The STOBBE condensation. X. The cyclisation of (E)-3-methoxycarbonyl-4-(5′-methyl-2′-furyl)-but-3-enoic acid†, and (E)−3-methoxycarbonyl-4-(2′-furyl)-pent-3-enoic acid, to Benzofuran Derivatives
i.e. COOCH3 group is in trans-position to furan ring. The nomenclature of cis/trans isomers used herein follows the IUPAC 1968 Tentative Rules, Section E, Fundamental Stereochemistry, J. org. Chemistry 35, 2849 (1970). See also: J. E. BLACKWOOD, C. L. GLADYS, K. L. LOENING, A. E. PETRARCA and J. E. RUSH, J. Amer. chem. Soc. 90, 510 (1968); Introduction to Subject Index of Chem. Abst. 66 (1963).
Abstract
The condensation of 5-methyl-furan-2-aldehyde and 2-furyl-methyl ketone with dimethyl succinate using either potassium t-butoxide or sodium hydride as condensing agents, gives predominantly (E)-3-methoxycarbonyl-4-(5′-methyl-2′-furyl)-but-3-enoic acid 1a and (E)-3-methoxy-carbonyl-4-(2′furyl)-pent-3-enoic acid 5 respectively. Their configurations are inferred by cyclisation with sodium acetate in acetic anhydride to the corresponding benzofuran derivatives 2,6. Alcoholysis of (E)-3-carboxy-4-(5′-methyl-2′-furyl)-but-3-enoic anhydride 3 gives the half-ester 1c which is isomeric with the half-ester 1a. A competing side reaction also gives the self-condensation product of the succinic ester 4.
