Dienolate ions

Z-Substituted Allyl Anions—Dienolate Ions. Z-Substituted allyl anions 4.45a are typified by dienolate ions 4.45b, which is how they are best drawn. They almost always react faster at the a carbon than at the y carbon, both with soft and hard electrophiles. 

The same factors may well account for the fact that protonation takes place faster at the a-carbon of a dienolate ion (36) than at the y-carbon. Again, a calculation35 on a... 

The enamine (419) with oxygen undergoes338 a radical-chain reaction, leading, after hydrolysis, to the 1,4-diketone (420). The site of attack contrasts with that we would expect by analogy with the site of protonation of a dienolate ion (p. 46), but the heteroatom is different, and that may be decisive. 

Z-Substituted Allyl Anions—Dienolate Ions. Electrophilic attack on Z-substituted allyl anions is almost always selective for attack at the a position. The problem is to explain it. 

Arylbutyraldehydes. These aldehydes are produced when 5-aryl-4,5-dihydro-l,3-dioxepins are subjected to reduction with Na-NHs. The transformation may involve elimination to furnish dienolate ions, which undergo reduction at the styrenic double bond. 

Alkylation of the dienolate ion derived from 4-methyl-19-nortestosterone (313) with CD3I in benzene solution gave the 4,4-dimethyl compound (314), with 70% of the labelled methyl groups in the 4j5-orientation. With t-butyl alcohol as solvent the product showed predominant (90%) introduction of 4a-CD3 (iWs latter conclusion, which reverses that reached in an earlier study, depended upon application of the nuclear Overhauser effect for the assignment of n.m.r. signals of the C-4 methyl groups). Solvent-dependence of the stereochemistry of alkylation has not been suspected before. ... 

Reactions Involving Enols or Enolic Derivatives.—Protonation of the dienolate ions was shown to be the rate-determining step in amino-catalysed isomeriz-ations of androst-5-ene-3,17-dione and 17a-ethynyl-17/8-hydroxyoestr-5(10)-en-3-one to the corresponding A -3-oxo-derivatives. The 6-methylenepreg-nenone (41) was available from the dimethylaminopregnenone (40), which was... 

The isomerization can be acid or base catalyzed. In the base-catalyzed reaction, the intermediate is the conjugated dienolate ion, which is reprotonated at the carbon terminus. 

An unexpected result from the same series of work as above by Perumal et al was obtained when cycloalkenones were used as dienophiles [187]. Instead of the imino Diels-Alder product, azabicyclic ketones were obtained (Figure 8.79). The authors proposed a novel Diels-Alder reaction between In-dienolate ions as dienes, and the imines as dienophiles. Another possibility brought up by the authors was that the products were the result of a tandem aldol reaction on the imine, followed by a Michael-type cyclization reaction. 

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