I. Platinum, gold, and ruthenium-catalyzed reactions of multi-ynes: Reactivity and selectivity. II. Synthetic studies on natural products: 3R,9R,10R-panaxytriol and (-)-dactylolide

Facile metallotropic [1,3]-shifts have been observed in reactions mediated by various transition metals. We employed carbophilic transition metals such as Pt and Au, which have emerged as powerful homogeneous catalysts in the past few years. The reaction of 1,3-diynes containing tethered alkenes with PtCl2/CO involves metallotropic [1,3]-shift via the formation of 5-exo-type Pt-alkylidenes, where the preference for 5- exo or 6-endo alkylidene formation depends on the heteroatom substituent in the tether. 1,3-Diynes containing propargylic ester functionality with Pt- or Au-complexes generate [1,2]-acetate migrated alkylidene intermediates which undergo the metallotropic [1,3]-shift. An alkyne moiety in 1,3-diynes as an electron-withdrawing substituent steers [1,2]-acyl migration over [1,3]-migration. We also found that a propargylic alkoxy group promots [1,2]- over [1,3]-migration to generate Pt-carbenoids as intermediates. In addition, the complementary and contrasting reactivity of vinyl vs. alkynyl Pt-carbenoids has been discovered. Experimental data indicate that vinyl Pt-carbenoids are more prone to a [1,2]-H shift, whereas alkynyl Pt-carbenoids tend to undergo addition to double and triple bonds in both intra- and intermolecular reactions.;We have realized an unusually high level of regio- and site-selectivity in the Alder ene reactions of 1,3-diynes and 1,3,5-triynes. The selectivity profile clearly indicates that one of the alkynyl moieties of 1,3-diynes not participating in the reaction determines the regiochemistry, whereas the interplay between steric hindrance and polar substituents at the propargylic sites determines the site-selectivity.;The synthetic utility of the Ru-catalyzed Alder ene reaction of multi-ynes was demonstrated in the total synthesis of (3R,9R,10R)-panaxytriol by providing enyne sustrates as starting materials. Furthermore, a tandem sequence of relay metathesis--metallotropic [1,3]-shift--cross metathesis was used for the synthesis, showing the utility of a metathesis-based tandem reaction for the synthesis of natural products with highly unsaturated carbon skeletons.;Our synthesis for (--)-dactylolide highlights some synthetic methodologies including a two-step tetrahydropyran synthesis via the Ru-catalyzed Alder ene reaction followed by a Pd-catalyzed pi-allyl etherification and a Re-catalyzed allylic rearrangement. More exploratory work and optimization of the established route are in progress.