Synthesis of chiral amines using terpenyl spiroborate esters as catalysts

Pure chiral amines play a key role in many branches of human life, because they are widely applied as starting materials for the total syntheses of natural products or drugs. Moreover, a large number of them exhibit a wide range of bioactivities by acting on the central nervous system, decreasing a blood pressure, and smooth muscle relaxation. On the other hand, optically active amines and their derivatives have been utilized successfully in a widespread modern asymmetric syntheses as chiral catalysts and building blocks.

Several strategies for the synthesis of chiral amines have been reported in the literature. However, there is still a wide requirement to develop better approaches. Herein, we present an alternative method based on the enantioselective reduction of cyclic and acyclic imines with borane using chiral terpenyl spiroborate esters as catalysts. Our earlier studies showed that spiroborate esters can be successfully utilized as catalysts in the enantioselective reduction of ketones, and their derivatives, with borane giving the corresponding alcohols with very good yields and enantiomeric excesses. On the other hand, this class of catalysts are much more stable towards moisture and air than other boron catalysts because of the unique structure containing a characteristic O₃BN framework, where B-N bond is a coordinate bond. These facts make spiroborate esters an interesting and hopeful variant for obtaining chiral compounds.