Lindlar reduction alkyne ring closing metathesis

Using this catalyst system for ring-closing alkyne metathesis, Fiirstner successfully synthesized ambrettolide and yuzu lactone from alcohol 125. " " Treatment of diyne 126a with Mo(CO)6 (5 mol%) and /i-chlorophenol (1 equiv.) in chlorobenzene at 140 °C leads to cycloalkyne 127a in 69% yield. Subsequent Lindlar reduction proceeds smoothly in a stereoselective manner to afford ambrettolide. Similarly, 126b affords cycloalkyne 127b in 62% yield. From this compound, yuzu lactone has been synthesized (Scheme 45). " " ... 

RCM of dienes to cycloalkenes provides a useful method for the syntheses of carbo- and heterocycles and thus has been proved to be extremely effective in total synthesis of various natural products. Usually, however, mixtures of (E)- and (Z)-olefms result. In contrast, ring-closing alkyne metathesis provides a reliable route for synthesis of both (E)- and (Z)-macrocycloalkenes in a stereoslective manner taking advantage of stereoselective partial reduction of resulting cycloalkynes. A Lindlar reduction gives (Z)-cycloalkenes, whereas a hydroboration/ protonation sequence afford ( )-cycloalkenes (Equation (23)). Recently, Trost reported an alternative procedure for the synthesis of (E)-olefins from alkynes through hydrosilylation by a ruthenium catalyst. This procedure converts cycloalkyne 130, for example, to vinylsilane 131 and then to (E)-cycloalkene 132 in a stereoselective manner (Scheme 46)7 ... 

---