DIASTEREOSELECTIVE CYCLOISOMERIZATIONS OF ENEDIYNES VIA PALLADIUM CATALYSIS

Considerations of atom economy drive a search for reactions that are simple additions which, performed intramolecularly, are cycloisomerizations. Exposure of acyclic enediynes to a catalyst generated by mixing a Pd(O) complex with acetic acid normally in the presence of a phosphine ligand creates [5.6.51 and [6.6.51 tricycles with extraordinary diastereoselectivity of remote stereogenic centers. Effects of substituents on the tethers as well as the olefinic and acetylenic bonds reveal a wide tolerance of functionality. While geminal substitution facilitates the cycloisomerization, it is not required. Allylic oxygen plays a role as a regioselectivity control element. Cycloisomerization dominates over allylic ionization in such cases by proper tuning of the ligand. The mechanism of this reaction appears to vary depending upon the structure of the substrate. In the normal cases, the process involves three stages, initiation, propagation, and termination. Chemoselective initiation at the acetylenic linkage closest to one of the chain's termini occurs by hydropalladation. Propagation entails intramolecular carbametalations. Termination by beta-hydrogen elimination generates a hexatriene that undergoes high rotoselectivity in its disrotatory cyclization to generate the final product. Blocking formation of the hexatriene shuts down reaction. With substrates bearing a gamma-siloxypropiolate as the acetylenic initiator, cycloisomerization forms a tricycle with different positions of the double bonds. In contrast' to the case of the other substrates, blocking formation of a hexatriene does not shut down cycloisomerization. Invoking a novel intramolecular Diels-Alder reaction of a dienylpalladium intermediate derived from the diyne moiety with the olefin, likely assisted by coordination to palladium, accounts for our observations. The ease of availability of the acyclic substrates because of the versatility of the acetylenes combined with the high chemo-, regio-, and diastereoselectivity makes this atom-economical reaction a very practical approach for the construction of polycycles.