Cyclopentene Rearrangement

Adducts derived from cyclopropyl-TMM reactions are versatile synthetic intermediates. Alkylidenecyclopropanes have been proven useful in further Pd-cata-lyzed transformations [4], On the other hand, vinylcyclopropanes can undergo smooth thermal ring-expansion to cyclopentenes. Thus, a total synthesis of 11-hy-droxyjasionone (27) was achieved with the cyclopropyl-TMM cycloaddition as the crucial step, and the thermal rearrangement of the initial adduct (28) as an entry to the bicyclo[6.3.0]undecyl compound (29), a key intermediate in the synthetic sequence (Scheme 2.9) [19]. 

The thermal rearrangement of vinylcyclopropanes 1 to yield cyclopentenes 2 is called the vinylcyclopropane rearrangement. 

An illustrative example for the generation of cyclopentenes from vinylcyclopropanes is the formation of bicyclo[3.3.0]oct-l-ene 10 from 1,1-dicyclopropylethene 9 by two consecutive vinylcyclopropane cyclopentene rearrangements. ... 

The vinylcyclopropane rearrangement is an important method for the construction of cyclopentenes. The direct 1,4-addition of a carbene to a 1,3-diene to give a cyclopentene works only in a few special cases and with poor yield. The desired product may instead be obtained by a sequence involving the 1,2-addition of a carbene to one carbon-carbon double bond of a 1,3-diene to give a vinylcyclopropane, and a subsequent rearrangement to yield a cyclopentene ... 

Vinyl-substituted cyclopropanes undergo thermal rearrangement to yield cyclopentenes. Propose a mechanism for the reaction, and identify the pericyclic process involved. 

Sect. 2.1.1) and [3C+2S] cyclopentene derivatives. The product distribution can be controlled by choosing the appropriate reaction conditions [72]. Moreover, the cyclopentene derivatives are the exclusive products from the coupling of fi-pyrrolyl-substituted carbene complexes [72b,c] (Scheme 25). The crucial intermediate chromacyclobutane is formed in an initial step by a [2+2] cycloaddition. This chromacyclobutane rearranges to give the rf-complex when non-coordinating solvents are used. Finally, a reductive elimination leads to the formal [3C+2S] cyclopentene derivatives. 

Small amounts of cyclopentene derivatives are detected in cyclopropanation reactions of electron-deficient dienes, but they may result from thermal rearrangement of the corresponding vinyl cyclopropanes and not from a direct [4+1] cycloaddition... 

Water can be added to the mother liquor, and the mixture extracted with chloroform to increase the diazide recovery to nearly quantitative (95-98%). During the course of any purification method that might be employed the diazide should not be heated above 50°, since decomposition occurs quite noticeably at that temperature. It is best to store the pure product below —5° in the dark, since it undergoes a facile photochemical rearrangement to the cyclopentene-dione. 

The anions of vinyl cyclopropanols (16), conveniently released from ethers (15) with BuLi, rearrange rapidly to cyclopentenes (17). 

One other 100% atom economic rearrangement is worth mentioning briefly. When vinylcyclopropanes are heated they readily undergo ring expansion to cyclopentenes. The temperature required varies significantly, depending on the molecule for example l-phenyl-2-vinylcyclopropane may be converted to phenylcyclopentene in reasonable yield at 200 °C. 

Temperatures in parentheses refer to thermal vinylcyclopropane -> cyclopentene rearrangement. 

In order to establish the correct absolute stereochemistry in cyclopentanoid 123 (Scheme 10.11), a chirality transfer strategy was employed with aldehyde 117, obtained from (S)-(-)-limonene (Scheme 10.11). A modified procedure for the conversion of (S)-(-)-limonene to cyclopentene 117 (58 % from limonene) was used [58], and aldehyde 117 was reduced with diisobutylaluminium hydride (DIBAL) (quant.) and alkylated to provide tributylstannane ether 118. This compound underwent a Still-Wittig rearrangement upon treatment with n-butyl lithium (n-BuLi) to yield 119 (75 %, two steps) [59]. The extent to which the chirality transfer was successful was deemed quantitative on the basis of conversion of alcohol 119 to its (+)-(9-methyI mande I ic acid ester and subsequent analysis of optical purity. The ozonolysis (70 %) of 119, protection of the free alcohol as the silyl ether (85 %), and reduction of the ketone with DIBAL (quant.) gave alcohol 120. Elimination of the alcohol in 120 with phosphorus oxychloride-pyridine... 

The photoindueed 1,7-cycloaddition of carbon monoxide across the divinyl-cyclopropane derivative 32 yields the two cyclic dienyl ketones 34, via the ferracyclononadiene intermediate 33 [18]. (Scheme 11) cyclopentene rearrangement. The dienylcyclopropane 35 is capable of forming the complex 36, followed by ring enlargement to 37 [19]. 1,1-Dicyclopropylethylene 29 is also converted to the 1-cyclopropyl-1-cyclopentene 38. The additional functionality of vinylcyclopropanes is necessary to serve as a 7t-donor... 

The addition of anisole to cyclopentene gives an 1,3-adduct which rearranges to tricycloundecenone in the presence of traces of acid (4.41)446). 

The cyclononadiene in (4.72) is obtained by thermal rearrangement of the photoadduct of a cyclobutene carboxylic acid ester and cyclopentene 484). 

On the other hand, bicyclopentane rearranges to cyclopentene. In this case, a /3-hydrogen shift occurs analogous to pathway fin Eq. (26) above ... 

The calculations indicate that only the most highly strained bridgehead olefins, specifically of the rrans-cyclopentene type, are candidates for a rearrangement to carbenes. Only at high temperature is there a chance for tams-cyclohexene analogues to undergo such a conversion. 

Upon flash vacuum pyrolysis or under silver nitrate catalysis, a variety of 2-ethenyl-sub-stituted cydopropylamines 146 cleanly undergo a vinylcyclopropane to cyclopentene rearrangement [129] and afford high yields (up to 95%) of 4-aminocyclopent-l-enes 147, some of which have unprecedented substitution patterns (Scheme 11.37) [130],... 

Scheme 11.38. Vinylcyclopropane to cyclopentene rearrangement in (n+3)-(dimethylamino)-l-ethenyl bicyclo[n.l. OJalkanes 66...

Vinylcyclopropanes are now widely used as precursors to five-membered rings. Various methods have been designed to construct vinylcyclopropanes and bring about their rearrangement — most frequently thermolytically — to cyclopentenes. 

The isopropenyl side chain may derive by elimination of a tertiary alcohol or ether as in 202. Such a masking of the olefin avoids a possible competing vinylcyclopropane rearrangement. The correspondence of the cyclopentene of 202 with the vinylcyclopropane in 203 now becomes obvious. The presence of the dimethylcarbinol side chain now also offers the opportunity for its introduction by addition of a cyclopropyl anion to acetone. The feasibility of creating such an anion by fluoride initiated desilylation... 

The extrapolation of the vinylcyclopropane-cyclopentene rearrangement to a vinyl-cyclobutaiie-cyclohexene synthesis begins to create new insights into the synthesis of six membered ring natural products. The eudesmane sesquiterpene (—)-P-selinene, 217 illustrates such a strategy as summarized in Scheme 14 80). A suitable cyclohexene... 

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