Photochemistry of 7-azide-1-ethyl-3-carboxylate-6,8-difluoroquinolone: a novel reagent for photoaffinity labeling Elisa Leyva * , Denisse de Loera, Socorro Leyva Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, Av. Nava No. 6, San Luis Potosí, S.L.P. México 78210, Mexico article info Article history: Received 13 June 2008 Revised 24 July 2008 Accepted 28 July 2008 Available online 13 August 2008 Keywords: Photochemistry Aryl azide Singlet nitrene Photoaffinity labeling Fluoroquinolone abstract A novel reagent for photoaffinity labeling, 7-azido-1-ethyl-3-carboxylate-6,8-difluoroquinolone, was obtained by nucleophilic substitution of 1-ethyl-3-carboxylate-6,7,8-trifluoroquinolone. Photochemical reaction of this azide with diethyl amine gave 7-hydrazino-derivative as the major product. This compound was generated by singlet nitrene N–H insertion. In addition, 7-amino-1-ethyl-3-carboxyl- ate-6,8-difluoroquinolone was also obtained. Ó 2008 Published by Elsevier Ltd. The photochemical reaction of phenyl azide 1 has been de- scribed as wonderfully complex. 1–4 This reaction is initiated by the elimination of molecular nitrogen from phenyl azide to give a C 6 H 5 N fragment. At least four reactive intermediates have been associated with this fragment: singlet phenyl nitrene 2, triplet phe- nyl nitrene 3, benzazirine 4, and 1,2-azacycloheptatetraene 5 (Scheme 1). The singlet phenyl nitrene 2 is considered the first intermediate formed upon photolysis of phenyl azide. Improved spectroscopic techniques have allowed direct observation of this short-lived intermediate. 5–7 It was also suggested 8 that singlet phenyl nitrene generates bicyclic benzazirine 4 which can undergo ring cleavage to 1,2-azacycloheptatetraene 5. This latter intermedi- ate has been trapped to give azepine type products. 9 Aromatic azides are widely used in the biochemical technique called photoaffinity labeling (PAL). 10 This technique is used to identify amino acid residues present in the binding site of biologi- cal receptors. In this experiment, a light-sensitive moiety is ap- pended to a natural substrate of an enzyme. Photolysis of the enzyme–substrate complex releases a reactive intermediate that, in an ideal experiment, will react with the first bond of the protein it encounters to form a robust new linkage between the reactive intermediate and the target biomolecule. Unfortunately, the trap- pable intermediate produced upon photolysis of phenyl azide is azacycloheptatetraene. 1–4 This intermediate is not the ideal species for an efficient PAL experiment since it reacts rather sluggishly. 11 We have observed that the ring expansion of an aromatic sin- glet nitrene to 1,2-azacycloheptatetraene is strongly dependent on the substituents on the ring as well as the reaction tempera- ture. 3,12 Singlet phenyl nitrene 2 undergoes intersystem crossing (ISC) to triplet nitrene 3 at 77 K while it undergoes ring expansion 0040-4039/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.tetlet.2008.07.156 * Corresponding author. Tel.: +52 519 8262440x508; fax: +52 519 8262371. E-mail address: elisa@uaslp.mx (E. Leyva). Scheme 1. Tetrahedron Letters 49 (2008) 6759–6761 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet