Aromatic molecules enones

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. 

Most of the examples in this chapter have been of molecules without selectivity. They have indeed all been self condensations. We hope this has established the basic disconnections and the chemistry but we must now turn to examples where selectivity is needed. So the ketone 46 was made to study aldol reactions with aromatic aldehydes.13 They found that, in acid or base, the enone 52 was the main product with the best yield from HCI in EtOH. The product 52 was isolated as its HCI salt. In this case it is easy to see that only the ketone can enolise, that the aldehyde is more electrophilic than the ketone and that the geometrical isomer shown is the more stable. Such considerations are the substance of the next chapter. 

Na, or Li in liquid ammonia, for example) to reduce aromatic rings and alkynes. The dissolving metal reduction of enones by lithium metal in liquid ammonia is similar to these reactions—the C=C bond of the enone is reduced, with the C=0 bond remaining untouched. An alcohol is required as a proton source and, in total, two electrons and two protons are added in a stepwise manner giving net addition of a molecule of hydrogen to the double bond. 

In neutral media, they leave carbonyl derivatives intact but reduce tosylhydrazones to the corresponding hydrocarbons under reflux of CHCI3 (Section 3.3.4). This reduction is compatible with a-enone, epoxide, or lactone groups present in the molecule [GL3]. In cold acetone, these reagents reduce acid chlorides to aldehydes [FHl] (Section 3.2.7). In the presence of Lewis acids or gaseous HCl in CHjClj, they reduce aldehydes and ketones. The selective reduction of aldehydes in the presence of ketones can also be realized (Section 3.2.1). These reagents also reduce aromatic azides to amines (Section 5.2). 

The most obvious pointer to its success is if the double bond is roughly in the middle of the molecule, or if it separates two rings. This means that the disconnection will quickly lead to much simpler starting materials. We obviously want to make the enone 15 from two different aromatic starting materials and the aldol disconnection gives us an unenolisable aldehyde 16 and a very enolisable methyl ketone 17 with no problems of selectivity. Simply mixing 16 and 17 with KOH gives3 an 87% yield of the enone 15. 

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