Stereoselective formation of triphenylene ketals.
The oxidative trimerization of catechol ketals by MoCl(5) or MoCl(5)/TiCl(4) mixtures leads preferentially to the all-syn stereoisomer of the corresponding triphenylene ketal. The concomitant metal salts of the oxidative coupling most probably form a multinuclear template that directs the diastereoselectivity in a subsequent isomerization step under electrophilic conditions. Several functionalities can serve as coordination sites for the multinuclear metal chloro clusters. Suitable functional groups have to be stable towards the strong electrophilic and oxidizing conditions. Therefore, esters, nitriles, nitro derivatives, triazoles, and pyridines are successfully employed. Based on the flexibility and size of the substrate, different reagent mixtures lead to the stereoselective formation of the all-syn derivatives.
Boshta
Nader M
NM
Kekulé Institute for Organic Chemistry and Biochemistry, Bonn University, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany.
Bomkamp
Martin
M
Schnakenburg
Gregor
G
Waldvogel
Siegfried R
SR