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

Conformational Pathways of Saturated Six-Membered Rings. A Static and Dynamical Density Functional Study

The conformation of the six-membered ring of pyranosyl sugars has pronounced effects on the physical and chemical properties of carbohydrates. We present a method to determine key features of the potential energy surfaces, such as transition states associated with the inversion pathways of the model compounds cyclohexane, tetrahydropyran, p-dioxane, m-dioxane, s-trioxane, and 2-oxanol. Finally, we make the first determination of the pathways for inversion of penta-O-methyl-alpha-D-glucopyranose and penta-O-methyl-beta-D-glucopyranose. For both anomers, a transition state with five coplanar atoms with appreciable (O)E character was found. The method is based on constrained Car-Parrinello ab initio molecular dynamics, as implemented in the projector augmented-wave (PAW) method. The constraints are derived from the normal modes of six-membered rings and are described in terms of the canonical conformations (1)C(4) chair, (1,4)B boat, and (O)S(2) skew-boat. The PAW derived trajectories are in agreement with previous suggestions in the literature that pseudorotation is an important feature of such conformational interconversions. The dynamic nature as well as the internal coordinate-based constraints provide a method which can reliably accommodate pseudorotation. To determine semiquantitative energies, we recalculate key conformations using standard quantum mechanical calculations while keeping the ring dihedral angles frozen at their values found in the dynamics. In all cases where experimental barriers are known, our results are in excellent agreement.

Synthesis of optically active 2-alkoxy-2H-pyran-3(6H)-ones. Their use as dienophiles in Diels-Alder cycloadditions

Optically active 2-alkoxy-2H-pyran-3(6H)-ones (4a-d) were synthesized in one step by the tin(IV) chloride-promoted glycosylation and rearrangement of the 2-acetoxy-3,4-di-O-acetyl-D-xylal (3) prepared from D-xylose (1). The absolute configuration of the new stereocenter at C-2 was determined by chemical transformation of the dihydropyranones 4a and 4b into the known alkyl pentopyranosides (7a and 7b, respectively). Also, from (1)H NMR experiments using a chiral ytterbium shift reagent, the enantiomeric excesses for 4a (>86%) and 4b (>77%) were established. Enantiomerically pure 4c and 4d were obtained by reaction of 3 with chiral 2-octanol (R and S, respectively). Dihydropyranones 4a-d were employed as dienophiles in Diels-Alder cycloadditions with 2,3-dimethylbutadiene and butadiene. Under thermal conditions, only moderate yields (approximately 50%) of cycloadducts 9a-c and 10a were respectively obtained with good diastereofacial selectivity (>80%). Optimized Lewis acid promoted cycloadditions led to 9a-d and 10a,c in higher yields (approximately 80%) and with higher diastereoselectivities (>94%). The major products were formed by approach of the dienes from the less hindered face of the dihydropyranones, and the minor products (such as 11a) were formed by addition from the opposite side. Furthermore, cycloadduct 9a was stable in an alkaline solution, whereas 11a underwent epimerization under the same conditions.