Preparation and conformation of dioxocalix[4]arene derivatives

CrO₃ oxidation experiments conducted on atropisomeric forms of 2 (2^paco, 2^1,3-alt and 2^1,2-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.