Synthesis and kinetic evaluation of substrate-based phospholipid analogues and studies towards the synthesis of 5-hydroxyaloin A

Abstract

In the studies to establish structure-reactivity relationships in the hydrolytic reaction of phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidyl L-serine (PS) catalyzed by the phospholipase C from Bacillus cereus (PC-PLCBc), a number of water-soluble, non-hydrolyzable substrate based inhibitors of PLCBc were prepared. These include ω-hydroxy phosphorodithioates and phosphonates of choline, ethanolamine and L-serine. Kinetic assays reveal that all the ω-hydroxy phosphorodithioates are good inhibitors with good aqueous solubilities. However, ethanolamine and L-serine derivatives of phosphonates fail to inhibit PLCBc at their maximum solubilities. The three-dimensional structures of phosphonate-PS with E4G, E4Q and wild type PC-PLCBc revealed that these mutants bind the PS analogue in a very different manner than wild-type does a PC analogue. The structural difference shed new views on our understanding of the mechanistic and kinetic aspects of PLCBc catalyzed hydrolytic reactions. In the application of our glycosyl furan/benzyne cycloaddition methodology towards natural product synthesis, a two-stage benzyne/furan cycloaddition strategy was used to assemble the anthrone core of Group I C-aryl glycoside 5-hydroxyaloin A. Proof of concept was established in the generation of benzyne from a chloronaphthol precursor 4.142 and subsequent cycloaddition with furan afford the cycloadduct 4.143. However, the cycloadducts of 4.142 and alkoxy/silyloxy furans were unstable, and attempts to convert them into 5-hydroxyaloin A were unsuccessful. During the course of the investigation, cycloadditions using glycal-substituted furans were investigated and a one-step novel approach to the C-aryl glycal was established starting from 2-deoxy sugar lactone.