Cycloheptadienes, from divinylcyclopropanes

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 naturally occurring divinylcyclopropane derivative 20, an antifungal antibiotic from Polyangium cellulosum var. fulvum, also rearranges at 240 °C, to the cycloheptadiene 21. ... 

Without the disadvantage of using diazo compounds in the first step, Wenkert s latest monoterpenoid syntheses would be most efficient approaches, and in any case represent novel routes to well-known materials. Nezukone (754) was the result of examining the reaction between butadiene and diazopyruvic ester catalyzed by rhodium tetraacetate. The major product of the addition was the cyclopropane 758 (Scheme 62). It was known that divinylcyclopropanes could be thermolyzed to cycloheptadienes (Vol.4, p.537, Ref.600). The Wittig product from 758 thus gives a cycloheptadiene, and subsequent steps are shown in the scheme. The last step involves Grignard addition to the ester function of the enolate, then loss of water and redistribution of the double bonds.(Further examples of the use of diazo compounds will be found under perillene in the section on furans.)... 

H shift in the biosynthesis of vitamin D3. Recently Boland et al. reported, that the ectocarpenes 19 and related substances, which were isolated from the brown algae Ectocarpus siliculosus, are not the sexual pheromones of the algae, but that these cycloheptadienes 19 are formed from the actual pheromones, namely the thermolabile cis-divinylcyclopropanes 18, by a spontaneous Cope rearrangement (Scheme 5) [9]. Thus the Cope rearrangement of 18 resembles a deactivation pathway of the pheromones. 

Carbene transfer to the more electron-rich C=C bond in diene 31 generates the cfr-divinylcyclopropane 32 which undergoes a spontaneous ring expansion to give cycloheptadiene 33 in a regio- and stereoselective one-pot reaction. A similar reaction sequence has been applied in the synthesis of dihydroazepines from aza-l,3-dienes. [38]... 

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