Cycloadditions of Allyl Cations, 29. Acid Catalyzed Reactions of 2,3‐Dimethyl‐3‐penten‐2‐ol and Cyclopentadiene in Two Phases. Formation of Seven‐ and Five‐membered Rings

2,3–Dimethyl‐3‐penten‐2‐ol (3) and cyclopentadiene were stirred in dilute aqueous sulfonic acid/pentane, giving 2,3,4,4‐tetramethylbicyclo[3.2.1]octa‐2,6‐diene (4) , the epimeric 3‐methyl‐ene isomers 5a, b and, for the first time, also a five‐membered ring, namely 2,3,4,4‐tetramethylbicyclo[3.3.0]octa‐2,6‐diene (6) in 69 – 73% overall yield. The major bicyclic diene 4 was conveniently isolated as crystalline silver nitrate complex (C 12 H 18 ) 2 ·AgNO 3 (41 – 44%). In a modified approach the two phase reaction was carried out at 0°C, the resulting alcohols were separated by chromatography and cyclized at 50°C, giving an increased proportion of the thermodynamically more stable, novel [3.3.0] bicyclic isomer 6 (4:6 = 2.6:1)