675120-16-8Relevant articles and documents
Photochemical cleavage and release of para-substituted phenols from α-keto amides
Ma, Chicheng,Chen, Yugang,Steinmetz, Mark G.
, p. 4206 - 4215 (2007/10/03)
In aqueous media α-keto amides 4-YC6H4OCH 2COCON(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-YC 6H4OH 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 H2C=C(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 5a,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 for photoelimination significantly decrease and 1,3-photorearrangment of the phenolic group is observed. The 1,3-rearrangement involves excited state ArO-C bond homolysis to give para-substituted phenoxyl radicals, which can be observed directly in laser flash photolysis experiments.
Photochemical cleavage and release of carboxylic acids from α-keto amides
Ma, Chicheng,Steinmetz, Mark G.,Kopatz, Erica J.,Rathore, Rajendra
, p. 4431 - 4442 (2007/10/03)
In aqueous media, α-keto amides LGCH2COCON(R)CH(R′) CH3 (1a, R = Et, R′ = H; 1b, R = iPr, 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-hydroxyoxazolidin-4-ones 2a,c in the cases of 1a,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 1a 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 1a, 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 1a (LG = CH3CO2-, PhCH 2CO2-) in neat CH3CN results in carboxylate elimination to form imminium ion 5a, followed by internal return to give aminals.
Time-resolved pH jump study of photochemical cleavage and release of carboxylic acids from α-keto amides
Ma, Chicheng,Steinmetz, Mark G.,Kopatz, Erica J.,Rathore, Rajendra
, p. 1045 - 1048 (2007/10/03)
Time-resolved pH jump experiments, using laser flash photolysis and bromocresol green as an indicator, showed that photochemical cleavage and release of carboxylic acids from various α-keto amides derivatives in aqueous media occurs on the microsecond tim
Substituent Effects on Competitive Release of Phenols and 1, 3-Rearrangement in α-Keto Amide Photochemistry
Ma, Chicheng,Steinmetz, Mark G.
, p. 629 - 632 (2007/10/03)
(Matrix presented) Photolysis of α-keto amides bearing 4-YC 6H4O leaving groups at the position α to the keto group efficiently produces high yields of phenols when Y is an electron-withdrawing group or H. The photoelimination likely
Photochemical cleavage and release of carboxylic acids from α-keto amides
Ma, Chicheng,Steinmetz, Mark G.,Cheng, Qing,Jayaraman, Vasanthi
, p. 71 - 74 (2007/10/03)
(Matrix presented) Carboxylate groups incorporated at the position α to the keto carbonyl of α-keto amides 1 were photochemically cleaved in aqueous media to give carboxylic acids in 70-90% yields with quantum yields of 0.3. The cleavage coproducts were diastereomeric hemiacetals 2. Prompt release of acetate and γ-aminobutyrate (GABA) in buffer was observed by difference FT-IR spectroscopy upon 355 nm laser flash photolysis. The time-constant for release of GABA was 30 ms.
