Vinylcyclopropane-cyclopentene rearrangement transform

Since the pioneering work by Sarel and co-workers on the iron carbonyl promoted transformation of vinylcyclopropanes and related compounds [1], a variety of transition metal complexes have been examined to achieve effective activation of the vinylcyclopropane-cyclopentene rearrangement which usually requires pyrolytic conditions. These reactions have been applied to natural product synthesis in some cases and have already been reviewed in several excellent articles [2-4]. 

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. 

Vinylcyclopropane-cyclopentene rearrangement (VCP-CP) reaction under thermal conditions is a useful transformation and has been extensively utilized in the synthesis of number of cyclopentanoid natural products.However, cw-alkyl-vinylcyclopropanes pose a serious problem since the retro-ene reaction, a lower energy pathway occurs readily instead. To obviate this problem, we sought to explore the excited state chemistry of c/s-alkyl-vinylcyclopropanes.t ] Thus, sensitized photolysis of (+)-A -carene readily afforded the VCP-CP product exclusively in a preparative yield. The obtention of a racemic product can be rationalized by involving a diradical intermediate.l l... 

A similar acceleration has most recently been observed in the rearrangement of vinylcyclopropanes of type (39 Scheme 8). This fluoride-mediated vinylcyclopropane-cyclopentene isomerization proceeds at -78 C to give (40) in 85% yield this is to date the mildest condition available. Two possible intermediates, the enolate anion (39a) or the diradical anion (39b), may be responsible for such acceleration in analogy to the enolate anion accelerated divinylcyclobutane rearrangement recently reported." The mechanism of this transformation is unclear but may involve anion acceleration similar to that observed in the rearrangement of sulfonyl anions derived from (42 Scheme 8). By comparison the thermolysis of (39) produced exclusively the endo isomer of (41) at 580... 

The photochemical transformations of vinylcyclopropanes are of mechanistic interest, but because of the high energy of the reaction intermediates, especially in the triplet manifold, practical applications are scarce. The regio- and stereo-selective aspects are, however, similar to those of the thermal processes. Additional methods that provide for the cyclopentene rearrangement involve the degenerate photochemical rearrangements observed by Wender during meta-photocycloaddition of arenes and alkenes and utilized extensively in the synthesis of triquinane sesquiterpenes. -" -" (See Section 8.1.9 or ref. 41 for a recent summary.)... 

Several vinylcyclopropane to cyclopentene rearrangements have been reported in which a cationic substituent appears to facilitate the reaction. For example, exposure of (150 equation 23) to excess di-ethylaluminum chloride at 0 C for 12 min furnished (151), which served as a key intermediate in Corey and Myers synthesis of the plant hormone antheridogen-An. Attempts to effect this transformation thermally were unsuccessful. In a similar fashion, treatment of (152 equation 24) with boron tribromide induced VCP rearrangement of this compound at room temperature, probably via initial cleavage to the allylic carbocation (153). The reaction of the analogous vinylcyclopropane lacking a phenyl group failed to go to completion under these conditions. 

Occasionally, photolysis of vinylcyclopropanes has been used for preparative purposes to effect the vinylcyclopropane to cyclopentene rearrangement in good yield. Bicyclic ketones and alkenes are available by the photolysis of endocyclic vinylcyclopropanes of type 1, see also Table 3. For a more detailed listing of photochemical transformations consult the recent reviews. ... 

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 rearrangements of vinylcyclopropanes to form cyclopentenes as well as 1,4-hexadienes by homodienyl [l,5]-shift are well-known16,49-51 and even described in textbooks (see, e.g., Chapter 18 in Reference 4). However, the heteroanalogous transformations are less known. 

Palladium-catalyzed rearrangements of dienylcyclopropanes have also been reported. Vinylcyclopropane (64) gave cyclopentene (65) without regard to the ( /(Z) composition of the diene (equation 27). Similar results with respect to (E) and (Z) mixtures were also observed in the more-substituted case of (66) with moderate stereoselectivities (3 1) observed upon reclosure (equation 28). It has been suggested that the palladium-catalyzed rearrangement is in fact a nucleophilic-like opening followed by reclosure. Such transformations are discussed in Section 8.1.3.4 in the context of the stepwise opening of activated vinylcyclopropanes with iodide and other nucleophiles. 

Nickel(0)-catalyzed rearrangement of l-/er/-butyldimethylsiloxy-l-vinylcyclopropane proceeds in refluxing toluene to give l-(tert-butyldimethylsiloxy)cyclopentene in high yield (Table 6, entry 10). The same transformation by thermal means alone requires a very high temperature (350°C)T ... 

The rearrangement of vinylcyclopropane to cyclopentene was first observed by Neureiter in 1959. The mechanism of this transformation has been studied extensively and the rearrangement has enjoyed numerous applications in methodology development and total synthesis. The topic has been the subject of many recent reviews. It is generally accepted that the transformation of a vinylcyclopropane to a cyclopentene proceeds through diradical intermedi-... 

The synthesis of useful organic motifs via molecular rearrangement of strained molecules continues to be an area of profound interest to synthetic chemists. The strain energy of a variety of three-membered carbocycles has been utilized to develop new synthetic transformations. One of the important reactions that has been ejq)lored within this class of molecules is the vinylcyclopropane (VCP)-cyclopentene (CP) rearrangement, which involves the formation of a cyclopentene ring from the ring expansion of the VCP via C—C bond cleavage fScheme 11.11. 

The Diels-Alder-type cycloaddition between 1,3-dithienium fluoroborate (106), obtained from 1,3-dithian and trityl fluoroborate, and 1,3-dienes formed the basis of a new and efficient synthesis of A -cyclopentenones. Simple 1,3-dienes such as butadiene, isoprene, or 2,3-dimethylbutadiene reacted rapidly with (lOQ to give (107) (107b) was obtained stereospecifi-cally from (106) and isoprene in 95% yield. Treatment of the sulphonium salts (107) with butyl-lithium at -78 °C gave an ylide which rearranged on warming to the cyclopropyl-dithian (108). The latt , v/hea heated further to 200 C, underwent the well-known vinylcyclopropane to cyclopentene transformation. Hydrolysis of the dithian thus obtained furnished cyclopen-tenones. The overall yield of 3-methylcyclopent-3-enone from (106) and isoprene was greater than 83%. 

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