Fw. Lichtenthaler et al., ENANTIOPURE BUILDING-BLOCKS FROM SUGARS .16. SOLID-STATE CONFORMATIONS OF 2,6-CIS-SUBSTITUTED AND 2,6-TRANS-SUBSTITUTED DIHYDROPYRAN-3-ONES, Carbohydrate research, 249(2), 1993, pp. 305-326
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.