Cyclopropane to Cyclopentene Rearrangement

This rearrangement was found to be independent of the nature of the substituents present in 125. 

Synthesis of Fused, Five-Membered Ring Compounds 

Mechanistically, in these reactions the lone pair of electrons on the heteroatom contributes to the 6n electron system, which undergoes conrotatory electrocycliza-tions with a conservation of orbital symmetry. The dehydrogenation of zwitterionic 

The expected intramolecular 1,3-dipolar cydoaddition product 171 was only a minor product (3%). The formation of major product 169 was explained through an intramolecular Michael reaction of the enolate ion. 

Synthesis of Five-Membered Carbocyclic and Heterocyclic Ring Systems 

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One of the classical papers in this area is Neureiter s discovery of the vinyl-cyclopropane to cyclopentene rearrangement [185]. This important process was first observed on a l,l-dichloro-2-vinylcyclopropane derivative, which on heating yielded a chlorocyclopentadiene, presumably by the loss of hydrochloric acid from the initially formed 4,4-dichlorocyclopentene. 

This reaction does not create a ring but there is an important group of sigmatropic rearrangements that make five-membered rings—the vinyl cyclopropane to cyclopentene rearrangement. 

The pyrolysis of vinylcyclopropanes 78 to 79 was accompanied by the competing vinyl-cyclopropane to cyclopentene rearrangement, which furnished sesquiterpene intermediates 80.8 ... 

Unlike the well-known chemistry of the vinylcyclopropane-cyclopentene rearrangement, there is no general method for the rearrangement of alkynyl-cyclopropane to cyclopentene derivatives. One specific example is the pyrolysis of l-ethynyl-2-methylcyclopropane to methylenecyclopentene and other compounds [5]. At 530°C, l-ethynyl-2-methylcyclopropane (1) undergoes a [1,5]-hydrogen shift to give hexa-l,2,5-triene (2), which further isomerizes to methy-lenecyclopentenes 3 and 4 in 38 and 29% yield, respectively (Scheme 1). 

Cyclopentene annelation. The known rearrangement of vinyl cyclopropanes to cyclopentenes (cf. 7, 190 9, 83, 265) can be used to obtain silyl-substituted cyclopentenes. The precursors, (l-trimethylsilylcyclopropyl)ethylenes (2), are usually prepared by addition of 1 to an aldehyde or a ketone followed by cyclopropanation (C2HjZnI, CH2I2) of the adduct. The products are then dehydrated by TsOH to 2 (equation I).1... 

AryI-2-vinylcyclopropanes 21 undergo both vinylcyclopropane to cyclopentene and divinyl-cyclopropane to benzocycloheptene rearrangements competitively (see also Section 2.4.5.) at temperatures in the range of 150-200°C. ... 

Cyclopropyl ketones are readily isomerized to dihydrofuran derivatives thermally or under catalytic conditions.For example, cyclopropyl ketones 2 and 4 yield dihydrofurans 3 and 5, respectively, thermally or under rhodium catalysis. Such rearrangements occur normally under acid catalysis whereas thermolysis favors the vinylcyclopropane to cyclopentene rearrangement, except for highly functionalized (R = SOjPh) cyclopropanes. 

At higher temperatures, bicyclo[3.3.0]oct-2-ene was formed from the equilibrium mixture, and the fact that 8,8-dideuteriobicyclo[5.1.0]oct-2-ene gave 3,3-dideuter-iobicyclo[3.3.0]oct-2-ene indicates that cyclopropane ring opening occurs and results in the vinylcyclopropane to cyclopentene rearrangement (see Chapter 6, Section 2.1). 

In addition to the two- and three-component entries to five-membered rings in Scheme 3-39, the nickel-catalyzed rearrangement of vinyl cyclopropanes to cyclopentenes has also been developed. Whereas various catalysts promote the rearrangement of vinyl cyclopropanes under harsh conditions, nickel-iV-heterocyclic carbene (NHC) complexes were found to catalyze the rearrangement under mild conditions with relatively broad scope (Scheme 3-40). ... 

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

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 thermal rearrangement of vinylcyclopropane to cyclopentene was uncovered in I96090 91. That vinylcyclopropanes, like other cyclopropanes, may undergo cis, trans iso-merizations was inferred in 1964 when trans-l-vinyl-2-methylcyclopropane was thermally converted to mostly (4Z)-1,4-hexadiene, a product formed at much lower temperatures from cw-1-vinyl-2-methylcyclopropane92. The reversible interconversion of the cis and trans isomers of l-vinyl-2-d-cyclopropane (equation 2) was reported soon thereafter, in 196793"96. Additional examples, including cases showing both geometrical isomerization and enantiomerization processes, soon followed. 

On the other hand, Rh1 in the absence of CO leads to 12 [12] formed via a vinyl cyclopropane/cyclopentene rearrangement. Such a rearrangement without a catalyst would require temperatures between 300 and 400 °C ... 

The substituent effects have been quantitatively addressed in the context of specific transformations, for example the vinylcyclopropane-cyclopentene rearrangement, and will be discussed in the appropriate sections. The donor/acceptor principles have been applied to thermal, heterolytic and transition metal catalyzed rearrangements and have been reviewed. These principles take into account the possible intermediate structures listed in Table 1 and are used to explain the reactivity of a particular cyclopropane system. In the discussion that follows emphasis will be given to the processes that are uniformly selective with regard to regio-, stereo- and enantio-chemical integrity of the products. 

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