Synthesis of Biomimetic Thioesters for Studies of Ketoreductase Domains from the Biosynthesis of Cytotoxic Polyketides

Abstract

The synthesis of biomimetic thioesters for enzymatic studies of ketoreductase (KR) domains from polyketide synthases is described. A TBS-protected dihydroxyalkene fragment was synthesised by a sequence containing Nagao acetate aldol reaction, Mukaiyama propionate aldol reaction and methylene Wittig olefination. Fragment coupling to N-acetylcysteamine (SNAC) (E)-3-hydroxyhex-4-enethioates using olefin cross-metathesis (OCM) and deprotection gave the potential KR product stereoisomers. Analogous OCM with a SNAC (E)-3-ketohex-4-enethioate did not give the desired KR precursor, but could successfully be substituted by a Horner-Wadsworth Emmons olefination between a SNAC 3-ketothioesterphosphonate and a TBS-protected dihydroxyaldehyde. After deprotection, an intramolecular cyclisation was observed that needs to be considered as a spontaneous side reactivity in the enzymatic assays.