Crown Catalyzed Phenacyl Ester Synthesis

A particularly useful application of the crown mediated ester synthesis is in the preparation of phenacyl esters [21, 22]. The crown catalyzed reaction of potassium carboxylates with a,4-dibromoacetophenone yields derivatives which are usually crystalline and which can be readily detected by uv spectroscopy [23]. Moreover, the phenacyl ester is a useful protecting group which can be removed under mild conditions [24]. Phenacyl ester formation has also been catalyzed by tertiary amines [5]. Examples of the condensation according to equation 6.7 are given in Table 6.6. 

Likewise a-keto and a-aldehydo esters have been prepared by the reaction of potassium phenylacetate with a-bromo ketones or aldehydes by an 18-crown-6 catalyzed displacement reaction in acetonitrile or benzene solution. These compounds could be isolated after short reaction times, or they could be converted on continued heating to 2(5H)furanones, the product of intramolecular aldol condensation and 

Phenylacetate ion appears to be an excellent nucleophile in nonpolar media, but a relatively weak base. Substitution of the a-bromoaldehydes seems very much more favorable than an aldol type condensation, even for the unhindered aldehydes [25]. A related example of this nucleophile/base balance has been observed in the case of 4-bromocyclohexenone [8]. Riase transferred acetate ion displaces bromide (73%) rather than dehydrohalogenating the system to phenol (Eq. 6.9) [8]. 

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