Observation of a stable carbocation in a consecutive Criegee rearrangement with trifluoroperacetic acid

Selective oxidative cleavage-cyclization of adamantane through the bridge carbon was developed in trifluoroperacetic acid (TFPAA). The methyl group in the bridge position was found to be the substituent that directs consecutive oxygen insertion into the cage structure during the course of a Criegee rearrangement. The formation of stable 5-methyl-4,6-dioxabishomoadamant-5-yl cation at -25 degrees C was observed. Stable carboxonium ion formation allows control of the selectivity of further transformations. Hydrolysis leads to the stereospecific formation of endo,endo-3-hydroxy-7-acetoxybicyclo[3.3.1]nonane. Its single-crystal X-ray structure was obtained. An increase in temperature results in deprotonation of the 5-methyl-4,6-dioxabishomoadamant-5-yl cation to endo3-trifluoroacetoxybicyclo[3.3.1]non-6-ene which undergoes further epoxidation with TFPAA and acidic transannular cyclization in trifluoroacetic acid (TFAA). The described reactions can be used as a convenient method for the synthesis of bicyclo[3.3.1]nonane and oxaadamantane derivatives. The proposed mechanism for each transformation, as well as supporting ab initio theoretical calculations of the strain energy and the stabilization energy of the relevant oxacage structures, are discussed.