Utility of polymer-supported reagents in the total synthesis of lamellarins

Abstract

Four solid-supported reagents have been utilized in the multistep synthesis of lamellarins. The use of Amberlyst A-26 Br3- and polymer bound pyridine hydrobromide perbromide (PVPHP) for keto α-bromination of the less studied ortho-substituted acetophenone derivatives selectively furnished the corresponding monobromination products (phenacyl bromide derivatives), which were used directly in condensation reactions with benzyldihydroisoquinoline mediated by Amberlyst A-26 NaCO 3-. The 2H-pyrrole carbonates subsequently underwent intramolecular Friedel-Crafts transacylation followed by lactonization to provide the lamellarin skeleton. Alternatively, Amberlyst A-26 NaCO 3- effectively served as base in condensation reaction of benzyldihydroisoquinoline with α-nitrocinnamate derivatives to provide the corresponding 2-ethoxycarbonyl pyrroles, which smoothly underwent O-debenzylation reaction followed by lactonization to furnish the lamellarin skeleton. The novel Amberlyst-15 mediated lactonization reactions effectively combined the otherwise two separate steps into a single transformation. © 2005 American Chemical Society.