Divinylcyclopropane cycloheptadiene rearrangement

Early synthetically oriented divinylcyclopropane-cycloheptadiene rearrangements were performed with hydrocarbons and were in part due to the interest in syntheses of marine brown algae constituents. The preparation of ectocarpene is displayed in equation 164. The corresponding divinylcyclopropanes have either been constructed by Wittig methodology or, as in the case shown, by addition of diazoalkanes to apt polyenes followed by photolysis or thermolysis . 

The stereocontroiied total synthesis of (+)-gelsemine was accomplished by T. Fukuyama and co-workers using the Knoevenagel condensation to prepare a precursor for the key divinylcyclopropane-cycloheptadiene rearrangement. The use of 4-iodooxindole as the active methylene component allowed the preparation of the (Z)-alkylidene indolinone product as a single stereoisomer. 

With the exception of vinylcyclopropane-cyclopentene rearrangements (cf. Section VII) radical reactions have so far rather rarely been used synthetically. This is also true for pericyclic modes of cyclopropane cleavage which are mainly restricted to divinylcyclopropane-cycloheptadiene type expansions. Cycloadditions, whether concerted or stepwise, occur only with very specific compounds. ... 

One example of an acid-catalyzed divinylcyclopropane to cycloheptadiene rearrangement which has been documented is the formation of tricyclic compound 3. ... 

In accordance with this, the reaction of the electron-donor-substituted butadienes 170 (R=Ph, OMe) with the arylcarbene complexes 163 yields divinylcyclopropane intermediates 168 with high chemoselectivity for the electron-rich double bond in 170, which readily undergo a [3,3]-sigmatropic rearrangement to give the as-6,7-disubstituted 1,4-cycloheptadiene derivatives... 

If 1,3-butadienes are cyclopropanated by use of vinylcarbene complexes, the divinylcyclopropanes which result can rearrange to cycloheptadienes [71,241,384 -387] (Figure 2.37). 

In particular the synthetic approach to dihydrofurans (first equation in Figure 4.23) represents a useful alternative to other syntheses of these valuable intermediates, and has been used for the preparation of substituted pyrroles [1417], aflatoxin derivatives [1418], and other natural products [1419]. The reaction of vinylcarbene complexes with dienes can lead to the formation of cycloheptadienes by a formal [3 + 4] cycloaddition [1367] (Entries 9-12, Table 4.25). High asymmetric induction (up to 98% ee [1420]) can be attained using enantiomerically pure rhodium(II) carboxylates as catalysts. This observation suggests the reaction to proceed via divinylcyclopropanes, which undergo (concerted) Cope rearrangement to yield cycloheptadienes. 

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... 

The vinylcarbenoid [3-1-4] cycloaddition was applicable to the short stereoselective synthesis of ( )-tremulenolide A 73 and ( )-tremulenediol A 74 (Scheme 14.7) [81]. The key step is the cyclopropanation between the cyclic vinyldiazoacetate 69 and the functionalized diene 70, which occurs selectively at the ds-double bond in 70. Because of the crowded transition state for the Cope rearrangement of the divinylcyclopropane 71, forcing conditions are required to form the fused cycloheptadiene 72. The stereo-... 

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... 

Cycloheptadiene (340) is obtained by the Cope rearrangement of cis-divinylcyclopropane (339.) Based on this reaction, highly diastereoselective and enantioselective construction of the 1,4-cycloheptadiene 343 (98% ee) was achieved by domino asymmetric cyclopropanation to generate cA-divinylcyclopropane... 

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. 

Pickenhagen, W., Naef, F., Ohioff, G., Mueller, P., Perlberger, J. C. Thermal and photochemical rearrangements of divinylcyclopropanes to cycloheptadienes. Model for the biosynthesis of the cycloheptadiene derivatives found in a seaweed (Dictyopteris). Helv. Chim. Acta 1973, 56, 1868-1874. 

Under the same conditions, reaction of 1 with buta-1,3-diene afforded the isomeric 1-chloro-l, 2-divinylcyclopropanes 3. However, trans-3 with its two vinyl groups cis to each other immediately rearranges via a 1,3-sigmatropic shift to produce 2-chloro-l,4-cycloheptadiene (4) (see also Sections 1.2.1.2.2 and l.B.2.4.5.1 for similar examples of the formation of seven-membered rings). 

Vinylcyclopropylcuprates 5 were coupled with )S-haloenones giving divinylcyclopropanes 6 and 8 which rearranged to annulated or spiroannulated cycloheptadienes 7 and 9. ... 

Formation of Seven-Membered Rings The Divinylcyclopropane to Cycloheptadiene (Cope) Rearrangement... 

In those cases where a m-divinylcyclopropane is an intermediate in a reaction, the rearrangement proceeds at ambient temperature, as in the formation of cycloheptadiene 6. ° The rearrangement of cyclopropane 5 is sensitive to the substitution pattern and the alkene geometry. ... 

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. ... 

Sealed-tube thermolysis is used to effect the rearrangement of divinylcyclopropanes such as 55, 58 and 61 to the corresponding annulated cycloheptadienes. ... 

---