Claisen rearrangement, Lewis acid

This section deals with Bronsted acid and Lewis acid catalyzed reactions, excluding Friedel-Crafts reactions, but including reactions such as nitrations, halogenations, and Claisen rearrangements. Friedel-Crafts reactions are discussed in the subsequent Sections 5.1.2.2 and 5.1.2.3. 

Lewis acid catalysis of Ireland-Claisen rearrangements by TiCl4 has been observed.251 This methodology was employed in the synthesis of a novel type of anti-inflammatory drug candidate.252... 

Catalysis of Claisen rearrangements has been achieved using highly hindered bis(phenoxy)methylaluminum as a Lewis acid.156 Reagents of this type also have the ability to control the E Z ratio of the products. Very bulky catalysts tend to favor the Z-isomer by forcing the a substituent of the allyl group into an axial conformation. 

Isolation of intermediates by treatment of 2 with 3 equiv. of I at room temperature lot 12 hours. Presumably (CH3)2A1C1 is liberated from 1 during methylenation and this Lewis acid catalyzes the Claisen rearrangement. Five other examples of this transformation are cited. 

Aluminium and copper Lewis acids effect a regioselective [l,3]-rearrangement of allyl vinyl ethers in moderate to good yields (Scheme 30). The use of trisubstituted alkenes leads to depressed levels of Claisen products.53... 

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