Page 77 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Carbohydrates 385

SCHEME 9 Formation of acyclic aldoses from diothioacetals.

these molecules possess one nucleophile acceptor (C-1 of
the acyclic form; the carbon atoms in positions 2, 3, 4,
5, and 6 are not counted, because their susceptibility to
nucleophiles by S N reactions is much smaller than that
of C-1, which undergoes carbonyl addition reactions) and
(2) the nucleophilicity of the attacking oxygen atom (as
determined from tables) is 4. Saccharide hydrazones and
oximes have similar NQ values, even though they possess
additional nucleophiles (NH or OH groups) whose nucle-
ophilicity is enhanced by adjacent nitrogen atoms. This is
because these additional nucleophiles are not suitably sit-
uated to attack the nucleophile acceptor (the C N group).
Saccharide phenylosazones, on the other hand, are much
more reactive, and this is reflected by the high NQ value of
FIGURE 10 Envelope and twist conformations of furanoses.
2.6. They possess two nucleophile acceptors, namely, C-1
and C-2, which form part of C NR systems, and contain a
derivatives possess more nucleophilic species than nucle- nitrogen atom, which is suitably situated to attack a nucle-
ophile acceptors (e.g., there are five nucleophilic oxygen ophile acceptor (the C C C N group present in one of
atoms vs. one carbonyl group in the acyclic form of an al- the tautomeric forms in equilibrium). The nucleophilicity
dohexose); accordingly, the number of nucleophile accep- of this atom is enhanced by the α effect of the adjacent
tors exerts a greater, and sometimes controlling, influence nitrogen atom, giving it an n value of 6. Finally, if it is de-
on the capacity of the molecule to cyclize, whereas the sired to obtain compounds that are even more reactive, an-
nucleophilicity of the attacking groups plays a lesser role. other nucleophile acceptor could be added to the osazone
For this reason, integers are used to designate the number molecule, for example, a carbonyl group in the saccharide
of nucleophile acceptors (such groups as C O, C NR, moiety or in the hydrazone residue, which would bring
C C C O, and C C C NR, which can undergo ad- their NQ values to 3.6. Examples of such highly reactive
dition reactions), and fractions are used to designate the molecules are L-ascorbic acid bis(phenylhydrazones) and
nucleophilicity (n) of attacking species (e.g., OH or NH glycosulose bis(benzoylhydrazones).
groups) that are suitably situated to react with a nucle-
ophile acceptor. The value of n, which is a measure of the 3. Nucleophilic Substitution Reactions of the
affinity of the nucleophile to a particular acceptor, can be Anomeric Carbon Atom
obtained from tables or can be determined experimentally
from kinetic measurements using a Hammett-like equa- The anomeric carbon atom is the most reactive carbon
tion, namely, log(k/k 0 ) = ns, where s is the sensitivity of atom in furanoses, pyranoses, and their derivatives. Al-
the nucleophile acceptor to the attacking nucleophile. Free though it is less susceptible to nucleophilic attack than
aldoses have an NQ value of 1.4, which signifies that (1) the carbonyl group of acyclic sugars (because it under-
goes nucleophilic substitution instead of addition), it is
significantly more reactive than the remaining hydroxyl-
bearing carbon atoms. The affinity of the anomeric carbon
atom to nucleophiles is attributed to its capacity to form
resonance-stabilized carbonium ions that can undergo di-
rect reactions via S N 1 mechanisms or synchronous reac-
FIGURE 11 Acetals hemiacetals, and their sulfer (thio)
derivatives. tions by S N 2 mechanisms. The rates of these reactions