Substituent‐Controlled Electrocyclization of 2,4‐Dienones: Synthesis of 2,3,6‐Trisubstituted 2 H ‐Pyran‐5‐carboxylates and Their Transformations

A facile access to 2,3,6‐trisubstituted 2 H ‐pyran‐5‐carboxylates is developed by employing 2‐alkyl‐2‐enals as reactants with acetoacetates. The reaction involves Knoevenagel condensation followed by a 6π‐electrocyclization, in which the presence of the C2 alkyl substituent in the enals favors the formation of ( E )‐Knoevenagel adducts for the ensuing electrocyclization. The resulting 2 H ‐pyrans are hydrogenated to form 3,4‐dihydro‐2 H ‐pyrans and converted into the endoperoxides by singlet‐oxygen cycloaddition.