Abstract Selective terminal oxidation of 3, 3-dimethyl-4-pentenoates does occur under chloride-free Wacker conditions [Pd (OAc) 2/O 2] in AcOH to give 5-acetoxy-3, 3-dimethyl-4- pentenoates (7) and their analogues (2, 8) in good yields. Successive cyclization of 7 and 8 at vapor phase pyrolysis on SiO 2 affords 3, 4-dihydro-4, 4-dimethyl-2H-pyran-2-one (1).