Regio- and diastereoselective synthesis of cyclobutylated phenothiazines via [2 + 2] photocycloaddition: demonstrating wavelength-gated cycloreversion inside live cells

Herein, we unveiled a regio- and diastereoselective synthesis of cyclobutylated phenothiazines, a unique class of structural congeners of phenothiazines via visible-light-irradiated intermolecular [2 + 2]-cycloaddition reaction, from readily available naphthoquinones, 2-aminothiophenols, and styrenes, either in a two-step or three-component coupling process. By varying substitutions in all three coupling partners, a library of cyclobutylated phenothiazines, including late-stage derivatization with five commercial drugs, has been realized with up to 97% isolated yield. In contrast to the reported pathways, the developed [2 + 2]-photocycloaddition seems to proceed via a 'photoinduced-electron-transfer' (PET) mechanism, which is well corroborated with the experimental observations, Rehm-Weller equation, and computation studies. Delightfully, a wavelength-gated reversibility of the [2 + 2]-photocycloaddition reaction has been accomplished on the synthesized cyclobutylated phenothiazines. By monitoring the rate of the cycloreversion reactions for different derivatives, a structure-activity relationship has also been achieved. Interestingly, this phenomenon was further replicated inside living cells, which leads to turn-on emission and is applied for photoresponsive cell imaging. This marks the first report of a light-triggered [2 + 2]-cycloreversion phenomenon occurring inside a live cell, leading to cell imaging. Moreover, the synthesized drug derivatives were utilized for synchronous cell imaging as well as drug delivery through the developed [2 + 2]-photocycloreversion process, which demonstrated the potential applicability of this class of molecules.

Photoinduced Regioselective Decarbonylative and Decarboxylative C-O Bond Functionalizations: Approach toward Chemoselective Scissions of Isatoic Anhydride and Unraveling the Enroutes through Control Experiments and DFT Studies

Distinctive, green, innovative, and well-organized photoinduced (metal- or photocatalyst-free) regioselective decarbonylative and decarboxylative C-O bond functionalization protocols to access aryl 2-aminobenzoates and 2-substituted benzoxazinone derivatives in excellent yields have been devised. These are achieved through the chemoselective scission of isatoic anhydride with ketones, diaryliodonium triflate, nitroalkene, phthalazinone, and phenol derivatives, which, in turn, served as the representative "electrophilic and nucleophilic" coupling partners. Control experiments and DFT calculations reveal that electrophilic radical-bearing coupling partners specifically follow the decarbonylation pathway, while nucleophilic radical-bearing conjugates facilitate the decarboxylation process. Thus, the devised methods represent the chemoselective fragmentation of isatoic anhydride, which occurs due to the electronic nature of the coupling partners. Again, the regioselective C-O/O-C bond formation is also a novel outcome of this methodology. We have also devised a green method for synthesizing 2-aminobenzoate-subtituted paracetamol through a decarboxylation route. A fluorescence quenching study indicates that phenyl 2-aminobenzoate specifically detects Fe(II) ions, exhibiting no reactivity toward various other metal ions. Additionally, transition-metal-catalyzed C-H bond functionalization of 2-substituted benzoxazinone with phenyl vinyl sulfone was performed at ease with significant yields, which appreciated the strategy developed by us.

Ligand Effects in Carboxylic Ester- and Aldehyde-Assisted β-C-H Activation in Regiodivergent and Enantioselective Cycloisomerization-Hydroalkenylation and Cycloisomerization-Hydroarylation, and [2 + 2 + 2]-Cycloadditions of 1,6-Enynes

Herein, we report room temperature, atom-economic protocols for high regio- and enantioselective tandem cycloisomerization-hydroarylation and cycloisomerization-hydroalkenylation of 1,6-enynes leading to vicinal carba-functionalized pyrrolidines, tetrahydrofurans, and cyclopentanes. The latter steps in these processes involve carbonyl-coordination-assisted ortho-C-H activation of aromatic aldehydes and esters, and, a similar, yet rarely seen, β-C-H activation in the case of the acrylates. Synthetically useful enantioselective versions of such reactions are rare and are limited to the C2-H activation of indoles and pyrroles. A similar reaction is also observed with N-vinylphthalimide, which also has a carbonyl group suitable for C-H activation. A dibenzooxaphosphole ligand, (2S,2S',3S,3S')-MeO-BIBOP was uniquely identified as crucial to achieving the challenging regio- and enantioselectivity. This methodology gives access to substituted five-membered carbo- and heterocyclic compounds in good yields and excellent enantioselectivities under a low catalyst loading. A primary KIE of 3.5 is observed in an intermolecular competition experiment with methyl benzoate and d5-methyl benzoate, which indicates that the C-H cleavage is the turnover-limiting step of this process. Unlike the acrylates, which undergoes exclusive hydroalkenylation, a β, γ-unsaturated ester, methyl but-3-enoate, undergoes the highly enantioselective cycloisomerization-coupling sequence with a 1,6-enyne giving either a [2 + 2 + 2]-cycloaddition with (S, S)-BDPP or hydroalkenylation with (2S,2'S,3S,3'S)-MeO-BIBOP depending on the ligand employed. The (E)-configuration of the newly formed double bond at the terminal alkynyl carbon (of the starting enyne) in the hydroalkenylation product of β,γ-unsaturated ester suggests a more classical migratory insertion-β-hydride elimination route for the formation of this product.

Chiral bisphosphine Ph-BPE ligand: a rising star in asymmetric synthesis

Chiral 1,2-bis(2,5-diphenylphospholano)ethane (Ph-BPE) is a class of optimal organic bisphosphine ligands with C2-symmetry. Ph-BPE with its excellent catalytic performance in asymmetric synthesis has attracted much attention of chemists with increasing popularity and is growing into one of the most commonly used organophosphorus ligands, especially in asymmetric catalysis. Over two hundred examples have been reported since 2012. This review presents how Ph-BPE is utilized in asymmetric synthesis and how powerful it is as a chiral ligand or even a catalyst in a wide range of reactions including applications in the total synthesis of bioactive molecules.

Recent Progress in Asymmetric Domino Intramolecular Cyclization/Cascade Reactions of Substituted Olefins

The domino cyclization/coupling strategy is one of the most effective methods to produce cyclized and multi-functionalized compounds from olefins, which has attracted huge attention from chemists and biochemists especially for its considerable potential of enantiocontrol. Nowadays, more and more studies are developed to achieve difunctionalization of substituted olefins through an asymmetric domino intramolecular cyclization/cascade reaction, which is still an elegant choice to accomplish several synthetic ideas such as complex natural products and drugs. This review surveys the recent advances in this field through reaction type classification. It might serve as useful knowledge desktop for the community and accelerate their research.

Asymmetric alkene-alkene reductive cross-coupling reaction via visible-light photoredox/cobalt dual catalysis

The first example of asymmetric alkene-alkene reductive coupling is demonstrated via visible-light-fueled photoredox/cobalt dual catalysis. The desymmetrization reaction provided products (>20 examples) with up to five chiral centers in single-step operation in up to 95% yields with very high relative (>99 : 1 dr) and absolute stereochemistry (up to 98 : 2 er) control. The preliminary mechanistic investigations suggested that the critical mechanistic steps involved light-mediated controlled low-valent cobalt complex generation, oxidative ene-ene cyclization, and protonation.

Cobalt-catalyzed enantioselective desymmetrizing reductive cyclization of alkynyl cyclodiketones

A highly enantioselective cobalt-catalyzed desymmetrizing reductive cyclization of alkynyl cyclodiketones has been developed. Under mild reaction conditions by employing HBpin as a reducing agent and ferrocene-based PHOX as a chiral ligand, a series of polycyclic tertiary allylic alcohols bearing contiguous quaternary stereocenters are achieved in moderate to excellent yields with excellent enantioselectivities (up to 99%). Broad substrate scope and high functional group compatibility are observed in this reaction. A CoH-catalyzed pathway involving alkyne hydrocobaltation followed by nucleophilic addition to the C[double bond, length as m-dash]O bond is proposed. Synthetic transformations of the product are conducted to demonstrate the practical utilities of this reaction.