Cycloadditions of Allyl Cations, 281)Novel Diels‐Alder Additions via Acid Catalyzed Reactions of 4‐Methyl‐3‐penten‐2‐ol with Cyclopentadiene and Cyclohexadiene in Two Phases. Formation of Monocyclic, Bicyclic, and Tricyclic Adducts and Double Fragmentation of a Norbornenylcarbinol

Abstract 4‐Methyl‐3‐penten‐2‐ol (1) and cyclopentadiene react in an acidic two phase system at 0°C to form allylcyclopentenols 2 and norbornenylcarbinols 4. At 50°C and under similar conditions, 2 as well as 4 are equilibrated to allylcyclopentadienes 3, bicyclic olefins 5 and 6, and tricyclic ether 7. With a comparatively reactive dehydrating agent (SOCl2/pyridine), norbornenylcarbinol 4 suffers double fragmentation to give dimeric dienes C12H20, which were also obtained independently by acid catalyzed dehydrative dimerization of allyl alcohol 1. – The endo/exo selectivity for the formation of epimeric norbornenylcarbinols 4 from cyclopentadiene and 1 is similar to that of the AlCl3‐catalyzed cycloaddition of methyl 2‐butenoate und cyclopentadiene (13:1). The acid catalyzed two phase reaction of cyclohexadiene and 1 at room temperture gives the substituted bicyclo[2.2.2]octene endo‐13 in 19 – 23% yield.