The phenacyl PPG is the archetypal example of a carbonyl-based PPG. Under this motif, the PPG is attached to the protected substrate at the αβ-carbon, and can exhibit varied photodeprotection mechanisms based on the phenacyl skeleton, substrate identify and reaction conditions. Overall, phenacyl PPGs can be used to protect sulfonates, phosphates, carboxylates and carbamates.
As with nitrobenzyl-based PPGs, several modifications are known. For example, the 3’,5’-dimethoxybenzoin PPG (DMB) contains a 3,5-dimethoxyphenyl substituent on the carbonyls α-carbon. Under certain conditions, DMB has exhibited quantum yields as high as 0.64. Additionally, the p-hydroxyphenacyl PPG (pHP) has been designed to react through a photo-Favorskii rearrangement. This mechanism yields the carboxylic acid as the exclusive photoproduct; the key benefit of the pHP PPG is the lack of secondary photoreactions and the significantly different UV absorption profiles of the products and reactants. While the quantum yield of the p-hydroxyphenacyl PPG is generally in the 0.1-0.4 range, it can increase to near unity when releasing a good leaving group such as a tosylate. The photoextrusion of the leaving group from the pHP PPG is so effective, that it also releases even poor nucleofuges such as amines (with the quantum yield in the 0.01-0.5 range, and dependent on solution pH). The Additionally, photorelease occurs on the nanosecond timeframe, with k' > 10 s.
The o-hydroxyphenacyl PPG has been introduced as an alternative with absorption band shifted closer towards the visible region, however it has slightly lower quantum yields of deprotection (generally 0.1-0.3) due to excited state proton transfer available as an alternative deactivation pathway.
The phenacyl moiety itself contains one chiral carbon atom in the backbone. The protected group (leaving group) is not directly attached to this chiral carbon atom, however has been shown to be able to work as a chiral auxiliary directing approach of a diene to a dienophile in a stereoselective thermal Diels–Alder reaction. The auxiliary is then removed simply upon irradiation with UV light.