Reactivities of Stable Rotamers. XXII. Lewis Acid-Catalyzed Reactions of 3-(1,4-Disubstituted 9-Triptycyl)-3-methylbutanoyl Chloride: Isolation of Rotameric Ketones and Competition between Cyclizations to Substituted and to Unsubstituted Benzene Rings

Abstract

Abstract Rotational isomers of 3-(1,4-disubstituted 9-triptycyl)-3-methylbutanoic acid, where the substituent is either a methoxyl or a methyl, have been prepared, starting from 1,4-disubstituted 9-(1,1-dimethyl-3-butenyl)triptycene. Lewis acid-catalyzed reactions of the ap-acid chloride afforded corresponding ketones expected from the intramolecular Friedel–Crafts reaction. The sc form of the 1,4-dimethoxy compound gave a ketone that was formed by the reaction of the unsubstituted benzene ring and a lactone which was derived by the attack of the acylium ion on the methoxy-oxygen followed by demethylation. The sc form of the 1,4-dimethyl compound afforded a ketone which was expected from cyclization to the unsubstituted benzene ring and another ketone that was derived from the ipso attack of the acylium ion followed by 1,2-methide migration. In both cases, the ketone derived from the ap form of the acid chloride was different from that obtained from the sc. The barriers to isomerization of the ap-ketone to the sc-ketone was 29.6 kcal mol−1 at 100.5 °C and 25.6 kcal mol−1 at 56.0 °C for the methoxy and the methyl compounds, respectively. In both cases, the ketone derived from the sc-form was more stable than that from the ap, the sc/ap ratio at the equilibrium being 20.0 (56.0 °C) and 5.9 (100.5 °C), respectively, for the methyl and the methoxy compounds. In the reaction of the sc form, the competitive results between the cyclization toward the unsubstituted benzene ring and that toward the substituted vary according to the solvent and the Lewis acid used.