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Sequential claisen-ene approach to carbocyclization: A new entry to a steroid c/d ring synthon

✍ Scribed by Kazuhiko Takahashi; Kōichi Mikami; Takeshi Nakai

Publisher
Elsevier Science
Year
1988
Tongue
French
Weight
218 KB
Volume
29
Category
Article
ISSN
0040-4039

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✦ Synopsis

A new and facile entry to a steroid C/D ring synthon is described which relies on the sequential combination of the Claisen rearrangement with the intramolecular ene reaction involving an acetylenic bond with methoxycarbonyl group as the enophile. The intramolecular ene reaction has currently been emerging as one of efficient tools for carbocyclization.' Recently we have reported that the thermal ene cyclization of the crotyl propargyl ether 1 can create a quarternary center in a highly diastereoselective fashion (es lL2 This observation prompted us to apply this ene cyclization methodology to the synthesis of a steroid C/D ring synthon.3 Scheme 1 illustrates our basic strategy which we now describe in this communication. Its key feature is that the requisite ene substrate 4 can be easily derived via the Claisen rearrangement. Thus, an intriguing problem in this strategy is whether the Claisen-ene sequence4 can be achieved in tandem or not.

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