Total syntheses of (+)-ricinelaidic acid lactone and of (-)-gloeosporone based on transition-metal-catalyzed C-C bond formations

Total syntheses of the macrolides (R)-(+)-ricinelaidic acid lactone (6) and (-)-gloeosporone (7), a fungal germination self-inhibitor, are presented, which are distinctly shorter and more efficient than any of the previous approaches to these targets reported in the literature. Both of them benefit from the remarkable ease of macrocyclization of 1,omega-dienes by means of ring-closing olefin metathesis (RCM) using the ruthenium carbene la as catalyst precursor. The diene substrates are readily formed via the enantioselective addition of dialkylzinc reagents to aldehydes in the presence of catalytic amounts of Ti(OiPr)(4) and bis-triflamide 18 and/or the stereoselective allylation of aldehydes developed by Keck et al, using allyltributylstannane in combination with a catalyst formed from Ti(OiPr)(4) and (S)-(-)-1,1'-bi-2-naphthol. Comparative studies show this latter procedure to be more practical than the stoichiometric allylation reaction employing the allyltitanium-alpha,alpha,alpha',alpha'-tetraaryl-1,3-dioxolane-4,5-dimethanol complex 3b. Finally, a method for the efficient ring closure of 4-pentenoic acid esters by RCM is presented that relies on the joint use of la and Ti(OiPr)(4) as a binary catalyst system. These results not only expand the scope of RCM to previously unreactive substrates but also provide additional evidence for the important role of ligation of the evolving ruthenium carbene center to a polar relay substituent on the substrate which constitutes the necessary internal bias for the RCM-based macrocyclization process.