Structural Basis for Superiority of N-Methyl-N-(4,6-dimethoxy-1,3,5-triazin-2-yl)morpholinium Tetrafluoroborate as Modular Coupling Reagent.Crystal Structures of N-Methyl-N-(4,6-dimethoxy-1,3,5-triazin-2-yl)morpholinium and-piperidinium Tetrafluorobo

摘要

Structural characteristics of tetrafluoroborates of N-methylated N-(4,6-dialkoxy-1,3,5-triazin-2-yl)-morpholine and-piperidine,as determined by X-ray diffraction study,not only explain their good performance as modular coupling reagents in the peptide synthesis but also their higher stability than appropriate chlorides,which degrade easi ly through demethylation.Not expected axial orientation of 4,6-dialkoxy-1,3,5-triazinyl substituent at the quaternary N atom of the aliphatic ring observed in the crystal state has been confirmed in a solution by NMR technique.Quarternary triazinyl ammonium salts have been found as excellent modular coupling reagents,particularly in peptide syntheses.Crystal structure determination of N-methyltriazinylmorpholinium and-piperidinium tetrafluoroborates as well as their demethylated products provides a structural reason for the observed reaction.Namely,formation of very stable guanidine system around C(2)atom in 2-morpholine-and 2-piperidine-s-triazines is controlled thermodynamically,while the difference in stability of chlorides and tetrafluoroborates(being superior)of quarternary N-methyl-N-triazinylmorpholinium(or piperidinium)results from conservative conformation of their cations,which hinders an access of large and less nucleophilie tetrafluoroborate anion than Cl~-to the methyl carbon atom.