Crustosic acid (1) differs from terrestric acid (2) by a 5β-carboxylmethyl at the tetronate ring instead of a 5α-methyl group in Penicillium crustosum. The formation of 1 via carboxylcrustic and viridicatic acid was confirmed by gene deletion and heterologous expression. The conversion of 1 to 2 requires a decarboxylation-mediated olefination by TraH and subsequent reduction by TraD. The redox-assisted decarboxylation and stereoisomerization proved the biosynthetic relationships of fungal acyltetronates with different stereochemistry.