Conjugated Dienyne‐Imides as Robust Precursors of 1‐Azatrienes for 6π Electrocyclizations to Furo[2,3‐b]dihydropyridine Cores

A novel strategy to generate functionalized 1‐azatriene intermediates for 6π electrocyclizations was developed by using readily accessible dienyne‐imides and various terminal olefins under Pd^II catalysis. Taking advantage of the sequential cooperation between preloaded and incorporated functional handles at 1,3‐dien‐5‐yne skeletons, this method not only enables the selective generation of putative 1‐azatrienes but significantly accelerates their thermal 6π‐electrocyclic ring‐closure processes to a series of highly substituted furo[2,3‐b]dihydropyridine derivatives in good yields.     

Library‐directed Solution‐ and Solid‐phase Synthesis of 2,4‐Disubstituted Pyridines: One‐pot Approach through 6 π‐Azaelectrocyclization

An efficient one‐pot synthetic procedure for the synthesis of 2,4‐disubstituted pyridines has been successfully established. The method proceeds through a 6π‐azaelectrocyclization‐aromatization sequence. Using this method, a wide variety of pyridine structures substituted at the 2‐position have been rapidly constructed from vinyl stannanes, vinyl iodide, sulfonamide, and a palladium catalyst. The method was further applied to the solid‐phase synthesis wherein the use of a “traceless” sulfonamide linker enabled the rapid preparation of a small library of pyridines with high purity, without any chromatographic separation.     

Zinc‐Catalyzed Alkyne Oxidation/CH Functionalization: Highly Site‐Selective Synthesis of Versatile Isoquinolones and β‐Carbolines

An efficient zinc(II)‐catalyzed alkyne oxidation/C? H functionalization sequence was developed, thus leading to highly site‐selective synthesis of a variety of isoquinolones and β‐carbolines. Importantly, in contrast to the well‐established gold‐catalyzed intermolecular alkyne oxidation, over‐oxidation can be completely suppressed in this system and the reaction most likely proceeds by a Friedel–Crafts‐type pathway. Mechanistic studies and theoretical calculations are described.     

Asymmetric Triple Relay Catalysis: Enantioselective Synthesis of Spirocyclic Indolines through a One‐Pot Process Featuring an Asymmetric 6π Electrocyclization

A rare example of a one‐pot process that involves asymmetric triple relay catalysis is reported. The key step is an asymmetric [1,5] electrocyclic reaction of functionalized ketimines. The substrates for this process were obtained in situ in a two‐step process that involved the hydrogenation of nitroarenes with a Pd/C catalyst to yield aryl amines and their subsequent coupling with isatin derivatives in a Brønsted acid catalyzed ketimine formation reaction. The electrocyclization was catalyzed by a bifunctional chiral Brønsted base/hydrogen bond donor catalyst. The one‐pot process gave the desired products in good yields and with excellent enantioselectivity.     

Transition from π Radicals to σ Radicals: Substituent‐Tuned Cyclization of Hydrazonyl Radicals

Hydrazonyl radicals are known for their π‐electronic structures; however, their σ‐electronic structures have not been reported as yet. Herein, we show that readily accessible β,γ‐ and γ,δ‐unsaturated N‐trichloroacetyl and N‐trifluoroacetyl hydrazones can be conveniently converted into hydrazonyl σ radicals, which subsequently undergo 5‐exo‐trig radical cyclization at the N¹ or N² atom to form pyrazolines and azomethine imines, respectively.     

Copper‐Catalyzed Amine–Alkyne–Alkyne Addition Reaction: An Efficient Method For the Synthesis of γ,δ‐Alkynyl‐β‐amino Acid Derivatives

A simple and efficient method for the synthesis of γ,δ‐alkynyl‐β‐amino acid derivatives by a copper‐catalyzed three‐component amine–alkyne–alkyne addition reaction was developed. Various γ,δ‐alkynyl‐β‐amino acid derivatives were synthesized in moderate to good yields in one step. With chiral prolinol derivatives employed as the amine component, excellent diastereoselectivities (up to >99:1 diastereomeric ratio (dr)) were obtained. The scope of the reaction and further transformations of the resulting amino acid derivatives, such as deprotection and cyclization are also described.     

A Visible Light Photocatalytic Cross‐Dehydrogenative Coupling/Dehydrogenation/6π‐Cyclization/Oxidation Cascade: Synthesis of 12‐Nitroindoloisoquinolines from 2‐Aryltetrahydroisoquinolines

A visible light‐induced photocatalytic dehydrogenation/6π‐cyclization/oxidation cascade converts 1‐(nitromethyl)‐2‐aryl‐1,2,3,4‐tetrahydroisoquinolines into novel 12‐nitro‐substituted tetracyclic indolo[2,1‐a]isoquinoline derivatives. Various photocatalysts promote the reaction in the presence of air and a base, the most efficient being 1‐aminoanthraquinone in combination with K₃PO₄. Further, the 12‐nitroindoloisoquinoline products can be accessed directly from C1‐unfunctionalized 2‐aryl‐1,2,3,4‐tetrahydroisoquinolines by extending the one‐pot protocol with a foregoing photocatalytic cross‐dehydrogenative coupling reaction, resulting in a quadruple cascade transformation.     

Synthesis of Diversely Functionalized Oxindoles Enabled by Migratory Insertion of Isocyanide to a Transient σ‐Alkylpalladium(II) Complex

Palladium‐catalyzed intramolecular carbopalladation of N‐aryl acrylamides followed by migratory insertion of an isocyanide‐coordinated C(sp³)?Pd intermediate afforded an alkylimidoyl?Pd^II complex, which can be intercepted by a nucleophile, including heteroarenes. In addition to amides, the alkylimidoyl?Pd^II complex was successfully converted into esters, ketones, and bis‐heterocyclic compounds. An unprecedented palladium‐catalyzed enantioselective domino process involving isocyanide was also documented.     

Synthesis of ent‐Ketorfanol via a C–H Alkenylation/Torquoselective 6π Electrocyclization Cascade

The asymmetric synthesis of ent‐ketorfanol from simple and commercially available precursors is reported. A Rh^I‐catalyzed intramolecular C? H alkenylation/torquoselective 6π electrocyclization cascade provides a fused bicyclic 1,2‐dihydropyridine as a key intermediate. Computational studies were performed to understand the high torquoselectivity of the key 6π electrocyclization. The computational results demonstrate that a conformational effect is responsible for the observed selectivity. The ketone functionality and final ring are introduced in a single step by a redox‐neutral acid‐catalyzed rearrangement of a vicinal diol to give the requisite carbonyl, followed by intramolecular Friedel–Crafts alkylation.     

1,4‐Dihydropyrrolo[3,2‐b]pyrrole and Its π‐Expanded Analogues

Various approaches to the synthesis of 1,4‐dihydropyrrolo[3,2‐b]pyrroles are summarized. Many two‐ and three‐step reaction sequences have been developed, and have allowed access to a broad variety of structures, including not only the parent 1,4‐dihydropyrrolo[3,2‐b]pyrroles, but also their π‐expanded analogues. The newest approaches are critically compared with older strategies. The reactivity of these compounds is also reviewed, with special emphasis on electrophilic aromatic substitution. The synthesis of indolo[3,2‐b]indole derivatives has been the subject of intense investigation. Overall, a few interesting and ingenious approaches toward these ladder‐type heteroacenes have been proposed, reaching total yields in the region of 30 %. Finally, the optical, electrochemical, and other physicochemical properties are presented in the broader perspective of heteropentalenes. The parent 1,4‐dihydro‐pyrrolo[3,2‐b]pyrroles constitute the most electron‐rich, simple, aromatic heterocycles, and their simple derivatives and π‐expanded analogues possess strong violet, blue, or green fluorescence both in solution and in the solid state.     

Visible Light Activation of Boronic Esters Enables Efficient Photoredox C(sp2)–C(sp3) Cross‐Couplings in Flow

We report herein a new method for the photoredox activation of boronic esters. Using these reagents, an efficient and high‐throughput continuous flow process was developed to perform a dual iridium‐ and nickel‐catalyzed C(sp²)–C(sp³) coupling by circumventing solubility issues associated with potassium trifluoroborate salts. Formation of an adduct with a pyridine‐derived Lewis base was found to be essential for the photoredox activation of the boronic esters. Based on these results we were able to develop a further simplified visible light mediated C(sp²)–C(sp³) coupling method using boronic esters and cyano heteroarenes under flow conditions.