Diastereoselective Synthesis of an Advanced Intermediate of Thapsigargin and Other 6,12-Guaianolides Using a RCEYM Strategy

Recent drug design strategies and identification of key heterocyclic scaffolds for promising anticancer targets

Cancer, a noncommunicable disease, is the leading cause of mortality worldwide and is anticipated to rise by 75% in the next two decades, reaching approximately 25 million cases. Traditional cancer treatments, such as radiotherapy and surgery, have shown limited success in reducing cancer incidence. As a result, the focus of cancer chemotherapy has switched to the development of novel small molecule antitumor agents as an alternate strategy for combating and managing cancer rates. Heterocyclic compounds are such agents that bind to specific residues in target proteins, inhibiting their function and potentially providing cancer treatment. This review focuses on privileged heterocyclic pharmacophores with potent activity against carbonic anhydrases and kinases, which are important anticancer targets. Evaluation of ongoing pre-clinical and clinical research of heterocyclic compounds with potential therapeutic value against a variety of malignancies as well as the provision of a concise summary of the role of heterocyclic scaffolds in various chemotherapy protocols have also been discussed. The main objective of the article is to highlight key heterocyclic scaffolds involved in recent anticancer drug design that demands further attention from the drug development community to find more effective and safer targeted small-molecule anticancer agents.

Rh(I)-Catalyzed Allenic Pauson-Khand Reaction to Access the Thapsigargin Core: Influence of Furan and Allenyl Chloroacetate Groups on Enantioselectivity

Thapsigargin (Tg) is a potent SERCA pump inhibitor with the potential to treat cancer and COVID-19. We have extended the scope of the asymmetric allenic Pauson-Khand reaction to furan-tethered allene-ynes, a stereoconvergent transformation affording the 5,7,5-ring system of Tg in good yields and high enantioselectivity. Computational studies of the oxidative cyclization step show that the furan and chloroacetate groups contribute to this high selectivity.

Function-Oriented and Modular (+/-)-cis-Pseudoguaianolide Synthesis: Discovery of New Nrf2 Activators and NF-κB Inhibitors

Described herein is a function-oriented synthesis route and biological evaluation of pseudoguaianolide analogues. The 10-step synthetic route developed retains the topological complexity of the natural product, installs functional handles for late-stage diversification, and forges the key bioactive Michael acceptors early in the synthesis. The analogues were found to be low-micromolar Nrf2 activators and micromolar NF-κB inhibitors and dependent on the local environment of the Michael acceptor moieties.

Reactions of Allylmagnesium Reagents with Carbonyl Compounds and Compounds with C═N Double Bonds: Their Diastereoselectivities Generally Cannot Be Analyzed Using the Felkin-Anh and Chelation-Control Models

This review describes the additions of allylmagnesium reagents to carbonyl compounds and to imines, focusing on the differences in reactivity between allylmagnesium halides and other Grignard reagents. In many cases, allylmagnesium reagents either react with low stereoselectivity when other Grignard reagents react with high selectivity, or allylmagnesium reagents react with the opposite stereoselectivity. This review collects hundreds of examples, discusses the origins of stereoselectivities or the lack of stereoselectivity, and evaluates why selectivity may not occur and when it will likely occur.

Allylative Approaches to the Synthesis of Complex Guaianolide Sesquiterpenes from Apiaceae and Asteraceae

With hundreds of unique members isolated to date, guaianolide lactones represent a particularly prolific class of terpene natural products. Given their extensive documented therapeutic properties and fascinating chemical structures, these metabolites have captivated the synthetic chemistry community for many decades. As a result of divergent biosynthetic pathways, which produce a wide array of stereochemical and oxidative permutations, a unifying synthetic pathway to this broad family of natural products is challenging. Herein we document the evolution of a chiral-pool-based synthetic program aimed at accessing an assortment of guaianolides, particularly those from the plant family Apiaceae as well as Asteraceae, members of which possess distinct chemical substructures and necessitate deviating synthetic platforms. An initial route employing the linear monoterpene linalool generated a lower oxidation state guaianolide but was not compatible with the majority of family members. A double-allylation disconnection using a carvone-derived fragment was then developed to access first an Asteraceae-type guaianolide and then various Apiaceae congeners. Finally, using these findings in conjunction with a tandem polyoxygenation cascade, we developed a pathway to highly oxygenated nortrilobolide. A variety of interesting observations in metal-mediated aldehyde allylation and alkene polyoxygenation are reported and discussed.

Ti(II) and Rh(I) Complexes as Reagents toward a Thapsigargin Core

A novel approach toward the [5-7]fused bicyclic core of thapsigargin, a subnanomolar inhibitor of the endo/sarcoplasmic calcium ATPase (SERCA), is presented. The synthetic route includes an original Ti(II)-mediated hydroxy-directed reductive coupling of an enantiomerically enriched propargylic alcohol and an intramolecular Rh(I)-catalyzed cyclocarbonylation reaction as key steps. Interestingly, through the first experiments of titanocene-mediated reductive cyclization of a 1,8-enyne, a seven-membered cycle was isolated as a unique product with a total diastereoselectivity.

Total Synthesis of (-)-Mitrephorone A

The first synthesis of (-)-mitrephorone A is disclosed along with discussion and study of synthetic strategies. The natural product includes a highly congested hexacyclic ent-trachylobane diterpenoid framework featuring a rare, embedded oxetane. The synthetic analysis presented dissects a number of approaches for the synthesis of the central oxetane, including carbonyl-olefin photocycloadditions, Prins-type cyclizations, and oxidative ring closures. In the successful route, three [4 + 2] cycloadditions enable rapid construction of all carbocycles. A novel late-stage oxidative cyclization of a hydroxy diosphenol with Koser's reagent furnishes the pivotal oxetane moiety.