title: A fluorine scan at the catalytic center of thrombin: C--F, C--OH, and C--OMe bioisosterism and fluorine effects on pKa and log D values.
abstract: A series of 16 tricyclic thrombin inhibitors was prepared by using the 1,3-dipolar cycloaddition of azomethine ylides derived from 3- or 4-hydroxyproline and 4-bromobenzaldehyde, with N-(4-fluorobenzyl)maleimide as the key step. The terminal pyrrolidine ring of the inhibitors was systematically substituted to explore the potential bioisosteric behavior of C-F, C-OH, and C-OMe residues pointing into the environment of the catalytic center of a serine protease. X-ray crystal structure analyses revealed a distinct puckering preference of this ring. Substitution by F, HO, and MeO has a strong effect on the basicity of the adjacent pyrrolidine nitrogen center which originates from two sigma-inductive pathways between this center and the electronegative O and F atoms. gem-Difluorination decreases the pKa value of this tertiary amine center to <2, making the conjugated ammonium ion a moderately strong acid. Unexpectedly, F substitution next to the nitrogen center reduced the lipophilicity of the ligands, as revealed by measurements of the logarithmic partition coefficient log D. The biological assays showed that all compounds are thrombin inhibitors with activities between Ki=0.08 and 2.17 microM. Bioisosteric behavior of F, HO, and MeO substituents was observed. Their electronegative F and O atoms undergo energetically similar polar interactions with positively polarized centers, such as the N atom of His 57 which is hydrogen bonded to the catalytic Ser 195. However, for energetically similar polar interactions of C-F, C-OH, and C-OMe to occur, sufficient space is necessary for the accommodation of the Me group of the C-OMe residue, and a H-bond acceptor must be present to prevent unfavorable desolvation of the C-OH residue.
authors.affiliation: Laboratorium für Organische Chemie, ETH Zürich, Hönggerberg, HCI, 8093 Zürich, Switzerland.
authors.forename: Eliane
authors.initials: E
authors.lastname: Schweizer
authors.forename: Anja
authors.initials: A
authors.lastname: Hoffmann-Röder
authors.forename: Kaspar
authors.initials: K
authors.lastname: Schärer
authors.forename: Jacob A
authors.initials: JA
authors.lastname: Olsen
authors.forename: Christoph
authors.initials: C
authors.lastname: Fäh
authors.forename: Paul
authors.initials: P
authors.lastname: Seiler
authors.forename: Ulrike
authors.initials: U
authors.lastname: Obst-Sander
authors.forename: Björn
authors.initials: B
authors.lastname: Wagner
authors.forename: Manfred
authors.initials: M
authors.lastname: Kansy
authors.forename: François
authors.initials: F
authors.lastname: Diederich
mesh_terms: D020134:Catalytic Domain; D005461:Fluorine / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D013917:Thrombin / Q000737:chemistry* / Q000494:pharmacology
chemical_list: D005461:Fluorine; D013917:Thrombin