Synthesis, optical resolution and complexation properties of inherently chiral monoalkylated p-tert-butyl-(1,2)-calix[4]crown ethers

Reaction of mono-O-alkylated calix[4]arenes 2 with tri-to pentaethylene glycol ditosylates and K2CO3 affords asymmetrical (1,2)-calix[4]crown ether derivatives 3 as the main product, along with minor amounts of calix[4]arene dimers 4 and mixed syn-distal di-O-alkylated calix[4]arenes 5. A reaction mechanism for the formation of 3-5 is proposed, and the NMR spectral features of these products are briefly discussed. Evidence of the chirality of 3 was provided by diastereomeric interaction with enantiopure alkylammonium salts. The enantiomeric resolution of racemates 3 was achieved by direct HPLC separation, using chiral stationary phases. A screening of the complexing abilities of pyridino-(1,2)-calix[4]crown ethers 3a-c by extraction studies from water into CH2Cl2 showed a low extraction level of alkali, alkaline earth, and heavy metal picrates, while up to 25% extraction was found for Ag+. UV and pH-metric measurements of 3a-c with silver picrate in THF indicate the formation of 1:1:1 (metal:ligand:picrate) species, with log K's in the range 3.1-3.7.