Mannopyranoside, methyl 3 anomer

Similar results have been obtained with the tetra-O-trimethylsilyl derivatives of the anomers of methyl glucopyranoside, methyl galacto-pyranoside, and methyl mannopyranoside. The 17 ev mass spectrum of the tetra-O-trimethylsilylated form of the anomers of methyl glucoside are shown in Figure 7. Again the traces are similar, but intensity differences are discernible. The ratio (377/361 )i7ev has proved most useful. 

Both of the steric effects mentioned above are operative in the o-manno-pyranosides. Thus, in methyl a-o-mannopyranoside, the 4,6-borate complex is hindered, but the 2,3-complex is not. The reverse situation obtains for the /3 anomer. Since the 2,3-complex is formed more readily than the 4,6-complex, the a anomer has the greater Mg value. 

The methylsulfonyloxy group at C-4 of methyl 2,3,6-tri-O-benzoyl-4-0-(methylsulfonyl)-a-D-mannopyranoside was found to be resistant to nucleophilic displacement,344 probably due to the fi-trans axial group at C-2 which hinders approach of the nucleophile to C-4. This reasoning was used to explain the observation that, whereas 1,2,4,6-tetra-0-benzoyl-3-0-p-tolylsulfonyl-/3-D-glucopyranose (61) undergoes benzoate displacement at C-3 to give penta-0-benzoyl-/8-D-allopyranose, the a-D anomer (62) does not.258,10 (No displacement was observed with either of the corresponding furanose anomers.)... 

With paraldehyde and sulfuric acid at room temperature, methyl 4,6-0-ethylidene-a-D-mannopju anoside (32%) and methyl 2,3 4,6-di-0-ethyli-dene-a-D-mannopyranoside (55%) were isolated. The corresponding (8-d anomers have also been prepared. Assuming that the CA conformation is the most probable, the 4,6-acetals of methyl mannopyranosides are readily available model compounds for comparing the reactivities of equatorial (C-3) and axial (C-2) hydroxyl groups in the pyranoid ring. 

The effect of Q-alkylation on the anomeric solution equilibrium of D-glucopyranose and of methyl D-glucopyranoside has been studied. A considerable increase in the proportion of a-anomer for 2-Q-methyl-D-glucopyranose was observed, but there was no change for methyl D-glucopyranoside on Q-alkylation. A method has been reported for the preparation of all of the methyl ethers of methyl 2-acetamido-2-deoxy-a-D-glucopyranoside. Methylation of methyl (methyl o-D-mannopyranosid)uronate, methyl (methyl o-D-glucopyranosid)uronate, and methyl (methyl p-D-... 

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