Preparation and Conformation of Dioxocalix[4]arene Derivatives

CrO3 oxidation experiments conducted on atropisomeric forms of 2 (2paco, 21,3-alt, and 21,2-alt) indicate
that under the reaction conditions only methylene groups located between pairs of geminal rings
oriented in an anti disposition are oxidized to carbonyls. NMR data suggest that the tetrahydroxydioxocalix[4]arenes 7 and 9 adopt the partial cone and 1,2-alternate conformations, respectively. In the crystal
structure of 7·2EtOAc the dioxocalixarene adopts a partial cone conformation, whereas 9 adopts
in the crystal a 1,2-alternate conformation. In both conformations, pairs of geminal rings connected
to a carbonyl are oriented in an anti fashion. The relative stability of the partial cone and 1,2-alternate conformations of 7 and 9 is underestimated by MM3 calculations. The topomerization
barriers of 7 and 9 are 12.8 and 13.6 kcal mol-1, respectively.