Alanes hindered

Reagents of choice for reduction of epoxides to alcohols are hydrides and complex hydrides. A general rule of regioselectivity is that the nucleophilic complex hydrides such as lithium aluminum hydride approach the oxide from the less hindered side [511, 653], thus giving more substituted alcohols. In contrast, hydrides of electrophilic nature such as alanes (prepared in situ from lithium aluminum hydride and aluminum halides) [653, 654, 655] or boranes, especially in the presence of boron trifluoride, open the ring in the opposite direction and give predominantly less substituted alcohols [656, 657,658]. As far as stereoselectivity is concerned, lithium aluminum hydride yields trans products [511] whereas electrophilic hydrides predominantly cis products... 

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]. 

Primary and secondary alkyl bromides, iodides, and sulfonates can be reduced to the corresponding alkanes with LiBHEt3 (superhydride) or with lithium aluminum hydride (LiAlH4, other names lithium tetrahydridoaluminate or lithium alanate). If such a reaction occurs at a stereocenter, the reaction proceeds with substantial or often even complete stereoselectivity via backside attack by the hydride transfer reagent. The reduction of alkyl chlorides to alkanes is much easier with superhydride than with LiAlH4. The same is true for sterically hindered halides and sulfonates ... 

Hydroalummation. Titaniuin(IV) chloride is a very effective catalyst for addition of LiAlH, to olefinic double bonds. Relative rates for various olefins are RCH=CH2 > RzC =CH2 > RCH=CHR. Alane is somewhat less reactive. Selective addition to the least hindered C=C bond of noncoiyugated dienes is possible. Ether is not an effective solvent THF, ethylene glycol, aud diglyme are satisfactory solvents. 

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