Ketones hydride reducing agents

For most laboratory scale reductions of aldehydes and ketones catalytic hydro genation has been replaced by methods based on metal hydride reducing agents The two most common reagents are sodium borohydride and lithium aluminum hydride... 

Sml2 reduces aldehydes and ketones to the corresponding alcohols. As there are numerous hydride reducing agents for the reduction of aldehydes and ketones, this functional group transformation has not been used widely. In some cases, however, Sml2 can display useful reactivity, stereoselectivity and chemoselec-tivity that more conventional reagents cannot achieve. 

Keck recently applied the diastereoselective reduction of ( S-hydroxy ketones in an approach to the anti-tumour natural products epothilone A and B. Sml2-mediated reduction of p-hydroxy ketone 15 in THF-MeOH gave anti- 1,3-diol 16 in 94% yield and as a 96 4 mixture of diastereoisomers.18 Attempts to carry out the reduction using common hydride reducing agents gave 16 with poor levels of diastereoselectivity (Scheme 4.10).18... 

Data from NOE experiments nicely supplement information from coupling constants in the determination of three-dimensional stereochemistry loo. Reduction of this bicyclic ketone with a bulky hydride reducing agent gives one diastereoisomer of the alcohol, but which Irradiation of the proton next to the OH group leads to an NOE to tlie green proton. 

The total synthesis of vindoline (205) from the pentacyclic ketone (206), whose formation was discussed in last year s Report, has now been completed (Scheme 27), essentially according to the route adopted earlier in the synthesis of vin-dorosine. The reduction of the C-17 ketone function in the penultimate stage with normal hydride reducing agents was not stereospecific. However, this was circumvented by addition of aluminium chloride, which presumably formed a complex involving the C-16 hydroxy-group and N reduction by a bulky complex hydride then ensured preferential a-attack with formation of deacetylvindoline. ... 

Diisobutylaluminium hydride (DIBAL-H) is a bulky hydride reducing agent that is very useful for the stereoselective reduction of prochiral ketones and reductions at... 

Lithium tri-fert-Butoxyaluminohydride is a bulky chemo- and stereoselective hydride reducing agent. Aldehydes are reduced chemoselectively in the presence of ketones and esters at low temperature. Ethers acetals, epoxides, chlorides, bromides, and nitro compounds are unaffected by this reagent. Reviews (a) Seyden-Penne, J. Reductions by the Alumino- and Borohydrides in Organic Synthesis Wiley-VCH NewYork, 1997, 2" edition, (b) Malek, J. Org. React. 1985, 34, 1-317. 

L-Selectride is a bulky stereoselective hydride reducing agent for prochiral ketones. Review Seyden-Penne, J. Reductions by the Alumino- and Borohydrides in Organic Synthesis, Wiley-VCH New York, 1997, 2" edition. 

The positive mesomeric effect (+M) of the OMe group ensures that the carbonyl carbon atom of the ester is less electrophilic than that of the ketone. Therefore, the ketone is more readily attacked by nucleophiles, and mild hydride reducing agents such as NaBH4 only react at the ketone. 

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