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'' obtain ed from 2-methylbenzothiazolium and -selenazolium salts and aqueous so dium hydroxide, whereas monomeric structures 3 are confirmed for methi ne 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-me thylbenzothiazole 10a and related compounds are assigned (Z) configura tions on the basis of NOE difference spectra. Molecular mechanics calc ulations predict a slightly higher steric energy for the observed s-ci s conformer of 10a than for the s-trans, indicating a stabilizing S .. . O 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-ci s 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 con version of 10a to 12a by thioacetic acid.