Abstract
Asymmetric reduction of several kinds of 2-alkyl-1, 3, 4-cyclopentanetriones (1) by the use of lithium aluminum hydride (LAH) partially decomposed by optically active β-aminoalcohols (7), was studied. It was found that the reduction of 2-propyl derivative (1c) with LAH decomposed by 3.0 eq. of (-)-N-methylephedrine (7a), in THF at -70°for 3 hr, followed by acetylation, gave (R)(-)-4-acetoxy-2-propyl-1, 3-cyclopentanedione ((R) (-)-3c), 58% e.e., in 42% yield. When the same reduction procedure was applied to 2-allyl derivative (1b), (R) (-)-4-acetoxy-2-allyl derivative ((R) (-)-3b), 55% e.e., was obtained in 48% yield. Recrystallization of (R)(-)-3b from ether could improve the optical purity up to 97% e.e. The absolute configuration and optical purity of (R) (-)-3b and (R) (-)-3c were established by the chemical correlation with (R) (-)-allethrolone ((R) (-)-5). When 2-(6-carbomethoxyhexyl)-1, 3, 4-cyclopentanetrione (1a) was treated by the exploited reduction condition, the corresponding (R) (+)-alcohol ((R) (+)-2a), 54±6% e.e., from which PGE1 had been synthesized, was prepared in 58% yield.
