Empirical and theoretical fusion barriers for1H and4He: Connections to evaporation from hot nuclei

Fusion excitation functions for¹H and⁴He have been compared to a one-dimensional, barrier-penetration model. In contrast to fusion for heavier nuclei this simple model is completely adequate, except for the statically deformed targed²³³U. Empirical barrier heights are obtained and compared to those from two theoretical nuclear potentials. These empirical barriers (from cold reactants) are used as input for calculating evaporation spectra that arise from hot nuclear emitters. The excess of observed low-energy¹H and⁴He emission signals information content concerning distortions of the hot nuclei.     

Photochemical Reaction between Tertiary Allylic Amines and Chromium Carbene Complexes: Synthesis of Lactams via a Zwitterion Aza Cope Rearrangement

Photolysis of chromium carbene complexes in the presence of tertiary allylic amines resulted in a zwitterionic aza Cope rearrangement to produce unsaturated lactams in fair yield.     

Cyclization of all-L-Pentapeptides by Means of 1-Hydroxy-7-azabenzotriazole-Derived Uronium and Phosphonium Reagents

Due to their restricted conformational flexibility, cyclic peptides are of great interest in connection with structure-activity relationships, especially the elucidation of bioactive conformations. For linear peptides that do not contain turn structure-inducing amino acid residues, the cyclization reaction may be an inherently improbable or slow process, and side reactions, such as cyclodimerization and epimerization at the C-terminal residue, may dominate. A number of 1-hydroxy-7-azabenzotriazole-based onium salts were examined for cyclization of thymopentin-derived pentapeptides and the results compared with data from more conventional coupling reagents. The azabenzotriazol-derived coupling reagents stood out as being the most effective by far. The cyclizations proceed extremely rapidly, and in contrast to other coupling reagents, C-terminal epimerization was generally less than 10%. C-terminal D-amino acid residues favor the formation of monocyclic pentapeptide rings. A similar effect was observed for cyclization of linear N-methylamino acid-containing peptides, suggesting that reversible amide bond alkylation such as Hmb-modification should be useful in promoting the cyclization of pepitdes devoid of turn-inducing amino acid residues.