Enantioselective epoxidation with chiral MnIII(salen) catalysts: Kinetic resolution of aryl-substituted allylic alcohols

Article abstract of DOI:10.1021/jo010350j

A set of aryl-substituted allylic alcohols rac-2 has been epoxidized by chiral Mn(salen*) complexes 1 as the catalyst and iodosyl benzene (PhIO) as the oxygen source. Whereas one enantiomer of the allylic alcohol 2 is preferentially epoxidized to give the threoor cis-epoxy alcohol 3 (up to 80% ee) as the main product (dr up to >95:5), the other enantiomer of 2 is enriched (up to 53% ee). In the case of 1,1-dimethyl-1,2-dihydronaphthalen-2-ol (2c), the CH oxidation to the enone 4c proceeds enantioselectively and competes with the epoxidation. The absolute configurations of the allylic alcohols 2 and their epoxides 3 have been determined by chemical correlation or CD spectroscopy. The observed diastereo- and enantioselectivities in the epoxidation reactions are rationalized in terms of a beneficial interplay between the hydroxy-directing effect and the attack along the Katsuki trajectory.

Full text of DOI:10.1021/jo010350j

5796
J . Org. Chem. 2001, 66, 5796-5800
En a n tioselective Ep oxid a tion w ith Ch ir a l Mn III(sa len ) Ca ta lysts:
Kin etic Resolu tion of Ar yl-Su bstitu ted Allylic Alcoh ols
Waldemar Adam,*,† Hans-Ulrich Humpf,‡ Konrad J . Roschmann,† and
Chantu R. Saha-Mo¨ller†
Institute of Organic Chemistry and Department of Food Chemistry, University of Wu¨rzburg,
Am Hubland, 97074 Wu¨rzburg, Germany
adam@chemie.uni-wuerzburg.de
Received April 3, 2001
A set of aryl-substituted allylic alcohols rac-2 has been epoxidized by chiral Mn(salen*) complexes
1 as the catalyst and iodosyl benzene (PhIO) as the oxygen source. Whereas one enantiomer of the
allylic alcohol 2 is preferentially epoxidized to give the threo- or cis-epoxy alcohol 3 (up to 80% ee)
as the main product (dr up to >95:5), the other enantiomer of 2 is enriched (up to 53% ee). In the
case of 1,1-dimethyl-1,2-dihydronaphthalen-2-ol (2c), the CH oxidation to the enone 4c proceeds
enantioselectively and competes with the epoxidation. The absolute configurations of the allylic
alcohols 2 and their epoxides 3 have been determined by chemical correlation or CD spectroscopy.
The observed diastereo- and enantioselectivities in the epoxidation reactions are rationalized in
terms of a beneficial interplay between the hydroxy-directing effect and the attack along the Katsuki
trajectory.
Sch em e 1. Kin etic Resolu tion of a Ch ir a l Allylic
In tr od u ction
Alcoh ol in th e Sh a r p less-Ka tsu k i Ep oxid a tion
Optically active epoxy alcohols are valuable building
blocks for the asymmetric synthesis of biologically active
molecules; for example, the â-blockers Propanolol and
Falintolol have been prepared from (R)-propenol oxide.1
For such enantioselective transformation, the Sharpless-
Katsuki epoxidation represents one of the most efficient
routes to optically active epoxy alcohols from primary
allylic alcohols.2 When a racemic allylic alcohol is used
as substrate, kinetic resolution is a necessary conse-
quence, in which one enantiomer is epoxidized preferably
to the corresponding erythro-epoxy alcohol, while the
other enantiomer of the allylic alcohol is enriched (Scheme
1).2 Recently we have reported that secondary allylic
alcohols are epoxidized in high threo diastereoselectivity
by an achiral MnIII(salen) complex with iodosyl benzene
as oxygen source.3 This raises the question whether
racemic allylic alcohols may be enantioselectively epoxi-
dized with chiral MnIII(salen*) catalysts 1 through kinetic
resolution. Although optically active MnIII(salen*) com-
catalysts in the enantioselective epoxidation of function-
alized alkenes such as allylic alcohols appears to have
not been attempted to date. Of course, for this purpose
racemic aryl-substituted substrates should be employed,
since these are destined to work best in the asymmetric
J acobsen-Katsuki epoxidation.5
Resu lts
The allylic alcohols rac-2a -f (for structures, see Table
1) were synthesized according to literature methods.6-11
(3) Adam, W.; Stegmann, V. R.; Saha-Mo¨ller, C. R. J . Am. Chem.
Soc. 1999, 121, 1879-1882.
(4) (a) Vander Velde, S. L.; J acobsen, E. N. J . Org. Chem. 1995, 60,
5380-5381. (b) Noguchi, Y.; Irie, R.; Fukuda, T.; Katsuki, T. Tetra-
hedron Lett. 1996, 37, 4533-4536. (c) Noguchi, Y.; Katsuki, T. Synlett
1998, 543-545. (d) Linker, T.; Peters, K.; Peters, E.-M.; Rebien, F.
Angew. Chem., Int. Ed. Engl. 1996, 35, 2487-2489.
(5) (a) Katsuki, T. J . Mol. Catal. A 1996, 113, 87-107. (b) J acobsen,
H.; Cavallo, L. Chem. Eur. J . 2001, 7, 800-807.
plexes have already been used in the kinetic resolution
of unfunctionalized olefins,4 the use of such J acobsen’s
(6) (a) Denis, J . N.; Greene, E.; Aara˜o Serra, A.; Luche, M. J . J . Org.
Chem. 1986, 51, 46-50. (b) Pelter, A.; Ward, R. S.; Little, G. M. J .
Chem. Soc., Perkin Trans. 1, 1990, 2775-2790.
* Fax: +49-931/888-4756.
(7) Friedrich, E. C.; Taggart, D. B. J . Org. Chem. 1975, 40, 720-
723.
† Institute of Organic Chemistry.
‡ Department of Food Chemistry.
(8) (a) Wenkert, E.; Youssefyeh, R. D.; Lewis, R. G. J . Am. Chem.
Soc. 1960, 82, 4675-4680. (b) Kelly, D. P.; Leslie, D. R.; Smith, B. D.
J . Am. Chem. Soc. 1984, 106, 687-694.
(9) Zimmermann, H. E.; Factor, R. E. J . Am. Chem. Soc. 1980, 102,
3538-3548.
(1) Hanson, R. M. Chem. Rev. 1991, 91, 437-475.
(2) (a) Katsuki, T.; Sharpless, K. B. J . Am. Chem. Soc. 1980, 102,
5974-5976. (b) Martin, V. S.; Woodard, S. S.; Katsuki, T.; Yamada Y.;
Ikeda, M.; Sharpless, K. B. J . Am. Chem. Soc. 1981, 103, 6237-6240.
10.1021/jo010350j CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/02/2001

Resolution of Aryl-Substituted Allylic Alcohols
J . Org. Chem., Vol. 66, No. 17, 2001 5797
Ta ble 1. En a n tioselective Ep oxid a tion of th e Ra cem ic Allylic Alcoh ols 2a -f
a
b
Determined by 1H NMR spectroscopy of the crude product (error ( 5%) or by chiral HPLC analysis (error ( 2%). Molar ratio of
2/1/PPNO (4-phenylpyridine N-oxide)/PhIO ) 1.0:0.1:0.2:0.6. c Mass balance. cis:trans or threo:erythro ratio. e Calculated according to
d
krel ) ln(1 - conv)[1 - ee(2)]/ln(1 - conv)[1 + ee(2)]. f Calculated according to conv ) ee(2a )/[ee(2a ) + ee(3a )]. 50:50 mixture of cis- and
g
h
trans-epoxides 3a , ee of cis-3a . ee of trans-3a .
Racemic samples of both diastereomers of the epoxides
3a ,b,d ,e,f were prepared by dimethyldioxirane (DMD)
epoxidation of the corresponding allylic alcohols 2, while
epoxy alcohol 3c (cis:trans ) 67:33) was obtained by
10, data not shown) by the use of the MnIII(salen*)
catalyst with PF6- as counterion.12 At about 20% conver-
sion (observed) in the kinetic resolution of (Z)-4-phenyl-
3-buten-2-ol (2a ) with the catalyst (S,S)-1a (entry 1), the
unreacted (R)-configured allylic alcohol 2a was obtained
in 46% ee, while the (S) enantiomer was epoxidized to
the (2S,3R,4S)-cis-epoxide 3a (73% ee) and the (2S,3R,4R)-
trans-epoxide 3a (80% ee). From these enantioselectivi-
ties a conversion of 37% is calculated; this discrepancy
may be explained in terms of the low mass balance (61%).
Complementary results were observed for the substrate
2a with the (R,R)-1b catalyst (entry 2), except that now
preferentially the (R)-2a enantiomer was epoxidized.
oxidation of 2c with PhIO, catalyzed by an achiral MnIII
-
(salen) complex.
The manganese-catalyzed asymmetric epoxidation of
the allylic alcohols 2 was carried out with 10 mol % of
catalyst 1, 20 mol % of 4-phenylpyridine N-oxide (PPNO)
as additive, and 0.6 equiv of iodosyl benzene (PhIO) as
oxygen source. The results are summarized in Table 1.
Since it is well-known5 that the J acobsen-Katsuki
epoxidation works best for cis-disubstituted olefins with
aryl groups, the substrates 2a -c were employed first.
The acyclic allylic alcohol 2a was epoxidized in excellent
chemo- and diastereoselectivity (>95:5, entries 1 and 2)
to afford the corresponding threo-configured epoxy alcohol
3a as a 50:50 mixture of the cis- and trans-epoxides threo-
3a . Such cis/trans isomerization has been observed in the
J acobsen-Katsuki epoxidation of cis-â-methylstyrene
and cis-stilbene5 and may be suppressed (cis:trans ) 90:
Under the same reaction conditions, the indenol 2b
(entries 3 and 4) was converted to the epoxy-alcohols 3b
in good chemoselectivity (epoxide/enone 89:11) and dia-
stereoselectivity (90:10), with cis-3b as the main product.
At a conversion of 51% with the (S,S)-1a catalyst (entry
3), the (1R)-cis-epoxide was formed in 66% ee and the
(1S)-trans-epoxide in 81% ee, while (R)-indenol [(R)-2b]
was enantiomerically enriched (36% ee). Again, comple-
mentary results were obtained for the (R,R)-1b catalyst
(entry 4), for which expectedly the optically active (S)-
2b enantiomer was left behind. Also minor (11-14%)
(10) (a) Bussas, R.; Mu¨nsterer, H.; Kresze, G. J . Org. Chem. 1983,
48, 2828-2832. (b) Akhtar, M.; J allo, L.; J ohnson, A. H. J . Chem. Soc.,
Chem. Commun. 1982, 1, 44-46. (c) Zimmermann, H. E.; Factor, R.
E.; J . Am. Chem. Soc. 1980, 102, 3538-3548.
(11) (a) Michel, J .; Canonne, P. Can. J . Chem. 1971, 49, 4084-4095.
(b) Kelly, D. P.; Leslie, D. R.; Smith, B. D. J . Am. Chem. Soc. 1984,
106, 687-694.
(12) Adam, W.; Roschmann, K. J .; Saha-Mo¨ller, C. R. Eur. J . Org.
Chem. 2000, 3519-3521.

5798 J . Org. Chem., Vol. 66, No. 17, 2001
Adam et al.
Sch em e 3. Assign m en t of th e Absolu te
Con figu r a tion s of th e Allylic Alcoh ols 2a ,e by
Ch em ica l Cor r ela tion
Sch em e 2. Oxid a tion of Ra cem ic a n d
En a n tiom er ica lly En r ich ed Allylic Alcoh ol 2c by
th e (S,S)-1a Ca ta lyst
was formed in 38% ee and the (2R)-erythro-epoxide 3e
in 52% ee, while (R)-(E)-4-phenyl-3-penten-2-ol (2e) was
moderately enantiomerically enriched (22% ee). Comple-
mentary results were found for the (R,R)-1b catalyst
(entry 10).
When the diastereomeric (Z)-4-phenyl-3-penten-2-ol
(2f) was submitted to the MnIII(salen*)Cl/PPNO/PhIO
oxidation (entries 11and 12), analogous to its (E)-dia-
stereomer 2e (entry 9), the threo-3f epoxide was formed
as the major diastereomer, but in slightly reduced (90:
10) chemoselectivity (entry 11). Also, no cis/trans isomer-
ization occurred. The ee values of the threo-epoxy alcohol
3f were substantially lower, that is, 19% for the (S,S)-
1a catalyst (entry 11), and for (R,R)-1b merely racemic
epoxy alcohol 3f was obtained (entry 12). Consequently,
there was essentially no enantiomeric enrichment of the
remaining allylic alcohol 2f.
The absolute configurations of the acyclic allylic alco-
hols 2a and 2e were determined by chemical correlation,
as outlined in Scheme 3. Thus, (+)-(Z)-4-phenyl-3-buten-
2-ol (2a ) was converted to the known (S)-phenylbutan-
2-ol13 by catalytic hydrogenation, which establishes that
the (+)-2a enantiomer possesses the (S) configuration.
The (-)-(E)-4-phenyl-3-penten-2-ol (2e) was first benzy-
lated and then cleaved by ozonolysis to the known (S)-
2-benzyloxypropanal,14 which allowed assignment of the
(S) configuration to the (-)-2e enantiomer.
To determine the absolute configuration of the cyclic
allylic alcohol 2c, circular-dichroism (CD) spectroscopy
was employed. After enzymatic kinetic resolution of rac-
2c [(+) alcohol 81% ee, (-) acetate 92% ee at 47%
conversion] with the lipase from Burkholderia sp. (CHIRA-
ZYME L-1), the required second chromophore in the
enantiomerically enriched (+)-2c was introduced by
treatment with benzoyl chloride in pyridine to give the
optically active benzoate of 2c (data not shown). Its CD
and UV spectra are given in Figure 1.
amounts of CH oxidation to the enone 4b took place
(entries 3 and 4).
When 1,1-dimethyl-1,2-dihydronaphthalen-2-ol (2c)
was submitted to the MnIII(salen*)Cl/PPNO/PhIO oxida-
tion (entries 5 and 6), the corresponding enone 4c (allylic
CH oxidation) was the main product; epoxidation oc-
curred only in 45% and 43% yields. As in the case of the
indenol 2b, the cis-3c epoxide was formed as the major
diastereomer (entry 5), but in slightly reduced (83:17)
diastereoselectivity. Also, the ee values of the remaining
allylic alcohol 2c were substantially lower, that is, 19%
(R) for the (S,S)-1a catalyst (entry 5) and 11% (S) for
(R,R)-1b (entry 6). The best enantioselectivity for the
epoxy-alcohol cis-3c (56% ee) was obtained with the
(R,R)-1b catalyst (entry 6).
The possibility of an enantioselective CH oxidation was
examined next. For this purpose, enantiomerically en-
riched allylic alcohol 2c [the ee values were 92% (S) and
88% (R), Scheme 2] was treated with the (S,S)-1a catalyst
and iodosyl benzene. While the (S)-2c enantiomer was
oxidized preferentially to the enone 4e (27:73 chemo-
selectivity), the (R)-2c enantiomer was epoxidized more
readily (66:34 chemoselectivity). For comparison, the
chemo- and diastereoselectivity of the reaction with rac-
2c and (S,S)-1a (Table 1, entry 5) are also shown in
Scheme 2.
Since in 1,1,2-trimethyl-1,2-dihydronaphthalen-2-ol
(2d ) the CH oxidation is totally suppressed, this tertiary
allylic alcohol was examined to assess the enantioselec-
tivity of the epoxidation (entries 7 and 8). While the
diastereoselectivity of the epoxidation was excellent (>95:
5), the epoxide cis-3d was formed in a rather poor
enantioselectivity [6% ee with catalyst (S,S)-1a and 32%
ee with (R,R)-1b]. Consequently, essentially no enantio-
meric enrichment (<10% ee) of the remaining allylic
alcohol 2d was observed.
The diastereomeric, trisubstituted 4-phenyl-3-penten-
2-ols 2e,f were examined to determine the steric effects
on the various selectivities. The (E)-diastereomer 2e
(entries 9 and 10) was epoxidized in excellent chemo-
selectivity (>95:5) and diastereoselectivity (93:7), to
afford the corresponding threo-configured epoxy-alcohol
3e. Unlike (Z)-4-phenyl-3-buten-2-ol (2a ), no cis/trans
isomerization was detected. At a conversion of 54% with
the (S,S)-1a catalyst (entry 9), the (2S)-threo epoxide 3e
A word of caution must be expressed, in view of the
fact that the 1,2-dihydronaphthalene chromophore shows
itself a large Cotton effect at about 260 nm [cf. CD
spectrum of the acetate, light line]. This peak must be
ignored, because it does not arise from the coupling of
the two chromophores. Thus, the negative Cotton effect
observed at 228 nm allows one to infer the (R) configu-
ration for the parent alcohol (+)-2c.
(13) Burgess, K.; J ennings, L. D. J . Am. Chem. Soc. 1991, 113,
6129-6139.
(14) Solladie, G.; Arce, E.; Bauder, C.; Carreno, M. C. J . Org. Chem.
1998, 63, 2332-2337.

Resolution of Aryl-Substituted Allylic Alcohols
J . Org. Chem., Vol. 66, No. 17, 2001 5799
ment through 1,3-allylic strain and hydrogen bonding
between substrate and reagent cooperate.
To rationalize the observed enantioselectivity, the
Katsuki trajectory 3 needs to be examined in more
detail.5 Katsuki postulated that the allylic alcohol 2a
should approach the Mn(V)oxo functionality of the (S,S)-
Mn(V)oxo species as shown in Figure 2, that is, the
substrate should come from the right side and the phenyl
group should point away from the right-hand aryl group
of the salen ligand to avoid π-π interaction. Since along
this attack only the (S) enantiomer of allylic alcohol 2a
may hydrogen-bond 2 effectively, the (S)-2a enantiomer
is oxidized preferentially to the threo-configured (2S,-
3R,4S)-cis-epoxide 3a and the (2S,3R,4R)-trans-epoxide
3a , while the (R)-2a enantiomer is enriched. In this way,
the enantioselectivity displayed in the manganese-
catalyzed kinetic resolution of allylic alcohol 2a may be
accounted for in terms of the mutual assistance of the
hydroxy-directing effect and the attack along the Katsuki
trajectory.
While this rationale also applies to the substrates 2b
(Table 1, entries 3 and 4) and 2e,f (entries 9-12), the
stereochemical results (entries 5 and 6) for 1,1-dimethyl-
1,2-dihydronaphthalen-2-ol (2c) cannot be explained in
this way. According to the Katsuki rationale, the epoxi-
dation of the cyclic allylic alcohol 2c with the (S,S)-1a
catalyst should preferentially lead to the (2R)-configured
epoxide cis-3c; however, the opposite enantiomer, namely,
(2S)-cis-3c, is observed (Table 1, entry 5). This discrep-
ancy may be explained in terms of the allylic CH
oxidation of substrate rac-2c, which constitutes the main
reaction pathway. Thus, the kinetic resolution in this step
rather than in the epoxidation is responsible for the
observed enantioselectivity. Indeed, in a control experi-
ment with the enantiomerically enriched allylic alcohol
2c (cf. Results section, Scheme 2), it was shown that the
(S)-2c enantiomer was oxidized preferentially to the
enone 4c by the (S,S)-1a catalyst (3c:4c ) 27:73), while
the (R)-2c enantiomer was epoxidized more readily (3c:
4c ) 66:34). If the enantioselectivity in the CH oxidation
were the same for both enantiomers (S)-2c and (R)-2c,
the kinetic resolution of allylic alcohol rac-2c would be
dictated by the asymmetric epoxidation and not by the
enantioselective enone formation. Consequently, when a
racemic mixture of the allylic alcohol rac-2c and the
catalyst (S,S)-1a is subjected to the standard reaction
conditions, the (S)-2c enantiomer is oxidized preferen-
tially to the enone 4c rather than being epoxidized to the
(2R)-configured cis-3c epoxy alcohol, and therefore, the
(R)-2c enantiomer is enriched by the former reaction
mode. The epoxidation of the substrate 2c to the epoxy
alcohols 3c could also proceed enantioselectively, but that
this is evidently not the case is already suggested by the
fact that (2R)-cis-3c should be formed preferentially
according to the Katsuki rationale. More significant, the
manganese-catalyzed epoxidation of the methyl deriva-
tive 2d , which is prohibited to undergo CH oxidation,
proceeded in poor enantioselectivity (Table 1, entries 7
and 8). Thus, the (R)-2c enantiomer, which was enriched
by enantioselective CH oxidation, is epoxidized to the
(2S)-cis- and (2S)-trans-3c epoxy alcohols without sub-
stantial additional kinetic resolution (Table 1, entries 5
and 6).
F igu r e 1. CD (upper) and UV (lower) spectra of the benzoate
(dark line) and acetate (light line) derivatives of the allylic
alcohol 2c.
Since the manganese-catalyzed epoxidation of the
substrates 2d and 2f were poorly selective (cf. Table 1,
entries 7, 8, 11, and 12), the absolute configurations of
the allylic alcohols 2d ,f and their epoxides 3d ,f were not
determined. The enantiomerically enriched allylic alco-
hols 2a -c,e (20-53% ee), whose absolute configurations
are known, were epoxidized to give both diastereomers
of the epoxy-alcohols 3a -c,e. By comparison of HPLC
retention times and the signs of optical rotation (see
Supplementary Information), the absolute configurations
of the epoxy alcohols 3a -c,e were assessed.
Discu ssion
As shown in the Results section, when the model
substrate (Z)-4-phenyl-3-buten-2-ol (2a ) was allowed to
react with the (S,S)-1a catalyst under the standard
conditions, the (S)-2a enantiomer was epoxidized to the
threo-configured cis-(2S,3R,4S)-epoxide 3a (73% ee) and
the trans-(2S,3R,4R)-epoxide 3a (80% ee), while the (R)-
configured allylic alcohol 2a was enantiomerically en-
riched up to 46% ee (Table 1, entry 1). The diastereo-
and enatioselectivity of this manganese-catalyzed epoxi-
dation may be rationalized in terms of the synergistic
interplay between the hydroxy-directing effect3,15 and the
attack along the Katsuki trajectory5 (Figure 2). On the
basis of 1,3-allylic strain 1, the preferred conformer is
given in Figure 2, in which the hydrogen atom at the C-2
position of the substrate points toward the phenyl ring.
Favorable hydrogen bonding 2 between the hydroxy
functionality of substrate 2a and the Mn(V)oxo function-
ality obliges attack from that π face of the double bond
such that the oxygen transfer leads to the threo-epoxides
3a as the main diastereomer. Thus, the observed threo
diastereoselectivity (Table 1, entries 1 and 2) underlies
hydroxy-directivity control, in which conformational align-
It remains to explain why (E)-4-phenyl-3-penten-2-ol
(2e) is epoxidized selectively (up to 51% ee), while its (Z)-
diastereomer 2f is not (Table 1, entries 9-12). As shown
(15) Adam, W.; Wirth, T. Acc. Chem. Res. 1999, 32, 703-710.

5800 J . Org. Chem., Vol. 66, No. 17, 2001
Adam et al.
F igu r e 2. Controlling factors on the diastereo- and enantioselectivity in the epoxidation of the chiral allylic alcohol 2a .
In conclusion, the kinetic resolution of aryl-substituted
allylic alcohols 2 in the enantioselective epoxidation by
the chiral MnIII(salen*) catalysts 1 provides a promising
alternative to the established Sharpless-Katsuki epoxi-
dation for the synthesis of optically active threo- or cis-
configured epoxy alcohols 3. As a bonus, enantiomerically
enriched allylic alcohols 2 are obtained. The stereochem-
ical control in this asymmetric oxidation is accounted for
in terms of the beneficial interplay between the hydroxy-
F igu r e 3. Steric effects in the epoxidation of substrates 2e
(top) and 2f (bottom), viewed along the Katsuki trajectory.
directing effect and the attack along the Katsuki trajec-
tory.
Ack n ow led gm en t. This work was financially sup-
ported by the Deutsche Forschungsgemeinschaft (Sonder-
forschungsbereich 347 “Selektive Reaktionen metallak-
tivierter Moleku¨le”) and the Fonds der Chemischen
Industrie, for which we are grateful. We thank Roche
Diagnostics GmbH, Penzberg, for generous samples of
lipases.
in Figure 3, the diastereomeric allylic alcohols 2e,f may
orientate themselves along the Katsuki trajectory in two
different ways toward the Mn(V)oxo functionality of the
(S,S)-1a catalyst. For the substrate 2e, the upper right-
hand approach is disfavored, since its phenyl group would
be placed proximate to the aryl group of the salen ligand
and π-π repulsion would be expected to build up. Thus,
the asymmetric epoxidation of the 2e diastereomer occurs
in good enantioselectivity along the upper left-hand
approach. Contrarily, the phenyl group of the 2f diaste-
reomer points away almost perpendicularly from the
plane of the salen ligand, and thus, π-π repulsion is
minimized such that both attacks become energetically
equal. Expectedly, the epoxidation of the Z-configured
allylic alcohol 2f is not enantioselective.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails of the epoxidation reactions mentioned in the text;
analytical and spectral characterization data of the alcohols
2, the epoxides 3 and the enones 4; experimental details of
the chemical correlation of the alcohols 2a ,c,e. This material
is available free of charge via the Internet at http://pubs.acs.org.
J O010350J