Phosphine-free palladium catalyzed Heck reaction for the synthesis of novel arotinoic acids

A phosphine-free palladium-catalyst condition for the Heck reaction using dicyclohexylamine as ancillary ligand was investigated, establishing a new Heck reaction system using phosphine-free palladium-catalyst, which can synthesize di- and tri-substituted stilbene compounds in high yields (72%similar to 84%) and high stereoselectivities (83%similar to 95%). Four novel stilbene derivatives of arotinoic methyl ester (3a, 3b, 4a and 4b) were synthesized by above method using 2,4,5-trimethoxystyrene and a-asarone as starting materials. Their hydrolysates the arotinoic acids (5a, 5b, 6a and 6b) were easily accessible under mild hydrolysis conditions using the LiOH, THF/H2O (V : V=5 : 1) system, and all the structures of synthesized compounds were confirmed by H-1 NMR, C-13 NMR, IR and MS techniques.