Switching on elusive organometallic mechanisms with photoredox catalysis.

Transition-metal-catalysed cross-coupling reactions have become one of the most used carbon-carbon and carbon-heteroatom bond-forming reactions in chemical synthesis. Recently, nickel catalysis has been shown to participate in a wide variety of C-C bond-forming reactions, most notably Negishi, Suzuki-Miyaura, Stille, Kumada and Hiyama couplings. Despite the tremendous advances in C-C fragment couplings, the ability to forge C-O bonds in a general fashion via nickel catalysis has been largely unsuccessful. The challenge for nickel-mediated alcohol couplings has been the mechanistic requirement for the critical C-O bond-forming step (formally known as the reductive elimination step) to occur via a Ni(III) alkoxide intermediate. Here we demonstrate that visible-light-excited photoredox catalysts can modulate the preferred oxidation states of nickel alkoxides in an operative catalytic cycle, thereby providing transient access to Ni(III) species that readily participate in reductive elimination. Using this synergistic merger of photoredox and nickel catalysis, we have developed a highly efficient and general carbon-oxygen coupling reaction using abundant alcohols and aryl bromides. More notably, we have developed a general strategy to 'switch on' important yet elusive organometallic mechanisms via oxidation state modulations using only weak light and single-electron-transfer catalysts.

A general and enantioselective approach to pentoses: a rapid synthesis of PSI-6130, the nucleoside core of sofosbuvir.

An efficient route towards biologically relevant pentose derivatives is described. The de novo synthetic strategy features an enantioselective α-oxidation reaction enabled by a chiral amine in conjunction with copper(II) catalysis. A subsequent Mukaiyama aldol coupling allows for the incorporation of a wide array of modular two-carbon fragments. Lactone intermediates accessed via this route provide a useful platform for elaboration, as demonstrated by the preparation of a variety of C-nucleosides and fluorinated pentoses. Finally, this work has facilitated expedient syntheses of pharmaceutically active compounds currently in clinical use.

Invention of MK-7845, a SARS-CoV-2 3CL Protease Inhibitor Employing a Novel Difluorinated Glutamine Mimic.

As SARS-CoV-2 continues to circulate, antiviral treatments are needed to complement vaccines. The virus's main protease, 3CLPro, is an attractive drug target in part because it recognizes a unique cleavage site, which features a glutamine residue at the P1 position and is not utilized by human proteases. Herein, we report the invention of MK-7845, a novel reversible covalent 3CLPro inhibitor. While most covalent inhibitors of SARS-CoV-2 3CLPro reported to date contain an amide as a Gln mimic at P1, MK-7845 bears a difluorobutyl substituent at this position. SAR analysis and X-ray crystallographic studies indicate that this group interacts with His163, the same residue that forms a hydrogen bond with the amide substituents typically found at P1. In addition to promising in vivo efficacy and an acceptable projected human dose with unboosted pharmacokinetics, MK-7845 exhibits favorable properties for both solubility and absorption that may be attributable to the unusual difluorobutyl substituent.

Rhodium(II)-catalyzed enantioselective C-H functionalization of indoles.

A catalytic, enantioselective method for the C-H functionalization of indoles by diazo compounds has been achieved. With catalytic amounts of Rh(2)(S-NTTL)(4), the putative Rh-carbene intermediates from α-alkyl-α-diazoesters react with indoles at C(3) to provide α-alkyl-α-indolylacetates in high yield and enantioselectivity. From DFT calculations, a mechanism is proposed that involves a Rh-ylide intermediate with oxocarbenium character.

Synthesis of C6-Substituted Purine Nucleoside Analogues via Late-Stage Photoredox/Nickel Dual Catalytic Cross-Coupling.

Nucleoside analogues have been and continue to be extremely important compounds in drug discovery. Despite the significant effort dedicated to their synthesis, medicinal chemistry campaigns around these structures are often hampered by synthetic challenges. We describe a strategy for the functionalization of purine nucleosides via photoredox and nickel-catalyzed sp2-sp3 cross-coupling. The conditions described herein allow for coupling of unprotected nucleosides with readily available alkyl bromides, providing opportunities for their application to parallel medicinal chemistry.

Native functionality in triple catalytic cross-coupling: sp³ C-H bonds as latent nucleophiles.

The use of sp(3) C-H bonds--which are ubiquitous in organic molecules--as latent nucleophile equivalents for transition metal-catalyzed cross-coupling reactions has the potential to substantially streamline synthetic efforts in organic chemistry while bypassing substrate activation steps. Through the combination of photoredox-mediated hydrogen atom transfer (HAT) and nickel catalysis, we have developed a highly selective and general C-H arylation protocol that activates a wide array of C-H bonds as native functional handles for cross-coupling. This mild approach takes advantage of a tunable HAT catalyst that exhibits predictable reactivity patterns based on enthalpic and bond polarity considerations to selectively functionalize α-amino and α-oxy sp(3) C-H bonds in both cyclic and acyclic systems.