6 For a theoretical study on the BF3ꢀOEt2-promoted HDA reaction
between 2-aza-3-silyloxy-1,3-butadiene and formaldehyde, see:
A. Bongini and M. Panunzio, Eur. J. Org. Chem., 2006, 972–977.
7 Panunzio and co-workers reported that the HDA reaction between
2-aza-3-siloxydienes and aromatic aldehydes in the presence of
5 mol% of Eu(fod)3 or Yt(fod)3 (fod = 6,6,7,7,8,8,8-heptafluoro-
2,2-dimethyl-3,5-octanedionate) under microwave heating gave
1,3-oxazinan-4-ones in good yields and with modest diastereo-
selectivities. See ref. 4b.
8 (a) M. P. Doyle, I. M. Phillips and W. Hu, J. Am. Chem. Soc.,
2001, 123, 5366–5367; (b) M. P. Doyle, M. Valenzuela and
P. Huang, Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 5391–5395;
(c) M. Valenzuela, M. P. Doyle, C. Hedberg, W. Hu and
A. Holmstrom, Synlett, 2004, 2425–2428; (d) X. Wang, Z. Li and
M. P. Doyle, Chem. Commun., 2009, 5612–5614.
Fig. 1 Plausible stereochemical course.
9 For the [2+2]-cycloaddition reaction of (trimethylsilyl)ketene with
ethyl glyoxylate, see: R. E. Forslund, J. Cain, J. Colyer and
M. P. Doyle, Adv. Synth. Catal., 2005, 347, 87–92.
(up to 98% ee). Using this catalytic protocol, we developed a
novel approach to syn-b-hydroxy-a-methyl amide terminus of
cruentaren A. Further application of this method to catalytic
asymmetric synthesis of biologically active natural products is
currently in progress.
10 For the effective use of chiral cationic dirhodium(II,III) carbox-
amidate complexes in asymmetric Lewis acid catalysis, see:
(a) Y. Wang, J. Wolf, P. Zavalij and M. P. Doyle, Angew. Chem.,
Int. Ed., 2008, 47, 1439–1442; (b) X. Wang, C. Weigl and
M. P. Doyle, J. Am. Chem. Soc., 2011, 133, 9572–9579.
11 (a) M. Anada, T. Washio, N. Shimada, S. Kitagaki, M. Nakajima,
M. Shiro and S. Hashimoto, Angew. Chem., Int. Ed., 2004, 43,
2665–2668; (b) T. Washio, R. Yamaguchi, T. Abe, H. Nambu,
M. Anada and S. Hashimoto, Tetrahedron, 2007, 63, 12037–12046;
(c) T. Washio, H. Nambu, M. Anada and S. Hashimoto, Tetra-
hedron: Asymmetry, 2007, 18, 2606–2612; (d) Y. Watanabe,
T. Washio, N. Shimada, M. Anada and S. Hashimoto, Chem.
Commun., 2009, 7294–7296; (e) M. Anada, T. Washio,
Y. Watanabe, K. Takeda and S. Hashimoto, Eur. J. Org. Chem.,
2010, 6850–6854.
This research was supported, in part, by a Grant-in Aid for
Scientific Research on Innovative Areas ‘‘Organic Synthesis
Based on Reaction Integration’’ (No. 2105) from the Ministry
of Education, Culture, Sports, Science and Technology,
Japan. We thank Ms. S. Oka, and M. Kiuchi of the Center
for Instrumental Analysis at Hokkaido University for technical
assistance in the MS and elemental analyses.
12 C. Xu and C. Yuan, Tetrahedron, 2005, 61, 2169–2186.
13 The product yields and enantioselectivities obtained with other
solvents are as follows: acetone, 12 h, 89% yield, 89% ee; toluene,
24 h, 86% yield, 45% ee; diethyl ether, 48 h, 87% yield, 63% ee.
14 K. Takeda, T. Oohara, M. Anada, H. Nambu and S. Hashimoto,
Angew. Chem., Int. Ed., 2010, 49, 6979–6983, and references cited
therein.
15 M. Anada, M. Tanaka, T. Washio, M. Yamawaki, T. Abe and
S. Hashimoto, Org. Lett., 2007, 9, 4559–4562, and references cited
therein.
Notes and references
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2 For reviews on [4+2] cycloaddition using 2-azadienes, see:
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in Organic Chemistry, Academic Press, San Diego, 1987,
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´
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17 The reaction between 2a and 3a at room temperature in the
presence of chiral bis(oxazoline)copper(II) complex, which has
previously been shown to be an effective catalyst for the asym-
metric Diels–Alder reaction between 2-aza-3-silyloxy-1,3-buta-
dienes and 3-(2-alkenoyl)-1,3-oxazolidin-2-ones, led to a complex
mixture of products arising from degradation of 2a along with a
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Yb(OTf)3 or Sc(OTf)3 also met with little success.
18 The reaction of 2a with 3a at room temperature using 1 mol% of
Rh2(pfb)4 (pfb = perfluorobutyrate) went to completion in 48 h
and gave 5a as the sole product in 64% yield, while Rh(II)
complexes such as Rh2(OAc)4, Rh2(tpa)4,19 Rh2(piv)4, Rh2(oct)4
and Rh2(cap)4 (tpa = triphenylacetate, piv = pivalate, oct =
octanoate, cap = caprolactamate) were less effective.
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6969–6971 6971