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far, our group has developed poly(ethylene glycol)-polysty-
rene (PEG-PS) resin-supported peptide catalyst 1 (Figure 1)
the conjugate additions to unsaturated iminium cations, provided
that the reaction intermediate takes a planar structure.
We chose the R-oxyamination of aldehydes reported by
Sibi as a target reaction because the enantioselectivity
leaves room for improvement.13 In the presence of peptide
catalyst 1, the oxyamination of 3-phenylpropanal with
TEMPO using iron(III) chloride as an SET reagent was
performed in THF/H2O ) 1/1 (v/v) at room temperature
(Table 1, entry 1). The enantioselectivity was better than
Figure 1. Resin-supported peptide catalyst.
possessing polyleucine as a chiral hydrophobe. The catalyst
has a ꢀ-turn pentapeptide connected to the N-terminus of
the R-helical polyleucine chain. With this catalyst, asym-
metric hydrogenation7 and Friedel-Crafts-type asymmetric
alkylation8 proceeded efficiently in aqueous media. Despite
its simple primary structure, the polyleucine moiety was
essential for both catalytic efficiency and enantioselectivity
in these reactions.9
Table 1. R-Oxyamination Using N-Proryl Peptide Catalyst
Recently, Sibi et al.,10 MacMillan et al.,11 and other
groups12 have reported a new class of chiral-amine-
catalyzed asymmetric reactions which involve radical
cation intermediates. For example, Sibi has developed
enantioselective R-oxyamination of aldehydes using TEMPO
in DMF or THF. In that reaction, the enamine intermediate
formed between the amine catalyst and the aldehyde is oxidized
through a single electron transfer (SET) mechanism, then the
resulting planar radical cation undergoes enantioface-selective
C-O bond formation with TEMPO. The reaction was achieved
with chiral 4-imidazolidinone catalysts, while proline and
diphenylprolinol were not suitable as catalysts because of quite
low enantioselectivity.10 In the reactions proceeding via iminium
ion intermediates, we have shown that prolyl peptide 1 is the
effective asymmetric catalyst even for the ones in which simple
proline is a poor catalyst. Therefore, it is expected that prolyl
catalyst 1 is also applicable to asymmetric reactions other than
a Determined by chiral HPLC analysis using Chiralcel OD-H. b Not
determined.
(5) (a) Kobayashi, S.; Manabe, K. Acc. Chem. Res. 2002, 35, 209. (b)
Iimura, S.; Manabe, K.; Kobayashi, S. Tetrahedron 2004, 60, 7673, and
references therein.
that reported for the same reaction by the imidazolidinone
catalyst (82% ee) at lowered temperature.10 The increase
in the ratio of water in the solvent system to THF/H2O )
1/2 brought about the acceleration of the reaction probably
because of intensified hydrophobic interactions between
the substrate and the catalyst (entry 2). It is noteworthy
that the reaction proceeded smoothly even in the absence
of the organic cosolvent (entry 3). The reaction was
sluggish with proline (entry 4), PEG-PS resin-supported
proline, or Pro-Leu-Leu (entries 5 and 6). On the contrary,
introducing polyleucine between the terminal prolyl
residue and the solid support effectively enhanced the
reaction rate with a considerable change in enantioselec-
tivity (entry 7).14 This indicates that the polyleucine
moiety not only provides a hydrophobic environment but
also affects the stereochemical outcome of the reaction.
The importance of polyleucine was further endorsed by
the fact that catalyst 2 lacking the polyleucine chain showed
(6) (a) Wei, S.; Wang, J.; Venhuizen, S.; Skouta, R.; Breslow, R. Bioorg.
Med. Chem. Lett. 2009, 19, 5543. (b) Javor, S.; Delort, E.; Darbre, T.;
Reymond, J.-L. J. Am. Chem. Soc. 2007, 129, 13238. (c) Kofoed, J.;
Reymond, J.-L. Curr. Opin. Chem. Biol. 2005, 9, 656.
(7) (a) Akagawa, K.; Akabane, H.; Sakamoto, S.; Kudo, K. Tetrahedron:
Asymmetry 2009, 20, 461. (b) Akagawa, K.; Akabane, H.; Sakamoto, S.;
Kudo, K. Org. Lett. 2008, 10, 2035.
(8) Akagawa, K.; Yamashita, T.; Sakamoto, S.; Kudo, K. Tetrahedron
Lett. 2009, 50, 5602.
(9) Simple polyleucine has been used as a catalyst for Julia´-Colonna
epoxidation. For a review, see: Porter, M. J.; Roberts, S. M.; Skidmore, J.
Bioorg. Med. Chem. 1999, 7, 2145.
(10) Sibi, M. P.; Hasegawa, M. J. Am. Chem. Soc. 2007, 129, 4124.
(11) (a) Amatore, M.; Beeson, T. D.; Brown, S. P.; MacMillan, D. W. C.
Angew. Chem., Int. Ed. 2009, 48, 5121. (b) Conrad, J. C.; Kong, J.;
Laforteza, B. N.; MacMillan, D. W. C. J. Am. Chem. Soc. 2009, 131, 11640.
(c) Wilson, J. E.; Casarez, A. D.; MacMillan, D. W. C. J. Am. Chem. Soc.
2009, 131, 11332. (d) Nicewicz, D. A.; MacMillan, D. W. C. Science 2008,
322, 77. (e) Graham, T. H.; Jones, C. M.; Jui, N. T.; MacMillan, D. W. C.
J. Am. Chem. Soc. 2008, 130, 16494. (f) Kim, H.; MacMillan, D. W. C.
J. Am. Chem. Soc. 2008, 130, 398. (g) Jang, H.-Y.; Hong, J.-B.; MacMillan,
D. W. C. J. Am. Chem. Soc. 2007, 129, 7004. (h) Beeson, T. D.;
Mastracchio, A.; Hong, J.-B.; Ashton, K.; MacMillan, D. W. C. Science
2007, 316, 582.
(12) (a) Nicolaou, K. C.; Reingruber, R.; Sarlah, D.; Bra¨se, S. J. Am.
Chem. Soc. 2009, 131, 2086. (b) Jang, D. O.; Kim, S. Y. J. Am. Chem.
Soc. 2008, 130, 16152. (c) Bui, N.-N.; Ho, X.-H.; Mho, S.-i.; Jang, H.-Y.
Eur. J. Org. Chem. 2009, 5309.
(13) For examples of peptide-based oxidations, see: (a) Jakobsche, C. E.;
Peris, G.; Miller, S. J. Angew. Chem., Int. Ed. 2008, 47, 6707. (b) Peris,
G. P.; Jakobsche, C. E.; Miller, S. J. J. Am. Chem. Soc. 2007, 129, 8710.
(c) Peris, G.; Miller, S. J. Org. Lett. 2008, 10, 3049.
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