Conjugate enones

An interesting case are the a,/i-unsaturated ketones, which form carbanions, in which the negative charge is delocalized in a 5-centre-6-electron system. Alkylation, however, only occurs at the central, most nucleophilic position. This regioselectivity has been utilized by Woodward (R.B. Woodward, 1957 B.F. Mundy, 1972) in the synthesis of 4-dialkylated steroids. This reaction has been carried out at high temperature in a protic solvent. Therefore it yields the product, which is formed from the most stable anion (thermodynamic control). In conjugated enones a proton adjacent to the carbonyl group, however, is removed much faster than a y-proton. If the same alkylation, therefore, is carried out in an aprotic solvent, which does not catalyze tautomerizations, and if the temperature is kept low, the steroid is mono- or dimethylated at C-2 in comparable yield (L. Nedelec, 1974). 

IV. REDUCTION OF CONJUGATED ENONES AND DIENONES, SATURATED KETONES AND KETOL ACETATES. 

Metal-ammonia solutions reduce conjugated enones to saturated ketones and reductively cleave a-acetoxy ketones i.e. ketol acetates) to the unsubstituted ketones. In both cases the actual reduction product is the enolate salt of a saturated ketone this salt resists further reduction. If an alcohol is present in the reaction mixture, the enolate salt protonates and the resulting ketone is reduced further to a saturated alcohol. Linearly or cross-conjugated dienones are reduced to enones in the absence of a proton donor other than ammonia. The Birch reduction of unsaturated ketones to saturated alcohols was first reported by Wilds and Nelson using lithium as the reducing agent. This metal has been used almost exclusively by subsequent workers for the reduction of both unsaturated and saturated ketones. Calcium has been preferred for the reductive cleavage of ketol acetates. 

Reduction of a conjugated enone to a saturated ketone requires the addition of two electrons and two protons. As in the case of the Birch reduction of aromatic compounds, the exact order of these additions has been the subject of study and speculation. Barton proposed that two electrons add initially giving a dicarbanion of the structure (49) which then is protonated rapidly at the / -position by ammonia, forming the enolate salt (50) of the saturated ketone. Stork later suggested that the radical-anion (51), a one electron... 

Lithium-ammonia reductions of most steroidal enones of interest create one or two new asymmetric centers. Such reductions are found to be highly stereoselective and this stereoselectivity constitutes the great utility of the reaction. For conjugated enones of the normal steroid series, the thermodynamically most stable products are formed predominantly and perhaps exclusively. Thus the following configurations are favored 5a, 8/ , 9a, and in certain cases 14a (see page 35). Starr has listed numerous examples illustrating these facts and Smith " and Barton have tabulated similar data. 

If the equilibrium were established rapidly, reduction of the free ketone as it formed would result in a substantial loss of product. Lithium enolates are more covalent in character than are those of sodium and potassium and consequently are the least basic of the group. This lower thermodynamic basicity appears to be paralleled by a lower kinetic basicity several workers have shown that lithium enolates are weaker bases in the kinetic sense than are those of sodium and potassium." As noted earlier, conjugated enones... 

Reaction times of from 5 to 60 minutes have been employed for the reduction of conjugated enones. Although the longer times apparently do not seriously diminish the yields of products, they usually are not necessary. If a conjugated enone is sufficiently soluble in the reaction medium, it is reduced almost instantly when added to lithium-ammonia solutions. 

A widely used procedure for the reduction of conjugated enones to saturated ketones is that of Bowers, Ringold and Denot, Procedure 5 (section V). Ether-dioxane (1 1) is used as the organic co-solvent and solid ammonium... 

For the reduction of conjugated enones to saturated alcohols, Procedure 5 (section V) may be modified by adding methanol in place of ammonium chloride a sufficient excess of lithium is present to effect reduction of the intermediate saturated ketone to the alcohol. Procedure 2 (section V) for effecting Birch reductions is also useful for reduction of conjugated enones to saturated alcohols. Thus, 17-ethyl-19-nortestosterone affords crude 17a-ethyl-5a-estrane-3) ,17) -diol of mp 174-181°, reported mp 181-183°, in quantitative yield. 

As first demonstrated by Stork,the metal enolate formed by metal-ammoni reduction of a conjugated enone or a ketol acetate can be alkylated in liquic ammonia. The reductive alkylation reaction is synthetically useful since ii permits alkylation of a ketone at the a-position other than the one at whicf thermodynamically controlled enolate salt formation occurs. Direct methyl-ation of 5a-androstan-17-ol-3-one occurs at C-2 whereas reductive methyl-... 

Deuterium exchange of conjugated enones and dienones on pretreated gas chromatography columns has been found useful for the characterization of these compounds by combined gas chromatography-mass spectrometry. ... 

There are ample precedents for reductions of double bonds in conjugated enones with lithium in deuterioammonia (see section V-C). Examples of the reduction of saturated ketones in deuterated media appear only as side reactions (over reductions) during the above mentioned conversions. For experimental details, therefore, one should consult the literature for the analogous reductions in protic medium (see also chapter 1). The use of deuterioammonia is essential for labeling purposes since by using liquid ammonia and methanol-OD the resulting alcohol contains no deuterium. For the preparation of deuterioammonia see section IX-D. 

In section V-A it has been pointed out that catalytic reduction of conjugated enones is usually a good method for the preparation of p- or y-labeled ketones. To overcome certain stereochemical problems, however, it is occasionally necessary to use the lithium-ammonia reduction. In this case deuteration takes place at the / -carbon and generally leads to the thermodynamically more stable product (see chapter 1). 

Until recently, pyridine-type bases have been commonly used to produce conjugated enones from 2-halo ketones yields are usually poor °° and these reactions are frequently accompanied by rearrangement, reduction and salt formation. Thus, Warnhoff found that dehydrobromination of (28) with 2,4-lutidine gave a mixture of (29), (30) and (31) in the ratio 55 25 20. Collidine gave a ratio of 38 25 37, whereas pyridine gave mainly the salt (32). 

The conjugated enone (177) is treated withp-toluenesulfonic acid in refluxing toluene to form the more stable product (178). The A -17-keto-system is formed by acid catalyzed cleavage of the A -17-ketal (see page 304), but the conditions are not drastic enough to cause equihbration to the more stable A " -compound. (The latter may be ketalized to form the A -17-ketal.) ... 

A useful alternate procedure which allows the generation and alkylation of the less stable enolate anion has been reported by Stork.This method takes advantage of the fact that the thermodynamically less stable enolate anion formed in the lithium ammonia reduction of a conjugated enone... 

Although at equilibrium the j5,y-tautomer (16a) is preferred, some of the conjugated enone (17) can be obtained by acid-catalyzed equilibration. Hydrogenation of the A-homo-enone (16a) gives a mixture from which A-homo-5a-cholestan-3-one (5b) can be isolated. 

Other kinds of conjugated systems, such as conjugated enones and aromatic rings, also have characteristic UV absorptions that are useful in structure determination. The UV absorption maxima of some representative conjugated molecules are given in Table 14.2. 

The jS-hydroxy aldehydes or ketones formed in aldol reactions can be easily dehydrated to yield a -unsaturated products, or conjugated enones. In fact, it s this loss of water that gives the condensation reaction its name, because water condenses out of the reaction when the enone product forms. 

The reaction conditions needed for aldol dehydration are often only a bit more vigorous (slightly higher temperature, for instance) than the conditions needed for the aldol formation itself. As a result, conjugated enones are usually obtained directly from aldol reactions without isolating the intermediate jS-hydroxy carbonyl compounds. 

Conjugated enones are more stable than nonconjugated enones for the same reason that conjugated dienes are more stable than nonconjugated dienes (Section 14.1). Interaction between the tt electrons of the C=C bond and the tt electrons of the C=0 group leads to a molecular orbital description for a conjugated enone that shows an interaction of the tt electrons over all four atomic centers (Figure 23.3). 

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