A re-investigation of Modena's protocol for the asymmetric oxidation of prochiral sulfides

Article abstract of DOI:10.1016/j.tetasy.2005.05.015

The reactivity of commercially available or easily accessible hydroperoxides has been conveniently exploited for the achievement of highly efficient and enantioselective catalytic modifications of Modena's protocol for the asymmetric oxidation of sulfides. A notably enhanced enantioselectivity has been obtained by exploiting a concomitant process of stereoconvergent kinetic resolution taking place under catalytic conditions.

Full text of DOI:10.1016/j.tetasy.2005.05.015

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 2271–2275
A re-investigation of Modena’s protocol for the
asymmetric oxidation of prochiral sulfides
Antonio Massa, Valeria Mazza and Arrigo Scettri*
`
Dipartimento di Chimica, Universita di Salerno, 84081 Baronissi, Salerno, Italy
Received 29 April 2005; accepted 10 May 2005
Available online 8 June 2005
Abstract—The reactivity of commercially available or easily accessible hydroperoxides has been conveniently exploited for the
achievement of highly efficient and enantioselective catalytic modifications of ModenaÕs protocol for the asymmetric oxidation of
sulfides. A notably enhanced enantioselectivity has been obtained by exploiting a concomitant process of stereoconvergent kinetic
resolution taking place under catalytic conditions.
Ó 2005 Elsevier Ltd. All rights reserved.
˚
1. Introduction
4 A molecular sieves were found to have a beneficial
effect on the level of enantioselectivity. In the following
years, the interest of several research groups was focused
on the influence exerted by different chiral ligands so
that many modifications of KaganÕs catalytic procedure
were reported in the literature and involved the use of
1,2-diols,8 binaphthols,9 triethanolamines10 and 1,2-
amino alcohols11 as chiral auxiliaries.
The metal-catalyzed asymmetric oxidation of prochiral
sulfides represents one of the most popular approaches
for the synthesis of chiral sulfoxides, whose importance
both as chirality controllers and biologically significant
compounds is well documented.1
In 1984, Kagan2 and Modena3 discovered independently
that the combinations of Ti(Oi-Pr)4/(R,R)-diethyl tar-
trate (DET)/H2O (1:2:1) and Ti(Oi-Pr)4/(R,R)-DET
(1:4) promoted the asymmetric oxidation of sulfides by
using tert-butyl hydroperoxide (TBHP) as oxidant.
The early suggestion of the involvement of the same
catalytic species in both protocols was then excluded
because of the differences observed in the oxidation of 2-
substituted 1,3-dithiane derivatives4 and in the influence
of temperature.5
Rather surprisingly, over the same period, very little
attention was paid to ModenaÕs protocol in spite of its
operational simplicity and good levels of diastereo-
and enantioselectivity observed in the asymmetric oxida-
tion of 1,3-dithiolanes,12 1,3-dithiane-2-carboxylates.4c
In recent years the ready availability of a new class of
renewable hydroperoxides of type 113 (Fig. 1) has
allowed the achievement of stoichiometric and cata-
lytic procedures14 for the enantioselective oxidation of
sulfides using, respectively, Ti(Oi-Pr)4/(R,R)-DET (1:4)
and Ti(Oi-Pr)4/(R)-BINOL/H2O (1:2:1) catalysts. It is
noteworthy, however, that in both cases, high values
of enantiomeric excess (ee) could be obtained through
a combined process of asymmetric sulfoxidation and
Subsequently, an extensive investigation by Kagan
resulted in the achievement of a highly enantioselective
stoichiometric procedure6 and pointed out the necessity
of a very careful control of the experimental conditions
(temperature, mode of stirring, order and time of addi-
tion of the reagents) required for the preparation of
Ti(Oi-Pr)4/(R,R)-DET/H2O (1:2:1) system. Further-
more, the catalytic version4a,7 of a modified KaganÕs
procedure was based on the employment of Ti(Oi-Pr)4/
(R,R)-DET/i-PrOH (1:4:4) catalyst and the presence of
R1
R2
R
O
a: R=H, R1=R2=Me
b: R=Me, R1=H, R2=n-hexyl
1
OOH
*
Corresponding author. Tel.: +39 089 965 374; fax: +39 089 965
296; e-mail: scettri@unisa.it
Figure 1.
0957-4166/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.05.015

2272
A. Massa et al. / Tetrahedron: Asymmetry 16 (2005) 2271–2275
stereoconvergent kinetic resolution of the enantio-
enriched sulfoxides with a reduction of the chemical
yields.
Conversely, the employment of 1b resulted in a very
chemoselective oxidation, which was found to proceed
at ꢀ20 °C in a highly efficient and enantioselective way
(entry 6). Furthermore, in both cases the decrease of cat-
alyst loading (entries 7 and 8) caused the formation of
the sulfoxide in rather lower yields and ees.
Since the achievement of new catalytic procedures
affording chiral sulfoxides in high yields and ees still rep-
resents a very appealing target, the original stoichio-
metric ModenaÕs protocol was re-investigated with
particular attention paid to the influence exerted by dif-
ferent oxygen donors ROOH on the efficiency and
enantioselectivity of the conversion 2!3 (Scheme 1).
In order to assess the general validity of the above
reported results, several sulfides were submitted to treat-
ment with furyl hydroperoxides 1a and b under the opti-
mized conditions [0.3 equiv of Ti(Oi-Pr)4/(R,R)-DET
(1:4) system]. In the case of 1a, although a very good
efficiency could be observed in all the entries 1–4 (Table
2), the asymmetric oxidation afforded the corresponding
sulfoxides with ees only in the 53–78% range. Very inter-
estingly, the employment of 1b, as oxidant, allowed the
attainment of high yields and ees (up to 91%) (entries 5–
12) and, furthermore, because of the enhanced chemose-
lectivity, only in entry 11 was the formation of the over-
oxidation product in a non-negligible amount (10%)
detected.
:
O
Ti(OiPr)4 / (R,R)-DET
S
S
R1
R1
R2
R2
ROOH
2
3
Scheme 1.
2. Results and discussion
In the preliminary phase, methyl p-tolyl sulfide was cho-
sen as a model compound and the reactivity of commer-
cially available TBHP and cumyl hydroperoxide (CHP)
was examined under the typical conditions of ModenaÕs
protocol in the presence of catalytic amounts of Ti-
(Oi-Pr)4/(R,R)-DET (1:4) system (Scheme 1, Table 1).
At ꢀ20 °C, the oxidation by TBHP proceeded with a
moderate level of efficiency and enantioselectivity (entry
1) while at a higher temperature (entry 2), only a notable
increase of yield could be observed. The use of CHP, as
oxidant, at ꢀ20 °C afforded much more satisfactory
results since very good yields and ees could be obtained
in a much reduced reaction time (entry 3). However, the
occurrence of the reaction at 0 °C (entry 4) resulted only
in a lower chemoselectivity since an appreciable amount
of sulfone (16%) could be isolated.
Table 2. Catalytic asymmetric sulfoxidation of R1–S–R2 by 1a,b
Entry ROOH R1
R2 Product Yielda
(%)
eeb
(%)
1
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
1b
p-Tolyl Me 3a
p-MeOC6H4 Me 3b
93(3)
92(4)
73(6)
98(0)
92(4)
96(3)
89(6)
92(6)
77(0)
78
75
76
53
89
86
87
91
81
2
3
p-ClC6H4
Ph
p-Tolyl
Me 3c
Et 3d
Me 3a
4
5
6
p-MeOC6H4 Me 3b
7
p-ClC6H4
Ph
Me 3c
Me 3e
Me 3f
8
9
2-Naphthyl
Ph
10
11
12
Et
3d
81(n.d.)d 77
p-NO2C6H4 Me 3g
n-Octyl Me 3h
80(10)
67(0)
87
72c
a All the yields refer to isolated chromatographically pure compounds.
The values in parentheses refer to sulfone yields. In all entries,
1.7 equiv of 1a and b were used in the presence of Ti(Oi-Pr)4/(R,R)-
DET in the ratio 1/4.
Table 1. Asymmetric sulfoxidation of Me–S–p-tolyl by different
ROOH
Reaction Temp Yielda eeb
b Predominant (R) enantiomer.
Entry ROOH Ti(IV)
(equiv) time (h)
c The ee was determined by 1H NMR (400 MHz) in the presence of
(R)-(ꢀ)-(3,5-dinitrobenzoyl)-a-phenylethylamine as shift reagent.
d Not determined.
(°C)
(%)
(%)
1
2
3
4
5
6
7
8
TBHP
TBHP
CHP
CHP
1a
0.3
0.3
0.3
0.3
0.3
0.3
0.2
0.2
46
23
6
ꢀ20
0
54(0)
83(4)
93(4)
64
64
85
ꢀ20
0
4
80(16) 85
It is noteworthy that the catalytic procedure of asym-
metric sulfoxidation seemed to be more enantioselective
than the stoichiometric one: in fact, the oxidation of
Me–S–p-tolyl with 1b, performed in the presence of
Ti(Oi-Pr)4/(R,R)-DET (1:4) under conditions ensuring
significant reduction of the concomitant process of
kinetic resolution, was reported14 to take place in 74%
ee (97% yield), while in entry 5, methyl p-tolyl sulfoxide
was obtained in 89% ee (92% yield). In a similar way, a
notable improvement of enantioselectivity was observed
in the case of Me–S–Ph, as clearly shown by the compar-
ison of the literature data for the stoichiometric sulfox-
idation (78% ee, 89% yield) with the ones of entry 8
(91% ee, 92% yield).
3
0
93(3)
92(4)
82(3)
79(9)
78
89
69
62
1b
1a
24
2
ꢀ20
0
1b
23
ꢀ20
a All the yields refer to isolated chromatographically pure compounds.
The values in parentheses refer to sulfone yields. In all entries,
1.7 equiv of ROOH were used in the presence of Ti(Oi-Pr)4/(R,R)-
DET in the ratio 1/4.
b Predominant (R)-enantiomer.
Methyl p-tolyl sulfide was then submitted to treatment
with the easily accessible furyl hydroperoxides 1a and
b14 under the same conditions as TBHP and CHP. It
is notable that at ꢀ20 °C, 1a proved to be almost com-
pletely unreactive; however a good yield and ee were ob-
tained by carrying out the reaction at 0 °C (entry 5).
The promising results obtained in the preliminary phase,
Table 1 (entries 3 and 4), stimulated a further investiga-

A. Massa et al. / Tetrahedron: Asymmetry 16 (2005) 2271–2275
2273
Table 4. Catalytic asymmetric sulfoxidation of R1–S–Me by CHP
tion on the reactivity of CHP in the presence of varying
R1
Ti(IV)
(equiv)
Product
Yielda
(%)
eeb
(%)
amounts of Ti(Oi-Pr)4/(R,R)-DET (1:4) catalyst. Me–S–
p-tolyl was again chosen as the model compound. As
clearly shown in Table 3, rather similar levels of enantio-
selectivity have been observed by using catalyst loading
in the range 0.1–1.0 equiv (entries 1–4), while a sharp
drop has been observed both in yield and ee derived
from the employment of 0.05 equiv of catalyst (entry 5).
Entry
1
p-Tolyl
p-MeOC6H4
Ph
0.30
0.30
0.30
0.30
0.30
0.30
0.15
0.15
0.15
0.15
0.15
0.10
0.10
3a
3b
3e
3g
3c
3h
3a
3c
3b
3e
3h
3a
3b
93(4)
87(5)
95(3)
66(10)
95(5)
90(5)
95(5)
93(7)
92(8)
89(9)
72(25)
96(2)
93(6)
85
85
91
84
89
83c
91
89
89
92
69c
85
80
2
3
4
p-NO2C6H4
p-ClC6H4
n-Octyl
5
6
7
p-Tolyl
Table 3. Asymmetric sulfoxidation of Me–S–p-tolyl by CHP
8
p-ClC6H4
p-MeOC6H4
Ph
Entry
Ti(IV) (equiv)
Time (h)
Yielda (%)
eeb (%)
9
1
2
3
4
5
1.00
0.30
0.15
0.10
0.05
6
6
84(0)
93(4)
95(5)
96(2)
79(2)
90
85
90
85
60
10
11
12
13
n-Octyl
p-Tolyl
p-MeOC6H4
24
24
26
a All the yields refer to isolated chromatographically pure compounds.
The values in parentheses refer to sulfone yields. In all entries,
1.7 equiv of CHP were used in the presence of Ti(Oi-Pr)4/(R,R)-DET
in the ratio 1:4. Reaction time: 6 h (entries 1–6), 24 h (entries 7–13).
b Predominant (R)-enantiomer.
a All the yields refer to isolated chromatographically pure compounds.
The values in parentheses refer to sulfone yields. In all the entries,
1.7 equiv of 1a and b were used in the presence of Ti(Oi-Pr)4/(R,R)-
DET in the ratio 1:4.
b Predominant (R)-enantiomer.
c The ee was determined by 1H NMR (400 MHz) in the presence of
(R)-(ꢀ)-(3,5-dinitrobenzoyl)-a-phenylethylamine as shift reagent.
A previous report15 pointed out the occurrence of a pro-
cess of kinetic resolution of racemic sulfoxides by treat-
ment with CHP in the presence of stoichiometric
amounts of Ti(Oi-Pr)4/(R,R)-DET (1:4). This finding
suggested the possibility of increasing the level of
enantioselectivity of the catalytic procedure by a com-
bined process of asymmetric sulfoxidation and kinetic
resolution of the enantioenriched sulfoxides. Therefore,
the experiment of entry 2 was repeated in the presence of
1.4 equiv of CHP and, after the oxidation was pro-
longed for 48 h, the expected (R)-sulfoxide was isolated
in 80% yield and 97% ee. The appropriate choice of
CHP/sulfide stoichiometric ratio allowed the achieve-
ment of a very high ee without affecting in a dramatic
way the efficiency of the process, since methyl p-tolyl sul-
fone was obtained in only 16% yield.
4 and 11 over-oxidation was greatly reduced, the sulfox-
ides were obtained in high ees and yields.
In an attempt to increase the level of enantioselectivity
the conversion, 2!3 was performed under the typical
conditions required by the combined process of asym-
metric sulfoxidation and kinetic resolution (Table 5).
Table 5. Combined asymmetric oxidation of R1–S–Me and kinetic
resolution of R1–SO–Me by CHP
Entry
R1
Product
Ti(IV)
(equiv)
Yielda
(%)
eeb
(%)
1
2
3
4
5
p-Tolyl
p-MeOC6H4
Ph
3a
3b
3e
3a
3a
0.3
0.3
0.3
0.15
0.1
80(16)
65(32)
82(16)
86(12)
96(4)
97
>99
97
The involvement of a stereoconvergent process of
kinetic resolution in this latter reaction was further con-
firmed by submitting rac-methyl p-tolyl sulfoxide 3a
to treatment with CHP in the presence of 0.3 equiv
of Ti(Oi-Pr)4/(R,R)-DET (1:4) under the conditions
depicted in Scheme 2.
p-Tolyl
p-Tolyl
91
80
a All the yields refer to isolated chromatographically pure compounds.
The values in parentheses refer to sulfone yields. In all entries,
1.7 equiv of CHP were used in the presence of Ti(Oi-Pr)4/(R,R)-DET
in the ratio 1:4.
b Predominant (R)-enantiomer.
:
O
S
O
O
O
Ti(OiPr)4 / (R,R)-DET
+
S
S
However, appreciable improvements were achieved only
in the presence of 0.3 equiv of Ti(Oi-Pr)4/(R,R)-DET
(1:4) catalyst (entries 1–3), although in entry 2 the for-
mation of the corresponding sulfoxide took place in
the highest ee at the expense of the chemical yield.
R1
R2
R1
R1
R2
R2
CHP (0.6 eq.), CH2Cl2, -20˚C
R-3a
rac-3a
Scheme 2.
In fact, after 24 h, unreacted sulfoxide was isolated in
70% yield and 35% ee [as the predominant (R)-enantio-
As previously reported in the introduction, the stoichio-
metric Modena protocol for the enantioselective oxida-
tion of sulfides has scarcely been explored and some
modifications of the original procedure are based on
the employment of simple or polyfunctional furyl hydro-
peroxides in substitution of TBHP.13 Therefore, in order
to broaden the field of this investigation, CHP was re-
acted with several sulfides in the presence of a stoichio-
metric amount of Ti(Oi-Pr)4/(R,R)-DET (1:4) complex.
mer] and
calculated.16
a
stereoselection factor S = 13.9 was
As shown in Table 4, the catalytic approach proved to
be successful with a series of sulfides, especially in the
presence of 0.3 and 0.15 equiv of Ti(Oi-Pr)4/(R,R)-
DET (1:4) complex and, with the exception of entries

2274
A. Massa et al. / Tetrahedron: Asymmetry 16 (2005) 2271–2275
Table 7. Asymmetric mono-sulfoxidation of 4 by CHP
Under the optimized conditions of entries 1–6 (Table 6)
the asymmetric sulfoxidation was found to proceed
with complete chemoselectivity, high efficiency and
enantioselectivity.
Entry
R
n
Time (h) Product Yielda (%) eeb (%)
1
2
3
4
H
1
1
1
2
23
24
6
5a
5b
5a
5c
73(>99/1)
95(>99/1)
95(>99/1)
68(>99/1)
89
91
74c
10
Me
H
H
40
Table 6. Stoichiometric asymmetric oxidation of R1–S–R2 by CHP
a All the yields refer to isolated chromatographically pure compounds.
In all entries, 1.2 equiv of CHP were used in the presence of 1 equiv of
Ti(Oi-Pr)4/(R,R)-DET in the ratio 1:4. Values in parentheses refer to
trans/cis diastereoisomeric ratios calculated according to Refs. 17
(5a,b) and 18 (5c).
Entry R1
R2
Product 3 Time Yielda eeb
(h)
(%)
(%)
1
2
p-Tolyl
2-Naphthyl
o-MeOC6H4 Me 3i
Me 3a
Me 3f
6
24
40
40
24
24
24
72
26
24
84(0)
90(0)
99(0)
95(0)
60(0)
73(0)
95(0)
73(15) 75d
72(20) 97d
80(15) 96d
90
92
92
70
87
91c
91
b Ees were determined by HPLC by using a Daicel Chiralcel OD
column.
3
4
Ph
Et
3d
c In this entry 0.3 equiv of Ti(IV) complex were used.
5
p-NO2C6H4
n-Octyl
p-Tolyl
Ph
Me 3g
Me 3h
Me 3a
6
7
3. Conclusion
8
Et
3d
9
10
Ph
p-ClC6H4
Me 3e
Me 3c
In conclusion, a careful investigation into the original
stoichiometric ModenaÕs procedure has allowed the
achievement of highly enantioselective catalytic modifi-
cations based on the use of CHP or easily recyclable
furyl hydroperoxides, as oxygen donors, in the presence
of reduced amounts (down to 10%) of Ti(Oi-Pr)4/(R,R)-
DET (1:4) complex. A further enhancement of the level
of enantioselectivity has been obtained through a stereo-
convergent process of kinetic resolution accompanying
the asymmetric sulfoxidation under catalytic conditions.
Finally, it is noteworthy that the stoichiometric version
of ModenaÕs protocol (oxidant CHP) afforded very high
ees and yields both in the oxidation of sulfides and the
stereoselective mono-sulfoxidation 2-aryl-substituted
1,3-dithiolanes.
a All the yields refer to isolated chromatographically pure compounds.
The values in parentheses refer to sulfone yields. In all the entries
1.2 equiv of CHP were used in the presence of 1 equiv of Ti(Oi-Pr)4/
(R,R)-DET in the ratio 1:4.
b Predominant (R)-enantiomer.
c The ee was determined by 1H NMR (400 MHz) in the presence of
(R)-(ꢀ)-(3,5-dinitrobenzoyl)-a-phenylethylamine as shift reagent.
d In these entries 1.4 equiv of CHP were used.
More prolonged reaction times (entries 7 and 8) or the
use of slightly increased CHP/sulfide ratios (entries 9
and 10) promoted the usual process of stereoconvergent
kinetic resolution, leading to the chiral sulfoxides in
higher yields or ees. The stereochemical outcome of
entry 6 (91% ee) can be considered of particular impor-
tance since dialkyl sulfides are known to usually suffer
asymmetric oxidation with moderate enantioselectivity.
4. Experimental
Finally the modified ModenaÕs protocol allowed signifi-
cant improvements in the asymmetric sulfoxidation of
cyclic 2-subsituted 1,3-thioacetals of type 4 (Scheme 3,
n = 1) since the usual treatment afforded the correspond-
ing mono-sulfoxides 5 in complete diastereoselectivity
and high enantioselectivity (Table 7, entries 1 and 2).
4.1. Materials and general methods
All the reactions were performed in flame-dried glass-
ware under an atmosphere of dry argon. All the solvents
were of reagent grade and were dried and distilled imme-
diately before use (CH2Cl2 from calcium hydride). Puri-
fications were performed by flash chromatography
(Merck silica gel), by elution with light petroleum (40–
70 °C)/ethyl acetate mixtures. Starting materials and
all other reagents, unless otherwise indicated, were pur-
chased from Aldrich or Fluka and used without further
purification. All the reactions were monitored by thin
layer chromatography (TLC) on Merck silica gel plates
(0.25 mm) and visualized by UV light or by KmnO4
spray test. The NMR spectra (Bruker DRX 400 (1H
400 MHz; 13C 100 MHz)), were performed in CDCl3
solution and referenced to residual CHCl3 (7.26 ppm
(1H); 77.23 ppm (13C)). Optical rotations were measured
on a Jasco Dip-1000 using the Na lamp. HPLC analyses
were performed with Waters Associates equipment
(Waters, 2487 Dual k absorbance detector) using a Dai-
cel Chiralcel OB column with the exception of 5a–c
(Daicel Chiralcel OD column). Furyl hydroperoxides
1a and b were prepared according to Ref. 14. Structures
and absolute configurations of compounds 3a and b,6c
3c,6a 3d,19 3e–i6c were assigned by comparison with liter-
ature data (1H NMR and sign of the specific rotation).
( ) n
( ) n
Ti(OiPr)4 / (R,R)-DET
S
S
S
S
O
CHP, CH2Cl2, -20˚C
Ph
R
Ph
R
5
4
a: n=1, R=H
b: n=1, R=Me
c: n=2, R=H
Scheme 3.
It is notable that, under catalytic conditions [0.3 equiv
of Ti(IV) complex], 5a was obtained as single diastereo-
isomer in 95% yield and 74% ee (entry 3). Conversely,
in spite of a high diastereoselectivity, a very low ee
was observed in the case of the six-membered thioacetale
4c (entry 4), confirming the poor results previously
reported for the oxidation of 2-alkyl- or 2-aryl-2-substi-
tuted 1,3-dithianes by the typical Kagan and Modena
procedures.

A. Massa et al. / Tetrahedron: Asymmetry 16 (2005) 2271–2275
2275
4.2. Standard procedure for the stoichiometric
asymmetric sulfoxidation
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4.3. Standard procedure for the catalytic asymmetric
sulfoxidation
In all the catalytic procedures, a Ti(IV) complex was
used at the same concentration (0.047 M). In a typical
experimental procedure, a solution of (R,R)-diethyl tar-
trate (0.124 g, 0.60 mmol), titanium tetraisopropoxide
(0.043 g, 0.15 mmol) and sulfide (1.0 equiv) in dry
CH2Cl2 (1.6 mL) under an argon atmosphere was stirred
at room temperature for 5 min. Then the temperature
was cooled to ꢀ20 °C and after 20 min a solution of
hydroperoxide (1.7 equiv in 1.6 mL of dry CH2Cl2)
slowly added. After the appropriate reaction time, the
above reported procedure for stoichiometric sulfoxida-
tion was followed for the work-up.
10. Di Furia, F.; Licini, G.; Modena, G.; Motterle, R. J. Org.
Chem. 1996, 61, 5175–5177.
11. Peng, Y. G.; Feng, X. M.; Cui, X.; Jiang, Y. Z.; Choi, M.
C.; Chan, A. S. C. Synth. Commun. 2003, 33, 2793–
2801.
4.4. Standard procedure for the catalytic kinetic
resolution
12. Bortolini, O.; Di Furia, F.; Licini, G.; Modena, G.; Rossi,
M. Tetrahedron Lett. 1986, 27, 6257–6260.
13. For a recent review, see: Lattanzi, A.; Scettri, A. Curr.
Org. Chem. 2004, 8, 607–621.
14. Massa, A.; Siniscalchi, F. R.; Bugatti, V.; Lattanzi, A.;
Scettri, A. Tetrahedron: Asymmetry 2002, 13, 1277–
1283.
15. Lattanzi, A.; Bonadies, F.; Senatore, A.; Soriente, A.;
Scettri, A. Tetrahedron: Asymmetry 1997, 8, 2473–
2478.
16. Kagan, H. B.; Fiaud, J. C. Top. Stereochem. 1988, 18,
249.
The above reported procedure for the catalytic sulfoxi-
dation was used with the substitution of sulfide with
racemic methyl p-tolyl sulfoxide. The reagents were
added to dry CH2Cl2 in the following order: sulfoxide
(1.0 equiv), (R,R)-diethyl tartrate (1.2 equiv), titanium
tetraisopropoxide (0.3 equiv), 0.6 equiv of cumyl hydro-
peroxide were employed.
17. Di Furia, F.; Licini, G.; Modena, G. Gazz. Chim. It. 1990,
120, 165–170.
Acknowledgements
18. Bulman Page, P. C.; Wilkes, R. D.; Namwindwa, E. S.;
Witty, M. Tetrahedron 1996, 52, 2125–2154.
19. Kokubo, C.; Katsuki, T. Tetrahedron 1996, 52, 13895–
13900.
We are grateful to MIUR (Ministero dellÕ Istruzione,
`
Universita e della Ricerca Scientifica) for financial
support.