The Deoxy-hexose sugars have four potentially acidic sites (i.e.,
C1-HO1,
C2-HO2,
C3-HO3 and
C4-HO4, see Figure 1c). Based on the
predicted acidity values, the OH groups in equatorial position in
CH3-pentose sugars have the weakest acidity site. For
instance, C4-HO4 site of L-fucose and
C2-HO2 site of L-rhamnose have the
acidity values of 355 and 369 kcal.mol-1,
respectively. The strongest acidic site in L-rhamnose is
C3-HO3 with ∆Hacid = 349 kcal.mol-1 and in L-fucose it is
C1-HO1 with∆Hacid = 346 kcal.mol-1 (Figure
1a). While, the calculated acidity values
(∆Hacid ) of
C1-HO1,
C2-HO2,
C3-HO3 and
C4-HO4 sites in aldo-pentose sugars are
in the range 343- 355 kcal.mol-1 (Table 2). As seen in
Table 2, the strongest acidic sites are
C2-HO2 in L-lyxose,
C3-HO3 in D-ribose, and
C4-HO4 in L-arabinose with the∆Hacid values of 343, 345, 348
kcal.mol-1, respectively (Figure 1b). It is concluded
that OH group in the axial position in aldo-pentose is the strongest
acidic site and thus the most favored deprotonation site. Moreover, the
acidity of ethanol and 2-propanol are 378.3 and 375.1
(kcal.mol-1) in the gas phase, respectively [42].
In comparison with these alcohols, the acidity values of methyl-pentose
and aldo-pentose sugars are 346-369 and 343-355
kcal.mol-1 (Table 2); these results suggest the role
of H-bonds in increasing the acidity of Deoxy-hexose sugars. As shown in
Scheme 1 (as a sample) and Figure 2, the conjugate bases of these sugars
are stabilized via intramolecular H-bonds, which are studied in details
using AIM and NBO analyses in this study. The geometries of the
conjugate bases of these sugars are discussed in the following section.