Macrotricyclic steroid receptors by Pd0-catalyzed cross-coupling reactions:: Dissolution of cholesterol in aqueous solution and investigations of the principles governing selective molecular recognition of steroidal substrates

Three double-decker cyclophane receptors, (+/-)-2, (+/-)-3, and (+/-)-4 with 11-13-Angstrom deep hydrophobic cavities were prepared and their steroid-binding properties investigated in aqueous and methanolic solutions. Pd-0-Catalyzed cross-coupling reactions were key steps in the construction of these novel macrotricyclic structures. In the synthesis of D-2-symmetrical (+/-)-2, the double-decker precursor (+/-)-7 was obtained in 14% yield by fourfold Stille coupling of equimolar amounts of bis(tributylstannyl)acetylene with dibromocyclophane 5 (Scheme I). For the preparation of the macrotricyclic precursor (+/-)-15 of D-2-symmetrical (+/-)-3, diiodocylophane 12 was dialkynylated with Me3SiC=CH to give 13 using the Sonogashira cross-coupling reaction; subsequent alkyne deprotection yielded the diethynylated cyclophane 14, which was transformed in 42% yield into (+/-)-15 by Glaser-Hay macrocyclization (Scheme 2). The synthesis of the C-2-symmetrical conical receptor (+/-)-4 was achieved via the macrotricyclic precursor (+/-)-25, which was prepared in 20% yield by the Hiyama cross-coupling reaction between the diiodo[6.1.6.1]paracyclophane 19 and the larger, dialkynylated cyclophane 17 (Scheme 4). Solid cholesterol was efficiently dissolved in water through complexation by (+/-)-2 and (+/-)-3, and the association constants of the formed 1:1 inclusion complexes were determined by solid-liquid extraction as K-a = 1.1 x 10(6) and 1.5 x 10(5) l mol(-1), respectively (295 K) (Table 1). The steroid-binding properties of the three receptors were analyzed in detail by H-1-NMR binding titrations in CD3OD. Observed steroid-binding selectivities between the two structurally closely related cylindrical receptors (+/-)-2 and (+/-)-3 (Table 2) were explained by differences in cavity width and depth, which were revealed by computer modeling (Fig. 4). Receptor (+/-)-2, with two ethynediyl tethers linking the two cyclophanes, possesses a shallower cavity and, therefore, is specific for flatter steroids with a C(5)=C(6) bond, such as cholesterol. In contrast, receptor (+/-)-3, constructed with longer buta-1,3-diynediyl linkers, has a deeper and wider hydrophobic cavity and prefers fully saturated steroids with an aliphatic side chain, such as Scc-cholestane (Fig. 7). In the 1:1 inclusion complexes formed by the conical receptor (+/-)-4 (Table 3), testosterone or progesterone penetrate the binding site from the wider cavity side, and their flat A ring becomes incorporated into the narrower [6.1.6.1]paracyclophane moiety. In contrast, cholesterol penetrates (+/-)-4 with its hydrophobic side chain from the wider rim of the binding side. This way, the side chain is included into the narrower cavity section shaped by the smaller [6.1.6.1]paracyclophane, while the A ring protrudes with the OH group at C(3) into the solvent on the wider cavity side (Fig. 8). The molecular-recognition studies with the synthetic receptors (+/-)-2, (+/-)-3, and (+/-)-4 complement the X-ray investigations on biological steroid complexes in enhancing the understanding of the principles governing selective molecular recognition of steroids.