Vinylcyclopropanes preparation

Preparation of cyclopropylacetylene (37) Cyclopropylacetylene (37) was a known compound and its synthetic method from vinylcyclopropane via dibromination had been reported [27] when we started our investigation. Large scale preparation of 37 was not an easy task. Actually, many chemists in our department worked on establishing the process for such a simple compound as 37 at the peak of the project. 

Radical [3 + 2 cycloaddition. Cyclopentanes can be prepared by addition of alkenes across vinylcyclopropanes catalyzed by phenylthio radicals formed from (C6H5S)2 and AIBN. A Lewis acid such as A1(CH3)3 can increase the rate and the stereoselectivity of this radical initiated cycloaddition. Thus the combination of the vinylcyclopropyl ester 1 with f-butyl acrylate (2) provides the four possible cyclo-... 

Generally, cycloproylallenes are prepared by the same methods as employed for the synthesis of other allenic hydrocarbons. Thus, cyclopropylallene (17) itself has been obtained from vinylcyclopropane (139) via its dibromocarbene adduct 140 using the DMS method (Scheme 5.19) [54],... 

The vinylcyclopropane 10 is a useful chiral building block for organic synthesis, as the vinyl group can be oxidatively cleaved if desired and further functionahzed (Scheme 14.1). Either diastereomer 20 or 21 of the cyclopropane analog of phenylalanine can be readily prepared from 10 [40]. Corey has reported another elegant appHcation of the vinylcyclopropane 10 in the asymmetric synthesis of the antidepressant (-i-)-sertraline 22 [52]. 

In order to sec the influence that a trifluoromethyl substituent has on cyclopropane reactivity, l-(trifluoromethyl)-2-vinylcyclopropane (18) was prepared and its thermal reactions studied.13 Both the cisitrans isomerization and the vinylcyclopropane rearrangement were slower than those of any 2-substituted vinylcyclopropane previously reported. When m-18 is heated at 275 C, a clean and reversible cisitrans isomerization takes place (Kequi, = 5.33). At 332-364 C, 18 undergoes normal vinylcyclopropane rearrangement to 4- and 3-(tri-fluoromethyl)cyclopentenc (19) and (20). respectively. 

The history of vinylcyclopropane goes back to Gustavson s reported synthesis of vinyl-cyclopropane ( vinyltrimethylene ) in 189678, a publication which occasioned considerable controversy, for the major C5H8 compound actually formed in his preparation proved to be spiropentane. Authentic vinylcyclopropane was secured by Demjanow and Dojarenko in 1922", but nevertheless both structures remained in dispute. Authorities as eminent as... 

The Cope rearrangement of c/j-di vinylcyclopropanes is thermally allowed and offers an attractive stereoselective approach to cycloheptadienes. Cyclopropanation reactions can be used to prepare divi-nylcyclopropanes, as shown in Scheme 23.120 Reaction of ethyl diazopyruvate with butadiene generated... 

FVP of styrene at 850 °C gives benzene, ethylbenzene, naphthalene and other minor hydrocarbon products, while a mixture of isomeric methyl cyclohexa-1,3-dienes is converted under similar conditions mainly into benzene, toluene and ethylbenzene33. A vinylcyclopropane rearrangement occurs on FVP of 48 at 600 °C to give the cis fused cyclopentanone 49 and this product may also be prepared by FVP of the 1,4-diene 5034. The highly reactive 1,6-azulylene 52 can be generated by reversible dedimerization of the... 

Bailey, W. F. Tao, Y. Efficient preparation of vinylcyclopropane by SN cyclization of the organolifhium derived from (E)-5-iodo-l-mefh-oxy-2-pentene. Tetrahedron Lett. 1997, 38, 6157-6158. 

In fact from the synthetic point of view, the 1-ethynylcyclopropanols 9 have been mainly used as precursors of 1-donor substituted vinylcyclopropanes, the preparation and utility of which are discussed in the following sections. 

Preparation of 1-Donor Substituted Vinylcyclopropanes and Cyclopropylcarbonyl Compounds... 

While the nucleophilic addition of 1-lithio-l-bromocyclopropanes to ketones gave oxaspiropentanes, precursors of 1-donor substited vinylcyclopropane derivatives vide supra, Sect. 4.5, Eq. (28)), addition of n-BuLi at low temperature to 1,1-di-bromocyclopropane 199 (prepared in 75 % yield from the addition of dibromocarbene... 

Since thiophenol was formed in the reaction, this by-product trapped the intermediate cation to give the bis(phenylthio)vinylcyclopropane 244 and so limited the formation of the desired cyclobutanone. To overcome this problem, a substitution pattern providing electronic acceleration for the cyclopropyl bond migration but also a steric bulk to inhibit the nucleophilicity of the thiol was required. For this purpose, l-(2,6-di-methoxyphenylthio)vinylcyclopropanes such as 242b were prepared the yield and cleanliness of the reaction were effectively increased, allowing by this route the isolation of pure cyclobutanones 243 63). 

Eq. 52 and 53 demonstrate remarkable characteristics of this [3 + 2]-cycloaddition starting with a pure diastereomer 130, two stereoisomeric cyclopentanes 131 are obtained. This stereorandom outcome is most simply rationalized assuming a stepwise mechanism with a 1,5-zwitterion as an intermediate in the cycloaddition. The vinylcyclopropane 132 only gives five-membered ring products 133 and no cyclo-heptene derivative, which would result from a conceivable [5 + 2]-cycloaddition. Less activated olefins or cyclopropanes do not undergo a similar [3 + 2]-cycloaddition. Due to the specific substitution pattern, the cyclopentane formation from these siloxycyclopropanes is of no preparative value. 

The described reaction was applied to the synthesis of various vinylcyclo-propanes. However, obvious limitations remained in the scope of the reaction at this stage (1) only 1-substituted vinylcyclopropanes could be obtained and (2) the reaction couldn t be applied to the preparation of cyclopropane derivatives other than vinylcyclopropanes. To overcome the first limitation, the methyl-substituted zirconacyclopropane [13] was stabilized by adding PMe3... 

Cycloheptane annelation (7, 212). The mixed cuprate 1 reacts with acid chlorides to afford vinylcyclopropyl ketones. Previously these ketones were prepared from aldehydes by condensation with l-lithio-2-vinylcyclopropane followed by oxidation (7, 192-193). These compounds are rearranged to 4-cycloheptenones on conversion to trimethylsilyl enol ethers, thermolysis, and hydrolysis. ... 

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