@article{KumkeEidnerKrueger2005,
  author    = {Kumke, Michael Uwe and Eidner, Sascha and Kr{\"u}ger, Tobias},
  title     = {Fluorescence quenching and luminescence sensitization in complexes of Tb3+ and Eu3+ with humic substances},
  year      = {2005},
  abstract  = {Intrinsic fluorescence quenching of humic substances (HS) and the sensitization of Ln(3+) luminescence (Ln3+ Tb3+, Eu3+) in HS complexes were investigated. Both measurements yielded complementary information on the complexation of metals by HS. Large differences between fulvic acids(FA)and humic acids (HA) were found. From time-resolved luminescence measurements it is concluded that a combination of energy transfer and energy back transfer between HS and Ln(3+) is responsible for the observed luminescence decay characteristics. In the case of Eu3+, an additional participation of charge-transfer states is suggested. A new concept for the evaluation of the sensitized luminescence decays of Ln(3+) was adapted},
  language  = {en}
}
@article{KruegerVorndranLinker2009,
  author    = {Kr{\"u}ger, Tobias and Vorndran, Katja and Linker, Torsten},
  title     = {Regioselective arene functionalization : simple substitution of carboxylate by alkyl groups},
  issn      = {0947-6539},
  doi       = {10.1002/chem.200901774},
  year      = {2009},
  abstract  = {Arenes with various alkyl side-chains were synthesized in high yields and excellent regioselectivities. Starting from toluic and naphthoic acids, the carboxylate group was conveniently substituted by alkyl halides by Birch reduction and subsequent decarbonylation. The method is characterized by inexpensive starting materials and reagents, and methylation of arenes was realized. Besides simple alkyl substituents, the scope of arene functionalization was extended by benzyl, fluoro, amino, and ester groups. We were able to control the alkylation of 1-naphthoic acid during Birch reduction by the addition of tert-butanol. This allowed the regioselective synthesis of mono and bis-substituted naphthalenes from the same starting material.},
  language  = {en}
}
@article{KruegerLinker2021,
  author    = {Kr{\"u}ger, Tobias and Linker, Torsten},
  title     = {Synthesis of gamma-spirolactams by Birch reduction of arenes},
  series = {European journal of organic chemistry},
  volume    = {2021},
  journal   = {European journal of organic chemistry},
  number    = {10},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1099-0690},
  doi       = {10.1002/ejoc.202100056},
  pages     = {1585 -- 1591},
  year      = {2021},
  abstract  = {A convenient method for the synthesis of gamma-spirolactams in only three steps is described. Birch reduction of inexpensive and commercially available aromatic carboxylic acids in the presence of chloroacetonitrile affords nitriles in moderate to good yields. Suitable precursors are methyl-substituted benzoic acids, naphthoic, and anthroic acid. Subsequent catalytic hydrogenation proceeds smoothly with PtO2 or Raney Ni as catalysts and lactams are isolated in excellent yields and stereoselectivities. Thus, up to 3 new stereogenic centers can be constructed as sole diastereomers from achiral benzoic acids. Furthermore, it is possible to control the degree of saturation at different pressures, affording products with 0, 1, or 2 double bonds. Overall, more than 15 new gamma-spirolactams have been synthesized in analytically pure form.},
  language  = {en}
}
@article{KruegerKellingSchildeetal.2016,
  author    = {Kr{\"u}ger, Tobias and Kelling, Alexandra and Schilde, Uwe and Linker, Torsten},
  title     = {Simple Synthesis of gamma-Spirolactams by Birch Reduction of Benzoic Acids},
  series = {European journal of organic chemistry},
  journal   = {European journal of organic chemistry},
  number    = {6},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1434-193X},
  doi       = {10.1002/ejoc.201601650},
  pages     = {1074 -- 1077},
  year      = {2016},
  abstract  = {A convenient synthesis of gamma-spirolactams in only two steps was developed. Birch reduction of benzoic acids and immediate alkylation with chloroacetonitrile afforded cyclohexadienes in high yields. The products could be isolated by crystallization on a large scale in analytically pure form. Subsequent hydrogenation with platinum(IV) oxide as the catalyst reduced the nitrile functionality and the double bonds in the same step with excellent stereoselectivity. The relative configurations were determined unequivocally by X-ray analyses. Direct cyclization of the intermediary formed amino acids afforded the desired gamma-spirolactams in excellent overall yields. The procedure is characterized by few steps, cheap reagents, and can be performed on a large scale, interesting for industrial processes.},
  language  = {en}
}
@article{KruegerBramborgKellingetal.2021,
  author    = {Kr{\"u}ger, Tobias and Bramborg, Andrea and Kelling, Alexandra and Sperlich, Eric and Linker, Torsten},
  title     = {Birch Reduction of Arenes as an Easy Entry to γ-Spirolactones},
  series = {European journal of organic chemistry},
  volume    = {2021},
  journal   = {European journal of organic chemistry},
  number    = {46},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1434-193X},
  doi       = {10.1002/ejoc.202101018},
  pages     = {6383 -- 6391},
  year      = {2021},
  abstract  = {A convenient method for the synthesis of γ-spirolactones in only 2-3 steps is described. Birch reduction of inexpensive and commercially available aromatic carboxylic acids in the presence of ethylene oxide affords hydroxy acids, which undergo direct lactonization during work-up. Suitable precursors are methyl-substituted benzoic acids, naphthoic, and dicarboxylic acids. Subsequent hydrogenation proceeds smoothly with Pd/C as catalyst and saturated γ-spirolactones are isolated in excellent yields and stereoselectivities. Thus, up to 3 new stereogenic centers can be constructed as sole diastereomers from achiral benzoic acids. Furthermore, it is possible to control the degree of saturation with Raney nickel or Wilkinson's catalyst to obtain products with 1 double bond. Overall, more than 30 new γ-spirolactones have been synthesized in analytically pure form.},
  language  = {en}
}
@article{KleinpeterHeydenreichKochetal.2017,
  author    = {Kleinpeter, Erich and Heydenreich, Matthias and Koch, Andreas and Krtitschka, Angela and Kr{\"u}ger, Tobias and Linker, Torsten},
  title     = {NMR spectroscopic conformational analysis of 4-methylene-cyclohexyl pivalateThe effect of sp(2) hybridization},
  series = {Magnetic resonance in chemistry},
  volume    = {55},
  journal   = {Magnetic resonance in chemistry},
  publisher = {Wiley},
  address   = {Hoboken},
  issn      = {0749-1581},
  doi       = {10.1002/mrc.4630},
  pages     = {1073 -- 1078},
  year      = {2017},
  abstract  = {The conformational equilibrium of the axial/equatorial conformers of 4-methylene-cyclohexyl pivalate is studied by dynamic NMR spectroscopy in a methylene chloride/freon mixture. At 153K, the ring interconversion gets slow on the nuclear magnetic resonance timescale, the conformational equilibrium (-G degrees) can be examined, and the barrier to ring interconversion (G(\#)) can be determined. The structural influence of sp(2) hybridization on both G degrees and G(\#) of the cyclohexyl moiety can be quantified.},
  language  = {en}
}
@article{FudickarMetzMaiLindeetal.2021,
  author    = {Fudickar, Werner and Metz, Melanie and Mai-Linde, Yasemin and Kr{\"u}ger, Tobias and Kelling, Alexandra and Sperlich, Eric and Linker, Torsten},
  title     = {Influence of functional groups on the ene reaction of singlet oxygen with 1,4-cyclohexadienes},
  series = {Photochemistry and photobiology : the official journal of the American Society for Photobiology},
  volume    = {97},
  journal   = {Photochemistry and photobiology : the official journal of the American Society for Photobiology},
  number    = {6},
  publisher = {Wiley},
  address   = {Malden, Mass.},
  issn      = {0031-8655},
  doi       = {10.1111/php.13422},
  pages     = {1289 -- 1297},
  year      = {2021},
  abstract  = {The photooxygenation of 1,4-cyclohexadienes has been studied with a special focus on regio- and stereoselectivities. In all examples, only the methyl-substituted double bond undergoes an ene reaction with singlet oxygen, to afford hydroperoxides in moderate to good yields. We explain the high regioselectivities by a "large-group effect" of the adjacent quaternary stereocenter. Nitriles decrease the reactivity of singlet oxygen, presumably by quenching, but can stabilize proposed per-epoxide intermediates by polar interactions resulting in different stereoselectivities. Spiro lactams and lactones show an interesting effect on regio- and stereoselectivities of the ene reactions. Thus, singlet oxygen attacks the double bond preferentially anti to the carbonyl group, affording only one regioisomeric hydroperoxide. If the reaction occurs from the opposite face, the other regioisomer is exclusively formed by severe electrostatic repulsion in a perepoxide intermediate. We explain this unusual behavior by the fixed geometry of spiro compounds and call it a "spiro effect" in singlet oxygen ene reactions.},
  language  = {en}
}