The enantiomers of the title compounds were separated for the first time by liquid chromatography (LC) on optically active sorbents analytically or preparatively. They were characterized mainly by their retention factors and by circular dichroism. Barriers to thermal cleavage of the C(sp(3))-O bond were mainly determined by a combination of polarimetry and LC without preparative enrichment of enantiomers, These Delta G(1)(not equal)-values (81.5 - 99.2 kJ/mol) are discussed with reference to substitution; it is concluded that the C-O bond in oxazines (Table IV, X = ($) over bar N) is broken less easily than the one in pyrans (X = CH). These thermal enantiomerizations (Scheme 1) are examples of fast valence isomerizations.