Fig. 2.1.6. Synthesis of (Sa)-10 and (all-Sa)-11. (a) t-BuLi, THF, -78 °C, then ZnCl2, -78 °C to rt, 2 h; (b) 2-chloro-5-trimethylsilylethynyl-pyridine, Pd2dba3-CHCl3, t-Bu3P, THF, 83%; (c) KF, MeOH, 97%; (d) NaH, DMF, MOMCl, 80%; (e) n-BuLi, Et2O, rt, then I2, Et2O, -78 °C, 42% monoiodo and 40% diiodo compound;

Fig. 2.1.7. 'H NMR spectra (500.1 MHz, [11 ]0 = ca. 10 mmol L-1 in THF-d8/CD3CN at 300 K) (a) (all-Sa)-11, (b) (all-Sa)-11 + 1/2 equiv. [Cu(CH3CN)4]BF4, (c) (all-Sa)-11 + 1/2 equiv. [Ag(CH3CN)2]BF4.

Fig. 2.1.8. Positive ESI MS of ca. 5 x 10"4 mol L"1 solution of [Ag{(all-Sa)-11}2]BF4 complex in CH2Cl2/CH3CN.

solutions. We first performed qualitative NMR studies with alkyl glycosides, for example n-octyl- and methyl hexopyranosides. These preliminary experiments gave very promising results indicating our aggregates can indeed function as receptors for monosaccharide derivatives [7]. Unfortunately, the solubility of all the dinuclear

Fig. 2.1.9. Crystal structure of (A,A)-[Zn2{(Sa)-10a}3](BF4)4 ■ 2.5 THF-5 CH3CN (counter-ions and solvent molecules omitted).

Fig. 2.1.10. Energy minimized structures of self-assembled [Cu{(all-Sa)-11}2]+-complex (MMFF-minimized, left) and of analogous covalently assembled tetra (BINOL) substituted spirobifluorene (all-Sa)-12 (MM2-minimized, right).

metal complexes of 10b so far investigated turned out to be too low to perform quantitative binding studies also. Thus, we are currently trying to improve solubility by using different counter-ions and introducing further groups that facilitate dissolution in different solvents.

The solubility of the coordination complexes of 11 and its covalent analog 12 proved to be much higher in organic solvents, however, so we could start to per-

Fig. 2.1.11. Synthesis of 12. (a) Pd2dba3 CHCl3, Mes3P, CuI, n-Bu4NI, DMF, THF, i-Pr2NEt, 62%; (b) conc. HCl, MeOH, THF, 86%.

Fig. 2.1.11. Synthesis of 12. (a) Pd2dba3 CHCl3, Mes3P, CuI, n-Bu4NI, DMF, THF, i-Pr2NEt, 62%; (b) conc. HCl, MeOH, THF, 86%.

Fig. 2.1.12. NMR studies of the binding of 12 to n-octyl fi-o-glucopyranoside. (I) 1H NMR spectra (500.1 MHz in CDCl3) (a) 0.4 mm sugar, (b) 0.4 mm sugar + 1.6 mm (all-Sa)-12; (II) Job-plot (cTotal = CGuest + cHost = 2 mm); (III) titration curve (csugar was kept constant at 0.1 mm).

form quantitative studies with (all-Sa)-12. Figure 2.1.12 shows results from one of these NMR binding studies (a more detailed explanation of NMR binding studies is given in Chapter 2.3). These indicate that (all-Sa)-12 and n-octyl ¡-d-glucopyranoside form a 1:1 complex. As expected from the results obtained for similar receptors, for example 8 and 9, the binding is rather weak and we calculated an association constant of KA = 25 + 1 by non-linear regression analysis of the titration curve. We are currently extending these studies to several n-octylhexopyranosides and methylhexopyranosides in solvents such as chloroform, benzene, and THF, to evaluate the hosts' affinity and diastereo-, and enantioselec-tivity when binding with these substrates.

0 0

Post a comment