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

4,5‐Dihydro‐3 H ‐naphtho[1,8‐ bc ]furans 4 and 6 which have various substituents (R 1 and R 2 ) 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 (R 1 and R 2 ) in acids 1 and 3 . As the substituent R 1 (R 2 = 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 R 2 (R 1 = 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‐3 H ‐naphtho[1,8‐ bc ]furans 4 and 4,5‐dihydro‐3 H ‐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 R 1 or R 2 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.