Heterocalixarenes Part 3: Bis-oxo-bridged calix[1]cyclicurea[3]arene and calix[1]cyclicurea[1]pyridine[2]arenes. Synthesis, X-ray crystal structure and conformational analysis 1
4(1H,3H)-dione to form 11 new bis-oxo-bridged heterocalix[4]arenes (11–19, 24, 25). The X-raycrystalstructure of the 11–benzene complex, 1H–1H COSY spectra and energy-minimization studies assign partial cone conformations to these heterocalix[4]arenes. The variation in the cyclicurea moiety controls the flexibility of these heterocalix[4]arenes.