Methine bases in the benzothiazole, benzoselenazole, and quinoline series, and geometry and conformational preferences of their acyl derivatives

Dimeric structures 2a–c are confirmed for the "methine bases" obtained from 2-methylbenzo-thiazolium and -selenazolium salts and aqueous sodium hydroxide, whereas monomeric structures 3 are confirmed for methine bases similarly derived from 2-methylquinolinium salts. The latter monomers dimerize slowly on standing. Monomeric methine bases 6–9, too hindered to dimerize, are described. 2-Acetonylidene-2,3-dihydro-3-methylbenzothiazole 10a and related compounds are assigned (Z) configurations on the basis of NOE difference spectra. Molecular mechanics calculations predict a slightly higher steric energy for the observed s-cis conformer of 10a than for the s-trans, indicating a stabilizing [Formula: see text] interaction in the s conformer that need only be a few kJ mol −1 in magnitude. For the quinolines 13a and b, the (E) configuration/s-cis conformation is favoured. The dielectric absorption method provides a rotational energy barrier of approximately 60 kJ mol −1 for acetyl group relaxation in 10a. Oxygen–sulfur exchange is observed in the conversion of 10a to 12a by thioacetic acid.