Sugar decorated receptors for chiral anions - Carbohydrate Chemistry: Chemical and Biological Approaches - page 457

titrations with Boc-N-phenylalanine and Boc-N-valine, good enatio-
selectivities were found. The above examples support the assumption
that design of proper anion binding pocket is important for chiral
recognition.
As shown above, most of the receptors analysed here contained
(thio)urea functionality as an anion binding pocket. Recently, we
explored anion binding properties of diindolylmethane-based anion
receptors (27 and 28) decorated with D -glucuronic acid 30 (Fig. 15). The
binding anities of these compounds were measured by 1 H NMR titra-
tions in DMSO-water (99.5 : 0.5 v/v) solution. Titrations of model chiral
carboxylate-mandelic acid, with 27 gave satisfactory results. The meas-
ured binding constants were 233 and 119 M 1 (K R /K S = 1.95) for (R)- and
(S)-mandelate, respectively. During 1 H NMR titration of mandelic acid
anions, significant chemical shift changes can be observed not only for
protons belonging to anion binding pocket but also for those belonging
to the glucopyranose ring as well as for acetyl groups. Such observation
provides important support for the crucial role of sugar-anion inter-
actions in chiral recognition. On the other hand, anion induced chemical
shift changes for sugar H's were different for both enantiomers (Fig. 16).

N
H

NH HN

O

NH

NH HN AcO

AcO

O

O

O

O O

OA OAc

AcO

AcO
AcO

OAc

OAc

OAc

OAc AcO

27 28

Fig. 15 Glucuronic acid based anion receptors 27 and 28.

Fig. 16 Stacked plots from 1 H NMR titrations of host 27 with S-mandelate (left) and
R-mandelate (right). H 5 protons in glucopyranuronic ring, H b bridging proton in

diindolylmethane in DMSO-d 6 þ0.5% H 2 O.