Acid-induced rearrangement of cycloadducts from N-aryl itaconimides and 1,3-diphenylisobenzofuran

Treatment of several Diels–Alder adducts of N-aryl itaconimides and 1,3-diphenylisobenzofuran with a strong acid triggers a skeletal rearrangement resulting in 2-aryl-6,10b-diphenylbenzo[h]isoquinoline-1,3(2H,10bH)-diones.     

1,3-Dipolar cycloadditions of acetylenic sulfones in solution and on solid supports

Several representative acetylenic sulfones were immobilized on a polymer support derived from Merrifield resin by means of ester linkers that were used to couple free carboxylic acid groups on the solid support with benzylic hydroxyl functions on the arylsulfonyl moieties of the acetylenes. Several examples of reversed ester linkers, using Merrifield resin directly, were also successfully prepared. The 1,3-dipolar cycloadditions of the solid-supported acetylenic sulfones were investigated with a series of 1,3-dipoles, including benzyl azide, ethyl diazoacetate, diazomethane, as well as representative nitrile oxides, nitrile imines, nitrile ylides, nitrones, azomethine imines, azomethine ylides, munchnones, and sydnones. In general, analogous cycloadditions were also performed with acetylenic sulfones in solution phase for comparison. The cycloadditions typically afforded good to excellent yields of the desired products in both solution and solid phase, although the latter reactions sometimes required more vigorous conditions. Except in the case of benzyl azide and diazo compounds, where mixtures of regioisomers were obtained, the other 1,3-dipoles reacted with high regioselectivity and afforded essentially unique regioisomers. Cleavage of the products from the resin was smoothly effected by alkaline hydrolysis, while several attempts at reductive desulfonylation with sodium amalgam or samarium diiodide-HMPA resulted in N-O or C-O scission, in addition to cleavage from the polymer. The method provides access to a number of important classes of heterocycles, including variously substituted and functionalized triazoles, pyrazoles, 1,2-oxazoles, pyrroles, as well as their dihydro and bicyclic analogues. The success of the cycloadditions on polymer supports paves the way to future investigations of sequential transformations leading to libraries of useful heterocycles.     

Ring-rearrangement metathesis of nitroso Diels-Alder cycloadducts

Strained nitroso Diels–Alder bicyclo[2.2.1] or [2.2.2] adducts functionalized with alkene side chains of diverse length undergo a ring‐rearrangement metathesis process with external alkenes and Grubbs II or Hoveyda–Grubbs II ruthenium catalysts, under microwave irradiation or classical heating, to deliver cis‐fused bicycles of various ring sizes, which contain a N? O bond. These scaffolds are of synthetic relevance for the generation of molecular diversity and to the total synthesis of alkaloids. The observation of unexpected reactions, such as epimerization or one‐carbon homologation of the alkene side chain, is also reported.     

Additions of thiols to acetylenic sulfones

Thiol addition to acetylenic sulfones in the 1 - methylsulfonyl - 2 - arylethyne series has been investigated. In analogy with earlier reports on thiol additions to arylsulfonyl - 1 - alkynes no violations of the trans-addition rule have been observed. Besides the expected nucleophilic attack on the β-C-atom, considerable α-addition occurred.     

Copper-promoted rearrangement of 1,3-cyclohexadiene-acylnitroso cycloadducts

The ring opening of [2.2.2]-acylnitroso cycloadducts was obtained for the first time by means of a cationic rearrangement promoted by Cu(OTf)₂-PPh₃ under homogeneous and polymer-supported conditions.     

Cyclizations and cycloadditions of acetylenic sulfones on solid supports

Acetylenic sulfones attached to solid supports by means of ester linkers were employed in a variety of cyclization and cycloaddition reactions, followed by cleavage of the products from the resin by ester hydrolysis or reductive desulfonylation.     

Retro iminonitroso Diels-Alder reactions: interconversion of nitroso cycloadducts

Retro iminonitroso Diels-Alder reactions were investigated in both solution and solid phase. In thermal or Cu(I)-mediated reactions, interconversion of various nitroso cycloadducts occurred in the presence of separate dienes. Up to 99% of conversion was observed. Use of chiral ligands in the Cu(I)-mediated reactions gave new cycloadducts enantioselectively.     

Synthesis of 1,3-dioxolanes from acetylenic 1,2,5-triols

The reaction of acetylenic 1,2,5-triols with vinyl butyl ether in the presence of catalytic amounts of hydrochloric acid leads to the formation of 4-(3-hydroxy-1-alkynyl)-1,3-dioxolanes.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1334–1335, October, 1988.     

Selective synthesis of multiply substituted 7-norbornenone derivatives or Diels-Alder cycloadducts from 1,2,3,4-tetrasubstituted 1,3-butadienes and maleic anhydride with or without Lewis acids

Reactions of 1,2,3,4-tetrasubstituted 1,3-butadienes with maleic anhydride and other dienophiles were investigated with or without addition of Lewis acid. When the silylated 1,3-butadienes, such as 1,4-bis(trimethylsilyl)-1,3-butadienes or 1-trimethylsilyl-1,3-butadienes, were treated with maleic anhydride in the presence of 2 equiv of AlCl₃, multi-substituted 7-norbornenones of well-defined exo,exo-disubstituted patterns were produced by an unprecedented and synthetically useful tandem process. Although some tetrasubstituted 1,3-butadienes could react directly with maleic anhydride under relative harsh conditions to afford Diels-Alder cycloadducts, the reactions, in the presence of 1 equiv of AlCl₃, afforded the corresponding cycloadducts in higher yields under mild conditions. These results showed that the size and substitution pattern of substituents on the butadienyl skeleton played a very important role in the reactivity of butadienes as a partner for the Diels-Alder reaction and Lewis acid could promote and/or realize the process of a Diels-Alder reaction.     

High triplet yield from singlet fission in a thin film of 1,3-diphenylisobenzofuran

Direct observation of triplet absorption and ground-state depletion upon pulsed excitation of a polycrystalline thin solid film of 1,3-diphenylisobenzofuran at 77 K revealed a 200 ± 30% triplet yield, which was attributed to singlet fission.     

Carbon-oxygen bond formation in electron-impact ions of acetylenic sulfoxides and sulfones

Migration of the ethynyl group, from sulfur to oxygen in acetylenic sulfoxides and sulfones leads to several documented examples of carbon-oxygen bond forming rearrangement reactions.     

Photobleaching of 1,3-diphenylisobenzofuran by novel phthalocyanine dye derivatives.

As part of a wider programme to identify novel photosensitizers for photodynamic therapy, the ability of a number of phthalocyanine dyes, including some novel copper phthalocyanine derivatives with a range of water solubilities, to produce potentially cytotoxic species in solution was examined. The experiments were performed in dimethylformamide using 1,3-diphenylisobenzofuran (DPIBF) as the scavenger. The study revealed that all the dyes tested produced DPIBF photobleaching on illumination in vitro, but with widely different (greater than 12x) rates. The possible correlation of DPIBF photobleaching rates with a number of the dyes' properties is discussed.     

Kinetics of the reaction between 1,3-diphenylisobenzofuran and nitrogen dioxide studied by steady-state fluorescence

1,3-Diphenylisobenzofuran (DPBF) is a fluorescent molecule which is believed to react highly specifically toward reactive oxygen species such as singlet oxygen (¹O₂), hydroxy (HO^·), alkyloxy (RO^·), and alkylperoxy (ROO^·) radicals. In all cases the reaction product is 1,2-dibenzoylbenzene. In order to prove that DPBF gives the same product in contact with reactive nitrogen species, its reaction with nitrogen dioxide radical has been studied in 2,2,4-trimethylpentane using the steady-state fluorescence method and mass spectrometry. The progress of the studied reaction was measured by observation of changes in fluorescence intensity of DPBF after addition of nitrogen dioxide (NO₂). The rate constants of DPBF fluorescence decay affected by NO₂ have been determined. Experiments were conducted over the temperature range of 13–37 °C and for NO₂ concentrations from 0.02 to 0.14 mmol dm^?3. It has been found that the reaction between 1,3-diphenylisobenzofuran and nitrogen dioxide proceeds in two steps. The first step is a very rapid reaction whose rate could not be measured under established experimental conditions. The second step is slower. The reaction product was identified by registration of mass spectra. The probable reaction mechanism is proposed.     

Random copolymerization of acetylenic monomers and 1,3-dienes

Copolymerization of acetylenic monomers and 1,3-dienes was carried out by use of nickel naphthenate–diethylaluminum chloride catalyst. The molecular weight of the copolymers is rather low, and the copolymers are suitable as prepared for direct use as coating vehicles. In the system of acetylene and 1,3-dienes, the order of the copolymerization rate decreases in the following order: butadiene > isoprene > 2,3-dimethylbutadiene > chloroprene. 1,3-Dienes substituted at 1-and/or 4-position were scarcely copolymerized with acetylene. Methylacetylene and dienes tend to form cyclized copolymers. In the system of phenylacetylene and dienes, polyphenylacetylene was the main product; the copolymer was not obtained. The copolymer composition and the sequence distribution of linear copolymers were evaluated by ¹H-NMR spectra. Comparison of dyad fractions of copolymers evaluated from NMR spectra and those calculated by assuming random polymerization indicated that the copolymers of acetylene and dienes were random, and that the copolymers of methylacetylene and dienes were somewhat blocky. The coordination of monomers on the catalyst may play an important role in controlling the copolymerizability.     

Competing Diels-Alder reactions of activated nitroethylene derivatives and [3,3]-sigmatropic rearrangements of the cycloadducts

Diels-Alder reaction of nitroethylene derivatives with cyclohexa-1,3-diene afforded three pericyclic products some of which could be converted to others via a new [3,3]-sigmatropic rearrangement or via a Claisen rearrangement.