Reactions Involving Enols or Enolic Derivatives

Base-catalysed autoxidation of either 3a,5-cyclo-5a-cholestan-6-one or 3a,5-cyclo-5a-cholestan-7-one gave the same products, which included the oxetanone (113). Similar oxidations of 2-oxo—steroids were reported, and of particular interest is the formation of the dioxide (115) from the 2,ll-dioxo-5/3-compound 

Kohoutova, J. Smolikova, and M. BudeJmsky, Coll. Czech. Chem. Comm., 1976, 

The full account of the use of sulphenylation-dehydrosulphenylation for the introduction of a -unsaturation into carbonyl compounds includes some steroidal examples. The similar selenylation-dehydroselenylation of 17-oxo-steroids resulted in the formation of the unsaturated lactones (118) owing to Baeyer-Villiger oxidation during the H2O2 oxidation of the a-phenyl selenyl intermediate (see also ref. 210). 

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Reactions Involving Enols or Enolic Derivatives.—Reviews on the use of... 

Reactions involving Enols or Enolic Derivatives.—Polyhalogenation studies on 5/8-cholestan-2-one show that chlorination occurs in the order l/8,3a,3/8, followed in the case of chlorination by the highly hindered la-position. Attempted bromination beyond the 1/8,3,3-tribromide gave only l,l,3-tribromo-5/8-cholest-3-en-2-one. ° Acid-catalysed bromination of la-methyl and la-phenyl-5a-cholestan-3-ones occurred normally to give 2a-bromo-derivatives. The bromination of 5/3-cholestan-3-one at the 4/8-position is also unaffected by the presence of a la-methyl... 

Reactions Involving Enols or Enolic Derivatives.—The full account was reported for the dehydrogenation of ketones using benzene seleninic anhydride.77 Further studies were reported on catalysts specific for the isomerization of A5-cholestenone to A4-cholestenone.78... 

Reactions involving Enols or Enolic Derivatives.—A review of the structure and reactivity of alkali-metal enolates includes some steroidal reactions. A study of the mechanism of isomerization of androst-5-en-17/3-ol-3-one to testosterone indicated that the acid-catalysed process proceeds through rate-determining enolization whereas the base-catalysed reaction proceeds through rate-determining protonation of an enolate ion. Bromination of the 4,4,6-trimethyl-A -3-oxo-compounds (111)—(113) gave the 2a-bromo-derivatives, each of which showed anomalous o.r.d. curves. Bromination at C-2 was favoured for the... 

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 prime functional group for constructing C-C bonds may be the carbonyl group, functioning as either an electrophile (Eq. 1) or via its enolate derivative as a nucleophile (Eqs. 2 and 3). The objective of this chapter is to survey the issue of asymmetric inductions involving the reaction between enolates derived from carbonyl compounds and alkyl halide electrophiles. The addition of a nucleophile toward a carbonyl group, especially in the catalytic manner, is presented as well. Asymmetric aldol reactions and the related allylation reactions (Eq. 3) are the topics of Chapter 3. Reduction of carbonyl groups is discussed in Chapter 4. 

Returning to the main theme in this section, another case where chelation to a metal centre controls reactions involving enolates is seen in complexes of amino acid derivatives. Amino acids are commonly found in metal complexes as the chelated anions in which the carboxylate oxygen and the amino group are co-ordinated to the metal. The co-ordinated amino acid anion could be in the keto (5.6) or enolate (5.7) form. 

The Reformatsky reaction involves condensation of ester-derived zinc enolates with aldehydes or ketones to furnish the con-esponding [3-hydroxy esters. The zinc enolates are generated by addition of an a-haloester in THF, DME, Et20, benzene, or toluene to an activated zinc, such as a Zn-Cu couple or zinc obtained by reduction of zinc halides with potassium (Rieke zinc). An example of a Reformatsky condensation using Rieke zinc is shown below. ... 

The reactions of ketenes or ketene equivalents with imines, discussed above, all involve the imine acting as nucleophile. Azetidin-2-ones can also be produced by nucleophilic attack of enolate anions derived from the acetic acid derivative on the electrophilic carbon of the imine followed by cyclization. The reaction of Reformatsky reagents, for example... 

A Michael reaction involves the conjugate addition of a stable enolate ion donor to an o,/3-unsaturated carbonyl acceptor, yielding a 1,5-dicarbonyl product. Usually, the stable enolate ion is derived from a /3-diketone, jS-keto ester, malonic ester, or similar compound. The C—C bond made in the conjugate addition step is the one between the a carbon of the acidic donor and the (3 carbon of the unsaturated acceptor. 

Reaction conditions that involve other enolate derivatives as nucleophiles have been developed, including boron enolates and enolates with titanium, tin, or zirconium as the metal. These systems are discussed in detail in the sections that follow, and in Section 2.1.2.5, we discuss reactions that involve covalent enolate equivalents, particularly silyl enol ethers. Scheme 2.1 illustrates some of the procedures that have been developed. A variety of carbon nucleophiles are represented in Scheme 2.1, including lithium and boron enolates, as well as titanium and tin derivatives, but in... 

Table 2-5 summarizes the results of the asymmetric alkylation (Scheme 2-17) of the lithium enolates derived from 22 or 23.28 When chiral auxiliary 22 or 23 is involved in the alkylation reactions, the substituent at C-4 of the oxazolidine ring determines the stereoselectivity and therefore controls the stereogenic outcome of the alkylation reaction. 

Although iV-acyloxazolidinones 88 and iV-acylthiazolidinethiones 90 lead to an anti aldol, the respective products 89 and 91 present a different anti configuration. Consequently, the corresponding derived magnesium enolates exhibit the opposite face selection in these reactions. On the basis of previous results involving enolates of various metal complexes such as boron, titanium, lithium or sodium enolates, the (Z)-metal enolate... 

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