Total Syntheses Of Sesquiterpenoids (±)-Omphadiol,(±)-Pyxidatol C,(+)-Leptographiol And (+)-Isoleptographiol

Terpenoids are a very important class of natural product, which are widely distributed in nature. They are mainly isolated from plants and marine organisms, and most show biological activities of great importance. Sesquiterpenes are the largest categories of terpenoids in terms of both isolated numbers and basic structural skeletons. This dissertation describes the total syntheses of africanane type sesquiterpenes omphadiol, pyxidatol C, leptographiol and isoleptographiol. The main content consists of five chapters.Chapter1, Introduction of terpene in general.Chapter2, Review on synthetic studies towards africanane type sesquiterpenoids.Chapter3, Racemic total syntheses of omphadiol, pyxidatol C, leptographiol, and isoleptographiol. Using a combination of aldol-Henry tandem reaction and Tiffenneau-Demjanov rearrangement, we have developed a robust multi-gram scale synthesis of cis-fused5,7-carbocyclic intermediate in only five steps from inexpensive chemicals4,4-dimelthyl cyclohexanone and4-nitrobutyraldehyde. Our key strategy involves converting our readily available cis-fused5,7-carbocyclic intermediates to more synthetically challenging trans-fused ones. This strategy allowed us for completion of racemic total syntheses of all four terpenoids bearing distict oxidation patterns through a common intermediate.Chapter4, Asymmetric total syntheses of (+)-leptographiol and (+)-isoleptographiol. By applying a novel strategy, we were able to achieved asymmetric total syntheses of two family members in this group of natural products. By exploiting the intrinsic structural nature of these terpenoids, we managed to convert both enantiomers of certain intermediate to natural enantiomer of leptographiol and isoleptographiol.Chapter5, Experimental details and analytical data.