Bromide ion → Methyl 3,7-dimethyl-4-nitro-6-octenoate

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The bromo compound, 8-bromo-2,6-dimethyl-5-[2-(tetrahydropyran-2-yloxy)]-2-octene, used to prepare the Grignard reagent in the first step in the synthesis is prepared by a series of reactions using 5-bromo-2-methyl-2-pentene as the starting material. The bromide is first converted to 2-methyl-5-nitro-2-pentene by reaction with sodium nitrite in dimethylsulfoxide. The nitro compound is then reacted with methyl crotonate to form methyl 3,7-dimethyl-4-nitro-6-octenoate. Reaction of the octenoate derivative with titanous chloride results in the formation of the oxo compound, methyl 3,7-dimethyl-4-oxo-6-octenate. The oxo compound is then reacted with lithium aluminum hydride to form 3,7-dimethyl-4-hydroxy-6-octen-1-ol. The diol is converted to the mono-acetate by reaction with acetic anhydride and the mono-acetate is then converted to the tetrahydropyranyl derivative by reaction with dihydropyran. The tetrahydropyranyl derivative is converted to the corresponding alcohol by reaction with potassium carbonate in a mixture of methanol and water. The bromo compound is then obtained by reacting the alcohol 3,7-dimethyl-4-[2-(tetrahydropyran-2-yloxy)]-6-octen-1-ol with phosphorous tribromide or triphenylphosphine and carbon tetrabromide.

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