Enol Alanes

The reason why the carbonyl group in -santonin remained intact may be that, after the reduction of the less hindered double bond, the ketone was enolized by lithium amide and was thus protected from further reduction. Indeed, treatment of ethyl l-methyl-2-cyclopentanone-l-carboxylate with lithium diisopropylamide in tetrahydrofuran at — 78° enolized the ketone and prevented its reduction with lithium aluminum hydride and with diisobutyl-alane (DIBAL ). Reduction by these two reagents in tetrahydrofuran at — 78° to —40° or —78° to —20°, respectively, afforded keto alcohols from several keto esters in 46-95% yields. Ketones whose enols are unstable failed to give keto alcohols [1092]. 

Nucleophiles relevant for this chapter are hydride ion, O-atom- and N-atom-centered nucleophiles and C-nucleo-philes like organometallic compounds, ketene acetals, and ester enolates as well as electron-rich heterocycles. Reductions of 1,2,3-triazines and 2-methyl-l,2,3-triazin-2-ium salts by sodium borohydride in methanol and lithium alanate in ether to yield 2,5-dihydro-l,2,3-triazines have been reviewed in CHEC(1984) and CHEG-II(1996). The reaction is explained by nucleophilic attack of the hydride ion at C-5. Borohydride reductions of 1,2,3-triazine... 

---