Dk. Smith et al., Dendroclefts: Optically active dendritic receptors for the selective recognition and chiroptical sensing of monosaccharide guests, HELV CHIM A, 82(8), 1999, pp. 1225-1241
The enantiomerically pure dendritic receptors with cleft-type recognition s
ites (dendroclefts) of generation zero ((-)-G0), one ((-)-G1), and two ((-)
-G2) (Fig. I) were prepared for the complexation of monosaccharides via H-b
onding. They incorporate a rigid, optically active 9,9'-spirobi[9H-fluorene
] core bearing 2,6-bis(carbonylamino)pyridine moieties as H-bonding sites
in the 2,2'-positions. The dendritic shells in (-)-G1 and (-)-G2 are made o
ut of a novel type of dendritic wedges of the first (8; Scheme 2) and secon
d (13; Scheme 3) generations, which contain only donor O-atoms and are atta
ched to the H-bonding edges of the core via glycine spacers (Scheme 4). The
formation of stable 1 :1 complexes (association constants K-a, between 100
and 600 M-1, T = 298 K; Table 2) between the three receptors and pyranosid
es in CHCl3, was confirmed by LH-NMR and CD binding titrations as well as b
y Job plot analyses. The degree of dendritic branching was found to exert a
profound effect on the stereoselectivity of the recognition processes. The
binding enantioselectivity decreases with increasing degree of branching,
whereas the diastereoselectivity increases. The H-1-NMR analysis showed tha
t the N-H...O H-bonds between the amide NH groups around the core and the s
ugar O-atoms become weakened with increasing dendritic generation, presumab
ly due to steric factors and competition from intramolecular H-bonding betw
een these amide groups and the O-atoms of the dendritic shell. The chiropti
cal properties of the dendroclefts respond to guest binding in a stereosele
ctive manner. Whereas large differential changes are seen in the circular d
ichroism (CD) spectra of (-)-G0 and (-)-G1 upon complexation of the enantio
meric monosaccharides (Figs. 3 and 4), the CD spectra of the higher- genera
tion derivative (-)-G2 respond to a lesser extent to guest complexation (Fi
g. 5). This is indicative of a different binding geometry, more remote from
the core chromophore. With their higher masses, the dendroclefts (-)-G1 an
d (-)-G2 are readily recycled from host-guest solutions by gel-permeation c
hromatography. The strong CD sensory response and the easy recyclability su
ggest applications of chiral dendroclefts as sensors for biologically impor
tant molecules.