Macrotricyclic Steroid Receptors by Pd°-Catalyzed Cross-Coupling Reactions: Dissolution of Cholesterol in Aqueous Solution and Investigations of the Principles Governing Selective Molecular Recognition of Steroidal Substrates

Article abstract of DOI:10.1002/hlca.19980810112

Three double-decker cyclophane receptors, (±)-2,(±)-3, and (±)-4 with 11-13-A deep hydrophobic cavities were prepared and their steroid-binding properties investigated in aqueous and methanolic solutions. Pd°-Catalyzed cross-coupling reactions were key steps in the construction of these novel macrotricyclic structures. In the synthesis of D₂-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 1). For the preparation of the macrotricyclic precursor (±)-15 of D₂-.symmetrical (±)-3, diiodocylophane 12 was dialkynylated with Me₃SiC≡CH to give 13 using the Sonogoshira 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 C₂-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 × 10⁶ and 1.5 × 10⁵ 1 mol^-1, respectively (295 K) (Table 1). The steroid-binding properties of the three receptors were analyzed in detail by ¹H-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 5α-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.