Gold(I)-Catalyzed Tandem Alkoxylation/Lactonization of γ-Hydroxy-α,β-Acetylenic Esters

The formation of 4-alkoxy-2(5H)-furanones was achieved via tandem alkoxylation/lactonization of g-hydroxy-a,b-acetylenic esters catalyzed by
2 mol% of [2,6-bis(diisopropylphenyl)imidazol-2-ylidine] gold bis(trifluoromethanesulfonyl)imidate [Au(IPr)(NTf2)]. The economic and simple procedure was applied to a series of various secondary propargylic alcohols allowing for yields of desired product of up to 95%. In addition, tertiary propargylic alcohols bearing mostly cyclic substituents were converted into the corresponding spiro derivatives. Both primary and secondary alcohols reacted with propargylic alcohols at moderate temperatures (65– 80 8C) in either neat reactions or using 1,2-dichloroethane as a reaction medium allowing for yields of
23–95%. In contrast to [Au(IPr)(NTf2)], reactions with cationic complexes such as [2,6-bis(diisopropylphenyl) imidazol-2-ylidine](acetonitrile)gold tetrafluoroborate
[Au(IPr)(CH3CN)][BF4] or (mhydroxy) bis{[2,6-bis(diisopropylphenyl)imidazol-2-
ylidine]gold} tetrafluoroborate or bis(trifluorometh- anesulfonyl)imidate – [{Au(IPr)}2(m-OH)][X] (X= BF4, NTf2) – mostly stop after the alkoxylation.
Analysis of the intermediate proved the exclusive formation of the E-isomer which allows for the subsequent lactonization.