Bioreduction of N-(3-oxobutyl)heterocycles with flexible ring by yeast whole-cell biocatalysts

This study explored the bioreduction of N-(3-oxobutyl)heterocycles with (partially) saturated heterocyclic moieties using whole-cell forms of wild-type yeast strains and commercially available baker's yeast (Saccharomyces cerevisiae). Eleven wild-type yeast strains and baker's yeast were screened for ketoreductase activity on a series of five flexible N-heterocycles with prochiral carbonyl group in the N-(3-oxobutyl) substituent. Among the yeast strains tested, Candida parapsilosis (WY12) proved to be the most efficient biocatalyst in the bioreductions, resulting in the corresponding enantiopure alcohols-being promising chiral fragments with high level of drug-likeness-with good to excellent conversions (83-99%) and high enantiomeric excess (ee > 99%). Other strains, such as Pichia carsonii (WY1) and Lodderomyces elongisporus (WY2), also showed promising ketoreductase activities with certain substrates. After screening as lyophilized whole cells, C. parapsilosis cells were immobilized in the form of calcium, zinc, nickel, and copper alginate beads. The whole-cell immobilization enabled recycling, with considerable residual activity of the biocatalyst over multiple cycles. Additionally, the study explored the scalability of these bioreductions, with immobilized C. parapsilosis delivering promising results. The use of immobilized cells simplified the work-up process and resulted in chiral alcohols with similar or even higher conversions to those observed in the screening reactions. Molecular docking of the five flexible N-heterocycles with prochiral carbonyl group into the active site of the experimental structure of the carbonyl reductase of C. parapsilosis rationalized their biocatalytic behavior and confirmed the assigned (S)-configuration of forming enantiopure alcohols. KEY POINTS: • Ketoreductase activity of eleven wild-type yeast strains and baker's yeast were examined. • Candida parapsilosis was subjected to whole-cell immobilization and recycling. • Enantiopure alcohols with flexible N-heterocyclic units were produced at preparative scale.

Transition-metal-catalyzed auxiliary-assisted C-H functionalization using vinylcyclopropanes and cyclopropanols

Transition-metal-catalyzed chelation-assisted C-H functionalization exploiting small strained rings has surfaced as an appealing strategy, offering a robust platform for the construction of complex molecules in a step- and atom-economic fashion. In this vein, three-membered rings, viz. vinylcyclopropanes (VCPs) and cyclopropanols, have emerged as staple coupling partners due to their inherent ring strain. Moreover, their strain release serves as a potent driving force, unlocking new possibilities in molecular engineering via sequential C-H functionalization and ring scission. Recently, significant progress has been made in this emerging domain employing the aforementioned rings. This review article focuses on directing group (DG)-assisted C-H functionalization adopting VCPs and cyclopropanols as potential coupling partners until November 2024. The advancements are organized based on the type of functionalizations achieved.

Synthesis of Benzoisochromene Derivatives via C-H Activation-Initiated Cascade Formal [4+2] and [2+4] Annulation of Aryl Enaminone with Vinyl-1,3-dioxolan-2-one

Cascade annulation reactions can assemble structurally intricate polycyclic molecules from simple starting materials with enhanced efficiency and minimized production of waste. Presented herein is a concise and effective synthesis of benzoisochromene derivatives based on a C-H activation-initiated cascade formal [4+2]/[2+4] annulation of aryl enaminone with vinyl-1,3-dioxolan-2-one. In constructing the six-membered carbocycle, aryl enaminone acted as the C4 synthon while vinyl-1,3-dioxolan-2-one acted as the C2 synthon. In constructing the six-membered O-heterocycle, on the other hand, the former acted as C2 synthon while the latter acted as C3O1 synthon. To our knowledge, this is the first simultaneous construction of both a carbocycle and an O-heterocycle via concurrent C-H/C-N/C-O bond cleavage and C-C/C-C/C-O bond formation. In general, this novel protocol features the use of readily obtainable substrates of broad scope, excellent atom- and step-economy, intriguing reaction pathway, and valuable products.

Advances in Transition Metal-Catalyzed C(sp2)-H Bond Functionalization Using Allyl Alcohols

The transition metal-catalyzed directed site-selective C─H bond functionalization utilizing allyl alcohols as coupling partner has been an intriguing area of research and has made considerable advances during the past decade. Multifunctional coupling characteristics of the allyl alcohol in the regioselective C(sp2)-H functionalization using transition metal-catalysis produces alkyl, alkenyl, allyl, and annulated products. These reactions provide an effective synthetic tool to afford diverse functionalized scaffolds that are of interest in synthetic and medicinal sciences. This review covers the developments of directed site-selective C(sp2)-H functionalization with unactivated allyl alcohols as the coupling partner using the transition metal-catalysis till December, 2024.

Rh-Catalyzed and Self-Directed Aromatic C-H Activation of Enaminones to Divergent Alkenylated and Annulated Compounds

By means of simple Rh catalysis, the direct activation of the ortho-C-H bond in aryl enaminones has been realized with the enaminone structure as a traceless directing fragment. The products resulting from C-H alkenylation and further annulation via intramolecular C-H bond addition could be accessed depending upon the structure of alkenes. The annulated products could be used for the easy synthesis of valuable 2-aza-fluorenones in a one-pot operation by employing NH4OAc.

Oxidative coupling of N-nitrosoanilines with substituted allyl alcohols under rhodium (III) catalysis

Rhodium(III) catalysis has been used for C-H activation of N-nitrosoanilines with substituted allyl alcohols. This method provides an efficient synthesis of the functional N-nitroso ortho β-aryl aldehydes and ketones with low catalyst loading, high functional group tolerance, and superior reactivity of allyl alcohols toward N-nitrosoanilines. We demonstrated that reaction also proceeds through the one-pot synthesis of N-nitrosoaniline, followed by subsequent, C-H activation. The protocol was also feasible with acyrlaldehyde and methyl vinyl ketone which furnished the same oxidative N-nitroso coupling product.

Regioselective ortho C-H insertion of N-nitrosoanilines with naphthoquinone carbenes

A Rh(III)-catalyzed ortho C-H migratory insertion of N-nitrosoanilines with naphthoquinone carbenes has been developed. The products were obtained in good yields under mild reaction conditions. Diverse elaborations of the products were explored. This method is valuable for the synthesis of biarylamines and their derivatives.

Tert-Butyl Nitrite-initiated C-N Bond Cleavage of 1-Nitromethyl-N-aryltetrahydroisoquinolines: Synthesis of Furoxans with N-NO Skeleton

A series of furoxan derivatives with N-nitroso groups were synthesized in good yields by TBN initiated radical sp³ C-N bond cleavage of 1-nitromethyl-N-aryltetrahydroisoquinolines. This reaction grafts the biologically important furoxan skeleton and N-nitroso group into on molecule, greatly improving the molecular complexity in one step transformation. The mechanistic study shows that this reaction is mediated by the in situ generated α-carbonyl nitrile oxide, which is afforded by TBN promoted C-N bond cleavage.

Scalability of U-Shape Magnetic Nanoparticles-Based Microreactor–Lipase-Catalyzed Preparative Scale Kinetic Resolutions of Drug-like Fragments

The production of active pharmaceutical ingredients (APIs) and fine chemicals is accelerating due to the advent of novel microreactors and new materials for immobilizing customized biocatalysts that permit long-term use in continuous-flow reactors. This work studied the scalability of a tunable U-shape magnetic nanoparticles (MNPs)-based microreactor. The reactor consisted of a polytetrafluoroethylene tube (PTFE) of various inner diameters (ID = 0.75 mm, 1.50 mm, or 2.15 mm) and six movable permanent magnets positioned under the tube to create reaction chambers allowing the fluid reaction mixture to flow through and above the enzyme-loaded MNPs anchored by permanent magnets. The microreactors with various tube sizes and MNP capacities were tested with the preparative scale kinetic resolution of the drug-like alcohols 4-(3,4-dihydroisoquinolin-2(1H)-yl)butan-2-ol (±)-1a and 4-(3,4-dihydroquinolin-1(2H)-yl)butan-2-ol (±)-1b, utilizing Lipase B from Candida antarctica immobilized covalently onto MNPs, leading to highly enantioenriched products [(R)-2a,b and (S)-1a,b]. The results in the U-shape MNP flow reactor were compared with reactions in the batch mode with CaLB-MNPs using similar conditions. Of the three different systems, the one with ID = 1.50 mm showed the best balance between the maximum loading capacity of biocatalysts in the reactor and the most effective cross-section area. The results showed that this U-shaped tubular microreactor might be a simple and flexible instrument for many processes in biocatalysis, providing an easy-to-set-up alternative to existing techniques.

Ligand-assisted olefin-switched divergent oxidative Heck cascade with molecular oxygen enabled by self-assembled imines

Divergent oxidative Heck reaction has proven to be reliable for the rapid construction of molecular complexity, while olefins switched the outcome that remained underexplored.