Use of temporary tethers in the intramolecular [2+2] photocycloaddition reactions of tetrahydrophthalimide derivatives: a new approach to complex tricyclic lactones

The intramolecular [2+2] photocycloaddition reactions of a series of alkenols tethered to ethanolamine, l-(+)-valinol and R-(−)-2-phenylglycinol derived 3,4,5,6-tetrahydrophthalimides via a carbonate or silicon linkage have been examined. These [2+2] photocycloadditions gave the corresponding cyclobutanes in high yield with complete endo control in all cases and with diastereoselectivities as high as 8:1 with the chiral tethers. The cleavage of the temporary tethers by either desilylation or hydrolysis provided the endo diols. Cleavage of the ethanolamine linkage by reduction/hydrolysis precipitated an acid-catalysed fragmentation/ring expansion sequence to generate complex tricyclic lactones.     

Deuterium isotope effects in the meta photocycloaddition of aromatic compounds to alkenes

Secondary deuterium isotope effects were observed in the meta photocycloaddition of several aromatic compounds to cyclopentene.     

The meta photocycloaddition of anisole and benzonitrile to 1,3-dioxol-2-one

The meta photocycloadditions of benzonitrile and anisole to 1,3-dioxol-2-one both yield two major products. In both cases the adducts are those which would be expected if some form of polar species were to develop along the reaction pathway. The absence of exo adducts in the case of benzonitrile and 1,3-dioxol-2-one is discussed.     

The meta photocycloaddition of benzonitrile and α,α,α-trifluorotoluene to cyclopentene

The photocycloaddition of benzonitrile to cyclopentene yields four major cyano-substituted tetracyclo[6.3.0.0^2,11.0^3,7]undec-9-enes, all with the cyano group at positions 9 or 11. This is in agreement with a reaction pathway involving polarized structures as are the results of the irradiation of α,α,α-trifluorotoluene in the presence of cyclopentene.     

Regio- and stereo-selectivities of the ortho and meta photocycloaddition reactions of ethylenes to benzene and its simple derivatives

The photoreactions of benzene, toluene, anisole, and benzonitrile with acrylonitrile, methacrylonitrile, and vinyl acetate, and of toluene and o- and p-xylene with maleic anhydride are described. The acrylonitriles do not react with benzonitrile but yield mixtures of ortho photocycloadducts with the other arenes. Contrary to previous findings both exo and endo stereoisomers of the ortho cycloadducts of benzene and acryloitrile are formed: the reaction is selective towards the exo isomer but the stereoisomers from methacrylonitrite and benzene are formed with approximately equal efficiencies. Complex mixtures of regio- and stereoisomers of the ortho cycloadducts are formed between toluene and the acrylonitriles but their addition to anisole is more selective and in acetonitrile essentially only 1,2-attack of the ethylene on the arene is observed. The 2:1 photoadducts of maleic anhydride with toluene and o- and p-xylene reflect formation of two regio ortho photocycloadducts in each case. The variation in the ratios of these isomers with temperature and light intensity is interpreted in terms of the differing photolabilities of the 1:1 adducts and their reactivities towards the thermal addition of the second molecule of maleic anhydride. Vinyl acetate undergoes 1,2-cycloaddition to benzonitrile but with the other arenes, meta cycloadducts are favoured. These latter additions are specifically 2,6- with respect to toluene and anisole but there is little regioselectivity with respect to the ethylene although the 7-endo acetate of the meta cycloadduct with benzene does constitute 60% of the reaction mixture.     

First example of an atropselective dehydro-Diels-Alder (ADDA) reaction

A new concept of a stereoselective synthesis of axially chiral biaryls, formed in the course of the dehydro-Diels-Alder (DDA) reaction, has been disclosed. It is based on asymmetric induction of the newly formed chirality axis by a chirality center, which is present in the two synthesized DDA reactants. Depending on the different length of the linkers joining the alkyne moieties the DDA reaction may be triggered photochemically or thermally, where only the thermal variant was stereoselective.     

X-ray crystallographic analysis of N,N-diallylcoumarincarboxamides and the solid-state photochemical reaction

X-ray crystallographic analysis and the photochemical aspects of N,N-diallylcoumarincarboxamides were investigated. Irradiation of the corresponding amides promoted stereoselective intramolecular cyclobutane formation exclusively. The solid-state photoreaction of the coumarinamide without substituent on the 4-position proceeded in a crystal-to-crystal manner. On the other hand, photolysis of the amide possessing a methyl group at the 4-position also effected 2+2 cycloaddition; however, the reaction proceeded much slower. The difference in the reactivity was explainable on the basis of the molecular conformation in the crystal lattice.     

New deconjugation reaction of (E)-1-indanylidene methylarene brought by photolysis with protic acid

A new photodeconjugation reaction of (E) indanylidene methylarenes (1a-d) carried out by photolysis in the presence of protic acid are reported with 80% to >95% yields. The reaction mechanism is through the protonation of the less stable (Z) isomer to form stable indanyl cation followed by deprotonation.     

Photosensitized intramolecular cyclization of furan and non-activated alkene: pathway switching by the substituent on the furan ring

A photosensitized reaction of furan with a non-activated simple alkene was investigated. Intramolecular Diels-Alder-type cycloaddition reactions between furan and a trisubstituted alkene were found to proceed in high yield in the presence of 9,10-dicyanoanthracene under UV irradiation to afford oxabicyclo[2.2.1]heptane derivatives in high stereoselectivity when the furan was alkyl substituted. On the other hand, the aryl-substituted furan cyclized via a completely different pathway to give spirocyclic and tricyclic products.     

The fragmentation reaction of 16R-bromopregnane-3S,20S-diol

16R-Bromopregnane-3S,20S-diol reacted with potassium t-butoxide to afford androst-16-en-3S-ol in a moderate yield via fragmentation reaction. The latter is a key intermediate for the synthesis of 5α-androst-16-en-3-one, as boar sex pheromone, and other steroidal drugs. In addition, 16R,20S-epoxypregnane-3S-ol was also obtained as a major product by changing the reaction solvent.     

Unsymmetrical polyheteropolyene: a versatile building block for the preparation of pyrrolo[2,1-b]thiazoles and 2H-thiopyrano[2,3-b]pyridines

The synthesis of two classes of bisheterocyclic compounds, pyrrolothiazoles and thiopyranopyridines, is reported. The 4-dimethylamino-2-dimethylaminomethylenamino-1-thiabuta-1,3-diene 1 is used like a useful building block for the chemoselective synthesis of these heterocycles. Indeed, synthon 1 can react as thiazabutadiene or thiabutadiene form to afford either five- or six-membered ring monocyclic azadienes, themselves being precursors to the bicyclic structures. Semi-empirical calculations were undertaken to explain this efficient chemoselectivity.     

In situ continuous monitoring of chloride, nitrate and ammonium in a temporary stream: Comparison with standard methods

A multi-parameter probe was used for in situ chloride, nitrate and ammonium measurements in a temporary stream (Ribeira da Pardiela, in the South of Portugal). Comparison with standard analytical methods was performed for all elements. For chloride, the results of the probe depicted the same behaviour as that obtained with the standard method, although it is clear that the matrix effects were present. For nitrate, the results obtained with the probe were in agreement with the other standard methods used, except for samples collected during drought, when the stream water became brownish and exhaled an offensive smell, due to the decomposition of organic matter. For ammonium, surprisingly the probe show to be the best option, the phenate method being affected by matrix effects. The results still suggest an interference of the bicarbonate ion on nitrate determination, but standard additions approach was shown to minimize most of the matrix effects. Recoveries were reasonable to good for all the three anions under scrutiny.     

Fluoropropenoates as dienophiles in the Diels-Alder reaction

Several fluorinated alkenes were prepared from known ethyl (R)-2-fluoro-4,5-dihydroxyisopropylidine-2-pentenoate (1). The fluoroalkenoates were tested as dienophiles with several dienes and showed cycloaddition only with the very reactive diene, 1,3-diphenylisobenzofuran (5). Fluorobutenolide 2 reacted with 5 to produce the endo-syn adduct 9 in a highly stereoselective manner.     

Chemoenzymatic epoxidation of alkenes based on peracid formation by a Rhizomucor miehei lipase-catalyzed perhydrolysis reaction

A chemoenzymatic and selective method for the epoxidation of a series of cyclic and linear alkenes is described. Epoxides have been obtained in moderate to excellent conversions under mild reaction conditions through a two-step sequence, carried out in one-pot. This chemoenzymatic approach is based on a Rhizomucor miehei lipase-catalyzed perhydrolysis reaction to form the corresponding peracid, and subsequent epoxidation of the corresponding alkenes. Reaction parameters with influence in the biotransformation have been optimized specially focusing in the efficient enzymatic peracid formation by means of the correct choice of solvent, oxidant, and peracid precursor. This chemoenzymatic approach has been efficiently applied for the first time, in the regioselective chemical oxidation of (S)-carvone and limonene, both showing an opposite behavior for the oxidation of the internal and external C–C double bond, respectively.     

New insights into the reaction of t-butylhydroperoxide with dichloro- and dimethyl(dioxo)molybdenum(VI)

Lewis-base adducts of dichlorodioxomolybdenum(VI) and dimethyldioxomolybdenum(VI) react in an equilibrium reaction with excess t-butylhydroperoxide (TBHP) under the formation of a seven-coordinated molybdenum(VI) complexes displaying a η¹-alkylperoxo-ligand. HCl/CH₄ elimination or the protonation of the Lewis-base ligand is not observed, the TBHP hydrogen atom is instead transferred to one of the terminal oxo ligands under the formation of a molybdenum bound OH moiety. The peroxo species is assumed to be the active catalyst in olefin epoxidation.     

Femtosecond photoelectron spectroscopy of trans-stilbene above the reaction barrier

Femtosecond photoelectron spectroscopy was employed to study the excitation of trans-stilbene above the isomerization reaction barrier. Apart from the S₁ contribution, evidence of a second electronic state is found based on two different transients measured across the photoelectron spectrum. Time Dependent Density Functional Theory calculations on S₀, S₁, S₂ and D₀, together with simulations of the electron energy distribution support the experimental findings for selective photoelectron energies of the S₀, S₁,… electronic states.     

Synthesis of benzo[c]fluorenone through a one-pot cascade reaction using inden-1-one derivatives

A novel one-pot thermal cycloaddition of two indenones followed by a decarbonylation and dehydrogenation cascade afforded benzo[c]fluorenones regioselectively. Various substituted indenone derivatives were converted into their corresponding benzo[c]fluorenones in good to excellent yields.     

Total synthesis and absolute stereochemistry of (9R,10S)-epoxyheptadecan-4,6-diyn-3-one, a diacylglycerol acyltransferase inhibitor from Panax ginseng

Asymmetric synthesis of four possible stereoisomers of (9,10)-epoxyheptadecan-4,6-diyn-3-one was accomplished, and the absolute configuration of the naturally occurring (9R,10S)-epoxyheptadecan-4,6-diyn-3-one (1) was elucidated.     

Stereoselective transformation of easily available (2Z,4E)-2,4-alkadien-1-ols into (E)-3-alkene-1,2,5-triol derivatives

Synthesis of 3-alkenyl-1,2,5-triol derivatives, potentially useful intermediates in organic synthesis, is established, which constitutes stereoselective epoxidation of the hitherto hardly accessible dienyl alcohols of the cis,trans stereochemistry followed by Pd-catalyzed reaction with AcOH.     

The nitroalkene showing dual behaviors in the same reaction system

The nitroalkene moiety of 1-methyl-3,6,8-trinitro-2-quinolone showed dual behaviors in the same reaction system, namely electron-poor heterodiene and dienophile, and affording polycyclic products.