18
1
General Principles
Comparison of the populated conformers of substituted and non-substituted aldopentopyranoses illustrates how various types of substitution influence the equilibria (see > Table 3). It
should be noted that Angyal’s interaction energies [125,126] predict the relative stabilities of
the 4 C 1 and 1 C 4 conformers of the unsubstituted aldopentopyranoses remarkably well, except
for those of arabinose, where the 1 C 4 conformer is more stable than expected. It may be that
the interaction energies overestimate the effect of having an axial hydroxyl group at C-2 or
C-4, where there is a gauche interaction with the ring oxygen [139]. The acetylated derivatives
generally agree with these relative stabilities as well [140], once allowance is made for the
greater stabilization of the 4 C 1 conformer of the α-anomer and the greater destabilization of
the 4 C 1 conformer of the β-anomer by the larger anomeric effects ( > Table 2) associated with
methoxy, acetoxy, and chloro groups than with the hydroxyl group. It is interesting that triO-acetyl-β-D-xylopyranosyl fluoride exists entirely in a conformation with the fluoride group
axial [141]. It is likely that 1,3-synaxial repulsive interactions for two acetates are much less
than expected based on hydroxyl or methoxyl values and the same is true to a greater extent
for 1,3-diaxially related sulfates [142].
⊡ Table 2
Other anomeric effects (kJ/mol)
Br a
> 13.4 Cl a
11.1
for OMe c
if O-2 is equatorial
5.6 d,e
12.8 if scaled b
if O-2 is axial
9.0 e
OAc f
5.9
a From tetrahydropyran equilibria [143]. b By the method of Franck [56]. c In methanol [2].
d [144]. e [145]. f [146]
1
General Principles
Comparison of the populated conformers of substituted and non-substituted aldopentopyranoses illustrates how various types of substitution influence the equilibria (see > Table 3). It
should be noted that Angyal’s interaction energies [125,126] predict the relative stabilities of
the 4 C 1 and 1 C 4 conformers of the unsubstituted aldopentopyranoses remarkably well, except
for those of arabinose, where the 1 C 4 conformer is more stable than expected. It may be that
the interaction energies overestimate the effect of having an axial hydroxyl group at C-2 or
C-4, where there is a gauche interaction with the ring oxygen [139]. The acetylated derivatives
generally agree with these relative stabilities as well [140], once allowance is made for the
greater stabilization of the 4 C 1 conformer of the α-anomer and the greater destabilization of
the 4 C 1 conformer of the β-anomer by the larger anomeric effects ( > Table 2) associated with
methoxy, acetoxy, and chloro groups than with the hydroxyl group. It is interesting that triO-acetyl-β-D-xylopyranosyl fluoride exists entirely in a conformation with the fluoride group
axial [141]. It is likely that 1,3-synaxial repulsive interactions for two acetates are much less
than expected based on hydroxyl or methoxyl values and the same is true to a greater extent
for 1,3-diaxially related sulfates [142].
⊡ Table 2
Other anomeric effects (kJ/mol)
Br a
> 13.4 Cl a
11.1
for OMe c
if O-2 is equatorial
5.6 d,e
12.8 if scaled b
if O-2 is axial
9.0 e
OAc f
5.9
a From tetrahydropyran equilibria [143]. b By the method of Franck [56]. c In methanol [2].
d [144]. e [145]. f [146]