Using enolates

Reductants such as zinc or sodium dithionite generate the semidione from diketones. Electrolytic reduction can also be used. Enolates can reduce diones to semidiones by electron transfer. 

In a recent publication, Perumal and coworkers [441] described the condensation of an aldehyde 2-863 with an aniline 2-864 to give an imine which is trapped by a dienophile. However, when using this approach an intermolecular cycloaddition takes place as the reaction is performed as a three-component process using enol ethers or cyclopentadiene as dienophiles (Scheme 2.192). When using enol ether 2-... 

Similar polymerization of MMA using enolate-zirconocene catalysts has also been found [223]. The mechanism of this catalytic reaction is related to the process described in Scheme XI because the cationic enolate complex is isolobal to that of the corresponding lanthanide complex. Recently, similar cationic... 

An interesting variant involves the use of an allylic alcohol as the alkene component. In this process, re-oxidation of the catalyst is unnecessary since the cyclization occurs with /Uoxygen elimination of the incipient cr-Pd species to effect an SN2 type of ring closure. Both five- and six-membered oxacycles have been prepared in this fashion using enol, hemiacetal, and aliphatic alcohol nucleophiles.439,440 With a chiral allylic alcohol substrate, the initial 7r-complexation may be directed by the hydroxyl group,441 as demonstrated by the diastereoselective cyclization used in the synthesis of (—)-laulimalide (Equation (120)).442 Note that the oxypalladation takes place with syn-selectivity, in analogy with the cyclization of phenol nucleophiles (1vide supra). 

Table 9 Vinylogous aldol reactions using enolate and aldehyde activation ...

Based on the same strategy, Denmark and coworkers developed a vinylogous aldol reaction using enolate activation with a catalyst derived from SiCl4 and dimeric phosphoramide 47 [24,25]. This strategy relies on the observation that not all Lewis acid - Lewis base interactions diminish the Lewis acidity [26-28]. Due to the formation of a pentacoordinated silicon cation (48), both the enolate and the substrate can be assembled in a closed transition state, giving rise to the observed high selectivities (Scheme 19) [29,30]. 

As will be discussed more thoroughly in Section 3.2.5, transition metal carbene complexes can mediate olefin metathesis. Because heteroatom-substituted carbene complexes are usually less reactive towards olefins than the corresponding nonheteroatom-substituted complexes, it is, e.g., possible to use enol ethers to terminate living polymerization or other types of metathesis reaction catalyzed by a non-heteroatom-substituted carbene complex. Olefin metathesis can also be used to prepare new heteroatom-substituted carbene complexes (Figure 2.15, Table 2.11). 

The D-ring of steroids has been cleaved frequently by a Beckmann fragmentation. Typical strategy uses enolate chemistry to introduce an oximino group at the a-carbonyl carbon. Reduction of the carbonyl (or addition of a carbanion) produces an a-hydroxy oxime which serves as good substrate for the fragmentation. Two examples of this strategy are shown in equations 213 and 214. 

Show how, using enolate chemistry and organoselenium reagents, you could convert... 

Trimethylsilyloxy-substituted alkenes are by far the most widely used enol ethers because of their straightforward preparation from the corresponding ketones (equation 20)78-82 -pjjg electron-rich character of silyl enol ethers allows for highly chemoselective cyclopropanations in the presence of additional double bonds (eqnation 21). ... 

Have you noticed that we have now fulfilled the specification of frame 219 we have used enols as nucleophiles in reaction of types a, b, and c ... 

Using enol ethers of acetophenone 207 or similar compounds in a reaction with 206 and 2 one primarily obtains 2-phenyl-substituted N-heterocydes 209, which can undergo hydrogenolysis under the reaction conditions to afford the corresponding open-chain compounds 210 as mixtures of several diastereomers (Scheme 5.41). 

Conia and his co-workers144 modified the Simmons-Smith reaction145,146 by using enol ether plus predominantly diiodomethane/zinc-silver couple instead of the common known diiodomethane/zinc-copper couple147. This method is an excellent access to cyclopropanols and related compounds148 149 (Scheme 29). Furthermore,... 

As discussed in Section III J, in general, catalytic asymmetric aldol reactions have been studied using enol silyl ethers, enol methyl ethers, or ketene silyl acetals as a starting material. So far several types of chiral catalysis have been reported.75-85 The chiral lanthanoid complex prepared from Ln(OTf)3 and a chiral sulfonamide ligand was effective in promoting an asymmetric Mukaiyama aldol reaction with a ketene silyl acetal.86 The preparation of the catalyst and a representative reaction are shown in Figure 45. 

The results obtained using enol ethers derived from enantiomerically pure / -ketosulfoxides as substrates were more interesting due to the higher stability of... 

Suggest how the following products might be made using enol or enolate alkylation as at least one step. Explain your choice of specific enol equivalents. 

In most of this chapter, we used enolates as our nucleophiles and worked under equilibrating conditions with alkoxide bases. There was alkoxide base present throughout the reaction, so the enolate didn t get used up deprotonating the product or, if it did, it could be re-deprotonated by the... 

The general route for the reactions leading to isoxazoles is shown in Scheme 2. A variation, using enolate ions as the substrate, is illustrated by the example shown in Scheme 3.101 As indicated, the isoxazole could be formed by cyclodehydration of an open-chain intermediate such as 35. 

An important advantage of this procedure is that vinyl triflates (e.g., 4 and 5) are obtained regioselectively using enolate chemistry - " and, therefore, regioselective formation of vinyl fluorides is also possible. 

Nevertheless, polymerizations other than the anionic ones have also been carried out using enolates as ligands of transition metal cations. Metal acetalyacetonates are indeed organometallic compounds that directly contributed to the control of radical polymerization, ring-opening polymerization and coordination polymerization of specific monomers, as discussed hereafter. 

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