Highly enantioselective oxidation of sulfides to sulfoxides by a new oxaziridinium salt

Article abstract of DOI:10.1021/ol0702573

The new oxaziridinium salt 5 (R² = TBDPS) is an effective reagent for the highly enantioselective oxidation of sulfides to sulfoxides with up to >99% ee and good yields. As such, it represents a new valuable nonmetallic alternative to the existing methods for asymmetric sulfoxidation.

Full text of DOI:10.1021/ol0702573

ORGANIC
LETTERS
2007
Vol. 9, No. 12
2265-2268
Highly Enantioselective Oxidation of
Sulfides to Sulfoxides by a New
Oxaziridinium Salt
R. E. del R´ıo, B. Wang, S. Achab, and L. Bohe´*
Institut de Chimie des Substances Naturelles, CNRS, AVenue de la Terrasse,
91198 Gif sur YVette, France
bohe@icsn.cnrs-gif.fr
Received February 1, 2007
ABSTRACT
The new oxaziridinium salt 5 (R2
to >99% ee and good yields. As such, it represents a new valuable nonmetallic alternative to the existing methods for asymmetric sulfoxidation.
) TBDPS) is an effective reagent for the highly enantioselective oxidation of sulfides to sulfoxides with up
Asymmetric sulfoxides are very useful as synthons and chiral
auxiliaries in asymmetric synthesis1 and also as biologically
active compounds.2 The asymmetric oxidation of sulfides is
one of the most straightforward and convenient approaches
to chiral sulfoxides. Biological and chemical processes may
achieve this transformation.1,2 Two main chemical methods
have been shown to lead to specific sulfoxides with high
enantioselectivity. They involve either an oxidant combined
with a chiral transition metal complex or a nonmetallic chiral
electrophilic oxidant. The area of metal-catalyzed sulfoxi-
dations has been actively investigated since the outstanding
results of Kagan3 and Modena,4 who obtained high enantio-
selectivities using modified versions of the Sharpless re-
agent.5 Other metals (such as vanadium, manganese, and
more recently iron) as well as different chiral ligands have
led to useful systems.2b
The area of nonmetallic electrophilic oxidants has known
a breakthrough with the remarkable work of Davis on chiral
sulfamyl- and sulfonyloxaziridines.6 The latter are still the
most efficient enantioselective nonmetallic reagents available
for asymmetric sulfoxidation.
Oxaziridinium salts are also electrophilic oxygen transfer
reagents, as established since Lusinchi first reported on the
synthesis and the chemistry of oxaziridiniums.7 Although the
great majority of the reports concerning oxaziridiniums deals
with the epoxidation of alkenes,8,9 only few studies have been
(5) Johnson, R. A.; Sharpless, K. B. In Catalytic Asymmetric Synthesis,
2nd ed.; Ojima, I., Ed.; Wiley-VCH: New York, 2000; pp 231-280.
(6) For leading references on asymmetric sulfoxidation, see: (a) Davis,
F. A.; Jenkins, R., Jr.; Awad, S. B.; Stringer, O. D.; Watson, W. H.; Galloy,
J. J. Am. Chem. Soc. 1982, 104 (20), 5412-5418. (b) Davis, F. A.;
McCauley, J. P.; Chattopadhyay, S.; Harakal, M. E.; Towson, J. C.; Watson,
W. H.; Tavanaiepour, J. J. Am. Chem. Soc. 1987, 109, 3370-3377. (c)
Davis, F. A.; Thimma Reddy, R.; Han, W.; Carroll, P. J. J. Am. Chem.
Soc. 1992, 114, 1428-1437.
(7) (a) Picot, A.; Milliet, P.; Lusinchi, X. Tetrahedron Lett. 1976, 17,
1573-1576. (b) Picot, A.; Milliet, P.; Lusinchi, X. Tetrahedron Lett. 1976,
17, 1577-1580. (c) Picot, A.; Milliet, P.; Lusinchi, X. Tetrahedron 1981,
37, 4201-4208.
(1) (a) Kagan, H. B. In Catalytic Asymmetric Synthesis, 2nd ed.; Ojima,
I., Ed.; Wiley-VCH: New York, 2000; pp 327-356. (b) Ferna´ndez, I.;
Kihar, N. Chem. ReV. 2003, 103, 3651-3705.
(2) (a) Carren˜o, M. C. Chem. ReV. 1995, 95, 1717-1760. (b) Legros, J.;
Dehli, J. R.; Bolm, C. AdV. Synth. Catal. 2005, 347, 19-31.
(3) For leading references, see: (a) Pitchen, P.; Kagan, H. B. Tetrahedron
Lett. 1984, 25, 1049-1052. (b) Brunel, J. M.; Diter, P.; Duetsch, M.; Kagan,
H. B. J. Org. Chem. 1995, 60, 8086-8088. (c) Brunel, J. M.; Kagan, H. B.
Synlett 1996, 404-406.
(4) Di Furia, F.; Modena, G.; Seraglia, R. Synthesis 1984, 325-326.
10.1021/ol0702573 CCC: $37.00
© 2007 American Chemical Society
Published on Web 05/09/2007

devoted to sulfide oxidation.8e,10 The action of a racemic
oxaziridinium on thioethers10c and the enantioselective sul-
foxidation of p-tolyl methyl sulfide, with moderate enantio-
selectivity, by chiral oxaziridiniums have been described.8e,10a,b
In a related but different reaction, the acid-promoted oxida-
tion of sulfides to sulfoxides by unactivated oxaziridines,10a,11
oxaziridinium-like intermediates are thought to be the active
species. Moderate enantioselectivities also resulted when
using chiral oxaziridines in this way.10a,11b
The development of new methods and reagents leading
to chiral sulfoxides with high enantioselectivity remains a
subject of both relevant synthetic and fundamental interest.
Thus, as part of our interest in oxaziridinium chemistry and
in the application of these reagents for asymmetric sulfoxi-
dation, we report herein on the synthesis of a newly
developed oxaziridinium salt and the oxidation of sulfides
with high enantioselectivities by this reagent.
substituent which is the weak point in this strategy. That
problem we found using the originally described reaction
conditions has also been pointed out by others.13
Thus, cholesterol 1 was protected as the tert-butyldiphen-
ylsilyl ether 1a14 using a standard protocol. The 5,6 double
bond was then oxidized in two steps. It was first epoxidized
at room temperature, and the resulting epoxides 1b (not
depicted in Scheme 1; mixture of diastereoisomers R/â in
ca. 2.7:1 molar ratio) were oxidized with the Jones reagent
at 50 °C.15 The (5,6-seco)-ketoacid 2, isolated in 60% yield
(56% from 1a), was converted to the acyl azide 2b through
the acid chloride 2a. The acyl azide moiety smoothly
rearranged into the isocyanate 2c on moderate heating, in
toluene. For the key step involving the cyclization of the
intermediate 2c leading to protected 6-azacholesterol 3, we
have established an alumina-promoted protocol. In our hands,
it proved more efficient than the silica gel induced pro-
tocol developed by Frye et al.13a for the purpose of preventing
the elimination of the 3-oxy substituent. Use of neutral
alumina shortened the reaction time with no need of heating.
Thus, the cyclization of 2c was performed at room temper-
ature, in 3 h, leading to 316 in 75% yield (62% from 2).
Hence, 3 was prepared from cholesterol 1 in 32% overall
yield.17
The oxaziridinium salt 5 was obtained in two steps from
the protected 6-azacholesterol 3 (Scheme 1). The synthesis
Scheme 1
(9) (a) Page, P. C. B.; Rassias, G. A.; Barros, D.; Ardakani, A.; Bethell,
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Buckley, B. R.; Ardakani, A.; Marpless, B. A. J. Org. Chem. 2004, 69,
3595-3597. (c) Page, P. C. B.; Buckley, B. R.; Blacker, J. A. Org. Lett.
2004, 6 (10), 1543-1546. (d) Page, P. C. B.; Buckley, B. R.; Heaney, H.;
Blacker, J. A. Org. Lett. 2005, 7, 375-377. (e) Page, P. C. B.; Barros, D.;
Buckley, B. R.; Marples, B. A. A. J. Tetrahedron: Asymmetry 2005, 16,
3488-3491. (f) Page, P. C. B.; Buckley, B. R.; Barros, D.; Blacker, J. A.;
Heaney, H.; Marpless, B. A. Tetrahedron 2006, 62, 6607-6613. (g) Page,
P. C. B.; Buckley, B. R.; Rassias, G. A.; Blacker, J. A. Eur. J. Org. Chem.
2006, 803-813. (h) Minakata, S.; Takemiya, A.; Nakamura, K.; Ryu, I.;
Komatsu, M. Synlett 2000, 1810-1812. (i) Washington, I.; Houk, K. N. J.
Am. Chem. Soc. 2000, 122, 2948-2949. (j) Wong, M.-K.; Ho, L.-M.; Zheng,
Y.-S.; Ho, C.-Y.; Yang, D. Org. Lett. 2001, 3, 2587-2590. (k) Lacour, J.;
Monchaud, D.; Marsol, C. Tetrahedron Lett. 2002, 43, 8257-8260. (l)
Vachon, J.; Pe´rollier, C.; Monchaud, D.; Marsol, C.; Ditrich, K.; Lacour,
J. J. Org. Chem. 2005, 70, 5903-5911. (m) Gonc¸alves, M.-H.; Martinez,
A.; Grass, S.; Page, P. C. B.; Lacour, J. Tetrahedron Lett. 2006, 47, 5297-
5301. (n) Biscoe, M. R.; Breslow, R. J. Am. Chem. Soc. 2005, 127, 10812-
10813.
of 3 (R ) TBDPS) was performed from cholesterol 1
following a strategy already described12 for the synthesis of
6-azacholesterol and other 6-azasteroids but using where
necessary different reagents and conditions to improve yields
and essentially to overcome the elimination of the 3-oxy
(10) (a) Bohe´, L.; Lusinchi, M.; Lusinchi, X. Tetrahedron 1999, 55, 155-
166. (b) Gluszynska, A.; Mackowska, L.; Rozwadowska, M. D.; Sienniak,
W. Tetrahedron: Asymmetry 2004, 15, 2499-2505. (c) Hanquet, G.;
Lusinchi, X. Tetrahedron Lett. 1993, 34, 5299-3302.
(11) (a) Hanquet, G.; Lusinchi, X.; Milliet, P. Tetrahedron Lett. 1988,
29, 2817-2818. (b) Schoumacker, S.; Hamelin, O.; Te´ti, S.; Pe´caut, J.;
Fontecave, M. J. Org. Chem. 2005, 70, 301-308.
(12) (a) Lettre´, H.; Knof, L. Chem. Ber. 1960, 93, 2860-2864. (b) Lettre´,
H.; Mathes, K.; Wagner, M. Liebigs Ann. Chem. 1967, 703, 147-151.
(13) (a) Frye, S. V.; Haffner, C. D.; Maloney, P. R.; Mook, R. A., Jr.;
Dorsey, G. F., Jr.; Hiner, R. N.; Batchelor, K. W.; Branson, H. N.; Stuart,
J. D.; Schweiker, S. L.; Van Arnold, J.; Bickett, D. M.; Moss, M. L.; Tian,
G.; Unwalla, R. J.; Lee, F. W.; Tippin, T. K.; James, M. K.; Grizzle, M.
K.; Long, J. E.; Schuster, S. V. J. Med. Chem. 1993, 36, 4313-4315. (b)
Brown, L.; Lyall, W. J. S.; Suckling, C. J.; Suckling, K. E. J. Chem. Soc.,
Perkin Trans. I 1987, 595-599.
(8) (a) Hanquet, G.; Lusinchi, X.; Milliet, P. Tetrahedron Lett. 1988,
29, 3941-3944. (b) Hanquet, G.; Lusinchi, X.; Milliet, P. C. R. Acad. Sci.
Paris 1991, 313 (II), 625-628. (c) Hanquet, G.; Lusinchi, X. Tetrahedron
1997, 53, 13727-13738. (d) Poisson, D.; Cure, G.; Solladie´, G.; Hanquet,
G. Tetrahedron Lett. 2001, 42, 3745-3748. (e) Bohe´, L.; Hanquet, G.;
Lusinchi, M.; Lusinchi, X. Tetrahedron Lett. 1993, 34, 7271-7274. (f)
Bohe´, L.; Lusinchi, M.; Lusinchi, X. Tetrahedron 1999, 55, 141-154. (g)
Bohe´, L.; Kammoun, M. Tetrahedron Lett. 2002, 43, 803-805. (h) Bohe´,
L.; Kammoun, M. Tetrahedron Lett. 2004, 45, 747-751. (i) Aggarwal, V.
K.; Wang, H. F. J. Chem. Soc., Chem. Commun. 1996, 191-192. (j)
Armstrong, A.; Ahmed, G.; Garnett, I.; Goacolou, K. Synlett 1997, 1075-
1076. (k) Armstrong, A.; Draffan, A. G. Synlett 1998, 646-648. (l)
Armstrong, A.; Ahmed, G.; Garnett, I.; Goacolou, K.; Wailes, J. S.
Tetrahedron 1999, 55, 2341-2352. (m) Page, P. C. B.; Rassias, G. A.;
Bethell, D.; Schilling, M. B. J. Org. Chem. 1998, 63, 2774-2777. (n) Page,
P. C. B.; Rassias, G. A.; Barros, D.; Bethell, D.; Schilling, M. B. J. Chem.
Soc., Perkin Trans. 1 2000, 3325-3334. (o) Page, P. C. B.; Rassias, G. A.;
Barros, D.; Ardakani, A.; Buckley, B.; Bethell, D.; Smith, T. A. D.; Slawin,
M. A. Z. J. Org. Chem. 2001, 66, 6926-6931.
(14) Hardinger, S. A.; Wijaya, N. Tetrahedron Lett. 1993, 34, 3821-
3824.
(15) Gao, L. J.; Zhao, T. Z. Tetrahedron Lett. 1999, 40, 131-132.
(16) The formation of 3 from 2c was characterized by an upfield shift
in the 13C NMR signal of C5 from 216.6 ppm in 2c to 173.6 ppm in 3
indicating thereby the disappearance of the ketone and the formation of
the imine bond.
(17) The overall yield is strongly dependent on the Jones oxidation step,
in which yields ranging from 60 to 80% have been obtained mainly as a
function of the efficiency of the workup, including chromatographic
purification and subsequent crystallization of compound 2.
2266
Org. Lett., Vol. 9, No. 12, 2007

The peracid oxidation of 3 led stereospecifically to the
â-oxaziridine 4 isolated in 76% yield. The NMR spectra of
the crude product unambiguously indicated the formation of
only one of the two possible diastereoisomers. On one hand,
only one signal corresponding to C5, the quaternary carbon
atom of the oxaziridine ring, was observed in the 13C
spectrum (at δ 87.7 ppm). On the other hand, the 1H spectrum
displayed a single set of signals for the protons in the
neighborhood of the three-membered heterocyclic ring (i.e.,
on C4, C7, and C19). This was particularly apparent for the
protons on C7, resonating at δ 3.40 ppm (dd, 2J ) 17.0 Hz,
Figure 1. Selected NOESY correlations observed for 5.19
2
3
3J ) 7.4 Hz) and δ 2.88 ppm (dd, J ) 17.0 Hz, J ) 9.9
conformation similar to those established for 5,6â-epoxides,20
hence involving ring B in a half chair-like conformation (with
C9 and C8, respectively, below and above the plane defined
by C10-C5-N-C7) and ring A in a twist-like conforma-
tion, both leading to an A/B cis junction which allows the
3â-substituent to be equatorial21 (Figure 1). The equatorial
N-Me group, lying almost in the C10-C5-N-C7 (half
chair) plane, showed NOE interactions with the equatorial
protons on C4 and C7 (H4R and H7â) but not with the axial
ones.
Hz). Each signal integrated as much as the multiplet at δ
3.82 ppm corresponding to C3-H. The J(H7,H8) coupling
3
constants gave insight into the positions of both C7-H relative
to the pseudoaxial C8-H. They were consistent with a
pseudoaxial-pseudoequatorial coupling for the more deshield-
ed proton at 3.40 ppm and with a pseudoaxial-pseudoaxial
coupling for the more shielded proton at 2.88 ppm. Therefore,
the first, syn to the pseudoaxial C8-H, was on the â face
and pseudoequatorial whereas the second, anti to C8-H, was
on the R face and pseudoaxial. From those relative positions,
the â configuration of the oxaziridine ring naturally followed
considering that the deshielding effect of the three-membered
ring is stronger on the cis-vicinal protons than on the trans-
disposed ones.18 Accordingly, the chemical shifts of the
protons on C4 were similarly affected. Although the R-proton
resonated within the methyne region at δ 1.94 ppm (br d, 2J
) 12.4 Hz), the â-proton appeared more deshielded at δ 2.32
With the oxaziridinium salt 5 in hand, we explored the
potential of this new reagent for asymmetric sulfoxidation.
A series of sulfide oxidations were thus performed (Table
1). We first studied the action of 5 on methyl p-tolyl sulfide.
Upon addition of 1 equiv of the oxaziridinium 5 to the sul-
fide, in dichloromethane at room temperature, the reaction
was almost instantaneous,22 complete, and quantitative. It
led to the sulfoxide with high enantioselectivity (ee 92%,
entry 1A). Interestingly, no overoxidation into sulfone could
2
3
ppm (dd, J ) 12.4 Hz, J ) 11.3 Hz). This signal also
integrated for one proton, as that of C3-H at δ 3.82 ppm,
and showed a multiplicity consistent with a trans-pseudodi-
axial arrangement with respect to the C3-H giving rise to a
3J(H3-H4â) very close in value to the geminal H4â,H4R
coupling constant.
1
be detected by TLC and H NMR in the crude reaction
mixture. On lowering the reaction temperature, we found
that the conversion remained quantitative while the enantio-
selectivity was improved to afford an almost enantiopure
product (ee > 99%, entry 1B). The oxidation of a variety of
sulfides involving different R1 and R2 residues was then
examined. In all cases, no further oxidation of the sulfoxide
was found.
With a few exceptions, the reactions proceeded with high
to excellent enantioselectivities. Thus, a less efficient asym-
metric induction resulted when the aryl group was a “bulky”
o-disubstituted phenyl ring (entries 5 and 6). A similar trend,
entailing lower enantioselectivities in the oxidation of sulfides
exhibiting an apparent larger difference in size between the
groups attached to the sulfur,6a has already been observed
in the acid-promoted oxidation of a series of aryl methyl
sulfides by a chiral oxaziridine.10a Once more, data in entries
1-6 indicate that the effective size of an aryl group in the
transition state of this type of sulfoxidation reactions may
be quite different from the apparent size.23
Methylation of the oxaziridine 4 with the Meerwein salt
trimethyloxonium tetrafluoroborate led to oxaziridinium 5
isolated in high yield (90%). The configuration of the three-
membered ring was obviously not affected on alkylation, but
its deshielding effect was in turn increased as a consequence
of the cationic nature of the oxaziridinium, which is far more
electrophilic than the parent oxaziridine. Accordingly, the
formation of the oxaziridinium salt 5 was characterized by
a downfield shift of C5 in the 13C spectrum to δ 102.0 ppm
1
and deprotection of the vicinal protons in the H spectrum
with respect to oxaziridine 4. Thus, C7-Hâ appeared at δ
2
3
4.19 ppm (dd, J ) 14.6 Hz, J ) 7.7 Hz) and C7-HR
2
3
appeared at δ 3.31 ppm (dd, J ) 14.6 Hz, J ) 10.8 Hz).
The signals of the methyl-19 and C4-Hâ are also shifted
downfield by ca. 0.3 ppm, appearing as a singlet at δ 1.41
ppm and a doublet of doublets at δ 2.60 ppm (2J ) 12.8 Hz,
3J ) 11.4 Hz). Moreover, the NOESY spectrum of 5
displayed correlations (Figure 1) in agreement with the
previous structural assignments. Thus, the NMR data for
oxaziridinium 5 suggested a nonsteroid-like predominant
(19) NOESY experiment carried out in CDCl3 at 400 MHz using a mixing
time of 0.6 s.
(20) See, for example: (a) Tavare`s, M.; Ramasseul, R.; Marchon, J.-C.;
Bachet, B.; Brassy, C.; Mornon, J.-P. J. Chem. Soc., Perkin Trans. 2 1992,
1321-1329. (b) Anta, C.; Gonzalez, N.; Rodriguez, J.; Jimenez, C. J. Nat.
Prod. 2002, 65, 1357-1359.
(18) See, for example, (a) the set: Dadoun, H.; Alazard, J.-P.; Lusinchi,
X. Tetrahedron 1981, 37, 1525-1540. And: Jeanniot, J. P.; Lusinchi, X.;
Milliet, P.; Parello, J. Tetrahedron 1971, 27, 401-410. (b) Ref 8f.
(21) A 3â-substituent is axial in a 5â-steroid with the normal steroidal
A/B cis junction.
1
(22) Actually too fast to be monitored by H NMR.
Org. Lett., Vol. 9, No. 12, 2007
2267

of 5 on more challenging sulfides. With this idea in mind,
we applied the new reagent to the asymmetric synthesis of
Table 1. Sulfoxidations with Oxaziridinium 5
the biologically active chiral sulfoxide lansoprazole,1a,2b
a
member of the well-known family of sulfinyl-substituted
benzimidazoles that behave as proton pump inhibitors and
are efficient antiulcer agents.30 Thus, the oxidation of sulfide
631 by oxaziridinium 5 led to (R)-lansoprazole32 7 with
excellent enantioselectivity (ee 97%) in good yield (Scheme
2). Moreover, the reaction was chemoselective.
Scheme 2
a -cSee footnotes b, c, and d in Table 1.
In conclusion, we have completed the first synthesis of a
new oxaziridinium salt (5) starting from cholesterol. Com-
pound 5 turned out to be a very effective reagent for the
highly enantioselective oxidation of sulfides to sulfoxides
(ee up to >99%). As such, it does represent a new valuable
nonmetallic alternative to the existing methods for asym-
metric sulfoxidation. Studies on the reactivity of 5 toward
other nucleophiles and the synthesis of new steroidal
oxaziridiniums displaying different substitution patterns or
bearing the oxaziridinium function upon other cycles of the
steroidal framework are underway.
a Conditions. A: CH2Cl2, rt. B: CH2Cl2, -70 °C to rt. bNot optimized;
sulfoxides isolated by CC. cDetermined by HPLC using a chiral OD
CHIRALCEL column. dAbsolute configuration assigned by comparing
HPLC elution order with known literature data. eAbsolute configuration
established by comparison of the sign of [R]D to literature data.
Acknowledgment. We thank Dr. K. Ben Ali for as-
sistance with HPLC analysis.
Supporting Information Available: Experimental pro-
cedures and spectral data for new compouds. General
procedure for sulfide oxidation and HPLC data. This material
is available free of charge via the Internet at http://pubs.acs.org.
The results for entries 7-9, 12, and 1529 suggested that
electronic effects play minor roles in the asymmetric induc-
tion. The oxidations of entries 9-11 suggested that for
primary and secondary alkyl residues the asymmetric induc-
tion is almost unaffected by the size of the group.
OL0702573
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The ee values depicted in Table 1 strongly suggested that
satisfactory results could also be expected from the action
(28) Boyd, D. R.; Sharma, N. D.; Gunaratne, N.; Haughey, S. A.;
Kennedy, M. A.; Malone, J. F.; Allen, C. C. R.; Dalton, H. Org. Biomol.
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(23) The difference between effective and apparent size has been
rationalized10a assuming a product-like TS where the preferred orientation
of the aromatic ring may vary from almost coplanar to the incoming SO
bond for unhindered phenyl rings to almost coplanar to the orbital containing
the lone electron pair for the ortho-disubstituted phenyl rings.
(24) Kagan, H. B.; Dun˜ach, B. NouV. J. Chim. 1985, 9, 1-3.
(25) Akazome, M.; Ueno, Y.; Ooiso, H.; Ogura, K. J. Org. Chem. 2000,
65, 68-76.
(29) As expected, the benzimidazole moiety was not affected, as observed
1
by H NMR in the crude reaction mixture.
(30) Shin, J. M.; Cho, Y. M.; Sachs, G. J. Am. Chem. Soc. 2004, 126,
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Bull. 1990, 38, 2853-2858.
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2268
Org. Lett., Vol. 9, No. 12, 2007