Chemically modified tetranitro-oxacalix[4]arenes: synthesis and conformational preferences of tetra-N-(1-octyl)ureido-oxacalix[4]arenes

Tetranitro-oxacalix[4]arenes 1-5, prepared by direct SNAr reaction of 1,5-difluoro-2,4-dinitrobenzene with the appropriate aromatic diol (pyrocatechol, resorcinol, hydroquinone, 2,7-dihydroxynaphthalene, and 4,4'-dihydroxybiphenyl), were subjected to Raney-nickel reduction to provide the corresponding tetraamino-oxacalix[4]arenes 6-10, which upon treatment with an excess of 1-octyl isocyanate were converted into the title compounds 11-15, featuring a pair of 1,3-bis-[N-(1-octyl)ureido]phenylene moieties doubly connected at their 4,6-positions by rigid spacers of varied geometry. All new oxacalix[4]arenes were characterized by MALDI-TOF spectrometry and NMR spectroscopy. H-1 NMR data and ab initio calculations support saddle-shaped conformations for oxacalix[4] arenes incorporating pyrocatechol, resorcinol and 2,7-dihydroxynaphthalene nucleophilic components, and boat-shaped conformations for derivatives possessing hydroquinone and 4,4'-dihydroxybiphenyl spacers.