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978-3-8439-1131-3, Reihe Organische Chemie
Cascade Reactions Initiated by Pi-Activation. A Flexible Way to Heterocycles & Studies toward the Total Synthesis of Melohenine B
288 Seiten, Dissertation Bergische Universität Wuppertal (2013), Softcover, A5
As part of the project on the cascade reactions initiated by π-activation, two sequences were developed to produce tri- and tetra-substituted furans.
On one hand, by exploring the cascade process of heterocyclization/1,2-migration to provide 2,3-dihydrofurans, an unexpected furan was isolated. Nevertheless, as poly-substituted furans are interesting target structures, the catalyzed route from certain cyclohexanones was investigated and a new and convenient way to access tri-substituted furans with an aldehyde-containing side chain was developed. The sequence is catalyzed by PtCl4 and probably proceeds via a heterocyclization followed by 1,2- shift ring-contraction and a Grob-type fragmentation.
On the other hand, in the realm of the study on catalyzed propargyl-Claisen rearrangements launched in the Kirsch group, a sequence of propargyl-Claisen rearrangement/condensation on propargylic vinyl ethers has generated tri- and tetra-substituted furans. The domino reaction was effectively catalyzed by AgBF4, and to facilitate the analytical analysis, the process was terminated by hydrogenation of the double bond. However, the method presents some limitations in the case of phenyl moiety in R1 or in the case of a terminal double bond, where the yields drop dramatically to 30-40%.