Photochemistry of Coumarin laser dyes: the role of singlet oxygen in the photo-oxidation of Coumarin 311

Enantioselective Synthesis and Application of Small and Environmentally Sensitive Fluorescent Amino Acids for Probing Biological Interactions

Environmentally sensitive fluorescent amino acids (FlAAs) have been used extensively to probe biological interactions. However, most of these amino acids are large and do not resemble amino acid side chains. Here, we report the enantioselective synthesis of two small and environmentally sensitive fluorescent amino acids bearing 7-dialkylaminocoumarin side chains by alkylation of a Ni(II) glycine Schiff base complex. These amino acids exhibit a large increase in fluorescence as environment polarity decreases. One of these FLAAs was incorporated into a highly active analog of the cyclic lipopeptide antibiotic paenibacterin by Fmoc solid-phase peptide synthesis via a new and very efficient route. This peptide was used to probe the interaction of the antibiotic with model liposomes, lipopolysaccharides, and live bacteria.

"Reduced" Coumarin Dyes with an O-Phosphorylated 2,2-Dimethyl-4-(hydroxymethyl)-1,2,3,4-tetrahydroquinoline Fragment: Synthesis, Spectra, and STED Microscopy

Large Stokes-shift coumarin dyes with an O-phosphorylated 4-(hydroxymethyl)-2,2-dimethyl-1,2,3,4-tetrahydroquinoline fragment emitting in the blue, green, and red regions of the visible spectrum were synthesized. For this purpose, N-substituted and O-protected 1,2-dihydro-7-hydroxy-2,2,4-trimethylquinoline was oxidized with SeO2 to the corresponding α,β-unsaturated aldehyde and then reduced with NaBH4 in a "one-pot" fashion to yield N-substituted and 7-O-protected 4-(hydroxymethyl)-7-hydroxy-2,2-dimethyl-1,2,3,4-tetrahydroquinoline as a common precursor to all the coumarin dyes reported here. The photophysical properties of the new dyes ("reduced coumarins") and 1,2-dihydroquinoline analogues (formal precursors) with a trisubstituted C=C bond were compared. The "reduced coumarins" were found to be more photoresistant and brighter than their 1,2-dihydroquinoline counterparts. Free carboxylate analogues, as well as their antibody conjugates (obtained from N-hydroxysuccinimidyl esters) were also prepared. All studied conjugates with secondary antibodies afforded high specificity and were suitable for fluorescence microscopy. The red-emitting coumarin dye bearing a betaine fragment at the C-3-position showed excellent performance in stimulation emission depletion (STED) microscopy.

One Photocatalyst, n Activation Modes Strategy for Cascade Catalysis: Emulating Coumarin Biosynthesis with (-)-Riboflavin

Generating molecular complexity using a single catalyst, where the requisite activation modes are sequentially exploited as the reaction proceeds, is an attractive guiding principle in synthesis. This requires that each substrate transposition exposes a catalyst activation mode (AM) to which all preceding or future intermediates are resistant. While this concept is exemplified by MacMillan's beautiful merger of enamine and iminium ion activation, examples in other fields of contemporary catalysis remain elusive. Herein, we extend this tactic to organic photochemistry. By harnessing the two discrete photochemical activation modes of (-)-riboflavin, it is possible to sequentially induce isomerization and cyclization by energy transfer (ET) and single-electron transfer (SET) activation pathways, respectively. This catalytic approach has been utilized to emulate the coumarin biosynthesis pathway, which features a key photochemical E → Z isomerization step. Since the ensuing SET-based cyclization eliminates the need for a prefunctionalized aryl ring, this constitutes a novel disconnection of a pharmaceutically important scaffold.

A singlet oxygen photosensitizer enables photoluminescent monitoring of singlet oxygen doses

Dichromophoric molecular dyad enables photosensitization and detection of sinlget oxygen.

Psoralen-olefin photoproducts: first observation of a photo-ene reaction

Methyl-substituted psoralens (4'-(hydroxymethyl)-4,5',8-trimethylpsoralen and 4,5',8-trimethylpsoralen) are found to yield an ene product as well as the expected [2 + 2] cycloaddition product from photochemical reaction with simple olefins. As determined by absorbance, liquid chromatography-mass spectrometry and nuclear magnetic resonance, both products are formed at the pyrone side of the respective psoralen. The product distribution is dependent on olefin concentration as well as the nature of the olefin. In deoxygenated solutions, cyclic olefins form as much as 50% ene product, while unsubstituted straight-chain olefins form as little as 3%. In oxygenated solutions, the product distribution is strongly affected by singlet oxygen.