The 3,4-dihydro-2,5-dimethyl-2H-pyran-2-carbaldehyde (methacrolein dimer) 1 was oxidized at 20-60 degrees C with air in the presence of polymeric Mn(II), Co(II) and Cu(II) complexes. The 3,4-dihydro-2,5-dimethyl-2H-pyran-2-carboxylic acid 2 formed at the first stage of oxidation undergoes self condensation with 1 to a monoester of hemiacetal 3. Gradual addition of hydroxyl groups of 3 to the vinyl-ether groups of 3 resulted in the formation of a mixture of cyclic oligo(acetal-esters) 5 (14-, 21- and 28-membered rings).