Unraveling the Photodeprotection Mechanism of Anthraquinon-2-ylmethoxycarbonyl-Caged Alcohols Using Time-Resolved Spectroscopy

Abstract

Anthraquinone (AQ) compounds have been used as photolabile protecting groups (PPGs) to protect alcohols, ketones, and carboxylic acids. However, because of the lack of direct spectroscopic information for the transient species and intermediates related with the deprotection reaction(s), the photorelease mechanism(s) of these systems are still largely unknown. In this contribution, we detail a time-resolved spectroscopic investigation using anthraquinone-protected galactose (1) and adenosine (2) to investigate the photodeprotection reaction mechanism(s) of these kinds of AQ-PPGs. It was found that, in THF-H2O solvents, the ketyl radical species generated for 1 and 2 was found to be a reactive intermediate that then formed a dihydroxyanthracene species. The photodeprotection then occurred with the accompaniment of a decarboxylation process. These results provide an improved understanding for how AQ-PPGs work and will help assist in the design and applications of selected anthraquinone derivatives as a PPG platform, especially in aqueous environments more relevant for use in biological systems.

2-Bromoanthraquinone as a highly efficient photocatalyst for the oxidation of sec-aromatic alcohols: experimental and DFT study

“…27 They also used time-resolved spectroscopy and density functional theory to unravel the photodeprotection mechanism of anthraquinon-2ylmethoxycarbonyl-caged alcohols. 28 In both studies, anthraquinone molecules formed a hydroanthraquinone structure, proving that this is a key intermediate in the anthraquinone molecule's redox reaction. Therefore, this study will also explore the key intermediates of anthraquinone molecules in organic solvents to reveal the anthraquinone redox cycle under free radical reaction conditions.…”

Biomimetic Enterobactin Analogue Mediates Iron-Uptake and Cargo Transport into E. coli and P. aeruginosa

“…IR (ATR) [cm -1 ]: 3369, 2976, 2940, 1714, 1522, 1442, 1394, 1365, 1274, 1274, 1169, 1037, 984, 871, 781, 716, 663, 616, 592, 565 6.88 (dt, J = 15.6, 6.9 Hz, 1H), 5.77 (dt, J = 15.6, 1.6 Hz, 1H), 4.50 (s, 1H), 3.66 (s, 3H), 3.07 (q, J = 6.9 Hz, 2H), 2.22 -2.11 (m, 2H), 1.66 -1.52 (m, 2H), 1.37 (s, 9H). 13 C-NMR (76 MHz, CDCl 3 ) δ [ppm]: 167.03, 156.03, 148.42 , 121.56, 79.39, 51.54, 40.13, 29.55, 28.66, 28.50. Methyl (2R,3S)-6-((tert-butoxycarbonyl)amino)-2,3-dihydroxyhexanoate (7) A solution of methyl (E)-6-((tert-butoxycarbonyl)amino)hex-2-enoate (6) (3.0 g, 12.3 mmol, 1.0 equiv) in THF (13 mL, 0.95 M) was added to a stirred mixture of AD-mix alpha (21.0 g ≈ 6.17 g, 44.64 mmol, 3.63 equiv K 2 CO 3 , 111.3 mg, 0.142 mmol, 0.012 equiv (DHQ) 2 PHAL,14.69 g,44.63 mmol,3.63 (200 mL). The organic extract was washed with brine (50 mL), dried over Na 2 SO 4 , filtered and concentrated under reduced pressure.…”

“…The organic extract was washed with brine (50 mL), dried over Na 2 SO 4 , filtered and concentrated under reduced pressure. The residue was purified by flash column chromatography through silica gel (CH 2 Cl 2 :MeOH/1:0 → 9:1) yielding methyl (2R,3S)-6-((tert-butoxycarbonyl)amino)-2,3-dihydroxyhexanoate (7) (2.8 g, 10.2 mmol, 83%) as a colorless, viscous liquid. TLC (CH 2 Cl 2 :MeOH/95:5) R f : 0.40 [Ninhydrin].…”

“…[α] D 23°C = -6.3° (c =8.5 mg/mL in CHCl 3 ). IR (ATR) [cm -1 ]: 3364,2937,1740,1686,1525,1446,1395,1367,1275,1250,1168,1133,1079,1043,984,863,781,713,633,546 173.96, 156.4 , 79.44, 73.62, 72.38, 52.84, 40.45, 30.62, 28.51, 26.60. Methyl (2S,3S)-2-azido-6-((tert-butoxycarbonyl)amino)-3-hydroxyhexanoate (8) Thionyl chloride (1.45 mL, 20.2 mmol, 2.0 equiv) was added dropwise to a solution of methyl (2R,3S)-6-((tert-butoxycarbonyl)amino)-2,3-dihydroxyhexanoate (7) (2.8 g, 10.2 mmol, 1.0 equiv) and triethylamine (8.4 mL, 60.5 mmol, 6.0 equiv) in dry CH 2 Cl 2 (340 mL, 0.03 M) under Argon atm. at 0 °C and the reaction mixture was stirred for 90 min at 0 °C.…”

“…The analytical data were in accordance with the literature. [7] (9,10-Dioxo-9,10-dihydroanthracen-2-yl)methyl (2S,3S)-2-(((benzyloxy)carbonyl)amino)-6-((tert-butoxycarbonyl)amino)-3-hydroxyhexanoate (11) [cm -1 ]: 3370, 2930, 1706, 1676, 1596, 1519, 1453, 1366, 1327, 1291, 1257, 1168, 1039, 925, 852, 710. 182.98, 182.76, 170.32, 156.52, 141.94, 136.10, 134.44, 134.35, 133.78, 133.56, 133.53, 133.31, 133.07, 128.68, 128.41, 128.32, 127.97, 127.45, 127.43, 126.16, 79.50, 72.87, 67.49, 66.19, 59.13, 40.08, 30.38, 29.84, 28.52, 26.91.…”