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 (C12H18)2·AgNO3 (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)