Access to Chromenopyrrolidines Enabled by Organophotocatalyzed [2 + 2 + 1] Annulation of Chromones with N-Arylglycines.

A facile approach toward chromenopyrrolidines was achieved under mild conditions via organophotocatalyzed aerobic decarboxylative [2 + 2 + 1] annulation of chromones with N-arylglycines, in which N-arylglycines perform dual roles (i.e., radical precursor and methylene provider). Mechanistic studies suggested that a Giese-type radical addition and consequent Mannich pathway were likely responsible for the annulation reaction.

A decade update on the application of ß-oxodithioesters in heterocyclic synthesis.

The diverse synthesis of heterocyclic compounds has always been one of the popular subjects of organic chemistry. To this end, great efforts have been devoted to developing new reagents and establishing new strategies and methods concerning efficiency, selectivity and sustainability. ß-Oxodithioesters and their enol tautomers (i.e., α-enolic dithioesters), as a class of simple and readily accessible sulfur-containing synthons, have been widely applied in the construction of various five- and six-membered heterocycles (e.g., thiophenes, thiopyrans, thiazoles, pyridines and quinolines) and other useful open-chain frameworks. Due to their unique chemical structures, ß-oxodithioesters bear multiple reaction sites, which enable them to participate in two-component or multicomponent reactions to construct various heterocyclic compounds. In the past decade, the application of ß-oxodithioesters in the synthesis of heterocycles has made remarkable progress. Herein, an update on the recent advances in the application of ß-oxodithioesters in the synthesis of heterocycles during the period from 2013 to 2023/06 is provided. According to the different types of rings concerning heteroatoms in products, this review is divided into five sections under discussion including (i) synthesis of sulfur-containing heterocycles, (ii) synthesis of sulfur and nitrogen-containing heterocycles, (iii) synthesis of nitrogen-containing heterocycles, (iv) synthesis of nitrogen and oxygen-containing heterocycles, and (v) modification to other open-chain frameworks.

Catalyst-free inverse-electron-demand aza-Diels-Alder reaction of 4,4-dicyano-2-methylenebut-3-enoates and 1,3,5-triazinanes: access to polysubstituted tetrahydropyridines.

An inverse-electron-demand aza-Diels-Alder reaction between 4,4-dicyano-2-methylenebut-3-enoates and 1,3,5-triazinanes under catalyst-free and additive-free conditions was developed, which provided a highly convenient and straightforward method to construct a series of polyfunctionalized tetrahydropyridines in high yields. This strategy features numerous advantages, including high efficiency, good functional group tolerance, broad substrate scope, and environmentally friendly conditions.

Recent advances in photochemical construction of aromatic C-P bonds via C-hetero bond cleavage.

Photochemical synthesis is flourishing in recent years, which has provided new and efficient methods for aromatic C-P bond formation. This review summarises the recent advances in photochemical C-P bond cross-couplings in aromatics, enriching the synthetic methods of phosphorus containing compounds. Photosensitizer-catalyzed and photosensitizer-free reactions are reviewed. Different types of bond cleavages for substrates, such as C-X (X = F, Cl, Br, I), C-N and C-O bond cleavage, are discussed in three categories.

Synthesis of Chromeno[2,3-d]pyrimidin-5-one Derivatives from 1,3,5-Triazinanes via Two Different Reaction Pathways.

1,3,5-Triazinanes, as a kind of versatile building block, are applied in the synthesis of chromeno[2,3-d]pyrimidin-5-one derivatives via two different reaction modes, which perfectly exhibits the powerful function of 1,3,5-triazinane as a three-atom synthon along with the structure variation of another substrate. The two annulation reactions proceed under mild conditions and bear broad substrate scope and high yield.

Synthesis of 5H-Chromeno[2,3-d]pyrimidin-5-one Derivatives via Microwave-Promoted Multicomponent Reaction.

A microwave-promoted multicomponent reaction of 3-formylchromones, amines, and paraformaldehyde was achieved under catalyst-free and solvent-free conditions, delivering 5H-chromeno[2,3-d]pyrimidin-5-one derivatives in good to excellent yields via an unexpected annulation pathway, which further expanded the synthetic application of paraformaldehyde as a C1 building block.

Total Synthesis of (-)-Picrinine, (-)-Scholarisine C, and (+)-5-ß-Methoxyaspidophylline.

The first asymmetric total synthesis of three picrinine-type akuammiline alkaloids, (-)-picrinine, (-)-scholarisine C, and (+)-5-ß-methoxyaspidophylline, has been accomplished. The synthesis features an efficient acid-promoted oxo-bridge ring-opening and further carbonyl O-cyclization to assemble the furoindoline scaffold, an unusual Dauben-Michno oxidation to construct the requisite α,ß-unsaturated aldehyde functionality, and a nickel-mediated reductive Heck reaction to forge the [3.3.1]-azabicyclic core.

Diversity-oriented synthesis of benzofuro[3,2-b]pyridine derivatives from aurone-derived α,ß-unsaturated imines and activated terminal alkynes.

An efficient annulation reaction of aurone-derived α,ß-unsaturated imines and activated terminal alkynes mediated by triethylamine is described, which enables the facile synthesis of 1,4-dihydrobenzofuro[3,2-b]pyridines in high yields. When the nucleophile of triethylamine was replaced with triphenylphosphine, another class of 1,4-dihydrobenzofuro[3,2-b]pyridines tethered with an additional acrylate motif were obtained instead. These two types of 1,4-dihydrobenzofuro[3,2-b]pyridines could be aromatized in the presence of DBU to afford benzofuro[3,2-b]pyridines, which could also be accessed via a one-pot procedure.

Synthesis of 1,3,4-Thiadiazoles and 1,4,2-Oxathiazoles from α-Enolic Dithioesters and Active 1,3-Dipoles.

The synthesis of two kinds of five-membered organosulfur heterocycles (i.e., 1,4,2-oxathiazoles and 1,3,4-thiadiazoles) from α-enolic dithioesters with active 1,3-dipoles (nitrile oxides and nitrilimines) generated in situ was achieved under mild reaction conditions. This transformation further expands the synthetic application of α-enolic dithioesters as the sulfur-containing building blocks.

Synthesis of 3,4-Dihydro-2H-1,3-thiazines from α-Enolic Dithioesters and 1,3,5-Triazinanes via a Formal (3 + 3) Annulation Reaction.

The synthesis of 3,4-dihydro-2H-1,3-thiazines from α-enolic dithioesters and 1,3,5-triazinanes has been achieved via a formal (3 + 3) annulation reaction under thermal conditions, where 1,3,5-triazinanes were utilized as three-atom synthons. This transformation is catalyst-free and additive-free.

Synthesis of indolizines from pyridinium 1,4-zwitterionic thiolates and propiolic acid derivatives via a formal [4 + 1] pathway.

A novel cyclization reaction of pyridinium 1,4-zwitterionic thiolates and propiolic acid derivatives mediated by triethylamine is described, which allows the facile synthesis of indolizines under mild reaction conditions. The net transformation involves an acetylide-driven formal [5 + 1] annulation reaction followed by a spontaneous ring-contraction/sulfur extrusion process of transient pyridothiazine intermediates.

Two Reaction Modes of Pyridinium 1,4-Zwitterionic Thiolates with Sulfenes: Synthesis of 3H-1,2-Dithiole 2,2-Dioxides, 1,9a-Dihydropyrido[2,1-c][1,4]thiazines, and Indolizines.

Two reaction modes of pyridinium 1,4-zwitterionic thiolates with sulfenes generated in situ from alkanesulfonyl chlorides are described with DIPEA as the base. 3H-1,2-Dithiole 2,2-dioxides could be obtained via a formal [3 + 2] pathway from alkylmethanesulfonyl chlorides, while 1,9a-dihydropyrido[2,1-c][1,4]thiazines were obtained via a stepwise [(5 + 2) - 1] pathway from phenylmethanesulfonyl chlorides. Moreover, as an application, indolizines could be accessed via a stepwise {[(5 + 2) - 1] - 1} pathway, with 1,9a-dihydropyrido[2,1-c][1,4]thiazines as the transient intermediates.

Synthesis of indolizines from pyridinium 1,4-zwitterionic thiolates and α-functionalized bromoalkanes via a stepwise [(5+1)-1] pathway.

The synthesis of pyridothiazines was achieved using pyridinium 1,4-zwitterionic thiolates and α-functionalized bromoalkanes via a formal [5+1] pathway. Further studies demonstrated that the synthesis of indolizines could be accomplished via a stepwise [(5+1)-1] pathway with unstable pyridothiazines as the transient intermediates from the same starting materials. The net transformation involves a sequence of annulation, oxidation and ring-contraction/desulfuration.

Cobalt-catalyzed 2-(1-methylhydrazinyl)pyridine-assisted cyclization of thiophene-2-carbohydrazides with maleimides: efficient synthesis of thiophene-fused pyridones.

A cobalt-catalyzed direct C-H/N-H functionalization of thiophene-2-carbohydrazides with maleimides by utilizing 2-(1-methylhydrazinyl)pyridine (MHP) as an easily removable bidentate directing group has been developed. This formal [4+2] cycloaddition has been achieved for the first time, and it provides an alternative and versatile approach to construct thiophene-fused pyridones using an inexpensive cobalt catalyst. The C-H/N-H activation cascade protocol showed a high efficiency and a broad substrate scope, and the products were obtained in good to excellent yields.

Synthesis of fully substituted pyrazoles from pyridinium 1,4-zwitterionic thiolates and hydrazonoyl chlorides via a [[3 + 3] - 1] pathway.

A novel and practical protocol for the synthesis of fully substituted pyrazoles from pyridinium 1,4-zwitterionic thiolates and hydrazonoyl chlorides in excellent yields under mild conditions is described. The transformation proceeds via an unusual [[3 + 3] - 1] pathway, which involves a formal [3 + 3] cascade cyclization followed by a spontaneous ring-contraction/sulfur extrusion reaction from 4H-1,3,4-thiadiazine intermediates.

Application of Pyridinium 1,4-Zwitterionic Thiolates: Synthesis of Benzopyridothiazepines and Benzothiophenes.

Pyridinium 1,4-zwitterionic thiolates, as a novel kind of sulfur-containing synthon, have been applied to the synthesis of 12aH-benzo[f]pyrido[1,2-d][1,4]thiazepines and benzo[b]thiophenes. Benzopyridothiazepines were produced through a 1,5-dipolar cycloaddition reaction from pyridinium 1,4-zwitterionic thiolates with arynes, whereas benzothiophenes as side products were generated via a [3 + 2] cascade cyclization reaction. The [5 + 2] reaction mode of pyridinium 1,4-zwitterionic thiolates is disclosed for the first time.

Synthesis of tetrasubstituted thiophenes via a [3+2] cascade cyclization reaction of pyridinium 1,4-zwitterionic thiolates and activated allenes.

A [3+2] cascade cyclization reaction of pyridinium 1,4-zwitterionic thiolates and activated allenes has been developed, which allows facile access to a library of tetrasubstituted thiophenes with great variety under thermal conditions.

Switchable Ring-Contractive Extrusion Reactions of 2,5-Dihydro-1,4,5-thiadiazepine S-Oxides: Entries to Pyridazines or Pyrazoles.

Switchable ring-contractive extrusion reactions of 2,5-dihydro-1,4,5-thiadiazepine S-oxides are described, which allow expedient access to pyridazines under thermal conditions or pyrazoles under Lewis acid-mediated conditions.

Pyridinium 1,4-zwitterionic thiolates as a useful class of sulfur-containing synthons: application to the synthesis of 2,5-dihydro-1,4,5-thiadiazepines.

A novel [4+3] cascade cyclization reaction of pyridinium 1,4-zwitterionic thiolates with azoalkenes derived from α-halo hydrazones in situ has been developed. This reaction was found to allow expedient access to an array of 2,5-dihydro-1,4,5-thiadiazepines. Libraries of highly functionalized sulfoxide and sulfone analogues of 2,5-dihydro-1,4,5-thiadiazepines were also obtained via selective oxidation with m-CPBA.

A 2-(1-methylhydrazinyl)pyridine-directed C-H functionalization/spirocyclization cascade: facile access to spirosuccinimide derivatives.

A cobalt-catalyzed oxidative coupling of benzoic hydrazides with maleimides by utilizing 2-(1-methylhydrazinyl)pyridine as a bidentate directing group has been developed. This C-H functionalization/spirocyclization cascade protocol shows high efficiency and remarkable functional group tolerance, and the ubiquitous spirosuccinimides were obtained in good to excellent yields with high regioselectivity. This strategy also provides a novel and efficient access to diverse symmetric and unsymmetrical bisspirosuccinimides.