Discovery of Silicon Enolate-mediated Crossed Aldol Reactions

Discovery of Silicon Enolate-mediated Crossed Aldol Reactions 

The driving force of the above-mentioned aldol reaction with boron eno-late is considered to be the interconversion of enol ketones (boron enolates) to their more stable ketones (j5-boryloxy ketones) [17]. When the boron 

It is reported that enol ethers react with acetals or ketals, promoted by Lewis acids, to give aldol-type adducts these reactions of alkyl enol ethers are, however, often accompanied by undesired side reactions [21]. Further, is difficult to perform crossed-aldol reactions selectively because conventional aldol reactions are conducted under equilibrium conditions using a basic or acidic catalyst in protic solvents [22]. Detailed studies of this new aldol reaction of silicon enolates, however, reveal a number of advantages over conventional methods. 

Second, functional group selectivity is observed - i.e. reactions with aldehydes proceed at —78 °C whereas those with ketones proceed at elevated temperatures (ca. 0 °C). Chemoselectivity is observed with acceptors having two different kinds of carbonyl function, for example aldehyde and ketone or ester, in the same molecule. Treatment of phenylglyoxal with silyl enol ether 38 at —78 °C affords a-hydroxy-y-diketone 39 (Eq. (20)) [20b]. The reaction of ketoesters 40 other than j5-ketoesters with silyl enol ether 38 gives hydroxyketoesters 41 as sole products (Eq. (21)) [23]. 

3 Crossed Aldol Reactions Using Silicon Enolates I 139 

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