Evolution of the Petasis-Ferrier union/rearrangement tactic: construction of architecturally complex natural products possessing the ubiquitous cis-2,6-substituted tetrahydropyran structural element.

The frequent low abundance of architecturally complex natural products possessing significant bioregulatory properties mandates the development of rapid, efficient, and stereocontrolled synthetic tactics, not only to provide access to the biologically rare target but also to enable elaboration of analogues for the development of new therapeutic agents with improved activities and/or pharmacokinetic properties. In this Account, the genesis and evolution of the Petasis-Ferrier union/rearrangement tactic, in the context of natural product total syntheses, is described. The reaction sequence comprises a powerful tactic for the construction of the 2,6- cis-substituted tetrahydropyran ring system, a ubiquitous structural element often found in complex natural products possessing significant bioactivities. The three-step sequence, developed in our laboratory, extends two independent methods introduced by Ferrier and Petasis and now comprises: condensation between a chiral, nonracemic beta-hydroxy acid and an aldehyde to furnish a dioxanone; carbonyl olefination; and Lewis-acid-induced rearrangement of the resultant enol acetal to generate the 2,6- cis-substituted tetrahydropyranone system in a highly stereocontrolled fashion. To demonstrate the envisioned versatility and robustness of the Petasis-Ferrier union/rearrangement tactic in complex molecule synthesis, we exploited the method as the cornerstone in our now successful total syntheses of (+)-phorboxazole A, (+)-zampanolide, (+)-dactylolide, (+)-spongistatins 1 and 2, (-)-kendomycin, (-)-clavosolide A, and most recently, (-)-okilactomycin. Although each target comprises a number of synthetic challenges, this Account focuses on the motivation, excitement, and frustrations associated with the evolution and implementation of the Petasis-Ferrier union/rearrangement tactic. For example, during our (+)-phorboxazole A endeavor, we recognized and exploited the inherent pseudo symmetry of the 2,6- cis-substituted tetrahydropyranone product to overcome the inherent chelation bias of an adjacent oxazolidine ring during the Lewis-acid-promoted rearrangement. In addition, we discovered that a more concentrated solution of Cp2TiMe2 (0.7 versus 0.5 M in THF) with the addition of ethyl pivalate dramatically improves the yield in the Petasis-Tebbe olefination. During the (+)-zampanolide and (+)-dactylolide programs, we observed that the addition of trifluoromethanesulfonic acid (TfOH), especially on a preparative scale, was crucial to the efficiency of the initial condensation/union reaction, while our efforts toward (-)-kendomycin led to the improved implementation of a modified Kurihara condensation of the beta-hydroxy acid and aldehyde involving i-PrOTMS and TMSOTf. Finally, the successful deployment of the Petasis-Ferrier tactic in our synthesis of (-)-clavosolide A validated the viability of this tactic with a system possessing the highly acid-labile cyclopropylcarbinyl moiety, while the challenges en route to (-)-okilactomycin demonstrated that a neighboring alkene functionality can participate in an intramolecular Prins cyclization during the TMSOTf-promoted union process, unless suitably protected.