Structure and Conformation of Carbohydrates
1.1
13
proven to be difficult. The current theoretical view, obtained using calculations at the MP2/ccpV5ZMP2/cc-pVTZ level, is that there are two minima, the most stable being the C s symmetric envelope conformer ( 1 E) (15), the next the C 2 symmetric twist conformer ( 2 T 3 ) (16),
0.55 kJ mol −1 higher, with the saddle point on the pseudorotational itinerary being a C 1 symmetric conformation 0.78 kJ mol −1 above the global minimum [95]. Older methods obtain
the twist form as more stable [96]. In the solid state, the twist conformer is observed in both
104 and 140 K X-ray [97] and 5 K high-resolution neutron powder [98] diffraction studies.
In solid tetrahydrofuran, the pseudorotational motion is a large-amplitude ring deformation
vibration with an amplitude of about 140 cm −1 [99]. The pseudorotational barrier increases in
water by about 1.0 kJ mol −1 to about 2.1 ± 0.8 kJ mol −1 [100]. When substituents are present,
their requirements become more important than the inherent tetrahydrofuran preferences. For
instance, the microwave spectrum of 3-hydroxytetrahydrofuran shows that it exists in a 2 E
conformation having an axial hydroxyl group hydrogen bonded to the ring oxygen (17), with
no evidence for pseudorotation both in the gas phase [101] and in aqueous solution [102].
1.3 Conformations of Monosaccharides
1.3.1 Conformations of Acyclic Carbohydrates
In the solid state, most acyclic carbohydrate derivatives adopt a conformation having the carbon atoms in an extended, planar zig-zag arrangement, unless there are parallel, 1,3-steric interactions between oxygen atoms (Hassel–Ottar effect), written in abbreviated form as
O//O interactions [103,104,105,106] ( > Fig. 10). In the last few years, a number of examples
have been observed where either O//O or CO interactions are present, particularly for compounds with chains longer than five carbons [107,108,109,110,111,112,113,114]. It is clear
that the magnitude of the destabilizing effect associated with these interactions is less than in
six-membered rings where torsional and bond angle relaxation is more difficult energetically [108,112]. O-Acetyl derivatives are more likely to adopt conformations with O//O or CO
interactions than unsubstituted compounds [115].
1.1
13
proven to be difficult. The current theoretical view, obtained using calculations at the MP2/ccpV5ZMP2/cc-pVTZ level, is that there are two minima, the most stable being the C s symmetric envelope conformer ( 1 E) (15), the next the C 2 symmetric twist conformer ( 2 T 3 ) (16),
0.55 kJ mol −1 higher, with the saddle point on the pseudorotational itinerary being a C 1 symmetric conformation 0.78 kJ mol −1 above the global minimum [95]. Older methods obtain
the twist form as more stable [96]. In the solid state, the twist conformer is observed in both
104 and 140 K X-ray [97] and 5 K high-resolution neutron powder [98] diffraction studies.
In solid tetrahydrofuran, the pseudorotational motion is a large-amplitude ring deformation
vibration with an amplitude of about 140 cm −1 [99]. The pseudorotational barrier increases in
water by about 1.0 kJ mol −1 to about 2.1 ± 0.8 kJ mol −1 [100]. When substituents are present,
their requirements become more important than the inherent tetrahydrofuran preferences. For
instance, the microwave spectrum of 3-hydroxytetrahydrofuran shows that it exists in a 2 E
conformation having an axial hydroxyl group hydrogen bonded to the ring oxygen (17), with
no evidence for pseudorotation both in the gas phase [101] and in aqueous solution [102].
1.3 Conformations of Monosaccharides
1.3.1 Conformations of Acyclic Carbohydrates
In the solid state, most acyclic carbohydrate derivatives adopt a conformation having the carbon atoms in an extended, planar zig-zag arrangement, unless there are parallel, 1,3-steric interactions between oxygen atoms (Hassel–Ottar effect), written in abbreviated form as
O//O interactions [103,104,105,106] ( > Fig. 10). In the last few years, a number of examples
have been observed where either O//O or CO interactions are present, particularly for compounds with chains longer than five carbons [107,108,109,110,111,112,113,114]. It is clear
that the magnitude of the destabilizing effect associated with these interactions is less than in
six-membered rings where torsional and bond angle relaxation is more difficult energetically [108,112]. O-Acetyl derivatives are more likely to adopt conformations with O//O or CO
interactions than unsubstituted compounds [115].