Acetals thiol ester silyl ketene

The enolates of other carbonyl compounds can be used in mixed aldol reactions. Extensive use has been made of the enolates of esters, thiol esters, amides, and imides, including several that serve as chiral auxiliaries. The methods for formation of these enolates are similar to those for ketones. Lithium, boron, titanium, and tin derivatives have all been widely used. The silyl ethers of ester enolates, which are called silyl ketene acetals, show reactivity that is analogous to silyl enol ethers and are covalent equivalents of ester enolates. The silyl thioketene acetal derivatives of thiol esters are also useful. The reactions of these enolate equivalents are discussed in Section 2.1.4. 

Unsaturated acyl derivatives of oxazolidinones can be used as acceptors, and these reactions are enantioselective in the presence of chiral to-oxazoline catalysts.321 Silyl ketene acetals of thiol esters are good reactants and the stereochemistry depends on the ketene acetal configuration. The Z-isomer gives higher diastereoselectivity than the Zf-isomer. 

Alkylidenemalonate esters are also good acceptors in reactions with silyl ketene acetals of thiol esters under very similar conditions.322... 

Alcohols can also be prepared from support-bound carbon nucleophiles and carbonyl compounds (Table 7.4). Few examples have been reported of the a-alkylation of resin-bound esters with aldehydes or ketones. This reaction is complicated by the thermal instability of some ester enolates, which can undergo elimination of alkoxide to yield ketenes. Traces of water or alcohols can, furthermore, lead to saponification or transesterification and release of the substrate into solution. Less prone to base-induced cleavage are support-bound imides (Entry 2, Table 7.4 see also Entry 3, Table 13.8 [42]). Alternatively, support-bound thiol esters can be converted into stable silyl ketene acetals, which react with aldehydes under Lewis-acid catalysis (Entries 3 and 4, Table 7.4). 

CH CI. solvent. OSiMe2Bu ether. Not specified.Note that silyl ketene acetals derived from thiol esters have opposite (Z)/( ) descriptors compared to esters due to change of sequence rule priority associated with the sulfur atom. STMS ether. Catalytic amount. 

Support-bound C-nucleophiles have also been successfully added to imines. Poly-styrene-bound thiol esters can be converted into ketene acetals by O-silylation, and then alkylated with imines in the presence of Lewis acids. Further examples include Mannich reactions of support-bound alkynes and indoles (Table 10.10). Some Man-nich-type products (e.g. 3-(aminomethyl)indoles, 2-(aminomethyl)phenols, (3-amino ketones) are unstable and can decompose upon treatment with acids. 3-(Amino-... 

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