The review covers the knowledge on photoremovable protecting
groups and includes all relevant chromophores studied in the
time period of 2000–2012; the most relevant earlier works are
also discussed.

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

2.3. [4 + 4] Cycloadditions 1058 2.4. Photocycloadditions of Aromatic Compounds 1058 2.4.1. Benzene Derivatives 1058 2.4.2. Condensed Aromatic Compounds 1060 3. Photochemical Rearrangements 1061 4. Cyclizations 1064 4.1. Pericyclizations 1064 4.2. Norrish−Yang Reaction 1066 5. Photochemical Extrusion of Small Molecules 1067 6. Photochemical Electron Transfer 1068 6.1. Photochemical Electron-Transfer Reactions with a Catalytic Sensitizer 1068

Photochemical Reactions as Key Steps in Organic Synthesis

Spin‐Center Shift (SCS) – A Versatile Concept in Biological and Synthetic Chemistry

Photolabile Protecting Groups: Structure and Reactivity

[3 + 2] Cycloaddition Reaction of in Situ Formed Azaoxyallyl Cations with Aldehydes: An Approach to Oxazolidin-4-ones

1 2 3 a (R = Et, R' = H) 94 : 6 b (R = iPr, R' = Me) 0 : 100 c (R = Ph, R' = H) 100 : 0 LG = CH 3 CO 2 - , PhCH 2 CO 2 - , C 6 H 5 CO 2 - , 4-NCC 6 H 4 CO 2 - , CH 3 CONHCH 2 CO 2 - , BocAla, GABA, Glu In aqueous media, α-keto amides LGCH 2 COCON(R)CH(R')CH 3 (la, R = Et, R' = H; 1b, R = i Pr, R' = Me; 1c, R = Ph, R' = H) with various carboxylate leaving groups (LG) at the C-3 position undergo photocleavage and release of carboxylic acids with formation of diastereomeric 5-hydroxy-oxazolidin-4-ones 2a,c in the cases of la,c or 5-methyleneoxazolidin-4-ones 3b in the case of 1b. For 1a,b, Φ(photocleavage) = 0.24-0.38, whereas Φ(photocleavage) = ca. 0.05 for 1c. The proposed mechanism involves transfer of hydrogen from an N-alkyl group to the keto oxygen to produce zwitterionic intermediates 4a-c that eliminate carboxylate anions. The resultant imminium ions, H2C=C(OH)CON+(R)=C(R')CH3 5a-c, cyclize intramolecularly to 3b or undergo intermolecular addition of water followed by tautomerization and cyclization to give 2a,c. These inter- or intramolecular trapping reactions of 5 release protons that decrease the pH and cause bleaching of the 620 nm band of the pH indicator, bromocresol green. Determination of the bleaching kinetics by laser flash photolysis methods in the case of la gives time constants of 18-137 μs, depending on the leaving group ability of the carboxylate anion, whereas amides 1b show only a small leaving group effect. For la, the large leaving group effect is consistent with rate-limiting carboxylate elimination from 4a, whereas the proton release step would be largely rate determining for 1b. Photolyses of la (LG = CH 3 CO 2 - , PhCH 2 CO 2 - ) in neat CH 3 CN results in carboxylate elimination to form imminium ion 5a, followed by internal return to give aminals.

Photochemical Cleavage and Release of Carboxylic Acids from α-Keto Amides

Substituent effects on competitive release of phenols and 1,3-rearrangement in alpha-keto amide photochemistry.

Time-resolved pH jump study of photochemical cleavage and release of carboxylic acids from α-keto amides

In aqueous media α-keto amides 4-YC6H4OCH2COCON(R)CH(R‘)CH3 (5a, R = Et, R‘ = H; 5b, R = iPr, R‘ = Me) with para-substituted phenolic substituents (Y = CN, CF3, H) undergo photocleavage and release of 4-YC6H4OH with formation of 5-methyleneoxazolidin-4-ones 7a,b. For both 5a,b quantum yields range from 0.2 to 0.3. The proposed mechanism involves transfer of hydrogen from an N-alkyl group to the keto oxygen to produce zwitterionic intermediates 8a−c that eliminate the para-substituted phenolate leaving groups. The resultant imminium ions H2CC(OH)CON+(R)C(R‘)CH3 9a,b cyclize intramolecularly to give 7a,b. The quantum yields for photoelimination decrease in CH3CN, CH2Cl2, or C6H6 due to competing cyclization of 8a,b to give oxazolidin-4-one products which retain the leaving group 4-YC6H4O- (Y = H, CN). A greater tendency to undergo cyclization in nonaqueous media is observed for the N,N-diethyl amides 5a than the N,N-diisopropyl amides 5b. With para electron releasing groups Y = CH3 and OCH3 quantum yields f...

Chicheng Ma

Papers

Photochemical Cleavage and Release of Carboxylic Acids from α-Keto Amides

Substituent effects on competitive release of phenols and 1,3-rearrangement in alpha-keto amide photochemistry.

Time-resolved pH jump study of photochemical cleavage and release of carboxylic acids from α-keto amides

Photochemical Cleavage and Release of Para-Substituted Phenols from α-Keto Amides

Substituent Effects on Competitive Release of Phenols and 1,3-Rearrangement in α-Keto Amide Photochemistry.