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The PIDA‐mediated oxidative decarboxylation of oxamic acids in the presence of alcohols is shown to afford the corresponding urethanes under thermal conditions. Computational and experimental mechanistic exploration allows to rationalize the different reactivity of PIDA as compared to related cyclic BI‐OAc and highlights the importance of the enhanced acidity of the proton in the carbamoyl radical intermediate. [ABSTRACT FROM AUTHOR]

The thermodynamics of regioselective change in an intramolecular Diel-Alder cycloadditions from allyl ether to allenyl ether is demonstrated. The allyl ether yields a fused tetrahydrofuran through a classical Diels-Alder reaction, whereas the allenyl ether yields a (methylidene)tetrahydrofuran through a stepwise process.

A density functional theory based computational approach to describing the mechanistic course of the allene azide cycloaddition cascade sequence, which explains the different behaviors observed in the experimental studies between conjugated and nonconjugated species, is presented.

A computational study of the sulfoxide-induced preference for a migrating trajectory in the vinylallene [1,5]-H sigmatropic shift is presented. The origin of the face selectivity is found to lie in a secondary orbital interaction (SOI) involving the termini of the pericyclic array and the sulfinyl group that is observed for this substituent.

A study is conducted of the mechanistic features of the cycloisomerization of protic and Lewis acid activated (2E,4Z)-heptatrienal and methyl (2E,4Z)-heptatrienoate to provide bicyclo[3.1.0]hexene derivatives, the skeleton of crispatenes. The reaction is neither strictly concerted nor stepwise but somehow intermediate to both.

The mechanism of the stereospecific gold(I)-catalyzed Rautenstrauch rearrangement of (E)-1-ethynyl-2-methyl-but-2-en-yl acetate to 2,4-dimethyl-cyclopent-2-enone is computationally addressed using density functional theory (DFT). Results indicate that the fidelity of the center-to-helix-to-center chirality transfer requires that the rates of helix interconversion and pivaloyl rotation are slower than the cyclization, as predicted by calculations.

Five multilevel model chemistries and seven hybrid density functional methods are applied to the calculation of gas-phase basicity and proton affinity values for a series of 17 molecules relevant to the study of biological phosphoryl transfer. Assessment of the accuracy of the methods and nature of systematic errors leads to the introduction of a set of effective bond enthalpy and entropy correction terms.

A theoretical study on the torquoselectivity of the electrocyclic ring opening of 2,3-disubstitutedmono and dibromo cyclopropanes was carried out at the B3LYP/6-311++G level. As a result of charge donation from the dissociating C-C bond to the antibonding orbital of the breaking C-Br bond, only one of the two allowed disrotatory paths for the disrotatory ring opening that takes place in concert with the C-Br bond cleavage, is energetically accessible, both in gas phase and in solution.

The characterization of two distinct competing mechanistic pathways for the rearrangement of vinyl allene oxides to cyclopent-2-en-1-ones is described. The concerted transition structure is shown as a monorotatory ring closure with concomitant epoxide ring opening

The electrocyclic ring closure reactions of (2Z)-hexa-2,4,5-trienals (vinylallenals) to alkylidene-2H-pyrans and of the corresponding Schiff base derivatives to alkylidenepyridines is shown to be concurrent. The cyclizations are torquoselective and the kinetically favored (E)-alkylidene heterocycles have evolved by electrocyclic ring opening/ring closure toward the thermodynamic Z isomers upon extended reaction times.

The thermal, photochemical and photochemical/CuI-mediated cascade cyclizations of a range od substituted 1-(2-azidophenyl)-3-alkenylallenes have provided both 1,2- and 2,3-cyclopentennelated indole products in varying ratios. High regioselectivity for the 2,3-annelated isomer is achieved and the studies have supported the intermediacy of indolidene intermediates whose electrocyclizations define the regioselectivity branch point in the sequence.

The three competing paths for the rearrangement of (3-acyloxyprop-1-ynyl)oxiranes involving 1,2- and 1,3-ester migration with alkyne or oxine activation have seen the multifaceted character of gold as a catalyst. The analysis of these paths has shown the use of a nonalkynophilic Lewis acid as a possible synthetic alternative for the transformation.

The disrotatory anti-Woodward-Hoffmann mechanism that might be responsible for the ring opening reactions of fused cyclobutenes is discussed. The findings revealed that the disrotatory transition state exchanged roles with a double-bond isomerization depending on the substituents around the bicyclic structure.

A combined experimental-theoretical study of the thermal isomerization of 11-cis-retinal to yield a mixture of double-bond isomers, namely 13-cis- and all-trans-retinal in a 65:35 ratio is described. The formation of all-trans-retinal occurs through direct internal rotation around the double bond at C11 via a diradical transition state, while the formation of 13-cis-retinal occurs through a complex pericyclic mechanism involving a pair of [1,7]-H sigmatropic shifts followed by a six electrons heteroelectrocyclic ring closure/opening process.

Detailed studies of the thermal conversion of 1-azidohepta-3,4,6-trienes into cylcopentennelated dihydropyrroles are presented. The calculation results provide insight into the intervention of unexpected mechanistic subtleties such as the planarity of an azatrimethylenemethane diyl intermediate and an apparent Woodward-Hoffmann-type electrocyclization of a five-atom diyl array.

Molecular dynamics simulations are used for examining the role of [Mg.sup.2+] in the full-length hammerhead ribozyme cleavage reaction. The simulations have shown that [Mg.sup.2+] is involved in the hammerhead ribozyme mechanism both at structural and catalytic levels.

A comprehensive scan of the transition state space for the reaction of 2-(trimethylsiloxy)furan and methacrolein offered a satisfactory explanation of the high like diastereoslectivity obtained experimentally in the Mukaiyama vinylogous aldol reaction of these and related partners. It was determined that the syn-gamma-hydroxyalkylbutenolides are formed preferentially following a g(super +) orientation of the two reactants with the aldehyde in the s-trans conformation.

In previous work, (E)-2-enynyl pyridines were reported to yield indolizinyl singlet carbenes through base-catalyzed E/Z isomerization followed by a 5-exo-dig pseudocoarctate cyclization. We report herein that in the presence of ethyl acrylate these carbenes undergo stereoselective cis-cyclopropanation due mainly to electrostatic interactions in the transition state. The scope of this carbene generation scheme has been further explored through the preparation of a symmetric bis(pyridylenyl)alkyne which spontaneously furnished the biindolizine core in a one-pot reaction. Computational characterization of this transformation suggests a highly asynchronous double cyclization. [ABSTRACT FROM AUTHOR]

The PIDA-mediated oxidative decarboxylation of oxamic acids in the presence of alcohols is shown to afford the corresponding urethanes under thermal conditions. Computational and experimental mechanistic exploration allows to rationalize the different reactivity of PIDA as compared to related cyclic BI-OAc and highlights the importance of the enhanced acidity of the proton in the carbamoyl radical intermediate., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The site-selective dearomatization of naphthols is realized in a straightforward manner through a gold(I)-catalyzed [3,3]-sigmatropic rearrangement/allene functionalization cascade sequence. The method employs readily available naphthylpropargyl ethers as starting materials. A range of densely functionalized dihydrofurylnaphthalen-2(1H)-ones are obtained in high yields (up to 98 %) and extremely mild reaction conditions (reagent grade solvent, air, 10 minute reaction time). A complete theoretical elucidation of the reaction machinery is also proposed, providing a rationale for important issues such as regio- and chemoselectivity., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)