ChemInform Abstract: Asymmetric Tandem Claisen-Ene Strategy for Convergent Synthesis of (+)- 9(11)-Dehydroestrone Methyl Ether: Stereochemical Studies on the Ene Cyclization and Cyclic Enol Ether Claisen Rearrangement for Steroid Total Synthesis.
Abstract
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ChemInform Abstract
In model studies it is shown that ene cyclizations of compounds such as (I) occur with high trans diastereofacial selectivity. This cyclization is markedly enhanced by the introduction of an electron- withdrawing ester group at the acetylenic terminus. Considering the Claisen rearrangements of enol ethers of cyclic ketones such as (V) it is found that the choice of catalyst employed permits the highly stereoselective formation of either the syn or anti product. A combination of these results is used for the carbocyclization of the D ring of the steroid skeleton. The reaction of (VIII) with (V) leads to (IX) which is transformed by Ziegler′s procedure into the title compound (X), a key intermediate for estrogen analogues.
