Stepwise synthesis and selective dimerisation of bis- and trisloop tetra-urea calix[4]arenes

Tetra-urea calix4]arenes substituted with four mono- or bisalkenyl residues have been converted into bis- or tetraloop compounds by intramolecular olefin metathesis, with use of a tetratosylurea calix4]arene as template. The same strategy has now been used to synthesise trisloop compounds and bisloop compounds with adjacent loops, completing the series of the loop-containing tetra-urea derivatives. A tetra-urea calix4]arene of the AABB type, where A stands for a bisalkenyl- and B for a monoalkenyl-substituted urea unit, was used as precursor for the three loops. It was easily synthesised from a tetraamino calix4]arene in which two adjacent amino groups were Boc-protected. The ABCB-type precursor for the two adjacent loops was prepared through protection of two opposite amino functions with trityl groups. The capabilities of the novel macrocyclic tetra-ureas for the selective formation of hydrogen-bonded dimers were studied.