Pyranoid rings

The features common to 2-deoxy ribose 5-phosphate and the two 3-deoxy hexulosonic acid 6-phosphates are that they all have a deoxy group next to the carbonyl group and that they can form only the furanoid ring (35, 78a, 80a) as long as the phosphate group is attached to the terminal, primary alcohol. However, they can all assume the pyranoid ring... 

Ring expansion of a furanoid ring. . 146 Ring expansion of a pyranoid ring. 148... 

C16H2207 l,5-Anhydro-2,3,4-tri-0-benzoylxylitol (ATBXYL10)114 PI Z = 2 Dx = 1.32 R = 0.053 for 3,536 intensities. The crystal structure contains centrosymmetrically related d and l enantiomers. The pyranoid conformation is an almost ideal (d) [1C4(l)]i withQ = 60 pm, 0=1°, with normal bond-lengths and valence-angles. The benzoyl groups are equatorial, with their planes approximately normal to the mean plane of the pyranoid ring. 

P212121 Z = 4 D = 1.25 R = 0.069 number of intensities not reported. This analysis was to confirm a configuration derived from n.m.r. data. The pyranoid rings are slightly distorted 4Q, due to fusion to the trioxacyclooctane ring, which has an almost ideal, boat-chair conformation. The overall molecule has a convex, sickle conformation. There is an error in the atomic coordinates reported, which do not correspond to the structure given in the paper. 

Ci9H2408Ss Ethyl 3,7-anhydro-6,8-0-benzylidene-4-deoxy-2-(ethyl-enedithio)-D-talo-2-octulosonate (ABTOET)265 P212121 Z = 4 Dx = 1.359 R = 0.045 for 1,520 intensities. The structure contains a fused pyranoid ring and a dioxolane ring. The conformation of the pyranoid part is a distorted 6C3, with Q = 57 pm, 6= 20°,

chair conformations of the dioxolane and the dithiene rings... 

A report on the acetylation of 2,7-anhydro-/3-D-aZtro-heptu-lopyranose (sedoheptulosan) with acetic anhydride further illustrates the low reactivity of an equatorial hydroxyl group on abridged, pyranoid ring under controlled conditions, 21% of the 1,3,5-tri-O-acetyl derivative may be isolated.60... 

An interesting observation was made by Ferrier and coworkers61 during the acylation of 2,4-boronic esters of methyl a- and /3-d-xylopyranosides, in both of which the pyranoid ring must be in the C.,(d) conformation, and the 3-hydroxyl group must be axially attached. On treatment with benzoyl chloride in pyridine, the a-d-xyloside derivative gave 37% of the 3-benzoate and 19% of unreacted starting-material, but the /3-D-xyloside derivative yielded... 

Figure 3. The conformational sphere for pyranoid rings. The perfect chairs are at the north and south poles (0=0 and 180 , respectively). The boat and skew (B and S designations) at the equator permit pseudorotation that is slightly hindered, at least for cyclohexane. The envelopes, E (also called sofas and half-boats), and half-chairs, H, are not observed for rings coiqposed of saturated carbon and oxygen atoms, but are iiqportant forms for rings with unsaturated carbon atoms. The aiqplitude of puckering corresponds to the radius of the sphere.

To date, only a few solution calculations for carbohydrates have been attempted (one such study of mannitol and sorbitol is described in the chapter by Grigera in this volume), but the results of these early studies bear out the expectation that solvation effects in carbohydrate systems can be both significant and difficult to predict. In the case of pyranoid rings, molecular solvation is further complicated by the close juxtaposition of these groups in essentially fixed relative orientations (assuming no conformational changes in the ring). Under such circumstances, molecular stereochemistry could play important physical roles, as is... 

The presence of aqueous solvent was found to have little effect upon the mean structure of the pyranoid ring in these MD simulations, with only slight deviations in the time-averaged structure away from that observed in vacuum simulations or in the crystallographic diffraction experiments ( ). However, the presence of the solute had substantial effects upon the average "structuring" of the solvent. Figure 7 displays a pair distribution function g(r), defined as (27)... 

Fig. 6. Superposition of ligands 1-deoxynojirimycin 2 (yellow, PDB 2J77) and castanospermine 8 (green, PDB 2CBU) bound to Thermotoga maritima p-glucosidase. The ring nitrogen atoms, all hydroxyl groups in the pyranoid rings, as well as 0-1 (8) and the primary 0-6 (2), respectively, are closely matched.

Fig. 7. Superposition from two different viewpoints of the ligands castanospermine, 8 (gray, PDB 2PWG) and 1-deoxynojirimycin 2 (green, PDB 2PWD) bound to trehalulose synthase from Pseudomonas mesoacidophila MX-45. The lateral inversion of 2 relative to the pyranoid ring of 8, with 0-6 of 2 matching 0-6 (corresponding to 0-2 in carbohydrate numbering) of 8 is clearly visible.

The force field calculations of the enthalpies (AAiT) of the four hemiketals of 6-methylpyrano[2,3-/ ]dioxane-8a-ols clearly indicate that the /3-m-fused link is the most favored, followed by the a-cis- and /3-/ra r-isomers, while the a-/ra r-compound, due to the pyranoid ring being forced into 4 conformation with an axial methyl group, is the least likely to be formed <1991MI235>. 

It is often possible to predict the reactivity of a chlorosulfonyloxy group by a consideration of the steric and polar factors affecting the formation of the transition state,27-28 as indicated in Section 11,1 (see p. 227) for nucleophilic-replacement reactions of sulfonic esters of carbohydrate derivatives. Thus, it has been found that the presence of a vicinal, axial substituent or of a (3-trans-axial substituent on a pyranoid ring inhibits replacement of a chlorosulfonyloxy group also, a chlorosulfate group at C-2 has been observed to be deactivated to nucleophilic substitution by chloride ion. 

Sedoheptulosan monohydrate, the only C7 sugar derivative whose crystal structure has been published, has two fused rings.27 One of these is a pyranoid ring in a chair conformation involving -C-2-C-3-C-4-C-5-C-6-0-6-, in which C-l, 0-3, and 0-4 are equatorially... 

A sugar derivative in which the iodine is covalently bound is methyl (methyl 4,5-di-0-benzoyl-3,7-dideoxy-7-iodo-a-D-arabtno-heptulopyranosid)onate.39 The pyranoid ring assumes that chair conformation in which the anomeric methoxyl group is axially attached and the (bulky) benzoyl groups are equatorially attached. 

The predictions for the conformations of pyranoses agree surprisingly well with the conformations actually existing in their crystals, despite the fact that the pyranoid rings are always somewhat distorted in the crystal. Perhaps the structure in solution is closer to the ideal... 

In a different synthesis of 1,5-anhydrohexitols from a compound having a pyranoid ring, Lehmann and Friebolin31 treated 1,5-anhydro-2-deoxy-D-arafemo-hex-l-enitol (D-glucal) (20) with a-toluenethiol in the presence of light, and obtained l,5-anhydro-2-S-benzyl-2-thio-D-mannitol (21) and the epimeric anhydro-D-glucitol in equal amounts. 

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