Conformational behaviour of medium-sized rings. Part 9. Disalicylides and trisalicylides

Abstract

The temperature dependences of the ¹H n.m.r. spectra of di-o-thymotide (13) and di-o-carvacrotide (14) have been interpreted in terms of ring inversion (12a)⇌(12b) between enantiomeric boat conformations. Comparison of the ΔG^‡ values (17.7 and 18.4 kcal mol^–1, respectively) for this conformational change suggests that it takes place by a pseudorotational process involving folded boat conformations (17) in which the substituents (methyl and isopropyl, respectively) on C-3 enter into 1,5-interactions with the carbonyl oxygen atoms in the eight-membered rings at the rate-determining transition states. The temperature dependences of the ¹H n.m.r. spectra of tri-o-cresotide (6) and tri-m-cresotide (7) provide further evidence that a mechanism involving pedalling of trans-ester linkages through the mean plane of the twelve-membered ring accounts for the conformational changes between enantiomeric propeller (4) and helical (5) conformations. The ΔG^‡ values for interconversion and inversion processes of eight- and twelve-membered ring compounds show an informative dependence upon the steric demands of the ortho-substituents on the aromatic rings.