Sugar Derivatives Having Fused-ring Systems

Conformations of Sugar Derivatives Having Fused-ring Systems 

Fusion of a second ring to a pyranoid ring often limits the number of possible conformations that the pyranoid part may adopt. For example, trans-fusion of a benzylidene acetal ring at C-4 and C-6 of a hexopyranose system effectively prevents attainment of the IC(d) conformation, although skew or boat conformations may be possible. Thus, a comparative study of the n.m.r. spectra of 36 methyl 4,6-0- 

The crystal structure of a pyranoid ring-system containing a three-atom bridge, namely, N-(p-bromophenyl)-a-D-ribopyranosylamine 2,4-benzeneboronate (29), has been reported. The presence of the 

Although there is some uncertainty as to the conformational effect of one cis-fiised, 5-membered acetal ring, evidence from both i.r.-and n.m.r.-spectral studies clearly indicates that the presence of two such cis-fiised rings forces the cyclohexane or pyranoid ring to adopt a skew conformation. Thus, the pyranoid ring in derivatives of 1,2 3,4-di-0-isopropylidene-j3-L-arabinose and -a-D-galactopyranose (34), and of 2,3 4,5-di-0-isopropylidene-/3-D-fructopyranose (35), and the 

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VII. Conformations of Sugar Derivatives Having Fused-ring Systems.114... 

The synthesis of pyranoid sugar derivatives having an episulfide group fused to the ring involves requirements of molecular geometry more critical than for episulfides on an acyclic chain. Conversion of an epoxide in a locked-ring system to an episulfide by the action of thiourea or thiocyanate ion is diflicult, because reactions of this type involve a trans-fused, cyclic intermediate (91), formation of which requires a [Pg.179]

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