Isotellurourea‐Catalyzed Enantioselective (4+2)‐Heterocycloadditions of But‐2‐ynoates and Michael Acceptors

Alk‐2‐ynoates are readily available starting materials which, in principle, can be utilized for cycloadditions in analogy to structurally similar allenoates. However, their significantly lower reactivity makes activation by established isothiourea and isoselenourea Lewis base catalysts difficult. Capitalizing on the higher nucleophilicity and basicity of our recently introduced class of isotellurourea catalysts we now succeeded in successfully employing alk‐2‐ynoates for asymmetric (4 + 2)‐heterocycloadditions as well, resulting in even improved enantioselectivities as compared to the use of analogous allenoates. These results demonstrate the high potential of the highly reactive isotelluroureas for the catalytic activation of less reactive starting materials where established catalyst classes come to their limits.