P-Methoxyphenacyl ester

Epstein, W.W. and Garrossian, M. (1987) p-Methoxyphenacyl esters as photodeblockable protecting groups for phosphates. Journal of the Chemical Society, Chemical Communications, 532-533. 

The p-methoxyphenacyl ester shows more efficient photolysis than that of the parent phenacyl ester, which can be rationalized by the added stabilization of radical intermediates and a bathochromic shift of the carbonyl chromophore through additional Ti-electron donation. a-Substitution appears to be important for... 

The second approach consisted in the separation of CLA isomers as the p-methoxyphenacyl esters with dichloromethane/hexane/acetonitrile mixtures as the mobile phase (14). In this instance, only a single chromatographic column was required. Because detection was by the absorbance of the p-methoxyphenacyl moiety at 270 nm, aU fatty acids were detected and quantified, not simply the conjugated dienes. Finally, good resolution of CLA as the free acids has recently been reported, with hexane/acetonitrile/acetic acid mixtures as the mobile phase with detection of the conjugated double bonds at 234 nm (15). This procedure may be of special value for commercial CLA samples supplied as the free adds because no derivatization step is required. 

Although most Ag-HPLC analyses have traditionally been done using CLA methyl esters, improved separation and quantitation of CLA isomers has been noted when phenacyl or p-methoxyphenacyl ester rather than methyl ester derivatives were analyzed (58). Nikolova-Damyanova et al. (59) demonstrated improved separation of the p-methoxyphenacyl derivatives of the cis/trans 8,10- through 11,13-18 2 isomers in a commercial sample of CLA when compared with CLAME. Only a single Ag-HPLC column was required. Resolution of the cis/ trans isomers was similar to that obtained for CLAME using Ag-HPLC systems with two or three Ag-HPLC columns connected in series (60). A stepwise solvent gradient of 100% hexane/dichloromethane/ACN (40 60 0.2 vol/vol/vol 30 min) to 100% dichloromethane/ACN (100 1) over 10 min with UV detection at 270 nm (for phenacyl esters) yielded a semiquantitative estimation of fatty acid composition comparable to, but more detailed than that obtainable by GC analysis as FAME. This method was also applied to detamine the CLA isomer composition of a sample of beef/pig fat, but prefractionation by RP-HPLC was required before Ag-HPLC analysis. The prefractionation step was required to concentrate the CLA isomers and to remove octadecenoate and other fatty acids that might interfere with Ag-HPLC analysis. 

Fig. 3.4. Silver-ion high-performance liquid chromatography of cis/trans-CLA as p-methoxyphenacyl esters with different mobile phases (A) hexane/dichloromethane/acetonitrile (ACN), 40 60 0.02 (vol/vol/vol) (B) dichlo-romethane/ACN, 100 0.1 (vol/vol) (C) hexane/dichloromethane/ isopropanol, 50 50 0.3 (vol/vol/vol) (D) dichlo-romethane/isopropanol, 100 0.2 (vol/vol). [Published with permission of J. High Resolut Chromatogr. (now ). Sep. Sci.) and redrawn from original.]... 

Momchilova, S.M. and Nikolova-Damyanova, B.M. (2010) Separation of isomeric octadecenoic fatty acids in partially hydrogenated vegetable oils as p-methoxyphenacyl esters using a single-column silver ion high-performance liquid chromatography (Ag-HPLC). Nat. Protoc. 5, 473-478. 

An alternative approach to improving resolution has been to use alternative derivatives to methyl esters. Utilizing only a single silver-ion HPLC column, the separation of a commercial CLA mixture was substantially improved as the p-methoxyphenacyl esters (Nikolova-Damyanova, MomchUova, and Christie,... 

Epstein and Garrossian reported the release of ethyl and phenyl phosphate esters from the corresponding p-methoxyphenacyl phosphates in 1,4-dioxane. The released phosphates were recovered in high yields (Et, 86% Ph, 74%) along with 13 (91-84%) as the only observed photoproducts. The absence of any rearranged ester products contrasted with reports by Anderson and Reese for substituted a-chloroacetophenones vide infra). The rationale for the discrepancy advanced by Epstein was an altered... 

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