Furan derivatives. Part 8 On substituent effects in the synthesis of 4,5‐dihydro‐3H‐naphtho[1,8‐bc]furans

Abstract

Abstract4,5‐Dihydro‐3H‐naphtho[1,8‐bc]furans 4 and 6 which have various substituents (R1 and R2) have been synthesized from 8‐oxo‐5,6,7,8‐tetrahydro‐1‐naphthyloxyacetic acids 1 and 3 or their ethyl esters 2. The reaction of acids 1 and 3 with sodium acetate in acetic anhydride gave a mixture of furans 4 and 6 and lactones 5 and 7. The ratios of the products were varied according to the types of substituents (R1 and R2) in acids 1 and 3. As the substituent R1 (R2 = hydrogen) in acids 1 was changed from hydrogen to a methyl, ethyl or isopropyl group, production of furans 4 became more difficult. However, when a phenyl group was used as the substituent, furan 4 was obtained in good yield. Similarly, as the substituent R2 (R1 = hydrogen) in acids 1 was changed from hydrogen to a methyl, ethyl or isopropyl group, furan formation was more difficult. In contrast, acids 3 which had electron‐withdrawing substituents such as chlorine, bromine or a nitro group at the 4‐position afforded furans 6 in good yield. 4,5‐Dihydro‐3H‐naphtho[1,8‐bc]furans 4 and 4,5‐dihydro‐3H‐naphtho[1,8‐bc]furan‐2‐carbocylic acids 8 were synthesized from the reaction of esters 2 and potassium hydroxide in dioxane. When the substituents R1 or R2 in esters 2 were varied from hydrogen to a methyl, ethyl or isopropyl group the total yields of furans 4 and furancarboxylic acids 8 were reduced.