C«-divinylcyclopropane

It was emphasized that a particular advantage of this approach over other synthetic strategies based on Cope rearrangement consists in the facile way of selectively preparing c -divinylcyclopropane intermediates262. 

Rhodium(II) (iV-dodecylbenzenesulfonyl)prolinate has been found to act as an effective catalyst for the enantioselective decomposition of vinyldiazoacetates to c -divinylcyclopropanes. Combination of this process with a subsequent Cope rearrangement has resulted in a highly enantioselective synthesis of a variety of cycloheptadienes containing multiple stereogenic centres (see Scheme 40). The tandem... 

Tricyclo[3.3.1.0 ]nonadiene (barbaralane) undergoes a degenerate 3,3-shift as a result of its c/ -divinylcyclopropane moiety with an activation free energy of only... 

The rate of the reaction is influenced by the energy of the starting materials, and strained compounds react at much lower temperatures than do unstrained molecules. The 1,2-divinyl derivatives of cyclopropane, cyclobutane, and cyclopentane illustrate this point. c -Divinylcyclopropane rearranges to qrcloheptadiene... 

A slight modification of the cyclopropyl conjunctive reagent transforms a cyclopentannulation into a cycloheptannulation. Thus, the 2-vinylcyclopropyllithium reagent 3, converted to its cuprate 4, generates a 1,2-divinylcyclopropane. Heating to only 180 °C leads to smooth Cope type rearrangement, driven by the release of the cyclopropyl strain, to create a perhydroazulene ring systerh of many sesquiterpenoids (Eq. 19) 20>. 

When ring strain is relieved, Cope rearrangements can occur at much lower temperatures and with complete conversion to ring-opened products. A striking example of such a process is the conversion of cw-divinylcyclopropane to 1,4-cycloheptadiene, a reaction which occurs readily at temperatures below —40°C.139... 

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. 

An interesting 1,2-divinylcyclopropane isomerization is observed when 5,5,10,10-tetrachlorotricyclo[7.1.0.09,6]deca-2,7-diene is subjected to flash vacuum pyrolysis at 700 °C and 10-4Torr a mixture of three isomeric dichloroazulenes is produced [215]. 

Indeed, c -l,2-divinylcyclopropanes give this rearrangement so rapidly that they generally cannot be isolated at room temperature,458 though exceptions are known.459 When heated, 1,5-diynes are converted to 3,4-dimethylenecyclobutenes.460 A rate-determining Cope rearrangement is followed by a very rapid electrocyclic (8-29) reaction. The interconversion of... 

Cis-1,2-divinylcyclopropane also rearranges rapidly (Equation 12.106). The free energy of activation is 20 kcal mole-1, and A Hi is 19.4 kcal mole-1.172 The trans isomer, in contrast, rearranges only at 190°C, presumably the temperature required for its isomerization to the cis form.173... 

Reaction of vinylcarbenoids with furans offers another level of complexity because the furanocyclo-propanes in this case would be divinylcyclopropanes capable of a Cope rearrangement as well as electro-cyclic ring opening to trienes.27b c As illustrated in Scheme 42, the product distribution was dependent on the furan structure. With 2,5-disubstituted furans [3.2.1 ]-bicyclic systems (202) were exclusively formed, but with furan or 2-substituted furans, trienes (203) were also produced. 

Not unexpectedly, Irons-1,2-divinylcyclopropane (4) is much more stable than the cis isomer (1) and is a readily isolable compound. Nevertheless, at elevated temperatures, e.g. 190 C, (4) undergoes smooth bond reorganization to provide 1,4-cycloheptadiene (2) in essentially quantitative yield Thus, at the time that the Cope rearrangement of 1,2-divinylcyclopropane systems was discovered, it was already clear that both cis and trans isomers could in principle, serve as suitable substrates for the reaction. As it turns out, this is an important reaction characteristic, since, in most (but not all) cases, it makes uiuiecess-ary the stereoselective preparation of either the cis or trans starting material. 

The highly substituted cir-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). ... 

With monocyclic tranj-1,2-divinylcyclopropanes, chirality transfer is, apparently, poor. Thermolysis of (+)-dictyopterene A (33) at 165 C for 48 h and of (-)-dictyopterene B (34) at 103-108 C for 40 h gives, in each case, a mixture of the two enantiomeric Cope rearrangement products, (30), (31) and (37), (40), respectively (Scheme 5). Although the uncertainty associated with the optical purities and (or) rotations of the various substances involved - made a quantitative determination of the product ratios dif-... 

Rearrangenient of the epimers (91) and (93) is, in each case, unexceptional (Scheme 11). Thermolysis of (91) in hexane provides the dienone (95), while heating either (91) or (95) at 110 C (neat) affords the conjugated ketone (96). Although, as expected, rearrangement of (93) requires much higher temperatures, the same product (96) is formed in good yield. Presumably, this conversion proceeds by way of the stabilized diradical (9 and the cir-divinylcyclopropane (91). 

The cis substrate (92) represents an interesting case. Thermolysis of this material under a variety of conditions produces mixtures of (94) and (98), in which the latter substance nearly invariably predominates (Scheme 12). For example, heating of (92) in collidine at 140-150 C gives (94) and (98) in a ratio of about 1 2. Under these and other thermolysis conditions, the tron -divinylcyclopropane (94) is stable. 

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