Pyranoid rings, conformation

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... 

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 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>. 

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... 

The acetoxyl-proton resonances were also in accordance with the 1C (d) conformation, namely, one equatorial and two axial acetoxyl resonances for 29, and the converse for 30 (see Table III), since methyl groups of axially oriented acetoxyl substituents of a pyranoid ring generally resonate at lower field than those of similar, equatori-ally attached groups. 

In the application of this conformational rationale simultaneously to the structures of o-L- and jS-D-cardenolides, the a-L- and the jS-D-rhamno-sides (19) and (44) serve to illustrate the hypothesis that a structural correspondence can be obtained, which permits a satisfactory explanation for the fact that a-L and 0-d isomers have potencies which are, approximately, the same. On the basis of the structures shown, the aglycon component is equatorially disposed, both pyranose rings have the same chair form, and the order of substituents on the ring is the same. The only difference between (19) and (44) is a reversal of configuration of substituents on the pyranoid ring, which is tantamount merely to a vertical displacement of... 

Pyranoid-ring sugars exist in rapid conformational equilibrium, and the flexibility of the ring differs little from that of tetrahydropyran. 

A further stereochemical factor (discussed next) that has bearing on the regiochemistry of some photobrominaiions is the relative difficulty with which equatorial hydrogen atoms on pyranoid rings may be abstracted. In the cases of the tri-0-acylated 1,6-anhydrohexopyranoses, reaction is diverted to 06, because the anhydro bridge holds the pyranoid ring in the iC4 conformation, in which H-l and H-5 are both equatorial. 

In the cyclodextrins readily obtainable from starch, the six (a-CD), seven (P-CD) and eight (y-CD) a(l- 4)-linked glucose units are "locked up" in a strait-jacket type belt due to adoption of 4Cj chair conformations of the pyranoid rings and a net of 2-OH — OH-3 hydrogen bonds (73, 74). As this structural rigidity even persists on inclusion complex formation, as exemplified by the three represenatives in Fig. 1 (75 - 78) ... 

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