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First Synthesis

Comparing the formulas of benzyl alcohol (C7H8O) with the intermediate (C9H12O2) shows a difference of C2H4O. Only two reactant compounds, ethylene oxide and acetic anhydride, have two-carbon chains that might provide this unit. The desired product has the benzyl group of the starting material joined to a two-carbon chain by an ether linkage. This arrangement is a common product of acid or base-catalyzed reactions of alcohols with epoxides. Therefore, it is reasonable to surmise that the C9H12O2 intermediate is formed by addition of benzyl alcohol to ethylene oxide. To effect such a reaction we must use either catalytic acid or base. To this end, reaction of benzyl alcohol with sodium hydride would generate an appropriate nucleophile (C7H7O(–)) for SN2 attack on the epoxide ring. The resulting alcohol (C6H5CH2–O–CH2CH2–OH) could then be oxidized to the product by PCC.

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Second Synthesis

Here we are allowed to choose our starting material. The proposed C3H6O2 intermediate suggests this be a three-carbon reactant. Since the desired product has a benzene ring, the second step will presumably use benzene as a reactant. One of the best methods for making aryl alkyl ketones is Friedel-Crafts acylation. For the product shown here this would require heating benzene with propanoyl chloride and aluminum chloride. Thus, the starting material is 1-propanol, and this is oxidized to propanoic acid by Jones' reagent. Reaction with thionyl chloride then gives propanoyl chloride.

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Third Transformation

From the structure of the desired product, it is clear that the cyclopentanol starting material must be converted to cyclopentanecarboxylic acid (formula C6H10O2). Since carbon dioxide is used in the third step (first arrow) a Grignard reagent is probably used here. Therefore the first two steps are reaction with PBr3 to give cyclopentylbromide, followed by reaction with magnesium in ether. Finally, the intermediate carboxylic acid is esterified by heating with 1-propanol and an acid catalyst

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Fourth Transformation

The desired product is a diacetate, and it is clear that it can be prepared from the corresponding diol by reaction with acetic anhydride (reactant F). Since the hydroxyl groups in the diol precursor are separated by three carbon atoms, this might be prepared by reduction of a β-hydroxy carbonyl compound or a β-dicarbonyl compound. Considering that the C3H6O intermediate has half the carbon atoms needed for the six-carbon diol, a dimerization reaction, such as an aldol condensation, would be a good choice for the carbon-carbon bond forming reaction. The first step will therefore be oxidation of 1-propanol to propanal by reaction with PCC. The base-catalyzed aldol condensation is then followed by reduction to the diol, and finally acetylation with acetic anhydride.

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Fifth Transformation

The final product is a symmetrical eight-carbon 3º-alcohol. The proposed synthesis proceeds by way of a three-carbon bromide. Since two of the alkyl substituents on the 3º-alcohol are isopropyl, an efficient synthesis would involve the reaction of ethyl acetate (reactant E) with two equivalents of isopropyl Grignard reagent. This could be prepared by reduction of acetone (NaBH4), conversion of the resulting isopropanol to the bromide (PBr3), and finally reaction with magnesium.

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