Stereospecific Cis Elimination

PBr3/CuBr, Zn Ph2PCl, I2, imidazole PhaP, I2, imidazole 

For other methods to eliminate j8-halo esters, see page 267, Section 8. 

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The first is a Wittig reaction with an unstabilized ylid, the second a Julia reaction, and the last two are Peterson reactions under different conditions. Each reaction is described in detail in the chapter. The Wittig reaction is under kinetic control and is a stereospecifically cis elimination. In this case the product is the Z-alkene. 

Dehydration. Burgess el a .2 reported that the reagent is useful for dehydration of simple alcohols. The reaction is a stereospecific cis elimination and follows Saytzeff s rule. Crabbe and Leon3 have used this procedure with various steroidal secondary and tertiary alcohols. They conclude that the nature of the alcohol group, the configuration, and the environment are the primary factors governing the course of dehydration. The reactions are carried out at room temperature in anhydrous benzene for 2 hr. followed... 

Trithiocarbonates also undergo elimination when heated with a trialky] phosphite to form olefins by stereospecific cis elimination, as illustrated by a practical synthesis of fratw-cyclooctene from the cis isomer ... 

Organic azides combine with open-chain and cyclic eno) ethers to give A -triazolines in high yield. The addition is stereospecific cis and the orientation process is determined by electronic effects.145-160 The triazolines are unstable above 100° and can decompose in mainly two ways. 44 Triazolines from the open-chain enol ethers, vinyl butyl ether, /S-ethoxypropene, and a- and /3-methoxystyrene, eliminate alcohol thermally and are converted into 1,2,3-triazoles. For example, the triazoline 67 from /J-ethoxypropene 66 and p-nitrophenyl azide decomposes quantitatively at 150° into l-(p-nitrophenyl)-5-methyl-1,2,3-triazole (68). 

The third method is a concerted stereospecific removal of two adjacent hydrogen atoms from the chain of a fatty acid after synthesis. This is an aerobic route as oxidation is required and is used by mammals such as ourselves. The stereochemistry of the reaction is known from labelling studies to be cis elimination. 

The observed stereospecificity of the reaction in the conversion of l-(p-nitro-benzoyl)-2-benzylaziridine in refluxing toluene into A -( ra/w)-cinnamyl-p-nitro-benzamide is best explained in terms of an intramolecular reaction mechanism in which cis elimination of a proton is concerted with opening of the aziridine ring (A)... 

Olefin inversion.2 Lithium diphenylphosphide in THF opens epoxides stereo-specifically quatemization of the crude product with methyl iodide leads to betaines, which fragment under mild conditions (25°) to give an olefin and methyldiphenyl-phosphine oxide. The product olefins are formed with inversion of stereochemistry relative to the starting epoxide owing to SN2 epoxide opening followed by cis elimination of the phosphine oxide. Thus the oxide of fraus-stilbene (1) is converted into the betaine (2), which at 25° gives cis-stilbene (3) in 95% yield overall. The conversion is stereospecific (> 98%). Similarly cis-stilbene can be converted into franx-stilbene. 

Another attractive feature of the phosphonamide route is that it can be controlled to give cis- and trans-olefins.4 Formation of the adduct is not stereospecific, but the elimination reaction is stereospecific (cis cycloelimination). The synthesis of cis- and trans-1 -phenylpropene is illustrative the former was prepared from the reaction of benzaldehyde with the a-lithio derivative of ethylphosphonic acid bis(dimethyl-amide) to give two diastereoisomeric /3-hydroxyphosphonamides (5) in the ratio of 3.5 1. The major isomer was separated by crystallization and on pyrolysis gave cis-1 -phenylpropene. The isomeric olefin was prepared by reaction of the a-lithio... 

The decomposition of /S-hydroxylsulfinamides occurs stereospecifically by cis elimination.3 Thus the tran.r /3-hydroxysulfinanilide (5) affords /rans-cyclododecene... 

The first reaction is stereospecific cis addition of hydrogen to an alkyne to give the c/s-alkene. The intermediate is therefore a cfs,cfs-diene and it may seem remarkable that it should become a trans,trans-diene on elimination. However, when we draw the mechanism for the elimination, we see that there need be no relationship between the stereochemistry of the intermediate and the product as this is an El reaction and the cationic intermediate can rotate into the most stable shape before conversion to the aldehyde. 

In the formation of the A bonds in the sterol side-chain studies with stereospecifically labeled MVA have revealed that stereoselectivity of removal of hydrogens from C-22 and C-23 varies with the organism examined (Table III). However, in all cases a cis elimination is involved (Goad and Goodwin, 1972). The mechanism concerned has not been elucidated. 

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