Diynes semihydrogenation

Diynes can be employed in intramolecular ring-closing metathesis. Several catalysts involving Mo and W have been investigated. These cyclizations can be combined with semihydrogenation to give macrocycles with Z-double bonds. 

While unmodified Pd/Al203 was an effective catalyst for the semihydrogenation of the amino alkyne, 9, (Eqn. 16.8), the hydrogenation of the amino diyne, 10, over palladium in absolute ethanol gave the products resulting from the cyclization of the partially saturated triple bonds on the catalyst surface (Eqn. 16.9). ... 

The methyl esters of endiandric acids B, C, F, and G were prepared according to the reactions that are outlined in Scheme 19.21. Semihydrogenation of diyne 80 followed by heating to 100°C in toluene solution led in 28% yield to a ca. 4.5 1 mixture of endiandric acids B and C as their methyl esters (81 and 82, respectively). If the product of the semihydrogenation reaction was not warmed above 25°C, endiandric acid F and G methyl esters (83 and 84, respectively) could be isolated in ca. 15% and 12% yield. As was the case for the reactions of Scheme 19.20, none of the intermediate tetraene or cyclooctadiene could be isolated at 25°C. Nicolaou and co-workers synthetic work substantiates Black et al. s hypothesis, that this family of polycyclic natural products self-assemble thermally from the linear acids and without assistance from an enzyme. 

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