Search

[Display omitted] Both concerted [4 + 2] cycloadditions and stepwise carbocation-alkene cyclizations have been proposed as means to construct the complex polycyclic cores of the artatrovirenols and daphenoid natural products during their biosyntheses. Here the energetic feasibility of each of these reactions is assessed using density functional theory calculations. While none of the previously proposed processes can be ruled out, an alternative process with a lower barrier is proposed. This process would allow for formation of a key C C bond during the cyclase phase of terpenoid biosynthesis. [ABSTRACT FROM AUTHOR]

Abstract: The results of quantum chemical calculations on a previously proposed biosynthetic pathway for the formation of calophyline A from rhazimol, which features formally forbidden 4-electron [1,2] and [1,3] sigmatropic shifts, are discussed. The proposed pathway was not found to be energetically viable, despite a surprisingly low predicted barrier for the [1,3] shift, but an alternative route involving a series of tautomerizations and carbonyl addition reactions is shown to be more energetically feasible. [Copyright &y& Elsevier]

Abstract: Results from density functional theory calculations on a proton-promoted [2+2] cycloaddition to form plumisclerin A’s cyclobutane ring are described. On the basis of these results, it is proposed that protonation allows for an energetically viable stepwise cycloaddition that provides an alternative to photochemical cyclobutane formation. [Copyright &y& Elsevier]

Abstract: Syntheses of several (sulfonyl)methylphosphonate analogs of geranyl, neryl, and farnesyl diphosphates are described. Key steps include utilization of an (E)-selective Horner–Wadsworth–Emmons olefination which couples an aldehyde to the sulfone phosphonate moiety, and a selective reduction of the resulting dienyl sulfone phosphonate substrates. [Copyright &y& Elsevier]

Abstract: Quantum chemical calculations (B3LYP/6-31G(d)) on carbocation rearrangements that are proposed to occur in the biosynthesis of aspernomine are described. Based on these calculations, a pathway involving a concerted but asynchronous [4+2] cycloaddition that avoids the formation of a secondary carbocation is proposed for small model systems. [Copyright &y& Elsevier]

Abstract: 3,4-Dihydro-2H-pyrrole derivatives were synthesized by the 1,3-dipolar cycloaddition reaction of nitrile ylides with acrylamides. Acrylamide substitution patterns and benzimidoyl chloride equilibration were investigated. [Copyright &y& Elsevier]

[Display omitted] Oxidopyrylium-based [5 + 2] cycloadditions between diethanolamine appended silyloxypyrones and organoboronic acid derivatives were achieved. Evidence for the diethanolamine boronate (i.e. DABO) complex was observed by 1H NMR at room temperature with boronic acids and [5 + 2] cycloaddition was achieved in a single reaction mixture in DCE. Pinacol boronates required elevated temperature (140 °C) in refluxing p-xylene but afforded access to products not readily available with boronic acids. Theoretical calculations provide evidence for an asynchronous concerted transition state. Good functional group tolerance was observed and products were isolated as the oxabicyclo[3.2.1]-octane DABO cycloadducts. [ABSTRACT FROM AUTHOR]

Abstract: The Davis–Beirut reaction, which provides an efficient synthesis of 2H-indazoles and, subsequently, indazolones, is shown to proceed rapidly from o-nitrosobenzaldehyde and primary amines under both acid and base catalysis. Experimental and theoretical evidence in support of a reaction mechanism is provided in which o-nitrosobenzylidine imine is a pivotal intermediate in this N,N-bond forming heterocyclization reaction. The Davis–Beirut reaction is also shown to effectively synthesize a number of novel 3-amino-2H-indazole derivatives. [Copyright &y& Elsevier]