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Synlett 2004(4): 726-728  
DOI: 10.1055/s-2004-817748
LETTER
© Georg Thieme Verlag Stuttgart · New York

Synthesis of 2H-Labelled α-Azidoisobutyryl Chloride (D6-Azib-Cl) as 2H6-Aib Equivalent Building Block in Peptide Synthesis

Sven Weigelt, Norbert Sewald*
University of Bielefeld, Department of Chemistry, Organic and Bioorganic Chemistry, Universitätsstr. 25, 33615 Bielefeld, Germany
Fax: +49(521)1068094; e-Mail: norbert.sewald@uni-bielefeld.de;
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Publication History

Received 23 October 2003
Publication Date:
29 January 2004 (online)

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Abstract

A new synthesis of 2H-labelled α-azidoisobutyryl ­chloride for application as a building block in peptide synthesis is presented. Starting from readily available acetone cyanohydrin-D6, an efficient synthetic protocol provides the deuterated target molecule α-azidoisobutyryl chloride (D6-Azib-Cl) in good yield with very high deuterium incorporation. This compound represents an activated, N-protected equivalent of 2H-labelled α-aminoisobutyric acid (D6-Aib), which will be especially useful for NMR investigations on peptides with a high Aib content.

Key words

isotopic labelling - azido acids - peptides - bioorganic chemistry - deuterium

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Typical procedure: 4-Toluene sulfonyl chloride (17.2 g, 90.0 mmol) is added at 0 °C to a solution of acetone cyanohydrin-D6 (D 6 -1) (5.47 g, 60.0 mmol) and DABCO (11.2 g, 100 mmol) in dry CH2Cl2 (100 mL). The reaction mixture is stirred at 0 °C for 5 min and then for an additional 1.5 h at r.t. EtOAc (500 mL) is added, the solid is filtered off and washed with EtOAc. The filtrate is washed with water and sat. NaHCO3, dried over Na2SO4 and concentrated to 50 mL. The pure product is obtained by crystallization from EtOAc. Further concentration and crystallization gives a mixture of the product, 4-toluene sulfonyl chloride and DABCO hydrochloride, which can be separated by flash-chromatography (silica gel, petroleum ether-EtOAc, 12:1); D 6 -2 is obtained as a colorless solid (10.2 g, 70%). Mp: 98 °C; 1H NMR (250.1 MHz, CDCl3): δ = 2.46 (s, 3 H), 7.36 (m, 2 H), 7.85 (m, 2 H); 13C NMR (62.9 MHz, CDCl3): δ = 21.7, 28.4 (m), 74.6, 118.2, 127.9, 129.9, 134.3, 145.5; IR (KBr): ν [cm-1] = 3061, 2360, 1594, 1359 (st), 1181 (st), 1089, 885 (sst), 715, 676; MS (EI, 70 eV): m/z = 245 (29%), 173 (99%), 155 (25%), 91 (100%); degree of deuteration (1H NMR): 92%.

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Typical procedure: 2-(4′-Toluene sulfonyloxy)-2-methyl propionitrile-D6 (D 6 -2) (7.19 g, 29.3 mmol) is added to a stirred solution of NaN3 (2.86 g, 44.0 mmol) in DMSO (60 mL) at 40 °C. After stirring overnight at 40 °C, aqueous work up (300 mL of water) and extraction with Et2O, the organic layers are washed with water and brine, dried over Na2SO4 and evaporated. The pure product D 6 -3 is obtained as a colorless liquid (2.54 g, 75%). 13C NMR (62.9 MHz, CDCl3): δ = 25.7 (sept, J = 20 Hz), 54.5, 119.4; IR (Film): ν [cm-1] = 2953, 2458, 2244, 2111 (sst), 1456, 1239 (st), 1160, 1101, 1032 (st), 810, 775, 672; GC-MS: t R = 3.4 min (100%); MS: m/z = 116 (11%), 74 (87%), 46 (100%).

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Typical procedure: 2-Azido-2-methyl propionitrile-D6
D 6 -3 (6.39 g, 55.0 mmol) is stirred in dry THF (400 mL) at -15 °C under argon. DIBALH (110 mL, 110 mmol, 1.0 M in THF) is added and the mixture is stirred at -15 °C for 30 min and then at r.t. for 2 h. Water (100 mL) is added carefully at 0 °C and the THF is evaporated in vacuo. After acidic workup (500 mL of water, 150 mL of H2SO4) and extraction with CH2Cl2, the organic layers are dried over Na2SO4 and concentrated to 50-100 mL at ambient temperature and pressure. The crude azido aldehyde D 6 -4 and sodium chlorite (67.8 g, 600 mmol, 80%) are dissolved in a mixture of tert-butanol (60 mL) and water (500 mL). Amido sulfuric acid (58.3 g, 600 mmol) is added carefully and the reaction mixture is stirred at r.t. for 1 h. The aqueous mixture is extracted with Et2O, the organic layers are washed with brine, dried over Na2SO4 and the solvent is evaporated. Dicyclohexyl amine (16 mL, 80.0 mmol) in dry Et2O (200 mL) is added to the crude azido acid D 6 -5 and the solvent is evaporated. The residue is recrystallized from Et2O, CH2Cl2-n-hexane (1:1). The isolated colorless solid is dissolved in a minimum of water and the solution is acidified to pH 1-2 with concentrated HCl. The precipitate is filtered off and the aqueous solution is extracted with Et2O. The organic layers are washed with 1 M HCl, dried over Na2SO4 and the solvent is evaporated to give an oily colourless substance, which crystallizes at reduced temperature to give colorless needles (0.76 g, 10%). Mp: 31-32 °C; 1H NMR (250.1 MHz, CDCl3): δ = 9.02 (s); 13C NMR (62.9 MHz, CDCl3): δ = 23.4 (sept, J = 20 Hz), 62.6, 178.5; IR (Film): ν [cm-1] = 2934 (sst), 2858 (st), 2361, 2111 (st), 1627 (st), 1451, 1383, 1267; MS (EI, 70 eV): m/z = 89 (13%), 76 (32%).