Transmetallation zinc derivatives from

The addition of carbonyl compounds towards lithiated 1-siloxy-substituted allenes does not proceed in the manner described above for alkoxyallenes. Tius and co-work-ers found that treatment of 1-siloxy-substituted allene 67 with tert-butyllithium and subsequent addition of aldehydes or ketones led to the formation of ,/i-unsaturated acyl silanes 70 (Scheme 8.19) [66]. This simple and convenient method starts with the usual lithiation of allene 67 at C-l but is followed by a migration of the silyl group from oxygen to C-l, thus forming the lithium enolate 69, which finally adds to the carbonyl species. Transmetalation of the lithiated intermediate 69 to the corresponding zinc enolate provided better access to acylsilanes derived from enolizable aldehydes. For reactions of 69 with ketones, transmetalation to a magnesium species seems to afford optimal results. 

When 303 was directly treated with Me2Cu(CN)Li2, the transmetallation failed to discriminate between the two carbon-metal bonds. By contrast, the allylzincation of the alkynyllithium derived from the propargylic alcohol 309 produced the alkenyl 1,1-dimetallic species 310, in which the two carbon-metal bonds exhibit different reactivities due to the presence of a metal-alkoxide. Indeed, transmetallation with Me2Cu(CN)Li2 led to the alkenyl copper-zinc species 311, which was relatively poorly reactive towards electrophiles but underwent successful 1,4-addition to ethyl propiolate leading to 312 in satisfactory overall yield (equation 145)180. 

Heterocycles. A route to 2,3-disubstituted furans takes advantage of the Cu-Zn-transmetallation (with ZnCl2) from enolates derived from conjugate organocuprate addition to enones, and aldol reaction of zinc enolates to an alkoxyacetaldehyde. ... 

Karoyan et al. accomplished an asymmetric synthesis of prolino-homotrypto-phan 50 via amino-zinc-ene-enolate cyclization of 47 followed by transmetalation of the cyclic zinc intermediate 48 with indolyliodide 49 [84]. The use of a Pd catalyst derived from Fu s [f-BusPHj-BFa was required to avoid the undesired p-hydride elimination. Proline chimeras such as 50 are useful tools for medicinal chemistry and/or biological applications. 

Marek and co-workers found that magnesium-to-zinc transmetalation of the alkoxides derived from addition of acetylenic Grignard reagents to acyl silanes promoted the 1,2-Brook rearrangement. The resulting propargyl zinc intermediates (e.g., 37), in equilibrium with the silyl allenol ethers of type 38, underwent diastereoselective carbocyclization in suitable systems, providing cyclopentanol 39, for example, after acid quench and desilylation. ... 

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