Phenacyl esters carboxy-protecting groups

In a formal sense, alkyl ester type carboxy protecting groups, e.g. benzyl, allyl, 4-picolyl, and (9,10-dioxo-2-anthryl)methyl esters, can be viewed at as modified methyl esters. However, because of their specific properties and cleavage conditions these esters will be discussed separately. In this section a series of acetal, phthalimidomethyl, various phenacyl, acetonyl, 9-anthryhnethyl, and cyclopropyhnethyl esters will be presented. 

The general mechanism of the light-induced fragmentation of phenacyl groups has been discussed in Section 2.4.1.2. In the case of phenacyl esters, irradiation promotes bond homolysis, which directly produces the deprotected carboxy group as a radical. H-atom abstraction from the solvent affords the product (Scheme 17). To date, phenacyl esters have been used only for a-carboxy protection. 

The preparation in solution of the preformed handle requires the protection of its carboxy group prior to the formation of the ester bond. Phenacyl esters, which can be removed with zinc in acetic acid and are therefore compatible with the presence of both Boc- and Fmoc-protecting groups, have been used.f l An alternative procedure, however, involves the use of 2,6-dichloro- or 2,4,5-trichlorophenyl esters,which serves a dual purpose of first protecting and then activating the carboxy group of the handle. 

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