Syntheses of 6,7‐Benzotropolone by Using Ring‐Closing Metathesis Variants Obviating a Strongly Acidic Hydrolysis Thereafter

摘要

Abstract A Wittig reaction of 3‐hydroxyphtalide (11) gave ortho‐styrene‐2‐carboxylic acid (12). Its Weinreb amide 13 acylated heterosubstituted methyllithiums. This led to aryl “methyl” ketones whose sp3‐carbon was substituted by Cl+OMe (in 14 a), 2×Cl (in 14 b), OtBu (in 17 a), SPh (in 17 b) or SO2Ph (in 17 c). The enolates of these ketones were C‐allylated. This furnished the benzene‐fused nona‐1,8‐dienones 7 a–b and 9 a–c, respectively. Ring‐closing metatheses provided the corresponding benzocycloheptadienones 8 a‐b and 10 a–c. Their ketone moieties were α,α‐disubstituted (in 8 a–b) or α‐monosubstituted (in 10 a–c). Each substitution pattern allowed to generate the hydroxyenone motif of the corresponding benzocycloheptatrienone—or 6,7‐benzotropolone—6 a: The MeO‐ and PhS‐containing benzocycloheptadienone 8 a gave 6,7‐benzotropolone methyl ether (15) by sulfoxide formation and pyrolysis; 15 was demethylated with BCl3. The same intermediate resulted from the dichlorobenzocycloheptadienone 8 b and NaOMe. The benzocycloheptadienone 10 a was de(tert‐butylated) with formic acid; a subsequent Pfitzner‐Moffatt oxidation completed 6,7‐benzotropolone (6 a) once more. The PhS‐containing benzocycloheptadienone 10 b was α‐acetoxylated with Pb(OAc)4; the resulting O,S‐acetal gave 6,7‐benzotropolone (6 a) by hydrolysis. The PhSO2‐containing substrate 10 c was deprotonated whereupon an oxidation with Me3Si−O−O−SiMe3 gave 6,7‐benzotropolone (6 a).