Enolates Regioselective formation

Scheme 10.6 Regioselective formation of 3-hydroxylated enol esters resulting from anti-Markovnikov addition.

The regioselective formation of a cw-hexalone containing a homonuclear diene which is useful for elaboration into ocddentalol [178] by the condensation of 4-methyl-3-cyclohexenone with methyl coumalate is rather surprising. It seems that C-3 of the cyclohexenone, under the influence of the ketone group (perhaps via the enol), prefers an acceptor role. 

Scheme 7.9 Regioselective formation of oxetanes derived from silyl enol ethers and allylic silanes.

The reaetion of ketones (with the exception of methyl ethyl ketone) with 1 and diiso-propy lethyl amine (DPEA) leads to regioselective formation of alkcny loxy dialkoxy boranes. A methyl group is deprotonated with high selectivity. A group is deprotonated almost exclusively in the presence of a CHR, group to form the (Z)-enolate (equation 1). ... 

This coupling in conjunction with the regioselective formation of vinyl triflates from enolates (see N-phenyltrifluoromethanesulfonimide, this volume) provides a short convergent synthesis of pleraplysillin-1 (1, Chart I). 

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. 

Facile 6-elimination of the silyl group is also utilized in the intramolecular anodic olefin coupling reactions [159-161]. For example, the intramolecular anodic coupling of enol ether with allylsilane group has been reported [Eq. (44)]. This reaction seems to be quite useful for the construction of functionalized cyclic compounds because it leads to the regioselective formation of olefinic product via a facile 6-silyl elimination. 

Regioselective formation of thermodynamic enolates (or their corresponding silyl enol ethers) can be accomplished by treatment of unsymmetrical ketones with KH, or with KH, t-BuMe2SiCl in the presence of HMPAA ... 

If the electrophile is a vinyl triflate, it is essential to add LiCl to the reaction so that the chloride may displace triflate from the palladium o-complex. Transmetallation takes place with chloride on palladium but not with triflate. This famous example illustrates the similar regioselectivity of enol triflate formation from ketones to that of silyl enol ether formation discussed in chapter 3. Kinetic conditions give the less 198 and thermodynamic conditions the more highly substituted 195 triflate. 

This approach can use the inherent regioselectivity of silyl enol ether formation (chapter 3) using kinetic or thermodynamic enolisation. Hence kinetic enolisation of enones (chapter 11) occurs on the a side leading to 2-Me3SiO-butadienes such as 222. Epoxidation of this silyl enol ether gives the unstable silyloxy ketone 223 which can be desilylated by fluoride ion and hence transformed into the hydroxyketone 225 or acetoxy ketone 224. These transformations are useful because the hydroxy ketones can be unstable34 (see below). 

The construction of the pyranooxepin system shown in equation (52) proceeds by way of a dibenzox-epinone acetic acid, followed by intramolecular enol ester formation.67 The regioselective cyclization of 4-(2-naphthyloxy)butanoyl chloride proceeds as expected (equation 53) using tin(lV) chloride as the catalyst, but a mixture of products was obtained when the related carboxylic acid was treated with poly-phosphoric acid.68 The preparation of a thieno[fr]suberanone in 40% yield has been achieved by the interaction of tin(IV) chloride with 5-(2-thienyl)valeryl chloride.69 Once again, the expected increase in yield is obtained when cyclization occurs to the 2-position in thiophene. The product shown in equation (54) was isolated in 81 % yield.70 Other intramolecular reactions involving thiophenes have been reported.71... 

Scheme 2.24 Regioselective formation of boron enolates according to Mukaiyama s protocols transition-state model 91 for kinetically controlled enolate generation.

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