Domino 6π‐Electrocyclization/Diels–Alder Reactions on 1,6‐Disubstituted ( E , Z , E )‐1,3,5‐Hexatrienes: Versatile Access to Highly Substituted Tri‐ and Tetracyclic Systems

The ( E , Z , E )‐1,3,5‐hexatrienes 1 a , 2 a ,  b and 3 b undergo 6π‐electrocyclization within 15–30 min upon heating to 200–215 °C. While the cyclohexene‐annelated products 8 a ,  b were stable, the analogous cyclopentene‐ and cycloheptene‐annelated derivatives 7 a and 9 b easily underwent dehydrogenation to the corresponding aromatic compounds 10 a and 12 b during the work‐up. The cyclohexadiene derivatives 8 a ,  b were employed in thermal Diels–Alder reactions with 4‐phenyl‐3 H ‐1,2,4‐triazoline‐3,5‐dione (PTAD) and tetracyanoethylene (TCNE) to give the expected [4+2] cycloadducts 13 a and 14 a in good yields (60 and 78 %). The initially formed cycloadduct of 8 a and dimethyl acetylenedicarboxylate (DMAD) underwent a subsequent retro‐Diels–Alder reaction to give the tetrahydronaphthalene 11 b (47 %). Under high pressure (10 kbar), the cycloadduct 15 a was formed at room temperature and could be isolated in 44 % yield. TCNE and N ‐phenylmaleimide with 8 a under high pressure also led to the [4+2] cycloadducts 14 a and 16 a in good yields (60 and 77 %). The 6π‐electrocyclization and subsequent Diels–Alder reaction, when performed as a one‐pot domino process, provided direct access to Diels–Alder products of intermediately formed 6π‐electrocyclization products, for example from the 1,3,5‐hexatrienes 1 a ,  b , 2 a ,  b , 3 b and TCNE to the corresponding tricyclic products 17 a ,  b , 14 a ,  b , 18 b in moderate to good yields (27–80 %) depending on the nature of the alkoxycarbonyl group. Such sequential reactions with N ‐phenylmaleimide, maleic anhydride, dimethyl maleate and fumarodinitrile, the latter two under high pressure (10 kbar), worked as well to yield 16 b (70 %), 19 a ,  b (19, 32 %) and 20 b (39 %) and 21 b (76 %), respectively. With PTAD, however, the hexatrienes 2 a ,  b reacted at ambient temperature without 6π‐electrocyclization to give the formal [4+2] cycloadducts 27 a ,  b (48 and 46 %), most probably via zwitterionic intermediates 23 a ,  b and 25 a ,  b .