Involved in the biosynthesis of tri- and tetraketide alpha-pyrones. Pks18 catalyzes the extension of medium- and long-chain aliphatic acyl-CoA substrates by using malonyl-CoA as an extender molecule to synthesize polyketide products. 2 H(+) + hexanoyl-CoA + 2 malonyl-CoA = 4-hydroxy-6-pentylpyran-2-one + 2 CO2 + 3 CoA 2 H(+) + 2 malonyl-CoA + octanoyl-CoA = 4-hydroxy-6-heptylpyran-2-one + 2 CO2 + 3 CoA dodecanoyl-CoA + 2 H(+) + 2 malonyl-CoA = 4-hydroxy-6-undecylpyran-2-one + 2 CO2 + 3 CoA dodecanoyl-CoA + 3 H(+) + 3 malonyl-CoA = 4-hydroxy-6-(2-oxotridecyl)pyran-2-one + 3 CO2 + 4 CoA 2 H(+) + hexadecanoyl-CoA + 2 malonyl-CoA = 4-hydroxy-6-pentadecylpyran-2-one + 2 CO2 + 3 CoA 3 H(+) + hexadecanoyl-CoA + 3 malonyl-CoA = 4-hydroxy-6-(2-oxoheptadecyl)pyran-2-one + 3 CO2 + 4 CoA 2 H(+) + 2 malonyl-CoA + octadecanoyl-CoA = 4-hydroxy-6-heptadecylpyran-2-one + 2 CO2 + 3 CoA 3 H(+) + 3 malonyl-CoA + octadecanoyl-CoA = 4-hydroxy-6-(2-oxononadecyl)pyran-2-one + 3 CO2 + 4 CoA eicosanoyl-CoA + 2 H(+) + 2 malonyl-CoA = 4-hydroxy-6-nonadecylpyran-2-one + 2 CO2 + 3 CoA eicosanoyl-CoA + 3 H(+) + 3 malonyl-CoA = 4-hydroxy-6-(2-oxohenicosyl)pyran-2-one + 3 CO2 + 4 CoA 9-decenoyl-CoA + 2 H(+) + 2 malonyl-CoA = 4-hydroxy-6-(non-8-en-1-yl)-pyran-2-one + 2 CO2 + 3 CoA 9-decenoyl-CoA + 3 H(+) + 3 malonyl-CoA = 4-hydroxy-6-(2-oxoundec-10-en-1-yl)pyran-2-one + 3 CO2 + 4 CoA 3-hydroxytetradecanoyl-CoA + 2 H(+) + 2 malonyl-CoA = 4-hydroxy-6-(2-hydroxytridecyl)-pyran-2-one + 2 CO2 + 3 CoA 15-hydroxypentadecanoyl-CoA + 2 H(+) + 2 malonyl-CoA = 4-hydroxy-6-(14-hydroxytetradecyl)-pyran-2-one + 2 CO2 + 3 CoA 15-hydroxypentadecanoyl-CoA + 3 H(+) + 3 malonyl-CoA = 4-hydroxy-6-(16-hydroxy-2-oxohexadecyl)pyran-2-one + 3 CO2 + 4 CoA 16-hydroxyhexadecanoyl-CoA + 2 H(+) + 2 malonyl-CoA = 4-hydroxy-6-(15-hydroxypentadecyl)-pyran-2-one + 2 CO2 + 3 CoA 16-hydroxyhexadecanoyl-CoA + 3 H(+) + 3 malonyl-CoA = 4-hydroxy-6-(17-hydroxy-2-oxoheptadecyl)pyran-2-one + 3 CO2 + 4 CoA 12-hydroxyoctadecanoyl-CoA + 2 H(+) + 2 malonyl-CoA = 4-hydroxy-6-(11-hydroxyheptadecyl)-pyran-2-one + 2 CO2 + 3 CoA 12-hydroxyoctadecanoyl-CoA + 3 H(+) + 3 malonyl-CoA = 4-hydroxy-6-(13-hydroxy-2-oxononadecyl)pyran-2-one + 3 CO2 + 4 CoA 2-methylhexadecanoyl-CoA + 2 H(+) + 2 malonyl-CoA = 6-(hexadecan-2-yl)-4-hydroxy-pyran-2-one + 2 CO2 + 3 CoA 2-methylhexadecanoyl-CoA + 3 H(+) + 3 malonyl-CoA = 4-hydroxy-6-(3-methyl-2-oxoheptadecyl)pyran-2-one + 3 CO2 + 4 CoA 3-methylundecanoyl-CoA + 2 H(+) + 2 malonyl-CoA = 4-hydroxy-6-(2-methyldecyl)-pyran-2-one + 2 CO2 + 3 CoA 3-methylundecanoyl-CoA + 3 H(+) + 3 malonyl-CoA = 4-hydroxy-6-(4-methyl-2-oxododecyl)pyran-2-one + 3 CO2 + 4 CoA 12-methyltridecanoyl-CoA + 2 H(+) + 2 malonyl-CoA = 4-hydroxy-6-(11-methyldodecyl)-pyran-2-one + 2 CO2 + 3 CoA 12-methyltridecanoyl-CoA + 3 H(+) + 3 malonyl-CoA = 4-hydroxy-6-(13-methyl-2-oxotetradecyl)pyran-2-one + 3 CO2 + 4 CoA 12-methyloctadecanoyl-CoA + 2 H(+) + 2 malonyl-CoA = 4-hydroxy-6-(11-methylheptadecyl)-pyran-2-one + 2 CO2 + 3 CoA 12-methyloctadecanoyl-CoA + 3 H(+) + 3 malonyl-CoA = 4-hydroxy-6-(13-methyl-2-oxononadecyl)pyran-2-one + 3 CO2 + 4 CoA Lipid metabolism; fatty acid biosynthesis. Homodimer. Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.