Cyclopentenes VCP- CP rearrangement

CHAPTER 11 VINYLCYCLOPROPANE-CYCLOPENTENE REARRANGEMENT Scheme 11.1 VCP-CP rearrangement. 

Although the previously mentioned heteroatom-substituted VCPs (see Schemes 11.3-11.6 underwent conversion to cyclopentenes at somewhat lower tenperatures than the original chloro-substituted and all-carbon systems, lowering reaction tenperatures even further would make the rearrangement synthetically more useful. The Danheiser group demonstrated a significant rate enhancement of the VCP-CP rearrangement by the use of lithium salts of vinylcyclopropanols. Cyclopentenol formation proceeded effectively at room tenperature in low to excellent yields (Scheme 11.7). ... 

The Hudlicky group also took advantage of anion-accelerated VCP-CP rearrangements by treating (siloxyvinyl)cyclopropanes to the tetra-butylammonium fluoride (TBAF) or trimethylsilyl iodide in the presence of hexamethyldisilazane (TMSI/HMDS). These conditions promoted rearrangement to form [5-5] and [5-6] annulated cyclopentenes at low temperatures with high selectivities (Scheme 11.9T ... 

In 2009, Lambert reported a mild, efficient, and stereoselective synthesis of fused cyclopentenes via Mgl2-promoted isomerization of activated VCPs. The VCPs were prepared by Pd-catalyzed cyclopropanation of 1,3-dienyl p-ketoesters in moderate to high yields and good diastereoselectivities. Typical standard reaction conditions for the VCP-CP rearrangement (i.e., pyrolysis, transition metal catalysis, standard Lewis acid catalysis) were found to be ineffective for these substrates. However, the use of 1.5 equiv. of Mgl2 led to conplete conversion, providing bicyclic cyclopentenes in high yields fScheme 11.15L... 

Sonawane et al. further utilized the photoinduced VCP-CP rearrangement in a formal s5mthesis of the sex pheromone of the male cotton boll weevil, (ij-grandisol. The toluene-sensitized irradiation of enantiopure (+)-A -carene 69 afforded the annulated cyclopentene ( )-72 in... 

Sonawane, H. R., Bellur, N. S., Kulkarni, D. G., Ahuja, J. R. Photoinduced vinylcyclopropane-cyclopentene rearrangement (photo-VCP-CP) a methodology for chiral bicyclo[3.2.0]heptenes and their application in natural product syntheses. Syn/eff 1993, 875-884. 

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... 

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. 

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