Semisynthetic Di- and Tri-Functionalized Non-Immunosuppressive Cyclosporin A Derivatives as Potential Anti-HIV 1 Drugs

Abstract

A regio- and stereoselective synthesis of original semisynthetic di- and tri-functionalized non-immunosuppressive ­cyclosporins by way of a Barton ester decarboxylation and a C-thioalkylation starting from cyclosporin A (CsA) and [4′-hydroxy-­MeLeu]⁴-CsA is described.

Key words

cyclosporin A derivatives - anti-HIV 1 drugs - Barton ester radical decarboxylation - sulfide derivatives - thioxo-2H-pyridin-1-yl ester - C-thioalkylation

References

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• For other examples using sodium amide as a base in liquid ammonia, see:
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Compound 5a: white solid, mp 140 °C; TLC R_f = 0.72 (10% MeOH in CH₂Cl₂). ¹H NMR (CDCl₃): two conformers (75-25 ratio), partial description, δ = 1.37 (d, J = 7.5 Hz, 3 H, CH₃ 7β), 2.00 and 2.03 (s, 3 H, SCH₃ 1δ), 2.30-2.90 [m, 11 H, SCH₂CH₂N(CH₃)₂ 3α, SCH₂ 1δ and CH 5β], 3.80 (m, 1 H, CH 1β), 3.90 (broad d, J = 6.0 Hz, 1 H, OH in 1β), 4.54 and 4.60-4.70 (two m, 1 H, CH 7α), 4.60-4.70 (m, 1 H, CH 5α), 4.80 (m, 1 H, CH 8α), 4.80-5.40 (m, 4 H, CH 2α and CH α of three leucine residues), 5.49 and 5.66 (two d, J = 6.0 Hz and J = 9.0 Hz, respectively, 1 H, CH 1α), 5.72 (dd, J = 10.5 and 4.0 Hz, 1 H, CH α, leucine residue), 5.79 and 5.98 (2 s, 1H, CH 3α), 7.05 and 7.17 (two d, J = 8.0 Hz, 1 H, CONH in 8), 7.32 and 7.35 (two d, J = 9.0 Hz, 1 H, CONH in 5), 7.42 and 7.76 (two d, J = 7.5 Hz, 1 H, CONH in 7), 7.94 (broad d, J = 10.0 Hz, 1 H, CONH in 2). IR (CH₂Cl₂): γ_max = 3415, 3325, 2990, 2875, 1637, 1510, 1470, 1035 cm^-1. LSI-MS: m/z = 1340 (M + H)⁺.

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Typical Experimental Procedures are as follows: Compound 2b: To a solution of 1b (12 g, 9 mmol) in tert-butanol (750 mL) were added successively a solution of Na₂CO₃ (7.6 g, 72 mmol) in H₂O (200 mL), NaIO₄ (15.7 g, 73.5 mmol) in H₂O (200 mL) and dropwise a solution of KMnO₄ (0.29 g, 1.8 mmol) in H₂O (200 mL) at 30 °C. The reaction mixture was stirred for 36 h at r.t. then a solution of KMnO₄ (0.1 g, 0.62 mmol) in H₂O (70 mL) was added dropwise at r.t. The solution was stirred for an additional 3 h at r.t. Then, a solution of NaHSO₃ (47 g, 0.45 mmol) in H₂O (200 mL) was added dropwise in 30 min, followed by 2 N H₂SO₄ (200 mL). The reaction mixture was washed with Et₂O (3 × 300 mL), and the combined organic extracts were dried (MgSO₄) and concentrated under reduced pressure to give a solid which was triturated in H₂O (300 mL), filtered and washed with H₂O (200 mL). The residue was mixed with H₂O (200 mL), and the suspension treated with ethanolamine (1.1 mL). The mixture was filtered, and the resulting solution was washed with Et₂O (4 × 100 mL). The aqueous layer was acidified to pH 1 with 5 N HCl, extracted with EtOAc (2 × 300 mL) and the combined organic extracts were washed with H₂O (2X100 mL), dried (MgSO₄) and concentrated in vacuo to afford 2b (6.8 g, 57%) as a white powder, TLC R_f = 0.42 (5% MeOH in CH₂Cl₂). LSI-MS: m/z = 1306 (M + H)⁺.
Compound 3b: To a solution of 2b (6 g, 4.6 mmol) in 1,2-dichloroethane (60 mL) under nitrogen at -15 °C in a flask protected from light (aluminum foil), were successively added distilled N-methylmorpholine (0.56 mL, 5 mmol) and distilled isobutyl chloroformate (0.66 mL, 48.5 mmol). The reaction mixture was stirred for 2.5 h at the same temperature. Then, a solution of 2-mercaptopyridine N-oxide (0.7 g, 5.5 mmol) and Et₃N (0.78 mL, 5.5 mmol) in 1,2-dichloroethane (15 mL) was added dropwise in 15 min, and the reaction mixture was stirred for 17 h at -20 °C. Finally, distilled dimethyl disulfide (4.1 mL, 45.5 mmol) was added dropwise, and the reaction mixture was subjected to irradiation by two 60-W tungsten filament lamps that were placed close to the reaction vessel at 0-10 °C for 3 h, and then allowed to warm to r.t. The reaction mixture was concentrated under reduced pressure, and the residue was triturated in Et₂O (200 mL) and 1 N HCl (20 mL). The organic layer was successively washed with H₂O (3 × 50 mL), 1 M aq ethanolamine (50 mL) and H₂O (3 × 50 mL), dried (MgSO₄) and concentrated in vacuo to give a solid which was purified by chromatography on silica gel eluting with EtOAc-MeOH (19:1) to give 3b (1.6 g, 27%) as a white powder, TLC R_f = 0.58 (10% MeOH in EtOAc), LSI-MS: m/z = 1308 (M + H)⁺. Compound 5b: Sodium (0.05 g, 2.2 mmol) and Fe(NO₃)₃·9 H₂O (0.01 g, 0.027 mmol) were added to a stirred solution of anhyd liquid NH₃ (70 mL) at -33 °C. When the reaction mixture changed from a blue solution to a grey suspension, additional sodium (0.6 g, 26 mmol) was introduced portionwise in 15 min, and the reaction mixture was stirred at -33 °C for 1.5 h. Then, 4b (2.4 g, 2 mmol) in tert-butylmethylether (80 mL) was added dropwise in 30 min at -33 °C, followed by the addition of di-(3-dimethylaminoethyl) disulfide (1.6 g, 7.8 mmol) in tert-butylmethylether (10 mL). The reaction mixture was stirred at the same temperature for 30 min, and NH₄Cl (2.3 g) was added portionwise. The reaction mixture was stirred overnight and allowed to warm up to r.t. Then, the reaction mixture was diluted with Et₂O (100 mL) and filtered. The solid was washed with Et₂O (3 × 100 mL), and the combined organic extracts were concentrated under reduced pressure to give an oil which was triturated in pentane (200 mL) for 48 h. The resulting solid was filtered and washed with pentane (3 × 50 mL). Finally, the solid was purified by chromatography on silica gel eluting with EtOAc-MeOH (9:1) to give 5b (0.9 g, 35%) as a white solid, mp 154 °C; TLC R_f = 0.38 (10% MeOH in CH₂Cl₂). ¹H NMR (CDCl₃): δ = 1.36 (d, J = 7.5 Hz, 3 H, CH₃ 7β), 2.04 (s, 3 H, SCH₃ 1ε), 2.25 [s, 6 H, N(CH₃)₂ 3α], 2.20-2.80 (m, 7 H, CH₂ 1γ -NCH₂CH₂S 3α and 1 H of CH₂ 4β), 2.70-2.72, 3.13-3.18, 3.28-3.45 and 3.51 (seven s, 3 H each, 7 NCH₃), 3.64 (d, J = 6.5 Hz, 1 H, OH 1β), 3.90 (m, 1 H, CH 1β), 4.50 (m, 1 H, CH 7α), 4.67 (t, J = 9.0 Hz, 1 H, CH 5α), 4.80 (m, 1 H, CH 8α), 4.90-5.10 (m, 3 H, CH 2α and CH α of two leucine residues), 5.15 (d, J = 11.0 Hz, 1 H, CH 11α), 5.42 (d, J = 6.5 Hz, 1 H, CH 1α), 5.46 (t, J = 6.0 Hz, 1 H, CH 4α), 5.71 (dd, J = 11.0 and 4.0 Hz, 1 H, CH α of one leucine), 6.00 (s, 1 H, CH 3α), 7.22 (d, J = 8.0 Hz, 1 H, CONH in 8), 7.50 (d, J = 9.0 Hz, 1 H, CONH in 5), 7.72 (d, J = 7.5 Hz, 1 H, CONH in 7), 8.03 (d, J = 10.0 Hz, 1 H, CONH in 2). IR (CH₂Cl₂): γ_max = 3409, 3319, 2965, 2873, 1637, 1514, 1469, 1411, 1095 cm^-1. LSI-MS: m/z = 1327 (M + H)⁺. Anal. Calcd for C₆₄H₁₁₈N₁₂0₁₃S₂: C, 57.89; H, 8.96; N, 12.66; O, 15.66; S, 4.83. Found: C, 57.06; H, 8.47; N, 12.59; S, 3.48; water, 1.1%.

16

The in vitro anti-HIV activity of compounds 5a and 5b was measured in established cell line cultures (see ref. [4] ). Thus, the CEM4 cell line was infected with HIV-1_Lai strain. The inhibition of HIV replication in the culture was estimated by the measure of the reverse transcriptase (RT) produced in the supernatant. Anti-viral activity was expressed as the IC₅₀ RT, the concentration required to reduce replication of HIV by 50%, and was determined by linear regression. Compounds 5a and 5b displayed IC₅₀’s lower than 100 nM.