Structure and Conformation of Carbohydrates
1.1
29
⊡ Figure 21
Pseudorotational itinerary for a D-aldopentofuranose
northern half or N while that with phase angles of 180 ± 90° (the lower half of the pseudorotational itinerary) is referred to as the southern half or S. For nucleotides and nucleosides, the
preferred conformations lie in part of the N section with P angles of 0 to 36° and in the S
section with phase angles of 144 to 190° [272,275,276,277,278,279,280]. In the N section,
these correspond to the three conformations with C-3 above the plane of the ring (C3-endo)
while the favored conformations in the S section have C-2 above the plane of the ring (C2endo). Interconversion of N and S conformers is rapid, on the same time-scale as molecular
reorientation [281]. Alteration of substitution and regiochemistry markedly alters the populated conformations [276,280,282,283]. Conformational studies on nucleosides and nucleotides
have shown that the N S equilibrium is biased towards S in 2-deoxyribofuranose derivatives
but is approximately 1:1 for ribofuranoses [273,284].
Lowary, Haddad and coworkers studied methyl 3-O-methyl-α-D-arabinofuranoside computationally at the B3LYP/6–31G* level unsolvated and in aqueous solution using the B3LYP/6–
31+G**SM5.42/BPW91/631G* method and obtained conformer distributions close to those
derived from 3 J H,H values, mainly a mixture of the °T 4 or E 4 conformers (N) and the 2 T 1
or 2 E o conformers (S) [285]. The same group studied all of the methyl aldofuranosides using
1.1
29
⊡ Figure 21
Pseudorotational itinerary for a D-aldopentofuranose
northern half or N while that with phase angles of 180 ± 90° (the lower half of the pseudorotational itinerary) is referred to as the southern half or S. For nucleotides and nucleosides, the
preferred conformations lie in part of the N section with P angles of 0 to 36° and in the S
section with phase angles of 144 to 190° [272,275,276,277,278,279,280]. In the N section,
these correspond to the three conformations with C-3 above the plane of the ring (C3-endo)
while the favored conformations in the S section have C-2 above the plane of the ring (C2endo). Interconversion of N and S conformers is rapid, on the same time-scale as molecular
reorientation [281]. Alteration of substitution and regiochemistry markedly alters the populated conformations [276,280,282,283]. Conformational studies on nucleosides and nucleotides
have shown that the N S equilibrium is biased towards S in 2-deoxyribofuranose derivatives
but is approximately 1:1 for ribofuranoses [273,284].
Lowary, Haddad and coworkers studied methyl 3-O-methyl-α-D-arabinofuranoside computationally at the B3LYP/6–31G* level unsolvated and in aqueous solution using the B3LYP/6–
31+G**SM5.42/BPW91/631G* method and obtained conformer distributions close to those
derived from 3 J H,H values, mainly a mixture of the °T 4 or E 4 conformers (N) and the 2 T 1
or 2 E o conformers (S) [285]. The same group studied all of the methyl aldofuranosides using