Abstract
Protocols have been devised whereby all four possible stereoisomeric α-alkyl-β-hydroxy carboxylic acids can be derived from a single aldol reagent, hydroxy ketone 3. 3 is used for aldol reactions in the form of its trimethylsilyl and tert-butyldimethylsilyl derivatives, 4 and 5. The Z lithium and Z boron enolates of 4 react with various aldehydes to give aldols 8 and 9, respectively. Deprotonation of 4 by bromomagnesium 2,2,6,6-tetramethylpiperidide (MTMP) gives the E enolate, which may be trapped by trimethylsilyl chloride to obtain the E silyl enol ether 11. The E bromomagnesium enolate of 4 reacts with aldehydes to give aldols of structure 15. Transmetalation of the bromomagnesium enolate of keto ether 5 is accomplished by reaction with (triisopropoxy)titanium chloride. The resulting E (triisopropoxy)titanium enolate reacts with aldehydes to provide aldols of structure 17. The aldols resulting from the foregoing reactions are hydrolyzed to keto diols 19-22, which are oxidized to the stereoisomeric α-methyl-β-hydroxy carboxylic acids 23-26
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