Asymmetric Total Synthesis and Evaluation of Antitumor Activity of Ophiorrhisine A and Its Derivatives

Recent studies on chemical constituents of Ophiorrhiza plants

Ophiorrhiza plants (Family Rubiaceae) are known to produce diverse monoterpenoid indole alkaloids including camptothecin with potent antitumor activity. This review contains a summary of recent chemical studies reported over the past 10 years regarding alkaloids (monoterpenoid indole and tetrahydroisoquinoline alkaloids, and cyclopeptide) in Ophiorrhiza plants. In addition, the alkaloid biosynthetic pathways based on their reported structures were proposed.

Approaches to Cyclophane-Types of Cyclopeptide Alkaloids

The cyclopeptide alkaloids are cyclic depsipeptides incorporating cyclophanes with polyamide units 13-, 14- and 15-membered macrocyclic systems. Although various pharmacological activities have been ascribed to cyclopeptide alkaloids from plants of the Rhamnacea family, these studies have been hampered by their low availability due to the lack of reasonable amounts distributed in nature. Therefore, novel and efficient synthetic approaches should be an important aim, which inspired us to examine how to diversely construct the unique structures of this type of natural products. In this account, several typical strategies are presented in terms of efficient, stereocontrolled and regioselective synthesis of cyclopeptide alkaloids.

Structure-guided evolution of a ketoreductase for efficient and stereoselective bioreduction of bulky α-amino β-keto esters

Chiral vicinal amino alcohols were generated with excellent stereoselectivity and high conversion from bulky α-amino β-keto esters by an engineered ketoreductase called M30.

Yb(OTf)3-Catalyzed and Di-tert-butyl Dicarbonate-Mediated Decarboxylative Etherification and Esterification Reactions

Protecting group chemistry has invariably captured the fascination of chemists because of its extensive viability in chemical synthesis. The present report describes our pioneer work of applying ytterbium triflate as a catalyst, for the reaction of alcohols with di-tert-butyl dicarbonate (Boc2O) leading to the formation of tert-butyl ethers. There exists no recorded evidence for the use of Yb(OTf)3 as a catalyst for the protection of alcohols to tert-butyl ethers, despite its excellent utility in various reactions. Yb(OTf)3 has been used predominantly in the catalytic deprotection studies such as selective deprotection of tert-butyl esters to carboxylic acids as well as prenyl ethers to alcohols. This study involved the critical evaluation of solvent, time, and temperature that finally led to an efficient protocol for the formation of tert-butyl ethers. Yb(OTf)3 catalyzed the formation of tert-butyl ethers, notably reducing the reaction time, which is exemplified by the achievement of up to 92% conversion of alcohols to tert-butyl ethers within an hour. Additionally, the report demonstrates the utility of this synthetic protocol for the protection of carboxylic acids.

Asymmetric Total Synthesis of Biphenylquinolizidine Alkaloids 4″-O-Demethyllythridine and 14-epi-4″-O-Demethyllythridine

The first asymmetric total synthesis of new biphenylquinolizidine alkaloids 4″-O-demethyllythridine and 14-epi-4″-O-demethyllythridine isolated from Heimia salicifolia was accomplished. The key steps in the synthesis were a copper(I)-catalyzed asymmetric intramolecular aza-Michael reaction to build a chiral 4-arylquinolizidine unit and an intramolecular Suzuki-Miyaura cross-coupling reaction to construct a macrolactone ring comprising a biphenyl moiety.