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I'm trying to design a synthesis of D from C. All I can think of is using $\ce{Me2CuLi}$ to add the methyl group to the alkene. I thought maybe then if isobutyl bromide was in the reaction mixture when $\ce{Me2CuLi}$ pushed the negative charge onto the oxygen and it came back down that could do an $\mathrm{S_N2}$ with isobutyl bromide, but I've never seen that before. Then for converting the carbonyl to an alkene I have no idea.

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    $\begingroup$ The Wittig reaction may be useful. $\endgroup$
    – bon
    May 6, 2016 at 18:35
  • $\begingroup$ Ah of course, the Wittig reaction with $H_3C-^+PPh_3$ as the phosphonium salt? Just the dialkylation I'm not sure about now. $\endgroup$
    – draksi
    May 6, 2016 at 18:57
  • $\begingroup$ I was thinking of Baylis-Hillman to add the isobutyl group, but I don’t think that works well enough with alkyl halides =C $\endgroup$
    – Jan
    May 6, 2016 at 19:34

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You are right about using the Gilman in a 1,4-Michael addition. The enolate created from this can then be used in the nucleophilic attack of a haloalkane. Then all that's left to do is a Wittig reaction, swapping the carbonyl oxygen for a carbon. The full stepwise synthesis would be:

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    $\begingroup$ Could also use Tebbe/Petasis methylenation $\endgroup$
    – SendersReagent
    May 8, 2016 at 2:56
  • $\begingroup$ Alkylation of the enolate with isobutyl bromide will likely give substantial elimination. 3-Bromo-2-methylprop-1-ene will be more reactive and not undergo elimination. Reduce the double bond with Pd/C/H2. Alternatively, trap the enolate with isobutyraldehyde, dehydrate with acid and hydrogenate. $\endgroup$
    – user55119
    Mar 4, 2018 at 3:52

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