Intramolecular vinylcyclopropanes

Ashfeld and Martin have used an intramolecular vinylcyclopropane and triple bond (5 + 2) which leads almost directly into sesquiterpenes (Scheme 40) [77]. 

An efficient carboannulation proceeds by the reaction of vinylcyclopropane (135) or vinylcyclobutane with aryl halides. The multi-step reaction is explained by insertion of alkene, ring opening, diene formation, formation of the TT-allylpalladium 136 by the readdition of H—Pd—I, and its intramolecular reaction with the nucleophile to give the cyclized product 137[I08]. 

The retro-ene reaction also is of synthetic importance. While the application of high pressure facilitates the ene reaction, the retro-ene reaction is favored at higher temperatures. Furthermore small-ring strain can shift the equilibrium towards the side of the dienes. The vinylcyclopropane 11 rearranges by a synchronous process to the open-chain diene 12. Formally this process is the reverse of an intramolecular ene reaction ... 

Scheme 16.75 Rh-catalyzed intramolecular [5 + 2]-cycloadditions of allenes and vinylcyclopropanes.

As a consequence of the above observations, the intramolecular capture of the vinylcyclopropane radical cation was attempted next. Thereby, chrysanthemol and homochrysanthemol both yielded the expected cyclization products (Scheme 35) [55],... 

Scheme 35 Cyclization by intramolecular capture of vinylcyclopropane radical cations.

The addition of 3-(phenylthio)allylzinc bromide to 1-octenylmagnesium bromide leads to the formation of 2-hexyl-3-vinylcyclopropylzinc bromide (equation 32)83, presumably by intramolecular cydization of the initially formed gem-dimetallic homoallylic intermediate. Hydrolysis gave a mixture of two isomeric 2-hexyl vinylcyclopropanes in a 7 3 ratio. 

The [Rh(NHC)Cl(COD)]/AgSbF6-catalysed intramolecular 4 + 2-cycloaddition of dienynes and the intramolecular 5 + 2-cycloaddition of alkyne vinylcyclopropanes formed the corresponding bicyclic cycloadducts in 91-99% yields within 10 min.8... 

As a related reaction, the bicyclo[5.3.0]decane derivative 64 was obtained at 30 °C by the Rh-catalysed intramolecular [5+2] cycloaddition of the alkyne with the vinylcyclopropane moiety in 61. The latter behaves as a pseudo-1,3-diene in oxidative addition, and generates 62. This is followed by rearrangement to 63, whose reductive elimination gives 64 [20]. [Rh(CO)2Cl]2 is a better catalyst than RhCUPl+P. The reaction can be extended to alkenes [20a],... 

When the double bond of the enyne possesses a cyclopropyl substituent, an intramolecular [5+2] cycloaddition of alkyne and vinylcyclopropane takes place [75, 76]. The ruthenacycle does not undergo /l-hydride elimination but a rearrangement of the cyclopropane to produce a ruthenacyclooctadiene. Thus, a variety of bicyclic and tricyclic cycloheptadienes were obtained in good yields [75] (Eq. 55). 

The protocol for this tandem cyclization involves deprotonation of allylic phenyl thioethers such as 218136. The authors observed that transmetalation with LiBr was required to obtain high yields of the cyclization product. Conversion of the resulting allyllithium 219 to the monocyclic intermediate 220 followed by intramolecular displacement of the thiophenoxide ion efficiently afforded the fused vinylcyclopropane 221 (Scheme 66). Formation of the five-membered ring was quantitative, starting from a substrate in which both alkene functions were monosubstituted126. 

In a study of vinylcyclopropanes with tetranitromethane (TNM), from 1,1-divinylcyclopropane 156, a specific 1,4-diene, the unexpected product 118, was obtained. The formation of the nitronic ester was accompanied by homo-allylic rearrangement, followed by the intramolecular 1,3-dipolar cycloaddition (Equation 9) <2002DOC9>. 

Intramolecular Coupling of Alkynes with Enones and Vinylcyclopropanes... 

Compounds with an additional 2-vinyl group, easily available in two steps from a,J -unsaturated ketones, are of special interest. If the reactive vinyl ketone moiety is liberated, it can be trapped in situ by suitable nucleophiles, e.g. CH-acids, generating polyfunctional compounds or by a diene unit which undergoes an intramolecular Diels-Alder reaction (equation 93). Besides, radical additions to the vinylcyclopropane are also possible giving silyl enol ethers as ring-opened products . Future synthetic applications of theses processes are obvious. 

The Hudlicky route making vinylcyclopropanes by intramolecular car bene additions to dienes has led to a variety of polycyclic cyclopentanoic terpenes. The crucial step in a synthesis of isocomenic acid is illustrated by equation 157. ... 

Various reactions discussed so far have been applied to cleave complex vinylcyclopropanes which are available by a novel method of extreme convergency. In 1981 Wender and Howbert reported their first example for an intramolecular 1,3-photoaddition of an olefin unit to an adjacent arene moiety This ingenious strategy generates functionalized polycyclic vinylcyclopropanes in a regio- and stereocontrolled fashion which is perfect to synthesize certain terpenes. The key steps of a short route to a-cedrene are displayed in equation 182. ... 

An elegant route to 11-deoxyprostaglandins uses a 1,5 attack of an enolate on such an activated cyclopropane. The ring-opening of activated cyclopropanes by intramolecul-arly situated nucleophiles has also been achieved An interesting case of intramolecular 1,7 attack on an activated vinylcyclopropane was observed upon treatment with dimsyl sodium-Me2SO at 90°C (equation 18) . 

JCycloadditions. Both intermolecular and intramolecular versions of the cycloaddition involving a vinylcyclopropane and an alkyne have been developed. The Rh-catalyzed reaction is very valuable for rapid assembly of unsaturated seven-membered... 

Special attention should be given to the MIRC reaction of 2-bromobut-2-enoates with cyclohexenones, e.g. 1 and Table 10. When first cyclohexenone was added to a lithium diiso-propylamide solution, tricyclo[3.2.1.0 ]octan-6-ones were formed in 20-55% yield via double Michael addition, followed by intramolecular displacement. However, when ethyl 2-bro-mobut-2-enoate was first added to a lithium diisopropylamide solution, vinylcyclopropanes were formed in good yield (Table 10, entries 1-2). Other chalcones have also been used to produce vinylcyclopropanes (entries 3-8) with reasonable success. 

Thus, ( + )-isocomene was prepared in five steps from 2-bromotoluene, methyl vinyl ketene and (Z)-2-bromobut-2-ene. According to the synthetic strategy the key step is the intramolecular meta photocycloaddition of (Z)-3-methyl-6-(2-tolyl)hept-2-ene. By irradiation in cyclohexane at room temperature for four hours, a 1 1 mixture of isomeric photoadducts was obtained in 72 % yield, the constituents of which were easily interconverted photochemically. Thermolysis of the vinylcyclopropanes at 235-240 C gave dehydroisocomene in 57% yield, which was finally hydrogenated over 5 % palladium on carbon to give the expected sesquiterpene in 98% yield. ... 

Vinylcyclopropanes are readily transformed to the corresponding oxiranes by treatment with a peracid or singlet oxygen or by a multistep procedure involving an intramolecular substitution reaction. Of these methods peracid oxidation is the most commonly used. 3-Chloro-peroxybenzoic acid has been predominantly utilized, " ... 

Alkylidenecyclopropanes bearing a carbanion center in the side chain of the alkylidene group react with iodobenzene in the presence of catalytic amounts of a palladium(0) complex to give various compounds depending on the chain length between the carbanion center and the alkylidene group. With a one-carbon chain vinylcyclopropanes are formed via intramolecular carbanion attack of an intermediate allyl system, with two- and four-carbon chains cyclohexenes or vinylcyclohexanes are formed, while with a three-carbon chain a cyclopropyl-substituted cyclopentane ring is obtained. 

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