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Enantiomerically Pure Thrombin Inhibitors for Exploring the Molecular‐Recognition Features of the Oxyanion Hole
Author(s) -
Schärer Kaspar,
Morgenthaler Martin,
Seiler Paul,
Diederich François,
Banner David W.,
Tschopp Thomas,
ObstSander Ulrike
Publication year - 2004
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.200490225
Subject(s) - chemistry , oxyanion hole , stereochemistry , thrombin , oxyanion , discovery and development of direct thrombin inhibitors , hydrogen bond , molecule , organic chemistry , platelet , immunology , biology , catalysis
A new route via intermediate pseudoenantiomers was developed to synthesize racemic and enantiomerically pure new non‐peptidic inhibitors of thrombin, a key serine protease in the blood‐coagulation cascade. These ligands feature a conformationally rigid tricyclic core and are decorated with substituents to fill the major binding pockets (distal (D), proximal (P), selectivity (S1), and oxyanion hole) at the thrombin active site ( Fig. 1 ). The key step in the preparation of the new inhibitors is the 1,3‐dipolar cycloaddition between an optically active azomethine ylide, prepared in situ from L ‐(4 R )‐hydroxyproline and 4‐bromobenzaldehyde, and N ‐piperonylmaleimide ( Scheme 1 ). According to this protocol, tricyclic imide (compounds (±)‐ 15 ‐(±)‐ 18 and (+)‐ 21 ) and lactam (compound (+)‐ 2 ) inhibitors with OH or ether substituents at C(7) in the proline‐derived pyrrolidine ring were synthesized to specifically explore the binding features of the oxyanion hole ( Schemes 2–4 ). Biological assays ( Table ) showed that the polar oxyanion hole in thrombin is not suitable for the accommodation of bulky substituents of low polarity, thereby confirming previous findings. In contrast, tricyclic lactam (+)‐ 2 ( K i =9 n M , K i (trypsin)/ K i (thrombin)=1055) and tricyclic imide (+)‐ 21 ( K i =36 n M , K i (trypsin)/ K i (thrombin)=50) with OH‐substituents at the ( R )‐configured C(7)‐atom are among the most‐potent and most‐selective thrombin inhibitors in their respective classes, prepared today. While initial modeling predicted H‐bonding between the OH group at C(7) in (+)‐ 2 and (+)‐ 21 with the H 2 O molecule bound in the oxyanion hole ( Fig. 2 ), the X‐ray crystal structure of the complex of (+)‐ 21 ( Fig. 7 , b ) revealed a different interaction for this group. The propionate side chain of Glu192 undergoes a conformational change, thereby re‐orienting towards the OH group at C(7) under formation of a very short ionic H‐bond (OH⋅⋅⋅ − OOC; d (O⋅⋅⋅O)=2.4 Å). The energetic contribution of this H‐bond, however, is negligible, due to its location on the surface of the protein and the unfavorable conformation of the H‐bonded propionate side chain.