Alkene Inversion

See also page 251, Section 2, for elimination via diols, epoxides, /1-dihalides, /J-halo esters and bromohydrins. Reviews  

---

Occasionally, chemists need to invert the stereochemistry of an alkene—that is, to convert a cis alkene to a trans alkene, or vice versa. There is no one-step method fordoing an alkene inversion, but the transformation can be carried out by combining several reactions in the proper sequence. How would you carry out the following reactions ... 

The aza-Diels-Alder reaction is an important and versatile tool for the preparation of nitrogen-containing heterocycles present in numerous natural products and drug candidates. It involves the [4 + 2] cycloaddition of either an imine with an electron-rich diene or an azabutadiene with an electron-rich alkene (inverse electron demand). Catalytic asymmetric variants employing not only metal complexes, but also organic molecules were disclosed over the last few years. 

Electrophilic addition reactions of tetravalent tellurium compounds have been reviewed.64 2-Naph-thyltellurium trichloride (ArTeCb) adds to alkenes in an anti stereospecific manner (equation 11), whereas tellurium tetrachloride gives mixtures of 2 1 adducts with both syn and anti addition.72 A one-pot alkene inversion procedure has been developed, based upon TeCU addition to alkenes followed by treatment of the (3-chloroalkyltellurium trichloride adduct with aqueous Na2S (Scheme 37).73 Tellurium compounds such as tellurinyl acetates, ArTe(0)0Ac, prepared in situ through reaction of tellurinic acid anhydrides with acetic acid, can be employed in oxytelluration and aminotelluration procedures (Schemes 38 and 39).74 In the oxytelluration reaction intermediate triacetates of the type RCH2Te(OAc)2Ph are reduced with hydrazine to the corresponding tellurides. 

A, vy/ -elimination of Ph2MeP=0 and simultaneous stereoselective alkene formation from an oxaphosphetane are shown in Figure 4.45 (note that this oxaphosphetane is not produced via a P-ylide). In Figure 4.45, this elimination is part of a an alkene inversion in which, via a four-step reaction sequence, an alkene such as cw-cyclooctene, a molecule with little strain, can be converted into its trans- and, in this case, more highly strained, isomer. 

Inversion of Olefin Stereochemistry The preparation of alkenes via inversion of the double bond geometry is an important synthetic transformation. For example, interconversion of the (Z)-alkene to the (ff-isomer depicted below involves treatment of the (Z)-epoxide with the nucleophilic LiPPh2 followed by phosphorus alkylation to furnish the betaine,which undergoes 5yn-elimination to produce the (Ef-alkene. The alkene inversion works for di-, tri-, and tetra-substituted olefins. 

The reductive elimination of v/c-bromochlorides and vic-dichlorides with metals such as zinc is much less stereoselective than the corresponding vic-dibromides. However, with Nal in DMF the reductions are highly stereoselective and proceed with a net syn elimination. These observations have been used to effect an alkene inversion as shown in Scheme 1 for the preparation of the sex pheromone (3) of the oriental fruit moth. 7 Treatment of v/c-dichlorides with sodium phenylselenolate also results in a syn elimination. ... 

Possible synthetic routes to frinvestigated using the /J-hydroxy-t/3-phosphane oxide alkene inversion procedure. A stepwise approach via the epoxy-trans-cyclononene derivative failed. However, consecutive double elimination involving the bis-/ -hydroxy-) 5-phosphane oxides 11 and 12 gives cis- and trails- 1,2-di-vinylcyclopentanes which are believed to be formed by a rapid Cope rearrangement of parallel and crossed r[Pg.362]

Thiophenol-mediated Alkene Inversion. /rans-Substituted... 

This effect has been used in a clever synthesis of butenolides 5.83 (Scheme 5.25). Heck reaction of the fra -4-alkoxycrotonate 5.81 resulted in the expected inversion of alkene stereochemistry, allowing lactonization on deprotection. The alkene inversion, in an intramolecular Heck reaction, is also apparent in an approach to vitamin D compounds (Scheme 5.26). ... 

---