Recent advances in ligand-enabled palladium-catalyzed divergent synthesis

Base-controlled regioselectivity via distinct mechanisms during C-H thionation of azinium salts with elemental sulfur

Regioselective C-H-thionation of pyridine and quinoline rings can be achieved through the reaction of the corresponding quaternary azinium salts with sulfur and a base via a radical- or carbene-involved reaction pathway. The selectivity of CS group installation is determined by the reaction mechanism and affected by the base used.

Aqueous/Nonaqueous DBU Mixtures: Versatile Switching Media for Chemoselective Aldol, Baylis-Hillman, and Aldol Condensation Reactions

Isophorone is a relatively small molecule with several neighboring reacting sites, making it susceptible to various competing reactions with aldehydes, including aldol, Baylis-Hillman (BH), aldol condensation, and Michael addition reactions. In the present work, we have designed a switchable 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-catalyzed procedure, where the reaction of isophorone with aldehydes is guided chemoselectively toward either aldol, BH, or aldol condensation reactions, depending on the use of water and/or heat. This controllable divergency likely stems from the ability to tune the dual nucleophilicity/basicity characters of the DBU/H2O medium. In other words, the nucleophilicity of DBU plays a crucial role in directing the process toward the formation of the BH adducts in the absence of water. At the same time, the aldol pathway dominates when water is present. The conditions were amenable for tandem processes, as demonstrated for an aldol condensation/Diels-Alder sequence.

Multimodal Precise Control Over Multiselective Carbonylation of 1,3-Enynes

Efficiently constructing structurally diverse and complex organic molecules through selective catalytic functionalization is a central goal in synthetic chemistry, yet achieving precise control over multiple reactive centers in multisite substrates remains a formidable challenge. Building on foundational advances in single- and dual-selective transformations, we report a multimodal strategy for the selective carbonylation of 1,3-enynes, a versatile class of multisite substrates. Through meticulous fine-tuning of the catalytic conditions, our approach enables five distinct regio- and stereoselective carbonylative transformations, including direct functionalization (1,2- and 2,1-hydroaminocarbonylation) and tandem cyclization pathways (2,4-, 1,3-, and 2,3-carbonylation). Furthermore, mechanistic studies suggested that multidimensional precise regulation enables the seamless relay of up to three tandem reactions (hydroaminocarbonylation-hydroamination-transamination) with exceptional accuracy. This unified platform not only establishes a robust framework for tackling the enduring challenges of selectivity control in multisite substrates but also broadens the chemical space accessible through 1,3-enyne transformations, exemplifying atom- and step-economic principles and paving the way for transformative advancements in drug discovery, materials science, and beyond.

Solvent-Dependent Divergent Cyclization of Bicyclo[1.1.0]butanes

Bicyclo[1.1.0]butanes (BCBs) have recently garnered significant research interest as versatile precursors for synthesizing potential [n.1.1] bioisosteres and multi-functionalized cyclobutanes in a straightforward and atom-economical manner. Here, we report a solvent-dependent divergent cyclization of BCBs that provides highly diastereospecific decorated cyclobutanes and oxygen-containing bicyclo[3.1.1]heptanes (BCHeps), which serve as bioisosteres of meta-substituted arenes. Additionally, an unprecedented 1,2-difunctionalization reaction mode for BCBs was explored, thus expanding the chemical space of arene bioisosteres and highly functionalized cyclobutanes.

Synthesis of β-lactam-zidovudine pronucleosides as potential selective narrow-spectrum antibacterial agents

Since the discovery of penicillin, the forerunner of the most widely used class of antibiotics (i.e. β-lactams), natural compounds and their derivatives represented a major source of antibacterial therapeutic products whose availability enabled modern medical practices (invasive surgery, organ transplant, etc.). However, the relentless emergence of resistant bacteria is challenging the long-term efficacy of antibiotics, also decreasing their economic attractiveness for big pharma, leading to a significant decay in antibacterial development in the 21st century and an increased use of last-resort drugs such as carbapenems or colistin. Indeed, bacteria evolved an arsenal of resistance mechanisms, leading to the emergence of totally-drug resistant isolates, already sporadically isolated among Gram-negative bacterial species. To face this deadly peril, it is fundamental to explore new ground-breaking approaches. In view of the significance of both β-lactam antibiotics and the production of one or more β-lactamases as the major resistance mechanism (especially in Gram-negative bacteria), we implemented an original approach to selectively deliver antibacterial zidovudine (AZT) exploiting the β-lactamase-mediated hydrolysis of a β-lactam-conjugate prodrug. The synthesis of the targeted pronucleosides was performed in 5-7 steps and based on an original Pd-catalyzed cross-coupling reaction. Enzymatic and microbiological evaluations were performed to evaluate the synthesized pronucleosides, yielding new insights into molecular recognition of β-lactamase enzymes. This approach would potentially allow a targeted and selective eradication of antibiotic-resistant β-lactamase-producing (opportunistic) pathogens, as the inactive prodrug is unable to harm the commensal microbial flora.

Recent advances in photocatalytic and transition metal-catalyzed synthesis of disulfide compounds

Disulfide bonds are essential in protein folding, cellular redox balance, materials science, and drug development. Despite existing synthetic methods, the efficient and selective synthesis of unsymmetrical disulfides remains challenging. This review highlights innovative approaches in visible light photocatalysis, including decarboxylation, deoxydisulfidation of alcohols, and direct C-H disulfidation, showcasing broad substrate applicability and functional group tolerance under mild conditions. Additionally, it explores transition metal-catalyzed systems with copper, nickel, palladium, chromium, Iridium, Rhodium molybdenum, and scandium, offering effective strategies for unsymmetrical disulfide bond formation and late-stage functionalization of complex molecules through reductive coupling, selective oxidation, and novel insertion reactions.

Recent Developments in Copper-Catalyzed Annulations for Synthesis of Spirooxindoles

Spirooxindoles represent a special scaffold for pharmaceuticals and natural products, and significant advancements have been achieved in their synthesis in recent years. Among these, transition metal catalysis, particularly copper catalysis, has emerged as an efficient and reliable method for the synthesis of spirooxindoles. Based on different reaction types, two distinct substrate types have been summarized and classified by us for constructing spirooxindole scaffolds via intramolecular and intermolecular annulations. This review outlines the latest advancements in copper-catalyzed cyclization reactions for synthesizing spirooxindoles and provides detailed insights into the types of annulation reactions and their possible reaction mechanisms.

Palladium-Catalyzed Ligand-Controlled Switchable Hetero-(5 + 3)/Enantioselective [2σ+2σ] Cycloadditions of Bicyclobutanes with Vinyl Oxiranes

For nearly 60 years, significant research efforts have been focused on developing strategies for the cycloaddition of bicyclobutanes (BCBs). However, higher-order cycloaddition and catalytic asymmetric cycloaddition of BCBs have been long-standing formidable challenges. Here, we report Pd-catalyzed ligand-controlled, tunable cycloadditions for the divergent synthesis of bridged bicyclic frameworks. The dppb ligand facilitates the formal (5+3) cycloaddition of BCBs and vinyl oxiranes, yielding valuable eight-membered ethers with bridged bicyclic scaffolds in 100% regioselectivity. The Cy-DPEphos ligand promotes selective hetero-[2σ+2σ] cycloadditions to access pharmacologically important 2-oxabicyclo[3.1.1]heptane (O-BCHeps). Furthermore, the corresponding catalytic asymmetric synthesis of O-BCHeps with 94-99% ee has been achieved using chiral (S)-DTBM-Segphos, representing the first catalytic asymmetric cross-dimerization of two strained rings. The obtained O-BCHeps are promising bioisosteres for ortho-substituted benzenes.

Functional-Group-Directed Regiodivergent (3 + 2) Annulations of Electronically Distinct 1,3-Dienes and 2-Formyl Arylboronic Acids

Presented herein is a palladium-catalyzed asymmetric (3 + 2) annulation reaction between 1,3-dienes and 2-formylarylboronic acids, proceeding in a cascade vinylogous addition and Suzuki coupling process. Both electron-neutral and electron-deficient 1,3-dienes are compatible under similar catalytic conditions, and distinct regioselectivity is observed via functional-group control of 1,3-diene substrates. A collection of 1-indanols with dense functionalities is constructed stereoselectively.