Abstract Olah's reagent (HF-pyridine) reacts easily at room temperature with 6, 6-dimethyl-6- sila-(and 6, 6-dimethyl-6-germa-)-cycloundecanols. In each case, the principal product formed is not the expected cyclic organosilicon (or organogermanium) fluoride but the compound resulting from a transannular substitution reaction at the heteroatom with cleavage of one intracyclic heteroatom-carbon bond. The structures of the derivatives ...