Acid-Catalysed Cyclisation of p-Mentha-1,8(9)-diene- and p-Mentha-1,8(10)-diene-9-carboxylic Acid. Novel Access to the Bicyclo[3.2.1]octane Skeleton

Treatment of the title compounds with either H₃PO₄ or BF₃ · Et₂O affords the bridged tricyclic lactones 3 and 7 as main products (57 and 70% yield, resp.). This is an efficient and novel access to specifically functionalised molecules possesing the bicyclo3.2.1]octane skeleton. Lactones 4 and 5 and the bicyclic ketone 6 were formed as by-products (2, 7, and 10% yield, resp.).