New Building Blocks for Peptide Analogue Synthesis: Reduced Diaza-β³-Dipeptides

Abstract

Reduction of N-protected aza-β³-amino esters and oxidation of the alcohols afford aza-β³-amino aldehydes. Condensation of unprotected aza-β³-amino esters to the resultant aldehydes leads to hydrazones, which have been converted by reduction to useful synthetic building blocks for solid-phase synthesis of new peptidomimetics.

aza-β³ amino aldehydes - pseudopeptides - reductive amination - reduced aza-β³-peptides - PNA aza analogues

References

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12

To a solution of CH₂Cl₂ (20 mL, freshly distilled on CaH₂), under nitrogen atmosphere, was added oxalyl chloride freshly distilled (1.35 mL, 1.2 equiv). The solution was cooled at -78 °C, and DMSO (2.23 mL, 2.4 equiv, freshly distilled on KOH) was carefully added. The solution was stirred for 15 min and Fmoc-aza-β³-Leu alcohol (2, 4.63 g, 1 equiv) in CH₂Cl₂ (20 mL) was added dropwise, the mixture was stirred for 15 min at -78 °C. After adding Et₃N (5.52 mL, 3 equiv, freshly distilled on CaH₂) the mixture was given up to r.t. during 45 min. Then, CH₂Cl₂ was added (50 mL) with a solution of NaHCO₃ (1 M, 20 mL). The organic layer was washed with brine, dried over Na₂SO₄ and concentrated to give a crude oil suitable for the next step. Purification by chromatography on silica gel (EtOAc-PE 3:7) gave 40% yield of 3a (1.84 g). ¹H NMR (CDCl₃): δ = 0.95 (br, 6 H, CH₃), 1.69 (m, 1 H, CH), 2.67 (br, 2 H, CH₂), 3.72 (br, 2 H, CH₂), 4.23 (br t, 2 H, J = 6.3 Hz, CH), 4.53 (br d, 2 H, J = 6.3 Hz, CH₂), 6.39 (br, 1 H, NH), 7.31-7.83 (m, 8 H, CH_ar), 9.12 (s, 1 H, CHO) ppm. The crude oil was diluted in CH₂Cl₂ (50 mL) and aza-β³-Tyr-OBn (0.9 equiv) was added, the mixture was stirred overnight at r.t. on Na₂SO₄. After filtration, the solution was concentrated and purified by chromatography on silica gel (EtOAc-PE 3:7) to give 2.64 g (55%) of corresponding hydrazone (5b) as a colorless oil. ¹H NMR (CDCl₃): δ = 0.93 (d, 6 H, J = 6.6 Hz, CH₃), 1.23 (t, 3 H, J = 7.0 Hz, CH₃), 1.72 (m, 1 H, CH), 2.47 (br, 2 H, CH₂), 3.50 (s, 2 H, CH₂), 3.73 (q, 2 H, J = 7.0 Hz, CH₂), 3.97 (s, 2 H, CH₂), 4.24 (t, 1 H, J = 6.9 Hz, CH), 4.36 (s, 2 H, CH₂), 4.42 (d, 2 H, J = 6.9 Hz, CH₂), 5.11 (s, 2 H, CH₂), 5.18 (s, 2 H, CH₂), 5.82 (br s, 1 H, NH), 6.66 (br, 1 H, CH), 6.93-7.80 (m, 17 H, CH_ar) ppm. ¹³C NMR (CDCl₃): δ = 170.08, 156.85, 155.81, 144.07, 141.38, 135.63, 131.66, 129.15, 128.61, 128.37, 127.73, 116.45, 127.73, 127.11, 125.28, 120.00, 93.14, 66.49, 65.20, 64.22, 60.44, 56.54, 54.61, 47.37, 26.37, 20.84, 15.21 ppm. HRMS (ESI): m/z calcd for C₄₀H₄₆N₄O₆Na [M + Na]⁺: 701.33151; found: 701.3331 (2 ppm).

13

The hydrazone 5b (2.64 g, 3.9 mmol) was dissolved in MeOH (20 mL). Sodium cyanoborohydride (0.62 g, 2.5 equiv) was added and pH was brought to 3 by slowly adding a solution of 2 N HCl. The mixture was stirred for 2 h, then the pH was adjusted to 1. After 10 min of stirring, the solution was neutralized with solid NaHCO₃, the mixture was filtrated, concentrated under vacum and the residue was taken up with EtOAc (50 mL) and washed with H₂O and brine. The organic layer was dried over Na₂SO₄ and the solvent was removed to give crude oil which was purified by chromatography on silica gel (EtOAc-PE 3:7 and 5:5) to give 1.91 g (72%) of corresponding hydrazine 6b as a colorless oil. ¹H NMR (CDCl₃): δ = 0.93 (d, 6 H, J = 6.7 Hz, CH₃), 1.24 (t, 3 H, J = 7.0 Hz, CH₃), 1.66 (m, 1 H, CH), 2.43 (d, 2 H, J = 6.6 Hz, CH₂), 2.86 (br, 4 H, CH₂), 3.45 (s, 2 H, CH₂), 3.74 (q, 2 H, J = 7.0 Hz, CH₂), 3.90 (s, 2 H, CH₂), 4.20 (t, 1 H, J = 6.3 Hz, CH), 4.48 (d, 2 H, J = 6.3 Hz, CH₂), 5.16 (s, 2 H, CH₂), 5.21 (s, 2 H, CH₂), 5.75 (br s, 1 H, NH), 6.95-7.82 (m, 17 H, CH_ar) ppm. ¹³C NMR (CDCl₃): δ = 171.02, 156.78, 155.89, 143.98, 141.40, 135.96, 130.40, 128.55, 128.33, 128.24, 116.17, 127.66, 127.11, 125.04, 120.01, 93.14, 66.20, 66.03, 65.90, 64.16, 58.93, 56.52, 55.93, 47.48, 46.10, 26.30, 20.83, 15.21 ppm. HRMS (ESI): m/z calcd for C₄₀H₄₉N₄O₆ [M + H]⁺: 681.36521; found: 681.3656 (1 ppm).

14 Cheguillaume A. Lehardy F. Bouget K. Baudy-Floc’h M. Le Grel P. J. Org. Chem.  1999,  64:  2924 

15

Busnel, O.; Bi, L.; Dali, H.; Cheguillaume, A.; Chevance, S.; Bondon, A.; Muller, S.; Baudy-Floc’h, M. manuscript submitted for publication.

16

To a solution of 6b (1.69 g, 2.5 mmol) in dioxane (15 mL) was added successively DIPEA (33 mg, 0.1 equiv) and Boc₂O (1.25 g, 2.3 equiv). The mixture was stirred during 24 h at 50 °C then added to a solution 1 M of NaHSO₄ (40 mL) and extracted twice with Et₂O. The organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The crude oil was purified by chromatography on silica gel (EtOAc-PE 1:9 and 2.5:7.5) to afford 0.79 g (41%) of 7b as a colorless oil. ¹H NMR (CDCl₃, 348 K): δ = 0.96 (d, 6 H, J = 5.8 Hz, CH₃), 1.11 (t, 3 H, J = 7.0 Hz, CH₃), 1.54 (s, 9 H, CH₃), 1.79 (m, 1 H, CH), 2.65 (br, 2 H, CH₂), 2.93 (br, 2 H, CH₂), 3.13 (br, 2 H, CH₂), 3.59 (q, 2 H, J = 7.0 Hz, CH₂), 4.18 (s, 2 H, CH₂), 4.25 (s, 2 H, CH₂), 4.38 (br, 1 H, CH), 4.53 (br, 2 H, CH₂), 5.06 (s, 2 H, CH₂), 5.10 (br s, 2 H, CH₂), 6.41 (br s, 1 H, NH), 7.12-7.70 (m, 17 H, CH_ar) ppm. ¹³C NMR (CDCl₃, 348 K): δ = 170.56, 157.50, 155.3, 144.41, 141.59, 136.25, 130.90, 128.42, 128.28, 128.05, 116.37, 127.56, 127.00, 125.14, 119.87, 95.33, 80.13, 66.19, 66.05, 64.87, 63.96, 58.62, 58.45, 54.18, 49.41, 47.79, 28.41, 26.41, 20.54, 14.88 ppm. HRMS (ESI): m/z calcd for C₄₅H₅₆N₄O₈Na [M +Na]⁺: 803.39959; found: 803.3992 (0 ppm).

17

To a solution of 7b (0.30 g, 0.39 mmol) in MeOH (10 mL) was added 10% Pd/C (20 mg). After purging 3 times with H₂, the resulting suspension was stirred for 30 min (end of reaction controlled by TLC). The catalyst was removed by filtration through Celite. The filtrate was evaporated and purified by chromatography on silica gel (EtOAc-CH₂Cl₂, 1:1 and Et₂O-MeOH, 40%) to afford 0.2 g (75%) of 8b as a yellow oil. ¹H NMR (CDCl₃, 353 K): δ = 1.01 (d, 6 H, J = 5.4 Hz, CH₃), 1.13 (t, 3 H, J = 6.8 Hz, CH₃), 1.50 (s, 9 H, CH₃), 1.81 (m, 1 H, CH), 2.56 (br, 2 H, CH₂), 2.98 (br, 2 H, CH₂), 3.26 (br, 2 H, CH₂), 3.60 (q, 2 H, J = 6.8 Hz, CH₂), 4.02 (br, 2 H, CH₂), 4.21 (s, 2 H, CH₂), 4.31 (br, 1 H, CH), 4.60 (br, 2 H, CH₂), 5.08 (s, 2 H, CH₂), 6.50 (br s, 1 H, NH), 7.12-7.75 (m, 12 H, CH_ar) ppm. ¹³C NMR (CDCl₃, 348 K): δ = 174.78, 157.54, 156.27, 144.39, 141.61, 131.28, 127.99, 127.70, 127.45, 116.38, 127.36, 127.07, 125.19, 119.86, 93.32, 80.84, 66.43, 63.94, 59.15, 58.36, 55.86, 47.83, 47.73, 28.33, 26.52, 20.62, 14.86 ppm. HRMS (ESI): m/z calcd for C₃₈H₅₀N₄O₈Na [M + Na]⁺: 713.35263; found: 713.3523 (0 ppm).