Rearrangement Approaches to Cyclic Skeletons. X. Pinacol-Type Rearrangement of 2-Substituted 1-Methoxybicyclo[2.2.2]oct-5-en-2-ols into Bicyclo[3.2.1]oct-6-en-2-ones. Remarkable Substituent Effects on Predominant Migration of the Unsaturated Bridge1

Article abstract of DOI:10.1246/bcsj.69.1727

Pinacol-type rearrangement of 2-substituted l-methoxybicyclo[2.2.2]oct-5-en-exo-2-ols (1) by treatment with p-toluenesulfonic acid in boiling benzene gave bicyclo[3.2.1]oct-6-en-2ones (2) and -3en-2-ones (4) in a ratio of 10 to 1 along with small amounts of dehydration products. Treatment of the endo isomers (3) under the same conditions gave 2, 4, and the dehydration products. The ratios of 2 to 4, in this case were within a certain range and were influenced by the substituent at the C-2 position. The ratio of the relative disappearance rate constant of 1 to that of the corresponding 3 was less than 10. This observation suggests that the unsaturated bridge does not offer remarkable anchimeric assistance in the reaction of 1. A similar acid treatment of the dehydration products gave 2 and 4 and their ratio was somewhat affected by the substituent. A mechanism is proposed for the pinacol-type rearrangement.