Photo-induced imino functionalizations of alkenes via intermolecular charge transfer

A catalyst-free photosensitized strategy has been developed for regioselective imino functionalizations of alkenes via the formation of an EDA complex. This photo-induced protocol facilitates the construction of structurally diverse β-imino sulfones and vinyl sulfones in moderate to high yields. Mechanistic studies reveal that the reaction is initiated with an intermolecular charge transfer between oximes and sulfinates, followed by fragmentation to generate a persistent iminyl radical and transient sulfonyl radical. This catalyst-free protocol also features excellent regioselectivity, broad functional group tolerance and mild reaction conditions. The late stage functionalization of natural product derived compounds and total synthesis of some bioactive molecules have been demonstrated to highlight the utility of this protocol. Meanwhile, the compatibility of different donors has proved the generality of this strategy.

Diastereoselective synthesis of tetrahydropyrrolo[1,2-d]oxadiazoles from functionalized Δ1-pyrrolines and in situ generated nitrile oxides

Tetrahydropyrrolo[1,2-d]oxadiazoles have been synthesized in good-to-excellent yields via the cycloaddition of nitrile oxides (in situ generated from aldoximes) to readily accessible functionalized Δ¹-pyrrolines. The reaction proceeds smoothly at room temperature in a two-phase system in the presence of sodium hypochloride as an oxidant to diastereoselectively afford pharmaceutically prospective 1,2,4-oxadiazolines fused with a five-membered ring. The reaction tolerates a broad range of substrates, including those with oxidant-sensitive functional groups and competitive reaction sites.

Recent Advances in N-O Bond Cleavage of Oximes and Hydroxylamines to Construct N-Heterocycle

Oximes and hydroxylamines are a very important class of skeletons that not only widely exist in natural products and drug molecules, but also a class of synthon, which have been widely used in industrial production. Due to weak N-O σ bonds of oximes and hydroxylamines, they can be easily transformed into other functional groups by N-O bond cleavage. Therefore, the synthesis of N-heterocycle by using oximes and hydroxylamines as nitrogen sources has attracted wide attention. Recent advances for the synthesis of N-heterocycle through transition-metal-catalyzed and radical-mediated cyclization classified by the type of nitrogen sources and rings are summarized. In this paper, the recent advances in the N-O bond cleavage of oximes and hydroxylamines are reviewed. We hope that this review provides a new perspective on this field, and also provides a reference to develop environmentally friendly and sustainable methods.

Microwave- and Thermally Promoted Iminyl Radical Cyclizations: A Versatile Method for the Synthesis of Functionalized Pyrrolines

A detailed study of iminyl radical cyclizations of O-aryloximes tethered to alkenes is reported. The reactions can be triggered by either microwave irradiation or conventional heating in an oil bath. A variety of radical traps can be employed, enabling C-C, C-N, C-O, C-S, or C-X bond formation and producing a diverse array of functionalized pyrrolines. Substrates containing an allylic sulfide furnish terminal alkenes by a tandem cyclization-thiyl radical β-elimination pathway. Cyclizations of hydroxylated substrates exhibit moderate diastereoselectivity that in some cases can partially be attributed to intramolecular hydrogen bonding. Computational studies suggested a possible role for thermodynamics in controlling the stereochemistry of cyclizations. The reaction temperature can be lowered from 120 to 100 °C by employing O-(p-tert-butylphenyl)oximes instead of O-phenyloximes as substrates, and these second-generation iminyl radical precursors can be used in a one-pot oxime ether formation-cyclization that is promoted by conventional heating. The functionalized pyrrolines obtained from these reactions can be conveniently transformed in several different ways.

Copper-catalyzed [4+2] oxidative annulation of α,β-unsaturated ketoxime acetates with ethyl trifluoropyruvate

A novel route for tandem C-N/C-O formation via copper-catalyzed [4+2] oxidative annulation of α,β-unsaturated ketoxime acetates with ethyl trifluoropyruvate to synthesize valuable trifluoromethyl-containing 2H-1,3-oxazines in moderate to good yields is developed. This procedure represents the first [4+2] oxidative annulation of oxime derivatives with activated CO bonds and provides an alternative route towards functionalized 2H-1,3-oxazines.

Synthesis of 1-pyrroline derivatives via cyclization of terminal alkynes with 2-azaallyls

An efficient transition-metal-free cyclization reaction to prepare 1-pyrroline derivatives bearing various functional groups is described. In this method, a simple combination of a base and a solvent allows the cyclization reaction of terminal alkynes and 2-azaallyls to be carried out efficiently under mild and metal-free conditions. This cyclization reaction will provide an efficient method for the synthesis of medicinally relevant polysubstituted and multifunctionalized pyrrolines.

Regioselective Synthesis of Spirocyclic Pyrrolines via a Palladium-Catalyzed Narasaka-Heck/C-H Activation/[4+2] Annulation Cascade Reaction

A novel palladium-catalyzed spirocyclization through sequential Narasaka-Heck, C-H activation and [4+2] annulation has been developed. In this reaction, cheap and readily available 2-chlorobenzoic acid or ethyl phenylpropiolate was employed as...

Copper-Catalyzed Cascade Annulation of Malonate-Tethered O-Acyl Oximes with Cyclic 1,3-Dicarbonyl Compounds for the Synthesis of Spiro-Pentacyclic Derivatives

A copper-catalyzed cascade annulation of malonate-tethered O-acyl oximes with cyclic 1,3-dicarbonyl compounds has been developed for the rapid synthesis of spiro-pentacyclic derivatives. This reaction allows the one-step formation of five C-C/N/O bonds and an angular tricyclic core under very mild conditions and shows excellent regioselectivity and stereoselectivity.

Tandem aza-Heck Suzuki and carbonylation reactions of O-phenyl hydroxamic ethers: complex lactams via carboamination

The palladium-catalysed tandem aza-Heck-Suzuki and aza-Heck-carbonylation reactions of O-phenyl hydroxamic ethers are reported. These formal alkene carboamination reactions provide highly versatile access to wide range complex, stereogenic secondary lactams and exhibit outstanding functional group tolerance and high diastereoselectivity.

Experimental and Computational Studies of Palladium-Catalyzed Spirocyclization via a Narasaka-Heck/C(sp3 or sp2)-H Activation Cascade Reaction

The first synthesis of highly strained spirocyclobutane-pyrrolines via a palladium-catalyzed tandem Narasaka-Heck/C(sp³ or sp²)-H activation reaction is reported here. The key step in this transformation is the activation of a δ-C-H bond via an in situ generated σ-alkyl-Pd(II) species to form a five-membered spiro-palladacycle intermediate. The concerted metalation-deprotonation (CMD) process, rate-determining step, and energy barrier of the entire reaction were explored by density functional theory (DFT) calculations. Moreover, a series of control experiments was conducted to probe the rate-determining step and reversibility of the C(sp³)-H activation step.

Synthesis of Functionalized Pyrrolines via Microwave-Promoted Iminyl Radical Cyclizations

O-Phenyloximes tethered to alkenes undergo 5-exo-trig iminyl radical cyclizations upon microwave irradiation. Trapping of the resulting cyclic radicals results in C-C, C-N, C-O, C-S, or C-X bond formation. Allylic sulfides undergo a tandem cyclization-thiyl radical β-elimination, affording terminal alkenes. The cyclizations exhibit a broad scope, and in some cases they are highly diastereoselective. The pyrroline adducts are versatile intermediates that can be transformed into a range of different species.

Mild and efficient copper-catalyzed oxidative cyclization of oximes with 2-aminobenzyl alcohols at room temperature: synthesis of polysubstituted quinolines

A simple and efficient ligand-free Cu-catalyzed protocol for the synthesis of polysubstituted quinolines via oxidative cyclization of oxime acetates with 2-aminobenzyl alcohols at room temperature has been developed. The presented approach provides a new synthetic pathway leading to polysubstituted quinolines with good functional group tolerance under mild conditions. Moreover, this transformation can be applied for the preparation of quinolines on a gram scale. Oxime acetates serve as the internal oxidants in the reactions, thus making this method very attractive.

Ni-Catalyzed direct iminoalkynylation of unactivated olefins with terminal alkynes: facile access to alkyne-labelled pyrrolines

The first example of iminoalkynylation of unactivated olefins with terminal alkynes was achieved by a nickel-catalyzed iminyl-radical cyclization/Sonogashira-type coupling sequence.

Copper-catalyzed regioselective [3+2] annulation of malonate-tethered acyl oximes with isatins

A copper-catalyzed regioselective [3+2] annulation of malonate-tethered acyl oximes with isatins was developed, affording valuable 2,3-dihydrooxazole-spirooxindoles in moderate to good yields with excellent diastereoselectivity.

Additive-Free Radical Cascade Reaction of Oxime Esters: Synthesis of Pyrroline-Functionalized Phenanthridines

A variety of dihydropyrrole-functionalized phenanthridines were efficiently synthesized by the metal-free, radical cascade cyclization reaction of 2-isocyanobiphenyls with γ,δ-unsaturated oxime esters. The C-N/C-C/C-C bonds were formed via the oil bath method in a one-pot procedure with broad substrate applicability. The radical process was supported by kinetic isotope effect studies and radical inhibition studies.

Metal-free synthesis of phosphinoylchroman-4-ones via a radical phosphinoylation-cyclization cascade mediated by K2S2O8

A variety of chroman-4-ones bearing phosphine oxide motifs were conveniently synthesized from readily available diphenylphosphine oxides and alkenyl aldehydes via a metal-free tandem phosphinoylation/cyclization protocol. The reaction utilizes K2S2O8 as oxidant and proceeds in DMSO/H2O at environmentally benign conditions with a broad substrate scope and afforded the title compounds in moderate yields.

Iron-catalyzed carbonylative cyclization of γ,δ-unsaturated aromatic oxime esters to functionalized pyrrolines

Herein, a new method of iron-catalyzed carbonylative cyclization of γ,δ-unsaturated aromatic oxime esters to functionalized pyrrolines has been developed. By using readily available substrates, 32 examples of functionalized pyrrolines were prepared in moderate to good yields. Notably, examples of reduction and cycloaddition reactions of the obtained product were given as well.

Visible-light-induced ligand-free RuCl3 catalyzed C-H phosphorylation in water

Visible-light-induced C-H phosphorylation of para-C_Ar-H and heteroarenes was realized using cost-effective RuCl₃ as a catalyst. The reaction conditions are green and environmentally friendly, using water as a solvent at room temperature and without ligands. A broad range of highly functional organophosphorus compounds were obtained via a cross-dehydrogenation-coupling (CDC) reaction. In addition, we also proved that RuCl₃ is a photocatalyst via its absorption spectrum and on/off light experiments.

Iron-catalyzed carbonylative cyclization of γ,δ-unsaturated aromatic oxime esters with amines

An efficient iron-catalyzed carbonylative cyclization of γ,δ-unsaturated aromatic oxime esters with amines has been developed.

Copper-catalyzed carbonylative synthesis of pyrrolidine-containing amides from γ,δ-unsaturated aromatic oxime esters

A new method of low-cost copper-catalyzed carbonylative cyclization for preparing pyrrolidine-containing amides from γ,δ-unsaturated aromatic oxime esters and amines has been described.

Recent advances in cyclization reactions of unsaturated oxime esters (ethers): synthesis of versatile functionalized nitrogen-containing scaffolds

We summarized recent advances in cyclization reactions of unsaturated oxime esters (ethers), which provide diversiform functionalized nitrogen-containing scaffolds.