| Research Abstract |
We investigated on two themes using dynamic stereocontrol : (i) the utilization of chromium(II) as a reductant, and (ii) stereo-controlled synthesis of fluorinated amino acids, and found several new reactions as follows : (1) The mild reductant chromium(II) chloride can discriminate between alkyl iodides, alkyl radicals, and conjugated carbon radicals. In the presence of chromium(II) the alkyl radical is so long-lived that it can undergo intermolecular addition to radical acceptors such as 1,3-dienes and α,β-unsaturated esters. (2) In an aprotic solvent, dimethylformamide, a coupling reaction between an α,β-unsaturated ketone and an aldehyde is achieved with chromium(II) chloride. An intermolecular aldol reaction and intramolecular cyclization proceed sequentially to give cis-2-hydroxyalkyl-substituted cyclopropanol stereoselectively. (3) In the presence of Et_3SiCl, a chromium enolate is trapped smoothly to give a silyl enol ether. This trapping changes the course of the reaction betw
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een an α,β-unsaturated ketone and an aldehyde from a sequential aldol-cyclopropanol formation to a cross pinacol-type coupling reaction. When aliphatic α,β-unsaturated ketones are employed, the anti / syn ratios of the 1,2-diols depend on the reaction temperature. (4) Stereo-controlled synthetic methods for fluorinated amino acids from cheap and easily available compounds. Under a hydrogen pressure, a catalytic amount of palladium(II) trifluoroacetate and BINAP promoted asymmetric hydrogenation of α-fluorinated iminoesters to afford the enantioenriched β-fluorinated α-amino esters. (5) Stereoselective syntheses of optically active trifluoro-norcoronamic acid and its diastereomer are studied. Stereospecific and diastereoselective cyclization of γ-cyanohydrins gave cyclopropyl nitriles. Further transformations of CN and Ar groups gave trifluoro-norcoronamic acid and trifluoro-allo-norcoronamic acid. (6) Chiral flurinated epoxide by chiral Shiff base of glycine nitrile gave both diastereomer of γ-hydroxynorvaline derivatives diastereoselectively depending on the base. This Shiff base is also applicable for preparation of optically pure bromodifluoroalanine derivative. Less
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