Rearrangement Lewis acid-promoted

The conversion of anomerically linked enol ethers 29 into either the cis- or trans-substituted pyranyl ketones with high diastereoselectivity and yield involves a Lewis acid-promoted O —> C rearrangement (Scheme 19) <00JCS(P1)2385>. Under similar conditions, homoallylic ethers 30 ring open and the oxonium ions then recyclise to new pyran derivatives 31. Whilst the product is a mixture of alkene isomers, catalytic hydrogenation occurs with excellent diastereoselectivity (Scheme 20) <00JCS(P1)1829>. 

Why do Lewis acids promote Claisen reactions Suggest another method for accelerating aza- and thia-Claisen rearrangements. 

An interesting double ring expansion sequence developed by Vogel involves the initial pinacol coupling of cyclobutanone, isomerization to ketone 79 in acid, followed by reduction and Lewis acid-promoted Wagner-Meerwein rearrangement with dehydration (Scheme 19).129) For preparative purposes, 80 can be more... 

Lewis acid-promoted (SnCU or Et2AlCl) rearrangements of allylic acetals provide substituted tetrahydrofurans [80]. Upon addition of SnCU, 4,5-dimethyl-2-phenyl-4-vinyl-l,3-dioxolane rearranges to the all-cw furan (Eq. 48). No racemization is observed with opti-... 

C6 stereocenters. R.K. Boeckman Jr. and co-workers had to overcome the stereochemical preference of the thermal rearrangement by using a bidentate Lewis acid promoter (TiCU) that coordinated to both the oxygen of the vinyl ether and the ester. This coordination enforced a boatlike conformation for the existing six-membered ring in the transition state. The rearrangement itself took place via a chairlike transition state. 

Cook, G. R., Stiiie, J. R. Stereochemicai consequences of the Lewis acid-promoted 3-aza-Cope rearrangement of N-aikyi-N-aiiyi enamines. Tetrahedron 1994, 50,4105 124. 

All of the examples described thus far have involved the use of bases to initiate the Favorskii rearrangements. However various Lewis acids have also been used for this purpose. An early example of this methodology is shown in Scheme A2P The Lewis acid promoted ring contractions depicted in Scheme 43 and Scheme 44 are also Favorskii-type rearrangements. 

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