.25 mL/min) 19.1 and 21.0 min.
4.3.10. 2-Trimethylsilyloxy-3-(4-methoxyphenyl)-2-methyl-
propanenitrile. 1H NMR (CDCl3) d 0.1 (s, 9H), 1.73 (s,
1H), 2.71–2.93 (m, 2H), 3.75 (s, 3H), 6.76–7.23 (m, 4H);
HRMS(M+) calcd for C14H21NO2Si: 263.1342; found:
4.3.3. 2-Trimethylsilyloxy-2-(4-chlorophenyl)propanenitrile.
1H NMR (CDCl3) d 0.21 (s, 9H), 1.83 (s, 3H), 7.38 (m,
2H), 7.49 (m, 2H); 13C NMR (CDCl3) d 1.05, 33.53,
71.06, 121.25, 126.07, 128.83, 134.60, 140.71;
i
263.1345 HPLC (DAICEL CHIRALCEL OJ-H, PrOH/
hexane = 0.25/99.75, flow = 0.5 mL/min) 28.5 and
29.9 min.
24
20
½aꢁD ¼ þ12:4 (c 1.68, CHCl3, 57% ee) {lit. ½aꢁ ¼ þ29:5
(c 1.04, CHCl3, for the (R)-enantiomer withD92% ee)}
HRMS(M+) calcd for C12H16ClNOSi: 253.0690; found:
253.0687 HPLC (DAICEL CHIRALCEL OJ-H, iPrOH/
hexane = 0.25/99.75, flow = 0.25 mL/min) 19.6 and
20.9 min.
4.3.11. 2-Methyl-2-(trimethylsilyloxy)-3-(3,4-dichlorophenyl)-
propanenitrile. 1H NMR (CDCl3) d 0.15 (s, 9H), 1.59 (s,
3H), 2.93 (m, 2H), 7.11–7.43 (m, 3H); HRMS(M+) calcd
for C13H17Cl2NOSi: 301.0456 found: 301.0461 HPLC
(DAICEL CHIRALCEL OJ-H, iPrOH/hexane = 1/99,
flow = 0.25 mL/min) 27.1 and 34.3 min.
4.3.4. 2-Trimethylsilyloxy-2-(4-bromophenyl)propanenitrile.
1H NMR (CDCl3) d 0.19 (s, 9H), 1.83 (s, 3H), 7.40–7.4 (m,
2H), 7.51–7.55 (m, 2H); 13C NMR (CDCl3) d 1.0, 33.4,
4.3.12. 2-Trimethylsilyloxy-2-benzyl-butanenitrile. 1H NMR
(CDCl3) d 0.1 (s, 9H), 1.1 (t, 3H), 1.79 (q, 2H), 2.99 (s, 2H),
7.25–7.36 (m, 5H); HRMS(M+) calcd for C14H21NOSi:
247.1392; found: 247.1398; HPLC (DAICEL CHIRALCEL
22
71.0, 121.1, 122.7, 126.3, 131.7, 141.2; ½aꢁD ¼ þ20:7 (c
1.65, CHCl3, 85% ee) HRMS(M+) calcd for C12H16BrNO-
Si: 297.0185; found: 297.0181 HPLC (DAICEL CHIRAL-

S. S. Kim et al. / Tetrahedron: Asymmetry 17 (2006) 1165–1169
1169
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and 21.5 min.
4.3.13. 2-Trimethylsilyloxy-2-methyl-4-phenylbutanenitrile.
1H NMR (CDCl3) d 0.27 (s, 9H), d 1.61 (s, 3H), 2.02 (m,
2H), 2.78 (d, 2H) 2.87 (d, 2H), 7.19–7.22 (m, 3H), 7.28–
20
7.3 (m, 2H); ½aꢁD ¼ þ9:2 (c 1.8, CHCl3, 60% ee) {lit.
24
½aꢁD ¼ þ13:3 (c 1.15, CHCl3, for the (R)-enantiomer with
81% ee)} HRMS(M+) calcd for C14H21NOSi: 247.1392;
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16. Tian, S.-K.; Deng, L. J. Am. Chem. Soc. 2001, 123, 6195.
17. Tian, S.-K.; Hong, R.; Deng, L. J. Am. Chem. Soc. 2003, 125,
9900.
2.97–3.02 (m, 1H), 3.10–3.15 (m, 1H), 7.28 (d, 1H), 7.31 (t,
23
1H), 7.36 (t, 1H), 7.55 (d, 1H); ½aꢁD ¼ þ14:4 (c 1.4, CHCl3,
58% ee) HRMS(M+) calcd for C13H17NOSi: 231.1079;
found: 231.1082 HPLC (DAICEL CHIRALCEL OD,
iPrOH/hexane = 1/99, flow = 0.25 mL/min) 4.1 and 4.7 min.
18. Ryu, D. H.; Corey, E. J. J. Am. Chem. Soc. 2005, 127, 5384.
19. Fuerst, D. E.; Jacobsen, E. N. J. Am. Chem. Soc. 2005, 127,
8864.
20. Liu, X.; Qin, B.; Zhou, X.; He, B.; Feng, X. J. Am. Chem.
Soc. 2005, 127, 12224.
Acknowledgement
We warmly thank The Center for Biological Modulators
for the financial support.
21. Kim, S. S.; Song, D. H. Eur. J. Org. Chem. 2005, 1777.
22. Kim, S. S.; Kwak, J. M. Tetrahedron 2006, 62, 49.
23. Kim, S. S. Pure Appl. Chem., in press.
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