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

Abstract

AbstractThree double‐decker cyclophane receptors, (±)‐2, (±)‐3, and (±)‐4 with 11–13‐Å 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 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 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 Ka = 1.1 × 106 and 1.5 × 105 l 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.