Palladium-catalyzed synthesis of indane and cyclobuta[a]indenes from homoallylic alcohols derived from Baylis–Hillman adducts: base-dependent stereoselectivity for the benzylidene group in cyclobuta[a]indene

Various indane and cyclobuta[a]indene derivatives were synthesized by palladium-catalyzed cyclization of homoallylic alcohol derivatives prepared from Baylis–Hillman adducts. Especially, cyclobuta[a]indene derivative was synthesized stereoselectively by palladium-catalyzed 5-exo-trig/4-exo-trig cascade cyclization, albeit in moderate yield. The Z isomer was formed exclusively in the presence of Et3N by usual Heck-type carbopalladation process while E isomer with Cs2CO3 most likely by a concerted metalation/deprotonation (CMD) process.