Heptadienes, Cope rearrangement

Although the possibility of antara-antara Cope rearrangements has been recognized and suggested as mechanistic alternatives for thermal reairangements of bicyclo[3.2.0]heptadienes, they appear unlikely in die latter case and have been ruled out for simple Cope leairangements. ... 

Therefore, a Cope rearrangement can be detected only when the diene is not symmetrical about this bond. Any 1,5-diene gives the rearrangement for example, 3-methyl-l,5-hexadiene heated to 300°C gives 1,5-heptadiene. However, the reaction takes place more easily (lower temperature required) when there is a group on the 3- or 4-carbon with leads to the new double bond being substimted. The reaction is obviously reversible and produces an equilibrium mixture of the two 1,5-dienes, which is richer in the thermodynamically more stable isomer. However, the equilibrium can be shifted to the right for 3-hydroxy-1,5-dienes, because the product tautomerizes to the ketone or aldehyde ... 

Diethenyloxiranes undergo thermal Cope-type rearrangements to oxacyclo-heptadienes (Scheme 16) (67AG(E)385) even when these are very strained intermediates (Scheme 17) (80JOC428). This process is particularly facile in fused-ring diethenyloxiranes like (16) (Chapter 5.17) and (17) (Chapter 5.07). 

The [2,3]-rearrangement of amine iV-oxides was utilized in an efficient S5mthesis of 2,6-dimethyl-l,5-heptadien-3-ol acetate 61, a pheromone of the insect Pseudococcus comstocki tScheme lS.12id Dimethylpyridine 55 was converted in two steps into silylated piperidine 56, which was oxidized with m-CPBA to generate cyclic amine A-oxide 57. Sila-Cope elimination furnished O-silylhydroxylamine 58, which was methylated and desilylated in the presence of Mel and CsF to yield acyclic amine A-oxide 59. Heating this ammonium zwitterion facilitated the [2,3]-Meisenheimer rearrangement to O-allylhydroxylamine 60, which was transformed to the desired pheromone 61 in three steps. 

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