Solid-state conformations of 2,6-cis- and 2,6-trans-substituted dihydropyran-3-ones

2,6-Dihydropyran-3-ones carrying substituents at C-2 and C-6 in cis-arrangement invariably adopt half-chair conformations in which the ring oxygen and the carbon atom next to the carbonyl group are above and below, respectively, the plane formed by the other four carbon atoms, i.e., the 2Ho or oH2 conformation. In the case of a trans-arrangement of 2,6-substituents, the geometry of the pyranoid ring falls into the Bo,6<-->Eo<-->2Ho or thet inverse o,6B<-->oE<-->oH2 section of the conformational cycle, depending on the absolute configuration of the compound; for two of the dihydropyranones, 4B and 6, a unique skew-boat (SBo,6) conformation, fixed between the Bo,6 and Eo geometries, was ascertained, which previously has only been observed for pyranoid enelactones.