A Convenient Synthesis of 5-Substituted-2,5-dihydro-2-furoic Acids

Nine 5-alkyl- and 5-aryl-2-furoic acids were reduced under controlled Birch conditions and then esterified with acidic methanol. 2-Furoic acids with 5-alkyl groups gave good yields of methyl 5-alkyl-2,5-dihydro-2-furoates, which were obtained as mixtures of cis- and trans-diastereoisomers. These 2,5-dihydrofuroates revealed interesting spectroscopic properties; (1) large long-range coupling constants, 5–7 and 3–4 Hz, between protons at C2 and C5 were observed in the NMR spectra of cis- and trans-diastereoisomers, respectively, and (2) two high absorption maxima due to the ester carbonyl group of each furoate appeared near 1750 and 1730 cm−1 in the IR spectrum. On the other hand, the furoic acids with 5-aryl substituents afforded 5-arylpentanoate or 5-arylpent-3-enoate as main products. These esters would be produced as a result of reductive cleavage at C5 of 5-aryl-2,5-dihydrofuroates formed as intermediates.