Allenyl electrophiles alkynylation

Although complex 43 does not react with phenylacetylene and methane, the products resulting from the formal addition of H-C bonds of these molecules to the C -Cp and Cp-C. bonds of the allenylidene ligand of 43 can be easily obtained [23]. Because in the allenylidene ligand of 43, the C and atoms are electrophilic centers, and the Cp atom is nucleophilic, the synthetic strategy involves the initial nucleophilic attack of the corresponding carbanion at the C or atoms and the subsequent protonation of the resulting allenyl or alkynyl derivatives. 

The alkynylation of propargyl and allenyl electrophiles gives satisfactory results for the synthesis of allenynes, as illustrated by the highly stereoselective conversion of the chiral propargyl tosylate 311 into the allenyne 312, which is the key step of a recent total synthesis of nemotin (Scheme 4.72) [225]. 

Allenyl ethers are useful key building blocks for the synthesis of a-methylene-y-butyrolactones [129, 130], The synthesis of the antileukemic botryodiplodin was accomplished with the crucial steps briefly presented in Scheme 8.56. Bromoallenyl ethers 225 were easily prepared by base-induced isomerization from the corresponding /3-bromoalkyl alkynyl ether compounds and then subjected to electrophilic bro-mination with NBS. The resulting acetals 226 were converted into 2-alkoxy-3-methy-lenetetrahydrofurans 227 by dehydrohalogenation of the alkenyl bromide unit to an alkyne and subsequent radical cyclization employing tributyltin hydride [130],... 

As previously mentioned, allenes can only be obtained by 1,6-addition to acceptor-substituted enynes when the intermediate allenyl enolate reacts regioselectively with an electrophile at C-2 (or at the enolate oxygen atom to give an allenyl ketene acetal see Scheme 4.2). The regioselectivity of the simplest trapping reaction, the protonation, depends on the steric and electronic properties of the substrate, as well as the proton source. Whereas the allenyl enolates obtained from alkynyl enones 22 always provide conjugated dienones 23 by protonation at G-4 (possibly... 

As indicated in Section ni.B, deprotonation of a carbamate affords a dipole-stabilized a-amino-organolithium that can be transmetalated with copper salts to form cuprates, thereby expanding the versatility of the organolithium. Suitable electrophiles include enones, alkenyl, alkynyl, allenyl and dienyl carboxylic acid derivatives, nitriles and sulfoxides. Dieter and coworkers have shown that the same process can be accomplished via transmetalation of a stannane (Scheme 36). The procedure is particularly... 

Similar to the addition reactions of acceptor-substituted dienes (Scheme 16), the outcome of the transformation depends on the regioselectivity of the nucleophilic attack of the organocopper reagent (1,4- vs. 1,6-addition) and of the electrophilic capture of the enolate formed. The allenyl enolate obtained by 1,6-addition can afford either a conjugated diene or an allene upon reaction with a soft electrophile, and thus opens up the possibility to create axial chirality. The first copper-mediated addition reactions to Michael acceptors of this type, for example, 3-alkynyl-2-cyclopentenone 75,... 

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