Pyranoids 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. 

The symbols CA and CE, appended to the names of the structures shown, follow an improved system for indicating the principal conformations of pyranoid sugars and derivatives [H. S. Isbell and R. S. Tipson, J. Ret. Natl. Bur. Sid., 64A, 171 (I860)]. Through the use of symbols, a precise specification of all of the principal pyranoid conformers can be made. 

Of the pyranoid conformations for which Reeves rules do not permit clearcut decisions, none have been studied examples include the a anomers of D-allose, D-altrose, D-idose, and D-gulose. It would be of interest to examine some of these, particularly those for which Lemieux was able, definitely, to assign the conformation of the polyacetates in nonpolar solvents, and those for which Tipson and Isbell have studied the in-... 

P2i2i2i Z = 4 D, = 1.53 R = 0.05 for 789 intensities. The pyranoid conformation is 4, with the C-1 -Br bond axial. A large disproportionation of the C-5-0-5,0-5-C-1 ring bond-lengths was reported that is, 142 and 137 pm. No atomic coordinates were reported. 

The pyranoid conformations of the amino-deoxyoctoses (6) and (7) have been calculated using modified Karplus equations. Enediol anion formation in inososes has been studied by u.v. and n.m.r. spectroscopy. The results suggested that the interpretation of n.m.r. data by Dufaye (Appl. Polym. Symp., 1976, 28, 955) was erroneous and that only small concentrations of enediolate ions are formed. 

Early structural studies were conducted by Kuhn and co-workers who determined the ring size of l-aminodeoxy-o-fructose derivatives by methylation and degradation experiments [63]. For the l-(p-toluidinyl)fructose derivative they proved with this method the consequently most stable pyranoid conformation. 

The pyranoid monosaccharides provide a wide range of asymmetric molecules for study by the c.d. spectroscopist. However, these compounds are not without their difficulties. In aqueous solution, these compounds exist in a complex equilibrium involving the two possible chair conformers of the pyranoses, the furanoses, a and p anomers, and the acyclic form, as well as septanoses for aldohexoses and higher sugars. 

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. 

P2j Z = 2 D = 1.17 R = 0.080 for 3,888 intensities. This is aconfigu-rational analysis of the macrolide antibiotic 23672RP from Streptomyces chryeus. All three sugar residues are pyranoid the conformation of the a-ketose is CX, with Q = 56 pm, 0 = 9° that of the / -D-mycinose (6-deoxy-2,3-di-0-methyl-D-allose) is 4Clt with Q = 59 pm, 0 = 6° and that of the / -L-mycarose (2,6-dideoxy-3-C-methyl-L-riho-hexose) is 1C4, with Q = 53 pm, 0= 177°. The O-5-C-l-O-l-C glycosidic torsion-angles are —71, —87, —83°. The atomic coordinates reported in the paper refer to the opposite enantiomer. 

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... 

While the broad mission of the National Bureau of Standards was concerned with standard reference materials, Dr. Isbell centered the work of his laboratory on his long interest in the carbohydrates and on the use of physical methods in their characterization. Infrared spectroscopy had shown promise in providing structural and conformational information on carbohydrates and their derivatives, and Isbell invited Tipson to conduct detailed infrared studies on the extensive collection of carbohydrate samples maintained by Isbell. The series of publications that rapidly resulted furnished a basis for assigning conformations to pyranoid sugars and their derivatives. Although this work was later to be overshadowed by application of the much more powerful technique of nuclear magnetic resonance spectroscopy, the Isbell— Tipson work helped to define the molecular shapes involved and the terminology required for their description. 

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... 

Similarly, pronounced decreases of inhibitory activity were observed with pyranoid inhibitors of other configurations upon deoxygenation, in conformity with the general picture provided by the modified substrates as mentioned earlier. 

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>. 

---