Chiral resolution and stereospecificity of 6-phenyl-4-phenylethynyl- 1,4-dihydropyridines as selective A(3) adenosine receptor antagonists.

Racemic 6-phenyl-4-phenylethynyl-1,4-dihydropyridine derivatives have been shown to be highly selective A(3) adenosine receptor antagonists (Jiang et al. J. Med. Chem. 1997, 40, 2596-2608). Methods for resolving the optical isomers at the C4 position, involving selective crystallization or chromatographic separation of diastereomeric ester derivatives, have been developed. Optically pure glycerol and threitol derivatives were used as chiral auxiliary groups for ester formation at the 3-position, resulting in diastereomeric mixtures of dihydropyridines. Esterification of a 6-phenyl-4-phenylethynyl-1,4-dihydropyridine derivative at the 3-position with a chiral, protected glycerol moiety, (S)-(+)-2, 2-dimethyl-1,3-dioxolane-4-methanol, allowed the selective crystallization of a pure diastereomer, 9. The (1)H NMR spectrum of 9 using the lanthanide shift reagent Eu(fod)(3) indicated optical purity, and the (4S,2'R)-configuration was assigned using X-ray crystallography. The noncrystalline (4R,2'R)-isomer 10 was also isolated and shown to be 3-fold more potent than the (4S,2'R)-isomer in binding to A(3) receptors. The 2,2-dimethyl-1,3-dioxolane moiety also served as a protected form of a diol, which showed selective reactivity versus a 5-ethyl ester in basic transesterification reactions. A racemic 5-carboxylic acid derivative could not be resolved through crystallization of diastereomeric salts. Enantiomers of 5-benzyl 3-ethyl 2-methyl-6-phenyl-4-phenylethynyl-1, 4-dihydropyridine-3,5-dicarboxylate (2) were obtained via an ester derived from (4R,5R)-(-)-2,3-O-isopropylidene-D-threitol at the 3-position, which was resolved using HPLC, and each diastereomer was subsequently deprotected in acidic conditions. The resulting diols were exchanged for ethyl ester groups by base-catalyzed transesterification. The binding of pure enantiomers of 2 at A(3) adenosine receptors indicated a 35-fold stereoselectivity for the (4S)-isomer 21. A receptor docking hypothesis, using a previously derived human A(3) receptor model, shows the bulkier of the two ester groups (5-Bn) of 21 oriented toward the exofacial side and the 4-position phenylethynyl group situated between transmembrane helical domain TM6 and TM7.

Structures of Cdc42 bound to the active and catalytically compromised forms of Cdc42GAP.

The Rho-related small GTP-binding protein Cdc42 has a low intrinsic GTPase activity that is significantly enhanced by its specific GTPase-activating protein, Cdc42GAP. In this report, we present the tertiary structure for the aluminum fluoride-promoted complex between Cdc42 and a catalytically active domain of Cdc42GAP as well as the complex between Cdc42 and the catalytically compromised Cdc42GAP(R305A) mutant. These structures, which mimic the transition state for the GTP hydrolytic reaction, show the presence of an AIF3 molecule, as was seen for the corresponding Ras-p120RasGAP complex, but in contrast to what has been reported for the Rho-Cdc42GAP complex or for heterotrimeric G protein alpha subunits, where AIF4- was observed. The Cdc42GAP stabilizes both the switch I and switch II domains of Cdc42 and contributes a highly conserved arginine (Arg 305) to the active site. Comparison of the structures for the wild type and mutant Cdc42GAP complexes provides important insights into the GAP-catalyzed GTP hydrolytic reaction.

A diterpenoid lactone from Aplysia juliana.

A new diterpenoid lactone, angasiol acetate [1], has been isolated from the sea hare Aplysia juliana collected from the Karachi coastline of the Arabian ocean. The structure, including the absolute configuration of 1, was determined by single-crystal X-ray diffraction and spectroscopic techniques.