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

Abstract

AbstractAn 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‐tBu, 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‐tBu, 22‐Me and 23 bearing alkyl (tert‐butyl, methyl) acrylate termini also selectively gave the tetracyclic dienes 48‐tBu, 48‐Me and 50 in 71–77 % yields by electrocyclizations and subsequent hydrogen shifts.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)