Diastereoselective single oxygen ene reaction (Schenck reaction) and diastereoselective epoxidations of heteroatom-substituted acyclic chiral olefins: A mechanistic comparison

Article abstract of DOI:10.1021/ja9530230

The directing propensity of allylic and homoallylic heteroatom substituents on the diastereoselective singlet oxygen ene reaction (Schenck reaction) of acyclic, chiral olefins has been investigated. These extensive stereochemical results offer evidence fo

Full text of DOI:10.1021/ja9530230

J. Am. Chem. Soc. 1996, 118, 1899-1905
1899
Diastereoselective Singlet Oxygen Ene Reaction (Schenck
Reaction) and Diastereoselective Epoxidations of
Heteroatom-Substituted Acyclic Chiral Olefins: A Mechanistic
Comparison
Waldemar Adam,*,† Hans-Gu1nter Bru1nker,† A. Sampath Kumar,†
Eva-Maria Peters,‡ Karl Peters,‡ Uwe Schneider,†,1 and Hans Georg von Schnering‡
Contribution from the Institut fu¨r Organische Chemie, UniVersitat Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany, and Max-Planck-Institut fu¨r Festko¨rperforschung,
Heisenbergstrasse 1, D-70506 Stuttgart, Germany
ReceiVed September 1, 1995X
Abstract: The directing propensity of allylic and homoallylic heteroatom substituents on the diastereoselective singlet
oxygen ene reaction (Schenck reaction) of acyclic, chiral olefins has been investigated. These extensive stereochemical
results offer evidence for our proposed mechanism of the diastereoselective singlet oxygen ene reaction. Thus,
provided a substrate possesses sufficient 1,3-allylic strain to populate preferentially the appropriate conformation,
the photooxygenation is directed threo-selectively by intramolecular hydrogen bonding in the threo-A exciplex of
allylic alcohols, while electron-accepting or bulky substituents result in the erythro-configurated products. A detailed
comparison of the diastereoselectivities for the singlet oxygen ene reaction with those of the epoxidation by
m-chlorobenzoic acid (mCPBA) and dioxirane of several judiciously chosen substrates showed that both oxygen
transfer processes are influenced similarly by allylic and homoallylic substituents. Thus, it is shown that the acyclic,
allylic alcohol 1a is epoxidized highly threo-selectively by methyl(trifluoromethyl)dioxirane due to hydrogen bonding
in the transition state and that bulky and/or electron-accepting substituents direct all three oxidants erythro-selectively.
These pronounced steering effects on the diastereoselectivity of oxygen transfer processes (photooxygenation and
1
epoxidation) should be valuable for the rational design of stereocontrolled oxyfunctionalizations by O2, mCPBA,
and dioxiranes.
Introduction
In regard to the diastereoselectivity for the 1O2 ene reaction,
π-facial differentiation has been abundantly documented9 for
cyclic and bicyclic substrates, whereas for acyclic olefins, such
diastereomeric control has been described only recently.10-12
Among the earliest examples count acyclic unsaturated silyl
cyanhydrines10 and olefins with phenyl substituents11 at an
allylic chirality center, for which again the cis effect in
conjunction with 1,3-allylic strain is held responsible as the
steering factor. Most recently we showed that hydroxy12 and
amino13 functionalities at an allylic chirality center direct the
singlet oxygen ene reaction threo-selectively, whereas acylated
chiral allylic amines are photooxygenated erythro-selectively.13
Unfortunately, not much is known to date on the directing effects
of other heteroatom substituents.
For mCPBA epoxidations of acyclic chiral olefins, it is
abundantly documented14 that the diastereoselectivity of the
epoxidation can be determined by the directing effect of an
allylic hydroxy group and steric interactions of the oxidant with
substituents at the stereogenic center. Thus, allylic alcohols,
which possess 1,3-allylic strain, are epoxidized in a highly threo-
The ene reaction of singlet oxygen (1O2) with alkenes, the
so-called Schenck reaction,2 constitutes a convenient route to
allylic hydroperoxides.3 Much work has been carried out in
recent years on such photooxygenations, particularly on its
diastereo- and regioselectivity.4-12 The prominent controlling
factor for the regioselectivity in the Schenck reaction2 is the by
now classical cis effect,4 but more recently, gem-directing5 and
“nonbonding large group” interactions have been recognized.6
Furthermore, very high regioselectivity is exercised by silyl7
and stannyl substituents.8
† Universitat Wu¨rzburg.
‡ Max-Planck-Institut fu¨r Festko¨rperforschung.
X Abstract published in AdVance ACS Abstracts, February 1, 1996.
(1) Undergraduate research participant, Spring 1994.
(2) Schenck, G. O.; Eggert, H.; Denk, W. Liebigs Ann. Chem. 1953,
584, 177-198.
(3) Matsumoto, M. In Singlet Oxygen; Frimer, A. A., Ed.; CRC Press:
Boca Raton, FL, 1985; Vol II, pp 212-272.
(4) (a) Orfanopoulos, M.; Grdina, M. B.; Stephenson, L. M. J. Am. Chem.
Soc. 1979, 101, 275-276. (b) Schulte-Elte, K. H.; Rautenstrauch, V. J.
Am. Chem. Soc. 1980, 102, 1738-1740.
(5) (a) Ensley, H. E.; Carr, R. V. C.; Martin, R. S.; Pierce, T. E. J. Am.
Chem. Soc. 1980, 102, 2836-2838. (b) Orfanopoulos, M.; Foote, C. S.
Tetrahedron Lett. 1985, 26, 5991-5994. (c) Adam, W.; Griesbeck, A.
Angew. Chem., Int. Ed. Engl. 1985, 24, 1070-1071. (d) Akasaka, T.;
Takeuchi, K.; Ando, W. Tetrahedron Lett. 1987, 28, 6633-6636. (e)
Akasaka, T.; Misawa, Y.; Goto, M.; Ando, W. Tetrahedron 1989, 45, 6657-
6666. (f) Clennan, E. L.; Chen, X.; Koola, J. J. J. Am. Chem. Soc. 1990,
112, 5193-5199. (g) Adam, W.; Bru¨nker, H.-G.; Nestler, B. Tetrahedron
Lett. 1991, 32, 1957-1960.
(9) (a) Schenk, G. O.; Neumu¨ller, O.-A.; Eisfeld, W. Liebigs Ann. Chem.
1958, 618, 202-210. (b) Furutachi, N.; Nakadaira, Y.; Nakanishi, K. J.
Chem. Soc., Chem. Commun. 1968, 1625-1626. (c) Paquette, L. A.; Liao,
C. C.; Liotta, D. C.; Fristad, W. E. J. Am. Chem. Soc. 1976, 98, 6412-
6413. (d) Lin, H.-S.; Paquette, L. A. Synth. Commun. 1986, 16, 1275-
1283. (e) Linker, T.; Fro¨hlich, L. Angew. Chem., Int. Ed. Engl. 1994, 33,
1971-1972.
(10) Adam, W.; Catalani, L. H.; Griesbeck, A. J. Org. Chem. 1986, 51,
5494-5496.
(6) Orfanopoulos, M.; Stratakis, M.; Elemes, Y. J. Am. Chem. Soc. 1990,
112, 6417-6419.
(11) (a) Adam, W.; Nestler, B. Liebigs Ann. Chem. 1990, 1051-1053.
(b) Kropf, H.; Reichwaldt, R. J. Chem. Res. 1987, 412-413.
(12) (a) Adam, W.; Nestler, B. J. Am. Chem. Soc. 1992, 114, 6549-
6550. (b) Adam, W. Nestler, B. J. Am. Chem. Soc. 1993, 115, 5041-5049.
(13) (a) Adam, W.; Bru¨nker, H.-G. J. Am. Chem. Soc. 1993, 115, 3008-
3009. (b) Bru¨nker, H.-G.; Adam, W. J. Am. Chem. Soc. 1995, 117, 3976-
3982.
(7) (a) Fristad, W. E.; Bailey, T. R.; Paquette, L. A.; Gleiter, R.; Bo¨hm,
M. C. J. Am. Chem. Soc. 1979, 101, 4420-4423. (b) Fristad, W. E.; Bailey,
T. R.; Paquette, L. A. J. Org. Chem. 1980, 45, 3028-3037. (c) Adam, W.;
Richter, M. Tetrahedron Lett. 1992, 33, 3461-3464.
(8) Adam, W.; Klug, P. J. Org. Chem. 1993, 58, 3416-3420.
0002-7863/96/1518-1899$12.00/0 © 1996 American Chemical Society

1900 J. Am. Chem. Soc., Vol. 118, No. 8, 1996
Adam et al.
selective manner, whereas olefins with a large substituent at
the allylic chirality center undergo epoxidation erythro-
selectively.15
intensive study which provides valuable mechanistic insight into
the diastereoselective oxidations by O2, mCPBA, and diox-
iranes.
1
In comparison, little is known presently on diastereoselective
epoxidations of chiral acyclic olefins with dioxiranes.16 Only
few examples show that the diastereoselectivity of this reaction
may also be influenced by large substituents at the allylic
chirality center or by an allylic ammonium or hydroxy group.
Thus, in a very recent publication, it was shown by Asensio16e
that the diastereoselectivity of the epoxidation of cyclic allylic
ammonium salts by dimethyl- and methyl(trifluoromethyl)-
dioxirane is directed by intramolecular hydrogen bonding, while
Murray16f showed the same for the epoxidation of cyclohex-2-
en-1-ol by dimethyldioxirane. Results similar to Murray’s for
the cyclic system were obtained by us for the epoxidation of
the acyclic, chiral alcohol 1a by dimethyldioxirane.16g
Results
Photooxygenations. Photooxygenation of the allylic sub-
strates 1a-m afforded the regioisomeric allylic hydroperoxides
2 and 2′ by means of the singlet oxygen ene reaction (eq 1). As
1
The main issue of this work was to investigate the directing
effect of several heteroatom substituents on the diastereoselec-
tivity of the singlet oxygen ene reaction of acyclic, chiral olefins.
On one hand, we were interested in knowing whether homoal-
lylic hydroxy functionalities can direct the singlet oxygen ene
reaction threo-selectively, like allylic alcohols,12 amines,13 and
ammonium chlorides13 do; on the other hand, we wanted to
probe whether electron-accepting or partially negatively charged
substituents at the stereogenic allylic position would generally
direct the photooxygenation erythro-selectively, as observed for
acylated allylic amines.13 This would provide relevant evidence
for the mechanism we have suggested for the singlet oxygen
ene reaction13b and would open new synthetic applications of
the diastereoselectively controlled Schenck reaction.
starting materials for the O2 oxidations of the hydroperoxide
1b, the alkene 1d, and the halides 1l,m, isomeric mixtures of
(E)-(2-methyl-3-penten-2-yl) hydroperoxide [(E)-1′b], of (E)-
2,2,3,5-tetramethyl-3-hexene [(E)-1′d], of (E)-4-chloro-4-meth-
yl-2-pentene [(E)-1′l], and of (E)-4-bromo-4-methyl-2-pentene
[(E)-1′m] were used. The (E)-1′b, (E)-1′l, and (E)-1′m isomers
survived the photooxygenation conditions and the substrates 1b
and 1l,m were photooxygenated selectively. (E)-1′d gave with
singlet oxygen the corresponding hydroperoxide, which was also
isolated and fully characterized, as described earlier.13b
The observed product ratios in Table 1 were determined by
1H NMR spectroscopy directly on the crude product mixtures.
Control experiments such as the irradiation of substrates 1a-m
in the absence of sensitizer and prolonged photooxygenation
of the corresponding hydroperoxides established that all starting
materials and products persisted under the oxidation conditions.
Photooxygenations in CCl4 and CDCl3 were performed at -25
°C and that of acetone at 0 °C. A control experiment with
substrate 1a showed that there is no significant temperature
effect.
The photooxygenation of the derivatives 1a,d-f in carbon
tetrachloride (entries 1, 23, 27, and 32) was described previ-
ously12,13 and was additionally performed in acetone. The allylic
alcohol 1a (entries 1-3) afforded in acetone, like in methanol,12
predominantly the threo-configurated hydroperoxide (R*,R*)-
2a in high regio- but only moderate threo-diastereoselectivity.
In contrast, from the allylic hydroperoxide 1b (entries 11 and
12), the bis-hydroperoxides 2b were obtained in a moderately
erythro-selective but also highly regioselective reaction. Due
to their instability under the conditions of silica gel chroma-
tography, the bis-hydroperoxides 2b were reduced to the
corresponding diols 3b, which were isolated and fully character-
ized. The photooxygenation of the homoallylic alcohol (Z)-1c
(entries 17 and 18) gave only the hydroperoxides 2c, but in
little if any diastereoselectivity.
The chiral olefin 1d (entries 23 and 24) and the acylated
allylic amines 1e (entries 27 and 28) and 1f (entries 32 and 33)
gave in carbon tetrachloride (entries 23, 27, and 32) predomi-
nantly the erythro-configurated products (R*,S*)-2d-f, in
diastereoselectivities identical within the error (ca. 5% of the
stated value) to those observed in acetone (entries 24, 28, and
33). The regioisomeric product was only detected in the case
of derivative 1f; its diastereomeric ratio (Z)-2′f:(E)-2′f was 95:5
in both solvents (entries 32 and 33).
The sulfone 1g afforded in carbon tetrachloride (entry 36) as
well as in acetone (entry 37) only one product, the erythro-
configurated (R*,S*)-2g. The photooxygenation of the sulfox-
ides 1h,i (entries 40 and 41) was complicated in view of the
fact that a 1:1 mixture of the diastereomers 1h,i had to be used
because all attempts to separate them by chromatography
Futhermore, it was of mechanistic interest to explore the
geometric details in the oxygen transfer process of the singlet
oxygen ene reaction by comparing its diastereoselectivities with
those of the epoxidation by mCPBA, since much valuable
information has been accumulated on the transition state for
the later oxidation through such stereochemical probes.12,14,16e-g
Additionally, the dioxirane epoxidations of the same set of chiral
substrates were to be examined to assess its propensity for
stereocontrolled oxyfunctionalizations. It was relevant to
determine whether the same substituent effects, which determine
the diastereoselectivity in the singlet oxygen ene reaction, may
serve for the design of diastereoselective epoxidations by
mCPBA and dioxiranes. Herein we present our results of this
(14) (a) Rao, A. S. ComprehensiVe Organic Synthesis; Ley, S. V., Ed.;
Pergamon Press: Oxford, U.K., 1991; Vol. 7, pp 357-387. (b) Sharpless,
K. B.; Verhoeven, T. R. Aldrichim. Acta 1979, 12, 63-74. (c) Henbest, H.
B.; Wilson, R. A. L. J. Chem. Soc. 1957, 1958-1965. (d) Berti, G. Top.
Stereochem. 1973, 7, 93-251. (e) Rao, A. S.; Paknikar, S. K.; Kirtane, J.
G. Tetrahedron 1983, 39, 2323-2367. (f) Rossiter, B. E.; Verhoeven, T.
R.; Sharpless, K. B. Tetrahedron Lett. 1979, 4733-4736. (g) Narula, A. S.
Tetrahedron Lett. 1983, 24, 5421-5424. (h) Hoveyda, A. H.; Evans, D.
A.; Fu, G. C. Chem. ReV. 1993, 93, 1307-1370.
(15) (a) Fleming, I.; Sarkar, A. K.; Thomas, A. P. J. Chem. Soc., Chem.
Commun. 1987, 157-159. (b) Vedejs, E.; McClure, C. K. J. Am. Chem.
Soc. 1986, 108, 1094-1096. (c) Hayashi, T.; Okamoto, Y.; Kabeta, K.;
Hagihara, T.; Kumada, M. J. Org. Chem. 1984, 49, 4224-4226. (d) Nagase,
T.; Kawashima, T.; Inamoto, N. Chem. Lett. 1985, 1655-1658. (e) Clayden,
J.; Collington, E. W.; Egert, E.; McElroy, A. B.; Warren, S. J. Chem. Soc.,
Perkin. Trans. 1 1994, 2801-2810. (f) Kocienski, P. J. Chem. Soc., Perkin
Trans. 1 1983, 945-948.
(16) (a) Nemes, C.; Le´vai, A.; Patonay, T.; To´th, G.; Boros, S.; Hala´sz,
J.; Adam, W.; Golsch, D. J. Org. Chem. 1994, 59, 900-905. (b) Adam,
W.; Hadjiarapoglou, L. Top. Curr. Chem. 1993, 164, 45-62. (c) Abou-
Elzahab, M.; Adam, W.; Saha-Mo¨ller, C. R. Liebigs Ann. Chem. 1991, 445-
50. (d) Adam, W.; Prechtl, F.; Richter, M. J.; Smerz, A. K. Tetrahedron
Lett. 1993, 34, 8427-8430. (e) Asensio, G.; Mello, R.; Boix-Bernadini,
C.; Gonza´lez-Nu´n˜ez, M. E.; Castellano, G. J. Org. Chem. 1995, 60, 3692-
3699. (f) Murray, R. W.; Singh, M.; Williams, B. L.; Moncrieff, H. M.
Tetrahedron Lett. 1995, 36, 2437-2440. (g) Adam, W.; Smerz, A. K. J.
Chem. Soc., Chem. Commun., submitted.

Heteroatom-Substituted Acyclic Chiral Olefins
J. Am. Chem. Soc., Vol. 118, No. 8, 1996 1901
Table 1. Regio- and Diastereoselectivity in the Photooxygenation and mCPBA and DMD Epoxidations of Chiral, Allylic Olefinsa
entry
substrate
X
R
oxidant
solvent
CCl4
methanol
acetone-d6
CCl4
time (h)
conv (%)
yieldb (%)
2:2′
erythro:threo
Z:E
1c
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
OH
OH
OH
OH
OH
OH
OH
OH
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
1O2
2
2
2
12
10
8
8
8
0.5
0.5
7
>95
>95
>95
>95
>95
>95
>95
>95
>95
60
89d
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
79d
96:4
>97:3
96:4
7:93
27:73
31:69
5:95
6:94
7:93
24:76
18:82
40:60
8:92
66:34
70:30
75:25
72:28
69:31
80:20
45:55
45:55
36:64
26:74
38:62
24:76
71:29
73:27
>95:5
>95:5
76:24
78:22
56:44
35:65
62:38
95:5
e
2c
1O2
f
3
4
5
1O2
e
mCPBA
mCPBA
mCPBA
DMD
DMD
TFD
methanol-d4
acetone-d6
acetone
6
7g,h
8h
i
CCl4
j
9
OH
OH
acetone-d6
CCl4
CCl4
acetone-d6
acetone-d6
CCl4
acetone
CCl4
10
11
12
13k
14k
15k
16k
17
18
19
20
21
22
23m
24
25
26
27m
28
29
30
31
32m
33
34
35
36
37
38
39
40o
41o
42
43
44
45
46
47
48
49q
50
51
52
53
54
55
56
57
TFD
OOH
OOH
OOH
OOH
OOH
OOH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
CH2OH
tBu
1O2
77
>95:5
>95:5
f
f
1O2
5
5
2
5
>95
>95
80
>95
80
mCPBA
mCPBA
DMD
TFD
0.5
2
(Z)-1c
(Z)-1c
(Z)-1c
(Z)-1c
(Z)-1c
(Z)-1c
1d
1O2
CCl4
51
>95:5
>95:5
f
f
H
H
H
H
1O2
acetone-d6
acetone-d6
CCl4
acetone
CCl4
2l
2
>95
>95
>95
>95
80
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
75
>95
>95
>95
>95
>95
88d
mCPBA
mCPBA
DMD
TFD
2
2
0.5
5
2
0.75
8
8
3
7
7
7
3
5
5
3
20
70
8
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
H
H
H
H
H
H
H
H
1O2
CCl4
>95:5
>95:5
f
f
1d
1d
1d
1e
1e
1e
1e
1e
1f
1f
1f
1f
1g
1g
1g
1g
tBu
tBu
tBu
1O2
acetone-d6
acetone-d6
acetone
CCl4
acetone-d6
acetone-d6
CDCl3
acetone
CCl4
acetone-d6
acetone-d6
acetone
CCl4
acetone-d6
acetone-d6
acetone
CDCl3
CDCl3
CCl4
acetone-d6
acetone-d6
acetone
CCl4
CCl4
acetone-d6
CCl4
acetone-d6
CCl4
acetone
CDCl3
acetone-d6
acetone-d6
acetone
CDCl3
>95
>95
82d
mCPBA
DMD
1O2
NHBoc
NHBoc
NHBoc
NHBoc
NHBoc
NBoc2
NBoc2
NBoc2
NBoc2
SO2Ph
SO2Ph
SO2Ph
SO2Ph
SOPh
SOPh
COOEt
COOEt
COOEt
COOEt
COOEt
COOH
COOH
COOH
COOH
COOH
COOH
Cl
76d
f,n
f,n
f
f
1O2
90d
mCPBA
mCPBA
DMD
1O2
>95
>95
90d
91d
86:14
85:15
95:5
95:5
1O2
>95
>95
88d
95:5
mCPBA
DMD
1O2
77:23
75:25
>95:5
>95:5
>95:5
>95:5
>95:5
85:15
78:22
65:35
49:51
51:49
49:51
79:21
70:30
85:15
60:40
46:54
46:54
85:15
>90:10
90:10
90:10
89:11
>95
87
75
87d
>95:5
>95:5
f
f
1O2
>95
>95
87d
mCPBA
DMD
1O2
>95
>95
>95
>95
>95
66
>95
>95
55
>90
80
50
>95
>95
>95
70
50
50
8
1h
1i
6.5
6.5
15
2l
2
94d
p
p
p
p
1O2
94d
(Z)-1j
(Z)-1j
(Z)-1j
(Z)-1j
(E)-1j
(Z)-1k
(Z)-1k
(Z)-1k
(Z)-1k
(Z)-1k
(Z)-1k
1l
1O2
77d
91:9
90:10
78:22
70:30
1O2
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
81d
mCPBA
DMD
1O2
2
60
13
2l
8
2
2
8
3.5
2
1
7
33:67
93:7
>95:5
95:5
44:56
77:23
68:32
74:26
1O2
1O2
1O2
mCPBA
mCPBA
DMD
1O2
H
H
Me
Me
Me
Me
Me
>95:5
>95:5
f
f
1l
1l
1l
1m
Cl
Cl
Cl
Br
1O2
>90
>90
>95
78
mCPBA
DMD
1O2
>95
65
1
f,r
f
a The photooxygenations were performed with two external 250 W sodium lamps and tetraphenylporphine as the sensitizer at -25 °C; control
experiments showed that there is no significant temperature effect on the diastereoselectivity. b The yields, diastereomeric ratios, and regioselectivities
1
were determined directly on the crude product mixture by H NMR spectroscopy (error (5% of the stated value); mass balances >95%, except
where stated by means of footnotes in this table. c Reference 12. d Yield of isolated material. e The regioisomer was observed in form of its ring
tautomer. f No regioisomeric products were detected. g Reference 16d. h Reference 16g. i A 90:10 mixture with acetone was used. j A 88:12 mixture
with CDCl3 was used. k After reduction of 4b with triphenylphosphine to 4a. l Two 400 W sodium lamps. m Reference 13b. n Mass balance 90%.
o Photooxygenation was carried out on a 1:1 mixture of the diastereomeric sulfoxides 1h,i; the relative configuration of the sulfoxides was not
assigned; one of the diastereomers gave the corresponding hydroperoxides 2h/i in a diastereometric ratio of >95:5, the other in 85:15. p The reaction
mixture contained 8% of (Z)-2′h/i, which can derive from 1h as well as from 1i. q In the presence of 2 equiv pyridine. r Mass balance 78%.
failed.17 Photooxygenation of 1h,i (1:1) gave a mixture of the
two erythro-configurated, diastereomeric hydroperoxy sulfoxides
erythro-2h/i, one of the two possible threo-configurated hydro-
peroxy sulfoxides threo-2h/i, and the Z-configurated regioiso-
meric product (Z)-2′h/i. This complex mixture of diastereomers
could not be separated by silica gel chromatography. Therefore,
from the diastereomeric ratio erythro(1)-2h/i:erythro(2)-2h/i:
threo-2h/i:(Z)-2′h/i ) 45:40:7:8, it was estimated that the two
diastereomeric sulfoxides 1h and 1i gave the corresponding
product pairs erythro-2h/threo-2h and erythro-2i/threo-2i in
diastereoselectivities of >95:5 and 85:15.
Photooxygenation of the ester (Z)-1j (entries 42 and 43)
(17) Tang, R.; Mislow, K. J. Am. Chem. Soc. 1970, 92, 2100-2104.
afforded in a highly regioselective but moderately diastereo-

1902 J. Am. Chem. Soc., Vol. 118, No. 8, 1996
Adam et al.
Scheme 1
selective reaction predominantly the erythro- and Z-configurated
products. The photooxygenation of the (E)-1j diastereomer
(entry 46), which unlike all other substrates possesses no 1,3-
allylic strain, showed no diastereoselectivity. Furthermore, the
regioselectivity was not only poor but even inverted in
comparison to its (Z)-1j isomer (entries 42 and 43).
The Schenck ene reaction of the carboxylic acid (Z)-1k
(entries 47 and 48) gave the corresponding hydroperoxides 2k
and 2′k in a regio- and diastereoselectivity quite similar to that
observed in the photooxygenation of the ester (Z)-1j (entries
42 and 43). In the presence of 2 equiv of pyridine, the
selectivities were slightly increased (entry 49).
From the allylic chloride 1l (entries 53 and 54) and the
bromide 1m (entry 57) also only the respective hydroperoxides
2l,m were obtained highly erythro-selectively. Due to their
instability under the conditions of silica gel chromatography,
the hydroperoxides were reduced to the corresponding alcohols
3l,m, which were isolated and fully characterized.
Epoxidations. Epoxidation of the substrates 1a-g, (Z)-1j,
(Z)-1k, and 1l with mCPBA, dimethyldioxirane (DMD) or bis-
(trifluoromethyl)dioxirane (TFD) led in very clean reactions to
the corresponding diastereomeric epoxides 4. The allylic
slight preference for the threo-product. The diastereoselectivi-
ties for the epoxidation of the imidodicarbonate 1f (entries 34
and 35) were somewhat higher. The epoxidations of the sulfone
1g led only to the erythro-configurated (R*,S*)-4g (entries 38
and 39).
The reaction of the ester (Z)-1j was not diastereoselective at
all both with mCPBA and DMD (entries 44 and 45). The
mCPBA epoxidation of the acid (Z)-1k in acetone showed a
moderate erythro-selectivity (entry 50), while the DMD epoxi-
dation in acetone and that of mCPBA in carbon tetrachloride
were unselective (entries 51 and 52). The oxidation of the allylic
chloride 1l with DMD and mCPBA afforded also in high
diastereoselectivities the erythro-configurated epoxide (R*,S*)-
4l (entries 55 and 56).
Stereochemical Assignments. The relative configurations
of the hydroperoxides 2a,d-f were already assigned previously.13b
The hydroperoxide 1b afforded after photooxygenation and
reduction directly the known12 diastereomeric 4-methyl-4-
pentene-2,3-diol (3b). The configurations of the hydroperoxides
2c were determined by chemical correlation with the literature-
known19 diastereomeric 2-methyl-4-pentene-1,3-diol (3c).
The erythro configuration of the hydroperoxide 2g was
deduced from the small coupling constant of the R protons at
the sulfur- and oxygen-bearing carbon atoms.20 Additionally,
the hydroperoxide was transformed by titanium-catalyzed
oxygen transfer to the epoxides (2R*,3R*,4S*)-5g and (2S*,-
3R*,4S*)-5g in a diastereomeric ratio of 91:9 (Scheme 1). For
the major isomer, an X-ray structure determination established
rigorously the (2R*,3R*,4S*)-5g geometry (Figure 1).
The configuration of the hydroperoxides 2h/i was determined
again from the coupling constants of the R protons at the sulfur-
and oxygen-bearing carbon atoms.20 Furthermore, the erythro-
configurated main isomers 2h/i were chemically correlated with
the hydroperoxide (R*,S*)-2g (Scheme 1). The configuration
of the regioisomeric product 2′h/i was established by NOE
experiments (Figure 2), in which a significant enhancement of
the signal for the olefinic proton was observed on irradiation
of the methyl group attached to the double bond.
alcohol 1a gave with mCPBA (entries 4-6) in very high
diastereoselectivities (identical within the experimental error)
to the threo-configurated epoxide 4a in acetone, methanol, and
carbon tetrachloride. The diastereoselectivity in the epoxidations
of 1a with DMD (entry 7) and TFD (entry 9) was significantly
lower in acetone. However, in both epoxidations, a significant
solvent effect was observed on the diastereoselectivity (entries
7-10), which was notably higher in CCl4 (entries 8 and 10)
than in acetone (entries 7 and 9).
Epoxidation of the allylic hydroperoxide 1b with mCPBA,
DMD, and TFD afforded after reduction the epoxides 4a in a
moderate erythro-selectivity (entries 13-16). In acetone as well
as in carbon tetrachloride, the epoxidations of the homoallylic
alcohol (Z)-1c (entries 19-22) gave the known18 epoxides 4c
in moderate threo-selectivity, which again was somewhat higher
in the nonpolar CCl4 (entries 20 and 22) Versus acetone (entries
19 and 21).
The configurations of the hydroperoxides 2j,k were deter-
mined by chemical correlation to the literature-known19 dia-
stereomeric diols 3c. The relative configuration of the regio-
isomeric products 2′j and 2′k was assessed by NOE experiments
(Figure 2); for the corresponding E isomers, no such effects
were observed.
(18) Prat, D.; Delpech, B.; Lett, R. Tetrahedron Lett. 1986, 27, 711-
714.
For the epoxidation of the chiral alkene 1d (entries 25 and
26) with mCPBA and DMD in acetone, only one diastereomer
was obtained. In contrast, the mCPBA and DMD oxidations
of the carbamate 1e (entries 29 and 31) gave in acetone the
epoxides 4e in only a slight excess of the erythro-configurated
stereoisomer, but in CDCl3 (entry 30), mCPBA expressed a
(19) Heathcock, C. H.; Finkelstein, B. L.; Jarvi, E. T.; Radel, P. A.;
Hadley, C. R. J. Org. Chem. 1988, 53, 1922-1942.
(20) (a) Carretero, J. C.; Garcia Ruano, J. L.; Martinez, M. C.; Rodriguez,
J. H.; Alcudia, F. Tetrahedron 1985, 41, 2419-2433. (b) Brunet, E.; Garcia
Ruano, J. L.; Martinez, M. C.; Rodriguez, J. H.; Alcudia, F. Tetrahedron
1984, 40, 2023-2034.

Heteroatom-Substituted Acyclic Chiral Olefins
J. Am. Chem. Soc., Vol. 118, No. 8, 1996 1903
Scheme 2
Figure 1. X-ray structure of sulfone (2R*,3R*,4S*)-5g.
Figure 2. NOE experiments of the diastereomeric hydroperoxides 2′.
The chloro- and the bromohydrins 3l and 3m were chemically
correlated to the known21 cis- and trans-configurated epoxides
(cis,trans)-2-methyl-3-(1-methylethenyl)oxirane (cis,trans-6).
The carbamate 1e gave with mCPBA in deuteriochloroform
(entry 30) a 65:35 diastereomeric mixture of epoxides 4e with
an inverse stereochemistry to that found in the epoxidations with
m-CPBA and DMD in acetone (entries 29 and 31). Since it is
known that in methylene chloride carbamates are epoxidized
by mCPBA in a moderately threo-selective manner,22 conse-
quently, the main diastereomer in the epoxidation of carbamate
1e with mCPBA and DMD in acetone should be erythro-
configurated.
The epoxide 4g was rearranged to the alcohol (R*,S*)-3g,
which was also obtained by reduction of the hydroperoxy
sulfone (R*,S*)-2g (Scheme 1). In view of the established
sterically controlled attack of mCPBA in the epoxidations of
allylic silanes,15a-c sulfones,15b and phosphine oxides,15e the
main diastereomers in the epoxidations of the olefins 1d,f are
assumed to have the erythro configuration.
NMR spectra. Thus, since for all erythro-configurated epoxides
investigated here the epoxy protons R to the stereogenic center
are shifted upfield compared to their threo-configurated dia-
stereomers, the major isomer of the epoxide 4l of the epoxi-
dation of chloride 1l is assumed to have the erythro configu-
ration.
Discussion
In recent publications12,13 we showed that the Schenck
reaction is a convenient tool for the diastereoselective synthesis
of allylic hydroperoxides with â-hydroxy and â-amino substit-
uents from chiral allylic alcohols and amines and selected
derivatives. Previously we suggested a mechanism,13b in which
the high diastereoselectivity is explained in terms of the
reversible formation of perepoxide-like structured diastereomeric
exciplexes A or bona fide perepoxide B intermediates during
the oxyfunctionalization step (Scheme 2).
The threo-selective photooxygenation of allylic alcohols,
amines, and ammonium salts, whose conformational preference
is fixed by 1,3-allylic strain,23 was rationalized in terms of
stabilization of the threo-configurated threo-A exciplex by
hydrogen bonding between the substituent X and the incipient
negatively charged dioxygen molecule. Thus, the threo-B
perepoxide and the threo-configurated hydroperoxide 2 are
formed preferentially. The erythro-selectivity found in the
Since the diastereomeric epoxides 4j,k were obtained in low
diastereoselectivities (entries 44-46 and 50-51), their relative
configurations were not assigned. An attempt to assign the
stereochemistry of the chloro epoxide 4l by chemically cor-
relating it to the epoxy alcohol 4a of known configuration34
failed unfortunately. For example, the conversion of 4l to 4a
with potassium hydroxide or (Bu)4NOH in methylene chloride
at room temperature or the conversion of 4a to 4l with SOCl2
in diethyl ether at 0 °C gave complex mixtures of unidentified
compounds. Therefore, the configurational assignments of the
1
diastereomeric epoxides are based on their characteristic H
(21) Sauleau, J. Bull. Soc. Chim. Fr. 1978, 474-480.
(22) (a) Roush, W. R.; Straub, J. A.; Brown, R. J. J. Org. Chem. 1987,
52, 5127-5136. (b) Luly, J. R.; Dellaria, J. F.; Plattner, J. J.; Soderquist,
J. L.; Yi, N. J. Org. Chem. 1987, 52, 1487-1492. (c) Hauser, F. M.;
Ellenberger, S. R.; Glusker, J. P.; Smart, C. J.; Carrell, H. L. J. Org. Chem.
1986, 51, 50-57.

1904 J. Am. Chem. Soc., Vol. 118, No. 8, 1996
Adam et al.
photooxygenation of the acylated allylic amines, which also
possess 1,3-allylic strain,23 was rationalized in terms of steric
and electrostatic repulsion of the X substituent and the incipient
negatively charged dioxygen molecule in the threo-A exciplex.
Thus, the threo-configurated exciplex is destabilized and the
erythro- and Z-configurated hydroperoxides are formed pref-
erentially.
The high regioselectivity of these photooxygenations was
accounted for in terms of the hindered rotation about the allylic
chirality center. Because of 1,3-allylic strain, the allylic
hydrogen atom at the tertiary carbon atom can hardly achieve
a conformation perpendicular to the olefinic plane and is not
abstracted.
In contrast to the high threo-selectivity in the photooxygen-
ation of the chiral allylic alcohol 1a (entry 1), the Schenck
reactions of the chiral allylic hydroperoxide 1b (entry 11) and
the homoallylic alcohol (Z)-1c (entry 17) exhibited little if any
stereocontrol. Clearly, the hydroxy functionality in these
substrates does not stabilize the threo-A exciplex. This can be
rationalized in terms of the threo-configurated exciplexes threo-
A/1a-c. Whereas favorable hydrogen bonding (six-membered
From the mechanism suggested in Scheme 2 we conclude
that quite generally acyclic, chiral olefins undergo regio- and
diastereoselective Schenck reactions provided two requisites are
met: (1) the chiral substrate possesses a preferred conformation
dictated by 1,3-allylic strain23 and (2) the allylic chirality center
bears a functionality which interacts with the incipient negatively
charged dioxygen molecule in the threo-A exciplex through
attractive (hydrogen bonding) or repulsive (electrostatic/steric)
means. We shall first analyze our present singlet oxygen
diastereoselectivities from this mechanistic perspective and
subsequently compare them with those of the peracid and
dioxirane epoxidations. We contend that the oxygen transfer
from the peracids to olefins is a particularly suitable model for
comparison because its transition state geometry is akin to the
perepoxide-like structures A and B in the singlet oxygen ene
reaction and fortunately its butterfly mechanism is well estab-
lished.24
ring) stabilizes the threo-A/1a complex, the seven-membered
rings in the threo-A/1b,c exciplexes do less so. Therefore, no
significant steering control is exercised by the OH group for
the hydroperoxide 1b and the homoallylic alcohol (Z)-1c and
the Schenck ene reaction proceeds in low diastereoselectivity.
In view of the epoxide-like geometry proposed13b for the
exciplex A and certainly for the perepoxide B (Scheme 2), it
should be mechanistically instructive to compare the singlet
oxygen diastereoselectivities with those of bona fide epoxida-
tions by peracids.14,15,24 It is widely accepted that in the peracid
epoxidation of alkenes the butterfly transition state applies,
which results from nucleophilic attack of the double bond on
the peroxide bond of the oxidant.24 In the case of allylic
alcohols, a strong hydrogen bond is formed14 and, provided 1,3-
allylic strain operates, the threo-C transition state is favored in
The results in Table 1 reveal that electron-accepting X
substituents like SO2Ph (entries 36 and 37), COOEt (entries 42
and 43), COOH (entries 47-49), Cl (entries 53 and 54), and
Br (entry 57) at the allylic chirality center all promote highly
regioselective singlet oxygen ene reactions in moderate to good
erythro stereocontrol. As proposed for the acylated allylic
amines, this can be explained by electrostatic and steric
repulsions between the substituent X and the negatively charged
oxygen atom in the threo-A exciplex (cf. Scheme 2). Thus,
the energy-favored erythro-A exciplex and erythro-B perepoxide
are formed, which lead to the erythro-2 and (Z)-2′ hydroper-
oxides.
Steric reasons alone cannot be responsible for the aforemen-
tioned erythro diastereoselectivities. This is clearly demon-
strated for the photooxygenation of the allylic halides 1l,m
(entries 53 and 57), the ester (Z)-1j (entry 42), and the acid
(Z)-1k (entry 47) in halogenated solvents (CCl4 and CDCl3).
Their diastereoselectivities are all higher than for the tert-butyl
derivative 1d (entry 23). The steric demand of the tert-butyl
group is certainly higher than that of the chloro substituent.
The importance of the 1,3-allylic strain for the regio- and
diastereoselectivity of the singlet oxygen ene reaction of acyclic
chiral olefins is once more shown by the photooxygenation of
the ester (E)-1j (entry 46). Since the chirality center in this
stereoisomer rotates essentially freely, the Schenck reaction is
not diastereoselective at all. Furthermore, the regioselectivity
is inverted in comparison to the (Z)-1j isomer (entry 42). The
reason is that in the (E)-1j isomer the hydrogen atom at the
chirality center readily aligns perpendicularly to the olefinic
plane and its abstraction leads preferentially to the R,â-
unsaturated ester hydroperoxides (Z,E)-2′j.
energy compared to that with the erythro-geometry in view of
methyl-methyl repulsion in the latter. Consequently, the threo-
configurated epoxy alcohols are formed preferentially. Quite
analogously, the preferred transition state geometry of the
photooxygenation possesses the structure represented by the
threo-A exciplex. In this context, it should be noted that
contrary to the six-membered ring proposed here for the
hydrogen bonding in the singlet oxygen ene reaction13b and other
publications,14g Sharpless favors a five-membered ring for the
hydrogen bonding in the diastereoselective epoxidation of allylic
alcohols.14c
As already pointed out in the Introduction, only little is known
on the diastereoselective epoxidation of allylic alcohols by
dioxiranes. Recently Murray25 proposed on the basis of solvent
effects on the rate of DMD epoxidations that a spiro-type
transition state with partial charge separation operates. More-
over, theoretical work by Bach26 on the reaction coordinate for
the parent dioxirane epoxidation of ethylene suggests that a
spiro-perpendicular transition state is the lowest energy pathway.
Consequently, analogous to the structures threo-A/1a for singlet
oxygen and threo-C/1a for mCPBA, the threo-D/1a transition
state for dioxirane should be favored in energy because of
stabilization of the incipient partially negatively charged external
(23) Hofmann, R. W. Chem. ReV. 1989, 89, 1841-1860.
(24) (a) Bartlett, P. D. Rec. Chem. Prog. 1950, 11, 47-51. (b) Hanzlik,
R. P.; Shearer, G. O. J. Am. Chem. Soc. 1975, 97, 5231-5233. (c) Plesnicar,
B.; Tasevski, M.; Azman, A. J. Am. Chem. Soc. 1978, 100, 743-746. (d)
Woods, K. W.; Beak, P. J. Am. Chem. Soc. 1991, 113, 6281-6283. (d)
Bach, R. D.; Owensby, A. L.; Gonzales, C.; Schlegel, H. B.; McDouall, J.
J. W. J. Am. Chem. Soc. 1991, 113, 2338-2341.
(25) Murray, R. W.; Gu, D. J. Chem. Soc., Perkin Trans. 2 1993, 2203-
2307.
(26) Bach, R. D.; Owensby, A. L.; Andre´s, J. L.; Schlegel, H. B. J. Am.
Chem. Soc. 1991, 113, 7031-7033.

Heteroatom-Substituted Acyclic Chiral Olefins
J. Am. Chem. Soc., Vol. 118, No. 8, 1996 1905
dioxirane oxygen atom through hydrogen bonding. This was
previously suggested16c and expected in view of minimized 1,3-
allylic strain.23 Very recent results by Asensio16e and Murray16f
for the epoxidations of cyclic allylic alcohols and ammonium
salts support this suggestion.
effects erythro-4c would have been expected as the main
product, a modest stabilization of threo-C and threo-D by
hydrogen bonding might be responsible for this moderate
diastereoselectivity.
Erythro-controlling effects on the epoxidation of acyclic chiral
olefins are displayed by large X groups at the allylic chirality
center. For example, it is known15 that substrates with large X
substituents are attacked by mCPBA preferentially from the side
opposite to X and, therefore, the erythro-configurated epoxides
are favored. A similar trend has been observed for the DMD16a,b
epoxidation of acyclic olefins.
The observed erythro selectivities in the oxidations of the
substrates 1d,g (entries 23-26 and 36-39) for the three oxidants
1O2, mCPBA, and DMD nicely corroborate these expectations.
Furthermore, the very high diastereoselectivities of the mCPBA
and DMD epoxidations of the tert-butyl-substituted alkene 1d
(entries 25 and 26) show that these epoxidations are more subject
to steric influence of the X substituent than the singlet oxygen
ene reaction. The latter exhibits only a moderate erythro-
selectivity in the oxidation of tert-butyl derivative 1d (entries
23 and 24).
Unlike the singlet oxygen ene reaction, the mCPBA and DMD
epoxidations of the substrates 1e,f (entries 29-31 and 34 and
35) and (Z)-1j,k (entries 44 and 45 and 50-52) exhibit only
moderate if any erythro-diastereoselectivity. For the singlet
oxygen ene reaction, the moderate to high diastereoselectivities
were explained by the electrostatic repulsion of the partially
negatively charged singlet oxygen and the X substituent. This
repulsion seems not to be decisive for the mCPBA and DMD
epoxidations. However, as was clearly shown by the highly
diastereoselective epoxidations of the allylic chloride 1l (entries
55 and 56), the mCPBA and DMD reactions depend not only
on steric but also on electronic influence.43 These electronic
effects might be of electrostatic nature, as described for the
singlet oxygen ene reaction.
Our results for the epoxidation of the acyclic allylic alcohol
1a by DMD in acetone display only a moderate threo
diastereoselectivity16d,g (entry 7), definitely lower than for
mCPBA (entry 6) but somewhat higher than for singlet oxygen
(entry 3). Since an appreciable solvent effect is observed on
the threo diastereoselectivities of the photooxygenation of the
allylic alcohol 1a in CCl4 Versus acetone (entries 1 and 3), it
was necessary to investigate the solvent effect on the epoxidation
of allylic alcohol 1a by dioxirane. Therefore, the epoxidation
with DMD was performed in the precence of a large excess of
carbon tetrachloride16g (90:10 CCl4/acetone; entry 8). Under
these conditions, the threo-configurated epoxide 4a was obtained
in a little higher diastereoselectivity than in acetone alone (entry
7). Furthermore, when the trifluoromethyl-substituted dioxirane
(TFD) was used as oxidant, since it can be prepared as ketone-
free solutions,27 indeed, in acetone-d4 (entry 9) only a low threo-
selectivity was obtained, while in carbon tetrachloride (entry
10) it was again high. Thus, we conclude that, in the
epoxidation of acyclic, allylic alcohols by dioxiranes, hydrogen
bonding operates in the transition state threo-D, quite analogous
to the transition states threo-C for mCPBA and threo-A for
singlet oxygen. However, despite the similarity in the transition
states for these three oxidants, marked differences apply in the
quality of hydrogen bonding. For example, for the mCPBA
epoxidations of the allylic alcohol 1a in methanol-d4, acetone-
d6, and carbon tetrachloride (entries 4-6), within the error limits
the same diastereoselectivities were obtained. This means that
the hydrogen bonding in the threo-C transition state for mCPBA
is so strong that even methanol cannot perturb it. In contrast,
for the singlet oxygen and TFD, hydrogen bonding in the
threo-A exciplex and the threo-D transition state is much weaker
and the diastereoselectivity is significantly lower in methanol-
d4 (entry 2) and acetone-d6 (entries 3 and 9) than in carbon
tetrachloride (entries 1 and 10).
Additional mechanistic insight is provided by the diastereo-
selectivities in the singlet oxygen, mCPBA, DMD, and TFD
oxidations of the allylic hydroperoxide 1b (entries 11-16) and
the homoallylic alcohol (Z)-1c (entries 17-22). The moderate
threo- or even erythro-diastereoselectivities observed for all
three oxidants imply little if any steering effect by the homo-
allylic hydroxy or homoallylic-like hydroperoxy functionality
through hydrogen bonding. As pointed out already for the
Schenck reaction of these substrates (entries 11, 12, 17, and
18), the threo-A/1b,c transition states possess an unfavorable
geometry for hydrogen bonding, which results in poor stereo-
control. Similarly, poor hydrogen bonding also operates in the
transition states for the mCPBA (threo-C) and DMD/TFD
(threo-D) epoxidations of the allylic hydroperoxide 1b (entries
13-16). Thus, not the threo- but the erythro-configurated
epoxide 4b is formed preferentially. Had hydrogen bonding
been appreciable, 1,3-allylic strain would have dictated a high
threo-selectivity. For the homoallylic alcohol (Z)-1c the situ-
ation seems to be somewhat different. Whereas the singlet
oxygen ene reaction of this substrate is not diastereoselective
at all (entries 17 and 18), the mCPBA and dioxirane epoxidations
give the epoxide 4c in a moderate threo-selectivity (entries 19-
22); the latter are even higher in the nonpolar CCl4 (entries 20
and 22) than in acetone (entries 19 and 21). Since due to steric
In conclusion, the above extensive stereochemical results
demonstrate that heteroatom-substituted acyclic, chiral olefins
with 1,3-allylic strain can nicely be employed for the diastereo-
selective synthesis of oxyfunctionalized molecules through the
singlet oxygen ene reaction and the mCPBA and DMD/TFD
epoxidations. Thus, threo-selectivity can be achieved by X
substituents, which act through hydrogen bonding. Indeed, it
has been convincingly demonstrated by the TFD epoxidation
of 1a that hydrogen bonding is the decisive factor in the
diastereoselective epoxidation of acyclic allylic alcohols by
dioxirane. In contrast, substrates which possess sterically
demanding and/or partially negatively charged X substituents
are oxidized erythro-selectively by all three oxidants.
Acknowledgment. We are grateful to the Deutsche For-
schungsgemeinschaft (Schwerpunktprogramm “Peroxidche-
mie: Mechanistische und pra¨parative Aspekte des Sauerst-
offtransfers”) for generous financial support and the Fonds der
Chemischen Industrie for a doctoral fellowship to H.G.B.
(1993-1995). We thank Drs. T. Linker and M. Prein for their
valuable discussions and constructively critical suggestions.
Supporting Information Available: Experimental details
for the preparation, purification, and characterization of all
mentioned compounds and the crystallographic data for
(2R*,3R*,4S*)-5g (24 pages). This material is contained in
many libraries on microfiche, immediately follows this article
in the microfilm version of the journal, can be ordered from
the ACS, and can be downloaded from the Internet; see any
current masthead page for ordering information and Internet
access instructions.
(27) Adam, W.; Curci, R.; Gonza´les Nun˜ez, M. E.; Mello, R. J. Am.
Chem. Soc. 1991, 113, 7654-7658.
(28) Fujita, M.; Ishizuka, H.; Ogura, K. Tetrahedron Lett. 1991, 32,
6355-6358.
JA9530230