Synthesis and Opioid Activity of Tyr1 -ψ[(Z)CF=CH]-Gly2 and Tyr1 -ψ[(S)/(R)-CF3 CH-NH]-Gly2 Leu-enkephalin Fluorinated Peptidomimetics

Abstract

We describe the design, synthesis, and opioid activity of fluoroalkene (Tyr¹ -ψ[(Z)CF=CH]-Gly² ) and trifluoroethylamine (Tyr¹ -ψ[(S)/(R)-CF₃ CH-NH]-Gly² ) analogues of the endogenous opioid neuropeptide, Leu-enkephalin. The fluoroalkene peptidomimetic exhibited low nanomolar functional activity (5.0±2 nm and 60±15 nm for δ- and μ-opioid receptors, respectively) with a μ/δ-selectivity ratio that mimics that of the natural peptide. However, the trifluoroethylamine peptidomimetics, irrespective of stereochemistry, did not activate the opioid receptors, which suggest that bulky CF₃ substituents are not tolerated at this position.

Keywords: amide bonds; enkephalin; fluorine; opioids; peptidomimetics.

Figure 1

Fluorinated isoelectronic and isopolar peptidomimetics

Figure 2

Target fluoroalkene 6 and trifluoroethylamine (S)-7/(R)-8 analogues of Leu-enkephlin

Scheme 1. Preparation of Tyr¹–ψ[(Z)CF=CH]–Gly² Leu-Enkephalin

Reagents and conditions: (a) 3 Å molecular sieves, THF, 0 °C, 4 h; (b) RhCl(PPh₃)₃ BrCF₂CO₂Et, Et₂Zn, 0 °C 30 min, 45%; (c) (Boc)₂O, 20% Pd(OH)₂/C, H₂, EtOH, rt, 48 h, 73%; (d) DIBAL–H, DCM, –78 °C; (e) a (EtO)₂P(O)CH₂CO₂Et, LiCl, DIPEA, CH₃CN, 0 °C→rt, 12 h, 60%; (f) SmI₂, THF:t-BuOH (3:1), 0 °C, 1 h, 75%; (g) LiOH, THF:H₂O (1:1), 0 °C→rt, 36 h, 78%; (h) DIPEA, HOBt, EDC.HCl, THF 0 °C→rt, 12 h, 65%. (i) LiOH, THF:H₂O (1:1), 0 °C→rt, 12 h, 75; (j) 4 N HCl in 1,4-Dioxane, 15 °C, 2 h, HPLC purification, 45%.

Scheme 2. Preparation of Tyr¹–ψ[(S)-CF₃CH–NH]–Gly² Leu-Enkephalin

Reagents and conditions: (a) Ti(OEt)₄, THF, rt, 6 h, 91%; (b) TMAF, TMSCF₃, THF, –35 °C, 2 h, 89%; (c) 4N HCl in 1,4-dioxane, MeOH, 70 °C, 2 h, 90%; (d) NaH, BrCH₂CO₂Et, HMPA, 48 h, 52%; (e) LiOH, THF:H₂O (1:1), 12 h, 88%; (f) DIPEA, HOBt, EDC.HCl, THF, 0 °C→rt, 12 h, 72 %; (g) LiOH, THF:H₂O (1:1) 0 °C→rt, 12 h, 80%; (h) Pd(OH)₂/C, methanol, 6 h, H₂ balloon, rt, HPLC purification, 37%

Scheme 3. Preparation of Tyr¹–ψ[(R)-CF₃CH–NH]–Gly² Leu-Enkephalin

Reagents and conditions: (a) Ti(OEt)₄, THF, rt, 6 h, 90%; (b) TMAF, TMSCF₃, THF, –35 °C, 2 h, 67%; (c) 4N HCl in 1,4-dioxane, MeOH, 70 °C, 2 h, 95%; (d) NaH, BrCH₂CO₂Et, HMPA, 3 d, 70%; (e) LiOH, THF:H₂O (1:1), 12 h, 91%; (f) DIPEA, HOBt, EDC.HCl, THF, 0 °C→rt, 12 h, 69 %; (g) LiOH, THF:H₂O (1:1) 0 °C→rt, 12 h, 85%; (h) Pd(OH)₂/C, methanol, 6h, H₂ balloon, rt, HPLC purification, 25%