合成路线1

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.