Main chain and side chain chiral methylated somatostatin analogs: syntheses and conformational analyses

Abstract

followed by filtration of the desired salt and vacuum drying.,' Then, 4.5 g (61 "01) of 2-butanol and 2.3 g (20 "01) of the formamidinium salt were stirred under reflux for 21 h. The mixture was cooled to room temperature, washed with 15 mL of Et,O, and concentrated under vacuum. The residue was dissolved in 2 mL of MeOH and chromatographed over silica gel with 1 3 1 CH2Cl2/MeOH eluent. Weobtained 1.29 g (8.4 mmol, 42%) of isouronium salt 8 as a highly hygroscopic, glassy solid (under vacuum), that fused to a slightly yellow oil in the atmosphere. J = 6 Hz, CHCH,); 1.45-1.65 (m, 2 H, CH,); 4.75-4.95 (m, 1 H, CH); 8.67 (br s, 4 H, 2N+H2). to (S)-8 were carried out analogously. Detailed descriptions of the optical rotations of these materials appear in the text.3-(2-Butoxy)-3-chlorodi.ziriw (9). The Graham oxidationI8 was carried out on 1.29 g (8.4 mmol) of isouronium salt 8. The salt was dissolved in 5 mL of DMSO, then, 15 mL of pentane and 0.1 g (2.4 "01)of LiCl were added. The mixture was stirred and cooled to 5 ' C while 220 mL of 12.5% aqueous NaOCl was added dropwise. The reaction temperature was kept at 10-13 ' C during the addition. Stirring was continued for 20 min at 13 OC after addition of the hypochlorite. Then, 150 mL of ice water was added, the pentane layer was separated, the aqueous layer was extracted with 10 mL of pentane, and the organic extracts were combined. The resulting pentane solution of 9 was washed with 50 mL of brine and then dried over CaCI,. Higher boiling solvents (MeCN, I-butanol) could then be added and pentane removed at 0 ' C under aspirator vacuum, to afford solutions of diazirine 9 in other solvents. The UV and NMR spectra of 9 are described above; it was too unstable to chromatograph. (a-9 was prepared from (S)-8 in the same way.(neat)), 6.2 mL (5.0 g, 67.5 mmol), and 4.6 mL of dichlorophenyl-'H NMR (DMSO-d6): 0.84 (t, J = 7 Hz, 3 H, CHzCHJ; 1.21 (d, (~)-2-chlorobut.ne (12). (R)-(-)-2-Butanol (AIdrich, [aI2'D -12.8' Abstract: We have developed an integrated approach for investigating the 'bioactive conformations" of the main chain and side chains for the somatostatin analog ~[Pro~-Phe~-~-Trp~-Lys~-Thr~~-Phe'~]. A series of analogs have been synthesizedincorporating a-methylated and 8-methylated residues at positions 7,8, and 11. These analogs display dramatic differences in in vitro binding affinities for somatostatin receptors. Using 500-MHz 'H NMR and computer simulations, we have assessed the effect of main chain and side chain chiral methylations on the overall structure. The analyses of the changes of side chain topologies and subsequent binding affinities in the 8-methylated analogs have provided definitive evidence about the 'bioactive conformation" of the side chains of Phe7, Trp8, and Phe". The analyses of the a-methylated analogs have defined a 'folded" feature for the peptide backbone. From this study, we have proposed a binding 'pocket" for somatostatin analogs which consists of the side chains of Trp' and Lys9, the peptide backbone, and the side chain of Phe" in a...

Synthesis and biological activities of potent peptidomimetics selective for somatostatin receptor subtype 2

“…2). This result is in accord with previously described results (20) -] with an attendant increase in binding to AtT20 cells. Also in agreement with these earlier studies, the (2R,3R)-isomer 6 bound much less tightly to sstr2.…”

Novel sst2-Selective Somatostatin Agonists. Three-Dimensional Consensus Structure by NMR

“…Interestingly, Goodman and coworkers illustrated that in SRIF analogues, which bind selectively to sst 2 2/3/5 , the side chains of DTrp 8 , Lys 9 , Phe 7 and Phe 2 constitute the most essential elements necessary for binding. 15,16 In their pharmacophore model, DTrp 8 and Lys 9 were at a close proximity of ∼5 Å, which is similar to the sst 2 pharmacophore. The aromatic ring at position 7, Phe 7 is farther away from DTrp 8 (7 to 9 Å) and Lys 9 (9 to 11 Å), and is not present in the sst 2 pharmacophore.…”

“…On the basis of these 3D structures of the free peptides, a pharmacophore model was proposed for SRIF analogues binding to sst 2/5 (and sst 3 ). [15][16][17][18] In this model, the side chains and the relative spatial arrangement of DPhe 2 , Phe 7 , DTrp 8 and Lys 9 constitute the most essential elements necessary for binding ( Figure 3C). The side chain of DTrp 8 is in close proximity to the side chain of Lys 9 (∼5 Å), whereas the side chain of Phe 7 is about 7 − 9 Å away from the side chain of DTrp 8 and 9 − 11 Å from the side chain of Lys 9 .…”

Structure of octreotide, a somatostatin analogue

“…The two basic ring conformations found in the crystal would then correspond to the two conformations found by NMR in solution. Huang et al (1992) made detailed predictions for 'flat' and 'folded' conformations of the same cyclic hexapeptide c[Pro6-Phe7-(D)-TrpS-Lys9-Thrl°-Phe ll] which are compared with our results in Table 4. It will be seen that the qo and ~b torsion angles of the -(D)-Trp 8-and -Lys 9-residues for both their structures and our molecules I, II and III are all typical for type II'/3-turns.…”

“…NMR studies by Kessler et al (1983) also indicated a type II'/3-turn. Huang et al (1992) and He, Huang, Raynor, Reisine & Goodman (1993), using 500MHz two-dimensional NMR, binding studies and conformational analysis of methylated derivatives, showed that although the side chains are conformationally flexible, the biologically active conformation of this cyclic hexapeptide probably involves the trans rotamer of D-Trp 8 and the gauche-(g-) rotamer of Lys 9.…”