Unusual conformations of 1,3-dialkoxythiacalix[4]arenes in the solid state

The structures of three syn-1,3-dialkoxy-thia-calix[4]arenes with unusual conformations in the solid state are reported. The pinched cone conformation of syn-2(2),4(2)-dihydroxy-1(2),3(2)-bis-(prop-2-enyl-oxy)thia-calix[4]arene, C30H24O4S4, (3a), is stabilized by two intra-molecular hydrogen bonds, remarkably formed from both OH groups to the same ether O atom. In syn-2(2),4(2)-dihydroxy-1(5),2(5),3(5),4(5)-tetra-nitro-1(2),3(2)-bis-(prop-2-enyl-oxy)thia-calix[4]arene acetone disolvate, C30H20N4O12S4.2C(3)H(6)O, (3b1), the mol-ecule is found in the 1,3-alternate conformation. The crystallographic C2 symmetry is due to a twofold rotation axis running through the centre of the calixarene ring. The hydroxy groups cannot form intra-molecular hydrogen bonds as in (3a) and both are bonded to an acetone solvent mol-ecule. The mol-ecule of the pseudo-polymorph of (3b1) in which the same compound crystallized without any solvent, viz. (3b2), is located on a crystallographic mirror plane. Only one of the two hydroxy groups forms a hydrogen bond, and this is with a nitro group of a neighbouring mol-ecule as acceptor. Mol-ecular mechanics calculations for syn-1,3-diethers suggest a preference of the 1,3-alternate over the usual cone conformation for thia-calix[4]arene versus calix[4]arene and for para-nitro versus para-H derivatives.