Variously substituted linear enediynes phosphines oxides possessing the double bond at either the terminal or internal position and with the phosphine oxide appended onto the alkyne or the alkene terminus have been prepared. Their cobalt (I)-mediated cyclizations produce the η4-complexed tricyclic compounds in high yields. The endo/exo selectivity depends on both the position of the phosphine oxide on the enediyne and the position of ...