Enynes stereospecific synthesis

The ds-hydrogenation of cyclopropenes is unexceptional in itself but when preceded by the addition of diazopropane to the yne component of an enyne (Section II.B) it provides a stereospecific synthesis of chrysanthemic acid esters (natural pesticides) in high yield. Reduction of unsymmetrical cyclopropenes with lithium aluminium... 

Dienes and 1 -enynes. Negishi et al.9 have extended the synthesis of 1,3-enynes by palladium-catalyzed cross-coupling of alkynylzinc chlorides with an alkenyl halide (8, 472) to a similar stereospecific synthesis of 1,5-dienes or 1,5-enynes by coupling an alkenyl halide with a homoallyl- or homopropargylzinc chloride. An example is the synthesis of the 1,5-enync I (equation I). This reaction... 

Pal et al. reported a facile and mild procedure for the regio- and stereospecific synthesis of novel enyne derivatives 131 via the copper-free palladium-catalyzed sequential coupling of 3-iodo(thio)flavone 130 with terminal alkynes [53] (Schane 6.34). 

Enynes. In the presence of this Pd(II) catalyst, vinyl iodides couple with alkynyltrimethylstannanes with retention of the geometry of the vinyl iodide to provide 1,3-enynes in high yields.2 Since the conjugated triple bond can be reduced to a (Z)-double bond stereospecifically, the coupling also provides a route to (Z,E)-or(Z,Z)-l,3-dienes. An example is the synthesis of bombykol (1), the sex attractant of the silkworm moth. 

Enynes were prepared in good yield from alkynyl iodonium salts and alkenylcopper reagents, stereospecifically. This approach was suitable for the synthesis of conjugated enynes, using a trisubstituted alkene with complete retention of its geometry [47], 1,3-Diynes were similarly obtained by coupling alkynyl iodonium... 

Shortly after the discovery of enyne metathesis, Trost began developing cycloisomerization reactions of enynes using Pd(ll) and Pt(ll) metallacyclic catalysts (429-433), which are mechanistically divergent from the metal-carbene reactions. The first of these metal catalyzed cycloisomerization reactions of 1,6-enynes appeared in 1985 (434). The reaction mechanism is proposed to involve initial enyne n complexation of the metal catalyst, which in this case is a cyclometalated Pd(II) cyclopentadiene, followed by oxidative cyclometala-tion of the enyne to form a tetradentate, putative Pd(IV) intermediate [Scheme 42(a)]. Subsequent reductive elimination of the cyclometalated catalyst releases a cyclobutene that rings opens to the 1,3-diene product. Although this scheme represents the fundamental mechanism for enyne metathesis and is useful in the synthesis of complex 1,3-cyclic dienes [Scheme 42(fe)], variations in the reaction pathway due to selective n complexation or alternative cyclobutene reactivity (e.g., isomerization, p-hydride elimination, path 2, Scheme 40) leads to variability in the reaction products. Strong evidence for intermediacy of cyclobutene species derives from the stereospecificity of the reaction. Alkene... 

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