Radical Addition of Thiols to Oxa(aza)bicyclic Alkenes in Water

Engaging Isatins and Amino Acids in Multicomponent One-Pot 1,3-Dipolar Cycloaddition Reactions-Easy Access to Structural Diversity

Multicomponent reactions (MCRs) have undoubtedly emerged as the most indispensable tool for organic chemists worldwide, finding extensive utility in the synthesis of intricate natural products, heterocyclic molecules with significant bioactivity, and pharmaceutical agents. The multicomponent one-pot 1,3-dipolar cycloaddition reactions, which were initially conceptualized by Rolf Huisgen in 1960, find extensive application in contemporary heterocyclic chemistry. In terms of green synthesis, the multicomponent 1,3-dipolar cycloaddition is highly favored owing to its numerous advantages, including high step- and atom-economies, remarkable product diversity, as well as excellent efficiency and diastereoselectivity. Among the numerous pieces of research, the most fascinating reaction involves the utilization of azomethine ylides generated from isatins and amino acids that can be captured by various dipolarophiles. This approach offers a highly efficient and convenient method for constructing spiro-pyrrolidine oxindole scaffolds, which are crucial building blocks in biologically active molecules. Consequently, this review delves deeper into the dipolarophiles utilized in the 1,3-dipolar cycloaddition of isatins and amino acids over the past six years.

An Efficient Synthesis of Oxygen-Bridged Spirooxindoles via Microwave-Promoted Multicomponent Reaction

A microwave-promoted multicomponent reaction of isatins, α-amino acids and 1,4-dihydro-1,4-epoxynaphthalene is achieved under environmentally friendly conditions, delivering oxygen-bridged spirooxindoles within 15 min in good to excellent yields. The attractive features of the 1,3-dipolar cycloaddition are the compatibility of various primary amino acids and the high efficiency of the short reaction time. Moreover, the scale-up reaction and synthetic transformations of spiropyrrolidine oxindole further demonstrate its synthetic utility. This work provides powerful means to expand the structural diversity of spirooxindole as a promising scaffold for novel drug discovery.

The cobalt(ii)-catalyzed acyloxylation of picolinamides with bifunctional silver carboxylate via C–H bond activation

The cobalt(ii)-catalyzed C–H bond acyloxylation of picolinamides with bifunctional silver carboxylate has been developed. The mild and practical esterification provides an atom-economic route to access to polysubstituted naphthalene compounds.

Copper-Catalyzed Hydroamination of Oxa- and Azabenzonorbornadienes with Pyrazoles

An efficient and highly chemo- and stereoselective copper-catalyzed hydroamination of oxa- and azabenzonorbornadienes with various pyrazole derivatives is described. This catalytic process is promoted by the presence of N-heterocyclic carbene ligands and KOt-Bu under mild and simple reaction conditions, and allows for the direct synthesis of new and versatile functionalized oxa(aza)benzonorbornyl pyrazoles starting from readily available oxa(aza)bicyclic alkenes. The synthetic utility of this method was demonstrated by the transformation of the obtained products into pyrazolyl-substituted naphthalenes.

Copper-Catalyzed Cycloaddition of Heterobicyclic Alkenes with Diaryl Disulfides to Synthesize Dihydrobenzo[b]thiophene Derivatives

A novel copper-catalyzed cycloaddition of diaryl disulfides to heterobicyclic alkenes has been developed. The C-S and C-C bonds can be formed simultaneously on the C═C bond of the olefins via a single-step cycloaddition to afford a series of 2,3-dihydrobenzo[b]thiophene derivatives. This reaction exhibits excellent diastereoselectivity and relatively broad substrate scope. Various functional groups attached to the substrates are tolerated in this protocol to give the corresponding exo adducts in moderate yields.

Ruthenium-Catalyzed [2 + 2] versus Homo Diels-Alder [2 + 2 + 2] Cycloadditions of Norbornadiene and Disubstituted Alkynes: A DFT Study

The ruthenium-catalyzed [2 + 2] and homo Diels-Alder [2 + 2 + 2] cycloadditions of norbornadiene with disubstituted alkynes are investigated using density functional theory (DFT). These DFT calculations provide a mechanistic explanation for observed reactivity trends with different functional groups. Alkynyl phosphonates and norbornadiene form the [2 + 2 + 2] cycloadduct, while other functionalized alkynes afford the respective [2 + 2] cycloadduct, in excellent agreement with experimental results. The computational studies on the potential energy profiles of the cycloadditions show that the rate-determining step for the [2 + 2] cycloaddition is the final reductive elimination step, but the overall rate for the [2 + 2 + 2] cycloaddition is controlled by the initial oxidative cyclization. Two distinct mechanistic pathways for the [2 + 2 + 2] cycloaddition, cationic and neutral, are characterized and reveal that Cp*RuCl(COD) energetically prefers the cationic pathway.

Copper-catalyzed diastereoselective hydrothioetherification of oxa(aza)benzonorbornadienes

A novel copper-catalyzed hydrothioetherification of oxa(aza)bicyclic alkenes with potassium thioacetate and aryl or alkyl iodides to synthesize unsymmetrical thioethers has been developed. Notably, the reaction with complete diastereoselectivity went through a syn-selective addition process to give exo-adducts. In addition, this protocol exhibited high efficiency and good functional group tolerance to afford the target thioethers in moderate to good yields. Based on the results of mechanistic investigations, a plausible mechanism was proposed.

Cobalt(II)-Catalyzed [4+2] Annulation of Picolinamides with Alkynes via C-H Bond Activation

A cobalt(II)-catalyzed [4+2] annulation of picolinamides with alkynes via C-H bond activation has been developed. The operationally simple annulation reaction allows for the synthesis of acyl-substituted 1H-benzoquinoline bearing multiple aromatic rings (up to 96 % yield) without co-oxidant or other oxidation factors under mild conditions. Several control experiments were carried out. This practical [4+2] annulation provides an efficient route to access highly functionalized compounds.

Three-Component Cycloaddition To Synthesize Aziridines and 1,2,3-Triazolines

An efficient three-component cycloaddition of oxa(aza)bicyclic alkenes/norbornene in the presence of NaN₃ and arylsulfonyl chlorides was developed, affording the corresponding aziridine products in good yields (up to 82%) with moderate to good endo/exo selectivities (up to >99:1 endo/exo). Further studies showed that the cycloaddition of oxa(aza)bicyclic alkenes in the presence of NaN₃ and chloroalkanes could afford the exo-cycloadduct 1,2,3-triazolines in good to excellent yields (up to 95%). Compared with the existing methodologies, the current protocol demands very simple and mild reaction conditions and is a metal-free catalyzed reaction. In addition, a plausible mechanism for the cycloaddition reaction was also proposed.

1,3-Dipolar cycloaddition of nitrones to oxa(aza)bicyclic alkenes

A new 1,3-dipolar cycloaddition of oxa(aza)bicyclic alkenes with nitrones has been developed without any catalyst and additive under mild conditions. The proposed concerted mechanism is investigated by DFT calculations of the reaction pathways.