γ-Regioselective Functionalization of 3-Alkenylindoles via 1,6-Addition to Extended Alkylideneindolenine Intermediates

Alkylideneindolenines are widely employed key electrophilic intermediates for the -functionalization of the C-3 side chain of indoles. However, the reactivity of their extended (vinylogous) counterparts has not been carefully explored so far. These intermediates can undergo 1,4- or 1,6-addition with functionalization at - or -position of the side chain, resulting in regioisomeric mixtures of products. This work demonstrates that a complete -regioselectivity can be achieved in the reaction of 3-indol-3-yl allylic alcohols with various nucleophiles. This process is catalysed by just 1mol% zinc(II) triflate at room temperature and entails the 1,6-selective addition of the nucleophile to an extended protonated alkylideneindolenine generated insitu. Indoles, pyrroles, anilines and thiols can be efficiently used as nucleophilic partners for this reaction, delivering the corresponding 3-vinyl substituted, -functionalised indole products in moderate to good yields.