Tricyclic [10]annulenes. Part 5. Phenol–keto tautomerism in the 2- and 5-hydroxy derivatives of 7b-methyl-7bH-cyclopent[cd]indene

Abstract

The 2- and 5-hydroxy derivatives of the 10π aromatic system 7b-methyl-7bH-cyclopent[cd]indene have been synthesised and their properties compared. The 2-hydroxy isomer (7) exists entirely in the non-aromatic keto form (8)(+1.41) but its lithium enolate (13; M = Li)(–1.45), like its methyl(–1.51) and trimethylsilyl (–1.51) ethers, (14) and (12), sustains a substantial diamagnetic ring current. The ketone (8) is methylated on oxygen and on carbon, at the C-2a position only, the ratio varying with conditions in the expected manner. In contrast, the 5-hydroxy isomer (9) exists entirely in the annulenol form (–1.49), as does the 5-hydroxy-diester (25)(–1.15) and the 5-hydroxy-dialdehyde (32)(–1.00). These three compounds are thus the first [10]annulenols to be isolated.