The oxidation of protected hydroxymethyl lactone (I) with DCC/DMSO gives the aldehyde (II), which is condensed with phosphorane (III) to yield the pentenone (IV). The reduction of (IV) with NaBH4 in methanol affords the pentenol (Va-b), which is further reduced with DIBAL to provide the lactol (VIa-d). The condensation of (VIa-d) with phosphorane (VII) by means of KOtBu gives the trihydroxyheptenoic acid (VIIIa-b), which is esterified with isopropyl iodide (A) and DBU, yielding the isopropyl ester (IXa-b). Finally, this compound is selectively oxidized with DDQ to afford the target compound.