The 5-, 6-, and 10-Fluoro-1,2,3,4-tetrahydro-7,12-dimethylbenzanthracenes. Comparative Acid-Catalyzed Isomerization to Exo Methylene Tautomers: A 6-Fluoro Peri Effect

Article abstract of DOI:10.1021/jo00374a001

Syntheses for 5-, 6-, and 10-fluoro-1,2,3,4-tetrahydro-7,12-dimethylbenzanthracenes (3-5) from an aminotetralin-HCl 6, fluorotetralin 14, and tetrahydronaphthaldehyde 29, respectively, are described.Comparative acid-catalyzed isomerization studies in refluxing benzene revealed that the 6-F analogue 4 equilibrates with 12- and 7-methylene tautomers 20 and 21 within 1 h.When the 12-methylene tautomer 20 was treated with p-toluenesulfonic acid in refluxing benzene the thermodynamic product ratio (4/20/21 = 1.0:0.6:2.8) was obtained after 72 h.These data are markedlydifferent than results observed for acid-catalyzed isomerization of 1,2,3,4-tetrahydro-7,12-dimethylbenzanthracene (TH-DMBA, 1), 5F-TH-DMBA (3) or 10F-TH-DMBA (5) which after 72 h only afforded aryl to 7-methylene isomer ratios of 14:1, 8:1 and 8:1, respectively.The additional peri interaction between the 7-CH3 and 6-F functions of 4 serves as explanation for the facile acid-catalyzed isomerization of 4 to the sterically least crowded, thermodynamically more stable 7-methylene tautomer 21.