Allenyl enolate protonation

Scheme 2.27 Regioselectivity of the protonation of the allenyl enolate obtained by 1,6-cuprate addition to enynoate 74.

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... 

In contrast to the protonation, the regioselectivity of reactions between other electrophiles and allenyl enolates derived from 2-en-4-ynoates is independent of the steric and electronic properties of the reaction partners.7,7a... 

Regioselective Protonation of Enolates. In a study of the regioselectivity of protonation of allenyl enolates, pivalic acid demonstrated superior selectivity in the example shown (eq 2), allene 1, resulting from protonation at C-2, was formed exclusively when the intermediate enolate was quenched with pivalic acid at-80°C. ... 

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