I. Photochemical catalytic [6pi + 2pi] higher-order cycloadditions. II. Toward the development of a chiral catalyst for chromium(0) higher-order cycloadditions. III. Synthetic studies toward a synthesis of taxol: Investigation of an intramolecular cycloaddition approach to the taxane core.

摘要

The preparation of complex natural compounds with important biological activity has driven the development of organic chemistry. The discovery of new complex functionally diverse natural products continues to test the limits of science. Researchers continue to develop new reaction methodologies to meet the demands of these complex molecular systems by convergent routes.; Chromium promoted higher-order cycloaddition reactions have become a valuable tool for the synthesis of bicyclic compounds, which are used as building blocks for the total synthesis of natural products. A number of advances have led to the development of catalytic reactions as well as the preparation of planar chiral chromium complexes that have produced enantiomerically enriched bicycles. The search for a chiral catalyst for these metal promoted higher-order cycloadditions has led to a number of new developments.; A novel room temperature photoactivated [6π + 2π] cycloaddition reaction utilizing substoichiometric amounts of chromium(0) precatalyst has been achieved with good to excellent yields. Unlike, the recently described catalytic room temperature reaction this variation is tolerant of a variety of trienes and trienophiles. The milder reaction conditions provided a significant improvement in diastereoselectivity with chiral auxiliaries over the polymer supported metal catalyst system. These chemical conditions were successfully applied to the multi-component [6π + 2π + 2π] higher-order cycloaddition process, to provide several polycyclic products in moderate yields.; The development work on preparation of a chiral catalyst for higher-order cycloaddition chemistry provided new insight into the reactivity of these chromium species. The first generation catalyst, (η2-BINAP)Cr(CO) ₃ was prepared in good yields and fully characterized for the first time, unfortunately it did not provide any of the desired cycloadduct, because of the extra electron density on the metal center. The second-generation catalyst, (η6-toluene)Cr(CO)₃(η1-BINAP) promoted the [6π + 2π] cycloaddition in high yields, but did not provide any enantioselectivity. Initial cycloaddition studies using electrophilic phosphite ligands were found to completely suppress the catalytic pathway.