Vinylcyclopropane-cyclopentene rearrangement thermal

Vinylcyclopropane-cyclopentene rearrangement Thermal or photochemical rearrangement of substituted vinylcyclopropanes to substituted cyclopentenes. 470... 

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

Because of the discovery of a growing number of naturally occurring substances of biological importance that contain the five-membered ring moiety 178), the synthesis of cyclopentanoid compounds is a subject of present interest. Indeed, among the various approaches recently investigated, the thermal vinylcyclopropane-cyclopentene rearrangement of readily available 1-siloxy-l-vinylcyclopropanes (vide supra, Sect. 4.1.5) constitutes an efficient three-carbon annelation process 179). 

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. 

The thermal vinylcyclopropane-cyclopentene rearrangement discovered by Neureiter in 1959 has been the subject of many mechanistic and theoretical studies (equation 121)206... 

Usually the activation free energy required for the thermal vinylcyclopropane-cyclopentene rearrangement ranges from 48 to 53kcal mor A quantitative treatment of the substituent effect has been conducted It appears that the trimethyl-... 

Houk, K. N., Nendel, M., Wiest, O., Storer, J. W. The Vinylcyclopropane-Cyclopentene Rearrangement A Prototype Thermal Rearrangement Involving Competing Diradical Concerted and Stepwise Mechanisms. J. Am. Chem. Soc. 1997, 119,10545-10546. 

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... 

Use of chiral cyclopropylethynyllithium derivatives permits the elegant selective synthesis of labeled chiral vinylcyclopropanes, for stereochemical studies of the thermal vinyl cyclopropane-cyclopentene rearrangement. Thus, reductive elimination of (15, rra )-(2,2-dibromoethenyl)-l-methylcyclopropane with BuLi in pentane, followed by hydrolysis of the lithium acetylide, afforded (15, rraw5)-2-ethynylmethylcyclopropane (equation 157). 

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. 

Reactions of phenylthiocyclopropyl lithium with alkyl halides and epoxides have also been reported to deliver products opened to a variety of carbonyl derivatives jS-(l-Phenylthio)cyclopropyl enones have been prepared by using lithium salts of a-hydroxymethylene ketones as electrophiles and dehydrating with acid (equation 110). Rearrangement to cyclobutanones occurs with trifluoroacetic acid, whereas thermal vinylcyclopropane-cyclopentene expansion sometimes gives mixtures of regioisomers . 

Baldwin, J. E., Shukla, R. Thermal Stereomutations and Vinylcyclopropane-to- Cyclopentene Rearrangement of 2-Methylene-3-spiro-cyclopropanebicyclo[2.2.1]heptane. J. Am. Chem. Soc. 1999, 121, 11018-11019. 

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. 

Some particularly striking examples of Cope rearrangements can be found in the rearrangement reactions of cw-divinylcyclopropanes. But before we go into these, let us examine vinylcyclopropane itself, which is known to rearrange thermally to cyclopentene ... 

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. 

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