Divinylcyclopropanes, Cope rearrangement

Photochemical rearrangements have also been reported, as shown in equation (53). " Transition metal catalyzed rearrangement [palladium(O)] of a dienylaziridine has been reported in one case, and a radical opening of a dienylaziridine led to pyrroline formation under the conditions of radical initiation with AIBN/PhsSnH (equation 54). For those vinylaziridines that contain additional unsaturation, the corresponding aza equivalent of a divinylcyclopropane Cope rearrangement is the usual pathwayThe subject of heterodivinylcyclopropane Cope rearrangement is covered in detail elsewhere." The... 

Piers and Reissig have developed a route to the spiro[6.7]tetradecane system based on the divinylcyclopropane Cope rearrangement [e.g. (247) -> (248)]. 

A dramatic diflference in reactivity is evident when cb-divinylcyclopropane is compared wifli vinylcyclopropane. ciy-Divinylcyclopropane can only be isolated at low temperature because it very rapidly imdeigoes Cope rearrangement to 1,4-cycloh ta-... 

The Cope rearrangement is of great importance as a synthetic method e.g. for the construction of seven- and eight-membered carbocycles from 1,2-divinylcyclopropanes and 1,2-divinylcyclobutanes respectively (e.g. 11 12),... 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

Indeed, cw-l,2-divinylcyclopropanes give this rearrangement so rapidly that they generally cannot be isolated at room temperature,though exceptions are known. When heated, 1,5-diynes are converted to 3,4-dimethylenecyclobu-tenes. A rate-determining Cope rearrangement is followed by a very rapid electro-cyclic (18-27) reaction. The interconversion of 1,3,5-trienes and cyclohexadienes... 

In contrast to the synthesis of carbocyclic rings, the Cope rearrangement has been used sparsely for generating azepinones. Recently, the enantioselectivity of the conversion of 2-aza-divinylcyclopropane 286 has been investigated. The synthesis started from the optically active cyclopropanecarboxylic acid (90% ee), which had been converted into the isocyanate 286 by initial azidation to 285 and a consecutive Curtius rearrangement. Furthermore, the conditions of the iso-... 

Entry 2 illustrates the reversibility of the Cope rearrangement. In this case, the equilibrium is closely balanced with the reactant benefiting from a more-substituted double bond, whereas the product is stabilized by conjugation. The reaction in Entry 3 involves a cz s-divinylcyclopropane and proceeds at much lower temperature that the previous examples. The reaction was used in the preparation of an intermediate for the synthesis of pseudoguiane-type natural products. 

Cyclopropanation of l,3-dienes. a,0-Unsaturated carbenes can undergo [4 + 2]cycloaddition with 1,3-dienes (12, 134), but they can also transfer the carbene ligand to an isolated double bond to form cyclopropanes. Exclusive cyclopropanation of a 1,3-diene is observed in the reaction of the a,(3-unsaturated chromium carbene 1 with the diene 2, which results in a frans-divinylcyclopropane (3) and a seven-membered silyl enol ether (4), which can be formed from 3 by a Cope rearrangement. However, the tungsten carbene corresponding to 1 undergoes exclusive [4 + 2]cycIoaddition with the diene 2. 

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. 

An interesting approach to form a divinylcyclopropane structure capable of rearranging into seven-membered functionalized derivatives consists of the silyloxylation of cyclic ketones 541 followed by a spontaneous Cope rearrangement to produce the cyclic enol esters 542 which then hydrolyzed to ketones 543 (equation 2 1 3)265. 

In principle, the divinylcyclopropane structure discussed here is incorporated into very well known systems such as bullvalene 547, barbaralane 548 and semibullvalene 549, which very easily undergo a Cope rearrangement. 

In this compound the Cope rearrangement, which occurs with cis-1,2-divinylcyclopropane, will not produce a new compound, viz.. 

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... 

One of the early examples demonstrating the efficiency of this chemistry was the [3+4] cycloaddition reaction of diazoglutaconate 42 with cyclopentadiene, in which the endo-isomer 43 is exclusively formed in 98% yield (Eq. 5) [73]. The intermediacy of a cis-divinylcyclopropane is consistent with the stereochemical outcome because it would rearrange to the endo-product. Indeed in the case of more highly functionalized vinyl-diazoacetates, the ds-divinylcyclopropane was isolable, in which elevated temperatures were required for the Cope rearrangement [73]. 

A direct entry into hydroazulenes has also been achieved through [3-I-4] cycloaddition chemistry, using the more elaborate cyclic vinyldiazoacetate 62 and acetoxybuta-dienes 63 and 64 (Scheme 14.6) [80]. The tandem process is facile with the trons-iso-mer of butadiene 64 leading to the [3-r4] cycloadduct 68 in 67% yield. Reaction with cis-acetoxybutadiene 63, however, forms the isolable cis-divinylcyclopropane 65 (80% yield), which required forcing conditions to initiate the Cope rearrangement to form 67 [80]. 

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... 

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. 

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