SEQUENTIAL GENERATION OF CARBON RADICALS AND CARBANIONS : DYNAMIC STEREOCONTROL IN THE REACTION SEQUENCE

我们利用动态立体控制研究了两个主题：(i)铬（II）作为还原剂的利用，以及（II）立体控制氟化氨基酸的合成，并发现了以下几个新反应：(1)温和还原剂氯化铬（II）可以区分烷基碘化物，烷基自由基和共轭碳自由基。在铬（II）存在下，烷基自由基的寿命很长，可以在分子间加成到自由基受体上，如1,3-二烯和α，β-不饱和酯。(2)在非质子溶剂二甲基甲酰胺中，α，β-不饱和酮与醛与氯化铬（II）发生偶联反应。分子间醛醇反应和分子内环化依次进行，立体选择性地得到顺-2-羟基烷基取代环丙醇。(3)在Et_3SiCl存在下，铬烯酸酯被顺利捕获，生成硅烯醇醚。这种捕获改变了α，β-不饱和酮与醛之间的反应过程，从序贯醛-环丙醇生成到交叉蒎醇型偶联反应。当采用脂肪族α，β-不饱和酮时，1,2-二醇的抗合成比取决于反应温度。(4)用廉价易得的化合物立体控制合成氟化氨基酸的方法。在氢压力下，一定催化量的三氟乙酸钯和BINAP促进了α-氟化亚胺酯的不对称加氢反应，生成富对映体的β-氟化α-氨基酯。(5)研究了光学活性三氟去冠酸及其非对映体的立体选择性合成。γ-氰丙烷的立体定向和非对映选择性环化得到环丙基腈。CN和Ar基团进一步转化得到三氟-去冠酸和三氟-异氟-去冠酸。(6)甘氨酸腈手性移位碱的手性氟化环氧化物，根据碱的不同，可选择性地得到γ-羟基正缬氨酸衍生物的非对映体。该Shiff碱也可用于制备光学纯的溴氟丙氨酸衍生物。少

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 … More 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