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Abstract: Different alkyl/aryl 7-hydroxy-8-coumarinyl ketones were converted to 7-O-allyl and 7-O-cyclohexenyl ethers and the study of hitherto unreported sigmatropic rearrangement on 7-O-allyl and 7-O-cyclohex-2'-ene-1'-ylcoumarinyl ketones prepared is accounted herein. The [...]

Abstract: In the presence of a catalytic amount of bismuth triflate, 2-alkoxycarbonyl-substituted allyl vinyl ethers as a mixture of enol ether double bond isomers were smoothly converted into the β, [...]

Abstract: Regioselective synthesis of pentacyclic heterocycles containing oxygen, nitrogen, and sulfur has been achieved in good to excellent yields by the sequential Claisen rearrangement of but-2-ynyl ethers and sulfides containing [...]

Appropriately prepared 1-alkenyl-4-pentyn-1-ol systems have served as useful precursors to many substituted cyclohept-4-enone derivatives via a microwave-assisted tandem oxyanionic 5-exo dig cyclization-Claisen rearrangement process. The reactions involving terminally substituted 4-pentyn-1-ols are highly stereoselective, with the [alpha] and [beta] groups in the final product showing a strong preference for the trans orientation.

The potential energy surfaces for the unimolecular rearrangement reactions of [beta]-dicarbonyl ylides and [beta]-carbonylimidoyl ylides were studied. The study predicted that the barrier height for the unimolecular aryl migration in the [beta]-dicarbonyl ylide bearing a halogen atom decreases in the order F>Cl>Br>I.

A mechanism is postulated for the reaction of sulfuric acid with stabilized Criegee intermediates (sCIs) and Gibbs free energies are computed for the reaction of sulfuric acid with two biogenic sCIs and three smaller model species. The reaction is strongly exothermic for all species and activation barrier calculations on the [(C[H.sub.3]).sub.2]COO model species have shown that the reaction with sulfuric acid might proceed faster than the sink reaction with water.

The synthesis of bisurea calix[4]arenes possessing L-amino acid moieties at the lower rim by the reaction of the methyl esters of glycine, L-alanine or L-isoleucine with the appropriate isocyanate, obtained with Curtis rearrangement from the corresponding carboxylic acid derivatives and their conformational properties, are presented. It is found that these ligands are affected by strong intramolecular H-bonds between the urea NH groups and the vicinal phenolic oxygen atoms and that the binding ability of these bisurea hosts has decreased by increasing the side chain size of the amino acid.

A kinetic and computational analysis of the palladium(II)-catalyzed asymmetric allylic trichloroacetimidate rearrangement is presented. The study leads to the development of a model, where in the planar chirality of the catalysts controls the enantioselectivity and hence can be used for rational designing of more enantioselective catalysts.

The reaction of a galactosylated 2-C-(hydroxymethyl)-tetrofuranose with paramolybdate ion-exchange resin in aqueous solution has given an equilibrium mixture containing the reactant aldofuranose and a 2-ketopentofuranose galactosylated at O1. The stereospecific rearrangement has provided a convenient route for 2-ketosugars glycosylated at the exocyclic C1 position.

Indium promoted coupling of propargyl aldehydes with allyl halides has resulted in the synthesis of propargyl alcohols under aqueous and organic conditions. Oxy-Cope rearrangement of 4-hydroxy-1-ene-5-ynes followed by a secondary addition step under highly basic conditions has led to the formation of lactol.

A new approach to the ring EFHG-tetracyclic core fragment of the marine secondary metabolite diazonamide A is described. This methodology could be applied to the tyrosine-indole derivative to give the EFHG-tetracyclic core of diazonamide A.

The structure, spectra, and rearrangement mechanisms of P[H.sub.2][F.sub.3], the first member of the P[H.sub.n][F.sub.5-n] series and a prototype for molecules that undergo rotational isomerism, are analyzed. The results have identified the spectral fingerprints that could distinguish various rotational isomers that could form a powerful basis for the characterization of various rotational isomers of P[H.sub.2][F.sub.3] when used along with high resolution measurements.

The electronic structure of the complexes formed from the coordination of [VO.sub.2.sup.+] by hydroxyurea was investigated by density functional theory (DFT). The weakly bonded species, oxo, hydroxy and water ligands displayed substantial conformational flexibility and a large number of isomers were found in a relatively small energy range.

The construction of complex architectures from simple building blocks is feasible using subcomponent self-assembly by forming covalent bonds around metal templates. Several rearrangement reactions involving substitution at both intraligand and metal-ligand bonds are possible as both covalent and coordinative bonds are formed reversibly, as also parallel preparation of structures from nonorthogonal combinations of subcomponents.

The metadynamics approach is applied to the intramolecular hydrogen migration reactions of acetone in the gas phase. The results have concluded that the enol-oxo tautomerism is the most frequent intramolecular proton exchange process, the acetone undergoes in the gas phase.

Competitive experiments with isotopically labeled chorismate derivates have provided direct information about the nature of the transition state of the chorismate mutase reaction both in solution and at the active sites of evolutionarily unrelated enzymes. The competing reaction of chorismate leading to 4-hydroxybenzoate, an intermediate in ubiquinone biosynthesis, likewise appears to occur via an asynchronous concerted mechanism.

The atomic properties of neutral and protonated forms of uracil and some model compounds, computed with the Quantum Theory of Atoms in Molecules (QTAIM) theory, indicate that sigma electron reorganization that plays a significant role in the protonation processes is studied. The stability sequence of N-protonated forms can be rationalized in terms of the closer position of the proton when attached to N3, to regions of larger electron population.

The thermal rearrangement reactions of 1-silylprop-2-en-1-ol H(sub 3)SiCH(OH)CH=CH(sub 2) by ab initio calculations at the G2(MP2) and G3 levels are studied. The influences of the vinyl group attached to the center carbon of the alpha-silyl alcohols on reactions are discussed.

The oxy-Cope rearrangement reaction in the antibody AZ28 is investigated using ab initio molecular orbital calculations and molecular mechanical dynamics simulations. It is shown that the differences in flexibility between the germ line and the mature form and the differences in structure between TSA and the transition state are the origin of the negative correlation in AZ28-catalyzed oxy-Cope rearrangement.

Negatively and positively chirped pulses used to excite particular vibrationally coherent wave packets in the ground and excited stages in an azobenzene derivative molecule in solvent are studied. Powerful dynamic mode link between the N=N and C-N stretching modes is observed in the excited state of the molecule.

Abietadiene synthase catalyzes the committed step in resin acid biosynthesis, forming a mixture of abietadiene double-bond isomers by two sequential, mechanistically distinct cyclizations at separate active sites. The first reaction, protonation-initiated cyclization, converts the universal diterpene precursor geranylgeranyl diphosphate to the stable bicyclic intermediate copalyl diphosphate. In the second, magnesium ion-dependent reaction, diphosphate ester ionization-initiated cyclization generates the tricyclic perhydrophenanthrene-type backbone and is coupled, by intramolecular proton transfer within a transient pimarenyl intermediate, to a 1,2-methyl migration that generates the C13 isopropyl group characteristic of the abietane structure. Alternative deprotonations of the terminal abietenyl carbocation provide a mixture of abietadiene, levopimaradiene, and neoabietadiene, and this product profile varies as a function of pH. Mutational analysis of amino acids at the active site of a modeled structure has identified residues critical for catalysis, as well as several that play roles in specifying product formation, apparently by ligation of a magnesium ion cofactor. These results strongly suggest that choice between alternatives for deprotonation of the abietenyl intermediate depends more on the positioning effects of the carbocation-diphosphate anion reaction partners than on the pKa of multiple participating bases. In one extreme case, mutant N765A is unable to mediate the intramolecular proton transfer and aborts the reaction, without catalyzing 1,2-methyl migration, to produce only sandaracopimaradiene, thereby providing supporting evidence for the corresponding stereochemistry of the cryptic pimarenyl intermediate of the reaction pathway.

Results demonstrate the reactivity of C-5, C-8 dioxygenated panasinsane derivatives in the preparation of compounds containing ginsenane or undecane skeleton through successive generation and trapping of carbocations from alcohols at C-5 and C-8 panasinsane derivatives.

Research describing the photochemical properties of 3-styrylpyridines under anaerobic conditions is presented. The paper suggests that regioselective photocyclization is dependent on relative rate constants of the dihydrophenanthrene intermediaries.

The cyclopropenyl anions with 3-aryl substitutions are unstable in the gas phase during their transition to 6-substituted 3-indenyl anions. The acidity of 3-(4-methylphenyl)cyclopropene is determined by the DePuy method and the pH value agrees with that ab initio value.

Allylmetal addition to vinylmetal is examined in relation to the metalla-Claisen rearrangement.

Research investigating the hydrogen bond basicity of several ylides is presented. Particular attention is given to ylides containing oxygen, nitrogen or carbon in their composition. The influence of hydrogen bond formations on the ylides structures is analysed.

A comprehensive analyzes of the (CH)(sub 2)(BR)(sub 2) potential energy surface is carried out at the [MP4/6-311+G(d,p)]/MP2/6-31G(d) level. The rearrangement of 1,3-diamino-1,3-diboretene (1-NH2) to 1,-diamino-1,2-diamino-1,2-diboretene (2-NH2) was computed and compared to the rearrangement in the parent and the effect of the amino group was to substantially increase the barrier height and stabilize the product, 2-NH2.

The thermal rearrangement kinetics of (1.1.1)propellane were studied using a gas phase pyrolysis system.

Polymer blends into a molecular dispersion transition induced by solid-state tensile deformation was observed using photoluminiscence microscopy.

A study of the allenyl cope rearrangement of syn-7-allenylnorbornene is reported.

Density functional theory was used to investigated the effects of thiomethoxy and methoxy substituents on anionic oxy-Cope rearrangements.

The 1,2 rearrangement of housane radical cations was studied and the role of a transient cyclopentane-1,3-diyl intermediate discussed.

B3LYP/6-31G calculations were performed on the chair Cope rearrangements of a range of substituted 1,5-hexadienes.

Research is presented demonstrating the rationalized behaviour of different intramolecular rearrangement reactions in terms of the principle of maximum hardness and the Hammond postulate.

A detailed ab initio molecular orbital calculations for the mechanisms of the 1,3-migrations of group-14 element groups in HC(=O)CH2MH3 and HC(=NH)CH3SiH3 were conducted. Results indicate that the retention 1,3-silyl migration in formylmethylsilane can be best described as an intramolecular nucleophilic substitution at silicon while the corresponding migration in allylsilane is as an electrocyclic sigmatropic rearrangement controlled by subjacent orbital interactions.

The sequence of rearrangements of carbenes (6) was investigated. Precursors for 6 were 1-halo-3-(n-halomethyl)bicyclo(1.1.1)pentane derivatives (11), which were produced by addition of polyhalomethanes to the central bond of (1.1.1)propellane derivative (13) via radical chain processes. Findings showed that the selectivity for these rearrangements is low. Results DFT computations on the rearrangement of 8 -> 9 and on the alternative CH insertion reactions of carbene (9) are in accordance with the experimental results.

Research was conducted to examine the generality, scope and reaction pathways of a new rearrangement of dithianylcyclobutachromanol derivative to 1,2,2a,8,9,9a-hexahydrocyclobuta(b)(1)benzoxepin-8,9-dione on treatment with mercuric oxide and fluoroboric acid in aqueous tetrahydrofuran. The general criteria and best experimental conditions for the rearrangement and its possible mechanism were postulated.

Research was conducted to examine the photochemical transformations of some selected examples to investigate the role of both steric and electronic effects of the bridgehead substituents on the regioselectivity of the photorearrangements. The thermal isomerization of the thermal photoproducts formed in the reactions is also the subject of research. The structures of the primary photoproducts were determined through X-ray crystallographic analysis.

Research was conducted to determine the absolute rate constants for radical transformation reactions in complex reaction systems by time-resolved electron spin resonance during intermittent photochemical radical production. The method was used to achieve absolute rate constants and their activation parameters for various reactions and has proven its versatility. Kinetic investigations of the neophyl and the 2-methyloxiran-2-yl rearrangements and the decarboxylation of the tert-butoxycarbonyl radical to benzene and fluorobenzene.

A useful synthesis for benzophenanthridines and annulated derivatives is presented. The process employs the thermolysis (refluxing chlorobenzene) of squaric acid-derived 4-(3-N-methyl-N-arylpropynyl)-cyclobutenones through a mechanism which involves an electrocyclic ring opening of the cyclobutenone to the corresponding enynylketenes. Subsequent ring closure to a diradical intermediate followed by radical arylation afforded the benzophenanthridines.

Research was conducted to examine the interconversions of norbornadiene using multireference ab initio methods and density functional theory with a 6-31G* basis set. The interconversions include the (1,5)carbon shift in norcaradiene, the formation of norcaradiene from cycloheptatriene, the ring flip of cycloheptatriene and the (1,5)hydrogen shift in cycloheptatriene. Results suggest that the transition state for the formation of toluene from norcaradiene proceeds through a hydrogen transfer transition state that is formed from the allowed transition state of the walk rearrangement but not from the forbidden transition state.

An investigation of the isomerization process for an azetidine derivative to the corresponding oxazine was conducted to verify the possibility of an SNi-like process. The MO calculations, performed with the aid of the GAUSSIAN94 program, revealed that O-protonation is a much more favorable reaction than N-protonation. The results provide the support needed in the discussion of the adverse effects of O-coordination in the SNi process.

Liquid chromatography and electrospray mass spectrometry have been used in studying the adducts produced by synthetic levuglandin E2 and prostaglandin H2-derived levuglandins with lysine. Prostaglandin H2 has been observed to rearrange the gamma-ketoaldehyde prostanoids E2 and D2. Results indicate that lysyl-levuglandin Schiff base adducts that immediately dehydrate to form lysyl-anhydrolevuglandin Schiff base adducts are formed from the reaction. The levuglandins and E2 generate pyrrole-derived lactam and hydroxy-lactam adducts when reacted with lysine.

Research was conducted to examine the photosolvolytic rearrangement of various p-hydroxyphenacyl esters and related compounds in solutions containing up to 50% aqueous content. Product studies, triplet quenchers and nanosecond laser flash photolysis were performed to investigate the mechanism and to provide evidence for excited-state intramolecular proton transfer (ESIPT) as the main photochemical step. Results are consistent with a mechanism of reaction involving ESIPT or ESPT as a required step which is mediated most efficiently by water.

Research was conducted to examine an energy barrier on the order of 60 kcal/mol which is predicted for the 1,2-shifts of hydrogen atoms in aryl radicals. The 2-benzo(c)phenanthryl radical was produced using flash vacuum pyrolysis of the corresponding aryl bromide to demonstrate the rearrangement. The cyclization and rearomatization of the resulting radical followed a 1,2-shift of hydrogen out of the sterically congested cove region of the 2-benzo(c)phenanthryl radical. Computational results indicate that aryl radicals should be good candidates to exhibit 1,2-shift rearrangements of hydrogen atoms.

Research was conducted to examine two procedures leading to smooth rearrangement complexation of compounds of type 7-XH-nido-7,8,9-C3B8H10. A series of new metallatricarbollide isomers having chemically flexible amine functionalities in p-positions with respect to the metal center and are isomeric with the unsubstituted complex (2-(eta(super 5)-C5H5)-closo-2,1,7,10-FeC3B8H11). High-field NMR spectroscopy and mass spectrometry were used to establish the structures of all compounds.

The isomerizations of glucose's molecular formula C6H12O6 and crystalline alpha and beta anomers were examined with ab initio calculations to study the mechanism of the mutarotation of glucose assisted by a few water molecules. The results showed that the water dimer and trimer were reasonably involved in the isomerization. In particular, the trimer constituted a strain-free hydrogen-bond network for ready proton transfers. Outer solvent H2O molecules changed the extent of transfers in the network and energies.

Research was conducted on the tautomeric rearrangements in the 6,8-dithioguanine using ab initio quantum chemical calculations. An optimization of molecular geometries of 35 tautomeric structures was performed without constraints imposed at the HF/3-21G level. Results show that the global minimum on the potential energy surface in the gas surface is the standard 6,8-dithioguanine at all applied levels of theory.

The primary DNA lesion induced by malondialdehyde, a byproduct of lipid peroxidation and prostaglandin synthesis, is 3-(2[prime]-deoxy-[Beta]-o-erythro-pentofuranosyl)-pyrimido[1,2-a]purin-10(3H)-one ([M.sub.1]G). When placed opposite cytosine (underlined) at neutral pH in either the d(GGTMTCCG)[center dot]d(CGGACACC) or d(ATCGCMCGGCATG)[center dot]d(CATGCCGCGCGAT) duplexes, [M.sub.1]G spontaneously and quantitatively converts to the ring-opened derivative [N.sup.2]-(3-oxo-1-propenyl)-dG. Ring-opening is reversible on thermal denaturation. Ring-opening does not occur at neutral pH in single-stranded oligodeoxynucleotides or when T is placed opposite to [M.sub.1]G in a duplex. The presence of a complementary cytosine is not required to stabilize [N.sup.2]-(3-oxo-1-propenyl)-dG in duplex DNA at neutral pH. When [N.sup.2]-(3-oxo-1-propenyl)-dG is placed opposite to thymine in a duplex, it does not revert to [M.sub.1]G. A mechanism for the conversion of [M.sub.1]G to [N.sup.2]-(3-oxo-1-propenyl)-dG is proposed in which the exocyclic amino group of the complementary cytosine attacks the C8 position of the [M.sub.1]G exocyclic ring and facilitates ring opening via formation of a transient Schiff base. Addition of water to the Schiff base regenerates the catalytic cytosine and generates [N.sup.2]-(3-oxo-1-propenyl)-dG. These results document the ability of duplex DNA to catalyze the transformation of one adduct into another, which may have important consequences for mutagenesis and repair.

Studies have shown that 2-fold Heck couplings of 1,2-dibromocyclopentene and 1,2-dibromocyclohexene result in good yields of 1,6-disubstituted (E,Z,E)-1,3,5-hexatrienes. Similar couplings provide an attractive, fast and flexible route to oxygen-bridged bridgehead cyclodecadienes. Furthermore, an increase in strain has been found on reactions involving 1,6-oxygen-bridged 1,5-cyclodecadiene to lower homologue and cabocyclic analogues.