Synthesis of 2,6‐Bis(oxazolyl)pyridine Ligands for Luminescent Ln^III Complexes

Abstract

New bis(oxazolyl)pyridine ligands for LnIII ions were prepared by using an expeditious methodology from threonine and dipicolinic chloride. The synthetic strategy includes a dehydration step to give a bis(dehydroaminobutyric acid) derivative followed by bromination and cyclization with DBU. Photophysical studies of EuIII and TbIII complexes of these ligands showed that the 2,6‐bis(oxazolyl)pyridine moiety acts as an effective sensitizer for lanthanide luminescence and indicate the formation of 3:1 complexes [Ln{bis(oxazole)pyridine}3]3+.

Dehydration reactions in polyfunctional natural products

“…A number of reports display the utility of di-tertbutylcarbonate in combination with DMAP to activate serine into a mixed carbonate, followed by treatment with tetramethylguanidine to effect elimination to dehydroalanine containing motifs (see Scheme 15A). [97][98][99][100] The reactivity of carbonyl di-imidazole has been investigated to indicate a requirement of sufficiently acidic protons positioned on the α-carbon of β-hydroxy amino acid residues in dehydration. 101 Dehydration of C-terminal serine derivatives with methyl ester protection of the carboxyl moiety afforded the corresponding dehydroalanine residue, whereas serine-containing dipeptides with the C-terminal acyl-moiety bound as an amide did not undergo dehydration.…”

New 2,6-Bis(5-phenyloxazolyl)pyridine Ligands for Luminescent LnIII Complexes

“…These bis-oxazoles are also fluorescent, with emission quantum yields of around 50% for compound 3 and 30% for compound 4. The fluorescence quantum yields of ligands 3 and 4 are significantly lower than fluorescence quantum yields of analogous ligands with a methyl group at the 5-position of the oxazole ring [7] A red-shift in emission is observed for compound 4 relative to compound 3 (Table 1) which could be attributed, not only to solvent effects, but also to the additional intramolecular charge transfer (ICT) character of the excited state of bis-oxazole 4. This was also observed for other 5-substituted bis-oxazolyl-pyridine derivatives [7].…”

“…The fluorescence quantum yields of ligands 3 and 4 are significantly lower than fluorescence quantum yields of analogous ligands with a methyl group at the 5-position of the oxazole ring [7] A red-shift in emission is observed for compound 4 relative to compound 3 (Table 1) which could be attributed, not only to solvent effects, but also to the additional intramolecular charge transfer (ICT) character of the excited state of bis-oxazole 4. This was also observed for other 5-substituted bis-oxazolyl-pyridine derivatives [7]. Fluorescence titration of the ligand 4 in acetonitrile with a solution of Eu(III) trifluoromethane-sulfonate (Eu(Tf)3) in acetonitrile (Figure 3A) was carried out to determine the stoichiometry of the complex.…”

Hairpin oligonucleotides anchored terbium ion: a fluorescent probe to specifically detect lead(ii) at sub-nM levels

“…9 Applications of lanthanide ions as uorophores needs sensitizers to enhance emissions. 10,11 Oligonucleotides are another type of sensitizer, which via energy transfer from oligonucleotides to their anchored lanthanide ions was rstly described in 1973. 12 Based on previous studies of terbium and oligonucleotides, there are three aspects of enhancement of uorescence of terbium in general: (1) guanine was more efficient compared with other normal oligonucleotides, 12 because the resonance energy levels of terbium ion overlap with the triplet energy state of guanine; 13 (2) ssDNA was more efficient compared with dsDNA; [14][15][16] (3) the conformation with cavity, such as G-quadruplexes, 17 may be an important factor.…”