A catalytic cycle for the asymmetric synthesis of epoxides using sulfur ylides

Full text of DOI:10.1021/jo015588m

5620
J . Org. Chem. 2001, 66, 5620-5623
Sch em e 1
A Ca ta lytic Cycle for th e Asym m etr ic
Syn th esis of Ep oxid es Usin g Su lfu r Ylid es
J acques Zanardi, Catherine Leriverend, Dimitri Aubert,
Karine J ulienne, and Patrick Metzner*
Laboratoire de Chimie Mole´culaire et Thioorganique
(UMR CNRS # 6507), ISMRA-Universite´ de Caen,
6 Boulevard du Mare´chal J uin, 14050 Caen, France
Resu lts a n d Discu ssion
During the development of the asymmetric conversion
of aldehydes into oxiranes, our group (and probably
others) has observed that the overall process tends to be
sluggish when one introduces bulky groups in the initial
sulfide. A preliminary study4b of the reaction mechanism
led us to believe that there are two slow steps in the
epoxidation: formation of the sulfonium salt and addition
of the ylide to the carbonyl compound. In contrast,
deprotonation of the sulfonium salt and ring closure of
the betaine are much faster. As the second slow step
determines the enantioselectivity, we wished to acceler-
ate the first one. We now report a simple way to achieve
this acceleration along with an optimization of the
reactant concentrations and the reaction temperature.
patrick.metzner@ismra.fr
Received February 22, 2001
Since the pioneering work1 of Furukawa in the late
1980s, some examples2 of nonracemic epoxidation have
been reported using aromatic aldehydes and chiral sulfur
ylides. Enantiopure oxiranes are versatile compounds for
organic synthesis. They can be transformed3 into 1,2-
difunctionalized derivatives by nucleophilic ring opening
reactions. Our group has recently developed4 an efficient
asymmetric conversion of aldehydes into epoxides via a
simple C2 symmetric sulfide, (2R,5R)-2,5-dimethylthi-
olane 1. We reported that trans epoxides could be
obtained with high yields (87%-92%) and enantiomeric
excesses (86-96%) in a one-pot synthesis employing mild
reaction conditions and a stoichiometric amount of sulfide
(Scheme 1).
Sch em e 2
In addition, the feasibility of a catalytic process was
demonstrated. The use of 0.1 equiv of sulfide 1 led to an
excellent yield and ee, but the reaction time at room-
temperature had to be prolonged to one month! We now
wish to present5 the results of a new study devoted to (i)
reducing the reaction time while maintaining the yields
and enantiomeric excesses, (ii) applying these conditions
to a variety of aromatic aldehydes, and (iii) demonstrat-
ing greater chiral control by using both enantiomers of
1 and a new chiral sulfide, (2R,5R)-2,5-diethylthiolane
2.
* To whom correspondence should be addressed. Fax: +33 231 452
877.
(1) Furukawa, N.; Sugihara, Y.; Fujihara, H. J . Org. Chem. 1989,
54, 4222-4224.
We have expanded the scope of the epoxidation to
include the aldehydes 6,7,9-12 (Scheme 2). The results
of the epoxidations are summarized in Table 1. Under
our standard noncatalytic conditions, 4-methoxybenzal-
dehyde 7 was converted in 80% yield with 88% ee (Table
1, entry 15) despite the acid sensitivity of the resulting
epoxide. Similar results were obtained with 2-naphthal-
dehyde 9 (entry 17) and a vinyl-oxirane was obtained
from cinnamaldehyde 10 in 93% yield and 87% ee (entry
20).
The epoxidation of heteroaromatic aldehydes has proved
problematic. While no epoxidation was reported using
camphor-derived sulfonium ylides,2j examples of this type
of reaction were recently reported.2n,6 In our hands the
reaction of 2-furaldehyde 11 and 2-thienylaldehyde 12
proceeded nicely at ambient temperature with good
conversion and enantioselectivities (93 and 89%, entries
23 and 24). However, one has to be very careful during
the isolation of the resulting oxiranes which exhibit great
acidic sensitivity.
(2) (a) Li, A.-H.; Zhou, Y.-G.; Dai, L.-X.; Hou, X.-L.; Xia, L.-J .; Lin,
L. Ibid. 1998, 63, 4338-4348. (b) Breau, L.; Ogilvie, W. W.; Durst, T.
Tetrahedron Lett. 1990, 31, 35-38. (c) Aggarwal, V. K.; Kalomiri, M.;
Thomas, A. P. Tetrahedron: Asymmetry 1994, 5, 723-730. (d) Aggar-
wal, V. K.; Thompson, A.; J ones, R. V. H.; Standen, M. C. H. Ibid. 1995,
6, 2557-2564. (e) Aggarwal, V. K.; Ford, J . G.; Thompson, A.; J ones,
R. V. H.; Standen, M. C. H. J . Am. Chem. Soc. 1996, 118, 7004-7005.
(f) Aggarwal, V. K.; Ford, J . G.; Fonquerna, S.; Adams, H.; J ones, R.
V. H.; Fieldhouse, R. J . Am. Chem. Soc. 1998, 120, 8328-8339. (g)
Aggarwal, V. K. Synlett 1998, 329-336. (h) Solladie´-Cavallo, A.; Adib,
A. Tetrahedron 1992, 48, 2453-2464. (i) Solladie´-Cavallo, A.; Diep-
Vohuule, A.; Sunjic, V.; Vinkovic, V. Tetrahedron: Asymmetry 1996,
7, 1783-1788. (j) Li, A.-H.; Dai, L.-X.; Hou, X.-L.; Huang, Y.-Z.; Li,
F.-W. J . Org. Chem. 1996, 61, 489-493. (k) Hayakawa, R.; Shimizu,
M. Synlett 1999, 1328-1330. (l) Saito, T.; Akiba, D.; Sakairi, M.;
Kanazawa, S. Tetrahedron Lett. 2001, 42, 57-59. (m) Myllyma¨ki, V.
T.; Lindvall, M. K.; Koskinen, A. M. P. Tetrahedron 2001, 57, 4629-
4635. (n) Aggarwal, V. K.; Alonso, E.; Hynd, G.; Lydon, K. M.; Palmer,
M. J .; Porcelloni, M.; Studley, J . R. Angew. Chem., Int. Ed. 2001, 40,
1430-1433.
(3) Gorzynski Smith, J . Synthesis 1984, 629-653. Rao, A. S.;
Paknikar, S. K.; Kirtane, J . G. Tetrahedron 1983, 39, 2323-2367.
(4) (a) J ulienne, K.; Metzner, P.; Henryon, V.; Greiner, A. J . Org.
Chem. 1998, 63, 4532-4534. (b) J ulienne, K.; Metzner, P.; Henryon,
V. J . Chem. Soc., Perkin Trans. 1 1999, 731-736.
(5) This work has been partly communicated at a French Chemical
Society Meeting, Le Mans, J une 20, 2001, Abstract P12.
(6) Solladie´-Cavallo, A.; Roje, M.; Isarno, T.; Sunjic, V.; Vinkovic,
V. Eur. J . Org. Chem. 2000, 1077-1080.
10.1021/jo015588m CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/07/2001

Notes
entry
J . Org. Chem., Vol. 66, No. 16, 2001 5621
Ta ble 1. Asym m etr ic Syn th esis of Va r iou s Oxir a n es
aldehyde
thiolane R )
Me (2R,5R)
iodide salt
sulfide (equiv)
time (days)
yield (%)a
de (trans) (%)b
ee (S,S) (%)c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
3
-
1
2
4
2
5
6
4
6
2
4
6
2
6
4
6
2
6
2
5
4
2
6
6
4
4
6
92d
82
80
74
95
97
90
88d
95
79
89d
77
88
82
80
33e
88
75
88
93
60
70
89
90
75
86
85
80
80
80
88
85
84
85
88
84
80
86
85
82
85
84
80
84
95
78
80
80
81
75
88
85
85
89
80 (R,R)
93
92
88
89
88
86
72
89
90
88
86
86
64
86
87
69
86
93
89
80
n-Bu4NI
n-Bu4NI
NaI
n-Bu4NI
0.1
0.2
0.1
0.2
1
0.1
1
1
0.1
1
0.1
1
0.1
1
0.1
1
Me (2S,5S)
Et (2R,5R)
-
n-Bu4NI
-
-
n-Bu4NI
-
n-Bu4NI
-
n-Bu4NI
-
n-Bu4NI
-
n-Bu4NI
-
-
4
5
Me (2R,5R)
Et (2R,5R)
Me (2R,5R)
Et (2R,5R)
Et (2R,5R)
Me (2R,5R)
Et (2R,5R)
Me (2R,5R)
6
7
8
9
0.1
1
1
0.1
0.1
1
Et (2R,5R)
Me (2R,5R)
10
n-Bu4NI
NaI
-
-
11
12
Me (2R,5R)
Me (2R,5R)
1
0.1
n-Bu4NI
a
Yield after purification on a silica gel column. b Diastereoisomeric excess determined on the 1H NMR of the crude product. c Enantiomeric
excess determined by chiral HPLC using a Daicel AD column. Our previous work.4b e Unstable product on silica gel.
d
Ta ble 2. Asym m etr ic Syn th esis of Stilben e Oxid e w ith
In some cases we have endeavored to increase enan-
tioselectivities by using new thiolanes. We began with
Va r iou s Con cen tr a tion s of Ald eh yd e
[PhCHO]
(M)
time
(days)
yield
(%)a
de (trans)
(%)b
ee (S,S)
the slighly bulkier and less volatile (2R,5R)-2,5-dieth-
ylthiolane 2 which was prepared in 94% yield from
(3S,6S)-3,6-octanediol7 using the procedure4 developed for
the synthesis of the dimethyl-substituted analogue 1.
Increased enantioselectivities were indeed achieved with
2. (S,S)-Stilbene oxide was obtained in 93% ee (entry 6),
showing a gain of 5% as compared to the dimethylthio-
lane 1 mediated reaction (entry 1). Three other examples
(tolualdehyde 4, 4-chlorobenzaldehyde 5, and 2-naph-
thaldehyde 9) led to similar selectivities (entries 9, 13,
and 19).
We have then developed a practical catalytic cycle, for
which very few examples have been reported.2e,j,l,8 In our
procedure the formation of the sulfonium salt is effected
by commercially available benzyl bromide, thereby avoid-
ing silver salts, triflates, or diazoalkanes. It should be
noted that under standard conditions, chlorides are not
electrophilic enough and that benzyl iodide is usually not
available with acceptable purity. We hoped to activate
the benzyl bromide through the addition of an iodide salt
leading to a metathetical halogen exchange and in situ
formation of benzyl iodide. In the racemic series with
thiolane itself, several iodide salts (NaI, Me4NI, n-Bu4-
NI) were tested and indeed showed an acceleration with
the shortest reaction times recorded for n-Bu4NI. Activa-
tion of the reaction was similarly observed using nonra-
cemic catalytic conditions with thiolane 1.
entry
(%)c
1
2
3
0.25
0.5
1
21
4
5
80
82
41
86
85
82
81
85
89
a
b
Yield after purification on a silica gel column. Diastereoiso-
meric excess determined on the 1H NMR of the crude product.
c Enantiomeric excess determined by chiral HPLC using a Daicel
AD column.
ments were carried out with 0.1 equiv of chiral sulfide 1
and benzaldehyde in concentrations of 0.25, 0.5, and 1
M (Table 2). With 0.5 M aldehyde (entry 2), the reaction
time was reduced to 4 days, with preservation of yield,
de, and ee values. With 1 M aldehyde (entry 3), the yield
of stilbene oxide dropped by approximately 40%. It is
possible that a higher concentration favors side processes
such as the Cannizzaro reaction.9 We then examined the
effect of temperature with a 0.5 M concentration of
aldehyde. At 35 °C or 50 °C the reaction proceeded
smoothly (73 and 70% yields) with shorter reaction times,
but caused a significant decrease of the ee (76 and 74%,
respectively, as compared to 85% at rt).
These studies led us to the following optimized catalytic
epoxidation conditions at room temperature (Scheme 3):
benzaldehyde (1 equiv, 0.5 M), benzyl bromide (2 equiv),
n-Bu4NI (1 equiv), NaOH (2 equiv) and the chiral sulfide
(0.1 equiv) in a mixture of 9/1 t-BuOH/H2O. In the case
of stilbene oxide (entry 2, Table 1), after 4 days, 82% of
oxirane was isolated with good ee and de values (85%,
85%). If the amount of sulfide was increased to 0.2 equiv,
the reaction could be achieved in only 2 days with
conservation of yield and selectivity (entry 3, Table 1).
For a generalized asymmetric synthesis, both enanti-
omers of the target molecule must be accessible. We have
demonstrated that this is true for our method by prepar-
Reaction conditions were optimized with respect to
reactant concentrations and temperature. Three experi-
(7) (a) Burk, M. J .; Feaster, J . E.; Harlow, R. L. Tetrahedron:
Asymmetry 1991, 2, 569-592. (b) Burk, M. J .; Feaster, J . E.; Nugent,
W. A.; Harlow, R. L. J . Am. Chem. Soc. 1993, 115, 10125-10138. (c)
Bach, J .; Berenguer, R.; Garcia, J .; Loscertales, T.; Manzanal, J .;
Vilarrasa, J . Tetrahedron Lett. 1997, 38, 1091-1094. (d) Bach, J .;
Berenguer, R.; Garcia, J .; Lo´pez, M.; Manzanal, J .; Vilarrasa, J .
Tetrahedron 1998, 54, 14947-14962.
(8) Aggarwal, V. K. Comprehensive Asymmetric Catalysis; J acobsen,
E., Pfaltz, A., Yamamoto, H., Eds.; Springer: Berlin, 1999; Vol. 2, pp
679-693.
(9) Geissman, T. A. Org. React. 1944, 2, 94-113.

5622 J . Org. Chem., Vol. 66, No. 16, 2001
Notes
Sch em e 3
ing the (2S,5S) enantiomer of 1. (2R,5R)-Hexanediol can
be purchased commercially and is also available10 by
lipase resolution10a-c of the meso + DL mixture of
hexanediols. Formation of the dimesylate of (2R,5R)-
hexanediol and nucleophilic displacement4a,11 with Na2S
provided the previously unreported (2S,5S)-2,5-dimeth-
ylthiolane in good yield (93%). This enantiomer was used
successfully for the synthesis of trans-(R,R)-stilbene oxide
(entry 5, Table 1).
22% with others). In this case, the nature of the iodide
salt is important: if NaI was used instead of n-Bu4NI
(sets of entries 1, 2, 4 and 20, 21, 22) ee variations were
not observed.
In conclusion we have developed a new catalytic
approach to optically actives oxiranes via a chiral sulfo-
nium ylide. We have found simple, practical and cheap
conditions under which epoxides can be obtained using
0.2 or 0.1 equiv of sulfide. Anhydrous solvents, inert
atmosphere, strong bases, low temperature, preformation
of sulfonium salts or use of phenyldiazomethane are not
required. We have reported the straightforward synthesis
of both enantiomers of dimethylthiolane 1 and their use
as chirality inductors. From 0.1 equiv of sulfide, in one
pot, trans oxiranes were obtained with good yields and
enantiomeric excesses, after a reasonable reaction time
(4 to 6 days). Moreover, diethylthiolane 2 is a very
promising new chiral sulfide (ee up to 93%). A variety of
aromatic aldehydes were converted using both sulfides.
These results provide a complementary route to chiral
oxiranes directly from readily available aldehydes, thereby
avoiding the Wittig synthesis and subsequent oxidation
of an intermediate alkene.14
The catalytic reaction conditions were extended to
various aromatic aldehydes using the three chiral sulfides
(Table 1). Chiral control was exerted by both enantiomers
of thiolane 1 with oxiranes isolated in good to excellent
yields (60-95%) after 4 to 6 days (entries 12, 18, 21, 25).
Four examples (entries 7, 10, 14, 16) were investigated
with a catalytic amount of (2R,5R)-2,5-diethylthiolane 2.
For stilbene oxide the highest ee for the catalytic series
was attained: 92%, with 90% yield (entry 7). Diastere-
omeric induction was generally good (up to 95%), but we
observed some decrease from the stoichiometric to the
catalytic series. The largest variation was observed with
cinnamaldehyde (entries 20, 21). A recent study by
Aggarwal and co-workers12 on the diastereoselectivity of
the reaction of sulfur ylides with aldehydes proposed that
the formation of anti betaines (leading to trans oxiranes)
is irreversible. On the other hand, they reported that the
formation of syn betaines is reversible and can lead to
mixtures of cis and trans oxiranes. Consequently, slower
reactions will favor reversibility and lead to a higher
proportion of trans epoxides. In contrast, here, we have
observed cases for which the slower catalytic reactions
led to a lower diastereoselection. To explain this, other
factors must be considered, including the effect of salts.13
We have also observed variations2l of the ee values with
some aldehydes between the catalytic and stoichiometric
series (small with benzaldehyde and tolualdehyde, 14-
Exp er im en ta l Section
Chromatographic purification of compounds was achieved
with Merck 60 silica gel (40-63 µm). Thin-layer chromatography
(TLC) was used routinely to monitor the progress of epoxidation
reactions. 1H NMR spectra were recorded using at 250 MHz.
Data appear in the following order: chemical shifts in ppm,
multiplicity (s, singlet; d, doublet; t, triplet; q, quartet; m,
multiplet), coupling constant J in Hz, number of protons. 13C
NMR spectra were acquired at 62.9 MHz with the same
spectrometer, operating with broad band 1H decoupling. TMS
is the internal standard for the CDCl3 solutions. Mass spectra
were obtained in EI mode at 70 eV. Specific optical rotations
(given in 10-1 deg cm2 g-1) were measured at 589 nm. Mps stand
uncorrected. High-pressure liquid chromatography (HPLC) was
performed with a diode array detector and a Daicel AD chiral
column 250 × 4.6 mm (length × i.d.) with n-hexane/isopropyl
alcohol 90/10 at a flow rate of 1 mL/min as the eluant.
Typ ica l P r oced u r e. Syn th esis of 2-(4-Ch lor op h en yl)-3-
p h en yloxir a n e. To a solution of (2R,5R)-2,5-dimethylthiolane
(0.05 mmol, 250 µL of a 0.2 M solution of dimethylthiolane in
t-BuOH/H2O 9/1, 0.1 equiv) in 750 µL of a mixture of t-BuOH/
H2O 9/1 were added benzyl bromide (120 µL, 1 mmol, 2 equiv),
powdered NaOH (40 mg, 1 mmol, 2 equiv), tetrabutylammonium
iodide (185 mg, 0.5 mmol, 1 equiv), and the 4-chlorobenzaldehyde
(72 mg, 0.5 mmol, 1 equiv). The reaction mixture was stirred at
room temperature. The reaction was judged complete by thin-
(10) (a) Nagai, H.; Morimoto, T.; Achiwa, K. Synlett 1994, 289-290.
(b) Kim, M.-J .; Lee, I. S.; J eong, N.; Choi, Y. K. J . Org. Chem. 1993,
58, 6483-6485. (c) Caron, G.; Kazlauskas, R. J . Tetrahedron: Asym-
metry 1994, 5, 657-664. (d) Solladie´, G.; Huser, N.; Garcia-Ruano, J .
L.; Adrio, J .; Carren˜o, M. C.; Tito, A. Tetrahedron Lett. 1994, 35, 5297-
5300. (e) Ikeda, H.; Sato, E.; Sugai, T.; Ohta, H. Tetrahedron 1996,
52, 8113-8122. (f) Persson, B. A.; Huerta, F. F.; Ba¨ckvall, J .-E. J . Org.
Chem. 1999, 64, 5237-5240.
(11) Otten, S.; Fro¨hlich, R.; Haufe, G. Tetrahedron: Asymmetry
1998, 9, 189-191. The absolute configuration of 1 has been con-
firmed: Wang, F.; Wang, H.; Polavarapu, P. L.; Rizzo, C. J . J . Org.
Chem. 2001, 66, 3507-3512.
(12) Aggarwal, V. K.; Calamai, S.; Ford, G. J . J . Chem. Soc., Perkin
Trans. 1 1997, 593-599.
(13) Hsi, J . D.; Koreeda, M. J . Org. Chem. 1989, 54, 3229-3231.
Vedejs, E.; Marth, C. F.; Ruggeri, R. J . Am. Chem. Soc. 1988, 110,
3940-3948. Maryanoff, B. E.; Reitz, A. B.; Mutter, M. S.; Inners, R.
R.; Almond, H. R.; Whittle, R. R.; Olofson, R. A. J . Am. Chem. Soc.
1986, 108, 7664-7678.
(14) J acobsen, E. N. Catalytic Asymmetric Synthesis; Ojima, I., Ed.;
VCH: Weinheim, 1993; pp 159-202. J acobsen, E. N.; Wu, M. H.
Comprehensive Asymmetric Catalysis; J acobsen, E., Pfaltz, A., Yama-
moto, H., Eds.; Springer: Berlin, 1999; Vol. 2, pp 648-677.

Notes
J . Org. Chem., Vol. 66, No. 16, 2001 5623
layer chromatography (TLC). TLC plates were visualized by UV
light and by treatment with a solution of 2,4-DNPH (400 mg in
100 mL of HCl 1 N). Water (5 mL) was added. The aqueous
phase was extracted with diethyl ether (10 mL, 3 times), and
the combined organic layers were dried over MgSO4 and then
concentrated to dryness. The crude product was subjected to
column chromatography (silica gel, 98/2 petroleum ether/diethyl
ether) to afford the oxirane (88 mg, 0.385 mmol, 77% yield, de:
80%, ee: 72%). Trans isomer: 1H NMR δ: 7.26-7.39 (m, 9H),
3.80 and 3.82 (AB syst, J ) 1.7, 2H); 13C NMR δ: 136.7, 135.6,
134.1, 128.7, 128.6, 128.5, 126.8, 125.4, 62.9, 62.1. Anal. Calcd:
C: 72.89, H: 4.81, O: 6,506.94. Found: C: 73.12, H: 4.90, O:
7.04. HPLC: cis: 4.9 and 5.0 [(R,S) and (S,R) enantiomers could
not be completely separated], trans (UV maximum absorption:
232 nm): 5.8 and 10.6 min [(R,R) and (S,S) enantiomers].
(2R,5R)-2,5-Dieth ylth iola n e 2. A solution of (3S,6S)-octane-
3,6-diol (1 g, 6.84 mmol) and triethylamine (2.3 mL, 16.4 mmol)
in dichloromethane (15 mL) was cooled at -18 °C. Methane-
sulfonyl chloride (1.23 mL, 15 mmol) was added dropwise while
the temperature was maintained between -20 °C and -15 °C.
After addition, the mixture was warmed to 0 °C. A 1 N HCl
solution (4 mL) was added. The extraction was carried out with
dichloromethane (15 mL, three times). The organic layer was
separated, washed with a saturated NaHCO3 aqueous solution
(10 mL), and dried over MgSO4. After filtration and evaporation
of the solvent, a colorless oil of (1S,4S)-4-methanesulfonyloxy-
1-ethylhexyl methanesulfonate (2.05 g, quantitative yield) was
obtained. 1H NMR δ: 4.70-4.75 (m, 2H), 3.03 (s, 6H), 1.81-
1.84 (m, 4H), 1.70-1.78 (m, 4H), 0.99 (t, J ) 7.4, 6H); 13C NMR
δ: 84.3, 39.1, 29.2, 27.9, 9.7. Sodium sulfide nonahydrate (3.4
g, 14 mmol) was added to a solution of the preceding dimesylate
in ethanol (20 mL). The mixture was stirred at room tempera-
ture for 15 days. A milky suspension of sodium methanesulfonate
was formed. It was poured into water (15 mL) and extracted
with pentane (4 mL, 4 times). The combined organic phase was
washed with brine and dried over MgSO4. After filtration,
pentane was evaporated with the evaporator water bath being
kept at 0 °C to avoid any loss of the volatile thiolane. A pale
yellow liquid with an unpleasant odor (700 mg, 4.85 mmol, 71%
yield) was obtained. It appeared unnecessary to further purify
(2R,5R)-2,5-diethylthiolane. 1H NMR δ: 3.25-3.40 (m, 2H),
2.15-2.20 (m, 2H), 1.48-1.70 (m, 6H), 0.95 (t, J ) 7.3, 6H); 13
C
NMR δ: 51.4, 36.9, 30.7, 13.4; EIMS m/z (%) ) 144 (16) [M+],
115 (100) [M+-C2H5], 81 (37), 69 (29), 59 (31), 55 (38), 41 (69);
HRMS: found 144.1017, requires 144.0973; [R]23D ) +164° (c )
1.27, CHCl3).
Ack n ow led gm en t. We thank Rhoˆne-Poulenc (now
Aventis) for their initial support of this work, Chirotech
(Cambridge, UK) for a generous gift of enantiopure
diols, and the PunchOrga network (Poˆle Universitaire
Normand de Chimie Organique) for support.
Su p p or t in g In for m a t ion Ava ila b le: Chromatographic
separations and spectral data of oxiranes. The material is
available free of charge via the Internet at http://pubs.acs.org.
J O015588M