A key missing tool in the chemist’s toolbox is an effective biocatalyst for macrocyclization. Macrocycles limit the conformational flexibility of small molecules, often improving their ability to bind selectively and with high affinity to a target, making them a privileged structure in drug discovery. Resorcylic acid lactones (RALs) are a class of fungal macrocyclic polyketides that exhibit anti-cancer and anti-malarial activity among others. The thioesterases (TEs) found in the biosynthetic pathways for the zearalenone (Zea) and radicicol (Rdc) resorcylic acid lactones are responsible for macrocyclization and show promising traits as biocatalysts. These RAL TEs show the highest substrate tolerance of any polyketide thioesterase to date. These TEs can efficiently cyclize 12- 18-membered rings, 14-membered macrolactams, and amino acid containing substrates. Their robustness is evident in their ability to retain activity after lyophilization/re-suspension and in high DMSO concentrations. Furthermore, the ability of Zea and Rdc TEs to macrocyclize depsipeptide substrates illustrates the first time a polyketide synthase TE has efficiently processed a peptide-containing substrate. The unique substrate tolerance of this class of TEs shows great potential as a viable biocatalyst. Herein we describe the synthesis and enzymatic results of diverse group of substrates, with the TEs from the radicicol and zearalenone biosynthetic pathways, as well initial results on the chemoenzymatic synthesis of asperterrestide A.