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B. (S)-3-Amino-2-oxetanone p-toluenesulfonic acid salt.4 A 500-mL, singlenecked, round-bottomed flask is equipped with a magnetic stirring bar and an argon inlet adaptor (Note 1). The flask is charged with a mixture of N-(tert-butoxycarbonyl)-L-serine |}-lactone (14.0 g, 74.8 mmol) and anhydrous p-toluenesulfonic acid (13.5 g, 78.5 mmol) (Note 9). The argon inlet adaptor is replaced with a rubber septum and an argon inlet and the flask is cooled in an ice bath for ca. 15 min. Anhydrous trifluoroacetic acid (200 mL, Note 10) is added by cannula along the sides of the flask over 20 min (stirring is initiated when possible). The pale yellow solution is stirred at
0°C for 10 min, the trifluoroacetio acid is removed on a rotary evaporator at below 30°C, and the resulting syrup is placed under high vacuum (~ 0.2 mm) for ca. 1 hr. Anhydrous ether (200 mL, Note 11) is added to the resulting solid, the mixture is triturated to break up lumps, and the suspension is filtered. The solid thus obtained is washed with ether (100 mL), air dried (5 min), and then dried under reduced pressure (0.2 mm) overnight to give 18.4 g (95% yield) of (S)-3-amino-2-oxetanone p-toluenesulfonic acid salt (Note 12).
1. The glass components of the apparatus are dried overnight in a 120°C oven, and then assembled and maintained under an atmosphere of dry nitrogen or argon before use.
2. Triphenylphosphine (obtained from Aldrich Chemical Company, Inc.) and N-tert-butoxycarbonyl-L-serine (obtained from U.S. Biochemical Corporation) were dried over P2O5 for 72 hr and 24 hr, respectively. Tetrahydrofuran was distilled from sodium benzophenone ketyl directly into the glassware (under argon) the day before and stored under argon overnight until used.
3. The temperature of the solution should be about -75°C before the diethyl azodicarboxylate is added.
4. Diethyl azodicarboxylate was obtained from Fluka AG. The material used was distilled, bp 82-83°C at 2 mm, in the hood behind a safety shield. It is important that the addition of this compound to the reaction mixture be done at a constant rate without interruption because of its tendency to freeze in the syringe needle. Dimethyl azodicarboxylate (17.7 mL, 160 mmol; d = 1.33 g/mL at 25°C; bp 71-72°C at 2 mm) can be used in place of diethyl azodicarboxylate. This compound is manfactured by Tokyo Kasei Kogyo Co. and is available from CTC Organics.
5. The solution of dried N-(tert-butoxycarbonyl)-L-serine was made up separately in ah addition funnel under an atmosphere of argon. This avoids complications that may arise if the funnel is prefitted on the reaction vessel.
6. The reaction vessel was placed in a water bath at room temperature after the temperature of the mixture was ca. 15°C.
7. The reaction usually works better on a smaller scale (25 mmol) where a yield of 70% or greater can be obtained. The flash column was eluted such that the solvent level dropped 1 cm/13 sec. This corresponds to an approximate rate of 362 mL/min. The concentration and purification of the reaction mixture should be done as quickly as possible on the same day. Although storage of the concentrated reaction mixture at -20°C overnight results in substantial decomposition of the lactone, column fractions containing pure lactone (after chromatography) can be stored at 4°C overnight and concentrated on the following day.
The p-lactone is readily visualized by TLC (Merck, Kieselgel 60 F254, 0.25 mm thickness, hexane/ethyl acetate (65/35) as solvent system) by staining in iodine or by using bromocresol green spray (0.04% in EtOH, made blue by NaOH) followed by heating the plate to detect the lactone as a yellow spot on a blue background.
8. The product exhibits the following properties: mp 119.5-120.5°C; [ot]q -26.2° (CH3CN, c 1); IRcm-1: 3358, 1836, 1678, 1533, 1290, 1104; 1H NMR (360 MHz, CD2CI2) S: 1.45 (s, 9 H, -C(Chb)3), 4.4-4.45 (m, 2 H, Cj±>), 4.95-5.05 (m, 1 H, NH-CH), 5.2-5.4 (br s, 1 H, NH); MS (Cl, NH3), m/z 205 (MNH4®, 100%). Anal. Calcd for C8H13N04: C, 51.33; H, 6.99; N, 7.48. Found: C, 51.28; H, 7.01; N, 7.42.
9. Anhydrous p-toluenesulfonic acid was prepared from the monohydrate (obtained from Aldrich Chemical Company, Inc.) by solution in hot benzene with the aid of ethyl acetate and azeotropic distillation to 50% volume to remove water. The solution was then concentrated to a syrup on a rotary evaporator. The syrup was dissolved in a minimum volume of acetone and excess benzene was added to
precipitate the anhydrous acid. This material was recrystallized from acetone/benzene and dried at 50°C for 12 hr. It was stored under reduced pressure over anhydrous CaSO/j. The material used melted at 104-105°C.
10. Trifluoroacetic acid (obtained from Aldrich Chemical Company, Inc.) was refluxed over P2O5 for ca. 3 hr and then distilled from P2O5 under an atmosphere of argon. The material used distilled at 68-71 °C. All manipulations involving trifluoroacetic acid (except removal on a rotary evaporator) were done in a fume hood.