Asymmetric sulfoxidation of prochiral sulfides using aminoalcohol derived chiral C3-symmetric trinuclear vanadium Schiff base complexes

Article abstract of DOI:10.1016/j.tetasy.2007.11.005

A series of trimeric variants of the efficient and well known Bolm's chiral vanadium salen catalysts are reported. These C₃-symmetric trinuclear chiral Schiff bases were synthesized by condensing a variety of trialdehydes with optically active aminoalcohols. The catalytic activity of the chiral vanadium complexes of these ligands was investigated for the enantioselective oxidation of prochiral sulfides using hydrogen peroxide as an oxidant. The procedure afforded the corresponding sulfoxide in good yield and the enantioselectivities were comparable to those obtained with the mononuclear complexes.

Full text of DOI:10.1016/j.tetasy.2007.11.005

Available online at www.sciencedirect.com
Tetrahedron: Asymmetry 18 (2007) 2820–2827
Asymmetric sulfoxidation of prochiral sulfides using aminoalcohol
derived chiral C3-symmetric trinuclear vanadium
Schiff base complexes
Paulsamy Suresh,a Sankareswaran Srimurugan,a Balaji Babub and Hari N. Patic,*
aDepartment of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
bDepartment of Chemistry, J.N.T. University, Hyderabad 500 072, India
cDepartment of Chemistry, Sambalpur University, Jyoti Vihar 768 019, India
Received 20 October 2007; accepted 7 November 2007
Abstract—A series of trimeric variants of the efficient and well known Bolm’s chiral vanadium salen catalysts are reported. These C3-
symmetric trinuclear chiral Schiff bases were synthesized by condensing a variety of trialdehydes with optically active aminoalcohols.
The catalytic activity of the chiral vanadium complexes of these ligands was investigated for the enantioselective oxidation of prochiral
sulfides using hydrogen peroxide as an oxidant. The procedure afforded the corresponding sulfoxide in good yield and the enantioselec-
tivities were comparable to those obtained with the mononuclear complexes.
ꢀ 2007 Elsevier Ltd. All rights reserved.
HO
1. Introduction
OH
N
R
Enantiomerically pure sulfoxides are widely used as drug
intermediates in pharmaceutical industries,1 and also chiral
sulfinyl groups have been used as auxiliaries in a variety of
highly diastereoselective carbon–carbon bond forming
reactions, including the synthesis of a-branched amines,
a- and b-amino acids, aziridines, and amino phosphonic
acids.2
R'
N
OH
Linker
R
OH
R
N
OH
R
HO
OH
N
Enantioselective sulfide oxidations catalyzed by chiral
complexes of transition metals such as titanium, manga-
nese, iron, and vanadium have produced interesting
results in the synthesis of optically active sulfoxides.3
Fujita et al. have introduced the use of alkyl peroxide
as the oxidant for vanadium catalyzed asymmetric sulf-
oxidation.4 Subsequently, Bolm et al.5 have employed a
highly selective and promising aminoalcohol based mono-
nuclear chiral Schiff base vanadium complexes (Fig. 1).
These catalyst systems gave high enantioselectivity with
very low catalyst loading in the presence of an environ-
mentally benign, nontoxic, and inexpensive stoichiometric
OH
R
Bolm's vanadium Schiff base ligands
Trimeric variant of Bolm's catalyst
Figure 1.
oxidant. Various modifications of Bolm’s vanadium Schiff
base complexes have been synthesized and explored. Poly-
mer supported modification6 of these vanadium Schiff base
complex has also been reported in recent years; however, it
gave moderate yield and enantioselectivity. Herein we
report the first synthesis of a series of chiral trinuclear
aminoalcohol based vanadium Schiff base complexes,
which when applied for asymmetric sulfide oxidation gave
higher yields with moderate to good enantioselectivities
of the sulfoxides.
*
Corresponding author. Tel.: +91 80 28394959; fax: +91 80 28394015;
e-mail: haripati@yahoo.com
0957-4166/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2007.11.005

P. Suresh et al. / Tetrahedron: Asymmetry 18 (2007) 2820–2827
2821
2. Results and discussion
acetylene backbone was synthesized. The trialdehyde,
5,50,500-(benzene-1,3,5-triyltris(ethyne-2,1-diyl))tris(3-tert-
butyl-2-hydroxybenzaldehyde) was synthesized from 1,3,5-
tribromobenzene according to Scheme 2. Accordingly,
1,3,5-tris((trimethylsilyl)ethynyl)benzene was synthesized
via a Pd-catalyzed Sonogashira–Hagihara cross-coupling
of ethynyltrimethylsilane with 1,3,5-tribromobenzene.
Deprotection of the TMS group gave 1,3,5-triethynyl-
benzene in 97% yield. Compound 1,3,5-triethynylbenzene
was unstable under normal conditions. However, under
the second Sonogashira–Hagihara cross-coupling with 3-
tert-butyl-2-hydroxy-5-iodobenzaldehyde, the trialdehyde
was formed in 52% yield (Scheme 3).10
2.1. Synthesis and characterization of the ligands
In asymmetric catalysis, chiral ligands with a C3-axis of
symmetry showed higher level of asymmetric induction.7
Similarly, multi-metallic complexes were generally found
to induce higher enantioselectivity in many asymmetric
transformations.8 Although the effect of additional chiral-
ity in the Bolm type catalyst had been explored, multi-
nuclear Bolm type catalysts have not been explored so
far. A series of trinuclear chiral vanadium Schiff base
complexes were synthesized and explored for the enantio-
selective oxidation of prochiral sulfides for the first time.
Trialdehydes with different architectures namely (i) trialde-
hydes with all the formyl groups in the same aromatic moi-
eties; and (ii) trialdehydes with formyl groups in different
aromatic moieties separated by tethers. Different chiral
aminoalcohols were also employed for this purpose.
C3-Symmetric tris(hydroxyaldehyde) without a spacer 1
was synthesized by Duff formylation of phloroglucinol.9
The chiral aminoalcohols for the ligand synthesis were syn-
thesized from the corresponding amino acids. (S)-Amino-
acids were converted to the corresponding methyl ester
hydrochloride and then reduced using sodium borohydride
to afford aminoalcohols in good yields.
Microwave irradiation (5 min) of a mixture of trialdehydes
and chiral aminoalcohols in the presence of potassium car-
bonate afforded chiral trinuclear Schiff bases in good yields
(89–96%) (Scheme 3). The condensation does not require
any anhydrous condition and in many cases the products
were purified by column chromatography over silica gel.
The yields of the chiral ligands formed are given in Table 1.
Structurally related triketones were synthesized for a
comparative study. Accordingly, 1,10,100-(2,4,6-trihydroxy-
benzene-1,3,5-triyl)triethanone 2 and (2,4,6-trihydroxy-
benzene-1,3,5-triyl)tris(phenylmethanone) 3 were synthe-
sized by Fries rearrangement of the corresponding
benzene-1,3,5-triyl triacetate and benzene-1,3,5-triyl
tribenzoate (Fig. 2). Trialdehydes possessing tunable
tethers (more than one atom) were synthesized from 3-
tert-butyl-2,5-dihydroxybenzaldehyde. The EDCI coupling
of 3-tert-butyl-2,5-dihydroxybenzaldehyde with benzene-
1,3,5-tricarboxylic acid in the presence of a catalytic
amount of DMAP and anhydrous CH2Cl2 afforded the
corresponding tris(hydroxyaldehydes) 4 in 74% yield
according to Scheme 1.
Trinuclear Schiff base ligands derived from structurally
related triketones were also attempted in a similar way.
Accordingly, a mixture of triketone 1,10,100-(2,4,6-trihydr-
oxybenzene-1,3,5-triyl)triethanone and (S)-2-amino-3-
phenylpropan-1-ol was subjected to microwave irradiation,
the reaction however did not produce any trinuclear ligand
as observed by NMR and MS. On the other hand, the reac-
tion afforded only the mono condensed product (Scheme
4), which did not undergo any further condensation with
higher equivalents of the aminoalcohol and also upon fur-
ther irradiation.
OH
ROC
HO
COR
OH
1; R = H
2; R = CH3
3; R = Ph
Proton NMR spectra of chiral ligands L7 and L8 showed a
single set of proton NMR signals indicating a highly sym-
metrical structure, and also indicating that the ligands were
perfectly C3-symmetric in nature. The proton NMR spec-
tra of C3-symmetric chiral ligands L1–L6 were surprisingly
complicated. In these cases, the 1H NMR spectra showed a
COR
Figure 2.
Apart from ligands with ester based tethers, the conjugated
acetylene tethered tris(hydroxyaldehyde) ligand with an
OH
CHO
O
O
CHO
OH
COOH
EDCl, DMAP
DCM, RT, 3 h, 74%
HO
CHO
+
O
O
HOOC
COOH
OH
O
O
OHC
OH
4
Scheme 1.

2822
P. Suresh et al. / Tetrahedron: Asymmetry 18 (2007) 2820–2827
Si
Br
Pd(PPh3)2Cl2, PPh3, CuI
K2CO3, MeOH, RT
TMS
Br
Br
2 h, 97%
Et3N, Reflux, 12 h, 82%
Si
Si
OH
CHO
OH
CHO
I
Pd(PPh3)2Cl2, PPh3, CuI
Et3N, Reflux, 12 h, 68%
OHC
HO
OH
CHO
5
Scheme 2.
HO
oxide was used as an oxidant and the reaction was carried
out at room temperature with dichloromethane as solvent.
The procedure afforded the chiral sulfoxide in good yields,
however, the enantioselectivities were moderate compared
to those reported with a monomeric chiral vanadium Schiff
base catalyst system.
OH
R
R
O
R
HN
R
MW, 5 min
EtOH
OHC
HO
CHO
OH
HO HN
+
H2N
OH
O
O
CHO
NH
OH
1
2.2.2. Effect of the substituents of the chiral aminoalcohol of
the ligand on thioanisole oxidation. A series of functionali-
zed trinuclear Schiff base ligands as shown in Table 2 were
used to explore the steric effect of the R group on the chiral
carbon of the aminoalcohol moiety toward the enantio-
selectivity of sulfide oxidation.
Scheme 3.
doublet around 8.0–8.5 ppm and doublet of doublet
around 11.5–12.5 ppm in CDCl3 instead of the expected
singlets around 8.0 ppm (imine NH) and ꢀ12 ppm (pheno-
lic OH). Similarly, 13C NMR showed the carbonyl reso-
nance of the central ring at ca. 185 ppm, characteristic of
the keto isomer.
All the ligands displayed poor enantioselectivity in combi-
nation with VO(acac)2. The poor selectivity could be either
due to the existence of the ligands as keto-enamine isomer
in comparison to the usual ONO imine environment ob-
served with other Schiff bases or due to steric crowding
of metal centers with respect to one another.
Both the spectral data indicated that the compound did not
exist in an imine form, but as the corresponding keto-
enamine isomers. Plausible isomeric structures of com-
pound L4 are shown in Figure 3 as compounds (A)–(C).
1H NMR and 13C NMR spectra of compound L4 shown
in Figure 4a and b, respectively, confirmed the presence
of symmetric isomers of a keto-enamine (B) in CDCl3.
The single crystal X-ray crystallographic structure of L2
also supported this hypothesis.11
2.2.3. Asymmetric oxidation of aryl methyl sulfide catalyzed
by VO(acac)2/trinuclear Schiff bases possessing tethers.
The lower activity of ligands L1–L5 toward the enantio-
selective oxidation of thioanisole was overcome by
employing trinuclear ligands possessing tethers. Ligands
L7 and L8 were used for this purpose. The use of tethers
in the ligand keeps all three catalytic centers far apart.
The ONO coordinating system possessing a tert-butyl
group resembles the Bolm’s ligand and behaved as three
individual moieties under the experimental conditions
exhibiting good reactivity and enantioselectivity. Both the
ligands with the ester and acetylene based tethers exhibited
similar reactivity during the oxidation of aryl methyl
sulfides affording sulfoxides in 81–98% yield and 54–89%
ee depending on the electronic effect of the substituent on
the para-position of the benzene ring of the substrate
(Table 3).
2.2. Asymmetric sulfoxidation studies catalyzed by chiral
vanadium Schiff base complexes
2.2.1. Asymmetric sulfoxidation of methyl phenyl sulfide
using C3-symmetric chiral trinuclear vanadium Schiff’s base
complexes. The catalytic efficiency of the trinuclear vana-
dium complexes was evaluated for the enantioselective oxi-
dation of prochiral sulfides. The well established Bolm’s
protocol was followed for the oxidation of thioanisole, in
which the catalytic system was formed in situ from the
reaction of VO(acac)2 and a C3-symmetric chiral ligand
(Scheme 5). The environmentally benign hydrogen per-

P. Suresh et al. / Tetrahedron: Asymmetry 18 (2007) 2820–2827
2823
Table 1.
Trialdehyde
Aminoalcohol
C3-Symmetric ligand
Yield (%)
HO
O
HN
HO HN
NH2
OH
1
O
O
96
NH
OH
L1
HO
HN
O
HO HN
1
1
1
86
74
93
O
O
H2N
OH
NH
OH
L2
HO
HN
O
HO HN
O
O
H2N
OH
NH
OH
L3
HO
HN
O
HO HN
O
O
H2N
OH
NH
OH
L4
HO
HN
O
HO HN
O
O
1
96
NH
OH
H2N
OH
L5
(continued on next page)

2824
P. Suresh et al. / Tetrahedron: Asymmetry 18 (2007) 2820–2827
Table 1 (continued)
Trialdehyde
Aminoalcohol
C3-Symmetric ligand
Yield (%)
HO
O
HN
HO HN
1
92
H2N
OH
O
O
NH
OH
L6
HO
N
OH
O
O
NH2
4
93
OH
O
O
O
N
HO
OH
N
O
OH
OH
L7
HO
N
OH
NH2
OH
5
98
N
HO
OH
OH
L8
N
OH
HO
N
selectivity (50–54%) though in good yields (Table 3, entry
4). As the nitro group is replaced by an electron donating
methyl or methoxy group, the ee values increased to 80–
82% (Table 3, entry 3).
O
OH
O
O
OH
MW,5 min
EtOH, 65%
+
HO
OH
H2N
OH
HO
OH
O
O
2
3. Conclusion
Scheme 4.
An efficient and facile method for the synthesis of trinu-
clear Schiff base ligands derived from chiral aminoalcohols
in good yields has been described. All the ligands in combi-
nation with VO(acac)2 resemble the most efficient Bolm’s
catalyst which forms a multi-metallic catalyst system for
an asymmetric sulfoxidation reaction. The vanadium
complex with all the three salen moieties in the same aryl
moiety displayed poor enantioselectivity toward the
The catalytic oxidation reaction of benzyl phenyl sulfide
provided high yields (92–98%) and remarkable enantio-
selectivity (86–89%) (Table 3, entry 6), whereas the
electron-deficient sulfide bearing a nitro group on the
para-position of the aryl group resulted in moderate enantio-

P. Suresh et al. / Tetrahedron: Asymmetry 18 (2007) 2820–2827
2825
OH
HO
HN
HO
HO
HN
NH
O
O
OH
N
O
HO
N
HO HN
OR
OR
O
HO
OH
O
O
NH
N
NH
OH
OH
OH
(C)
(A)
(B)
Figure 3.
Figure 4.
O-
S+
Table 3. Asymmetric oxidation of aryl methyl sulfides catalyzed by
VO(acac)2/L7 or L8
VO(acac)2 (1mol%) /Ligand (0.6mol%)
H2O2(1.1 eq.), CH2Cl2, RT
S
Entry Thioethers
Ligands employed Yield (%) ee (%)
1
2
3
4
5
6
PhSCH3
L7
L8
L7
L8
92
88
95
94
90
90
87
87
82
81
98
92
70
72
83
78
80
82
62
67
54
58
86
89
Scheme 5.
p-CH3PhSCH3
p-OCH3PhSCH3 L7
L8
Table 2. Asymmetric oxidation of thioanisole catalyzed by different
VO(acac)2/trinuclear Schiff base ligands derived from various chiral
aminoalcohols
p-BrPhSCH3
p-NO2PhSCH3
PhSCH2Ph
L7
L8
L7
L8
L7
L8
Entry
Ligands screened
Yield (%)
ee (%)
Configuration
1
2
3
4
5
L1
L2
L3
L4
L5
91
89
87
93
90
12
8
6
5
4
(R)
(S)
(S)
(S)
(S)
4. Experimental
4.1. General information
asymmetric oxidation of aryl methyl sulfides. However,
catalysts derived from ligands with the three salen moieties
separated from one another by a tether displayed high
enantioselectivities. The salen moieties of these complexes
behaved as individual units and both the ligands with a
flexible (ester) and rigid (acetylene) tether showed similar
reactivities and enantioselectivities. A more detailed mech-
anistic investigation of the presented catalyst systems as
well as broad catalytic studies is currently under way.
1H NMR spectra were recorded on 400 MHz Bruker
AVANCE 400 spectrometer and 13C NMR spectra were
recorded on 100 MHz Bruker AVANCE 400 spectrometer,
respectively, using TMS as an internal standard. IR spectra
were recorded on a Perkin–Elmer FT/IR 100 spectrometer.
Mass spectra were recorded on MALDI TOF mass spec-
trometer. Optical rotations were measured by a Rudolph
Autopol V polarimeter. All reactions were monitored by

2826
P. Suresh et al. / Tetrahedron: Asymmetry 18 (2007) 2820–2827
thin layer chromatography (TLC). TLC was performed on
F254, 0.25 mm silica gel plates (Merck). Plates were eluted
with appropriate solvent systems, and then stained with
either alkali KMnO4 or Ceric ammonium molybdate solu-
tions prepared in the laboratory. The developed plates were
first analyzed under UV 254 nm then stained with the
appropriate reagent. Column chromatography was per-
formed using silica gel with particle size 100–200 mesh.
High-performance liquid chromatograph with Daicel
Chiracel OD-H (25 cm · 0.46 cm i.d.) and Daicel Chiracel
OB-H (25 cm · 0.46 cm i.d.) chiral columns were used for
the measurements.
4.2.4. Spectral data for L4. Orange solid; yield 94%;
25
½aꢁD ¼ þ181:2 (c 0.5, CHCl3); IR (KBr): 3304, 2923,
2862, 1605, 1546, 1452, 1376, 1313, 1230, 1071, 832,
1
758 cmꢂ1; H NMR (CDCl3): d 3.81–3.87 (t, J = 9.6 Hz,
1H), 4.13–4.15 (d, J = 9.6 Hz, 1H), 4.63 (m, 1H), 6.77 (br
s, 1H) 7.33–7.43 (m 5H), 8.22–8.25 (d, J = 12 Hz, 1H),
12.18–12.24 (dd, J = 12 Hz and 12 Hz, 1H); 13C NMR
(CDCl3): d 67.6, 68.3, 104.6, 126.0, 128.3, 129.2, 136.5,
157.8, 184.7; mp slow decomposition >65 ꢁC; HRMS
(ESI) calcd for C27H46N3O6 [M+H]+: Calcd: 568.2448;
Obsd: 568.2464.
4.2.5. Spectral data for L5. Yellow solid; yield 95%;
25
½aꢁD ¼ ꢂ329:9 (c 0.48, CH2Cl2); IR (KBr): 3342, 3054,
4.2. General procedure for the synthesis of chiral
C3-symmetric aminoalcohol based Schiff’s base ligands
2930, 1609, 1546, 1456, 1381, 1266, 1053, 835, 735,
1
701 cmꢂ1; H NMR (CDCl3): d 2.78–2.82 (m 2H), 2.84–
2.92 (dd, J = 8 Hz and 1H), 3.55–3.6 (m, 1H), 3.69–3.72
(d, J = 12 Hz, 1H), 6.26 (br s, 1H), 7.11–7.37 (m 5H),
7.76–7.79 (d, J = 12 Hz 1H), 11.18–11.23, (dd, J = 12 Hz
and 12 Hz, 1H); 13C NMR (CDCl3): d 59.4, 68.9, 69.6,
105.7, 127.8, 129.6, 130.4, 137.6, 157.8, 184.7; mp 149–
150 ꢁC; HRMS (ESI) calcd for C36H40N3O6 [M+H]+:
Calcd: 610.2917; Obsd: 610.2907.
To a solution of trialdehyde (1 mmol) in ethanol (1 mL)
was added a solution of corresponding equivalent of chiral
aminoalcohol in ethanol (1 mL). The homogeneous mix-
ture was irradiated in an unmodified domestic microwave
oven at a low power setting for 5 min. The reaction mixture
was then cooled to room temperature and ethanol was re-
moved under reduced pressure to afford the crude product
which was purified by column chromatography over silica
gel using EtOAc–hexane to give pure C3-symmetric amino-
alcohol based Schiff’s base ligands as yellow solids (86–98%
yield).
4.2.6. Spectral data for L6. Orange syrup; yield 89%;
25
½aꢁD ¼ ꢂ154:2 (c 0.62, CHCl3); IR (KBr): 3338, 3021,
2932, 1612, 1552, 1454, 1391, 1256, 1049, 837, 729 cmꢂ1
;
1H NMR (CDCl3): d 1.15–1.16 (d, J = 6.5 Hz, 3H), 3.32–
3.57 (m, 2H), 3.71–3.73 (d, J = 10 Hz, 1H), 5.24 (br s,
1H), 7.81–7.84 (d, J = 12 Hz, 1H), 11.04–11.09 (dd,
J = 12 Hz and 12 Hz, 1H); 13C NMR (CDCl3): d 17.3,
60.1, 67.8, 104.6, 158.2, 185.1; HRMS (ESI) calcd for
C18H28N3O6 [M+H]+: Calcd: 382.1916; Obsd: 382.1931.
4.2.1. Spectral data for L1. Yellow solid; yield 96%;
25
½aꢁD ¼ þ28:5 (c 0.6, CHCl3); IR (KBr): 3338, 3050, 2975,
1
1619, 1547, 1448, 858, 841, 738 cmꢂ1; H NMR (CDCl3):
d 1.82 (br s, 1H), 2.87–3.04 (m, 2H), 4.42–4.56 (m, 2H),
7.21–7.43 (m, 5H), 7.91 (s, 1H), 11.25 (s, 1H); 13C NMR
(CDCl3): d 39.9, 67.6, 73.6, 105.2, 125.2, 127.2, 128.8,
139.4, 141.0, 157.1, 184.5; mp slow decomposition
>156 ꢁC; HRMS (ESI) calcd for C36H33N3O6 [M+H]+:
Calcd: 604.7317; Obsd: 604.7320.
4.2.7. Spectral data for L7. Yellow solid; yield 93%;
25
½aꢁD ¼ 32:3 (c 1.04, CHCl3); IR (KBr): 2957, 1740, 1630,
1434, 1213, 1096, 734, 597 cmꢂ1 1H NMR (CDCl3): d
;
1.42 (s, 6H), 3.19 (m, 3H), 4.60–4.73 (m, 2H), 7.13–7.30
(m, 6H), 8.20 (s, 1H), 9.09 (s, 1H); 13C NMR (CDCl3): d
29.2, 33.9, 35.1, 39.3, 75.1, 118.1, 122.5, 123.6, 124.8,
125.5, 127.1, 128.6, 131.2, 135.6, 139.7, 140.6, 140.8,
141.3, 159.4, 164.0, 166.0. ESI-MS [M+H]+: 1131.
4.2.2. Spectral data for L2. Yellow solid; yield 96%;
25
½aꢁD ¼ ꢂ274:8 (c 0.35, CHCl3); IR (KBr): 3322, 3054,
2965, 1613, 1546, 1458, 1377, 1307, 1265, 1074, 858, 841,
1
738 cmꢂ1; H NMR (CDCl3): d 1.04–1.06 (d, J = 4.9 Hz,
6H), 1.93–1.95 (m, 1H), 3.26 (m, 1H), 3.60–3.66 (t,
J = 12 Hz, 1H), 3.94–3.97 (d, J = 12 Hz, 1H), 6.43 (br s,
1H) 8.01–8.04 (d, J = 12 Hz, 1H), 11.57–11.63 (dd,
J = 12 Hz and 12 Hz, 1H); 13C NMR (CDCl3): d 17.9,
19.3, 29.3, 64.4, 70.4, 104.1, 158.5, 184.5; mp slow decom-
position >135 ꢁC; HRMS (ESI) calcd for C24H40N3O6
[M+H]+: Calcd: 466.2917; Obsd: 466.2940.
4.2.8. Spectral data for L8. Yellow solid; yield 93%;
25
½aꢁD ¼ ꢂ153:4 (c 0.94, CHCl3); IR (KBr): 3361, 2950,
2207, 1624, 1574, 1267, 1126, 735, 501 cmꢂ1 1H NMR
;
(CDCl3): d 1.42 (s, 6H), 3.12–3.14 (d, J = 5.4 Hz, 1H),
3.21–3.22 (d, J = 5.0 Hz, 1H), 4.68–4.69 (d, J = 5.0 Hz,
1H), 4.75–4.77 (d, J = 5.0 Hz, 1H), 7.16–7.35 (m, 5H),
7.51 (s, 1H), 7.60 (s, 1H), 8.47 (s, 1H), 14.2 (br s, 1H);
13C NMR (CDCl3): d 29.3, 34.9, 39.5, 72.9, 75.1, 86.4,
90.6, 112.1, 118.5, 124.9, 125.4, 125.6, 126.9, 127.9, 128.7,
133.8, 138.5, 140.5, 140.8, 143.7, 161.9, 166.6. ESI-MS
[M+H]+: 1073.
4.2.3. Spectral data for L3. Orange syrup; yield 91%;
25
½aꢁD ¼ ꢂ172:6 (c 0.85, CHCl3); IR (KBr): 3401, 2959,
1612, 1546, 1461, 1378, 1320, 1265, 1153, 1069, 836,
739 cmꢂ1 1H NMR (CDCl3): d 0.83 (s, 6H), 1.21–1.59
;
(m, 2H), 1.91–1.92 (m, 1H), 1.91–2.00 (dd, J = 16 Hz and
4 Hz, 1H), 3.36–3.38 (t, J = 12 Hz, 1H), 3.76–3.79 (d,
J = 12 Hz, 1H), 6.32 (br s, 1H) 7.86–7.89 (d, J = 12 Hz,
1H), 11.13–11.19 (dd, J = 12 Hz and 12 Hz 1H); 13C
NMR (CDCl3): d 22.6, 23.2, 24.8, 39.7, 62.8, 66.6, 104.0,
157.9, 184.4; HRMS (ESI) calcd for C27H46N3O6
[M+H]+: Calcd: 508.3342; Obsd: 508.3360.
4.3. General procedure for the preparation of chiral
sulfoxides
Vanadyl acetylacetonate (0.01 mmol) and the ligand
(0.006 mmol) were dissolved in dichloromethane (2 mL),
and the solution was stirred for 5 min at room temperature.
To this reaction mixture was added the corresponding sul-

P. Suresh et al. / Tetrahedron: Asymmetry 18 (2007) 2820–2827
2827
Elebring, T.; Larsson, M.; Li, L.; So¨rensen, H.; Unge, S. V.
Tetrahedron: Asymmetry 2000, 11, 3819–3825; (f) Padmanab-
han, S.; Lavin, R. C.; Durranat, G. J. Tetrahedron: Asym-
metry 2000, 11, 3455–3457; (g) Rouhi, A. M. Chem. Eng.
News 2003, 81, 56–61; (h) Fernandez, I.; Khiar, N. Chem.
Rev. 2003, 103, 3651–3705.
fide (1 mmol) followed by the dropwise addition of 30%
hydrogen peroxide (1.1 mmol) at 0 ꢁC. The mixture was al-
lowed to warm to room temperature and stirred for a fur-
ther 16 h at ambient temperature. After the completion of
the reaction, the aqueous layer was extracted with dichlo-
romethane (2 · 2 mL). The combined organic extracts were
dried over anhydrous sodium sulfate, filtered, and evapo-
rated under vacuum to afford the crude product which
was purified by column chromatography over silica gel.
´
2. Hanquet, G.; Colobert, F.; Lanners, S.; Solladie, G. Arkivoc
2003, 328–401.
3. Volcho, K. P.; Salakhutdinov, N. F.; Tolstikov, A. G. Russ.
J. Org. Chem. 2003, 1537–1552.
4. Nakajima, K.; Kojima, M.; Fujita, J. Chem. Lett. 1986, 1483–
1486.
5. Bolm, C.; Bienewald, F. Angew. Chem., Int. Ed. Engl. 1995,
34, 2640–2642.
Acknowledgement
6. Bolm, C.; Schlingloff, G.; Bienewald, F. J. Mol. Catal. 1997,
117, 347–350.
7. (a) Moberg, C. Angew. Chem., Int. Ed. 1998, 37, 248–268; (b)
Gibson, S. E.; Castaldi, M. P. Chem. Commun. 2006, 3045–
3062.
The authors acknowledge the help and support of Indian
Institute of Technology Madras, Chennai 600 036.
References
8. Shibasaki, M.; Yamamoto, Y. Multimetallic Catalysts in
Organic Synthesis; John Wiley: New York, 2004, pp 53–161.
9. Chong, J. H.; Sauer, M.; Patrick, B. O.; MacLachlan, M. J.
Org. Lett. 2003, 5, 3823–3826.
10. Busse, C.; Weigelt, S.; Petersen, L.; Lægsgaard, E.; Besenb-
acher, F.; Linderoth, T. R.; Thomsen, A. H.; Nielsen, M.;
Gothelf, K. V. J. Phy. Chem. B 2007, 111, 5850–5860.
11. Suresh, P.; Varadarajan, T. K.; Varghese, B.; Viswanathan,
B. Acta Crystallogr., Sect. E 2007, E63, o984–o986.
1. (a) Morita, S.; Matsubara, J.; Otsubo, K.; Kitano, K.;
Ohtani, T.; Kawano, Y.; Uchida, M. Tetrahedron: Asymme-
try 1997, 8, 3707–3710; (b) Carlsson, E.; Lindberg, P.; Unge,
S. V. Chem. Br. 2002, 38, 42–45; (c) Matsugi, M.; Fukudo,
N.; Muguruma, Y.; Yamaguchi, T.; Minamikawa, J.; Otsuka,
S. Tetrahedron 2001, 57, 2739–2744; (d) Naito, S.; Nishimura,
M. Yakugaku Zasshi 2001, 121, 989–994; (e) Cotton, H.;