Cyclopentenes, ring expansion

Petrov and Shchekin (297) showed that below the cracking temperature (250-316°C.) cyclohexene undergoes over silica-alumina hydrogen disproportionation and dimerization. Identical results were obtained from 1-methyl-l-cyclopentene. Ring expansion of lower alkylated cyclopentanes occurs simultaneously with polymerization. However, no bicyclic compounds with similar rings were formed. 

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]. 

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. 

Fragmentation of cyclopropylcarbinyl radicals has been incorporated into several synthetic schemes.357 For example, 2-dienyl-l,l-(dimethoxycarbonyl)-cyclopropanes undergo ring expansion to cyclopentenes. 

The similar insertion of 93 into 1,2-diphenylcyclopropenone results in cy-clobutenones or o- and p-alkoxyphenols [49]. In the reaction of 93 with 1-alkynylcyclobutenols, 2-alkenyl-4-cyclopentene-l,3-diones are obtained via alkyl shift-ring expansion [50]. (Scheme 33)... 

The observed ring expansion of 2-azido-2-phenylindandione (129) and 2,2-diazido-l,3-indandione (130), along with the established propensity of 2,6-diazido- and 2,5-diazido-l,4-quinones to undergo easy, thermally induced ring contractions to 2-cyano-4-azido-l,3-cyclopentene-diones suggested a unique synthetic route to 2-aza-3-cyano-l,4-quinones (75JA6181). 

Metal-promoted vinylcyclopropane C3 - C5 ring expansions have been reported. Thus, ethyl 2-methoxy-2-vinylcyclopropanecarboxylate 393 rearranged to a 2 1 mixture of 3-methoxy-2-cyclopentenecarboxylate 396 and 3-methoxy-3-cyclopentene-carboxylate 397 on heating at 160 °C in the presence of catalytic amounts of copper bronze or copper(I) chloride in contrast, platinum and rhodium complexes catalyzed... 

From four-membered rings An acid-catalyzed transformation has been observed in the conversion of l-[l-methylsulfinyl-l-(methylthio)alkyl]cyclo-butanol to 3-methyl-2-(methylthio)cyclopentanone [9]. - Rearrangement of a /3-lactone to a y-lactone derivative in the presence of magnesiumdibromide [10]. - A borontrifluoride catalyzed cyclobutene to cyclopentene rearrangement [11]. - Ring expansion of a [2+2] photoadduct to a five-membered ring [12]-... 

The following references contain additional examples of this method and corresponding reaction types Synthesis of 3-fluoroethyl-2-cyclopenten-l-ones [35], preparation of functionalized bicyclo[5.3.0]decane systems and conversion of 1,2-divinylcyclopropanes to functionalized cycloheptanes [36] e.g. karaha-naenone [37], ( )-/Thimachalene [38][39]. Metal-catalyzed ring expansions in which cyclopropane (e.g. [40] [41] [42] [43] [44]) or aziridines (e.g. [45] [46]) and diaziridines (e.g. [45]) function as essential moieties are well known reactions. 

Two examples that show selectivity in synthetic applications are the synthesis of cycloheptanone (19),7 which involves ring expansion via a [1,5] hydrogen shift, and the synthesis of sarkomycin (20 Scheme 5). 7 In both of these protocols, the intermediate cyclopropyl ketone or vinylcyclopropane system could be rearranged to dihydrofurans or cyclopentenes respectively by the appropriate adjustments in the experimental conditions. ... 

Trost developed a general method of synthesis of vinylcyclqnopanes (and their sulfur and oxygen analogs) through the application of ylide addition to carbmyl compounds. Three major methods involve the cyclopentene annulation, cyclobutanone synthesis, and ring expansion with concomitant alkylation (Scheme 51). °... 

Dinnocenzo and Conlon have described the remarkable effect of one-electron oxidation on the rate of certain vinylcyclopropane rearrangements. Exposure of several l-p-anisyl-2-vinylcyclopropane derivatives to a catalytic amount of tris(4-bromophenyl)aminium hexafluoroantimonate in acetonitrile at room temperature was found to induce ring expansion to form cyclopentenes (equation 25) temperatures in excess of 200 °C are required for the conventional thermal rearrangement cf these systems. At this time it is uncertain whether these reactions follow concerted mechanisms, or are stepwise processes involving trimethylene cation radical intermediates. 

The thermal expansion of a vinylcyclopropane to a cyclopentene ring is a special case of a [l,3]-sigmatropic migration of carbon, although it can also be considered an internal [ 2 + 2]-cycloaddition reaction (see 15-63). It is known as a vinylcyclopropane rearrangementThe reaction has been carried out on many vinylcyclopropanes bearing various substituents in the ring or... 

On heating at 160°C in the presence of catalytic amounts of copper bronze or copper(l) chloride, ethyl 2-methoxy-2-vinylcyclopropanecarboxylates (220) undergo ring expansion into the isomeric 3-methoxy-2-cyclopentene (221) and 3-methoxy-3-cyclo-pentenecarboxylates (222) equation 146) on the other hand platinum and rhodium complexes catalyse the ring-opening into ethyl-4-methoxy-3,5-hexadienoate. ... 

In 1967 Willcott and Cargle reported the pyrolysis and rearrangement of trans-vinylcyclopropane-2-d. They found that stereomutation occurred at least five times faster than rearrangement, a result that might seem inconsistent with the intermediacy of a common biradical for the two processes. It is important to note, however, that there are two geometrical isomers of the biradical, both of which can cause stereomutation but only one of which can allow ring expansion to the cyclopentene (see Figure 24). It is unlikely that the... 

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