An acyl-Claisen approach to the synthesis of lignans and substituted pyrroles

The acyl-Claisen rearrangement, also called a zwitterionic aza-Claisen rearrangement, allows for the synthesis of 2,3-syn-substituted morpholine pent-4-eneamides with high levels of diastereoselectivity. A wide variety of alkyl and aryl substituents can be introduced with yields highly dependent on the stoichiometry of the Lewis acid catalyst. The use of these morpholine amides in the synthesis of the tetrasubstituted tetrahydrofuran lignans fragransin A2, talaumidin, and galbelgin is summarized. The conversion of the Claisen-derived amides into aryl tetraline and 1,1-diarylbutanol lignans via alteration of the protecting groups is also described. Nucleophilic addition of an organometallic reagent to the morpholine amide followed by Wacker oxidation of the alkene gives highly substituted 1,4-diketones, which can be easily converted into fully substituted pyrroles.