A novel method for the introduction of a methyl group into the furan ring of a 2,5-disubstituted C₁₈ furanoid fatty estervia a malonic acid function

Abstract

Furan ring opening with benzohydroxamic acid of methyl 9,12-epoxy-9,11-octadecadienoate gave a mixture of positional isomers of conjugated methyl 3-phenyl-1,4,2-dioxazolyl C₁₈-enone esters 6a,6b. Michael addition of diethyl malonate anion to the conjugated enone system of 6a,6b furnished the corresponding malonyl intermediates 7a,7b, which upon removal of the dioxazole ring by hydrolysis gave methyl 10- and 11-dicarbethoxymethyl-9,12-dioxooctadecanoate 8a,8b. Cyclization of the latter gave the trisubstituted C₁₈ furanoid fatty esters 9a,9b, containing the malonate ester function at the 3-/4-position of the furan ring. Base hydrolysis of compounds 9a,9b gave the corresponding tricarboxylic acid derivatives 10a,10b, which were esterified to the trimethyl esters 11a,11b in BF₃/MeOH. When a mixture of 9a,9b was refluxed with Na₂CO₃/MeOH, hydrolysis of the malonate ester function was followed by decarboxylation to yield a-CH₂COOH substituent at the 3-/4-position of the furan ring (12a, 12b). Esterification of the latter with BF₃/MeOH gave the corresponding methyl diester derivatives 13a,13b. When a mixture of tricarboxylic acids 10a,10b was heated at 160–180°C for 6 hr, exhaustive decarboxylation of malonic acid function furnished a methyl group at the 3-/4-position of the furan nucleus. Esterification of the decarboxylated product gave a mixture of trisubstituted furanoid compounds 14a,14b (overall yield 28%). The procedure constitutes a novel method for the introduction of a methyl groupvia a malonic acid group to the 3-/4-position of the furan ring of a 2,5-disubstituted C₁₈ furanoid fatty ester.