Search

Oxidative functionalization of 3H-pyrazol-3-ones, isoxazol-5(2H)-ones, pyrazolidine-3,5-diones, and barbituric acids by malonyl peroxides results exclusively in C−O coupling products. Traditional hydroxylation, formation of carbonyl groups, or oxidative destruction of the heterocyclic ring are not observed. Under optimized reactions conditions – fluorinated alcohols as activating medium and at room temperature (20 – 25 °C) – the selective C−O coupling proceeds in high yields (up to 94 %). The oxidative insertion into the enolizable C−H bond of the substrate is mechanistically viewed as a nucleophilic attack by the heterocycle onto the electrophilically activated malonyl peroxides. For heterocyclic substrates with an active methylene group - 3H-pyrazol-3-ones, isoxazol-5(2H)-ones, and barbituric acids - both C−H bonds are oxidized to afford double oxidative C−O coupling products in good yields (up to 72 %).

The lanthanide-catalyzed oxidative C–O coupling of α-substituted 1,3-dicarbonyl compounds with diacyl peroxides (act both as oxidant and oxygen substituent) affords oxygen-functionalized adducts in up to 94% yield. The products of this convenient and efficient transformation serve as potentially valuable precursors for the synthesis of natural products and pharmaceuticals.

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 dark and photosensitized effects of alloimperatorin methyl ether 1 (hereafter simply alloimperatorin) and its photooxygenation product alloimperatorin hydroperoxide 2 were investigated on human erythrocytes. The results reveal that the furocoumarin 1 photosensitizes efficiently the hemolysis of erythrocytes. The rate of photohemolysis increases on raising the temperature of the postirradiated incubation from 4°C to 37°C. Thermal activation of the photohemolysis and inhibition by 2,6-di-tert-butyl-p-cresol (BHT) suggest that the furocoumarin 1 photosensitizes lipid peroxidation, increasing permeability in the erythrocyte membrane. The hydroperoxide 2 induces dark and photosensitized hemolysis more efficiently than the furocoumarin 1. The rate of hemolysis induced by 2 increases with the incubation temperature and decreases in the presence of tert-butanol and BHT. The hydroperoxide 2 photosensitizes the formation of lipid peroxidation products as shown by the reaction with thiobarbituric acid. This process is diminished by BHT. Our data imply that the photohemolysis sensitized by the furocoumarin 1 is caused by the in situ-formed photooxygenation product 2. Such hydroperoxides are potent hemolytic agents in the dark and especially on photosensitization.

The lanthanide-catalyzed oxidative C-O coupling of 1,3-dicarbonyl compounds with diacyl peroxides, specifically the cyclic malonyl peroxides, has been developed. An important feature of this new reaction concerns the advantageous role of the peroxide acting both as oxidant and reagent for C-O coupling. It is shown that lanthanide salts may be used in combination with peroxides for selective oxidative transformations. The vast range of lanthanide salts (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Y) catalyzes oxidative C-O coupling much more efficiently than other used Lewis and Bronsted acids. This oxidative cross-coupling protocol furnishes mono and double C-O coupling products chemo-selectively in high yields with a broad substrate scope. The double C-O coupling products may be hydrolyzed to vicinal tricarbonyl compounds, which are otherwise cumbersome to prepare. Based on the present experimental results, a nucleophilic substitution mechanism is proposed for the C-O coupling process in which the lanthanide metal ion serves as Lewis acid to activate the enol of the 1,3-dicarbonyl substrate. The side reactions-chlorination and hydroxylation of the 1,3-dicarbonyl partners-may be minimized under proper conditions.

The correlations between synaptophysin (SYP) plasma levels and the brain neurotransmission activity are still not strictly identified. However, the efficiency of neurotransmission depends, inter alia, on the age, hormonal status, and coexistence of a low-grade systemic inflammation (LGSI) which is regarded as a pathogenic link with obesity and insulin resistance, atherogenesis and aging per se.The aim of this study was to investigate the associations between synaptophysin serum levels and age, LGSI indices, homocysteine and selected hormonal parameters (dehydroepiandrosterone and its sulfate, free-testosterone, SHBG) and the prevalence of metabolic syndrome (MS) in men over the age of 40.After randomization, 157 male volunteers aged 40-80 years were included in a retrospective study. MS was diagnosed according to the International Diabetes Federation criteria. For the diagnosis of late-onset hypogonadism (LOH) we adopted the criteria proposed by the European Male Aging Study (EMAS).Synaptophysin plasma concentrations in respondents decreased with age, but only between the ages of 40 to 70 years. There were no differences in SYP plasma concentrations in men suffering from MS compared to healthy subjects (p=0.845). Men suffering from MS demonstrated while higher hs-CRP (high sensitive C - reactive protein) levels than healthy (p=0.019), contrary to the α1-antichymotrypsin and transferrin. A positive monotonic correlation between synaptophysin and hs-CRP was demonstrated (r=0.235; p=0.003). No statistically significant relationships between SYP and homocysteine plasma levels were presented (r=0.047; p=0.562), although in men diagnosed with MS higher homocysteine levels compared to healthy subjects were demonstrated. No correlations between synaptophysin and free testosterone (r=-0.036; p=0.651), DHEA (r=-0.122; p=0.128) and its sulphate (r=-0.024; p=0.764) as well as SHBG (r=-0.088; p=0.288) were demonstrated.Although the correlations between synaptophysin plasma levels and age as well as strong LGSI indicator (hs-CRP) have been demonstrated, the usefulness of determining SYP serum concentration as a marker of age-related studied diseases (MS, LOH) seems to be significantly limited.

The lanthanide-catalyzed oxidative C-O coupling of 1,3-dicarbonyl compounds with diacyl peroxides, specifically the cyclic malonyl peroxides, has been developed. An important feature of this new reaction concerns the advantageous role of the peroxide acting both as oxidant and reagent for C-O coupling. It is shown that lanthanide salts may be used in combination with peroxides for selective oxidative transformations. The vast range of lanthanide salts (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Y) catalyzes oxidative C-O coupling much more efficiently than other used Lewis and Bronsted acids. This oxidative cross-coupling protocol furnishes mono and double C-O coupling products chemo-selectively in high yields with a broad substrate scope. The double C-O coupling products may be hydrolyzed to vicinal tricarbonyl compounds, which are otherwise cumbersome to prepare. Based on the present experimental results, a nucleophilic substitution mechanism is proposed for the C-O coupling process in which the lanthanide metal ion serves as Lewis acid to activate the enol of the 1,3-dicarbonyl substrate. The side reactions-chlorination and hydroxylation of the 1,3-dicarbonyl partners-may be minimized under proper conditions.

The 1,2,4-trioxolane and 1,2,4,5-tetroxane pharmacophores are currently considered as the next generation of synthetic antimalarial drugs. This fact has stimulated the exploration of the chemiluminescence displayed by these cyclic peroxides in biomimetic reactions with Fe(II). The CL has been induced by FeSO4 and/or the FeCl3/L-cysteine/Rhodamine G system in aqueous (50%) acetonitrile. The light emission in the visible spectral region has been recorded for triterpenoid-based 1,2,4-trioxolanes, purely synthetic ozonide OZ03, bicyclic 1,2,4,5-tetroxanes, a tetroxane derived from deoxycholic acid, the diperoxide of trifluoroacetone, as well as the natural artemisinin. The herein discovered CL provides a promising perspective for the study of pharmacologically active peroxides in biomedical applications.

The mechanism of the catalytic chloride-ion-induced decomposition of dimethyldioxirane has been investigated from a theoretical point of view. The geometrical parameters and harmonic vibrational frequencies of the detected stationary points were calculated at the MP2/6-31+G(d) level of theory, the energies of all the stationary points were refined by means of the MP4(SDTQ)/6-31+G(d) approximation. It was found that the chloride ion forms complexes with acetone and dioxirane, the latter transforms to the anionic intermediate 2-chlorooxy-2-hydroxy propane alcoholate (5b) or ClO–C(CH3)2–O−. Electrostatic forces stabilize these complexes, supported by the similar reaction enthalpies (ΔrH°298.15 K) and ion–dipole interaction energy (E). Our results reveal that the chloride-ion-induced decomposition of the peroxide is a four-step process in which the ClO–C(CH3)2–O− anion 5b serves as a chain carrier in contrast to the earlier postulated hypochlorite ion. The critical step for the production of singlet oxygen is the strongly exothermic decomposition of the intermediate by its reaction with dimethyldioxirane. This chemiluminescent process takes place on the singlet potential energy surface (PES), for which spin conservation obliges the generation of electronically excited singlet oxygen. The present results require revision of the previously published mechanism.

The catalytic oxidations of chiral allylic alcohols 2 by manganese complexes of the cyclic triamine 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn) 1 and hydrogen peroxide as oxygen donor in the presence of co-catalyst are investigated to understand the factors that affect the catalyst selectivity. Chemoselectivity and diastereoselectivity of catalyst 1 are significantly affected by the structure of the allylic alcohol and the nature and amount of co-catalyst. More pronounced is the influence of the amount of added molar equivalents of H(2)O(2) (20-110 mol % with respect to the substrate). Our present results reflect the complex redox chemistry of the Mn catalyst 1/H(2)O(2)/co-catalyst system in the early phase of the alkene oxidation.

In the photochemical denitrogenation of 1,4-diaryl-2,3-diazabicyclo[2.2.1]heptane (AZ6) bearing sterically hindered substituents, a curious new absorption band at about 450 nm was observed under low-temperature matrix conditions, together with the previously well-characterized planar singlet diradical pl-(1) DR6 with λmax =≈580 nm. The 450 nm species was electron paramagnetic resonance (EPR)-silent. Instead of generating the planar diradical pl-(1) DR6 and the precursor azoalkane AZ6 upon warming, the ring-closed bicyclo[2.1.0]pentane derivative SB6, that is, the AZ6 denitrogenation product was identified. Based on product analysis, low-temperature spectroscopic observations, high-level quantum-mechanical computations, viscosity effect, and laser-flash photolysis, the puckered singlet diradicaloid puc-(1) DR6 was assigned to the new 450 nm absorption. The latter was detected experimentally at the same time as the planar singlet diradical pl-(1) DR6. Sterically demanding substituents as well as viscosity impediments were essential for the detection of the experimentally hitherto unknown puckered singlet cyclopentane-1,3-diyl diradicaloid puc-(1) DR6, that is, the third isomer in homolysis. The present findings should stimulate future work on the mechanistically fascinating stereoselectivity documented in the formation of bicyclo[2.1.0]pentanes during the 2,3-diazabicyclo[2.2.1]heptane denitrogenation.

The stereoselectivity in the reactions of the E/Z enecarbamates 1, equipped with the oxazolidinone chiral auxiliary, has been examined for singlet oxygen (1O2), ozone (O3), and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) in a variety of solvents as a function of temperature. The oxidative cleavage of the alkenyl functionality by 1O2 and O3 releases the enantiomerically enriched methyldesoxybenzoin (MDB) product. The extent (% ee) as well as the sense (R vs S) of the stereoselectivity in the MDB formation depends on the electronic nature of the oxidant. A high stereoselectivity, substantially dependent on solvent and temperature, is displayed for the reactions with 1O2, whereas the ground-state reactants O3 and PTAD are rather unaffected by solvent and temperature variations. The present comparative analysis clearly substantiates our hypothesis of stereoselective vibrational quenching of the attacking 1O2, whereas O3 and PTAD are only subject to steric impositions. The electronically excited 1O2 is sensitive to all three stereochemically relevant structural characteristics embodied in the chiral enecarbamates, namely the R/S configuration at the C4 position of the oxazolidinone chiral auxiliary, the Z/E geometry of the ‘alkene’ functionality, and R/S configuration at the C3′ position of the enecarbamate side chain.

The disproportionation of hydrogen peroxide catalyzed by molybdate anions provides an effective non-photochemical source of singlet oxygen 1O2, (1Δg). Microemulsions are the preferred media to carry out ‘dark’ singlet oxygenation of labile and hydrophobic substrates. Single-phase and multiphase microemulsion systems have been developed and improved for the last decade and their respective advantages and limitations are shortly reviewed and discussed.

At high concentrations of oxygen, the EPR spectrum of the nitroxide radical 4-oxo-TEMPO (= 4-oxo-2,2,6,6-tetramethylpiperidin-1-yloxy) is found to broaden significantly. In addition to the expected broadening, double integration of the EPR signals indicates that a significant fraction of the nitroxide spins has ‘disappeared’. In perfluoro(2-butyltetrahydrofuran) at 273 K, the extent of diminution of the EPR signal intensity is ca. 20%. The results are analyzed in terms of collision and supramolecular complexes between oxygen and 4-oxo-TEMPO. It is concluded that a supramolecular complex is responsible for the observed phenomenon.

On photooxygenation (methylene blue as sensitizer) of E/Z enecarbamates, equipped with the oxazolidinone chiral auxiliary, the oxidative cleavage of the alkenyl functionality releases the enantiomerically enriched methyldesoxybenzoin (MDB) product. The extent (% ee) as well as the sense (R vs S) of the stereoselectivity in the MDB formation depends on the choice of the alkene configuration; the efficacy of stereocontrol may be tuned by appropriate solvent and temperature conditions. Highlighted is the finding that the formation of the preferred MDB enantiomer (R or S) depends for the E isomer on the chosen solvent and temperature, but not for the corresponding Z isomer. The activation parameters for the various solvents disclose that differential entropy effects (ΔΔS‡) dominate the conformationally more flexible E diastereomers. As mechanistic rationale for this unprecedented conformationally imposed stereochemical behavior, we propose the competitive action of stereoselective vibrational quenching of the attacking singlet oxygen by the enecarbamate versus sterically controlled stereoselective oxidative cleavage of its double bond.

The versatility of the tricarbonyliron fragment as protecting group in the oxyfunctionalization of cyclohexadienyl-substituted furans with singlet oxygen (1O2), dimethyldioxirane (DMD) and m-chloroperoxybenzoic acid (mCPBA) was investigated. The photooxygenation of the (tricarbonylcyclohexadiene)iron-substituted furans 1a and 1b in methanol gave, after reduction with triphenylphosphane, the furanone 2a and the enediones 3b as mixture of Z and E isomers in good yields. Precursors to these oxyfunctionalized products were the corresponding furan endoperoxides, which were trapped by methanol. The enedione Z-3b was also obtained in the dimethyldioxirane oxidation, which was performed in boiling acetone to minimize the oxidative destructin of the tricarbonyliron moiety. In contrast, mCPBA was unsuitable for the oxyfunctionalization of the ligand sphere of such tricarbonyliron complexes due to the liberation of the organic ligand. While the decomplexation of the furanone 2a with cerium (IV) ammoniumnitrate (CAN) led selectively to the functionalized cyclohexadiene 4a, photooxygenation of the free furan 6a resulted in a complex mixture of oxidized compounds. Thus, protection of the 1,3-cyclohexadiene moiety by tricarbonyliron complexation, oxidation (1O2, DMD) and decomplexation (CAN) offers a convenient and useful sequence for the regioselective oxyfunctionalization of such complex furans.

Bei der Beschaftigung mit Chemilumineszenz- und Biolumineszenzvorgangen drangt sich unweigerlich der Gedanke an die viergliedrigen cyclischen Peroxide — 1,2-Dioxetane und α-Peroxylactone (1,2-Dioxetanone) — auf. Wahrend Chemilumineszenz und Biolumineszenz jedoch traditionelle Gebiete wissenschaftlicher Betatigung sind, befast man sich mit derartigen isolierbaren und charakterisierten hochenergetischen Molekulen erst seit etwa einem Jahrzehnt. Diese faszinierenden Verbindungen erzeugen in der Tat wahrend ihres thermischen Zerfalls elektronisch angeregte Produkte, die sich durch Lichtemission zu erkennen geben. Wichtiger ist noch, das solche Umwandlungen bei enzymkatalysierten Lumineszenz und Dunkelprozessen beteiligt sind. In diesem Fortschrittsbericht sollen die neuesten Entwicklungen zusammengestellt werden. Dabei erheben wir keinen Anspruch auf Vollstandigkeit, sondern konzentrieren uns auf jene Aspekte, die weitere Aktivitaten erfordern.

2,2-Dimethyloxazolidines have been utilized as chiral auxiliaries for the diastereoselective functionalization of the optically active tiglic acid derivatives (S)-1 by means of epoxidation with DMD or m-CPBA and ene reactions with 1O2 or PTAD. In the DMD and m-CPBA epoxidations, high diastereoselectivities but opposite senses of diastereomer selection were observed. In contrast, the stereochemistry of the 1O2 and PTAD ene reactions depended on the size of the attacking enophile: whereas essentially perfect diastereoselectivity was obtained with PTAD, much lower stereoselection was observed with 1O2. The stereochemical results for the DMD and m-CPBA epoxidations and the PTAD ene reaction are explained in terms of the energy differences for the corresponding diastereomeric transition states, dictated by steric and electronic effects. The PTAD ene reaction for these tiglic acids (S)-1 provides, after removal of the chiral auxiliaries, an attractive synthetic route for optically active β-amino acid derivatives.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

[reaction: see text]. Oxazolidinone-functionalized enecarbamates show contrasting behavior upon oxidation by singlet oxygen and by ozone. The observed stereoselectivity difference indicates that the oxidation with ozone is subject to classic steric effects, whereas the very high selectivity in the photooxidation with singlet oxygen is derived from vibrational deactivation.

An overview of the literature through May 2004 on the available chiral auxiliaries for stereoselective epoxidation is reviewed, with emphasis on synthetic utility. The mechanistic complexities of the n-facial selectivity are scrutinized and plausible transition structures for the oxygen-transfer process are proposed. Convenient methods for the removal of the chiral auxiliary and applications of the released optically active building blocks in synthesis are featured.

High diastereoselectivity, but of the opposite sense, is observed in the epoxidation (DMD or mCPBA) of α,β-unsaturated imides equipped with pyrrolidinone-type chiral auxiliaries that bear either a hydroxymethyl or trityloxymethyl side chain. This unprecedented reversed π-facial differentiation is promoted by chelation of a magnesium ion, which results in conformational control over the essential steric interactions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

The thermal cycloaddition of beta-disulfonyl iodonium ylides to cyclic alkenes affords exclusively 1,2,3-trisubstituted cis(1,2)/cis(2,3)-configured benzocyclopentenes by an electrophilic attack of the ylide on the olefinic double bond. This unsual transformation provides a convenient and direct method for the diastereoselective synthesis of functionalized bicyclo[3.3.0]octanes (characteristic structural units contained in polyquinane natural products), when cyclopentenes are used as cycloalkene partner.

The dipolar cycloaddition of thiobenzophenone S-oxide (1) and 2,2,4,4-tetramethyl-3-thioxocyclobutanone (2) generates the labile 1,2,4-oxadithiolane I, which in the presence of trans-cyclooctene (3) affords trans-episulfide (9). In this direct sulfur transfer, the oxathiirane II and/or the dithiirane IV, both derived from the 1,2,4-oxadithiolane I, are proposed as elusive episulfidating agents. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

The origin of the inversion stereoselectivity of housane formation via photochemical nitrogen extrusion of diazabicycloheptene (DBH) has been investigated using reaction path computations and multireference second-order perturbation theory within a CASPT2//CASSCF scheme. We show that the primary photoproduct of the reaction is an exo-axial conformer of the diazenyl diradical ((1) DZ) which displays a cyclopenta-1,3-diyl moiety with a Cs-like structure. (1) DZ is selectively generated via decay at a linear-axial conical intersection, and it is located in a shallow region of the ground state potential energy surface that provides access to five different reaction pathways. Reaction path analysis (including probing with classical trajectories) indicates that production of inverted housane can only occur via impulsive population of an axial-to-equatorial pathway, and it is thus inconsistent with thermal equilibration of the primary (1) DZ conformer. Similarly, according to the same analysis, the decrease of inversion stereoselectivity and even the retention (stereochemical memory effect) observed for suitably substituted DBHs are explained by dynamics effects where the axial-to-equatorial impulsive motion is restrained by the inertia and/or steric hindrance of the substituents. These results shade light on the poorly understood mechanisms that allow a photochemical reaction, in which a large amount of energy is deposited in the reactant by photon absorption, to show a high degree of stereoselectivity.

The kinetics of the chemical and physical quenching of the first excited singlet state of oxygen [1O2 (1delta(g))] by unfunctionalized alkenes 1-4, allylic alcohols 5-7 and 9, allylic acetates 8 and 11, and the allylic ether 10 display small solvent-polarity effects on the reactivity. The regioselectivity of the singlet oxygen ene reaction is solvent independent for the unfunctionalized alkenes as well as the prenol-type substrates, the latter showing substantial solvent effects on the diastereoselectivity. Pronounced physical quenching is detected only for the allylic alcohols 5 and 6. These results are interpreted in terms of the interactions between singlet oxygen and the allylic hydroxy groups, conformationally promoted by allylic strain which lead either to chemical activation or to physical quenching. The results for substrate 9 in deuterated v.s non-deuterated methanol are in accord with hydrogen bonding between the allylic alcohol and 1O2, which directs the diastereoselectivity of the ene reaction with chiral allylic alcohols.

The vanadium(V)-catalyzed asymmetric epoxidation of primary allylic alcohols by the optically active TADDOL-derived hydroperoxide as the asymmetric controller provides the corresponding (R)-epoxides in up to 72% ee. From this mechanistic study we conclude that this novel enantioselective oxygen transfer takes place via a hydrogen-bonded template, held together by the vanadium metal.

The electron-transfer-catalyzed rearrangement of the housanes 1 affords regioselectively the two cyclopentenes 2 and 3 by 1,2-migration of a group at the methano bridge. Appropriate ring annelation in the intermediary cyclopentane-1,3-diyl radical cation 1(*+) changes the stereochemical course of the rearrangement from complete stereoselectivity (stereochemical memory) for the structurally simple housane 1b to partial loss of stereoselectivity through competing conformational interconversion for the tricyclic housane 1c. Additional cyclohexane annelation, as in the tetracyclic housane 1a, results in complete loss of stereocontrol through Curtin-Hammett behavior, as substantiated by the viscosity dependence on the product ratio of the rearrangement. Whereas in the radical cations 1b(*+) and 1c(*+) the 1,2-shifts (k(2) and k(3)) are faster than the conformational anti==syn change (k(1), k(-1)), the reverse applies for the radical cation 1a(*+). Such structural manipulation of conformational effects in radical cation rearrangements has hitherto not been documented.

The photolysis of the cyclopentene-annelated DBH-type azoalkanes 1c,d with the n-propyl and acetoxymethyl substituents at the bridgehead positions affords under singlet conditions (high-temperature direct photolysis) predominantly the retained housanes anti-2c,d, while under triplet conditions (low-temperature direct or benzophenone-sensitized photolysis) the inverted diastereomer syn-2c,d is favored. The latter novel stereochemical feature of the triplet pathway is rationalized in terms of the unsymmetric nature of the n-propyl and acetoxymethyl substituents in regard to rotation about the bridgehead position of the planar cyclopentane-1,3-diyl triplet diradicals 33DR(c,d). In their lowest-energy conformation (AM1 calculations), the rotationally unsymmetric substituents prefer to be located opposite to the annelated cyclopentene ring. The ratio of the diastereomeric syn and anti housanes is determined by the equilibrium distribution of the bridgehead-substituent conformers for the triplet diradicals on ring closure after ISC.

The molybdenum oxo complexes 1a and 1b catalyze efficiently the sulfur transfer to a series of alkenes 4 and allenes 6, for which elemental sulfur, phenylthiirane, or methylthiirane have been employed as sulfur sources to afford the corresponding episulfides 5 and 7. The most effective catalytic episulfidation system to date is the combination of the dithiophosphate-ligated oxo complex 1b and phenylthiirane (Ibeta). This metathesis process is efficient enough to convert usually reluctant alkenes (cyclopentene, cycloheptene, Z-cyclooctene, Z-cyclononene, E-cyclodecene, norbornene, and even bicyclopropylidene) to their episulfides in good yields under mild conditions. The direct catalytic sulfuration of allenes (cyclonona-1,2-diene, cyclonona-1,2,5-triene, cyclodeca-1,2-diene, and 2,4-dimethylpenta-2,3-diene) to their labile methylenethiiranes is unprecedented.

The intramolecular and intermolecular kinetic isotope effects (KIE) have been determined for the nitrosoarene ene reaction with deuterium-stereolabeled 2,3-dimethyl-2-butenes (TME). trans-TME-d(6) (k(H)/k(D) = 3.0) and gem-TME-d(6) (k(H)/k(D) = 4.0) show large intramolecular primary isotope effects. In contrast, the intramolecular competition in cis-TME-d(6) (k(H)/k(D) = 1.5) and the intermolecular competition for the TME-d(0)/TME-d(12) pair (k(H)/k(D) = 1.98) show considerably smaller, but mechanistically significant kinetic isotope effects. The latter fact is rationalized in terms of reversible formation of a three-membered-ring intermediate, namely the aziridine N-oxide, or a similar unsymmetrical, polarized diradical in the first step of the reaction. Such reversibility has also been implied earlier for triazolinedione (TAD) and singlet oxygen ((1)O(2)) with deuterium-stereolabeled 2-butenes, but of the three enophiles, ArNO is the most sensitive toward reversibility, which is due to its moderate reactivity and its high steric demand.

The photolysis of the cyclopentene- and cyclopentane-annellated DBH-type azoalkanes 1a and 1b affords under singlet conditions (high-temperature direct photolysis) predominantly the inverted housanes syn-2 in similar amounts for both derivatives 2a and 2b. Under triplet conditions (low-temperature direct or benzophenone-sensitized photolysis), the photolysis leads to the retained housane anti-2 as the major diastereomer, but with a substantial difference in the syn/anti-housane ratio for 2a (38:62) and for 2b (6:94). This significant difference in the anti stereoselectivity of the triplet pathway is mechanistically rationalized in terms of long-range steric interactions between the annellated ring and the gem-dimethyl-substituted methylene bridge during the cyclization of the cyclopentane-1,3-diyl triplet diradical 3DR after ISC. In contrast, the denitrogenation of the intermediary diazenyl diradical 1DZ along the SH2 pathway (inversion) for the singlet process is quite insensitive to these remote steric effects. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

The asymmetric aldol reaction of the (S)-proline-derived bicyclic amide enolate 1 with benzaldehydes selectively affords the two diastereomeric aldol adducts (3S,3′R,8S)-3a and (3S,3′S,8S)-3a, of four possible diastereomers, in good yield. The high control of π-facial stereoselectivity at the endocyclic C-3 stereocenter is due to perfect shielding by the pseudo-axial phenyl ring at the C-1 position. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Sandwich-type polyoxometalates (POMs), namely [WZnM2(ZnW9O34)2]q- [M = Mn(II), Ru(III), Fe(III), Pd(II), Pt(II), Zn(II); q = 10-12], are shown to catalyze selectively the epoxidation of chiral allylic alcohols with 30% hydrogen peroxide under mild conditions (ca. 20 degrees C) in an aqueous/organic biphasic system. The transition metals M in the central ring of polyoxometalate do not affect the reactivity, chemoselectivity, or stereoselectivity of the allylic alcohol epoxidation by hydrogen peroxide. Similar selectivities, albeit in significantly lower product yields, are observed for the lacunary Keggin POM [PW11O39]7-, in which a peroxotungstate complex has been shown to be the active oxidizing species. All these features support a tungsten peroxo complex rather than a high-valent transition-metal oxo species operates as the key intermediate in the sandwich-type POM-catalyzed epoxidations. On capping of the hydroxy functionality through acetylation or methylation, no reactivity of these hydroxy-protected substrates [1a(Ac) and 1a(Me)] is observed by these POMs. A template is proposed to account for the marked enhancement of reactivity and selectivity, in which the allylic alcohol is ligated through metal-alcoholate bonding, and the H2O2 oxygen source is activated in the form of a peroxotungsten complex. 1,3-Allylic strain promotes a high preference for the threo diastereomer and 1,2-allylic strain a high preference for the erythro diastereomer, whereas tungsten-alcoholate bonding furnishes high regioselectivity for the epoxidation of the allylic double bond. The estimated dihedral angle alpha of 50-70degrees for the metal-alcoholate-bonded template of the POM/H2O2 system provides the best compromise between 1,2A and 1,3A strain during the oxygen transfer. In contrast to acyclic allylic alcohols 1, the M-POM-catalyzed oxidation of the cyclic allylic alcohols 4 by H2O2 gives significant amounts of enone.

The reaction of bis(arylsulfonyl)iodonium ylides with a variety of norbornene derivatives 3 affords functionalized indanes 4 in good yields through an unusual cycloaddition. This diastereoselective cycloaddition provides a convenient preparative route to multicyclic structures of well defined stereochemical configuration.

The cyclopentane-1,3-diyl triplet diradicals T and T' with the triplet-bonded acetylene, cyano, and isocyano functionalities at one of the radical sites are readily prepared from the corresponding azoalkanes by photodenitrogenation in a 2-methyltetrahydrofuran (2-MTHF) matrix at 77 K. The EPR-spectral D values of these triplet diradicals show that the spin delocalizing ability of the triple-bonded pi substituent follows the order -Ctbd1;CH > -NC approximately -CN. Good correlations of the D values have been obtained with the hyperfine coupling constants (a(H)) and with the calculated spin densities (PM3/AUHF-CI method) of the corresponding monoradicals M. The propargyl-type mesomeric structure is favored over the allenyl-type contributor for all three triple-bonded functionalities; spin delocalization is less pronounced in the heteropropargyl derivatives due to the electronegativity effect of the nitrogen atom.

Comparative analysis of the calculated gas-phase activation barriers (DeltaE++) for the epoxidation of ethylene with dimethyldioxirane (DMDO) and peroxyformic acid (PFA) [15.2 and 16.4 kcal/mol at QCISD(T)// QCISD/6-31+G(d,p)] and E-2-butene [14.3 and 13.2 kcal/mol at QCISD(T)/6-31G(d)//B3LYP/6-311+G(3df,2p)] suggests similar oxygen atom donor capacities for both oxidants. Competition experiments in CH(2)Cl(2) solvent reveal that DMDO reacts with cyclohexene much faster than peracetic acid/acetic acid under scrupulously dried conditions. The rate of DMDO epoxidation is catalyzed by acetic acid with a reduction in the classical activation barrier of 8 kcal/mol. In many cases, the observed increase in the rate for DMDO epoxidation in solution may be attributed to well-established solvent and hydrogen-bonding effects. This predicted epoxidative reactivity for DMDO is not consistent with what has generally been presumed for a highly strained cyclic peroxide. The strain energy (SE) of DMDO has been reassessed and its moderated value (about 11 kcal/mol) is now more consistent with its inherent gas-phase reactivity toward alkenes in the epoxidation reaction. The unusual thermodynamic stability of DMDO is largely a consequence of the combined geminal dimethyl- and dioxa-substitution effects and unusually strong C-H and C-CH(3) bonds. Methyl(trifluoromethyl)dioxirane (TFDO) exhibits much lower calculated activation barriers than DMDO in the epoxidation reaction (the average DeltaDeltaE++ values are about 7.5 kcal/mol). The rate increase relative to DMDO of approximately 10(5), while consistent with the higher strain energy for TFDO (SE approximately 19 kcal/mol) is attributed largely to the inductive effect of the CF(3) group. We have also examined the effect of alkene strain on the rate of epoxidation with PFA. The epoxidation barriers are only slightly higher for the strained alkenes cyclopropene (DeltaE++ = 14.5 kcal/mol) and cyclobutene (DeltaE++ = 13.7 kcal/mol) than for cyclopentene (DeltaE++ = 12.1 kcal/mol), reflecting the fact there is little relief of strain in the transition state. Alkenes strained by twist or pi-bond torsion do exhibit much lower activation barriers.

The electron-transfer-catalyzed rearrangement of the housanes 5 affords regioselectively only the two cyclopentenes 6 (CH(3) migration) and 7 (R migration) by 1,2-migration of the two groups at the methano bridge to the methyl terminus. The 1,2-shift of the CH(3) group prevails, and the rearrangement ratio is essentially insensitive to the migratory aptitude of the R substituent. This stereochemical memory effect derives from the conformational impositions on the stereoelectronic requirements during the 1,2-migration in the 1,3-radical-cation intermediates. Similar regioselectivities and diastereoselectivities are observed for the TFA-catalyzed and silver(I)-ion-promoted rearrangements, whereas the rearrangement catalyzed by HClO(4) affords a complete reversal in the product selectivity and both the regioselectivity and the diastereoselectivity are much reduced. Migration to the phenyl terminus is favored to afford the 6' and 7' cyclopentenes, of which the former (CH(3) migration) dominates. For the minor regioisomer, only the cyclopentene 6 is formed by an exclusive 1,2-shift of the CH(3) group. This dichotomy in product selectivities is rationalized in terms of two distinct mechanisms for the various activation modes: a common one for the electron-transfer-induced, TFA-catalyzed, and silver(I)-ion-promoted rearrangements and a different one for HClO(4).

The influence of structural changes on the α parameter, which represents the effective volume involved in molecular reorganizations, has been determined through the viscosity dependence of the stereoselectivity in the photodenitrogenation of the DBH-type azoalkanes 1a–c. The structural variation reveals that the stereoselective formation of housane is not only subject to the frictional impositions exerted by the solvent molecules on the flap motion of the methylene bridge, but also on the displacement of the bridgehead substituents during the inversion process.

The auxiliary-induced diastereoselectivities of [4+2] cycloaddition and the ene reaction of singlet oxygen and PTAD through a variety of auxiliaries is studied. The different trends in the diastereoselectivities that are observed for the diverse auxiliaries, which include 2,2-dimethyloxazolidines, a menthol derivative, several related cyclohexanes, and the Oppolzer sultam, are compared and backed by mechanistic interpretations. From this overview of two reaction modes for two electrophiles and four different types of auxiliaries, the advantages of the individual auxiliaries become evident and comprehensible. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

In this account, we present a brief overview of our interdisciplinary efforts on the metal-assisted selective oxyfunctionalization of organic substrates in the ‘Sonderforschungsbereich (SFB 347): Selektive Reaktionen Metall-aktivierter Molekule’. The reactivity and selectivity of peroxy-, peroxo-, and oxo-type metal oxidants have been studied to gain insight into the mechanism of the oxygen transfer by such catalytic oxidants. Based on our mechanistic work, effective catalytic oxyfunctionalization methods have been developed by employing Ti, V, Re, Mn or Cr complexes as catalysts for the chemo-, regio-, diastereo- and enantioselective synthesis of hydroxy epoxides (from olefins), sulfoxides (from sulfides), and silanols (from silanes). We show that the stoichiometric dimethyldioxirane (DMD) oxidation of ligands in selected transition-metal complexes provides an useful tool for the synthesis of oxyfunctionalized compounds.

High reactivity under mild reaction conditions, excellent selectivity, and good environmental tolerance establish enzymes as valuable biocatalysts in asymmetric synthesis. The catalytic, enantioselective hydroxylation of prochiral long-chain carboxylic acids by the α-oxidase system with molecular oxygen constitutes to date the only direct enzymatic oxyfunctionalization without recourse to the transformation of the oxygen functionalities in the substrate. A complementary method is the synthesis of optically active α-hydroxy acids with short and intermediate chains by kinetic resolution of racemic 2-hydroxy acids through enantioselective oxidation with glycolate oxidase (GOX). The horseradish-peroxidase (HRP)-catalyzed kinetic resolution of racemic secondary hydroperoxides affords their alcohols and the hydroperoxides in high enantiomeric excesses, the latter serve as valuable chiral oxygen sources for asymmetric oxidations.

In this account, we summarize the highlights of our cooperative research work on the biocatalytic preparation of enantiomerically enriched, oxyfunctionalized substances in the ‘Sonderforschungsbereich (SFB 347): Selektive Reaktionen Metall-aktivierter Molekule’. The biocatalytic kinetic resolution of secondary hydroperoxides with the metalloenzymes horseradish (HRP) and Coprinus (CiP) peroxidase affords the hydroperoxides and their alcohols in excellent enantiomeric excess. Also the semisynthetic enzyme selenosubtilisin catalyzes effectively the enantioselective reduction of racemic hydroperoxides to produce optically active hydroperoxides and alcohols in high ee values. The asymmetric CH oxidation of long- and medium-chain carboxylic acids by molecular oxygen, catalyzed with the α-oxidase from pea leaves or germinating peas yields enantiomerically pure 2-hydroxy acids. The enzymatic kinetic resolution of racemic 2-hydroxy acids through the enantioselective oxidation by glycolate oxidase (GOX) or the esterification by lipase provides an attractive route to such optically active building blocks. The bacteria Bacillus subtilis and Bacillus megaterium , isolated from topsoil by selective screening with hydrogen peroxide or allylbenzene, are efficient biocatalysts for the enantioselective reduction of racemic hydroperoxides and the asymmetric CH oxidation of alkanes. The latter enzymatic process is to date still difficult to emulate by chemical catalysis.

For the ene reaction of 4-nitronitrosobenzene (ArNO) with a variety of primary and secondary lone alkyl-substituted substrates, the twix/twin regioselectivity is constant at about 85:15. In contrast, for the lone tert-butyl group and for lone aryl substituents, the twix regioisomer is obtained exclusively. These regioselectivities have been rationalized in terms of steric interactions and coordination between the enophile and the substrates in the transition states of the first reaction step.