Enolate trapping

In general bromination of 20-ketones is directed to the introduction of functionality at C-21. However, on occasion 17-bromo compounds are required for dehydrobromination to A -20-ketones, although these are generally obtained in other ways. Kinetic enolization of a 20-ketone gives the A °-enol, whereas the thermodynamic product is the A kjsomer. An interesting enolate trapping reaction has been used recently to prepare 16-methyl-A -20-ketones ... 

Enolate trapping, 97 Epoxidation of steroidal dienes, 7 Epoxidation of steroidal monoenes, 2 4, 5/3-Epoxycholestan-3 -ol, 27 5a,6a-Epoxy-5a-cholestan-3 -ol, 82 5a,6a-Epoxy- 17-cy anoandr ost-16-en-3 jS-ol acetate, 20... 

Enolate generation, 106-7 Enolate trapping, 99-101 Enones, 34-5 Epoxidation, 21-3 a/3-Epoxysilanes, 21-4, 78 -Ethoxy acylsilane, 110 1-Ethoxy-l-trimethylsilyloxycyclo-propane,133 Ethyl bromoacetate, 123 Ethyl 2-chloropropanoate, 133 Ethyl glycinate, 87,88-9 Ethyl m-nitrobenzene, 137 Ethyl irimethylsilylacetate. 71, 123-4, 134 Ethyllithium, 66... 

C. Regioselective silylation of ketones by in situ enolate trapping... 

Chiang, Y. Kresge, A. J. Hellrung, B. Schunemann, P Wirz J. Flash photolysis of 5-methy 1-1,4-naphthoquinone in aqueous solution kinetics and mechanism of photoenoli-zation and of enol trapping. Helv. Chim. Acta 1997, 80, 1106-1121. 

Scheme 2.28 Functionalized allenes obtained by 1,6-cuprate addition to acceptor-substituted enynes and regioselective enolate trapping with methyl triflate (77), aldehydes (78, 79), ketones (80) and silyl halides (81).

Use of TMSCl in combination with HMPA, DMAP, or TMEDA all favored 1,2-addition over 1,4-addition. Sequential a-alkoxyalkylcuprate conjugate addition, enolate trapping with TMSCl, and silyl enol ether alkylation provides a one-pot synthesis of tetrahydrofurans (Scheme 3.35) [129]. Cyclic enones afford as-fused tetrahydrofurans, while acyclic systems give complex mixtures of diastereomers. a-Alkoxyalkylcopper reagents also participate in allylic substitution reactions with ammonium salts [127]. 

The [1,4]-Wittig rearrangement is potentially useful not only for the carbon-carbon bond formation but also for enolate formation. However, synthetic applications have been rather limited, because of the low yields and restricted range of substrates. Schlosser s group have developed a practical approach to aldehydes based on a [1,4]-rearrangement/ enolate trapping sequence. In contrast, standard aqueous workup gave poor yield of aldehyde. This protocol was employed as the key step in a synthesis of pheromone (102) from 99 via 100 and 101 (equation 56f. ... 

It has been demonstrated35 that enolate trapping by the electrophilic iron-carbene complex 1 provides 2. Alkylation at sulfur followed by a-elimination again generates a carbene, which then inserts stoichiometrically 1,5 into the C —H bond to give the octahydroindenone 3. Several other applications of this cyclization have been published 3fi. 

Due to their inherent polarizability, a-halo-/3-ketosulfoxides may be used as electrophilic partners in desulfination reaction to generate metal enolate. Therefore, treatment of a-halo-/3-ketosulfoxides with EtMgBr gives magnesium enolates. Trapping these reagents with various electrophiles allows the preparation of a-haloketones (equation 79, Table 10). 

Review Taylor has reviewed the conjugate addition-enolate trapping reactions of organocopper reagents, in particular of lithium dialkylcuprates (131 references). 

The general reaction sequence has been named more specifically as tandem vicinal dialkylation, nucleophilic-electrophilic carbacondensation,11 dicarbacondensation1213 or conjugate addition-enolate trapping,14 usually in reference to the fact that most of the reaction examples available create two new vicinal carbon-carbon bonds. Many noncarbon nucleophiles and electrophiles also are known, resulting... 

For the most part, alkynic and allenic ketones have found limited use in conjugate addition-enolate trapping sequences 69,81-83 their analogous esters have been used with far greater frequency (vide infra). Alkynic ketones, in particular, have found use in development of a new anionic polycyclizadon method consisting of intramolecular Michael addition followed by intramolecular alkylation (equation 15).84... 

In addition to the simple one-carbon electrophile formaldehyde, carbon dioxide67 292 and carbonyl sulfide273 also are excellent one-carbon electrophiles for use in conjugate enolate trapping. 

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