Modular type I polyketide synthases (PKSs) comprise a family of enzymes that synthesize a diverse class of natural products with medicinal applications. The biochemical features of these systems include the extension and processing of polyketide chains in a stepwise, stereospecific manner, organized by a series of modules divided into distinct catalytic domains. Previous work revealed that a primary hurdle for utilizing PKS modules to create diverse macrolactones hinges on the selectivity of the thioesterase (TE) domain. Herein, we generated novel hybrid 12-membered macrolactone/lactam ring systems employing unnatural amide-containing hexaketide intermediates in conjunction with an engineered TE S148C mutant from the pikromycin (Pik) biosynthetic pathway. Specifically, unnatural macrocycle (