Stereoselective Synthesis of New (2 S,3 R)-3-Carboxyphenyl)pyrrolidine-2-carboxylic Acid Analogues Utilizing a C(sp3)-H Activation Strategy and Structure-Activity Relationship Studies at the Ionotropic Glutamate Receptors

Abstract

Competitive antagonists for ionotropic glutamate receptors (iGluRs) are highly valuable tool compounds for studying health and disease states in the central nervous system. However, only few subtype selective tool compounds are available and the discovery of antagonists with novel iGluR subtype selectivity profiles remains a profound challenge. In this paper, we report an elaborate structure-activity relationship (SAR) study of the parental scaffold 2,3-trans-3-carboxy-3-phenyl-proline by the synthesis of 40 new analogues. Three synthetic strategies were employed with two new strategies of which one being a highly efficient and fully enantioselective strategy based on C(sp3)-H activation methodology. The SAR study led to the conclusion that selectivity for the NMDA receptors was a general trend when adding substituents in the 5'-position. Selective NMDA receptor antagonists were obtained with high potency (IC₅₀ values as low as 200 nM) and 3-34-fold preference for GluN1/GluN2A over GluN1/GluN2B-D NMDA receptors.

Keywords: C(sp3)−H activation; Electrophysiology; Ionotropic glutamate receptors; NMDA receptor antagonist; Proline analogues.

Figure 1.

A) X-ray crystal structure of 5a (green) which holds a 5’-OH substituent, in the LBD of GluA2 (PDB: 4YMA). Receptor surface is displayed in grey. B) X-ray crystal structure of 5a (green) in GluA2 (PDB: 4YMA). Protein surface is colored in grey. The 5’-position directs towards cave-shaped space with a depth of 10–12Å.

Figure 1.

A) X-ray crystal structure of 5a (green) which holds a 5’-OH substituent, in the LBD of GluA2 (PDB: 4YMA). Receptor surface is displayed in grey. B) X-ray crystal structure of 5a (green) in GluA2 (PDB: 4YMA). Protein surface is colored in grey. The 5’-position directs towards cave-shaped space with a depth of 10–12Å.

Figure 2.

Overview of herein designed and synthesized analogs of iGluR antagonist 4a: series 4–6,8,9 and compound 7.

Figure 3.

Compound 9a in the GluA1-LBD (PDB: 4YMB).

Figure 4.. Concentration-inhibition data of 4h and 6b at NMDA receptor subtypes.

A) Representative two-electrode voltage-clamp recordings showing inhibition of GluN1/2A and GluN1/2D by 4h. Receptors were activated by 1 μM glutamate in the continuous presence of 100 μM glycine. B) Concentration-inhibition data for 4h and 6b at different recombinant NMDA receptor subtypes (GluN1/2A-D) measured using two-electrode voltage-clamp recordings. Data are mean ± SEM from 5–6 oocytes for each receptor subtype. The IC₅₀ values were used to estimate K_i values using the Cheng-Prusoff relationship (Table 6).

Scheme 1.. Synthesis of analog 7 from enantiopure enone 10^a

^aReagent and conditions: (a) nBuLi, ((3-bromobenzyl)oxy)(tert-butyl)dimethylsilane, CuCN, thiophene, Et₂O, −78 °C to −42 °C, (84%); (b) BH₃·Et₂O, THF, 0 °C to reflux, 18h, (59%); (c) 1N TBAF, THF, rt, 18h, (88%); (d) NaIO₄, RuCl₃·H₂O, H₂O/MeCN/EtOAc, 0 °C, 1.5h, (86%); (e) TFA, DCM, rt, overnight, 99%.

Scheme 2.. Synthesis of boronic acid 17^a

^aReagents and conditions: (a) MeOH, H₂SO₄, reflux; (b) pinacol, Et₂O; (c) LiAlH₄, THF, 0–20 °C, (86% over three steps); TBDMSCl, imidazole, DMAP, DMF (quantitative).

Scheme 3.. Synthesis of 5e-i^a

^aReagents and conditions: (a) Boronic ester 17, Rh(I), CsCO₃, (86%); (b) DMS·BH₃, THF, reflux, (62%); (c) TBAF, THF, (68%); (d) Pd-catalyst, Ar-B(OR)₂, solvent, (Xantphos), heating, (31–87%); (e) RuCl₃, NaIO₄, H₂O, MeCN, EtOAc; (f) TFA, DCM, then 1M HCl, (6–98 %); (g) BBr₃, DCM, (4%).

Scheme 4.. Synthesis of 5j-m^a

^aReagents and conditions: (a) i) Et₃N, NCS; ii) Pyr, CbzCl, (55%); (b) NBS, DABCO, (80%); (c) 23, PdCl₂dppf, Cs₂CO₃, THF/H₂O, 80 °C, (85%); (d) H₂ (g), Pd/C, MeOH, rt, (quantitative); (e) Boc₂O, DCM, rt, (50% over two steps); (f) RCl, pyridine, rt, overnight, (85–92%); (g) i) (for 26a) LDA, THF, −78 °C to 0 °C, 10 min, (50%, d.r. 1.3:1) or (ii) (for 26b-d) 25wt% NaOMe in MeOH, MeOH, reflux, 24h, (quantitative); (h) 4N HCl, reflux, 24h, (16–74%); (i) i) LiOH, MeOH/H₂O, ii) TFA, DCM, (20%); (j) MeOH, SOCl₂, (73%).

Scheme 5.

Retrosynthetic analysis for CH-activation strategy

Scheme 6.. Synthesis of intermediate 28a^a

^aReagents and conditions: (a) 8-Aminoquinoline, EDCI, HOBt, DCM, rt, 24h, (85%); (b) Methyl-3-bromo-5-iodobenzoate, 10 mol% Pd(OAc)₂, 30 mol% PivOH, Ag₂CO₃, toluene, reflux, 3d, (40–48%); (c) NaOH, EtOH, reflux, 24h, (95%); (d) TBTA, DCM, rt, 24h, (74%).

Scheme 7.. Synthesis of 4a and 2,3-cis-4a^a

^aReagents and conditions: (a) NaOH, EtOH, reflux, 24h, (95%); (b) TFA, DCM, rt, (74%); (c) 40% H₂SO₄, H₂O, (27%).

Scheme 8.. Synthesis of 4f-h^a

^aReagents and conditions: (a) 36 or 38, 10 mol% Pd(OAc)₂, 30 mol% PivOH, Ag₂CO₃, toluene, reflux, 3d, (61% and 37%); (b) NaOH, EtOH, reflux, 24h, (86% and 86 %); (c) NaN₃, Cs₂CO₃, CuI, EtOH, reflux, 3d, (d) TFA, DCM (14–72%); (e) SOCl₂, MeOH, reflux, overnight, (70%); (f) Pd/C, H₂, MeOH, (95%); (g) I₂, tert-BuNO2, MeCN, toluene, 0 °C, then rt, 18h (46%).

Scheme 9.. Synthesis of 5b,c^a

^aReagents and conditions: (a) HCOOH, Ac₂O, Et₃N, Pd(OAc)₂, Xantphos, 80 °C, (90%); (b) TFA, DCM, (77% and 75%, respectively).

Scheme 10.. Synthesis of 5d^a

^aReagents and conditions: (a) TMSA, PdCl₂(PPh₃)₂, CuI, Et₃N, 60 °C, 12h, (77%); (b) 1N TBAF, THF, rt, 30min (82%); (c) TMSN₃, CuI, DMF-MeOH, (65%); (d) TFA, DCM, (92%)

Scheme 11.. Synthesis of 6a,b^a

^aReagents and conditions: (a) NIS, TfOH, (99%); (b) SOCl₂, MeOH, reflux, ON, (70%); (c) 44, 10 mol% Pd(OAc)₂, 30 mol% PivOH, Ag₂CO₃, toluene, reflux, 3d, (31%); (d) NaOH, EtOH, reflux, 24h, (88%); (e) TBTA, DCM, (52%); (f) HCOOH, Ac₂O, Et₃N, Pd(OAc)₂, Xantphos, 80 °C, (63%); (g) TFA, DCM, (96% and 93%, respectively).

Scheme 12.. Synthesis of amino acid conjugates 8a-s from intermediate 23a^a

^aReagents and conditions: (a) EDCI, HOBt, AA-OtBu, DMF, rt, 24h, (65–99%); (b) TFA, DCM, rt, overnight (33–97%); (c) (i) 1N LiOH, MeOH, rt, overnight, (ii) TFA, DCM, rt, overnight, (46%).

Scheme 13.. Synthesis of intermediate aldehyde 50^a

^aReagents and conditions: (a) LHMDS, then allylbromide, THF, DMPU, −78 °C to −40 °C, 4h, (36%); (b) (i) LiEt₃SiH, then H₂O₂; (ii) BF₃·Et₂O, Et₃SiH, (72%); (c) LiOH, H₂O, THF, (quantitative); (d) EDCI, HOBt, 8-Aminoquinoline, DCM, rt, overnight, (72%); (e) Methyl-3-iodobenziate, Pd(OAc)₂, PivOH, toluene, reflux, 3d, (35%); (f) (i) NaOH, EtOH, 100 °C, overnight, (ii) TBTA, DCM, rt, overnight, (35% over two steps); (g) OsO₄, NaIO₄, THF, H₂O, rt, 4h, (53%).

Scheme 14.. Synthesis of 9b-e from aldehyde 50.^a

^aReagents and conditions: (a) MeNH₂·HCl, NaBH₃CN, MeOH, rt, overnight, (48%); (b) BnNH₂·HCl, NaBH₃CN, MeOH, (70%); (c) NaBH₄, MeOH, rt, 30min, (83%); (d) NaO₂Cl, THF:H₂O:tert-BuOH 1:1:1, 2-methyl-butene, phosphate buffer, (94%); (e) TFA, DCM, rt, overnight, (53–99%).