Abstract-The synthesis of a large variety of hyperenergetic molecules such as the 1,2-dioxetanes, u-peroxylactones, 3-imino-l, 2-dioxetanes, benzo-l,4-dioxin dioxetanes, bisdioxetanes, u-pyrone endoperoxides and cyclic peroxalates are presented. The utilization of these unique substances for the thermal generation of electronically excited states is reviewed, with special reference to the total excitation yield ($T+S) and spin state selectivities ($T/$S), The physical and chemical methods for the determination of excitation yields are discussed and scrutinized for their reliability and effectiveness, Mechanistic problems such as energy transfer, heavy atom effects, electron exchange, etc. are analyzed. The biological implications of such "high energy" molecules is emphasized.
Mercury (Hg) has been identified as one of the most toxic nonradioactive materials known to man. Although mercury is a naturally occurring element, anthropogenic mercury is now a major worldwide concern and is an international priority toxic pollutant. It also comprises one of the primary constituents of dental amalgam fillings. Even though dental mercury amalgams have been used for almost two centuries, its safety has never been tested or proven in the United States by any regulatory agency. There has been an ongoing debate regarding the safety of its use since 1845, and many studies conclude that its use exposes patients to troublesome toxicity. In this review, we present in an objective way the danger of dental amalgam to human health based on current knowledge. This dilemma is addressed in terms of an integrated toxicological approach by focusing on four mayor issues to show how these interrelate to create the whole picture: (1) the irrefutable constant release of mercury vapor from dental amalgams which is responsible for individual chronic exposure, (2) the evidence of organic mercury formation from dental amalgam in the oral cavity, (3) the effect of mercury exposure on gene regulation in human cells which supports the intrinsic genetic susceptibility to toxicant and, finally, (4) the availability of recent epidemiological data supporting the link of dental amalgams to diseases such as Alzheimer’s and Parkinson.
The diastereoselectivity in the ene reactions of triazolinediaries) display a very low diastereoselectivity. No selectivity is observed in the singlet-oxygen ene reaction with dimethylonone and singlet oxygen with chiral oxazolidine or oxazolidinone derivatives of tiglic acid depends both on the chiral au-xazolidine amides of tiglic acid. These stereochemical results are rationalized in terms of the differences in the steric dexiliary used and the size of the attacking enophile. While chiral 2,2-dimethyloxazolidine amides of tiglic acid give with mand of the singlet oxygen and TAD enophiles rather than electronic factors. triazolinediones TAD only one single ene adduct, the corresponding chiral oxazolidinone derivatives (EvansЈ auxiliScheme 1. Preferred conformation A (syn/s-cis) versus B (anti/s-cis)The ene [1] reaction of triazolinediones (TAD) [2] and sinand favored electrophilic attack for N-acryloyl-2,2-di-[3] of alkenes with allylic hydrogens has methyloxazolidines; metal-locked conformation C of attracted much attention in the last years from both the N-acryloyloxazolidinones synthetic [4] and mechanistic [5] points of view. A problem that remains, however, is the stereochemical control of the new stereogenic center that is formed in this process.[6] In the present communication we report an unprecedented asymmetric ene reaction [7] of chiral amides of tiglic acid with 2,2-dimethyloxazolidines as chiral auxiliaries.[8] These auxiliaries are readily prepared in two steps from commercially available amino alcohols and are easily removed from the resulting product by acid hydrolysis. The stereoselectivity with these auxiliaries is based on the conformational control of the amide linkage (Scheme 1).[8c]In α,β-unsaturated acryloyl amide derivatives, the amide functionality predominantly occupies the syn/s-cis conformation A to minimize steric hinderance such that one of the diastereotopic faces of the reacting π system may be effectively shielded by the proximate substituent at the 4 position (Scheme 1). The related 2-oxazolidinones C are (phenyl) or MTAD (methyl) to give the corresponding ene well-known as EvansЈ chirality-controlling auxiliaries, [9] products 2 or 3 in 65Ϫ81% yield.13 C-NMR (63 MHz) specwith which highly efficient chiral induction has been troscopy unequivocally showed that only one diaachieved in asymmetric reactions when a Lewis acid or a stereomeric ene adduct (d.r. > 98:2) was formed (Scheme 2). metal ion is utilized for fixing the amide conformation.In contrast, the photooxygenation of substrates (S)-1 at 0°C in CCl 4 for 48 h with tetraphenylporphine (TPP) as The preparation of α,β-unsaturated tiglic acid amides (S)-1 derived from oxazolidines is straightforward and fol-sensitizer afforded a 50:50 mixture of the diastereomeric allylic hydroperoxides. These labile hydroperoxides were relowed analogous literature procedures.[8d] N-Tigloyl amides (S)-1 were allowed to react with the triazolinediones PTAD duced with triphenylphosphine at 0°C in dichloromethane Eur.
On heating hexamethylbicyclo-[2,2,0]-hexa-2,5,-diene (hexamethyldewarbenzene, wish to present the first application of this unique and novel mechanistic tool to the thermal isomerization of hexamethyldewarbenzene (11) and hexamethylprismane (111). In Figure I the thermograms for the isomerizations I11 ---f I + I1 (solid curve) and I1 -+ I (broken curve) are illustrated. The rate constant for the unimolecular reaction is conveniently given by the quotient of the ordinate (dH/dt) and the fractional area under the peak ( A -a) at a particular temperature in the thermogram, that is k( T ) = (dH/dt)/(A -a ) [S]. From
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