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Dolbier HW Solutions 230 - 224 ALKYNES(i The...

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( i ) The stereochemistry of meso -2,3-dibromobutane is most easily seen with a Fischer projection: Recalling that the addition of Br 2 to alkenes occurs with anti stereochemistry, rotate the sawhorse diagram so that the bromines are anti to each other: Thus, the starting alkene must be trans -2-butene. trans -2-Butene is available from 2-butyne by metal-ammonia reduction: 9.34 Attack this problem by fi rst planning a synthesis of 4-methyl-2-pentyne from any starting material in a single step. Two different alkyne alkylations suggest themselves: Isopropyl bromide is a secondary alkyl halide and cannot be used to alkylate accord- ing to reaction ( a ). A reasonable last step is therefore the alkylation of (CH 3 ) 2 CHC > CH via reac- tion of its anion with methyl iodide. The next question that arises from this analysis is the origin of (CH 3 ) 2 CHC > CH. One of the available starting materials is 1,1-dichloro-3-methylbutane. It can be converted to
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