Abiotic Anion Receptor Functions. A Facile and Dependable Access to Chiral Guanidinium Anchor Groups

We describe the synthesis of the chiral bicyclic guanidinium salts 3 and 4, which may be useful as anchor modules for oxoanionic functions of molecular guest species complexed by polytopic artificial receptors. Starting from the chiral amino acids asparagine and methionine a convergent strategy is followed to produce a thiourea compound 15 containing all the atoms necessary to construct the bicyclic skeleton. The key reaction is the double cyclization process of this thiourea initiated by S-alkylation. In a four-step one-pot reaction the Tos-protected bicyclic guanidines 18 are obtained, which are finally deprotected by electrolyses or aluminum amalgam reduction to give the target compounds. This route matches an older one with respect to the availability of chiral educts and the reliability of the stereochemical outcome, but is distinctly superior in terms of yield, managable scale, rapidity, and experimental ease.