Experimental and Computational Investigations of the Reactions between α,β-Unsaturated Lactones and 1,3-Dienes by Cooperative Lewis Acid/Brønsted Acid Catalysis

The reactions of α,β-unsaturated δ-lactones with activated dienes such as 1,3-dimethoxy-1-[(trimethylsilyl)oxy]-1,3-butadiene (Brassard's diene) are barely known in literature and show high potential for the synthesis of isocoumarin moieties. An in-depth investigation of this reaction proved a stepwise mechanism via the vinylogous Michael-products. Subsequent cyclisation and oxidation by LHMDS and DDQ, respectively, provided six mellein derivatives (30-84 %) and four angelicoin derivatives (40-78 %) over three steps. DFT-calculations provide insights into the reaction mechanism and support the theory of a stepwise reaction.

Acyl-chain elongation drives ketosynthase substrate selectivity in trans-acyltransferase polyketide synthases

Type I modular polyketide synthases (PKSs), which are responsible for the biosynthesis of many biologically active agents, possess a ketosynthase (KS) domain within each module to catalyze chain elongation. Acylation of the KS active site Cys residue is followed by transfer to malonyl-ACP to yield an extended β-ketoacyl chain (ACP = acyl carrier protein). To date, the precise contribution of KS selectivity in controlling product fidelity has been unclear. Six KS domains from trans-acyltransferase (trans-AT) PKSs were subjected to a mass spectrometry based elongation assay, and higher substrate selectivity was identified for the elongating step than in preceding acylation. A close correspondence between the observed KS selectivity and that predicted by phylogenetic analysis was seen. These findings provide insights into the mechanism of KS selectivity in this important group of PKSs, can serve as guidance for engineering, and show that targeted mutagenesis can be used to expand the repertoire of acceptable substrates.