Chiral guanidine catalyzed cyclization reactions of 1,3-enynes for lactone synthesis: switchable H-bond catalysis

Abstract

H-bonding catalysis has attracted much attention in asymmetric reactions. Taking advantage of guanidine-amides as bifunctional organocatalysts, we successfully achieved cyclization as well as 1,4-conjugate addition/cyclization of azlactones with acyclic and cyclic α-alkynyl-α,β-enones, respectively. The enol-type activation facilitated the generation of lactones via inverse electron-demand hetero-Diels–Alder reactions, diverging from the typical formation of allenes. A diverse array of δ-lactone derivatives were obtained in high yields and with excellent ee (up to >99%) and dr (>19 : 1). DFT calculations elucidate that the switchable hydrogen bonds formed by the positively charged guanidinium and neutral amide with various acceptors impart versatile activity and stereoselectivity.