Synthesis of Methyl dl-Jasmonate and Its Related Compounds from Methyl (E)-and (Z)-4,4-Dimethoxy-2-butenoates

A synthesis of methyl dl-jasmonate (1b) and its dehydro derivatives 2b and 3b from methyl (E)-and (Z)-4,4-dimethoxy-2-butenoates (4) is described. Dimethyl 2-acetyl-3-dimethoxymethylglutarate (5) could be obtained by Michael addition of 4 with methyl acetoacetate in excellent yields. Deacetalization of dimethyl 2-acetyl-3-dimethoxymethyl-2-(2-pentynyl)glutarate (7a) followed by cyclization with base after alkylation of 5 (R′ = Me) with 2-pentynyl bromide afforded 5-methoxycarbonyl-4-methoxycarbonylmethyl-5-(2-pentynyl)-2-cyclopentenone (10a). Reduction of 10 (R′ = Me) with NaBH₄ in MeOH giving 2-methoxycarbonyl-3-methoxycarbonylmethyl-2-(2-pentynyl)cyclopentanol (13a) and subsequent oxidation of 13 with chromic acid gave 2-methoxycarbonyl-3-methoxycarbonylmethyl-2-(2-pentynyl)cyclopentanone (14a), a precursor of lb. Cis hydrogenation of 7a→ 7b, 10a → 10b, 13a → 13b, and 14a → 14b using Lindlar catalyst proceeded in quantitative yields. Direct demethoxycarbonylation of 10b (R = 2-cis-pentenyl) with Me₂SO-H₂O-NaCl in a sealed tube afforded a mixture of 2b and 3b. However, acid-catalyzed de-teri-butoxycarbonylation of 10b (R′ = t-Bu), prepared from 5 (R′ = t-Bu) by alkylation followed with cyclization, under reflux in benzene gave 2b as a sole product. Hydrogenation of 10a with palladium on charcoal afforded 14c (R = pentyl). The products 2b and 3b could be converted into lb smoothly.