| Summary: | 碩士 === 國立中興大學 === 化學系 === 87 === The synthesis of two novel chiral ligands, Schiff base 5 and amide 11, from camphor and their application to the asymmetric formation of optically active cyanohydrins from aldehydes are described in this study.
Camphorquinone, prepared from camphor and SeO2 in quantitative yield, was converted to C-3 oxime 3 in 65% isolated yield (recrystallized from CH2Cl2). Reduction with LAH, oxime 3 was transformed into 3-exo-aminoborneol (4) in 91% yield. Reaction of compound 4 with salicylaldehyde and salicylic acid afforded Schiff base 5 (96%) and amide 11 (86%) respectively. Ligands 5 and 11 were treated with Ti(OPri)4 at 0 °C to form the catalytic complexes. Subsequent addition of aldehyde and TMSCN at -60 °C followed by reaction at -40 °C furnished the corresponding cyanohydrin in good yields (60-99%) and e.e. (62-97%). Noteworthily, S-cyanohydrin was obtained as the major product using Schiff base 5 while R-cyanohydrin was the predominant product employing amide 11.
In conclusion, two highly efficient chiral catalysts for the formation of cyanohydrins in good yields and enantioselectivities from the corresponding aldehydes have been developed. Both ligands were synthesized from camphor in four steps in good overall yields and could be readily purified by recrystallization. More importantly, the two ligands provide cyanohydrins in opposite configuration and therefore are complementary to each other in stereoselectivity. Consequently, a new, enantioselective and practical method for the synthesis of both stereoisomers of cyanohydrin has been established from a common intermediate (3-exo-aminoborneol).
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