Progress Toward the Enantioselective Total Synthesis of Ineleganolide and the Polycyclic Norcembranoid Diterpenes and Construction of the Ineleganoloids

Abstract

Ineleganolide, horiolide, kavaranolide, sinulochmodin C, scabrolide A, scabrolide B, and yonarolide are related polycyclic furanobutenolide norcembranoid natural products that exhibit potent biological activity, and feature highly oxygenate carbocyclic scaffolds with complex stereochemical frameworks. Herein, we describe a unified synthetic approach toward ineleganolide and the related furanobutenolide norcembranoids. Assembly of the tetracyclic scaffold of ineleganolide is accomplished in a convergent manner from (R)-carvone and an enantioenriched cis-1,3-cyclopentenediol fragment, which is constructed by a key enantioselective allylic alkylation. From the combined product of the two major fragments, we employ a tandem intramolecular cyclopropanation/Cope rearrangement to furnish the cycloheptene carbocyclic core characteristic of ineleganolide. Synthetic manipulations of this carbocyclic core are extensively investigated; several constitutional isomers of ineleganolide are synthesized through these studies. Many approaches are explored to assist in the completion of the synthesis. Computational studies inform our understanding of the conformational bias of late-stage intermediates. The retroaldol-aldol carbocyclic isomerization from the core of ineleganolide to the core of sinulochmodin C is also investigated. Additionally, our work on the (3 + 2) cycloadditions of heterocumulenes with donor–acceptor cyclopropanes and with N–H- and N–sulfonyl aziridines is disclosed. The exploration of substrate scope, reaction mechanism, and the enantiospecific formation of heterocyclic products in investigated.