Stereochemistry of 11-Hydroxy-1,3,5,7,11-pentamethyl-2,4,6,8-tetraoxatricyclo[3.3.3.0(3,7)]undecan-9-one and Related Diols

11-Hydroxy-1,3,5,7,11-pentamethyl-2,4,6,8-tetraoxatricyclo[3.3.3.0(3,7)]undecan-9-one adopts an equilibrium (in CD2CI2 at −90°C) between ca. 68% of the conformer with a pseudo-axial C-11 methyl substituent and ca. 32% of the alternative pseudo-equatorially substituted conformer. The derived cis-9,11-diol, cis-9,11-dimethyldiol and cis-9-phenyl-11-methyldiol adopt conformational equilibria heavily biased towards the diaxial OH conformations which are stabilised by internal H-bonds involving the dioxolan ring oxygen atoms.