Silver versus Gold Catalysis in Tandem Reactions of Carbonyl Functions onto Alkynes: A Versatile Access to Furoquinoline and Pyranoquinoline Cores

Abstract

An efficient and versatile tandem process of acetalization and cycloisomerization reactions has been developed for the reactions of 1-alkynyl-2-carbonylquinoline substrates. The reaction occurs thanks to Au(I) and Ag(I) catalysis. Silver(I) catalysis has been extensively studied (11 different silver species) on a broad range of quinoline derivatives (variation of alkyne substituent, of carbonyl function and of nucleophiles), leading to a variety of furoquinoline and pyranoquinoline moieties. An insight is given for the presumed mechanism along with DFT-B3 LYP/6-31G** calculations to address the 6-endo and 5-exo regioselectivities observed.

Silver-catalyzed direct couplings of 2-substituted furans with cyclic 1,3-dicarbonyls

“…By contrast, in all the cases we examined we observed only C-C coupling reaction on the carbonyl moiety. Overall the oxidative properties of silver catalysts may direct a stronger coordination than gold ones to the oxygen atom of the carbonyl function [15]. It is reasonable to assume that activation of the carbonyl group, followed by a Friedel-Crafts-type reaction could be responsible of the formation of the corresponding alkenyl derivative.…”

Tandem imination/annulation of γ- and δ-ketoalkynes in the presence of ammonia/amines

“…Indeed, by switching to the less reactive 2-alkynyl-acetophenones as substrates their reaction with ammonia is not observed at all in the absence of a catalyst. By choosing as model system the reaction of 1-{2-[(4-methylphenyl)ethynyl]phenyl} ethanone 19a with ammonia, we tried some metal catalysts potentially able to promote both the imine formation and the intermolecular hydroamination step (Scheme 11) [36] [37], gold [38] and indium [39], in the intramolecular annulation reactions involving alkynes bearing a proximate nucleophile. As reported in Scheme 11, a catalytic amount of TiCl 4 gave rise to a complex mixture of unidentified products (Scheme 11, entry 1) whereas the less acidic complex TiCl 4 $2THF gave very poor result also after an extended reaction time (Scheme 11, entry 2).…”

Silver‐Catalyzed Cycloisomerization Reactions

“…They accessed diverse furoquinoline and pyranoquinoline cores (e.g., 31 and 32, respectively, Table 5.1). 55 One particularly interesting aspect of this work is that, starting from the same quinoline derivatives and depending on the nature of the silver salt used, one could access either a furoquinoline or a pyranoquinoline product. This behavior was rationalized from the value of the pK a of the conjugate acid of the silver counterion (Table 5.1).…”