Torquoselectivity in the cationic cyclopentannelation of (2Z)-hexa-2,4,5-trienal acetals

Acid-promoted cycloisomerizations of phenylallenes bearing acetalic functions at the ortho position: a stereocontrolled entry to indeno-fused dioxepanes, dioxocanes and thioanalogues

Two different approaches convert ortho acetal-substituted phenylallenes into several classes of indeno-fused heterocycles.

Electrophilic activation of allenenes and allenynes: analogies and differences between Brønsted and Lewis acid activation

This review focuses on electrophilic activation of allenes by an acid. Key mechanistic features are presented together with recent synthetic applications.

Beyond the Divinyl Ketone: Innovations in the Generation and Nazarov Cyclization of Pentadienyl Cation Intermediates

The requirement for new strategies for synthesizing five-membered carbocycles has driven an expansion in the study of the Nazarov cyclization. This renewed interest in the reaction has led to the discovery of several interesting new methods for generating the pentadienyl cation intermediate central to the cyclization. Methods reviewed include carbon-heteroatom ionization, functionalization of a double bond, nucleophilic addition, or electrocyclic ring opening. Additional variations employ unconventional substrates to produce novel pentacycles, such as the iso- and imino-Nazarov. Herein, we provide an overview of these unconventional, yet highly useful versions of the Nazarov cyclization.

Density Functional Studies on the Nazarov Reaction Involving Cyclic Systems

Density functional theory (DFT) has been used to define the energy profiles of the Nazarov reaction involving cyclic systems. The calculations were carried out at the B3LYP/6-311G** level of theory and the solvent (dichloromethane) contribution was estimated by using the recently developed SM5.43R solvation model. DFT calculations were first carried out to determine the energy profiles associated with the electrocyclization reactions of 3-hydroxy- and 3-ethoxypentadienyl cations in which one of the double bonds is embedded in O-heterocyclic and carbocyclic systems. In particular, the effects on the reaction rate of modifications to the substrate, as well as the presence of the heteroatom in the cycle, have been investigated. The torquoselectivity of the electrocyclization reaction was then explored with substituted O-heterocycles to understand the factors that control the stereochemical outcome of the process that preferentially provides 2,5-trans-disubstituted products. These DFT-based results rationally explain most of the experimental observations related to the Nazarov reaction of the substrates herein investigated and could be useful in the rational interpretation, and likely in the prediction, of the outcome of Nazarov reactions involving other cyclic systems.

2-Alkylidenesulfol-3-enes by (regio- and) stereoselective cheletropic addition of SO2 to (di)vinylallenes

[reaction: see text] The cheletropic addition of SO(2) to divinylallenes is regioselective, taking place at the most substituted vinylallene and at the E unit if vinyl groups of opposite geometry are competing. Diastereofacial differentiation results from the approach of the reagent from the less-substituted direction of the allene and from the concomitant disrotatory movement of the termini of the vinylallene to afford the sterically more congested 2-alkylidenesulfol-3-ene isomer. DFT computations confirm the regio- and the stereoselectivity of these cheletropic additions.