| Alkaloids and terpenoids are two very important categories of natural products widely found in nature. While showing many different properties and functions, they often possess complex and fascinating molecular structures. Therefore, they attract the attention of many chemists, biologists and pharmacists. As one knows, natural products are usually hard to separate from its natural sources and large-scale extraction is a popular way to obtain them from nature. But such operation can cause serious or even devastating effects on natural and ecological systems, so chemical synthesis including semi-synthesis and total synthesis of natural products remains a necessary tool for human to utilize them. This dissertation describes our efforts in solving some synthetically chanllenging natural products, which consists of the following four parts:total syntheses of (+)-agelastatins A and B, total synthesis of longifolene, total synthesis of (+)-minfiensine, and studies toward total synthesis of daphniyunnine C.Chapter1, total syntheses of (+)-agelastatins A and B. Exploiting the intrinsic structural nature moleculars, we have designed a completely new synthetic strategy, based on a tandem a cationic polycyclization to construct the synthetically challenging tetrasubstituted cyclopropane core structure. Commencing from a commercially available enantiopure amino acid derivative L-aspartic acid diethyl ester hydrochloride, we achieved total syntheses of (+)-Agelastatin A and B in8or9steps, respectively.Chapter2, total synthesis of longifolene. Starting from4-nitro-butylaldehyde and3-methyl-2-cyclohexenone, two simple and inexpensive chemicals, we accomplished a total synthesis of longifolene through a10-step sequence. Key strategic steps involve a Micheal-aldol-Henry cascade reaction, a Tiffeneau-Demjanov rearrangement and an intramolecular radical cyclization.Chapter3, total synthesis of (+)-minfiensine. Using the modified Fischer indole sythesis methodology developed in our group, we completed a total synthesis of (+)-minfiensine. The key optically pure building block (S)-4-TBSO-cyclohex-2-enone was reaily derived from D-(-)-quinic acid. Our new chemistry allowed us to contribute a new entry to asymmetric total synthesis of (+)-minfiensine.Chapters4, studies toward total synthesis of daphniyunnine C. Starting from a readily available enantiopure building block derived from (R)-carvone, we prepared the complex [6-6-5] tricyclic core skeleton in daphniyunnine containing five stereogenic centers including an all-carbon quaternary center in only7steps. The key steps include an aza-Michael addition and a palladium-catalyzed intramolecular enolate α-vinylation. The chemistry laid solid ground for total synthesis of dampniynnine C and other family members in Daphniphyllum alkaloids. |
