Rotational isomerism in 3,4‐alkylenedioxy‐2,5‐bis[di(tert‐butyl)hydroxymethyl]thiophenes>

In 3,4‐ethylenedioxy‐2,5‐bis[di( tert ‐butyl)hydroxymethyl]thiophene there are three rotational isomers, syn,syn ( SS ), anti,syn ( AS ) and anti,anti ( AA ) ( syn , free; anti , hydrogen‐bonded), differing in the number (zero, one or two) of alcohol hydrogen atoms intramolecularly hydrogen bonded to the oxygen atoms of the bridge. When a methyl group is introduced into the bridge the AS rotamer can be distinguished from SA [where the first character indicates the orientation of the —C( t ‐Bu) 2 OH group closer to the substituent] by means of a 1 H NOESY experiment. Forms with ‘free’ OH groups are favoured by hydrogen‐bonding solvents, but the AS:SA ratio for the methyl derivative is solvent independent, slightly favouring the AS isomer. Whatever the solvent, the methyl group has no significant effect upon the equilibrium rotamer ratios. Rotation barriers for the various equilibria are of the order of 20 kcal mol −1 .