Dendroclefts: Optically active dendritic receptors for the selective recognition and chiroptical sensing of monosaccharide guests

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.