Activation-controlled hydrogen abstraction by benzophenone triplet

Through nanosecond flash photolysis measurements, activation energies for hydrogen abstraction by benzophenone triplet have been shown to correlate with solvent molecular structure. We demonstrate that two successive processes result in photoproduct generation. One, triplet quenching, is independent of macroscopic viscosity, and is not simply reflected in the observed quantum yield. The second process involves separation of two caged radicals, and does not occur in rigid media.     

Solvent effects in hydrogen abstraction from cholesterol by benzophenone triplet excited state

Hydrogen abstraction from the C-7 position of cholesterol (Ch) by triplet excited benzophenone (BZP) exhibits remarkable solvent-dependence in product studies. Kinetic measurements on the intramolecular version of the process in dyads containing covalently linked Ch and BZP units reveal important solvent effects and significant stereodifferentiation.     

Excited state enantiodifferentiating interactions between a chiral benzophenone derivative and nucleosides

Time-resolved measurements using nanosecond laser flash photolysis have revealed significant enantiodifferentiation in the interaction between ketoprofen (a chiral benzophenone derivative) and two relevant nucleosides, namely, thymidine and 2'-deoxyguanosine. In both cases, the highest quenching rate constants have been observed for (R)-ketoprofen, the enantiomer with lower pharmacological activity. Photoproduct studies performed in the case of thymidine suggest that the enantiodifferentiating process corresponds to a Paterno-Büchi reaction, leading to the formation of oxetanes. With 2'-deoxyguanosine, the quenching is associated with an electron-transfer process monitored through the generation of a ketyl radical.     

Stereoselective intramolecular nitrone cycloadditions to chiral allyl ethers

The Intramolecular nitrone cycloadditions to Z and E chiral allyl ethers afford annulated isoxazolidines with good to excellent stereocontrol in favour of the C-5/C-5' anti isomers. The relative stereochemistry at the stereocenters in C-3/C-4 depends on the length of the chain connecting dipole and dipolarophile.     

Photopolymerization induced by a benzophenone derivative photoinitiator

A benzophenone derivative photoinitiator, HBP-TDI-HQ-TDI-HBP （HTCTH）, was synthesized based on 4-hydroxy benzophenone （HBP）, toluene-2,4-diisocyanate （TDI） and hydroquinone （HQ）. HTCTH was a more effective photoinitiator which had longer wavelength absorption in the UV-vis absorption spectra than the low molecular counterpart benzophenone （BP）. It showed that both rate of polymerization （Rp） and final conversion （P） increased with increase of amine and HTCTH concentration in photopolymerization.     

Reactions of carbamyl radicals: intramolecular hydrogen abstraction reactions

ω-Phenylalkyl-N-methylcarbarnyl radicals undergo intermolecular addition to 3,3-dinethylbut-l-ene in preference to intramolecular hydrogen abstraction. Methyl N-(ω-phenylalkyl) carbanyl radicals and methyl N-pentylcarbamyi radicals readily abstract hydrogen through a six membered transition state or a seven membered transition state if the hydrogen is beniylic. The selectivities are interpreted in terms of the electrophilicity of the radical and the stereo-electronic requirements of hydrogen abstraction reactions.     

Facile synthesis of a chiral polymeric helix; folding by intramolecular hydrogen bonding

In a single condensation step, a poly-ureidophthalimide is synthesized, which folds into a chiral, helical architecture according to circular dichroism spectroscopy.     

Stereoselective synthesis of C-ketosides by sequential intramolecular hydrogen atom transfer-intermolecular allylation reaction

A tandem 1,5 or 1,6 hydrogen atom transfer (HAT)-radical allylation using carbohydrate models is described. The HAT reaction generated a C-glycos-1-yl radical intermediate, which added to allyltri-n-butyltin with high diastereoselectivity, to give C-ketosides with the quaternary carbon carrying two differently functionalized tethers.     

Efficient and selective photogeneration of cholesterol-derived radicals by intramolecular hydrogen abstraction in model dyads

Three model dyads have been synthesized by esterification of beta- and alpha-cholesterol (Ch) with (S)- and/or (R)-ketoprofen (Kp). The alpha-dyads are efficient photogenerators of the 7-allyl Ch radicals by intramolecular H abstraction. Subsequent cyclization via C-C coupling occurs in a stereoselective way.     

Radical polymerization behavior of ethyl ortho-formylphenyl fumarate involving intramolecular hydrogen abstraction

The radical polymerization behavior of ethyl ortho-formyl-phenyl fumarate (EFPF) using dimethyl 2,2′-azobisisobutyrate (MAIB) as initiator was studied in benzene kinetically and ESR spectroscopically. The polymerization rate (R_p) at 60°C was given by R_p = k[MAIB]^0.76[EFPF]^0.56. The number-average molecular weight of poly(EFPF) was in the range of 1600–2900. EFPF was also easily photopolymerized at room temperature without any photosensitizer probably because of the photosensitivity of the formyl group of monomer. Analysis of ¹H? and ¹³C-NMR spectra of the resulting polymer revealed that the radical polymerization of EFPF proceeds in a complicated manner involving vinyl addition and intramolecular hydrogen-abstraction. The polymerization system was found to involve ESR-observable poly(EFPF) radicals under the actual polymerization conditions. ESR-determined rate constant (2.4–4.0 L/mol s) of propagation at 60°C increased with decreasing monomer concentration, which is mainly responsible for the observed low de-pendency of R_p on the EFPF concentration. Copolymerizations of EFPF with some vinyl monomers were also examined. © 1995 John Wiley & Sons, Inc.     

Ortho effect in the Bergman cyclization: interception of p-benzyne intermediate by intramolecular hydrogen abstraction

Intramolecular hydrogen atom (H-atom) abstraction from the o-OCH3 group effectively intercepts the p-benzyne intermediate in the Bergman cycloaromatization of 2,3-diethynyl-1-methoxybenzene (1) before this intermediate undergoes either retro-Bergman ring opening or external H-atom abstraction. This process leads to the formation of a new diradical and renders the cyclization step essentially irreversible. Chemical and kinetic consequences of this phenomenon were investigated through the combination of computational and experimental studies.     

Hydrogen abstraction from ionic liquids by benzophenone triplet excited states

The activation energy for hydrogen abstraction from imidazolium-based ionic liquids is significantly higher than that observed in conventional solvents.     

Hydrogen abstraction from neurotransmitters by active oxygen species facilitated by intramolecular hydrogen bonding in the radical intermediates

The reactivity of neurotransmitters toward hydrogen abstraction by an active oxygen species (the cumylperoxyl radical) is comparable to that of a strong antioxidant such as catechin due to the strong intramolecular hydrogen bonding, which has been successfully detected by ESR.     

Stereoselective synthesis of a new perhydroindole derivative of chiral iminodiacid,a potent inhibitor of angiotensin converting enzyme

The stereoselective synthesis of N-[(s) 1 - carbethoxybutyl)] (s) alanine and of (2S, 3aS, 7aS) 2-t.butoxycarbonyl perhydroindole are described. Their coupling produces the title compound     

Second hydrogen atom abstraction by molecular ions

We report the observation of a new physical phenomenon of the addition of 2 hydrogen atoms to molecular ions thus forming [M + 2H]⁺ ions. We demonstrate such second hydrogen atom abstraction onto the molecular ions of pentaerythritol and trinitrotoluene (TNT). We used both gas chromatography mass spectrometry (GC‐MS) with supersonic molecular beam (SMB) with methanol added into its make‐up gas and electron ionization (EI) liquid chromatography mass spectrometry (LC‐MS) with SMB with methanol as the LC solvent. We found that the formation of methanol clusters resulted upon EI in the formation of dominant protonated pentaerythritol ion at m/z = 137 plus about 70% relative abundance of pentaerythritol molecular ion with 2 additional hydrogen atoms at m/z = 138 which is well above the 5.7% natural C¹³ isotope abundance of protonated pentaerythritol. Similarly, we found an abundant protonated TNT ion at m/z = 228 and a similar abundance of TNT molecular ion with 2 additional hydrogen atoms at m/z = 229. Upon the use of deuterated methanol (CD₃OD) as the solvent, we observed an abundant m/z = 231 (M + 2D)⁺ of TNT with 2 deuterium atoms. We found such abundant second hydrogen atom abstraction with butylglycolate and at low abundances in dioctylphthalate, Vitamin K3, phenazine, and RDX. At this time, we are unable to report the magnitude and frequency of occurrence of this phenomenon in standard electrospray LC‐MS. This observation could have important implications on the provision of elemental formula from mass spectra that are involved with protonated molecules. Accordingly, while accurate mass measurements can serve for the generation of elemental formula, their further support and improvement via isotope abundance analysis are questionable. Consequently, if a given compound can be analyzed by both GC‐MS and LC‐MS, its GC‐MS analysis can be superior for the provision of accurate elemental formulae if its EI mass spectrum exhibits abundant molecular ions such as with GC‐MS with SMB (also known as cold EI).     

Stereoselective synthesis of a new chiral synthon: a cyclic pseudodipeptide containing an aspartic acid derivative and l-valine

A simple stereoselective synthesis of cyclic synthons 4a,b and 10a,b, derived from an l-valine unit and an unnatural derivative of the aspartic acid, has been accomplished starting from the chiral synthon 1 and following the procedure already reported by us for the synthesis of similar compounds. These cyclic synthons are interesting substrates because they can behave as both electrophiles and nucleophiles and could be useful starting materials for the preparation of higher peptides such as, for instance, the unnatural tetrapeptides 15 and 18, through the condensation of 4a with 6a or 13 with 17, respectively.     

Stereoselective synthesis of chiral oxepanes and pyrans through intramolecular nitrone cycloaddition in organized aqueous media

[reaction: see text] A highly stereoselective surfactant-catalyzed intramolecular nitrone (formed by dehydration in water) cycloaddition in aqueous media leading to exclusive formation of a single isomer is reported. Either oxepane or pyran is formed from 3-O-allyl furanoside derivatives, which constitute the framework of a large number of biologically active compounds. Therefore, the environmentally friendly, efficient, and highly stereoselective syntheses of these chiral intermediates are still a meaningful pursuit.     

Stereoselective formal synthesis of (−)-mesembrane by intramolecular condensation of chiral amide and 1,3-cyclohexanedione moiety

A new stereoselective formal synthesis of (−)-mesembrane has been achieved by intramolecular condensation of chiral amide and 1,3-cyclohexanedione moiety. The precursor amide was readily prepared by condensation of the corresponding chiral amine and acid. Condensation provided moderate ratios of ene-amide derivatives and the following transformation of functional groups has yielded the known precursor for (−)-mesembrane, resulting in formal synthesis.