C/’.s-1,2-divinylcyclopropane

Entry 3 in Scheme 6.11 illustrates the application of a c/s-divinylcyclopropane rearrangement in the prepartion of an intermediate for the synthesis of pseudoguaiane-type natural products. 

From a synthetic viewpoint, a comparison of the thermolyses of the epimers (92) and (94) (Schemes 12 and 13) illustrates the point, previously mentioned, that, in certain cases in which alternative modes of rearrangement are possible, the stereoselective formation of the c/s-divinylcyclopropane substrate is important. Thus, while thermal rearrangement of the cis substrate (92) provides Ae Cope rearrangement product (98) in reasonable yield, thermolysis of the trans isomer (94) does not. 

It was emphasized that a particular advantage of this approach over otha- synthetic strategies based on Cope rearrangement consists in the facile way of selectively preparing C S-divinylcyclopropane intermediates. ... 

A dramatic change in stability is evident when c/s-divinylcyclopropane is compared with vinylcyclopropane. The presence of the cw-divinyl groups causes rearrangement to occur with great ease. The parent cw-divinylcyclopropane has only recently been isolated reactions designed to produce it usually lead to cyclohep-tadiene by spontaneous rearrangement. ... 

Dictyopterene C (15, R = H R = Bu) and dictyopterene D [15, R = H R = (Z)-CH = CHEt], the natural pheromones of brown algae, have been synthesized from both ci.i-16 and /ran.s-divinylcyclopropanes 14 via thermal rearrangements. Extensive racemiz-ation was observed when optically pure divinylcyclopropanes were used. Cycloheptadienes 15 (R = COjEt R = H, Ph) are also available heterocyclic analogs can be prepared via cyclopropanations of dienes with ethyl diazopyruvate (see Section 2.4.5.2.). ... 

The highly substituted d.s-divinylcyclopropanes (20) and (21) do not undergo sigmatropic rearrangement at all. Apparently, the highly sterically congested nature of the transition states (F) precludes this possibility. Thermolysis of (20) and (21) at 170-180 °C produces only equilibrium mixtures of these substances and the corresponding trans isomers (22) and (23), respectively (Scheme 3). ... 

If the vinyl groups of c/. s-l,2-divinylcyclopropane are joined by a methylene bridge. Cope rearrangement of the resulting homotropylidene yields a product identical with the starting material. 

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