Synthesis and optical resolution of axially dissymmetric pyrroles and pyrocolls: new catalysts for the enantioselective addition of diethylzinc to aromatic aldehydes
作者:Yoshio Furusho、Akihiro Tsunoda、Takuzo Aida
DOI:10.1039/p19960000183
日期:——
The novel axially dissymmetric pyrroles, 4-methyl-3-(2′-methoxy-1′-naphthyl)pyrrole-2-carboxylates 1 and 4-ethyl-3-(10′-methoxy-9′-phenanthryl)pyrrole-2-carboxylates 3 were synthesized from the corresponding nitroalkenes and ethyl isocyanoacetate. Optical resolution of compounds 1 was achieved via crystallization of a diastereoisomeric mixture of the (R)-1-phenylethyl esters 1d, while the antipodes of compound 3b spontaneously resolved upon crystallization of the racemate. The rotational barrier (ΔG‡) at 25 °C about the pyrrole–phenanthrene bond in compound 3b (160 kJ mol–1) was 30 kJ mol–1 higher than that about the pyrrole–naphthalene bond in compound 1b. The carboxylate 1b or 3b lost optical activity upon lactonization, while the resulting lactone 2 opened diastereoselectively by reaction with lithium (S)-1-phenylethylamide. The antipodes of compounds 1a and 3a were converted respectively without racemization into axially dissymmetric pyrocolls, 1,6-bis(2′-hydroxy-1′-naphthyl)-2,7-dimethylpyrocoll 5a and 2,7-diethyl-1,6-bis(10′-hydroxy-9′-phenanthryl)pyrocoll 6a, both of which effectively catalysed enantioselective addition of Et2Zn to aromatic aldehydes.
新型轴向不对称吡咯,4-甲基-3-(2′-甲氧基-1′-萘基)吡咯-2-羧酸酯 1 和 4-乙基-3-(10′-甲氧基-9′-菲基)吡咯-2-羧酸酯 3 是由相应的硝基烯类和异氰基乙酸乙酯合成的。通过结晶(R)-1-苯基乙基酯 1d 的非对映异构体混合物,实现了化合物 1 的光学解析,而化合物 3b 的对映异构体则在外消旋体结晶时自发解析。化合物 3b 中的吡咯-菲键在 25 °C 时的旋转障碍(ΔG‡)(160 kJ mol-1)比化合物 1b 中的吡咯-萘键的旋转障碍(ΔG‡)高 30 kJ mol-1。羧酸盐 1b 或 3b 内酯化后失去了光学活性,而生成的内酯 2 与锂 (S)-1 苯基乙酰胺反应后非对映地开放。化合物 1a 和 3a 的对映体分别转化为轴向不对称的吡咯、1,6-双(2′-羟基-1′-萘基)-2、2,7-二甲基吡咯 5a 和 2,7-二乙基-1,6-双(10′-羟基-9′-菲基)吡咯 6a,它们都能有效地催化 Et2Zn 与芳香醛的对映选择性加成。