| Research Abstract |
In this project, we studied on asymmetric synthesis based on intramolecular haloetherification of ene acetals and its application in two ways : 1) asymmetric desymmetrization of σ-symmetric dienes-synthesis of multi-chiral center, and 2) asymmetric desymmetrization of symmetric diols.
1) Asymmetric desymmetrization of σ-symmetric dienes-Synthesis of multi-chiral centers : An intramolecular haloetherification reaction, treatment with NBS in the presence of MeOH, of ene acetal, prepared from 2,5-cyclohexadiene-1-methylaldehyde and chiral hydrobenzoin, proceeded stereoselectively to give 8-membered acetal discriminated two olefins. Subsequent hydroboration of the remained olefin followed by oxidation afforded the cyclohexenone derivative. Stereoselective 1, 4-addition of Grignard reagent, silylation of enolated anion, excess MeLi treatment, then diallylation with excess allyl iodide afforded the compound with the same carbon skeleton of (-)-stenine, a major component of stemona alkaloids.
2) Asymmetric desymmetrization of symmetric diols : An asymmetric desymmetrization of unsaturated cyclic and acyclic meso-1, 2-diols has been developed from the ene acetals, prepared from the norbornene carboxyaldehyde and unsaturated meso-1, 2-diols. The intramolecular haloetherification of the ene acetals as a key step afforded 8-memberd acetals in a stereoselective manner just by the reaction of norbornene olefin. Subsequent reductive elimination, followed by protection of the hydroxy group and transacetalization, gave optically active 1, 2-diol derivatives and the starting ene acetals in good yields. Haloetherification reaction of diastereomeric mixture of the ene acetals, derived from racemic norbornene aldehydes and chiral non-racemic (R, R)-hydrobenzoin, proceeded in a kinetically controlled manner to give the optically pure 8-membered acetal along with the intact ene acetal. Both acetals were converted to the optically pure norbrnene aldehydes in both enantiomeric forms.
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