Ketone reduction trialkylborohydride

Sodium or tetramethylammonium triacetoxyborohydride has become the reagent of choice for diastereoselective reduction of P-hydroxyketones to antidiols. Trialkylborohydrides, eg, alkaH metal tri-j -butylborohydrides, show outstanding stereoselectivity in ketone reductions (39). 

The hydride-donor class of reductants has not yet been successfully paired with enantioselective catalysts. However, a number of chiral reagents that are used in stoichiometric quantity can effect enantioselective reduction of acetophenone and other prochiral ketones. One class of reagents consists of derivatives of LiAlH4 in which some of die hydrides have been replaced by chiral ligands. Section C of Scheme 2.13 shows some examples where chiral diols or amino alcohols have been introduced. Another type of reagent represented in Scheme 2.13 is chiral trialkylborohydrides. Chiral boranes are quite readily available (see Section 4.9 in Part B) and easily converted to borohydrides. 

Stereoselectixe reduction of ketones. Corey s synthesis of prostaglandins utilizes as a key intermediate the cnone (I). One major synthetic problem is the stereoselective reduction of the carbonyl group to the desired 15S alcohol (Ila). Reduction with various borohydridc reagents or various trialkylborohydrides (R, R2R3BH Li ) affords about... 

Uses. Brown has published details of his recent work on potassium hydride. It is particularly useful for preparation of hindered bases such as potassium bis-(trimethylsilyl)amide. It is useful for preparation of potassium enolates from ketones, without reduction or aldol condensation. Metalation of 2-methyl-cyclohexanone with KH gives an equilibrium mixture of potassium enolates. However, the less substituted enolate can be obtained as the product of kinetic control by use of potassium bis(trimethylsilyl)amide. Another important use of KH is for preparation of hindered potassium trialkylborohydrides, which are valuable for reduction of ketones to the less stable alcohols. 

Table 26.21 Reduction of representative ketones with chiral trialkylborohydride [ 1 ]...

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