Toward the Total Synthesis of Norzoanthamine: The Development of a Transannular Michael Reaction Cascade

Abstract

Norzoanthamine is a complex heptacyclic marine alkaloid isolated from colonial zoanthids. It potently inhibits loss of bone weight and strength in a postmenopausal osteoporosis mouse model, but its mode-of-action remains unknown. The scarcity of this natural product from its natural source and the need to access analogs for structure-activity relationship (SAR) study make it necessary to chemically synthesize this compound. However, the complex molecular skeleton, especially the highly functionalized and stereochemically complex ABC core structure of the natural product poses a significant challenge.

As part of our efforts to develop a practical synthetic route to norzoanthamine, we systematically explored a transannular Michael reaction cascade in the context of the synthesis of angular 6-6-6 tricyclic ring system, a mimic of the ABC core structure of norzoanthamine. Using 1,7-bis-enones in the form of 14-membered macrocyclic lactone as model substrates, we demonstrated that both E,Z- and E,E-macrocycles underwent facile transannular reactions to give cis-syn-cis and trans-anti-trans ring systems, respectively. However, Z,E- and Z,Z- macrocycles did not cyclize under similar reactions. The similarities and differences between transannular Diels-Alder reactions and this transannular cyclization process were also disclosed.

Building upon these preliminary studies, we developed a 12-linear step synthesis of the ABC carbocyclic core of norzoanthamine. It features an organocatalytic asymmetric intramolecular aldolization to set the stereochemistry of the entire molecule, a fragment coupling based on selective alkylation of a bis-enolate, and a transannular Michael reaction cascade for rapid and stereoselective synthesis of the polycyclic core. Subsequent Claisen rearrangement enabled installation of a handle for introduction of the bottom piece to complete the total synthesis. Other efforts toward the total synthesis have also been discussed.