Synthesis of polyhalo acridones as pH-sensitive fluorescence probes Chao Huang   , Sheng-Jiao Yan   , Yan-Mei Li, Rong Huang, Jun Lin * Key Laboratory of Medicinal Chemistry for Natural Resources, Ministry of Education, School of Chemical Science and Technology, Yunnan University, Kunming 650091, PR China article info Article history: Received 31 January 2010 Revised 10 May 2010 Accepted 27 May 2010 Available online 2 June 2010 Keywords: Polyhaloacridone Polyhalo isophthalonitrile Fluorescence probe abstract Polyhalo isophthalonitriles were reacted with substituted anilines and subsequently cyclocondensed in the presence of sulfuric acid to give polyhalo acridones. These polyhalo acridones were proven to be use- ful as pH-sensitive fluorescent probes for a wide range of acidic and basic conditions. Ó 2010 Elsevier Ltd. All rights reserved. Fluorescent probes that are pH-sensitive have been widely used in analytical chemistry, bioanalytical chemistry, cellular biology (particularly for measuring intracellular pH) and medicine, 1 pre- sumably due to their preferential properties, including nondestruc- tive character, high sensitivity and specificity, and the availability of a wide range of indicator dyes. 2 To date, a variety of pH-sensitive probes have been reported, 3 most of which can be used in either acidic or basic conditions. 4 In fact, new probes with specific fluo- rescence over a broad pH range under both acidic and basic condi- tions are continuously being developed. Acridone derivatives have received much attention over recent years because of their perfect fluorescent properties, such as high quantum yields and excellent photostability, 5 and their potential pharmaceutical applications, possibly being used as antitumor agents, 6 antivirals, 7 antimalarials, 8 and antibacterials, etc. 9 Acri- done derivatives can be used as fluorescent probes based on their unique interactions with biomolecules such as DNA and enzymes. Many documents 5a–c,10 regarding the ionization of acridones including their ammonium salts and neutral molecules in a low pH medium are widely available. To date, the ionization of acri- dones in a high pH medium has not been reported. This is probably because the higher pK a value of acridones limits their ionic disso- ciation. Therefore, it is crucial to improve the acidity of acridone derivatives in order to develop pH fluorescent probes that may be applied to a broad pH range. In this context, the acidity of acri- done derivatives 3a–e (Scheme 1) could be enhanced by introduc- ing an acyl group at site 6 of the acridone enatic structure and a halogen atom at the C-1 position. In this manner, it is envisioned that compounds 3 could act as potential acid–base dual-purpose pH fluorescent probes. Although acridones with 1 or 2 substituents have been synthe- sized, 7a,b,9d access to highly functionalized acridones is still limited, probably because introducing multiple substituents to the acri- done ring often requires multistep reactions and complicated prep- aration procedures. Therefore, the development of efficient and concise approaches for producing acridones that tolerate a wide variety of functional groups is desirable. Polyhalo isophthalonitriles, especially polyfluoro isophthalo- nitriles, have been widely used as reagents/intermediates in organ- ic synthesis. 11 Halogens and amido groups can be readily derived and thus provide opportunities for constructing molecule libraries for screening biological activity. This communication reports the 0960-894X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2010.05.101 * Corresponding author. Tel./fax: +86 871 5033215. E-mail address: linjun@ynu.edu.cn (J. Lin).   These authors contributed equally to this Letter. X CN NC X Y X NH 2 R X X Y N H R O H 2 N O 1a: X=Cl, Y=Cl 1b: X=F, Y=Cl 1c: X=F, Y=F X CN NC X Y N H R i) ii) R=H, Cl 2a: X=Cl, Y=Cl, R=H yield=78% 2b: X=F, Y=Cl, R=H yield=92% 2c: X=F, Y=F, R=H yield=92% 2d: X=F, Y=Cl, R=Cl yield=85% 2e: X=F, Y=F, R=Cl yield=89% 3a: X=Cl, Y=Cl, R=H yield=84% 3b: X=F, Y=Cl, R=H yield=76% 3c: X=F, Y=F, R=H yield=86% 3d: X=F, Y=Cl, R=Cl yield=85% 3e: X=F, Y=F, R=Cl yield=81% Scheme 1. Synthesis of polyhalo acridones. Reagents and conditions: (i) DMF, K 2 CO 3 , rt; (ii) H 2 SO 4 , 90 °C. Bioorganic & Medicinal Chemistry Letters 20 (2010) 4665–4669 Contents lists available at ScienceDirect Bioorganic & Medicinal Chemistry Letters journal homepage: www.elsevier.com/locate/bmcl