ChemInform Abstract: Synthesis of Phenyl-Substituted 2H,5H-Pyrano[4,3-b]pyran-5-ones and Related Heterocycles via a Domino Knoevenagel Condensation/6π-Electron Electrocyclization of 4-Hydroxy-6-phenyl-2H-pyran-2-one with Cyclic and Acyclic α,β-Unsaturated

Autor: Juergen Conrad, Sabine Mika, Uwe Beifuss, Iris Klaiber, Heiko Leutbecher, Sylvia Rieg
Rok vydání: 2010
Předmět:
Zdroj: ChemInform. 41
ISSN: 1522-2667
0931-7597
DOI: 10.1002/chin.201001147
Popis: Microwave-assisted syntheses are a particularly attractive alternative to syntheses under thermal conditions since they often proceed much faster and deliver productswith higher yields and higher purity [1]. Upon conventional heating using an external heat source like an oil bath or an electric heater the energy transfer depends on the thermal conductivity of the sample to be penetrated, which is relatively slow and inefficient. In contrast, the energy of the microwaves is directly transferred to the molecules of the reaction mixture via dielectric heating. The heating is largely caused by dipolar polarization and ionic conduction. Due to the transparency of the reaction vials normally used for microwave-assisted syntheses an inverted temperature gradient is established. Wall effects are minimized, and the reaction mixture is heated from inside. In addition to the purely thermal/kinetic effects and the “specific microwave effects” a number of scientists involved in microwave technology also consider “nonthermal microwave effects” which are supposedly based on a direct interaction between the electric field and specific molecules of the reaction mixture. It has been proposed that the electric field induces orientation effects of polar molecules/intermediates which affect the preexponential factor A or the activation energy (entropy
Databáze: OpenAIRE
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