ASYMMETRIC REDUCTIONS VIA TRIALKYLBORANES

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Chirality plays a central role in organic and bioorganic chemistry. Hence the synthesis of optically-active compounds is an important endeavor. We have found certain B-alkyl-9-borabiocyclo [3.3.1]nonanes(9-BBN) reduce aldehydes and alkynyl ketones under remarkably mild conditions. The reagent derived from Alpha-pinene and 9-BBN induces a high degree of optical activity into the alcohol product. For example benzaldehyde-Alpha-d and 4-methyl-1-pentyne-3-one are reduced to the corresponding alcohols with essentially complete asymmetric induction. Mechanistic investigations have shown that these reactions proceed via a six-centered cyclic process rather than a dehydroboration-reduction process. For other ketones, such as acetophenone, the dehydroboration-reduction process becomes important. By running the reaction at high pressure, the dehydroboration may be completely suppressed and a variety of ketones may be effectively reduced. It has been found that chiral ketones may be reduced in ananti-Cram sense with dialkylboranes. The scope of these reductions will be explored. A borohydride reagent which provides asymmetric inductions of up to 80% e.e. has been developed. New analogs of this reagent will be tested and mechanistic studies initiated. The synthesis of optically-active amines, amino alcohols and amino acids via the asymmetric reduction of imines or the cyclocondensation of dienes with imines and substituted ketones or aldehydes will be explored. The stereochemistry of [2,3]Wittig rearrangements has been elucidated. Mechanistic studies on this reaction will be initiated to explain differences in stereoselectivities observed from E and Z starting materials. The scope of this rearrangement will be explored. The synthesis of optically-active tertiary alcohols by cyclocondensations of alkoyx aldehydes and by asymmetric additions to propargyl ketones will be investigates.