Soluble polystyrene-based sulfoxide reagents for Swern oxidation reactions

Article abstract of DOI:10.1016/S0040-4020(03)01105-0

Two new soluble polystyrene-based sulfoxide reagents are introduced. These polymeric reagents are used in Swern oxidation reactions where the resulting sulfide polymers are easily separated from the product by simple precipitation and filtration. The reco

Full text of DOI:10.1016/S0040-4020(03)01105-0

Tetrahedron 59 (2003) 7171–7176
Soluble polystyrene-based sulfoxide reagents for
Swern oxidation reactions
*
Matthew Kwok Wai Choi and Patrick H. Toy
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China
Received 2 April 2003; revised 24 June 2003; accepted 4 July 2003
Abstract—Two new soluble polystyrene-based sulfoxide reagents are introduced. These polymeric reagents are used in Swern oxidation
reactions where the resulting sulfide polymers are easily separated from the product by simple precipitation and filtration. The recovered
reduced polymer reagents can be recycled by oxidation with tert-butyl hydroperoxide in the presence of acid. Attempts to use these reagents
in a multi-polymer Swern oxidation reaction system were unsuccessful.
q 2003 Elsevier Ltd. All rights reserved.
1
. Introduction
acid to commercial chloromethyl polystyrene beads
(Merrifield resin) to prepare an insoluble methyl sulfoxide
equivalent.
1
0,11
The oxidation of alcohols to carbonyl compounds is one of
the fundamental classes of reactions in organic chemistry.
Methyl sulfoxide-based oxidation is one such reaction and it
is among the most widely used methods for oxidizing
primary and secondary alcohols to aldehydes and ketones,
respectively. The broad range of oxidation applications in
which this reaction has been applied has meant that it is
continuously being refined and improved. In these reac-
tions, methyl sulfoxide is activated and reacts with an
alcohol to form a dimethylalkoxysulfonium salt. Upon
treatment with a base, the salt is deprotonated and affords
the desired carbonyl compound and methyl sulfide. A
variety of electrophilic reagents have been used to activate
They observed that this reagent afforded
high yields of oxidation products but that when the
recovered polymer was oxidized by sodium periodate, the
effectiveness of the recycled reagent was reduced. There-
fore, they then attached 6-(methylsulfinyl)hexanoic acid to
1
12
the soluble polymer poly(ethylene glycol). This polymer
supported reagent showed no loss of activity upon recycling.
Recently, Crich et al. have reported a fluorous Swern
oxidation reaction that uses tridecafluorooctylmethyl sulf-
oxide, which can be recovered and reused via a simple
continuous fluorous extraction procedure followed by
2
1
3
reoxidation with hydrogen peroxide. Finally, Node et al.
have also introduced a new odorless protocol for the Swern
oxidation that uses dodecyl methyl sulfoxide in place of
3
methyl sulfoxide including acetic anhydride, p-toluene-
4
sulfonyl chloride, phosgene, and cyanuric chloride.
5
6
14
methyl sulfoxide.
Among the various activators, oxalyl chloride has estab-
lished itself as the most broadly used reagent for methyl
sulfoxide-based alcohol oxidation in what is known as the
Soluble polymers are alternative supports to insoluble resins
for synthesis and reagent delivery and are becoming widely
7
15
Swern oxidation reaction. However, one drawback of all of
used. One such polymer, non-crosslinked polystyrene
(NCPS) has been used in a range of application that include
these reactions is that they produce the pungent smelling
and highly volatile byproduct methyl sulfide and, thus, the
modification of the Swern oxidation reaction to eliminate
the formation of this toxic substance has been a popular
research topic.
1
6
as a support for the synthesis of peptides and prosta-
1
7
18
19
glandins and for the delivery of tin and phosphorous
reagents. It is easily prepared with a wide variety of
functional groups by the radical co-polymerization of
2
0
styrene with a functionalized styrene. NCPS has even
been used as a support for the Dess–Martin periodinane
21,22
oxidation reagent.
The use of insoluble polymer supports to deliver reagents
8
,9
and remove their byproducts is now commonplace.
Vederas et al. have applied this methodology to the Swern
oxidation reaction by attaching 6-(methylsulfinyl)hexanoic
We have previously reported the use of NCPS to deliver
triethylamine equivalents in the REM resin-based solid-
2
3
phase synthesis of tertiary amines. This report is
noteworthy in that it is one of the few published examples
of a multi-polymer reaction system where a soluble polymer
Keywords: Swern oxidation; polymer; tert-butyl hydroperoxide.
*
Corresponding author. Tel.: þ852-2859-2167; fax: þ852-2857-1586;
e-mail: phtoy@hkucc.hku.hk
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4020(03)01105-0

7
172
M. K. W. Choi, P. H. Toy / Tetrahedron 59 (2003) 7171–7176
Table 1. Swern oxidation reactions using polymers 3a and 3b
is used to deliver a reagent or substrate to an insoluble
2
4,25
polymer.
Therefore, we sought to extend this method-
Entry
Alcohol
Product
Yields (%)
ology by using NCPS to deliver the base to methyl sulfoxide
derivatized NCPS in a multi-polymer Swern oxidation
reaction system.
Polymer 3a
Polymer 3b
1
2
3
4
92
65
55
61
82
2
. Results and discussion
75
79
63
In order to develop a multi-polymer Swern oxidation
reaction system, we prepared a soluble analogue of Crosby’s
1
1
thioanisole reagent as outlined in Scheme 1. Monomer 1a
2
6
was prepared according to the literature procedure and
was co-polymerized with styrene to afford polymer 2a.
5
6
60
61
80
71
(
Fig. 1). Unfortunately, attempts to use 4 for the
deprotonation step in a multi-polymer oxidation system
failed.
The failure of 3a to work in a multi-polymer system led
us to prepare polymer 3b (Scheme 1), in which the
sulfoxide moieties are further removed from the polymer
backbone than they are in 3a, to examine if steric effects
between the two polymer backbones caused the failure of 4.
Monomer 1b was prepared from 4-vinylbenzyl chloride and
3-(methylthio)propionic acid using potassium carbonate
and tetrabutylammonium iodide and then co-polymerized
with styrene to afford polymer 2b. This polymer was then
oxidized with t-BHP to afford 3b. The loading level of 3b
was determined by elemental analysis (S content) to be
0
.94 mmol/g. Again, this was in agreement to the loading of
1
Scheme 1. Synthesis of sulfoxide polymers 3a and 3b.
2b determined by H NMR analysis (Section 4). Polymer 3b
was used to oxidize the same set of alcohols that were
oxidized with 3a. As can be seen in Table 1, the two
polymers afforded comparable yields of ketones. However,
removing the sulfoxide moieties from the polymer back-
bone had no effect and 3b also failed to work with 4 in a
multi-polymer reaction system.
The sulfide moieties of 2a were oxidized using t-butyl
2
7
hydroperoxide (t-BHP) to afford sulfoxide polymer 3a.
The loading level of 3a was determined by elemental
analysis (S content) to be 1.25 mmol/g. This was in
1
agreement with the loading of 2a determined by H NMR
analysis. The fact that the ratio of monomer incorporation
(
styrene:1a) differs from the ratio of monomer input
To study the failure of 4 we prepared small molecule
analog 5 (Fig. 1). The successful application of 5 with
polymers 3a and 3b in the oxidation 1-(4-bromophenyl)-
ethanol (53 and 40% yield, respectively) indicates that the
polymeric nature of 4 is responsible for failure of the multi-
polymer system. Further substantiation of this was found
when 4 failed to afford the desired product when methyl
indicates that 1a more readily participates in the polymer-
ization process (Section 4). The use of 3a in the Swern
oxidation of secondary alcohols was examined and these
results are listed in Table 1. The reactions were carried out
using oxalyl chloride activation and with triethylamine as
the base. After addition of the base, the mixture was
partially concentrated in vacuo and poured into methanol to
precipitate the mixture of 2a and 3a. These polymers were
filtered off and the resulting methanol solution was again
concentrated in vacuo. The resulting product was then
filtered through a plug of silica gel to afford the product
1
ketone that was essentially pure as determined by H NMR
analysis. Having developed a protocol for the facile
isolation of pure oxidation products using 3a, we next
2
examined replacing triethylamine with the polymer 4
3
Figure 1.

M. K. W. Choi, P. H. Toy / Tetrahedron 59 (2003) 7171–7176
7173
sulfoxide was activated with oxalyl chloride. All attempts to
use 4 resulted in intractable polymer residues that were
difficult to separate from the reaction mixtures and analysis
of the reaction mixtures by thin layer chromatography
indicated that no desired product was present. To determine
if it is the soluble nature of 4 that is the problem, we then
examined commercially available insoluble diethylamino-
methyl-polystyrene resin and Amberlite IRA-67 ion-
exchange resin (basic form) as the base in the multi-polymer
system, but they too failed to produce the desired products.
solvents, such as t-BHP, are preferred for polymer
2
reoxidation.
7
3. Conclusions
In summary, two easily prepared and recyclable NCPS-
based sulfoxide polymers have been developed that can be
used in place of DMSO in Swern oxidation reactions. In the
oxidation of secondary alcohols, the polymers afforded
comparable yields of ketones that were isolated in
essentially pure form via two simple filtration operations.
The polymers were removed from the reaction product and
recovered for reuse by precipitation and filtration. Oxidation
of both the initially prepared and recovered polymers was
accomplished using t-BHP in the presence of acid.
Unfortunately, these polymers could not be successfully
applied in a multi-polymer oxidation system. Considering
the ease of preparation, cost of the starting materials, and
chemical stability, polymer 3a is preferred to 3b. Since
reagent recycling is not a major issue in modern drug
discovery, the observed diminishing yields upon reuse of the
polymers is not detrimental and 3a should be useful in
medicinal chemistry for the rapid synthesis of pure
compounds for biological screening. The use of polymer
3a in the high-throughput parallel synthesis of compounds
that can be isolated essentially pure is currently being
investigated.
2
8
w
We next examined the recycling of the polymers recovered
from the oxidation reactions. The mixtures of sulfide and
sulfoxide polymers (2a with 3a, and 2b with 3b) recovered
from the oxidation reactions were reoxidized using t-BHP
and the resulting polymers were reused. This cycle was
repeated three times for both 3a and 3b and the results are
listed in Tables 2 and 3, respectively. As can be seen from
these tables, the recycled polymers were essentially as
effective as were the original polymer with only a modest
decrease in yields between cycles. It should be noted that
even though reduced yields were observed, the products
were all essentially pure after isolation. Although we cannot
1
explain the decreased yields, we can say that based on H
NMR, 3a and 3b were essentially pure, with all of the sulfur
2
9
moieties in the sulfoxide oxidation state. Therefore,
incomplete oxidation or over oxidation is not the cause of
the observed results. Since the goal of modern combinatorial/
parallel synthesis in the context of medicinal chemistry and
drug discovery is the direct rapid production of pure
compound samples for biological screening, the diminish-
ing performance of 3a and 3b upon recycling may not limit
their utility.
4. Experimental
4.1. General
0
Table 2. Yield of 4 -bromoacetophenone from 1-(4-bromophenyl)ethanol
oxidation using recycled polymer 3a
All reagents were obtained from the Aldrich Chemical
Company or TCI Tokyo Kasei Organic Chemicals and
were used without further purification. All moisture
sensitive reactions were carried out in dried glassware
under a nitrogen atmosphere. Tetrahydrofuran was distilled
under a nitrogen atmosphere over sodium and benzo-
phenone. Dichloromethane and dimethyl formamide were
distilled from calcium hydride. Merck silica gel 60 (230–
Cycle Number
Yield (%)
1
2
3
4
92
86
79
75
400 mesh) was used for chromatography and filtration. Thin
layer chromatography analysis was performed using glass
plates coated with silica gel 60 F254. The NMR spectra were
recorded using a Bruker DRX 400 spectrometer. Chemical
shift data is expressed in ppm with reference to TMS.
Elemental analyses were conducted at the Analytical and
Testing Center of the Shanghai Institute of Organic
Chemistry.
Table 3. Yield of benzil from benzoin oxidation using recycled polymer 3b
Cycle Number
Yield (%)
1
2
3
4
80
70
63
59
2
6
4
.1.1. 4-Vinylphenyl methyl sulfide (1a). Methyl disul-
Interestingly, the fact that these polymers remain soluble
and can be recycled at all indicates that previous speculation
that the use of oxallyl chloride with polystyrene results in
fide (21.6 g, 229 mmol) was added slowly at 08C to a
solution of the Grignard reagent prepared from 4-bromo-
styrene (28.0 g, 153 mmol) and Mg (7.4 g, 305 mmol) in
dry THF (200 mL). The mixture was stirred at room
temperature for 3 h. At this time, the reaction mixture was
diluted with diethyl ether (500 mL), and then washed
sequentially with water (250 mL), 10% aqueous HCl
(250 mL), saturated aqueous NaHCO (250 mL) and brine
3
(250 mL). The organic layer was dried over MgSO , filtered
4
and concentrated in vacuo. The crude product was purified
by silica gel chromatography (5% EtOAc/hexanes) to afford
1
0
polymer cross-linking is not valid. If this were to happen,
cross-linking of polymers 3a and 3b would have made them
become insoluble and forced them to precipitate from the
reaction mixtures. Therefore, our findings confirm that poor
polymer compatibility with water was probably the cause
for the previously reported diminished yields upon recy-
cling of related insoluble polymer based sulfoxide reagents
and that the use of oxidizing reagents that require organic

7174
M. K. W. Choi, P. H. Toy / Tetrahedron 59 (2003) 7171–7176
1
1
(
0
a as a clear, colorless liquid (16.0 g, 70%). H NMR
(200 mL) was treated with 70% t-BHP (21.7 mL,
157 mmol) and para-toluenesulfonic acid (6.3 g, 33 mmol).
The mixture was stirred at room temperature for 24 h and
then concentrated in vacuo. The residue was taken up in
50 mL of THF and this solution was added dropwise to a
vigorously stirred cold methanol (1 L). The white precipi-
tate was filtered and dried to afford 3a as a white powder
(24.9 g, 94%). A sulfur loading level of 1.25 mmol/g was
400 MHz, CDCl ) d 2.45 (s, 3H), 5.21 (dd, 1H, J¼10.9,
3
.9 Hz), 5.70 (dd, 1H, J¼17.6, 0.9 Hz), 6.68 (dd, 1H,
1
3
J¼17.6, 10.9 Hz), 7.17–7.40 (m, 4H). C NMR (100 MHz,
CDCl ) d 15.7, 113.1, 126.6 (4C), 134.5, 136.2, 138.0. HR
3
FAB-MS: calcd for C H S, 150.0503; found, 150.0500.
9 10
4
.1.2. 4-Vinylbenzyl 3-(methylthio)propanoate (1b). To
1
an anhydrous DMF (200 mL) solution of 4-vinylbenzylchlor-
ide (21.7 g, 140 mmol) was added 3-(methylthio)propionic
acid (19.0 mL, 170 mmol), potassium carbonate (38.7 g,
80 mmol) and tetrabutylammonium iodide (5.2 g,
4 mmol). The reaction mixture was stirred for 24 h at 508C.
After cooling to room temperature, it was then filtered through
Celite and concentrated in vacuo. The crude product was
dissolved in CH Cl (300 mL) and washed sequentially with
determined by elemental analysis. H NMR (400 MHz,
CDCl ) d 1.26–2.05 (bm, 21H), 2.62 (bs, 3H) and 6.23–
3
7.19 (bm, 33H). The ratio of monomer incorporation into 3a
was determined by elemental analysis (4.03% S) to be 6.1:1
(styrene/1a). This corresponds to a loading level of
1.25 mmol/g of polymer.
2
1
4.3.2. Poly(styrene-co-4-vinylbenzyl 3-[methylsulfinyl]
propanoate) (3b). A solution of 2b (24.0 g, 22 mmol) in
CH Cl (200 mL) was treated with 70% t-BHP (13.8 mL,
2
2
1
0% aqueous HCl (250 mL), saturated, aqueous NaHCO3
250 mL) and brine (250 mL). The organic layer was dried
(
over MgSO , filtered, and concentrated in vacuo. The crude
2
2
100 mmol) and para-toluenesulfonic acid (4.2 g, 22 mmol).
The mixture was stirred at room temperature for 24 h and
then concentrated in vacuo. The residue was taken up in
50 mL of THF and this solution was added dropwise to
vigorously stirred cold methanol (1 L). The white precipi-
tate was filtered and dried to afford 3b as a white powder
(23.7 g, 95%). A sulfur loading level of 0.94 mmol/g was
4
product was purified by silica gel chromatography (5%
EtOAc/hexanes) to afford 1b as a pale yellow liquid (20.6 g,
2%). H NMR (400 MHz, CDCl ) d 2.11 (s, 3H), 2.67 (t, 2H,
3
1
6
J¼7 Hz), 2.76 (t, 2H, J¼7 Hz), 5.13 (s, 2H), 5.26 (dd, 1H,
J¼10.9, 0.8 Hz), 5.76 (dd, 1H, J¼17.6, 0.8 Hz), 6.72 (dd, 1H,
1
3
J¼17.6, 10.9 Hz), 7.25–7.47 (m, 4H). C NMR (400 MHz,
1
CDCl ) d 15.5, 29.1, 34.5, 66.3, 114.4, 126.4, 128.5, 135.2
3
determined by elemental analysis. H NMR (400 MHz,
CDCl ) d 1.27–2.00 (bm, 24H), 2.55 (bs, 3H), 2.87 (bs, 2H),
(2C), 136.3, 137.7 (2C), 171.8. HR FAB-MS: calcd for
C H O S, 236.0871; found, 236.0817.
3
3.00 (bs, 2H), 5.05 (bs, 2H) and 6.20–7.21 (bm, 36H). The
ratio of monomer incorporation into 3b was determined by
elemental analysis (3.06% S) to be 7.8:1 (styrene/1b). This
corresponds to a loading level of 0.94 mmol/g of polymer.
1
3 16 2
4
4
.2. NCPS-sulfide polymers
.2.1. Poly(styrene-co-4-vinylphenyl methyl sulfide) (2a).
To a solution of styrene (55.5 g, 533 mmol) and 1a (10.0 g,
4.4. General procedure for alcohol oxidation
6
3
7 mmol) in toluene (400 mL) was added AIBN (0.4 g,
mmol). The mixture was purged with N for 30 min and
A solution of 3a or 3b (4 equiv.) in anhydrous CH Cl
2
2
2
the solution was stirred at 908C for 24 h. The solution was
concentrated in vacuo and then the residue was taken up in
(30 mL) was cooled to 2708C and oxalyl chloride
(3.7 equiv.) was added dropwise. After 30 min a solution
of the alcohol (1 equiv.) in anhydrous CH Cl was added.
40 mL of THF. This solution was added dropwise to a
vigorously stirred cold methanol (1 L). The white precipi-
2
2
The mixture is stirred for at low temperature for 1 h and then
triethylamine (6 equiv.) was added. The solution is kept at
2408C for 1 h and then allowed to warm to room
temperature. The reaction mixture was then concentrated
in vacuo and then THF (10–15 mL) was added. The
solution was then poured into cold methanol (100 mL) to
tate was filtered and dried to afford 2a as a white powder
1
(
25.7 g, 36%). H NMR (400 MHz, CDCl ) d 1.26–2.05
3
(
ratio of monomer incorporation into 2a was determined by
bm, 21H), 2.42 (bs, 3H) and 6.23–7.19 (bm, 33H). The
1
H NMR to be 6:1 (styrene/1a).
3
0
precipitate the polymer. After filtration the filtrate is
concentrated in vacuo and the crude residue was filtered
through a plug of silica gel to provide essentially pure
oxidized product. (Table 1). These reactions were typically
performed on a 1 mmol scale.
4
.2.2. Poly(styrene-co-4-vinylbenzyl 3-[methylthio]pro-
panoate) (2b). To a solution of styrene (35.3 g, 339 mmol)
and 1b (10.0 g, 42 mmol) in toluene (400 mL) was added
AIBN (0.3 g, 2 mmol). The mixture was purged with N for
2
30 min and the solution was stirred at 908C for 24 h. The
solution was concentrated in vacuo and then the residue was
takenupin40 mLofTHF.Thissolutionwasaddeddropwiseto
vigorously stirred cold methanol (1 L). The white precipitate
4.5. General procedure for recycling of the sulfoxide
polymers 3a and 3b
was filtered and dried to afford 2b as a white powder (24.0 g,
The polymer mixtures (2a and 3a or 2b and 3b) recovered
from the oxidation reactions were treated with 70% t-BHP
(5 equiv.) and para-toluenesulfonic acid (1 equiv.) in
CH Cl (50 mL). The mixture was stirred at room
temperature for 24 h and then concentrated in vacuo. The
residue was taken up in 20 mL of THF and this solution was
added dropwise to vigorously stirred cold methanol
1
5
2
6
2
2%). H NMR (400 MHz, CDCl ) d 1.21–2.09 (bm, 24H),
3
.11 (bs, 3H), 2.67 (bs, 2H), 2.78 (bs, 2H), 5.05 (bs, 2H) and
.20–7.21(bm, 36H). Theratioofmonomerincorporationinto
2
2
1
b was determined by H NMR to be 8:1 (styrene/1b).
4
.3. NCPS-sulfoxide polymers
(100 mL). The white precipitate was filtered and dried.
The polymers were determined to be completely and cleanly
oxidized to either 3a or 3b by H NMR analysis.
4
(
.3.1. Poly(styrene-co-4-vinylphenyl methyl sulfoxide)
3a). A solution of 2a (25.7 g, 33 mmol) in CH Cl2
1
31
2

M. K. W. Choi, P. H. Toy / Tetrahedron 59 (2003) 7171–7176
7175
3
2
4
.6. N,N-Diethyl-4-methylbenzylamine (5)
J. Chem. Ed. 1980, 57, 438. (b) Frechet, J. M. J.; Darling, P.;
Farrall, M. J. J. Org. Chem. 1981, 46, 1728–1730. (c)
Bergbreiter, D. E.; Chandran, R. J. Am. Chem. Soc. 1985, 107,
4792–4793. (d) Hinzen, B.; Ley, S. V. J. Chem. Soc. Perkin
Trans. 1 1997, 1907–1908. (e) Dodd, D. S.; Wallace, O. B.
Tetrahedron Lett. 1998, 39, 5701–5704. (f) Ley, S. V.; Bolli,
M. H.; Hinzen, B.; Gervois, A. G.; Hall, B. J. J. Chem. Soc.
Perkin Trans. 1 1998, 2239–2241. (g) Bhalay, G.; Dunstan,
A.; Glen, A. Synlett 2000, 12, 1846–1859.
To a THF (100 mL) solution of 4-methylbenzyl chloride
21.2 g, 151 mmol) was added diethylamine (13.5 g,
86 mmol) and 30 mL of 5N aqueous NaOH. The reaction
(
1
mixture was stirred for 24 h at room temperature. At this
time, the reaction mixture was poured into water was
extracted with diethyl ether (4£100 mL). The combined
organic layers were dried with MgSO , filtered, and
4
concentrated in vacuo. The light-yellow crude product
was purified by distillation (908C, 0.4 mm Hg) to afford 5 as
10. Liu, Y.; Vederas, J. C. J. Org. Chem. 1996, 61, 7856–7859.
11. In related work, an earlier report described the use of an
insoluble methyl phenyl sulfoxide reagent that was activated
with chlorine Crosby, G. A.; Weinshenker, N. M.; Uh, H.-S.
J. Am. Chem. Soc. 1975, 97, 2232–2235.
1
a clear, colorless liquid (20.9 g, 78%). H NMR (400 MHz,
CDCl ) d 1.03 (t, 6H, J¼7.1 Hz), 2.33 (s, 3H), 2.51 (q, 4H,
3
J¼7.1 Hz), 3.52 (s, 2H), 7.11 (d, 2H, J¼8 Hz), 7.21 (d, 2H,
1
3
J¼8 Hz). C NMR (100 MHz, CDCl ) d 11.7, 21.0, 46.5,
3
12. Harris, J. M.; Liu, Y.; Chai, S.; Andrews, M. D.; Vederas, J. C.
J. Org. Chem. 1998, 63, 2407–2409.
13. (a) Crich, D.; Neelamkavil, S. J. Am. Chem. Soc. 2001, 123,
57.1, 128.7 (2C), 128.8 (2C), 135.9, 136.7. HR FAB-MS:
calcd for C H N, 177.1517; found, 177.1516.
1
2
19
7
449–7450. (b) Crich, D.; Neelamkavil, S. Tetrahedron 2002,
8, 3865–3870.
5
1
4. Nishide, K.; Ohsugi, S.; Fudesaka, M.; Kodama, S.; Node, M.
Acknowledgements
Tetrahedron Lett. 2002, 43, 5177–5179.
This research was supported financially by the University of
Hong Kong and the Research Grants Council of the Hong
Kong Special Administrative Region, P. R. of China
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(
Project No. HKU 7112/02P). We would also like to thank
Mr Michael McNello and the Aldrich Chemical Company
for their gift of many of the reagents used in this project.
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Kozhevnikova, I. V. Tetrahedron Lett. 1965, 6, 2323–2327.
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1
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