Synthetisch interessante Transformationen von Ring-anellierten (E,Z,E)-1,3,5-Hexatrienen

Abstract

Twofold palladium-catalyzed coupling reactions (Heck reactions) of 1,2-dibromocycloalkenes with styrene or acrylates afforded 1,6-diphenyl- and 1,6-dialkoxycarbonyl-substituted (E,Z,E)-1,3,5-hexatrienes in high yields. These hexatrienes are valuable intermediates for the synthesis of a broad spectrum of interesting compounds.Photochemical reactions of six- and seven-membered ring-annelated methoxycarbonyl-substituted hexatrienes led to ring-attached 8-oxabicyclo[3.2.1.]octa-2,6-diene derivatives, the formal products of intramolecular hetero Diels-Alder reactions. The acetal moiety of these products could be cleaved using Lewis acids to give the corresponding troponecarboxylates. By using hexatrienes with more bulky alkoxycarbonyl groups, the formation of mixtures containing the respective 8-oxabicyclo[3.2.1.]octa-2,6-diene derivative and a corresponding bicyclo[3.1.0.]hex-2-ene was observed.Reduction of the dialkoxycarbonyl-substituted hexatrienes led to the analogous diol-compounds. These 1,6-dihydroxymethyl-substituted hexatrienes were subjected to a thermal 6p-electrocyclization leading to the corresponding ring-annelated cyclohexadienes, which were not sensitive to oxygen regardless of the size of the annelated cycloalkane.Also dialkoxycarbonyl-substituted hexatrienes were subjected to a thermal electrocyclization. It appeared that only the six-ring-annelated cyclohexadienes were not sensitive to oxygen whereas the five- and seven-membered ring-attached cyclohexadienes aromatized rapidly in the presence of oxygen.The isolable six-ring-annelated electrocyclization products were successfully used as dienes in Diels-Alder reactions (thermal and under high pressure) with various dienophiles leading to functionalized tri- and tetracyclic systems.The 6p-electrocyclization and the Diels-Alder reaction were also carried out as domino reactions in a one-pot procedure. Thus the efficiency of this cascade was improved and products, which were not accessible in the two-step procedure because of sensibility of the intermediate towards oxygen, could be obtained.Furthermore the central double bond of the 1,3,5-hexatriene system can selectively be epoxidized. Palladium-catalyzed reductive ring openings of the epoxides yield 1,2-dialkenylcycloalkanols with a trans relationship of the alkenyl groups, which can be converted into disubstituted medium sized cycloalkenones by anionic oxy-Cope rearrangements. Depending on the reaction conditions up to three isomers can be obtained.By a chemoselective methylating epoxide opening the synthesis of a 1,2-dialkoxycarbonyl-2-methylcyclohexanol succeeded. This was also used in anionic oxy-Cope rearrangements and gave the corresponding cyclodecenone as a mixture of isomers.For the first time thermal oxy-Cope rearrangements of the 1,2-dialkenylcycloalkanols were carried out. These showed a much higher selectivity than the anionic rearrangements; only one of cycloalkenone isomers was isolated in each case.