Exploration of conjugate addition routes to advanced tricyclic components of mangicol A

Article abstract of DOI:10.1016/j.tet.2006.03.066

Two synthetic approaches to the cytotoxic marine natural product known as mangicol A are described. The starting material common to both pathways is the cyclopentenonecarboxylate 11. The first tactic involves the 1,4-addition to 11 of the cuprate derivable from iodide 10, while the second proceeds via base-promoted conjugate addition of the regiospecifically generated enolate anion of 41. The first strategy proceeds by a series of efficient steps to tricyclic aldol 21 and subsequently to β-diketone 7. The latter proved to be totally unresponsive to schemes aimed at introduction of a butenyl group. The second approach involves earlier introduction of this substituent as realized in stereocontrolled fashion via transition state 42. While further passage to 44 proved uneventful, this advanced intermediate and analogs thereof proved remarkably recalcitrant to cyclization in the precedented fashion. In no instance was generation of a suitable product realized. These studies serve to underscore the extent to which steric considerations can complicate matters and the extent to which they must be skirted. Finally, a direct enantioselective route to the side chain aldehyde 2 is detailed.