Endomorphin‐1 (EM1, Tyr‐Pro‐Trp‐Phe‐NH2) and endomorphin‐2 (EM2, Tyr‐Pro‐Phe‐Phe‐NH2) are natural tetrapeptide ligands of µ‐opioid receptors involved in the modulation and attenuation of pain. For a detailed examination of their receptor‐binding properties and their metabolic stability, tritium‐labelled EM1 and EM2 radioisotopomers were synthesized by catalytic dehalogenation or saturation of the precursor peptides with tritium gas. Amino acid analysis revealed that the tritium labelling was specific and the specifically labelled radioligands possessed high specific activity, ranging from 0.77 TBq/mmol to 2.35 TBq/mmol. The biological half‐lives of the peptides in the biological matrix (295 min and 230 min for EM1 and EM2, respectively) indicate that these radioligands are appropriate for binding assays in rat brain membrane preparations. The radioisotopomers of EM2 are not statistically different in the receptor‐ligand interaction, and they are excellent tools for further comparative biochemical studies. Copyright © 2001 John Wiley & Sons, Ltd.

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Zn­(NCO)2·2NH3 catalyst has been synthesized, characterized by X-ray diffraction and IR spectroscopy, and used as a catalyst for the cyclization of urea and ethylene glycol (EG) to ethylene carbonate (EC) for the first time. A maximum yield of 40% has been obtained with a urea/EG mole ratio of 1:1.5 and a temperature of 150 ± 2 °C. An attempt has been made to predict the mechanism of the reaction with the help of density functional theory calculations. Our calculations suggest the reaction to be a consecutive one. In the first step, urea decomposes to ammonia and isocyanic acid (HNCO). HNCO reacts with EG to produce 2-hydroxyethyl carbamate (2-HEC). At the last step, 2-HEC cyclizes over Zn­(NCO)2·2NH3 to EC. Calculations suggest the cyclization of 2-HEC to follow a charge-controlled intramolecular nucleophilic elimination cyclization path.