ENANTIOPURE BUILDING-BLOCKS FROM SUGARS .16. SOLID-STATE CONFORMATIONS OF 2,6-CIS-SUBSTITUTED AND 2,6-TRANS-SUBSTITUTED DIHYDROPYRAN-3-ONES

2,6-Dihydropyran-3-ones carrying substituents at C-2 and C-6 in cis-ar rangement invariably adopt half-chair conformations in which the ring oxygen and the carbon atom next to the carbonyl group are above and be low, respectively, the plane formed by the other four carbon atoms, i. e., the H-2(o) or H-o(2) conformation. In the case of a trans-arrangem ent of 2,6-substituents, the geometry of the pyranoid ring falls into the B(o,6) half arrow right over half arrow left E(o) half arrow right over half arrow left H-2(o) or the inverse (o,6)B half arrow right ov er half arrow left (o)E half arrow right over half arrow left H-o(2) s ection of the conformational cycle, depending on the absolute configur ation of the compound; for two of the dihydropyranones, 4B and 6, a un ique skew-boat (SB(o,6)) conformation, fixed between the B(o,6) and E( o) geometries, was ascertained, which previously has only been observe d for pyranoid enelactones.