Stereoselectivity in the Claisen rearrangement

The Claisen rearrangement was used in the asymmetric total synthesis of (+)-9(ll)-dehydroestrone methyl ether (5), a versatile intermediate in the synthesis of estrogens5 (Scheme 1. If). The key feature of the synthesis is the successful development of the asymmetric tandem Claisen-ene sequence. Thus, a solution of the cyclic enol ether 6 in toluene was heated in a sealed tube at 180 C for 60 hours to afford the product 9 in 76% isolated yield after deprotection of the silyl enol ether. The Claisen rearrangement of the allyl vinyl ether 6 occurred stereoselectively to give an intermediate (7), in which the 8,14-configuration was 90% syn. The stereoselectivity in the Claisen rearrangement can be explained... 

Scheme 30. Transition state geometry and stereoselectivity in the Claisen rearrangement of a crotyl sugar enol ether [61]...

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