48 メソ-ヘテロ環式化合物の酸化的不斉非対称化 : 不斉C-H酸化の合成的応用(口頭発表の部)
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Recent development of asymmetric metal catalysis has realized highly enantioface-selective oxidation of carbon-carbon double bonds. On the other hand, enantiotopic selective C-H bond oxidation has so far been met with limited success: moderate to good enantioselectivity has been achieved only in the oxidation of activated benzylic and allylic C-H bonds. However, we have demonstrated that (salen)manganese(III) complexes are excellent catalysts not only for asymmetric epoxidation of simple olefins but also for enantioselective hydroxylation of prochiral benzylic carbon. As an extension of enantiotopic selective C-H oxidation, we examined Mn-salen catalyzed oxidative asymmetric desymmetrization of meso-heterocycles. This class of compounds are readily available in bulk and the resulting oxidation products are useful building blocks in organic synthesis. At first we examined asymmetric desymmetrization of meso-cyclic ethers in the presence of various Mn-salen complexes and found that high enantioselectivity (up to 90% ee) was realized in the reactions of meso-tetrahydrofuran derivatives, when complex 3 was used as a catalyst. The resulting lactols are useful building blocks. As their application, we synthesized (-)-alloyohimbane (13) from lactol 7 only in seven steps in a straight forward manner. We next examined asymmetric desymmetrization of meso-pyrrolidine derivatives. Moderate to good enantioselectivity was also observed, though the selectivity depended on the N-protecting group. Enantioselectivity up to 82% ee was obtained in the reaction of pyrrolidine derivative N-protected with phenylacetyl group. It is noteworthy that the oxidation of C-H bond occurred chemoselectively. For example, acetonide is tolerable to the present conditions. In conclusion, we were able to disclose that salen-catalyzed enantiotopic selective C-H oxidation is of high synthetic use.
- 天然有機化合物討論会の論文
- 1998-08-31
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