Enolate moiety

The acetoxy dienone (218) gives phenol (220). Here, an alternative primary photoreaction competes effectively with the dienone 1,5-bonding expulsion of the lOjS-acetoxy substituent and hydrogen uptake from the solvent (dioxane). In the case of the hydroxy analog (219) the two paths are balanced and products from both processes, phenol (220) and diketone (222), are isolated. In the formation of the spiro compound (222) rupture of the 1,10-bond in the dipolar intermediate (221) predominates over the normal electron transmission in aprotic solvents from the enolate moiety via the three-membered ring to the electron-deficient carbon. While in protic solvents and in 10-methyl compounds this process is inhibited by the protonation of the enolate system in the dipolar intermediate [cf. (202), (203)], proton elimination from the tertiary hydroxy group in (221) could reverse the efficiencies of the two oxygens as electron sources. 

Allyl anion synthons A and C, bearing one or two electronegative hetero-substituents in the y-position are widely used for the combination of the homoenolate (or / -enolate) moiety B or D with carbonyl compounds by means of allylmetal reagents 1 or 4, since hydrolysis of the addition products 2 or 5 leads to 4-hydroxy-substituted aldehydes or ketones 3, or carboxylic acids, respectively. At present, 1-hetero-substituted allylmetal reagents of type 1, rather than 4, offer the widest opportunity for the variation of the substitution pattern and for the control of the different levels of stereoselectivity. The resulting aldehydes of type 3 (R1 = H) are easily oxidized to form carboxylic acids 6 (or their derivatives). 

As shown above, the electronic properties have a serious effect on the rate of the reaction. It means that the aromatic ring should occupy the same plane with that of the estimated intermediate enol moiety. Then, it is supposed that the conformation of the substrate is already restricted when it binds to the active site of the enzyme. The evidence that supports this estimation is the inactiveness of a-methyl-o-cWorophenyl and a-naphthylmalonic acids. This is a marked difference with the fact that a-methyl-p-Cl-phenyl and methyl-(3-naphthylmalonic acids are... 

Silyl enol ethers are an elegant means to protect the reactive and hence labile enolate moiety [15]. At the time of reaction, the enolate group is generated as an intermediate and reacts with the carbonyl-carrying compound. 

This finding is also in agreement with another three-component Michael/aldol addition reaction reported by Shibasaki and coworkers [14]. Here, as a catalyst the chiral AlLibis[(S)-binaphthoxide] complex (ALB) (2-37) was used. Such hetero-bimetallic compounds show both Bronsted basicity and Lewis acidity, and can catalyze aldol [15] and Michael/aldol [14, 16] processes. Reaction of cyclopentenone 2-29b, aldehyde 2-35, and dibenzyl methylmalonate (2-36) at r.t. in the presence of 5 mol% of 2-37 led to 3-hydroxy ketones 2-38 as a mixture of diastereomers in 84% yield. Transformation of 2-38 by a mesylation/elimination sequence afforded 2-39 with 92 % ee recrystallization gave enantiopure 2-39, which was used in the synthesis of ll-deoxy-PGFla (2-40) (Scheme 2.8). The transition states 2-41 and 2-42 illustrate the stereochemical result (Scheme 2.9). The coordination of the enone to the aluminum not only results in its activation, but also fixes its position for the Michael addition, as demonstrated in TS-2-41. It is of importance that the following aldol reaction of 2-42 is faster than a protonation of the enolate moiety. 

The synthesis of (-)-hirsutine (2-795) was concluded by removal of the Boc-group, condensation with methyl formate, and methylation of the formed enol moiety. In a similar manner as was described for 2-795, (+)-dihydrocorynantheine (2-797) [399] with the (3S)- and (15 k)-configuration was synthesized from ent-2-800. 

Diboration of a,/3-unsaturated esters is catalyzed by the platinum(0)/diimine catalyst, giving a,/3-diboryl esters, that is, 3,4-addition products (Equation (13)). Although the a,/3-diboryl ester products are hydrolytically more stable than the corresponding 1,4-addition products bearing a boron ester enolate moiety, they gradually undergo hydrolysis... 

Much attention has been devoted to the examination of chiral enolate systems in which metal ion chelation may play an important role in establishing a fixed stereochemical relationship between the resident chirality and the enolate moiety. This has resulted in the conclusion that enolate geometry is critical in the definition of 7r-l acial selection. The following sections discuss this effort in several different chemical systems. 

When the enol ring is adjacent to a cyclic moiety, then it is possible to achieve very short hydrogen bonds, as in the structure of usnic acid, a natural product found in lichens. A low-temperature X-ray diffraction analysis of this compound showed two enol moieties, one in which a carbon-carbon bond of the enol was part of a cyclohexenone ring, and this had... 

Vinyl oxetane 310 was opened with lithinm and a catalytic amonnt (ca 1%) of DTBB in THF at —78 to 0°C to give the intennediate 311, which reacted with electrophiles at —78°C yielding, after hydrolysis, the corresponding products 312 (Scheme 91) . As shown, the electrophile reacted with the more reactive enolate moiety of the intermediate 311 instead at the benzylic position. 

In the monoanionic enolates from the ethers and other 2-substituted 1-acylpyrrolidines, chelation should be much weaker or absent, leading to less restricted rotation of the enolate moiety. Furthermore, rotation of the pyrrolidine substituent and conformational mobility of the pyrrolidine ring can lead to inversion of the nitrogen lone pair. These factors may explain the reversed and lower diastereoselectivity observed with these substrates. 

The observed stereoselectivity of the reaction might be explained by transition state 18, as shown in Figure 9.5. The Si-face of the assumed Breslow-type intermediate would be shielded by the tert-butyl group of the bicyclic catalyst, thus limiting any attack to the. Re-face of the hydroxy enamine. Furthermore, the substituents of the enol moiety might cause a pre-orientation of the approaching... 

The authors used silver salts since gold salts catalyzed the reaction with R=H (giving oxazole 34, Scheme 5.16) but not with R=Me. Moreover, only traces of the desired furopyrrolidinone were formed with the use of a cationic gold species activated with silver additives. Therefore, silver traces were thought to be the active reagent. Indeed, on activation of compound 33 mediated by AgN03 in the presence of sodium acetate (Scheme 5.16), the enol moiety V can then accomplish a nucleophilic attack to produce the pyrrolidinone W and after protonolysis give compound X. Pyrrolidinone Y (the enol version of X) can, in turn, be subject to an oxidative cyclization to yield the furopyrrolidinone 35. Two equivalents of silver salts are needed for the activation step and the oxidative cyclization. 

The chiral alcohol group in Figure 13.42 was chosen to differentiate as much as possible between the half-spaces on both sides of the enolate plane. One half-space should he left entirely unhindered while the other should be blocked as completely as possible. The reaction of the alkylating reagent then occurs preferentially, and in the ideal case exclusively, from the unhindered half-space. The stereostructures of the two ester enolates of Figure 13.42 therefore model the enolate moieties of the (early ) transition states of these alkylations. The part of the transition state structure that contains the alkylating reagent is not shown. 

Metal-mediated intramolecular addition of silyl enolates to alkynes is also valuable for the synthesis of cyclic ketones. A stoichiometric amount of HgCl2 or EtAlCl2 effectively promotes the cycloalkenylation via anti-addition to alkynes (Equations (87) and (88)).319 320a The -addition mode can be explained by a metal coordination to the triple bond and subsequent attack of the enolate moiety from the opposite side to the metal. The resultant alkenylmetals can be used for carbon-carbon and carbon-heteroatom bond formation as well as protonation. 

Hereinafter, the extra- and intra- annular chirality transfer (CT) classification defined by Evans402 will be employed provided the original stereocenter is linked to the enol moiety by one or two points of anchorage, with the possibility of chelate-enforced chirality transfer due to internal chelation with heteroatoms (Figure 4)410. 

However, the (/i/X)-geometry of the Breslow intermediate has not yet been determined, but is of relevance for the pre-orientation of the second aldehyde molecule. As shown in transition state 103, the corresponding ( )-isomer would probably favor a Si-Si-attack and therefore a (R)-configuration in the product. An unfavorable sterical interaction between the phenyl substituent of the enol moiety and the phenyl sub-... 

Asymmetric synthesis via enolate intermediates has been extensively studied. Asymmetric induction can be divided into five main categories (1) a chiral auxiliary covalently linked to an enolate moiety,2,3 (2) a chiral ligand of a countercation of an enolate,4-6 (3) a chiral electrophile,7,8 (4) a chiral Lewis acid,9,10 and (5) a chiral phase-transfer catalyst.11,12 Rather than reviewing these examples, we introduce here the principle of asymmetric induction for... 

This allylic C-H activation can be, in some specific cases, in competition with the direct transformation of monosubstituted enol ether into vinylic organometallic derivatives. Compound 116 reacts faster with 21 by the enol moiety to lead to 117 than with the remote double bond of 116, which would have given dienyl 118 after hydrolysis (Scheme 43). For the isomerization reaction to proceed, higher substitution of the enol ether is necessary. 

---