Metathesis of dicobalt hexacarbonyl systems and subsequent applications

A new method for the preparation of seven to nine-membered rings using ringclosing metathesis of a dicobalt hexacarbonyl alkyne- complex as the critical step is described. Involvement of the organometallic complex introduces a conformational constraint at the metathetical stage and serves as a mask for the alkyne functionality. Metathesis of a variety of substrates containing various types of oxidation in the pseudoparpargyl position is described. Thus, silyl ethers, acetates, and ketones were found to be compatible with metathesis. Results in a homologous series of pseudopropargyllic alcohols were found to metathesize in somewhat lesser yield due to a suspected chelation mechanism. Applications of the complexes in synthesis were generally found consistent with known transformations of acyclics, for example, decomplexation of nine-membered ring complex yielded a cyclononyne. Bicyclic and tricyclic ring systems were found to be accessible using maleic anhydride conversion, Nicholas reaction, or a Pauson-Khand reaction. An approach to a functionalized B-ring related to the curcusone framework resulted in successful closure of the B-ring using ring-closing metathesis of a dicobalt hexacarbonyl alkyne complex. Preliminary studies on the cyclopentenone A-ring using a silicon-tethered reductive Pauson-Khand reaction are also described.