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@INPROCEEDINGS{Schfer:825270,
      author       = {Schäfer, Dominique and Weiss, Philipp and Castillo, Johnny
                      and Ermert, Johannes and Coenen, Heinrich Hubert},
      title        = {{A} new method for the two-step synthesis of n.c.a.
                      6-[{F}-18]fluorotryptophan},
      reportid     = {FZJ-2016-07741},
      year         = {2015},
      abstract     = {Objectives: Tryptophan was recently described as a possible
                      radiotracer for tumors diagnostics1,2. Furthermore the
                      amino acid is the precursor of serotonin biosynthesis
                      and acts as a part in neuronal functions which seems to be
                      inhibited by fluorinated tryptophan3. Earlier approaches
                      using the Balz-Schiemann reaction yielded
                      [18F]fluorotryptophan in low radiochemical yields (RCY).
                      Newer approaches as the isotopic exchange in the carbonyl
                      activated aromatic system with subsequent Baeyer-Villiger
                      oxidation required three radiosynthetic steps and
                      [18F]fluorotryptophan in carrier-added form. The aim of this
                      work was to develop a two-step synthesis of n.c.a.
                      6-[18F]fluorotryptophan with high RCY.Methods: The concept
                      of radiosynthesis is illustrated in figure 1. The precursor
                      2 for 6-[18F]fluorotryptophan was prepared from
                      6-bromo-indole. The precursor includes the Schöllkopf’s
                      amino acid auxiliary and a boronic ester group at the
                      desired position in the aromatic ring. The radiosynthesis
                      was performed by a novel copper(II) mediated
                      18F-fluorination4 and a subsequent hydrolysis as a two-step
                      reaction. Figure 1: Pathway to 6-[18F]fluorotryptophan 3
                      starting from 6-bromo indole 1 (PG = protecting
                      group).Results: Preparation of the precursor was
                      accomplished in a linear six-step synthesis with an overall
                      yield of 23 $\%.$ The following n.c.a. 18F-fluorination,
                      performed under modified conditions of the [Cu(OTf)2(py)4]
                      mediated n.c.a. 18F-fluorination4 gave radiochemical yields
                      of 60-65 $\%.$ After separating the product, an acidic
                      hydrolysis was performed with about 20 $\%$ RCY. Finally
                      L-6-[18F]fluorotryptophan was purified by HPLC with an
                      overall RCY of about 8 $\%$ and a molar activity of XX
                      MBq/mmol within 95 min total reaction time.Conclusions: The
                      radiosynthesis of n.c.a. L-6-[18F]fluorotryptophan was
                      performed in a short two-step synthesis. The radiotracer is
                      selectively labeled in 6-position and has a high molar
                      activity. Therefore it can be used for neuronal studies
                      without inhibitional effects. References: [1] J. Lin et al.,
                      J. Membrain Biol. 1988, 104, 1, [2] G.C. Prendergast, Nature
                      2011, 478, 192, [3] C. Jacquot et al., Biochem. Pharm.,
                      1992, 45, 1049, [4] V. Gouverneur et al., Angew. Chem. 2014,
                      53, 7751.},
      month         = {May},
      date          = {2015-05-26},
      organization  = {21st International Symposium on
                       Radiopharmaceutical Sciences, Columbia
                       (USA), 26 May 2015 - 31 May 2015},
      subtyp        = {Plenary/Keynote},
      cin          = {INM-5},
      cid          = {I:(DE-Juel1)INM-5-20090406},
      pnm          = {572 - (Dys-)function and Plasticity (POF3-572)},
      pid          = {G:(DE-HGF)POF3-572},
      typ          = {PUB:(DE-HGF)24},
      url          = {https://juser.fz-juelich.de/record/825270},
}