Solvolysen von 4‐Alkylidenbicyclo[3.2.0]hept‐2‐en‐6‐olen. Synthese von 1‐Vinylfulvenen und 8,8‐Diphenylheptafulven

Solvolysis of 4‐Alkydenbicyclo[3.2.0]hept‐2‐en‐6‐oles. Synthesis of 1‐Vinylfulvenes and 8,8‐Diphenylheptafulvene Four 4‐alkylidenebicyclo[3.2.0]hept‐2‐en‐6‐ones 2–5 , obtained via ketene cycloaddition to fulvenes, were reduced to separated mixtures of the ‘ endo ’ ‐alcohols ‘ endo ’‐ 6 to ‘ endo ’‐ 9 (68–73%) and ‘ exo ’‐ 6 to ‘ exo ’‐ 9 (3–20%). Treatment of some of these alcohols with (CF 3 SO 2 ) 2 O in CH 2 Cl 2 /pyridine caused a spontaneous solvolysis to yield unsaturated 7‐membered rings as pyridinium triflates 10–12 or 1‐vinylfulvenes 13 and 14 , a new class of reactive tetraenes: Both ‘ endo ’‐ 9 and ‘ exo ’‐ 9 , having two methyl groups at C(7), were converted into the vinylfulvene 13 (≈ 80%). The alcohols with two H‐atoms at C(7) exhibited a stereochemically controlled reaction selectivity, inasmuch as ‘ endo ’‐ 6 to ‘ endo ’‐ 8 afforded only the corresponding 7‐membered‐ring pyridinium salts 10–12 (66–79%), while ‘ exo ’‐ 6 produced only the vinylfulvene 14 (77%). A stereoelectronic control argument explains the C(1), C(5)‐bond cleavage with ‘ endo ’‐ B and ‘ endo ’– 6 ‐‘ endo ’‐ 8 , as well as the C(1), C(7)‐bond cleavage with ‘ exo ’‐ B , ‘ exo ’‐ 6 , and with both ‘ endo ’‐ and ‘ exo ’‐ 9 . Thermolysis (120°) of the pyridinium triflates 10 and 11 yielded the 3‐isopropenyl‐cycloheptatrienes 18 and 19 , respectively (≈90%); similar conditions (145°) applied to the triflate 12 produced the doubly cyclized fluorene derivative 21 (60%). When the iodide 22 derived from the triflate 12 with Nal was heated in refluxing toluene, 8,8‐diphenylheptafulvene ( 23 , 86%) was obtained.