Sodium triacyloxyborohydrides

Asymmetric reduction of imines. Japanese chemists2 have prepared a number of chiral sodium triacyloxyborohydrides from N-acyl derivatives of natural a-amino acids. The most effective are derived from (S)-proline. A particularly useful reducing agent is 1, derived from N-benzyloxycarbonyl-(S)-proline (equation I). 

The (S)-prolinate-borane complex (5)-(22) reduces ketones to the corresponding alcohols with optical yields up to 50%. The asymmetric reduction of cyclic imines (24) with chiral sodium triacyloxyborohydride (S)-(23) was utilized to prepare optically active alkaloids (25) with optical yields up to 86% (eq 9). ... 

The reduction of imines such as 3,4-dihydroisoquinolines with chiral sodium triacyloxyborohydrides [NaB(0acyl)3H] has been effected in optical yields of up to 1 % Two new methods for the preparation of 1-substituted tetrahydroisoquinolines have been reported. These both involve directed 1-lithiation and subsequent reaction with electrophiles. The directing groups on nitrogen are CH=N-R and P(0)(NMe2)2 (see above) respectively, both of which are readily removable. 

Secondary Amines.—The reduction of imines to the corresponding secondary amines can be effected by various methodologies. New additions are the sodium triacyloxyborohydrides (easily obtainable from sodium borohydride and AT-acyl derivatives of optically active amino-acids), which are used for the asymmetric reduction of cyclic imines. Also now available is a highly stereoselective reduction of N-benzylimines derived from substituted cyclohexanones, with alkali-metal borohydrides, in particular L-selectride. A fiuther addition is the first report of the reduction of aldimines by hydrogen transfer from propan-2-ol,... 

Asymmetric reduction of prostereogenic cyclic imines with chiral sodium acyloxyborohy-drides, which arc easily prepared by the reaction of sodium borohydride with various A-acyl-a-amino acids, has been investigated. Triacyloxyborohydrides, derived from sodium borohydride (1 equiv) and (S)-TV-acylproline (3 equiv), were found to reduce 3,4-dihydropapaverine (2) to (5 )-norlaudanosine (3)30. 

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