Diketopiperazine-derived hydroperoxide for chemoselective oxidations of sulfides and enantioselective Weitz-Scheffer epoxidations

Article abstract of DOI:10.1016/j.tetlet.2008.01.090

l-Proline-derived hydroperoxide (-)-2, which was obtained from the corresponding diketopiperazine by irradiation under oxygen atmosphere, was applied to the oxidation of a variety of sulfides and asymmetric Weitz-Scheffer epoxidations of cyclic and acycli

Full text of DOI:10.1016/j.tetlet.2008.01.090

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 1971–1974
Diketopiperazine-derived hydroperoxide for chemoselective oxidations
of sulfides and enantioselective Weitz–Scheffer epoxidations
Marcel Kienle, Wassiliki Argyrakis, Angelika Baro, Sabine Laschat *
Institut f u¨ r Organische Chemie, Universit a¨ t Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
Received 5 December 2007; revised 18 January 2008; accepted 21 January 2008
Available online 30 January 2008
Abstract
L-Proline-derived hydroperoxide (ꢀ)-2, which was obtained from the corresponding diketopiperazine by irradiation under oxygen
atmosphere, was applied to the oxidation of a variety of sulfides and asymmetric Weitz–Scheffer epoxidations of cyclic and acyclic
enones. The sulfoxidation, however, gave only racemic products. In contrast, depending on the base catalyst, enantioselectivities up
to 37% were achieved in the epoxidation of chalcone with (ꢀ)-2.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Asymmetric catalysis; Chirality; Hydroperoxide; Oxidation
The chemo- and stereoselective oxidation of sulfides to
the corresponding sulfoxides and the enantioselective
Weitz–Scheffer epoxidation of a,b-unsaturated carbonyl
compounds constitute two valuable synthetic transforma-
X
X
O
O
O
O
H
N
H
N
O2
N
O
N
(1 atm)
N
H
PhCOPh
(1 mol %)
H
OOH
1
,2
tions, which have received enormous interest. Organic
hydroperoxides of the general formula 1 (Scheme 1) are
known for more than two decades, mostly in the form of
flavin derivatives. Their use in the transfer of oxygen to
sulfides, amines and other substrates is well established in
O
EtOAc, hν
1 (X = NR, O, CR2)
(−)-2
cyclo(-L-Pro-L-Pro-)
Scheme 1. General formula of organic hydroperoxides 1 and cis-config-
ured L-proline-derived hydroperoxide (ꢀ)-2.
19
3
the literature. Most recent results along this direction were
the first who studied olefin epoxidations with chiral hydro-
4
5
reported by B a¨ ckvall and Murahashi employing flavins
as redox catalysts and hydrogen peroxide as stoichiometric
oxidant. Sulfoxidations were performed with excellent che-
moselectivities but no enantioselectivity was achieved.
Chiral hydroperoxides, which were obtained via kinetic
11,12
peroxides.
Optically active hydroperoxides have also
13
6,14
been employed in sulfoxidations by Wong, Adam,
Hamann and others as well as in asymmetric Weitz–
Scheffer epoxidations of enones.
15
16
16d,17
In most cases, the
hydroperoxide unit was generated by the oxidation of the
corresponding C–H bond with hydrogen peroxide rather
than by molecular oxygen. However, the formation of a
chiral hydroperoxide under aerobic conditions and its
subsequent use in oxidations would be highly desirable.
Surprisingly, besides a report by Oda on a Weitz–Scheffer
epoxidation under chiral phase transfer conditions with
in situ formed 9-hexyl-fluorenone-9-hydroperoxide as achi-
6
,7
8
resolution using peroxidases,
Sharpless epoxidation,
separation by column chromatography on a chiral station-
9
10
ary phase and via diastereomeric perketals, were success-
fully used in asymmetric oxidations. Rebek was probably
*
Corresponding author. Tel.: +49 711 685 64565; fax: +49 711 685
1
8
ral oxidant, this issue has never been investigated in
detail. Thus, we studied optically active L-proline-derived
64285.
E-mail address: sabine.laschat@oc.uni-stuttgart.de (S. Laschat).
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.01.090

1972
M. Kienle et al. / Tetrahedron Letters 49 (2008) 1971–1974
1
9–23
diketopiperazine hydroperoxide (ꢀ)-2
which is accessible by irradiation of precursor cyclo-
-L-Pro-L-Pro-) under O atmosphere, as oxygen source in
(Scheme 1),
O
O
(
−)-2 (1.1 equiv)
(
O
R2
2
NaOH (1.2 equiv)
THF, rt
R2
sulfoxidations and Weitz–Scheffer epoxidations.
n
n
5
6
First, various sulfides 3 were treated with a stoichiome-
tric amount of hydroperoxide (ꢀ)-2 in MeOH at room tem-
perature. Upon complete consumption of starting material
(monitored by NMR), the reaction mixture was submit-
ted to aqueous workup and sulfoxides 4 were isolated after
chromatography (Table 1).
n
R2 Time Conv.
h) (%)
Yield % ee
(%) (config.)
(
48
5
a
b
0
1
1
H
H
100 6a 31
12, (−)
3
5
1.5 100 6b 86
19, (+) (2
R
,3
R
)
5c
Me 14 d 75
6c 52
12, (+) (2
R
,3
R
)
Scheme 2. Weitz–Scheffer epoxidation of cyclic enones 5 by hydroperox-
ide 2. Yields refer to isolated yields, ee values were determined by HPLC
on a chiral phase Chiracel OD-H. The configuration of the major
enantiomer of 6b was derived from the comparison of optical rotation
Dibutylsulfide 3a reacted rapidly to sulfoxide 4a in
excellent yield of 86% whereas diphenylsulfide 3f required
5
days for completion (entries 1 and 6). The unsymmetri-
1
6d,24
cally substituted sulfides 3b–e gave sulfoxides 4b–e in
moderate to good yields (entries 2–5). The reactivity of
arylmethylsulfides 3g–i depended on the aryl substituent.
Whereas electron-withdrawing substituents such as cyano
with the literature data,
that of 6c was assigned by analogy.
Unfortunately, only poor enantioselectivities up to 19%
ee were obtained.
(
3i) strongly decreased the reactivity (entry 9), the reaction
times were improved for the electron-donating +M substi-
tuents: bromo and methoxy (entries 7 and 8). It must be
noted that even in the cases of moderate yields, oxidations
went with complete chemoselectivities, that is, no trace of
the corresponding sulfones was detected. However, despite
the use of enantiopure hydroperoxide (ꢀ)-2 no enantiose-
lectivity was observed (separation by HPLC).
In order to assess the reactivity and stereoselectivity of
hydroperoxide (ꢀ)-2, the base-catalyzed Weitz–Scheffer
epoxidation was performed with enones 5 (Scheme 2).
Cyclopentenone 5a and cyclohexenone 5b yielded the
corresponding epoxyketones 6a and 6b without any prob-
lem. The reduced yield of 6a is due to its high volatility.
Steric hindrance at the b-position of the enone moiety sig-
nificantly decreased the reactivity as shown for 3-methyl-
cyclohexenone 5c with a reaction time of 14 days.
Recently, Adam reported the important influence of the
base catalyst on the enantioselectivity in Weitz–Scheffer
1
7b,c
epoxidations.
We therefore studied the epoxidation of
4-phenylbut-3-en-2-one (7a) and chalcone 7b by hydroper-
2
5
oxide 2 in the presence of various bases (Table 2).
In most cases, conversions and yields for 7b are better
than those for 7a. The most surprising outcome, however,
was the different reactivity of enones 7a and 7b in the pres-
ence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (entries
3, 8, 9). Whereas 4-phenylbutenone 7a and hydroperoxide
(ꢀ)-2 did not give any trace of the desired product 8a,
epoxide 8b was isolated in excellent yields with the highest
enantioselectivities of 35–37% ee. From the results in Table
2 it is obvious that chalcone 7b displays a much higher
reactivity than 4-phenylbutenone 7a. Again, the absolute
configuration of products 8a,b (entries 4 and 10) was
Table 1
Oxidation of sulfides 3 to the corresponding sulfoxides 4 using hydroperoxide 2
O
H
N
(−)-2
N
O
S
OOH
O
S
R
R1
R
R1
MeOH, rt
3
4
1
Entry
Sulfide
R
R
Oxidant 2 (equiv)
Time
Sulfoxide
Yield (%)
1
2
3a
3b
Bu
Ph
Bu
Me
1.1
1.0
1.5 h
24 h
4a
4b
86
67
3
4
3c
3d
Ph
Ph
1.1
1.0
24 h
19 h
4c
4d
44
78
a
5
3e
Ph
1.0
24 h
4e
39
6
7
8
9
a
3f
3g
3h
3i
Ph
4-BrC
4-MeOC
4-CNC H
Ph
1.0
1.1
1.1
1.1
5 d
4f
4g
4h
4i
86
84
41
15
6
H
H
6 4
4
Me
Me
Me
72 h
20 h
9 d
6
4
2
0% of sulfide 3e was recovered.

M. Kienle et al. / Tetrahedron Letters 49 (2008) 1971–1974
1973
Table 2
Epoxidation of enones 7 with hydroperoxide 2 in the presence of various bases
O
H
3
O
(
—)-2 (1.1 equiv)
O
Ph
R2
Ph
R2
base (1.2 equiv)
THF
2
H
2
2
7
a R = Me
(—)-8a,b
7
b R = Ph
a
b
Entry
Enone
Base
T (°C)
Time
Conv. (%)
Product
Yield (%)
% ee
c
1
2
3
4
5
6
7
8
9
7a
7a
7a
7a
7a
7b
7b
7b
7b
7b
7b
KOtBu
LiOH
DBU
ꢀ20
7 h
24 h
7 d
24 h
5 h
42 h
44 h
6 d
66
93
0
72
60
77
100
95
100
100
100
8a
8a
8a
8a
8a
8b
8b
8b
8b
8b
8b
33
62
—
48
57
53
87
84
95
98
98
18
24
—
0
0
d
e
KOH
0
11
nBuLi
KOtBu
ꢀ20
18
11
22
37
35
0
f
LiOH
0
0
25
0
ꢀ20
DBU
DBU
KOH
nBuLi
6 d
8 h
8 h
e
1
1
0
1
a
b
c
2
26
Yields refer to isolated yields.
Enantioselectivities of 8a were detected by GC on a chiral stationary phase Bondex un-a, those of 8b by HPLC on a chiral phase Chiracel OD-H.
Low temperature to achieve higher enantioselectivity.
Complete reisolation of 7a.
d
e
16d,26
Configuration of (ꢀ)-8a and (ꢀ)-8b was assigned by comparison of optical rotation with the literature data
to be (2R,3S).
f
2.5 equiv.
assigned by comparison of optical rotation with the litera-
Russ. J. Org. Chem. 2003, 39, 1537–1552; (d) Bolm, C. Coord. Chem.
Rev. 2003, 237, 245–256; (e) Colonna, S.; Gaggero, N.; Carrea, G.;
Pasta, P. Asym. Oxid. React. 2001, 227–235; (f) Kagan, H. B. Asym.
Oxid. React. 2001, 153–170; (g) Bolm, C. Peroxide Chem. 2000, 494–
1
6d,26
17b
ture data.
According to Adam’s findings,
the
(
2R,3S)-configuration of the major enantiomer of epoxy-
ketone 8a and 8b indicates that hydroperoxide (ꢀ)-2 pref-
5
10; (h) van de Velde, F.; Arends, I. W. C. E.; Sheldon, R. A. Top.
erentially attacks the Re face of enones 7a,b.
Catal. 2000, 13, 259–265; (i) Page, P. C. B.; Heer, J. P.; Bethell, D.;
Lund, A. B. Phosphorus, Sulfur Silicon Relat. Elem. 1999, 153–154,
In conclusion, enantiomerically pure diketopiperazine
hydroperoxide (ꢀ)-2 could be successfully used for chemo-
selective sulfoxidations and enantioselective Weitz–Scheffer
epoxidations under mild conditions. The formation of
hydroperoxide 2 under oxygen atmosphere is particularly
important, thus avoiding the use of hydrogen peroxide.
However, further efforts are necessary to achieve enantio-
selectivity in sulfoxidations as well, to improve the chirality
transfer in Weitz–Scheffer epoxidations and to develop a
catalytic version of this process. Work along these lines
are currently under way in our laboratories.
2
47–258; (j) van de Velde, F.; Arends, I. W. C. E.; Sheldon, R. A. J.
Inorg. Biochem. 2000, 80, 81–89; (k) Bolm, C.; Muniz, K.; Hilde-
brand, J. P. In Comprehensive Asymmetric Catalysis I–III; Jacobsen,
E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer: Berlin, 1999; Vol. II,
pp 697–710; (l) Kagan, H. B.; Diter, P. Organosulfur Chem. 1998, 2,
1–39.
. Reviews on enantioselective Weitz–Scheffer and Julia–Colonna
epoxidations: (a) Hinch, M.; Jacques, O.; Drago, C.; Caggiano, L.;
Jackson, R. F. W.; Dexter, C.; Anson, M. S.; Macdonald, S. J. F.
J. Mol. Catal. A: Chem. 2006, 251, 123–128; (b) Kelly, D. R.;
Roberts, S. M. Biopolymers 2006, 84, 74–89; (c) Berkessel, A. Pure
Appl. Chem. 2005, 77, 1277–1284; (d) Lauret, C.; Roberts, S. M.
Aldrichim. Acta 2002, 35, 47–51; (e) Porter, M. J.; Skidmore, J. Chem.
Commun. 2000, 1215–1225; (f) Aggarwal, K. V. In Comprehensive
Asymmetric Catalysis I–III; Jacobsen, E. N., Pfaltz, A., Yamamoto,
H., Eds.; Springer: Berlin, 1999; Vol. II, pp 679–697; (g) Porter, M. J.;
Roberts, S. M.; Skidmore, J. Bioorg. Med. Chem. 1999, 7, 2145–
2156.
2
Acknowledgements
Generous financial support by the Deutsche For-
schungsgemeinschaft (SFB 706), the Fonds der Chemis-
chen Industrie, the Ministerium f u¨ r Wissenschaft,
Forschung und Kunst des Landes Baden-W u¨ rttemberg,
the BASF, Degussa and S u¨ dchemie are gratefully
acknowledged.
3. (a) Zheng, Y.-J.; Bruice, T. C. Bioorg. Chem. 1997, 25, 331–336; (b)
Oae, S.; Asada, K. O.; Yoshimura, T.; Fujimori, K. Heterocycles
1
992, 33, 189–194; (c) Mer e´ nyi, G.; Lind, J. J. Am. Chem. Soc. 1991,
13, 3146–3153; (d) Keum, S.-R.; Lee, K. B.; Bruice, T. C. Bull.
1
Korean Chem. Soc. 1990, 11, 95–99; (e) Keum, S.-R.; Gregory, D. H.;
Bruice, T. C. J. Am. Chem. Soc. 1990, 112, 2711–2715; (f) Harayama,
T.; Sakurai, O.; Sonehara, S.; Tezuka, Y.; Yoneda, F. Chem. Express
References and notes
1
988, 3, 187–190; (g) Miller, A. E.; Bischoff, J. J.; Bizub, C.;
1
. Reviews on enantioselective oxidation of sulfides: (a) Legros, J.;
Dehli, J. R.; Bolm, C. Adv. Synth. Catal. 2005, 347, 19–31; (b) Kagan,
H. B.; Luukas, T. O. In Transition Metals for Organic Synthesis, 2nd
ed.; Beller, M., Bolm, C., Eds.; Wiley-VCH: Weinheim, 2004; Vol. 2,
pp 479–495; (c) Volcho, K. P.; Salakhutdinov, N. F.; Tolstikov, A. G.
Luminoso, P.; Smiley, S. J. Am. Chem. Soc. 1986, 108, 7773–7778; (h)
Doerge, D. R.; Corbett, M. D. Mol. Pharmacol. 1984, 26, 348–352; (i)
Doerge, D. R.; Corbett, M. D. Biochem. Pharmacol. 1984, 33, 3615–
3619; (j) Oae, S.; Asada, K.; Yoshimura, T. Tetrahedron Lett. 1983,
24, 1265–1268; (k) Bruice, T. C. J. Chem. Soc., Chem. Commun. 1983,

1974
M. Kienle et al. / Tetrahedron Letters 49 (2008) 1971–1974
1
4–15; (l) Bruice, T. C.; Noar, J. B.; Ball, S. S.; Venkataram, U. V. J.
14. (a) Adam, W.; Korb, M. N.; Roschmann, K. J.; Saha-M o¨ ller, C. R. J.
Org. Chem. 1998, 63, 3423–3428; (b) Adam, W.; Korb, M. N.
Tetrahedron: Asymmetry 1997, 8, 1131–1142.
Am. Chem. Soc. 1983, 105, 2452–2463; (m) Miller, A. Tetrahedron
Lett. 1982, 23, 753–756; (n) Muto, S.; Bruice, T. C. J. Am. Chem. Soc.
1
982, 104, 2284–2290; (o) Ball, S.; Bruice, T. C. J. Am. Chem. Soc.
981, 103, 5494–5503 and references cited therein.
15. Hamann, H.-J.; H o¨ ft, E.; Mostowicz, D.; Mishnev, A.; Urbanczyk-
Lipkowska, Z.; Chmielewski, M. Tetrahedron 1997, 53, 185–192.
16. (a) Lattanzi, A.; Iannece, P.; Scettri, A. Tetrahedron: Asymmetry
2004, 15, 1779–1785; (b) Lattanzi, A.; Iannece, P.; Scettri, A.
Tetrahedron: Asymmetry 2004, 15, 413–418; (c) Massa, A.; Lattanzi,
A.; Siniscalchi, F. R.; Scettri, A. Tetrahedron: Asymmetry 2001, 12,
2775–2777; (d) Aoki, M.; Seebach, D. Helv. Chim. Acta 2001, 84, 187–
207; (e) Andersson, M.; Allenmark, S. Biocatal. Biotransform. 2000,
18, 79–86; (f) Palombi, L.; Bonadies, F.; Pazienza, A.; Scettri, A.
Tetrahedron: Asymmetry 1998, 9, 1817–1822; (g) Brunner, H.; Reimer,
A. Bull. Soc. Chim. Belg. 1997, 106, 267–272; (h) Scettri, A.; Bonadies,
F.; Lattanzi, A. Tetrahedron: Asymmetry 1996, 7, 629–632; (i) Takata,
T.; Wataru, A. Bull. Chem. Soc. Jpn. 1986, 59, 1275–1276.
1
4
. (a) Lind e´ n, A. A.; Johansson, M.; Hermanns, N.; B a¨ ckvall, J.-E. J.
Org. Chem. 2006, 71, 3849–3853; (b) Lind e´ n, A. A.; Hermanns, N.;
Ott, S.; Kr u¨ ger, L.; B a¨ ckvall, J.-E. Chem. Eur. J. 2005, 11, 112–119;
(
c) Lind e´ n, A. A.; Kr u¨ ger, L.; B a¨ ckvall, J.-E. J. Org. Chem. 2003, 68,
5890–5896; (d) Minidis, A. B. E.; B a¨ ckvall, J.-E. Chem. Eur. J. 2001,
7, 297–302; (e) Bergstad, K.; B a¨ ckvall, J.-E. J. Org. Chem. 1998, 63,
6650–6655.
5
. (a) Imada, Y.; Iida, H.; Ono, S.; Murahashi, S.-I. J. Am. Chem. Soc.
003, 125, 2868–2869; (b) Murahashi, S.-I.; Oda, T.; Masui, Y. J. Am.
2
Chem. Soc. 1989, 111, 5002–5003.
. Adam, W.; Mock-Knoblauch, C.; Saha-M o¨ ller, C. R. J. Org. Chem.
6
7
1
999, 64, 4834–4839.
17. (a) Lattanzi, A.; Cocilova, M.; Iannece, P.; Scettri, A. Tetrahedron:
Asymmetry 2004, 15, 3751–3755; (b) Adam, W.; Rao, P. B.; Degen,
H.-G.; Saha-M o¨ ller, C. R. Eur. J. Org. Chem. 2002, 630–639; (c)
Adam, W.; Rao, P. B.; Degen, H.-G.; Saha-M o¨ ller, C. R. J. Am.
Chem. Soc. 2000, 122, 5654–5655.
. (a) Adam, W.; Boss, B.; Harmsen, D.; Lukacs, Z.; Saha-M o¨ ller, C. R.;
Schreier, P. J. Org. Chem. 1998, 63, 7598–7599; (b) Adam, W.; Hoch,
U.; Saha-M o¨ ller, C. R.; Schreier, P. Angew. Chem. 1993, 105, 1800–
1801; Angew. Chem. Int. Ed. Engl. 1993, 32, 1737–1739; (c) Schneider,
C.; Schreier, P.; Humpf, H.-U. Chirality 1997, 9, 563–567; (d) H a¨ ring,
D.; Sch u¨ ler, E.; Adam, W.; Saha-M o¨ ller, C. R.; Schreier, P. J. Org.
Chem. 1999, 64, 832–835.
18. Baba, N.; Kawahara, S.; Hamada, M.; Oda, J. Bull. Inst. Chem. Res.,
Kyoto Univ. 1987, 65, 144–146.
19. (a) H a¨ usler, J.; Jahn, R.; Schmidt, U. Chem. Ber. 1978, 111, 361–366;
(b) Schmidt, U.; H a¨ usler, J. Angew. Chem. 1976, 88, 538–539; Angew.
8
9
. (a) H o¨ ft, E.; Hamann, H.-J.; Kunath, A. J. Prakt. Chem. 1994, 336,
¨
5
34–537; (b) H o¨ ft, E. Top. Curr. Chem. 1993, 164, 63–77.
Chem., Int. Ed. Engl. 1976, 15, 497–498; (c) Ohler, E.; Tataruch, F.;
Schmidt, U. Chem. Ber. 1973, 106, 165–176.
. (a) Wagner, J.; Hamann, H.-J.; D o¨ pke, W.; Kunath, A.; H o¨ ft, E.
Chirality 1995, 7, 243–247; (b) Kunath, A.; H o¨ ft, E.; Hamann, H.-J.;
Wagner, J. J. Chromatogr. 1991, 588, 352–355.
20. Borowitz, G. B.; Borowitz, I. J.; Wang, Y.; Yang, D.; Toupence, R.;
Persaud, N. A. J. Inclusion Phenom. Macrocyclic Chem. 2000, 38,
207–219.
21. de Costa, B. R.; He, X.-S.; Linders, J. T. M.; Dominguez, C.; Gu, Z.
Q.; Williams, W.; Bowen, W. D. J. Med. Chem. 1993, 36, 2311–2320.
22. Zadel, G.; Breitmaier, E. Chem. Ber. 1994, 127, 1323–1326.
23. A solution of cyclo(-L-Pro-L-Pro-) (0.492 g, 2.53 mmol) in EtOAc
(250 mL) was irradiated (kmax 254, 355 nm) in the presence of 1 mol %
1
0. (a) Porter, N. A.; Dussault, P.; Breyer, R. A.; Kaplan, J.; Morelli, J.
Chem. Res. Toxicol. 1990, 3, 236–243; (b) Dussault, P.; Porter, N. A.
J. Am. Chem. Soc. 1988, 110, 6276–6277.
1
1. Rebek, J.; McCready, R. J. Am. Chem. Soc. 1980, 102, 5602–
5
605.
2. (a) Lattanzi, A.; Piccirillo, S.; Scettri, A. Eur. J. Org. Chem. 2005,
669–1674; (b) Adam, W.; Alsters, P. L.; Neumann, R.; Saha-M o¨ ller,
C. R.; Seebach, D.; Beck, A. K.; Zhang, R. J. Org. Chem. 2003, 68,
222–8231; (c) Adam, W.; Alsters, P. L.; Neumann, R.; Saha-M o¨ ller,
1
1
2
benzophenone under 1 atm O , and the precipitate was collected
2
D
0
daily. After 7 days, hydroperoxide 2 (½aꢁ ꢀ135 (c 0.15, CHCl
3
)) was
8
obtained in a total yield of 39%. For comparison a racemic sample of
C. R.; Seebach, D.; Zhang, R. Org. Lett. 2003, 5, 725–728; (d) Bolm,
C.; Beckmann, O.; K u¨ hn, T.; Palazzi, C.; Adam, W.; Rao, P. B.;
Saha-M o¨ ller, C. R. Tetrahedron: Asymmetry 2001, 12, 2441–2446; (e)
Adam, W.; Beck, A. K.; Pichota, A.; Saha-M o¨ ller, C. R.; Seebach, D.;
Vogl, N.; Zhang, R. Tetrahedron: Asymmetry 2003, 14, 1355–1361; (f)
Lattanzi, A.; Iannece, P.; Scettri, A. Tetrahedron Lett. 2002, 43, 5629–
hydroperoxide 2 was prepared by treatment of proline methyl ester
with K CO in H O at 0 °C and subsequent heating of the neat
2 3 2
compound at 105 °C for 14 h followed by photooxidation.
24. (a) Sato, T.; Watanabe, M.; Honda, N.; Fujisawa, T. Chem. Lett.
1984, 1175–1176; (b) Wynberg, H.; Marsman, B. J. Org. Chem. 1980,
45, 158–161.
5
631; (g) Hamann, H.-J.; H o¨ ft, E.; Chmielewski, M.; Maciejewski, S.
25. With NaOH as the base conversions were quantitative, but no
enantioselectivities were achieved.
Chirality 1993, 5, 338–340.
1
3. Fu, H.; Kondo, H.; Ichikawa, Y.; Look, G. C.; Wong, C. H. J. Org.
26. Carde, L.; Davies, D. H.; Roberts, S. M. J. Chem. Soc., Perkin Trans.
1 2000, 2455–2463.
Chem. 1992, 57, 7265–7270.