Enolates, aluminum Claisen rearrangement

The quaternary center was constructed stereospecifically by Claisen rearrangement (Scheme 46). The necessary enol ether was obtained by reaction of the secondary alcohol of 399 with ethyl vinyl ether and mercuric acetate. To change the polarity of the endocyclic double bond, the unsaturated ketone was reduced with lithium aluminum hydride to the allylic alcohol, 400, at low temperature. Then, prolonged heating with xylene led to the aldehyde, 401. Protection of the secondary alcohol was achieved by bromoether formation with W-bromosuccinimide in acetonitrile before the aldehyde of 402 was reacted with methyllithium. The epimeric mixture of secondary alcohols was protected as acetates 403. Then, the cyclic ketone... 

A different type of ring contractions, involving dihydrofuranes 749, occurs under excitation by hght, where a stabihzed radical is formed which recombines to the cyclopropane 150 [237] Reaction scheme 95). Here it is the transition from the 5-membered ring to the 3-membered ring. Now it is a question of how to prepare dihydrofurans. One proposal uses the aluminum-catalyzed addition of an enol upon double bonds, as shown in Reaction scheme 96. The needed precursor is the product 152 of a Claisen-rearrangement based on the chloral-isobutene adduct 126 [238]. 

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