Photoremovable Protecting Groups

Abstract

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Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

“…84 The intriguing features of this protecting group are the skeletal rearrangement that accompanies the release of a substrate, the quantitative chemical yield of released product, and the necessary role of water. 7,9u,82,83,85 Advantageous properties are the hydrophilicity of the pHP ligand, the high quantum yields, and the unusually clean reaction that yields only one significant byproduct.…”

“…7,9u,82,83,85d,85e Most of the leaving groups have been introduced through a sequence of bromination of pHA followed by its S N 2 displacement with the conjugate base of the leaving group (X – ) under basic conditions 82,83c,85a−85c (Scheme 16a–c). In some instances, protection of the phenol group by benzylation, silylation, or acetylation is required.…”

“…The most efficacious leaving groups are conjugate bases of moderate to strong acids, e.g., sulfates, 85d,93 phosphates 7,9u,82−85,99 (thiophosphate), 100,111 carboxylates, 7b,9u,85a−85c,93−95,97b,97c,112 phenolates, 9u,93 and thiolates. 111,113 In general, quantum yields monotonically decrease with an increase in the acid leaving group’s p K a (Table 4), conforming to a Brønsted leaving group relationship (β LG ), which correlates the pHP release rate constant with the K a of the leaving group.…”

“…The latter authors introduced the convenient, if somewhat misleading, term “caged” to designate a compound protected by a PPG. Two general perspectives 7 and many more specialized reviews covering applications of PPGs in synthesis, 8 biochemistry and neurobiology, 9 biomedicine, 10 volatiles release, 11 polymerization, 12 and fluorescence activation 13 have been published during the past decade, and a journal issue themed on these topics has recently been published. 14 The present review covers recent developments in the field, focusing on the scope, limitations, and applications of PPGs, which are used to release organic molecules.…”

p-Hydroxyphenacyl photoremovable protecting groups — Robust photochemistry despite substituent diversity

“…9–11 The release rates, as well as the product distribution, are sensitive to the presence of H 2 O and other hydroxylic solvent combinations. 8–10 An adiabatic triplet sequence has been proposed 10,12 that involves both the deprotonation of the phenolic OH group and the heterolytic release of the leaving group 13 prior to formation of cyclopropanone I 2 (eq. [1]).…”