A Reverse Approach to the Total Synthesis of Halichondrin B

Synthesis of 4'-Thionucleoside Analogues Bearing a C2' Stereogenic All-Carbon Quaternary Center

The design of novel 4'-thionucleoside analogues bearing a C2' stereogenic all-carbon quaternary center is described. The synthesis involves a highly diastereoselective Mukaiyama aldol reaction, and a diastereoselective radical-based vinyl group transfer to generate the all-carbon stereogenic C2' center, along with different approaches to control the selectivity of the N-glycosidic bond. Intramolecular SN2-like cyclization of a mixture of acyclic thioaminals provided analogues with a pyrimidine nucleobase. A kinetic bias favoring cyclization of the 1',2'-anti thioaminal furnished the desired β-D-4'-thionucleoside analogue in a 7:1 ratio. DFT calculations suggest that this kinetic resolution originates from additional steric clash in the SN2-like transition state for 1',4'-trans isomers, causing a significant decrease in their reaction rate relative to 1',4'-cis counterparts. N-glycosylation of cyclic glycosyl donors with a purine nucleobase enabled the formation of novel 2-chloroadenine 4'-thionucleoside analogues. These proprietary molecules and other derivatives are currently being evaluated both in vitro and in vivo to establish their biological profiles.

1,3-Diamine-Derived Catalysts: Design, Synthesis, and the Use in Enantioselective Mannich Reactions of Ketones

1,3-Diamine-derived catalysts were designed, synthesized, and used in asymmetric Mannich reactions of ketones. The reactions catalyzed by one of the 1,3-diamine derivatives in the presence of acids afforded the Mannich products with high enantioselectivities under mild conditions. In most cases, bond formation occurred at the less-substituted α-position of the ketone carbonyl group. Our results indicate that the primary and the tertiary amines of the 1,3-diamine derivative cooperatively act for the catalysis.

An α-chloroaldehyde-based formal synthesis of eribulin

Eribulin (Halaven) is the most structurally complex non-peptidic drug made by total synthesis and has challenged preconceptions of synthetic feasibility in drug discovery and development. However, despite decades of research, the synthesis and manufacture of eribulin remains a daunting task. Here, we report syntheses of the most complex fragment of eribulin (C14-C35) used in two distinct industrial routes to this important anticancer drug. Our convergent strategy relies on a doubly diastereoselective Corey-Chaykovsky reaction to affect the union of two tetrahydrofuran-containing subunits. Notably, this process relies exclusively on enantiomerically enriched α-chloroaldehydes as building blocks for constructing the three densely functionalized oxygen heterocycles found in the C14-C35 fragment and all associated stereocenters. Overall, eribulin can now be produced in a total of 52 steps, which is a significant reduction from that reported in both academic and industrial syntheses.

Marine natural products

Covering: January to December 2021This review covers the literature published in 2021 for marine natural products (MNPs), with 736 citations (724 for the period January to December 2021) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1425 in 416 papers for 2021), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. An analysis of the number of authors, their affiliations, domestic and international collection locations, focus of MNP studies, citation metrics and journal choices is discussed.

Palladium(0)-Catalyzed Intermolecular Asymmetric Allylic Dearomatization of Substituted β-Naphthols with Morita-Baylis-Hillman (MBH) Adducts

Pd-catalyzed intermolecular asymmetric allylic dearomatization of substituted β-naphthol derivatives with Boc-protected Morita-Baylis-Hillman (MBH) adducts was developed. The reaction occurs smoothly in 1,4-dioxane at room temperature in the presence of [Pd(C₃H₅)Cl]₂ (2.5 mol %), (S, S_p)-PHOX ligand (5.5 mol %), and Li₂CO₃ (1.0 equiv). A series of dearomatized products were afforded in moderate to excellent yields and enantioselectivity (up to 99% yield, 97% ee). Furthermore, the compatibility with gram-scale reaction and mild conditions make the current method synthetically useful.

A unified strategy for the total syntheses of eribulin and a macrolactam analogue of halichondrin B

A unified synthetic route for the total syntheses of eribulin and a macrolactam analog of halichondrin B is described. The key to the success of the current synthetic approach includes the employment of our reverse approach for the construction of cyclic ether structural motifs and a modified intramolecular cyclization reaction between alkyl iodide and aldehyde functionalities to establish the all-carbon macrocyclic framework of eribulin. These syntheses, together with our previous work on the total syntheses of halichondrin B and norhalichondrin B, demonstrate and validate the powerful reverse approach in the construction of cyclic ether structural motifs. On the other hand, the unified synthetic strategy for the synthesis of the related macrolactam analog provides inspiration and opportunities in the halichondrin field and related polycyclic ether areas.

Branching out: redox strategies towards the synthesis of acyclic α-tertiary ethers

Acyclic α-tertiary ethers represent a highly prevalent functionality, common to high-value bioactive molecules, such as pharmaceuticals and natural products, and feature as crucial synthetic handles in their construction. As such their synthesis has become an ever-more important goal in synthetic chemistry as the drawbacks of traditional strong base- and acid-mediated etherifications have become more limiting. In recent years, the generation of highly reactive intermediates via redox approaches has facilitated the synthesis of highly sterically-encumbered ethers and accordingly these strategies have been widely applied in α-tertiary ether synthesis. This review summarises and appraises the state-of-the-art in the application of redox strategies enabling acyclic α-tertiary ether synthesis.

Borane- and Silylium-Catalyzed Difunctionalization of Carbohydrates: 3,6-Anhydrosugar Enabled 1,6-Site Selectivity

A novel diastereoselective, Lewis acid catalyzed 1,6-difunctionalization of galactose and mannose derivatives has been developed in one pot, via sequential nucleophile additions. Our studies point to the formation of a 3,6-anhydrosugar intermediate as key to the 1,6-site-selectivity. Starting material-specific reactivity occurs when competitive ring-opening C-O cleavage is possible, owed to basicity and stereoelectronic stabilization differences. Lastly, Mayr nucleophilicity parameter values helped predict which reaction conditions would be most suitable for specific nucleophiles.

A Highly Convergent Total Synthesis of Norhalichondrin B

A new synthetic strategy for the total synthesis of norhalichondrin B featuring a highly convergent approach and our recently disclosed reverse approach for the synthesis of cyclic ether structural motifs is disclosed. Resulting in the shortest route to norhalichondrin B disclosed thus far, the reported total synthesis was achieved through the synthesis of two almost equally complex fragments whose coupling and short elaboration sequence featured an essential epimerization of the C16 stereocenter occurring concurrently with a simple acid-induced deprotection, a tactic based on a prior study along the synthetic route. This unprecedented strategy within the halichondrin family of natural products could find practical application to the synthesis of other more or less complex natural or designed halichondrin analogues.