Treatment of ethyl N-Boc-(S)-pyroglutamate (I) with lithium hexamethyldisilazide in THF at -78 C, followed by aldol condensation of the resulting lithium enolate with cyclopentanone (II) in the presence of boron trifluoride etherate, furnished adduct (III). Dehydration of the alcohol group of (III) to produce olefin (IV) was achieved by treatment of (III) with methanesulfonyl chloride and triethylamine. Hydrolysis of the ester and lactam groups of (IV) by means of LiOH afforded diacid (V). The N-Boc group of (V) was finally cleaved with HCl in EtOH.
Treatment of ethyl N-Boc-(S)-pyroglutamate (I) with lithium hexamethyldisilazide in THF at -78 C, followed by aldol condensation of the resulting lithium enolate with 3-pentanone (II) in the presence of boron trifluoride etherate, furnished adduct (III). Dehydration of the alcohol group of (III) to produce olefin (IV) was achieved by treatment of (III) with methanesulfonyl chloride and triethylamine. Hydrolysis of the ester and lactam groups of (IV) by means of LiOH afforded diacid (V). The N-Boc group of (V) was finally cleaved with HCl in EtOH.