Cycloadditions of Allyl Cations, 26 1) Norbornene Derivatives from Cyclopentadiene and 2,4‐Dimethyl‐3‐penten‐2‐ol in an Acidic Two Phase System. A Stepwise Diels‐Alder‐like Cyclization

2,4‐Dimethyl‐3‐penten‐2‐ol ( 1 ) and cyclopentadiene react in an acidic two phase system at 0‐25°C in several steps yielding epimeric norbornenylcarbinols ( endo ‐ 4 and exo ‐ 4 ) and at least three isomeric allylcyclopentenols ( 2a, b , c ), whereas allylcyclopentadienes ( 3a , b ) and norbornene‐type olefins (endo ‐ 5 , exo ‐ 5 , and 6 ) are formed in traces under these conditions. All bicyclic products of this reaction were synthesized independently. On subjecting the pure product norbornenylcarbinols ) endo ‐ 4 and exo ‐ 4 ) to the acidic, two phase reaction conditions at slightly elevated temperature, i. e. 50°C, some epimerization occurs and the norbornene‐type olefins ( exo ‐ 5 , endo ‐ 5 , and 6 ) are mainly formed in addtion to allylcyclopentadienes ( 3a , b ) allylcyclopentenols ( 2a, b , c ), nortricyclic alcohol 7 , and tricyclic ether 8 . Independently, the forward reaction was traced by subjecting the isomeric allylcyclopentenols 2b , c to the acidic, two phase reaction conditions at 50°C: once again, endo / exo bicyclization gave the olefins 5 and 6 as well as alcohols 4 and 7 Hence, a stepwise Diels‐Alder‐like cyclization is proposed to account for the norbornenetype alcohols and bicyclic olefins, which are formed from the acid promoted reaction of cyclopentadiene and 1 .