Synthesis and use of new substituted 1,3,5-hexatrienes in studying thermally induced 6π-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-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.