Acyloxyborohydrides, reduction with

Acyloxyborohydrides [NaBH4 (02CR)n] are remarkably selective,as illustrated by the reduction of the aldehyde unit in 126, in the presence of a ketone moiety.Reduction with potassium triacetoxyborohydride (generated by dissolving potassium borohydride in acetic acid) gave 127 in 60% yield. It can also reduce... 

Unusual reducing properties can be obtained with borohydride derivatives formed in situ. A variety of reductions have been reported, including hydrogenolysis of carbonyls and alkylation of amines with sodium borohydride in carboxyHc acids such as acetic and trifluoroacetic (38), in which the acyloxyborohydride is the reducing agent. 

A new chiral acyloxyborohydride has been prepared by combining sodium borohy-dride with a tartaric acid-based reagent. This reagent effects the reduction of aromatic ketones to provide the product alcohols in ees of 93-98%. Several aliphatic ketones were also reduced with moderate to excellent enantioselectivity. A mechanism has been provided with supporting calculations for the proposed active species and tran- sition state.262... 

Table 1 demonstrates that NaBH4 may be used in the presence of heterocyclic rings (entries 8-10, 13-17), esters (entries 8-10, 17), amides (entries 1, 9), conjugated double bonds (entry 7), alkynes (entry 5) and acetals (entry 4). The process can also be used to methylate an amine with formaldehyde as the carbonyl (entry 13). With certain structures, further reactions may occur subsequent to reduction (e.g. entry 6). Entry 15 illustrates a synthetically useful amine alkylation reaction which may occur in acetic acid. This remarkable reaction is attributed to self-reduction of an acyloxyborohydride to an aldehyde... 

Reduction of ketones with sodium borohydride in the presence of a carboxylic acid and 1,2 5,6-di-0-cyclohexylidene-a -D-glucofuranose gave 35-50% enantiomeric enhancement values.Another group has reported a similar reaction with the corresponding di-O-isopropylidene-glucose derivative and prochiral aromatic ketones. Optical yields of up to 64% were claimed. The chiral reagents appear to be sodium acyloxyborohydrides, which complex with the carbohydrate before reduction takes place. 

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