Neue Heterazoliumsalze aus Prolin und Pyroglutaminsäure und Anwendungen in Carben-katalysierten asymmetrischen nucleophilen Acylierungen
The coenzyme thiamine (vitamin B1), a natural thiazolium salt, is involved in many enzymatic catalyses in vivo. It has been proposed that the catalytically active species of these reactions is a nucleophilic carbene. Consequently, enzyme mimetic asymmetric catalytic processes that are mediated by nucleophilic carbenes in vitro as, for instance, the benzoin condensation, the Stetter and other related nucleophilic acylation reactions have taken their place in the growing field of asymmetric organocatalysis. The synthesis of new carbene catalysts remains an important task aimed at the development of new reactions as well as unsolved synthetical problems, e. g. the asymmetric intermolecular Stetter reaction. Catalysis with bicyclic heterazolium salts: A chiral triazolium salt that had been developed in the group of Prof. Dr. D. Enders as precatalyst for an asymmetric benzoin condensation was employed in other carbene catalysed reactions (acyloin condensations, intermolecular Stetter reactions, reactions of the "conjugated umpolung"), but the nucleophilic carbene that resulted in situ from deprotonation with strong bases did not show any catalytic activity in any of these reactions. Bicyclic imidazolinium and thiazolinium salts could be obtained from proline derivatives, but none of these carbene precursors showed any catalytic activity in organocatalytic reactions. Triazolium salts derived from L-pyroglutamic acid in organocatalysis: In a one-pot synthesis protocol, several novel silyl-protected triazolium salts could be obtained from easily accessible L-pyroglutaminoles. The precatalysts were successfully utilised in the first asymmetric intramolecular crossed benzoin reaction generating acyloines in high yields and good enantioselectivities. A novel N-methyl triazolium salt gave the best results reported so far as the carbene precursor in an organocatalytic asymmetric intermolecular Stetter reaction.
info:eu-repo/classification/ddc/540, Chemie, asymmetrische Organokatalyse, nucleophile Acylierung, N-heterocyclische Carbene, asymmetric organocatalysis, nucleophilic acylation, N-heterocyclic carbenes
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