The intermediate (racemic)-2-(methoxycarbonylmethyl)-6,6-dimethyltetrahydropyran-2-carboxylic acid (VIII) has been obtained by several related methods: 1. The Grignard condensation of 4-methyl-3-pentenyl bromide (I) with diethyl oxalate (II) in HF gives the 2-oxoheptenoate (III), which is condensed with methyl acetate (IV) by means of LiHMDS in THF to yield 3-(ethoxycarbonyl)-3-hydroxy-7-methyl-6-octenoic acid methyl ester (V). The cyclization of (V) by means of Ts-OH in hot toluene or by means of hot aqueous formic acid affords 2-(methoxycarbonylmethyl)-6,6-dimethyltetrahydropyran-2-carboxylic acid ethyl ester (VI), which is hydrolyzed with KOH in boiling water to provide the corresponding dicarboxylic acid (VII). Finally, this compound is regioselectively monoesterified by means of BF3/MeOH in methanol to furnish the intermediate (racemic)-2-(methoxycarbonylmethyl)-6,6-dimethyltetrahydropyran-2-carboxylic acid (VIII). 2. The reaction of 3-(ethoxycarbonyl)-3-hydroxy-7-methyl-6-octenoic acid methyl ester (V) with HCl in hot methanol gives 3-(ethoxycarbonyl)-3,7-dihydroxy-7-methyloctanoic acid methyl ester (IX), which is then cyclized by means of ZnCl2 in hot dichloroethane to yield the previously described intermediate (VIII). 3. The hydrolysis of 3-(ethoxycarbonyl)-3-hydroxy-7-methyl-6-octenoic acid methyl ester (V) with KOH in refluxing methanol/water gives the corresponding diacid (X), which is regioselectively monoesterified by means of BF3/MeOH in methanol to yield 3-carboxy-3-hydroxy-7-methyl-6-octenoic acid methyl ester (XI). Finally, this compound is cyclized by means of Ts-OH in hot toluene to afford the previously described carboxylic intermediate (VIII). The racemic acid (VIII) is submitted to optical resolution by esterification with quinine (XII) by means of 2,4,6-trichlorobenzoyl chloride and TEA or DCC to give a diastereomeric mixture of esters (XIII) that is separated by preparative HPLC to obtain the desired diastereomer (XIV). The hydrolysis of (XIV) with KOH in refluxing ethanol/water gives the corresponding chiral dicarboxylic acid (XV), which is regioselectively monoesterified with BF3/MeOH in methanol to yield the chiral (R)-2-(methoxycarbonylmethyl)-6,6-dimethyltetrahydropyran-2-carboxylic acid (XVI). The esterification of (XVI) with cephalotaxine (XVII) by means of 2,4,6-trichlorobenzoyl chloride and TEA in toluene affords the corresponding ester (XVIII), which is treated with HBr in dichloromethane/HOAc, providing the bromoester (XIX). Finally, this compound is treated with NaHCO3, CaCO3 or BaCO3 in acetone/water to give the target hydroxyester.
The oxidation of 2-methyl-1-cyclopentene-1-carbaldehyde (I) with O3 and Ag2O gives 2,6-dioxoheptanoic acid (II), which is esterified with cephalotaxine (III) by means of (COCl)2, yielding the ester (IV). Reformatsky reaction of (IV) with methyl bromoacetate (V) and Zn affords the adduct (VI), which is treated with an excess of methylmagnesium iodide to provide the target homoharringtonine (as a single diastereomer), along with some starting cephalotaxine that is separated by chromatography.