Carboxylic acid potassium salt,
ethyl chloroformate,
BuLi,
anhydrous CH₂Cl₂,
(EtO)₂P(O)NH₂,

(2S)-g-N-Diethoxyphosphoryl-aN,aN-bis(tert-butyloxycarbonyl)-L-asparagine tert-butyl ester a-tert-Butyl N,N-bis(tert-butyloxycarbonyl)-L-aspartate potassium salt (515 mg, 1.20 mmol) was dissolved in anhydrous CH₂Cl₂ (11 mL) with some molecular sieves (4A, 1.6 mm pellets). Ethylchloroformate (855 uL, 970 mg, 8.7 mmol) was added dropwise and the mixture was stirred for 75 min. Some colourless solid was observed to precipitate at this point (KCl). In a second flask, diethylphosphoramidate ((EtO)₂P(O)NH₂, 1.1 g, 97%, 6.9 mmol) was dissolved in anhydrous CH₂Cl₂ (5mL), cooled to -78 ^oC and butyl lithium (2.76 mL, 2.5 M, 6.9 mmol) was added dropwise. The reaction mixture was stirred for 30 min, allowing it to reach RT. The deprotonated phosphoramidate solution was added dropwise to the first reaction mixture and then stirred overnight at RT. Diluted HCl was added to the crude reaction mixture until pH 3 was achieved. The product was extracted with CH₂Cl₂ (4 x 50 mL), the organic layers were collected together, dried (Na₂SO₄), filtered and the solvent was evaporated. A pale red oil was obtained (1.27g). The crude mixture was purified by column chromatography (30% EtOAc:70% Petrol (R_f 0.05) as eluent, increasing the ratio of EtOAc up to 100%, R_f 0.61) to give (2S)-g-diethoxyphosphoryl-aN,aN-bis(tert-butyloxycarbonyl)-L-asparagine tert-butyl ester as a colourless oil (310 mg, 588 mmol, 49%);

This reaction sequence is good for any system in which acid sensitive groups are present - it works well with compounds containing acetals, t-butyl esters or BOC groups. Carbamate protected nitrogens require two protecting groups (e.g. BoC₂) and base sensitive groups such as Fmoc are not suitable. The limitation is the solubility of the k salk, although this can be improved in some cases by the use of 18-crown-6. The low yield in this case was probably caused by a poor batch of BuLi - yields of up to 80% have been observed in other systems when the BuLi has been freshly titrated.

[a]_D25 -5.0 (c 0.88 in CH₂Cl₂); IR (oil): n_max = 3122 (CONH, N-H), 2981 (C-H), 2934 (C-H), 2256 (N-CO), 1734 (C=O), 1725 (C=O), 1366 (C(CH₃)₃, C-H), 1234 (P=O), 1141 (C-O) cm^-1; ¹H NMR (300 MHz, CDCl₃): d = 8.32-8.08 (bs, 1H, HN-P) 5.38 (dd, 1H, ³J(H,H) = 4.5Hz, ³J(H,H) = 7.6Hz, aCH), 4.25-4.05 (m, 4H, 2 x OCH₂), 3.29 (dd, 1H, ²J(H,H) = 16.0Hz, ³J(H,H) = 7.6Hz, bCH), 2.6 (dd, 1H, ²J(H,H) = 16.0Hz, ³J(H,H) = 4.5Hz, bCH), 1.34 (m, 6H, 2 x OCH₂CH₃); ³¹P NMR (121 MHz, CDCl₃, {¹H}): d = -1.8 (s); ³¹P NMR (121 MHz, CDCl₃): d = -1.8 to -1.9 (m); ¹³C NMR (75 MHz, D₂O): d = 171.4 (d, ²J(C,P) = 38Hz, CO-NH-P), 168.9 (CO₂^tBu), 151.9 (^tBuCO₂N), 83.2 (C(CH₃)₃), 81.8 (C(CH₃)₃), 64.0 (d, ²J(C,P) = 6Hz, OCH₂), 63.9 (d, ²J(C,P) = 5Hz, OCH₂), 55.3 (aCH), 36.3 (d, ³J(C,P) = 9Hz, bCH₂), 28.0 (N-(CO₂C(CH₃)₃)₂), 27.8 (CCO₂C(CH₃)₃), 16.1 (d, ³J(C,P) = 3Hz, CH₃CH₂), 16.0 (d, ³J(C,P) = 3Hz, CH₃CH₂); LCMS (ES⁺, CH₃CN/H₂O) RT 16.4 min, m/z (%): 525 (100, [M]H⁺), 425 (52, [M - Boc + H]H⁺), 369 (24, [M - Boc - C4H9 + 2H]H⁺), 325 (5, [M - 2Boc + 2H]H⁺), 269 (22, [M - 2Boc - C₄H₉ + 3H]H⁺); MS (CI⁺) m/z (%): 525 (1, [M]H⁺), 425 (12, [M - Boc + H]H⁺), 369 (74, [M - Boc - C₄H₉ + 2H]H⁺), 341 (20, [M - Boc - C₄H₉ - C₂H₅ + 3H]H⁺), 325 (12, [M - 2Boc + 2H]H⁺), 313 (22, [M - 2Boc - C₂H₅ + 3H]H⁺), 269 (20, [M - 2Boc - C₄H₉ + 3H]H⁺), 251 (28, [M - 2Boc - C₄H₉ - C₂H₅ + 4H]H⁺), 180 (18, [CONHP(O)(OCH₂CH₃)₂]⁺). HRMS (CI⁺) calcd for [M - Boc + H]H⁺, C₁₇H₃₄N₂O₈P 425.4041, observed 425.2052.

L. A. Adams, R. J. Cox. J. S. Gibson, M. B. Mayo-Martin, M. Walter and W. Whittingham, J. Chem. Soc., 2002, in press.

R. J. Cox, J. S. Gibson and M. B. mayo-Martin, ChemBioChem, 2002, in press.