Highly chemoselective oxidation of dithioester enethiolates to sulfenates: Application to the synthesis of ketene dithioacetal S-oxides

Article abstract of DOI:10.1021/jo0492362

Enethiolates derived from dithioesters were efficiently converted into the corresponding vinyl sulfenates by oxidation with a unique N-sulfonyloxaziridine 1a derived from pinacolone. Subsequent alkylation with alkyl halides led to ketene dithioacetal S-oxides in good to excellent yields.

Full text of DOI:10.1021/jo0492362

The two conventional preparative methods for these
sulfoxides are based upon (i) oxidation6 of the corre-
sponding ketene dithioacetal, the best reagents being
m-CPBA and NaIO4, and (ii) the use of methyl (methyl-
sulfanyl)methyl sulfoxide or functionalized derivatives as
starting materials and creation of the double bond by
Knoevenagel-type reactions,7 Wittig-Horner condensa-
tions,8 or elimination reactions.9 A high (E)-stereoselec-
tivity is generally achieved. A completely different ap-
proach involving a sulfine from a dithioester has also
been described by the group of Zwanenburg, but is limited
to a single substrate.10 In this synthesis, R-deprotonation
of the thiocarbonyl S-oxide generates a vinyl sulfenate
that is captured in situ at the sulfur center by an alkyl
halide.11 This deprotonation/alkylation sequence suffers,
however, from a few drawbacks such as the instability
of the starting sulfine5,12 and the requirement of a
thallium base [thallium(I) ethoxide].10,11 With all other
bases, O-alkylation of the ambident sulfenate surpris-
ingly occurs. The development of an efficient and general
alternative pathway to the sulfenate intermediates could
allow further investigation of this sequence.
High ly Ch em oselective Oxid a tion of
Dith ioester En eth iola tes to Su lfen a tes:
Ap p lica tion to th e Syn th esis of Keten e
Dith ioa ceta l S-Oxid es
Franck Sandrinelli, Gae¨lle Fontaine,
Ste´phane Perrio,* and Pierre Beslin
Laboratoire de Chimie Mole´culaire et Thio-organique
(UMR CNRS 6507), ENSICaen-Universite´ de Caen
Basse-Normandie, 6 Boulevard du Mare´chal J uin,
14050 Caen, France
stephane.perrio@ismra.fr
Received May 5, 2004
Abstr a ct: Enethiolates derived from dithioesters were
efficiently converted into the corresponding vinyl sulfenates
by oxidation with a unique N-sulfonyloxaziridine 1a derived
from pinacolone. Subsequent alkylation with alkyl halides
led to ketene dithioacetal S-oxides in good to excellent yields.
We have previously reported13 a highly efficient ap-
proach to aromatic sulfenates (ArSOLi) through oxidation
of the corresponding thiolates (ArSLi) in which the
unusual racemic N-sulfonyloxaziridine 1a , derived from
pinacolone,14 was introduced as the optimal reagent
(Scheme 1). Subsequent S-alkylation with aliphatic
Ketene dithioacetal mono-S-oxides have proven to be
useful intermediates in organic synthesis.1 They can act
as good Michael acceptors for a variety of nucleophiles,
including enamines, enolates, and carbanions derived
from imines or nitroalkanes.2 In addition, they are
efficient precursors of carboxylic acid derivatives3 or
thiocarbonyl compounds.4 An original and high-yielding
transformation into γ-hydroxyvinyl sulfides, which can
serve as synthetic equivalents of â-hydroxy aldehydes,
has also been reported.5
(5) Bonini, B. F.; Franchini, M. C.; Mazzanti, G.; Slief, J .-W.;
Wegman, M. A.; Zwanenburg, B. J . Chem. Soc., Chem. Commun. 1997,
1011-1012.
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1977, 522-523. (b) Kaya, R.; Beller, N. R. J . Org. Chem. 1981, 46,
196-197. (c) J ust, G.; Potvin, P.; Hakimelahi, G. H. Can. J . Chem.
1980, 8, 2780-2783. (d) Davis, F. A.; Mancinelli, P. A. J . Org. Chem.
1980, 45, 2597-2601.
* To whom correspondence should be addressed. Phone: Int. code
+23145-2884. Fax: Int. code +23145-2877.
(1) (a) Pinard, E.; Gaudry, M.; He´not, F.; Thellend, A. Tetrahedron
Lett. 1998, 39, 2739-2742. (b) Ishii, H.; Ozawa, M.; Ohta, S.;
Harayama, T.; Ishikawa, T. Heterocycles 1994, 37, 897-902. (c) Garner,
P.; Ho, W. B.; Shin, H. J . Am. Chem. Soc. 1993, 115, 10742-10753.
(d) Pearson, A. J .; Shin, H. Tetrahedron 1992, 48, 7527-7538. (e)
Schuda P. F.; Price W. A. J . Org. Chem. 1987, 52, 1972-1979. (f)
Rizzacasa, M. A.; Sargent, M. V. J . Chem. Soc., Perkin Trans. 1 1987,
2017-2022. (g) Krohn, K.; Klimars, M.; Ko¨hle, H.-J .; Ebeling, E.
Tetrahedron 1984, 40, 3677-3694. (h) Artico, M.; Corelli, F.; Massa,
S.; Stefancich J . Heterocycl. Chem. 1982, 19, 1493-1495. (i) Kiec-
zykowski, G. R.; Pogonowski, C. S.; Richman, J . E.; Schlessinger, R.
H. J . Org. Chem. 1977, 42, 175-176. (j) Romanet, R. F.; Schlessinger,
R. H. J . Am. Chem. Soc. 1974, 96, 3701-3702. (k) Bo¨s, M.; Stadler H.;
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Bull. 1982, 30, 2440-2446.
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Wepplo, P. J .; Schlessinger, R. H. Tetrahedron Lett. 1973, 14, 4711-
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1982, 47, 5017-5019. (d) Cloudsdale, I. S.; Kluge, A. F.; McClure, N.
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M.; Hanano, T.; Aoki, Y.; Mishina, T.; Arita, M.; Nakao, N.; Ohtsuki,
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Chem. 2000, 43, 2946-2961. (c) Wang, Z.; Li, Z.; Ren, J .; Chen, H.
Synth. Commun. 1999, 29, 2355-2359. (d) Wang, Z.; Li, Z.; Liu, T.;
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Chapat, J .-P. Heterocycles 1993, 36, 2513-2522. (b) Gairns, R. S.;
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Perin, G.; J acob, R. G.; Braga, A. L. Phosphorus Sulfur 2001, 172, 55-
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(9) (a) Lo´pez Sastre, J . A.; Mart´ın-Ramos, J . D.; Mart´ınez-Arago´n,
A. B.; Molina Molina, J .; Amo, J . R.; Solans, X. J . Chem. Res. (S) 1993,
60-61. (b) Herrmann, J . L.; Richman, J . E.; Wepplo, P. J .; Schlessinger,
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S.; J anousek, Z.; Merenyi, R.; Viehe, H. G. Tetrahedron 1985, 41, 2527-
2543. (e) Schaumann, E.; Winter-Extra, S.; Kummert, K.; Scheiblich,
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G.-i. Chem. Lett. 1974, 659-662.
(10) Veenstra, G. E.; Zwanenburg, B. Recl. Trav. Chim. Pays-Bas
1976, 95, 202-205.
(11) A similar result has also been described starting with thiocam-
phor S-oxide: Veenstra, G. E.; Zwanenburg, B. Recl. Trav. Chim. Pays-
Bas 1976, 95, 37-39.
(12) (a) Metzner, P. Top. Curr. Chem. 1999, 204, 127-181. (b)
Zwanenburg, B. Phosphorus Sulfur 1989, 43, 1-24.
(13) (a) Sandrinelli, F.; Perrio, S.; Beslin, P. J . Org. Chem. 1997,
62, 8626-8627. (b) Sandrinelli, F.; Perrio, S.; Averbuch-Pouchot, M.-
T. Org. Lett. 2002, 4, 3619-3622. For a very recently published review
on sulfenate salts, see: O’Donnell, J . S.; Schwan, A. L. J . Sulfur Chem.
2004, 25, 183-211.
(14) With the exception of our work, there is only one reported
example of the use of oxaziridine 1a (hydroxylation of a â-lactam).
Shimizu, M.; Ishida, T.; Fujisawa, T. Chem. Lett. 1994, 1403-1406.
10.1021/jo0492362 CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/27/2004
6916
J . Org. Chem. 2004, 69, 6916-6919

SCHEME 1. Ar om a tic Su lfoxid es via Su lfen a tes
simple example in which the stereochemistry was not a
concern. Dropwise addition of oxaziridine 1a (1 equiv)
to the anion of dithioacetic acid methyl ester 2a (R1 )
H), generated by deprotonation with methyllithium at
-78 °C, resulted in a spontaneous reaction in which all
the oxidant was consumed. Treatment of the resulting
colorless solution with methyl iodide (1 equiv) at -78 °C,
followed by warming to room temperature and hydrolysis
with ammonium chloride solution, gave an extremely
clean crude product, consisting of the anticipated dithio-
acetal S-oxide 3a 1 and benzenesulfonamide.20 The latter
was easily removed by precipitation from dichloromethane/
pentane and the sulfoxide 3a 1 purified by column chro-
matography and isolated in 79% yield (Table 1, entry 1).
Using benzyl bromide as an alternative electrophile, the
analogous benzylic sulfoxide 3a 2 was produced in 56%
yield (entry 2). An important point to note is that hydroxy
and methylsulfinyl products were not detected, thus
revealing complete chemoselectivity in favor of the
negatively charged sulfur center. Furthermore, alkylation
with the soft alkyl halide took place at the soft sulfur
center of the sulfenate.
With these results in hand, the procedure was next
applied to dithioester 2b (R1 ) Me). Treatment of the
intermediate sulfenate with methyl iodide at -78 °C
(conditions B) led to the formation of ketene dithioacetal
S-oxide 3b1 in 61% yield as a 76:24 mixture of the (Z)
and (E) isomers, which reflected perfectly the stereose-
lectivity of the initial deprotonation (entry 3).21 This
configuration was assigned on the basis of literature
data22 and further confirmed by NMR spectra in the
presence of europium23 shift reagents. The reaction was
then repeated on the same substrate, but this time the
temperature of the sulfenate solution was allowed to rise
slowly to -15 °C before exposure to the electrophile
(conditions C). This afforded sulfoxide 3b1 in 80% yield,
but with an almost completely inverted isomeric ratio (13:
87, entry 4). An intermediate situation with a 54:46
mixture of isomers was obtained when removing the cold
bath at -78 °C immediately after introduction of the
electrophile (entry 5, conditions D). A similar switch in
favor of the (E) isomer was likewise observed when using
benzyl bromide and ethyl iodide as alternative electro-
philes under conditions C (entries 6 and 7).24 The result-
ing diastereoisomeric sulfoxides of 3b1, 3b2, and 3b3 were
SCHEME 2. Keten e Dith ioa ceta l S-Oxid es 3 via
Vin yl Su lfen a tes
halides led to sulfoxides in good to excellent yield. Earlier
examples of this oxidation reaction in the sulfur series
are scarce, the few oxidizing reagents that were tested
(m-CPBA, hydroperoxides) having proven insuitable.15 As
a consequence, applications of this concept in organic
synthesis have hitherto remained unexploited. The mild
conditions we have developed (-78 °C) have broadened
the scope of this reaction quite considerably, with good
chemoselectivity and tolerance of a wide range of sub-
strates.
These results suggested to us that R,â-unsaturated
sulfenates could, in principle, be prepared by oxidation
of the corresponding enethiolates, which are readily
available by deprotonation16 of dithioesters (Scheme 2).
An important feature of this sequence would be to start
this time from a stable thiocarbonyl compound. Further-
more, if the overall sequence proceeds with retention of
the enethiolate geometry, the (Z)-isomer should be pre-
dominantly produced.17 Inadequate chemoselectivity is,
however, a potential problem as there could be competi-
tive oxidation of two other sites in the molecule, namely
the alkylthio substituent and the monosubstituted carbon
atom of the double bond. The latter is known to occur
readily at low temperature in the corresponding oxygen-
ated series, thereby yielding R-hydroxy carbonyl com-
pounds.18,19 In contrast, the thioether function should be
unaffected, as oxidation of the methylsulfanyl functional-
ity in thioanisole with oxaziridine 1a (1 equiv) took more
than 1 day at room temperature to go to completion.13a
In this paper we present the results obtained with this
oxidation approach.
The choice of base for the generation of the enethiolate
followed an extensive study that identified methyllithium
as the optimal reagent. Use of amides such as LDA,
NaHMDS, or KHMDS led to complex mixtures, suggest-
ing unwanted side reactions with the liberated secondary
amine. We began our investigations by examining a
(20) The imine byproduct of the oxidation reaction undergoes
hydrolytic cleavage to benzenesulfonamide and the volatile pinacolone.
(21) The cis:trans ratio of the enethiolates obtained by deprotonation
of dithioesters 2 with methyllithium was determined by conversion
into the corresponding ketene dithioacetals with an ethyl iodide quench
and 1H NMR analysis of the crude product, based on the vinyl hydrogen
and the SCH2 protons (see the Supporting Information).
(22) Remarkable differences are observed in the 1H NMR data of
the pair of (Z) and (E) isomers. The vinylic proton in the (Z) series
occurs at approximately δ 6.3 ppm but is substantially deshielded at
around 6.8 ppm in the corresponding (E) isomer. (a) Cazes, B.; Huynh,
C.; J ulia, S.; Ratovelomanana, V.; Ruel, O. J . Chem. Res. (M) 1978,
957-968. (b) Seebach, D.; Bu¨rstinghaus, R.; Gro¨bel, B.-T.; Kolb, M.
Liebigs. Ann. Chem. 1977, 830-845.
(15) (a) Heckel, A.; Pfleiderer, W. Tetrahedron Lett. 1983, 24, 5047-
5050. (b) Hogg, D. R.; Rashid, M. A. M. J . Chem. Res. (S) 1988, 160-
161. (c) Ishii, A.; Komiya, K.; Nakayama, J . J . Am. Chem. Soc. 1996,
118, 12836-12837.
(16) Metzner, P. Synthesis 1992, 1185-1199.
(17) The deprotonation of dithioesters leads predominantly to cis
enethiolates (SLi and R1 groups located on the same side of the CdC
bond). Beslin, P.; Valle´e, Y. Tetrahedron 1985, 41, 2691-2705. In
contrast, the analogous carbonyl compounds afford trans enolates
under similar kinetic conditions.
(18) Davis, F. A.; Chen, B.-C. Chem. Rev. 1992, 92, 913-934.
(19) For reported syntheses of R-hydroxydithioesters see: (a) Poupaert,
J .; Bruylants, A. Bull. Soc. Chim. Belg. 1976, 85, 431-434. (b) Li, J .;
Guha, K.; Creighton, D. J . Biochem. Biophys. Res. Commun. 1991, 181,
657-663. (c) Scheithauer, S. German (East) Patent 86823, 1973.
(23) The sulfinyl functional group is known to bind strongly to
lanthanide shift reagents, as a result of which any vinyl hydrogens
cis to this moiety are shifted noticeably downfield. Eu(fod)3 was used
for the spectra recorded with compounds 3b1 and 3b2. See the
Supporting Information. (a) Dannhardt, G.; Kiefer, W. Arch. Pharm.
(Weinheim, Ger.) 1994, 327, 509-514. (b) Mazzanti, G.; van Helvoirt,
E.; van Vliet, L. A.; Ruinaard, R.; Masiero, S.; Bonini, B. F.; Zwanen-
burg, B. J . Chem. Soc., Perkin Trans. 1 1994, 3299-3304. (c) Schwan,
A. L.; Roche, M. R.; Gallagher, J . F.; Ferguson, G. Can. J . Chem. 1994,
72, 312-324.
J . Org. Chem, Vol. 69, No. 20, 2004 6917

TABLE 1. Keten e Dith ioa ceta l S-Oxid es 3 fr om Dith ioester s 2 Accor d in g to Sch em e 2
alkylation
deprotonation
cis:transc
entry
dithioester
R1
R2
sulfoxide
conditionsa
yield (%)
(Z):(E)b
1
2
3
4
5
6
7
8
9
2a
2a
2b
2b
2b
2b
2b
2c
2d
2d
2e
H
H
Me
Me
Me
Me
Me
n-Pr
n-Oct
n-Oct
Ph
Me
Bn
Me
Me
Me
Bn
Et
Me
Me
Me
Me
3a 1
3a 2
3b1
3b1
3b1
3b2
3b3
3c1
3d 1
3d 1
3e1
A
A
B
C
D
C
C
B
B
C
B
79
56
61
80
76
66
76
58
85
62
26
76:24
13:87
54:46
22:78
15:85
79:21
74:26
42:58
58:42
75:25
75:25
75:25
75:25
75:25
81:19
76:24
76:24
63:37
10
11
a
Conditions A: Slow warming from -78 °C to rt in the presence of the electrophile (1.1 equiv). Conditions B: -78 °C/5 h in the
presence of the electrophile (5 equiv). Conditions C: Slow warming from -78 to -15 °C, followed by addition of the electrophile (1.1-2
b
equiv). Conditions D: Rapid warming from -78 °C to rt in the presence of the electrophile (1. equiv). Determined by 1H NMR of the
crude product. c Determined by reaction of the enethiolate with ethyl iodide.
SCHEME 3. Oxid a tion w ith Oxa zir id in e 1b
easily separated by column chromatography on silica gel.
Enethiolates and the final sulfoxides being configuration-
ally stable,16 these observations suggest that the inter-
mediate sulfenates, which initially have a cis geometry,
can undergo isomerization into the thermodynamically
stable trans anion. The reaction temperature is therefore
of critical importance to the product diastereoselectivity
(alkylation is rather slow at low temperature), though
the initial configuration remains stable for as long as the
low temperature is maintained.
The reaction has been further extended to three other
dithioesters 2c-e with iodomethane in each case em-
ployed as the electrophile. A series of ketene dithioacetal
monoxides 3 was thus prepared and, with performance
of the entire sequence at -78 °C (conditions B), the
diastereomeric ratios were identical with those of the
as a single diastereoisomer.27 Rationalizing this finding
by comparison with the results previously obtained by
us with the same reagent in the thiophenol series,28 it is
likely that (i) oxidation of the enethiolate selectively
afforded the corresponding sulfinate salt (i.e. the double
enethiolate precursors21 (entries 8, 9, and 11). In all cases,
both double bond isomers were separable by purification
on silica gel and the stereochemistry was assigned on the
basis of NMR data.22,25 Isomerization of the intermediate
sulfenate under conditions C was once again observed
in the case of dithioester 2d (entry 10).
The oxidation was also investigated by using the
classical benzaldehyde-derived N-sulfonyloxaziridine 1b
(Scheme 3).26 Upon addition of the oxidizing solution, an
obvious difference was apparent, with the reaction mix-
ture immediately developing a bright orange color char-
acteristic of a dithioester function. After treatment with
an alkyl halide and workup, no sulfoxide was produced
and instead amino dithioester 5 was isolated in 43% yield
oxidation product) with (ii) nucleophilic addition of the
enethiolate on the liberated N-sulfonylimine PhCHd
NSO2Ph resulting in adduct 5 as a sideproduct and that
(iii) the absence of any significant alkylation in the THF
solution led to extraction of the sulfinate into the aqueous
layer. Thus, with a modification of the original reaction
conditions, concentration of the aqueous layer followed
by treatment with methyl iodide in THF/DMF led to vinyl
sulfone 6 in 21% yield.29 Both diastereoisomers were
produced in a 56:44 ratio, but separation by column
chromatography on silica gel was unsuccessful. The
origin of this difference in behavior between 1a and 1b
is unclear: it might be a consequence of steric effects or
electronic factors or it may simply reflect a difference in
oxidizing power. A noteworthy observation that should
be pointed out is that sulfenate anions, whose structure
contains an unshared electron pair adjacent to the sulfur
center, are considered to be R-nucleophiles30 with height-
ened reactivity, therefore the double oxidation reaction
leading to the sulfinate ought to be preferred.31
(24) The sulfenate formed from dithioester 2b was also subjected
to a simple aqueous workup, resulting in the isolation of thiolester 4
in 25% yield as a 83:17 mixture of two diastereoisomers. Its formation
can be interpreted by the following pathway: protonation of the
oxidized anion to give the sulfenic acid, dimerization into the corre-
sponding thiosulfinate with elimination of H2O, Claisen rearrangement
into a sulfine, and finally rearrangement, via an oxathiirane, with loss
of elemental sulfur (see the Supporting Information).
(27) The relative configuration was not determined.
(28) Sandrinelli, F.; Perrio, S.; Beslin, P. Org. Lett. 1999, 1, 1177-
1180.
(29) The yield is calculated from the dithioester based on an
optimum value of 50%, as a single equivalent of oxidant is used.
(30) Kice, J . L.; Cleveland, J . P. J . Am. Chem. Soc. 1973, 95, 104-
109.
(31) (a) Davis, F. A.; Billmers, R. L. J . Am. Chem. Soc. 1981, 103,
7016-7018. (b) Davis, F. A.; Rizvi, S. Q. A.; Ardecky, R.; Gosciniak,
D. J .; Friedman, A. J .; Yocklovich, S. G. J . Org. Chem. 1980, 45, 1650-
1653. (c) Refvik, M. D.; Schwan, A. L. Can. J . Chem. 1998, 76, 213-
220.
(25) In the case of compound 3e1, which has a phenyl substituent,
a 0.7 ppm downfield shift is observed for the vinylic proton, but the
singlet for the (Z) isomer again occurs at higher field (δ 7.00 against
7.63 ppm).
(26) Davis, F. A.; Sheppard, A. C. Tetrahedron 1989, 45, 5703-5742.
6918 J . Org. Chem., Vol. 69, No. 20, 2004

was slowly added dropwise (exothermic reaction). The reaction
mixture was stirred at this temperature for 20 min and treated
with the alkyl halide (1-5 equiv). The solution was stirred at
-78 °C for 5 h and then hydrolyzed at this temperature with
saturated aqueous NaCl (3 mL). The product was extracted with
Et2O (3 × 30 mL) and the combined organic extracts were dried
over magnesium sulfate, filtered, and evaporated to dryness. A
7:1 pentane:dichloromethane mixture was added to precipitate
the benzenesulfonamide byproduct from the crude mixture. After
filtration and concentration under reduced pressure the resulting
yellow oil was analyzed by 1H NMR to determine the diastere-
oisomeric ratio and purified by column chromatography (silica
gel, petroleum ether/ethyl acetate mixtures) to give the antici-
pated sulfoxides 3. The individual diastereoisomers were sepa-
rated and isolated in the same proportions as were observed in
the crude material.
In conclusion, we have shown that our methodology
for the oxidation of anionic sulfur centers using the
pinacolone-derived N-sulfonyloxaziridine 1a can be suc-
cessfully extended to enethiolates from dithioesters as
substrates. S-alkylation of the resulting vinyl sulfenates
with an alkyl halide affords ketene dithioacetal S-oxides
in good yield. The overall sequence provides an original
and straightforward route to these compounds, the major
advantage of being carried out in one pot starting from
the thiocarbonyl precursor. Despite the sulfenate, which
has an initial cis geometry, being prone to isomerization,
complete retention of configuration is achieved if the
reaction temperature is maintained at -78 °C. The (Z)
geometry of the sulfoxide thus available is the opposite
of that observed with the more conventional routes
involving, for example, oxidation of the ketene dithio-
acetal. By contrast, using the classical oxaziridine 1b
derived from benzaldehyde, a competing double oxidation
produces a vinyl sulfinate, with isolation of a sulfone as
the alkylation product. Future work will seek to broaden
to other substrates the unique reactivity of oxaziridine
1a and to develop further applications of sulfenate
chemistry in organic synthesis.
Ack n ow led gm en t. We gratefully acknowledge fi-
nancial support from the “Ministe`re de la Recherche et
des Nouvelles Technologies” (F.S.), CNRS (Centre Na-
tional de la Recherche Scientifique), the “Re´gion Basse-
Normandie” and the European Union (FEDER funding).
We also thank Andreas Haeuseler (University of
Wu¨rzburg, Germany) for initial contributions to this
work.
Su p p or tin g In for m a tion Ava ila ble: General experimen-
tal methods, stereoselectivity for deprotonation of dithioesters
2, summary of lanthanide shift experiments with sulfoxides
3b1 and 3b2, full spectroscopic data for 3-6, experimental
procedures for 5 and 6, mechanism for the generation of 4,
Exp er im en ta l Section
Gen er a l P r oced u r e for th e Syn th esis of Keten e Dith io-
a ceta l S-Oxid es 3. A solution of dithioester 2 (1.00 mmol) in
anhydrous THF (5 mL) was cooled to -78 °C and MeLi (0.69
mL of a 1.6 M solution in Et2O, 1.1 mmol) was added dropwise.
Discoloration of the orange reaction mixture was immediately
observed. After being stirred at -78 °C for 15 min, a solution of
oxaziridine 1a (267 mg, 1.05 mmol) in anhydrous THF (2 mL)
1
and H-12C COSY of sulfone 6. This material is available free
of charge via the Internet at http://pubs.acs.org.
J O0492362
J . Org. Chem, Vol. 69, No. 20, 2004 6919