Synthesis and chemical application s of ansa-type compounds

点击翻译按钮获取中文摘要

Aromatic compounds bridged at the meta or para positions are referred to ansa compounds. This class of compounds is chemically and biologically useful. For example, ansamycin and its analogs are powerful antibiotics and pyridinophane-type crown ethers serve as excellent host for metals. However, these ansa-type compounds are not readily accessible due to their much strain by the strained bridged structure.We have studied a new method for synthesizing ansa-type compounds and their use in molecular recognition chemistry.Previously, we found that acylketenes can be generated most conveniently by thermal fragnentation of 1,3-dioxin-4-one derivatives. Based on this finding, we studied generation of bis(acylketene) and their intramolecular cyclization to produce ansa-type compounds. This project was very much successful. A series of 4-hydroxy-2-pyrones and 2-pyridines bridged at the 3- and 6-positions was synthesized by the intramolecular cyclization of bis(acylketene) intermediate highly effciently. These ansa-type compounds have planar chirality and served as excellent resolution reagent for chiral primary amines. Enantiomerically pure amines thus obtained were applied to the first synthesis of chiral imidazolium carbenes that proved to be useful catalyst for kinetic resolution of chiral alcohols. Pyridinophane-type 12-crown-4 was synthesized by ring transformation of the bridged 4-hydroxy-2-pyrones.We have also studied synthesis of bridged pyrones by metal-catalyzed ring-closing metathesis to that increasing attention has been diected toward the synthesis of medium and large rings inclucing cyclophanes. The results indicated that ring-closing metathesis afford the bridged pyrones in much lower yields than that by intramolecular cycloaddition of bis (acylketene).