B. Watanabe et al. / Tetrahedron Letters 45 (2004) 2767–2769
2769
H
R
18
H
R
β face
α face
R
H
H
O
R
AlL3
O
L3Al
O
H
L3Al
H
O
H
H
AlL3
TSA
(20S,22R)
TSB
TSC
(20R,22S)
TSD
(20R,22R)
20-‘unnatural’
(20S,22S)
20-‘natural’
Figure 1. Possible transition states for the carbonyl-ene reaction.
2. Khripach, V. A.; Zhabinskii, V. N.; de Groot, A. E.
Brassinosteroids: A New Class of Plant Hormones; Aca-
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1050.
was required for the reaction to proceed. In this regard,
Jackson and co-workers have reported that EtAlCl2
could mediate the intermolecular carbonyl-ene reactions
of olefins and ketones, an enophile with considerably low
reactivity, to give ene adducts in low to moderate yield.19
4. Wovkulich, P. M.; Batcho, A. D.; Uskokovic, M. R. Helv.
Chim. Acta 1984, 67, 612–615.
To verify the assumed reaction mechanism, we per-
formed a carbonyl-ene reaction between 2 and (E)-3b-
acetoxypregna-5,17-diene 11.20 The reaction proceeded
smoothly under the same conditions as those for (Z)-
isomer 8, and ene adduct 12 was obtained in 52% yield,
which was converted to triol 13. As expected, analytical
data of 13 was identical with those reported for 6-de-
oxoteasterone.21
5. Mikami, K.; Kishino, H.; Loh, T.-P. J. Chem. Soc., Chem.
Commun. 1994, 495–496.
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In summary, we have developed the shortest method
reported to date to construct the BL side chain using a
carbonyl-ene reaction as a key step. We have also syn-
thesized 6-deoxoteasterone and its 20-epimer. Further
investigation of the scope and limitations of this type
carbonyl-ene reaction are now in progress.
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€
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1989, 54, 1295–1304.
Acknowledgements
We are thankful to Dr. Craig Wheelock of the University
of California Davis for carefully reviewing manuscript.
This investigation was supported, in part, by a Grant-in-
Aid for Scientific Research from the Ministry of Educa-
tion, Culture, Sports, Science and Technology (formally
Ministry of Education, Science, and Culture) of Japan
(09660117, 10161207), and by the Fujisawa Foundation.
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M.; Honda, T. J. Org. Chem. 1988, 53, 1982–1991.
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References and notes
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