Enynes semihydrogenation

The ease of reduction is 1-alkynes > disubstituted alkynes > 1-alkenes, indicating that selective semihydrogenation of triple bonds can be achieved in molecules containing double bonds. For example, nonconjugated enynes can be hydrogenated regio-and stereospecifically in high yields. 

Synthesis of previtamin Ds. The reagent, bis(ethylenediamine)chromium(II), was employed by a British group8 in the first total synthesis of previtamin D3, the immediate product of irradiation of 7-dehydrocholesterol. Condensation of the lithium derivative of the trimethylsilyl ether of the enyne (2) with the chloroketone (1) gives the chlorohydrin (3). This is converted into the enynene (4) by reaction with bis(ethylenediamine)chromium(II). Semihydrogenation with Lindlar s catalyst completes the synthesis. The overall yield from (1) is over 25%. 

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