Cyclopentenes, rearrangements with

The vinylcyclopropane to cyclopentene rearrangement is seemingly broadly applicable to solution of the five-ring annulation problem. Besides the example cited in Scheme 8, routes provided by Trost,61 Miller,62a and Conia62b are conceptually attractive. Thus, we see that reaction of a cyclopentanone with the cyclopro-pyldiphenylsulfonium ylide provides an oxaspiropentane which can be opened efficiently with an amide base (Scheme 10). The resulting silylated cyclopropanol can be... 

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 conversation of free and Cr(CO)s-complexed 2-vinylphosphirane into 2,5-dihydro-17/-phosphole via 1,3-sigmatropic shifts was studied with DFT and compared with the corresponding vinylcyclopropane-cyclopentene rearrangement (Equation 2) <2002JA13903>. 2-Vinylphosphirane has smaller barriers as in the case of vinylcyclo-propane and the complexed species has still smaller one. The rearrangement of syn- and /F2-vinylphosphirane is a... 

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. 

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. 

The conversion of free and Cr(CO)5-complexed 2-vinylphosphiranes into 3-phospholenes has now been studied using density functional theory. It is concluded that this rearrangement has much in common with the vinylcyclo-propane-cyclopentene rearrangement, a pericyclic [l,3]-sigmatropic shift... 

With the exception of vinylcyclopropane-cyclopentene rearrangements (cf. Section VII) radical reactions have so far rather rarely been used synthetically. This is also true for pericyclic modes of cyclopropane cleavage which are mainly restricted to divinylcyclopropane-cycloheptadiene type expansions. Cycloadditions, whether concerted or stepwise, occur only with very specific compounds. ... 

The reactivity of these promising building blocks has very recently been reviewed by de Meijere S therefore only a few principles will be discussed in this text. Treatment with n-butyllithium as outlined in equation 187 will give certain functionalized alkynylcyclopro-panes , which can serve as active components in cycloadditions or reactions with olefins in the presence of Co2(CO)g. The latter [2 + 2 + l]-addition provides cyclopropyl cyclopentenones capable of undergoing vinylcyclopropane-cyclopentene rearrangement. 

The iridoid sesquiterpene (-)-specionin, an antifeedant to the spruce budworm, was synthesized by T. Hudlicky et al. using the low-temperature vinylcyclopropane-cyclopentene rearrangement as the key step. The substituted cyclopentenone precursor was first exposed to the lithium dienolate derived from ethyl 4-(dimethyl-fert-butylsilyloxy)-2-bromocrotonate at -110 °C to afford silyloxyvinylcyclopropanes as a mixture of exo and endo isomers (with respect to the vinyl group). The mixture was not separated but immediately subjected to TMSI/HMDS, and the corresponding tricyclic ketones were obtained in good yield. Similar results were obtained when TBAF in THF was used instead of TMSI. 

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