(i)The stereochemistry of meso-2,3-dibromobutane is most easily seen with a Fischer projection:Recalling that the addition of Br2to alkenes occurs with anti stereochemistry, rotate thesawhorse 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-butyneby metal-ammonia reduction:9.34Attack this problem by first planning a synthesis of 4-methyl-2-pentyne from any starting materialin 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 (CH3)2CHC>CH via reac-tion of its anion with methyl iodide.The next question that arises from this analysis is the origin of (CH3)2CHC>CH. One ofthe available starting materials is 1,1-dichloro-3-methylbutane. It can be converted to
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