Methyl glycosides, torsional

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. 

Fig. 5. Dependence of 3/C-i jro-2 on the glycosidic torsion angle a> and the C-l/HO-2 dihedral angle 0, as calculated by DFT for methyl a- and /(-D-glucopyranoside mimics (curves (A) and (B), respectively) and. methyl a- and /1-D-mannopyranoside mimics (curves (C) and (D), respectively), all having deoxy functions at C-3, C-4, and C-6. The curves display a strong dependence of 3JCcoh on 0, and a minimal dependency on to.

Lipkind et al. [85, 90] used their above mentioned procedure to calculate the conformations of methyl-P-cellobioside conformations with (p/ f of 30°/—40° (39%), 55°/20° (38%), -20°/-25° (10%), and 30°/175° (13%). They support their calculations with the 3Jch coupling constants across the glycosidic bond. Although these authors use a torsional potential around the glycosidic Cl-Ol bond, they assume that the exo anomeric effect does not exist. This was later criticized by Lemieux et al. [72] (cf. below). 

C2 Z = 8 Dx = 1.457 R = 0.032 for 1,535 intensities. The crystals contain two symmetry-independent molecules, both of which have the C4 conformation, but differ in the orientation of the thio-glycosidic bonds. The torsion angle S-5-C-1-S-1-C-6 is 159° in one molecule and 82° in the other. As in methyl jS-D-ribopyranoside, there are intramolecular hydrogen-bonds between O-2-H and O-4-H. The O-H O direction of the hydrogen bonds is opposite in the two molecules. There are small, but possibly significant, differences in the S-C bond lengths. 

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