Diastereoselective [2+2] photocycloaddition of polymer-supported cyclic chiral enone with ethylene

The phenylmenthyl derivative, previously shown to be very effective chiral auxiliaries in the diastereoselective [2+2] photocycloaddition of cyclic enones with the simplest olefin, ethylene, was attached to poly(ethylene glycol)-grafted Wang resin. We then investigated the diastereoselective [2+2] photocycloaddition on solid support in several solvents. As the result, we accomplished good selectivity in toluene as well as the recycle of the polymer-supported chiral auxiliary. This is the first example that bicyclo[4.2.0]octane derivative has been obtained photochemically on the solid support.     

Highly diastereoselective synthesis of bicyclo[4.2.0]octanone derivatives by the [2+2] photocycloaddition of chiral cyclohexenonecarboxylates to ethylene

The diastereoselective [2+2] photocycloaddition of a cyclohexenonecarboxylate containing (−)-8-(4-nitrophenyl)menthyl as a chiral auxiliary to ethylene gave the photocycloadduct, a bicyclo[4.2.0]octanone derivative, with a high degree of diastereoselectivity. A photoreaction, conducted in CH₂Cl₂ at −78 °C gave the corresponding photocycloadduct in 88% de. In the presence of Ti(OR)₄ or Me₃SnCl, the diastereoselectivity was increased up to 92% de.     

Ring-opening metathesis polymerization with [2+2]-crosslinking to create new materials

There are almost no examples of Grubbs’-type ring-opening metathesis polymers that have been [2+2]-crosslinked. Two new monomers, 6 and 10, were used to construct two novel linear soluble polymers 7 and 11, bearing photochemically labile moieties for [2+2]-cycloaddition reactions. These soft polymers were photocrosslinked into very hard polymers, 8 and 12. Potential advantages of these new bioinspired materials include: (1) ready processing into flat, round, and cylindrical shapes; (2) reasonably low toxicity due to trace amounts of catalyst needed; and (3) rapid rate of polymerization and crosslinking that avoids acids and bases.     

Synthetic studies of pestalotiopsin A: asymmetric synthesis of the 2-oxabicyclo[3.2.0]heptane substructure

A functionalized 2-oxabicyclo[3.2.0]heptan-3-one derivative, possessing all the skeletal carbons of pestalotiopsin A, has been synthesized. For the preparation of intermediary cyclobutane derivatives in enantioenriched form, the Lewis acid-catalyzed [2+2] cycloaddition of N-propiolated Oppolzer’s camphorsultam with dimethylketene bis(trimethylsilyl) acetal followed by a stereoselective 1,4-hydride addition/protonation, has been developed.     

Organocatalytic intermolecular [2+2] cycloaddition of norbornadienes by a stable organic radical compound

An organocatalytic [2+2] cycloaddition reaction of norbornadienes (NBDs) using catalytic amount of TEMPO was reported. Single crystal X-ray diffraction of the product revealed its detailed multicyclic structure containing a 4-membered ring, formed in intermolecular reaction. Addition of AIBN to the current catalytic system improved the product yield. Quantitative reaction of the NBD and TEMPO gave a 2:2 adduct of NBD and TEMPO, which was confirmed by HR-MS. This catalytic [2+2] addition of NBDs has great advantage in selective intermolecular coupling in comparison with [2+2] photocycloaddition.     

Studies in stereoselective [2+2]-cycloadditions with dichloroketene

During the investigation of the reaction of dichloroketene with cyclic enoxy-lactones and acyclic enoxy-ester substrates it was found that only the acylic variants effectively participated in the [2+2]-cycloaddition. Although a complete understanding of the reasons for this are lacking, molecular mechanics calculations do suggest that an out of plane twist of the cabonyl group in the acyclic compounds may be partially responsible. After screening a variety of chiral auxiliaries it was found that useful levels of diastereoselectivity (2.6-10.8:1) could be obtained in this cycloaddition reaction when (R)-2,2-diphenylcyclopentanol was used as the chiral auxiliary.     

Intermolecular [2+2] photocycloaddition of chalcones with 2,3-dimethyl-1,3-butadiene under neat reaction conditions

The intermolecular [2 + 2] photocycloaddition of chalcones with 2,3-dimethyl-1,3-butadiene under visible-light irradiation for the synthesis of cyclobutane derivatives has been developed. Without using any photosensitizer, metallic catalyst and solvent, the reaction proceeded with high regioselectivity and moderate to high stereoselectivity. Mild reaction conditions and no additives make the reaction easy to operate. Control experiments and density functional theory (DFT) computations demonstrated that the reaction takes place via visible-light activation of chalcones, which is different from the previously reported [2 + 2] cycloaddition of chalcones.     

Screening of chiral phosphines as catalysts for the enantioselective [3+2] annulations of N-tosylimines with allenic esters

The use of chiral, binaphthyl-based phosphepines as catalysts improves previous results for the enantioselective [3+2] cyclisation reactions between allenic esters and N-tosylimines, both in terms of conversion rate and enantioselectivity. Pyrrolines bearing 1-naphthyl, phenyl, o-tolyl and p-MeO-phenyl substituents on the stereogenic alpha-carbon have been obtained with enantiomeric excesses up to 64-80%.     

A [2+2+2]-cyclotrimerization approach for the synthesis of enantiopure isochromans using a carbohydrate derived dialkyne template

An easy access to enantiopure isochromans through cross alkyne trimerization of a glucose derived dialkyne was developed. One of the synthesized isochromans was converted into a novel tricyclic nucleoside by simple transformations.     

Reaction of ene-bis(phosphinylallenes): [2+2] versus [4+2] cycloaddition

Reaction of ene-bis(phosphinylallenes), derived from ene-bis(propargyl alcohols) and chlorodiphenylphosphine, was investigated. Benzene-bridged bis(phosphinylallenes) exclusively gave intramolecular [2+2] cycloadducts in the presence of dimethyl fumarate in sharp contrast to the reaction of benzene-bridged bis(sulfinylallenes), which gave the corresponding [4+2] cycloadducts. On the other hand, substituted ethylene- or five-membered heterocycle-bridged bis(phosphinylallenes) provided [4+2] cycloadducts. Reaction of benzene-bridged diallene bearing both a sulfinyl group and a phosphinyl group on the two allenyl groups was also described.     

Cyclobutane ring formation by triflic imide catalyzed [2+2]-cycloaddition of allylsilanes

Cyclobutane forming [2+2]-cycloaddition reactions of allylsilane with electron-deficient olefin is promoted by triflic imide (Tf₂NH). Triflic imide is converted in situ to silyl triflic imide (R₃SiNTf₂), which serves as the actual catalyst for this process. When these reactions take place at higher than ambient temperatures, thermodynamically more stable anti-cyclobutanes are generated preferentially by equilibration of the initially formed adducts via retro [2+2]-cycloaddition.     

Asymmetric synthesis of pyrrolo[1,2-a]indoles via organocatalytic[3 + 2] annulation of substituted 2-vinylindoles with azlactones

The chiral phosphoric acid catalyzed asymmetric[3+2]annulation of substituted 2-vinylindoles with azlactones has been established.This reaction represented a practical approach for the synthesis of structurally diverse pyrrolo[1,2-a]indoles with two vicinal stereocenters including one tetrasubstituted stereocenter in good yields and good stereoselectivities under mild conditions.     

Synthesis of (+)-(1S,2R) and (−)-(1R,2S)-2-aminocyclobutane-1-carboxylic acids

(+)-(1S,2R) and (−)-(1R,2S)-2-aminocyclobutane-1-carboxylic acids have been prepared in >97% ee and in 33% and 20% overall yields starting from a single, chiral, bicyclic compound perceived as a chiral uracil equivalent. Construction of the cyclobutane ring is achieved via a [2+2] photocycloaddition reaction of this chiral precursor with ethylene.     

Catalytic enantioselective [2+2]-cycloaddition reaction of 2-methoxycarbonyl-2-cyclopenten-1-one by chiral copper catalyst

A new catalytic system for enantioselective [2+2]-cycloaddition reaction of 2-methoxycarbonyl-2-cyclopenten-1-one with thioacetylene derivatives is described. The use of a catalytic amount (20-30 mol%) of copper(II) salt with chiral bis-pyridine ligand was found to be effective in promoting the [2+2]-cycloaddition reaction, furnishing the corresponding bicyclic compound in good yield and good enantioselectivity.     

Reactivity of allenoates toward aziridines: [3+2] and formal [3+2] cycloadditions

The reactivity of buta-2,3-dienoates toward aziridines is reported. Allenoates react as 2π-component in the [3+2] cycloaddition with the azomethine ylide generated from cis-1-benzyl-2-benzoyl-3-phenylaziridine affording 4-methylenepyrrolidines in a site-, regio-, and stereoselective fashion. Under conventional thermolysis, cis- and trans-2-benzoyl-1-cyclohexyl-3-phenylaziridines showed a different reactivity. These aziridines participate in formal [3+2] cycloadditions with allenes via C-N bond cleavage of the three-membered ring leading to functionalized pyrroles.     

Imine polymers containing chiral nanohoops in the backbone obtained through [2 + 2] cyclocondensation

A simple method for synthesis of polymers containing shape persistent imine macrocycles as nanohoops in the main chain is studied. It is based on the cyclocondensation reaction carried out in solution and room temperature of triphenylamine-based tetraaldehyde compounds with (R,R)-trans-1,2-diaminocyclohexane. The pristine polymer P1 bearing long alkyl groups is soluble during the synthesis but becomes insoluble after precipitation due to the strong and multiple C H/π and π-π stacking intermolecular interactions from arene–arene species and entanglement and interpenetrating of flexible alkyl groups inside of rigid macrocycle hollow. Polymer without any solubilizing groups ( P2 ) separates during the polymerization as an insoluble material. Both polymers are amorphous and have good thermal and environmental stability. They have a low surface area because discrete nanovoids introduced by macrocycles are disconnected in the amorphous polymers and non-accessible for gas adsorption. Polymers have inherent luminescent properties due to triphenylamine groups and chirality derived from (R,R)-conformation of the cyclohexane skeleton. In presence of picric acid (PA), the polymer fluorescence in solid state or suspension is strongly quenched, thus the polymers can work as efficient fluorescent probes toward nitrophenolic compounds. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2565–2573     

The Choice of Rhodium Catalysts in [2+2+2] Cycloaddition Reaction: A Personal Account

[2+2+2] Cycloaddition reaction is a captivating process that assembles six-membered rings from three unsaturations with complete atom economy. Of the multiple transition metals that can be used to catalyze this reaction, rhodium offers many advantages. These include high activity and versatility, but especially the ability to easily tune the reactivity and selectivity by the modification of the ligands around the metal. In this personal account, we summarize our endeavours in the development of efficient and sustainable [2+2+2] cycloaddition reactions to prepare products of interest, develop conditions in which the catalyst can be recovered and reused, and understand the mechanistic details that govern the selectivity of the processes.     

Synthesis of 2-iodoynamides and regioselective [2+2] cycloadditions with ketene

The first synthesis of 2-iodoynamides is described as well as the first [2+2] cycloadditions of ketene with iodo alkynes.     

Stereospecific [2+2] cycloaddition reactions of diphosphorus with alkenes

[2+2] Cycloaddition reactions of P₂ with alkenes were predicted to have concerted paths, that is, pseudoexcitation, distorted 2π_s+2π_s, and 2π_s+2π_a processes without any interventions of intermediates. The pseudoexcitation and/or distorted 2π_s+2π_s paths with retention of configuration of alkenes are kinetically preferred to the 2π_s+2π_a path with inversion of configuration. The reactions were predicted from the appreciable difference in the calculated enthalpies of activation to be stereospecific.