Amphiphilic allylation of arylidene-1,3-oxazol-5(4H)-one using bis-π-allylpalladium complexes: an approach to synthesis of cyclohexyl and cyclohexenyl α-amino acids

Abstract

An efficient method for synthesis of cyclohexyl and cyclohexenyl α-amino acids via palladium-catalyzed three-component assemblies followed by ring-closing metathesis (RCM) is described. The present catalytic reaction is successfully extended to substituted benzylidene azlactones 2a–j RCH=(1,3-oxazole): R = alkyl or aryl. The amphiphilic bis-allylation of these substrates has been achieved by replacing toxic allylstannanes with allyltrifluoroborate and the reaction proceeded smoothly to afford the corresponding 1,7-diene derivatives 3a–j in acceptable to good yields. RCM of the resulting octadienes using the first generation Grubbs catalyst gave easy access to stereodefined substituted cyclohexene derivatives 7–11 in high yields. Acid hydrolysis of the oxazolone ring of 7–10 gave protected amino acids 12–16. Debenzoylation of 13 and 15 afforded 1-amino-6-aryl-cyclohex-3-enecarboxylic acids 17 and 18 in excellent yields, respectively. Moreover, catalytic reduction of 13 gave the corresponding cyclohexane derivative 19 which could be debenzoylated to give 1-amino-2-phenylcyclohexene-1-carboxylic acid (20). The structures of compounds 9, 12 and 13 were confirmed by X-ray structural analysis. It is an excellent method for creating a wide range of cyclic α,α-disubstituted α-amino acids.