Synthesis TETRAHYDROISOQUINOLINE-BASED PEPTIDOMIMETICS MIMICKING REVERSE TURN SECONDARY STRUCTURES Nicola Landoni, Giordano Lesma, Alessandro Sacchetti and Alessandra Silvani Dipartimento di Chimica Organica e Industriale, Università degli Studi di Milano, via Venezian 21 – Milano, Italy The Tic heterocyclic frame (Tic = 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) is present in many biologically active natural alkaloids and in a great number of pharmaceutical compounds. Particularly, the presence of the Tic nucleus, which is a conformationally constrained Phe analogue, seems to be essential in many synthetic peptidomimetics showing agonistic and antagonistic activity towards many G protein-coupled receptors. This project is aimed to identify new Tic-based privileged structures having the capability to mimic reverse turn secondary motifs. Conformational analysis (intramolecolar hydrogen bond evaluation): - 1 H NMR; - IR; - CD. Peptidomimetics synthesis: - Control of stereochemistry. Peptidomimetics design through computational chemistry: - Conformational analysis (MM / MC); - Analysis of geometrical parameters of low energy conformers. Pyrrolo-tetrahydroisoquinoline (AHPIC) ( AHPIC = 2-amino-8,9-dimethoxy-3-oxo-1,2,3,5,6,10b-hexahydro-pyrrolo[2,1-a]isoquinoline-5-carboxylic acid ) Conformational analysis of the structures AHPIC (Spartan ‘06, MC search, MMFF94 force field): No. of conf. < 6 kcal/mol % dα < 7Å %  β  < 30° % HA bond% HB bond (32)52 (26)12 (6)34 (17) (21)83 (39)4 (2)38 (18) Results expressed as % of the conformers that meet the requirement. In parentheses, the number of conformers.  -turn type Conf. n°II’IIII’IIIIII’ Similarity analysis of Ac-AHPIC-NHMe 1 and 2 with standard type β-turns. The score is defined as [(1-R 2 )/N], where R 2 is the r.m.s. of the distances between points of similarity of structures and N is the number of centres. δ (ppm) b Δδ/ΔT (ppb/K) c IR (cm -1 ) 1 NHMe NHAc , NHMe NHAc , 3370 a All analyses were performed on 3.0 mM CDCl 3 solutions. b At 298 K. c Determined between 298 and 328 K. NMR and IR spectroscopical data. a The presence of a reverse turn was observed in both the structures, with the C1 stereochemistry playing a central role in determining stable conformations. In particular, all the analyses led to the conclusion that a type II’  -turn is mostly stabilized in tetrapeptide mimic 1, while a typical inverse γ-turn geometry is revealed for the diastereoisomer 2. Synthesis of a II’  -turn mimic fragment of HOE 140. B RADIKININ (B K ): an endogenous ligand for G protein- coupled receptors (GPCRs): GPCRs: membrane receptors involved in signal transduction; Control of different aspects of cell function, through the mediation of the response to different extracellular stimuli; Adjust many biological processes, including sensory (smell, taste, sight) and not sensory (appetite, digestion, blood pressure, reproduction, inflammation) activities; Approximately 50% of drugs on the market today exerts its therapeutic function through interaction with the GPCRs. B RADIKININ (B K ): H-Arg 1 -Pro 2 -Pro 3 -Gly 4 -Phe 5 -Ser 6 -Pro 7 -Phe 8 -Arg 9 -OH  -turn (type II’) Receptors (GCPRs)Function Concerned Pathologies B2 receptors (physiologically expressed) Mediators of physiological action of BK (eg vasodilatation) Bronchial asthma, allergic rhinitis, hypertension B1 receptors (overexpressed in case of trauma or infection) Mediators of BK action during pathologies (eg prostaglandin synthesis) Chronic pain, inflammation Spyro-pyrrolo-tetrahydroisoquinoline (SIPP) (SIPP = 2-(2'-oxo-2,4-dihydro-1H-spiro[isoquinoline-3,3'-pyrrolidine]-1'-yl)propanoic acid) HOE 140: A powerful antagonist for the bradykinin B2 receptor Type II’  - turn with D- Tic in the i+1 position. δ (ppm) Δ δ / Δ T (ppb/K)IR (cm -1 ) NHMen.d. 3323, 3435 NMR and IR spectroscopical data. CD (MeOH 0.2 mM) Type II’  -turn Biochemistry, 1978, 17, Synthesis % of conformers which meet the requirements. Synthesis δ (ppm) b Δ δ / Δ T (ppb/K) c IR (cm -1 ) NHMe NMR e IR spectroscopical data. a Titration with DMSO-d 6 a All analyses were performed on 3.0 mM CDCl 3 solutions. b At 298 K. c Determined between 298 and 328 K. X-Ray Structure (from isopropanol) dα (Å)β ( o )Φ 2 ( o )Ψ 2 ( o )Φ 3 ( o )Ψ 3 ( o ) Type II’  -turn X-Ray MM HF (6-31G*) Analysis of geometrical parameters of 3. Application of the SIPP scaffold to mimic a bioactive peptide G. Lesma, E. Meschini, T. Recca, A. Sacchetti, A. Silvani, Tetrahedron 2007, 63, ; N. Landoni, G. Lesma, A. Sacchetti, A. Silvani, J. Org. Chem. 2007, 72,