Ketene acetals rearrangement

The silyl ketene acetal rearrangement can also be carried out by reaction of the ester with a silyl triflate and tertiary amine, without formation of the ester enolate. Optimum results are obtained with bulky silyl triflates and amines, e.g., f-butyldimethylsilyl triflate and (V-methyl-Af, /V-dicyclohcxylaminc. Under these conditions the reaction is stereoselective for the Z-silyl ketene acetal and the stereochemistry of the allylic double bond determines the syn or anti configuration of the product.243... 

Entry 6 is analogous to a silyl ketene acetal rearrangement. The reactant in this case is an imide. Entry 7 is an example of PdCl2-catalyzed imidate rearrangement. Entry 8 is an example of an azonia-Cope rearrangement, with the monocylic intermediate then undergoing an intramolecular Mannich condensation. (See Section 2.2.1 for a discussion of the Mannich reaction). Entry 9 shows a thioimidate rearrangement. 

Ireland-Claisen (silyl ketene acetal) rearrangement... 

Allylic esters of fluoroacetic acid were used in the Ireland silyl ketene acetal rearrangement procedures by the Welch group at Albany [164]. For example, Eq. (53) shows a highly diastereoselective rearrangement which formed an early stage in syntheses of 2,3-dideoxy-2-fluoro-3-C-methyl pentose nucleosides [165, 166]. If a stereoselective synthesis of a functionalised monofluorocompound is... 

For the carvyl propanoate 4. both the /,)- and the (L )-silyl ketene acetal rearrange through a boat/chair transition state 6. The chair/chair transition state 5, however, is destabilized due to strong steric interactions of the isopropyl group and the silyloxy group 515 539. The two other possible transition states (boat/boat, chair/boat, not depicted) are disfavored because of interactions of the trialkylsilyl group with the ring. 

Si-transfer from oxygen to carbonIn the presence of a trialkylaluminum, particularly (CH3),AI, silyl ketene acetals rearrange at —78° to 20° to esters of trialkyl-silylacetic acid. This 1,3-rearrangemcnt of Si from oxygen to carbon is the reverse of the well-known Brook thermal rearrangement2 of Si from carbon to oxygen. But R AI does not rearrange trimethylsilyl enol ethers. 

Sol 1. (d) Abstraction of proton from a-methyl group of ester and subsequent treatment with trimethylsilyl chloride (TMSCl) resulted in the formation of cyclohexenyl silyl ketene acetal, which on heating undergoes [3,3] shift, i.e., Ireland—Claisen (silyl ketene acetal) rearrangement. The final step is the removal of the silyl group by acid hydrolysis to get the free acid. 

In contrast to the acyclic aUyl silyl ketene acetals, cyclic substrates may preferentially rearrange through either the chair or boat transitions states. In 1981 Bartlett and Pizzo reported that treatment of cyclohexenyl propionates under either set of conditions reported by Ireland resulted in the formation of the same major isomer (Scheme 4.17) [20]. They concluded that the -silyl ketene acetal rearranged preferentially via a chair-hke transition state, while the Z-silyl ketene acetal rearranged via a boat-like transition state. These conclusions were recently supported computationally by Houk et al., who reported a 1.0 kcal/mol preference for the boat transition state for the Z-geometry in the analogous OMe ketene acetal and a 1.4 kcal/mol preference for the chair transition state for the -geometry in the OMe ketene acetal [18]. 

Scheme 33. Uronate ketene acetal rearrangement in the total synthesis of monensin [64-66]...

Scheme 35. Low yields in the ketene acetal rearrangement of )8-benzyloxy uronates [69,70]...

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