Synthesis and Use of New Substituted 1,3,5‐Hexatrienes in Studying Thermally Induced 6 π ‐Electrocyclizations

An acyclic, two heterocyclic, and two bicyclic alkenylstannanes, 3 , 4a , 4b , 8 and 11 , respectively, were synthesized in yields ranging from 43 to 97 %, and each was subjected to a sequence of Stille and Heck couplings with 2‐bromocyclohexenyl triflate ( 13 ) and alkyl ( tert ‐butyl and methyl) acrylate to furnish seven new 1,3,5‐hexatrienes 19 , 20 , 21 , 22 ‐ t Bu, 22 ‐Me, 23 and 43 , respectively, in 58–84 % yields. For the alkenylstannanes 4a , b , 8 and 11 , customized combinations of catalysts had to be used. The Stille–Heck sequence involving 13 , 3 and tert ‐butyl acrylate could be performed in a one‐pot mode and proceeded in 75 % yield. The hexatrienes were heated in decalin solutions so as to effect 6π‐electrocyclization. Temperatures and reaction times were optimized individually. The hexatrienes 29 , 31 and 36 gave the bi‐ and tricyclic cyclohexadienes 28 , 30 and 34 , incorporating allylic alcohol and allyl ether termini, by 6π‐electrocyclization and subsequent [1,5]‐hydrogen shift, as single products in good yields (85–93 %). In contrast, the hexatrienes 19 , 20 , 21 and 39 furnished mixtures of the initial electrocyclization products 26 , 32 , 37 and 40 as well as the products of a subsequent [1,5]‐hydrogen shift 27 , 33 , 38 and 41 , respectively. The tricyclic hexatrienes 22 ‐ t Bu, 22 ‐Me and 23 bearing alkyl ( tert ‐butyl, methyl) acrylate termini also selectively gave the tetracyclic dienes 48 ‐ t Bu, 48 ‐Me and 50 in 71–77 % yields by electrocyclizations and subsequent hydrogen shifts.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)