Communications
examples of enantioselective reductions using chiral hydrogen-
atom transfer reagents, see: d) Y. Cai, B. P. Roberts, D. A.
(Table 3) and proceeded equally well with 30 mol% of chiral
Lewis acid as with stoichiometric amounts. Further reduction
of the catalyst load to 10 mol% resulted in a decrease of the
chemical yield and enantioselectivity (Table 3, entry 4). The
high Z/E selectivity of products 13 clearly indicates that the
iodine atom transfer from R2I to an intermediate radical
proceeded efficiently.
Tocher, J.Chem.Soc.Perkin Trans.1
2002, 1376; e) D.
Dakternieks, C. H. Schiesser, Aust.J.Chem. 2001, 54, 89; f) M.
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D. P. Curran, W. Liu, C. H.-T. Chen, J.Am.Chem.Soc. 1999, 121,
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Table 3: Cascade radical reaction of 12A and 12B with alkyl iodides.[a]
[5] M. Nishida, H. Hayashi, A. Nishida, N. Kawahara, Chem.
Commun. 1996, 579.
Entry Substrate R2
LA [equiv] Product Yield [%][b] d.r.[c]
ee [%][d]
[6] K. Hiroi, M. Ishii, Tetrahedron Lett. 2000, 41, 7071.
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see also reference [1a].
1
2
3
4
5
6
7
8
9
10
12A
12A
12A
12A
12A
12A
12B
12B
12B
12B
iPr
iPr
iPr
iPr
tBu 1.0
cHex 1.0
iPr
iPr
tBu 1.0
cHex 1.0
1.0
0.5
0.3
0.1
13Aa
13Aa
13Aa
13Aa
13Ab
13Ac
13Ba
13Ba
13Bb
13Bc
87
85
82
49[e]
85
82
86
74
94
87
>98:2 80
>98:2 81
>98:2 81
>98:2 47
>98:2 92
>98:2 81
>98:2 83
>98:2 81
>98:2 90
>98:2 85
1.0
0.3
[a] Reactions were carried out using 12A or 12B (1 equiv), R2I (30 equiv),
and Et3B in hexane (1.0m, 2.5 equiv) with Zn(OTf)2 and ligand 4. [b] Yield of
the isolated product. [c] Determined by 1H NMR spectroscopic analysis.
[d] Determined by HPLC analysis. [e] Compound 12A was recovered in 29%
yield.
In conclusion, we have succeeded in performing the
enantioselective radical addition–cyclization–trapping reac-
tion that provides a powerful synthetic approach to chiral g-
lactams.
[11] O. Corminboeuf, L. Quaranta, P. Renaud, M. Liu, C. P. Jasperse,
M. P. Sibi, Chem.Eur.J. 2003, 9, 28.
[12] M. Ueda, H. Miyabe, A. Nishimura, O. Miyata, Y. Takemoto, T.
Naito, Org.Lett. 2003, 5, 3835.
[13] For reviews, see: a) H. Yorimitsu, H. Shinokubo, K. Oshima,
Synlett 2002, 674; b) C. Ollivier, P. Renaud, Chem.Rev. 2001,
101, 3415.
Received: May 23, 2006
Published online: July 28, 2006
[14] A 1H NMR study of 1a in the presence of a chiral Lewis acid is
provided in the Supporting Information; a downfield shift in the
chemical shifts of hydrogen atoms around the hydroxamate ester
moiety was observed.
Keywords: asymmetric synthesis · enantioselectivity · lactams ·
.
lewis acids · radical reactions
[15] For a report on a similar inversion in configuration, see: M. P.
Sibi, J. Ji, J.Org.Chem. 1997, 62, 3800.
[1] For general information on enantioselective radical reactions,
see: a) P. Renaud, M. Gerster, Angew.Chem. 1998, 110, 2704;
Angew.Chem.Int.Ed. 1998, 37, 2562; b) M. P. Sibi, N. A. Porter,
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[16] In general, the combination of phenyl-substituted bis(oxazoline)
(box) ligand and zinc Lewis acid gives high selectivity, whereas
the aliphatic-substituted box ligands give high selectivity in
combination with magnesium Lewis acids; see: a) M. P. Sibi, J. Ji,
J.Am.Chem.Soc. 1996, 118, 9200; b) N. A. Porter, J. H. L. Wu,
G. R. Zhang, A. D. Reed, J.Org.Chem. 1997, 62, 6702; c) M. P.
Sibi, J. Zimmerman, T. Rheault, Angew.Chem. 2003, 115, 4659;
Angew.Chem.Int.Ed. 2003, 42, 4521.
[17] The absolute configuration at the newly formed stereocenter of 8
could not be determined. The relative configuration of trans and
cis diastereomers 8Aa was determined by NOESY experiments
(see the Supporting Information).
[2] For selected examples of enantioselective radical addition
reactions and allylations, see: a) M. P. Sibi, G. Petrovic, J.
Zimmerman, J.Am.Chem.Soc.
Friestad, Y. Shen, E. L. Ruggles, Angew.Chem. 2003, 115,
5215; Angew.Chem.Int.Ed. 2003, 42, 5061; c) M. P. Sibi, J.
2005, 127, 2390; b) G. K.
Zimmerman, T. Rheault, Angew.Chem. 2003, 115, 4659; Angew.
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D. P. Curran, S. Kanemasa, Tetrahedron: Asymmetry 1999, 10,
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Chem.Soc. 1997, 119, 11713; g) M. P. Sibi, J. Ji, J.Am.Chem.
Soc. 1996, 118, 9200; h) J. H. Wu, R. Radinov, N. A. Porter, J.
Am.Chem.Soc. 1995, 117, 11029.
[18] For discussions on atom-transfer cyclization, see: a) D. P.
Curran, J. Tamine, J.Org.Chem.
1991, 56, 2746; b) D. P.
Curran, W. Shen, J. Zhang, T. A. Heffner, J.Am.Chem.Soc.
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[19] a) A. L. J. Beckwith, Tetrahedron 1981, 37, 3073; b) D. C. Spell-
meyer, K. N. Houk, J.Org.Chem. 1987, 52, 959.
[20] The chiral substrate of (R)-9 (81% ee) was prepared by our
method using an iridium catalyst and the pyridinebis(oxazoline)-
(pybox) ligand; see: a) H. Miyabe, K. Yoshida, M. Yamauchi, Y.
[3] For selected examples of enantioselective hydrogen-atom trans-
fer reactions, see: a) M. P. Sibi, K. Patil, Angew.Chem. 2004, 116,
1255; Angew.Chem.Int.Ed.
2004, 43, 1235; b) M. P. Sibi, Y.
Takemoto, J.Org.Chem.
2005, 70, 2148; b) H. Miyabe, A.
Asano, J. B. Sausker, Angew.Chem. 2001, 113, 1333; Angew.
Chem.Int.Ed. 2001, 40, 1293; c) M. Murakata, H. Tsutsui, N.
Takeuchi, O. Hoshino, Tetrahedron 1999, 55, 10295; for selected
Matsumura, K. Moriyama, Y. Takemoto, Org.Lett. 2004, 6, 4631.
[21] Details are provided in the Supporting Information.
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 5863 –5866