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Org. Synth. 1993, 71, 226
DOI: 10.15227/orgsyn.071.0226
(S)-N-(9-PHENYLFLUOREN-9-YL)ALANINE AND (S)-DIMETHYL N-(9-PHENYLFLUOREN-9-YL)ASPARTATE
[L-Alanine, N-(9-phenyl-9H-fluoren-9-yl)- and L-Aspartic acid, N-(9-phenyl-9H-fluoren-9-yl-, dimethyl ester]
Submitted by T. F. Jamison1 and H. Rapoport2.
Checked by Carol M. Taylor and Amos B. Smith, III.
1. Procedure
Caution! Chlorotrimethylsilane is moisture-sensitive and should be kept under an inert atmosphere at all times. Persons following this procedure should be thoroughly familiar with the handling of moisture-sensitive materials. Lead nitrate [Pb(NO3)2] is toxic (1), and it and tripotassium phosphate (K3PO4) are hygroscopic.
A. (S)-N-(9-Phenylfluoren-9-yl)alanine (2).3 A 1-L, flame-dried, three-necked Morton flask (Note 1) equipped with an overhead stirrer, rubber septum, and reflux condenser (equipped with a rubber septum) under a nitrogen atmosphere is charged with L-alanine (1, 13.5 g, 150 mmol, (Note 2)), chloroform (375 mL, (Note 3)), acetonitrile (75 mL, (Note 4)), and chlorotrimethylsilane (19.04 mL, 150 mmol, (Note 5)). The rubber septum in the neck of the Morton flask is replaced with a glass stopper, and the mixture is heated at reflux for 2 hr with vigorous stirring under an inert atmosphere (Note 6). The mixture is cooled to room temperature under a stream of nitrogen, triethylamine (46.0 mL, 330 mmol, (Note 7)) is added via syringe at a rate sufficient to maintain a gentle reflux, and the mixture is stirred for 15 min after which Pb(NO3)2 (33.1 g, 100 mmol, (Note 8)) is added. The glass stopper is replaced with a rubber septum, and a solution of 9-bromo-9-phenylfluorene (57.8 g, 180 mmol, (Note 9)) in chloroform (180 mL, (Note 3)) is added via a Teflon cannula with a positive nitrogen pressure. The reflux condenser is replaced with a glass stopper, and an 18-gauge syringe needle equipped with an argon-filled balloon is inserted in the rubber septum. The heterogeneous, off-white mixture is stirred vigorously under this inert atmosphere for 48 hr. After about 20 hr, the reaction mixture becomes orange and darkens over time. Methanol (15.2 mL, 375 mmol, (Note 10)) is then added, and the mixture is stirred an additional 30 min.
The mixture is filtered using a sintered glass filter, the filter cake is washed by stirring with chloroform (3 × 50 mL), and the dark orange filtrate is evaporated to a residue that is partitioned between ether (750 mL, (Note 11)) and aqueous 5% citric acid (750 mL, (Note 12)). The layers are separated, and the aqueous layer is extracted with ether (4 × 250 mL). The combined organic solutions are extracted with 1 M sodium hydroxide (300 mL). The aqueous solution is washed with 300 mL of ether, cooled to 0°C with stirring using a magnetic stir bar, and the pH is adjusted to 7 by the dropwise addition of glacial acetic acid (approximately 17 mL, (Note 13)). The mixture now containing an off-white precipitate is extracted with 25% 2-propanol (Note 14) in chloroform (3 × 300 mL). The combined organic solutions are washed with 150 mL of saturated sodium chloride solution, dried (Na2SO4), filtered, and evaporated to a light yellow foam that is dried under reduced pressure to give 39.2 g (80% yield) of (S)-N-(9-phenylfluoren-9-yl)alanine (2) (Note 15).
B. (S)-Dimethyl N-(9-phenylfluoren-9-yl)aspartate (4).4 A 1-L, flame-dried, three-necked Morton flask (Note 1) equipped with an overhead stirrer and two rubber septa under a nitrogen atmosphere is charged with Pb(NO3)2 (28.2 g, 85.0 mmol, (Note 8)), K3PO4 (44.6 g, 210 mmol, (Note 16)), and acetonitrile (250 mL, (Note 4)). (S)-Dimethyl aspartate hydrochloride (3, 19.8 g, 100 mmol, (Note 17)) is added, followed by 9-bromo-9-phenylfluorene (40.15 g, 125 mmol, (Note 9)). The off-white, heterogeneous mixture is stirred vigorously for 22 hr, and to the mixture is added methanol (40.5 mL, 1.00 mol, (Note 10)); the mixture is stirred an additional 30 min and filtered through approximately 20 g of diatomaceous earth ("Celite") on a sintered glass funnel. The filter cake is washed by stirring with portions of chloroform until no UV chromophore can be detected in the filtrate (Note 18). Silica (60 g, (Note 19)) is added to the combined organic solutions, and the solvent is removed (rotary evaporator), leaving a dry powder. This powder is added to a column of silica (800 g, (Note 19)) and the column is eluted with 10% ethyl acetate in hexanes (Note 20) to give 9-methoxy-9-phenylfluorene (5.30 g, 19.5 mmol, (Note 21)) and then 9-phenyl-9-fluorenol (2.00 g, 7.7 mmol, (Note 22)). A 1/1 mixture of 9-phenyl-9-fluorenol and 4 is eluted next (2.13 g). After all the 9-phenyl-9-fluorenol has been eluted, the eluting solvent is changed to 25% ethyl acetate in hexanes (Note 20). The combined solutions of pure 4 are evaporated (rotary evaporator), yielding (S)-dimethyl N-(9-phenylfluoren-9-yl)aspartate (4) as a light yellow solid (36.1 g, 90 mmol, 90% yield, (Note 23)) after drying for 1 day at 0.1 mm. The 2.13-g mixture of 4 and 9-phenyl-9-fluorenol can be rechromatographed (100 g of silica, 10–25% ethyl acetate in hexane) to give an additional 0.8 g (3.1 mmol) of 9-phenyl-9-fluorenol (for a total of 2.80 g, 10.8 mmol) and 1.3 g (3.2 mmol, 3.2% yield) of 4, for a total of 37.4 g (93% yield) of 4.
2. Notes
1. Use of a Morton flask and an overhead stirrer allows for better mixing of the heterogenous system and gives conversion to product faster than does use of a standard round-bottomed flask with an overhead stirrer.
2. L-Alanine was purchased from Fisher Scientific company and used without further purification.
3. Chloroform was purchased from Fisher Scientific Company and distilled from phosphorus oxide (P2O5) immediately before use and added to the reaction mixture via syringe.
4. Acetonitrile was purchased from EM Science, distilled from calcium hydride (CaH2) immediately before use and added to the reaction mixture via syringe.
5. Chlorotrimethylsilane was purchased from Aldrich Chemical Company, Inc., distilled from CaH2 immediately before use, and added to the reaction mixture via syringe.
6. A syringe needle equipped with an argon-filled balloon should be inserted through the rubber septum on the reflux condenser. In addition, while the mixture is refluxing, the apparatus should be checked frequently for leaks.
7. Triethylamine was purchased from Fisher Scientific Company and distilled from barium oxide (BaO) immediately before use and added to the reaction mixture via syringe.
8. Lead nitrate (toxic!) was purchased from Fisher Scientific Company, dried in an oven at 160°C for 4 days, and cooled in a desiccator, yielding a freely-flowing, white granular solid. The checkers dried it at 100°C under 1.5 mm vacuum for 4 days.
9. 9-Bromo-9-phenylfluorene was prepared as described.5
10. Methanol was purchased from Fisher Scientific Company and used without further purification.
11. Ethyl ether was purchased from Fisher Scientific Company and used without further purification.
12. An insoluble brownish-orange polymer formed was carefully excluded from the organic extractions. Leaching of this material into the organic layer produces colored product.
13. Glacial acetic acid was purchased from Fisher Scientific Company and used without further purification. Near pH 7, much of the product precipitated, and the off-white mixture became difficult to stir. Distribution of the acetic acid was accomplished by manually swirling the flask.
14. 2-Propanol was purchased from Fisher Scientific Company and used without further purification.
15. Compound 23 thus obtained was of sufficient purity (>97%, as determined by elemental analysis) for direct use; but was contaminated by a small amount of highly colored impurities. IR cm−1: 3070 (m), 3005 (m), 2905 (m), 1765 (m), 1735 (m), 1640 (m), 1590 (m), 1450 (s), 1390 (s), 1375 (s), 1355 (s), 690 (m), 605 (m). 2 can be recrystallized (1:1 EtOAc/hexane) to give a white solid,3 mp 158–161°C. [α] −63.0° (EtOH, c 1.4); 1H NMR δ: 1.09 (d, 3 H, J = 7.2), 2.70 (q, 1 H, J = 7.2), 7.36 (m, 11 H), 7.71 (m, 2 H); 13C NMR δ: 19.2, 52.9, 73.0, 120.19, 120.21, 125.5, 125.7, 125.9, 127.6, 128.2, 128.6, 129.1, 140.5, 140.6, 141.8, 145.9, 146.5, 176.5 (Four 13C NMR signals appear to be missing, possibly due to overlapping of signals.); TLC Rf 0.25 (~8:1 EtOAc/hexane); UV (EtOH) λ, nm (ε): 310 (9,600), 298 (5,000), 266 (14,000), 238 (23,000), 209 (47,000). Anal. Calcd for C22H19NO2: C, 80.22; H, 5.81; N, 4.25. Found: C, 80.4; H, 5.6; N, 4.2.
16. Potassium phosphate was obtained from Mallinckrodt Chemical and was dried at >500°C for > 12 hr, cooled in a desiccator, ground to a fine powder with a mortar and pestle, and stored in a desiccator before use. It must be weighed quickly, as it is hygroscopic.
17. (S)-Dimethyl aspartate hydrochloride was prepared as described4 and recrystallized from acetone to give a white crystalline solid (mp 114.5–115°C, lit.6 mp 116–117°C), which was stored in a vacuum desiccator before use.
18. Eight or nine 100-mL portions of chloroform, obtained from EM Science and used without further purification, were required.
19. Silica gel of 230–400 mesh was obtained from EM Science. The checkers used silica gel from J. T. Baker.
20. Both ethyl acetate and hexanes were obtained from Fisher Scientific Company and used without further purification.
21. The physical properties of 9-methoxy-9-phenylfluorene are as follows:7 mp 93–94°C (lit.7 mp 94–95°C), 1H NMR δ: 2.96 (s, 3 H), 7.2–7.4 (m, 11 H), 7.7 (m, 2 H); 13C NMR δ: 51.3, 89.0, 119.9, 125.3, 125.5, 127.1, 128.0, 128.1, 128.9, 140,8, 143.4, 146.8; TLC (1/3 EtOAc/hexane, aluminum backed silica) Rf 0.85.
22. The properties of 9-phenyl-9-fluorenol are as follows: TLC (1/3 EtOAc/hexane, aluminum backed silica) Rf 0.67; see ref. 5 for additional spectral and physical data.
23. The physical properties of (S)-dimethyl N-(9-phenylfluoren-9-yl)aspartate (4) are as follows:4 mp 58–59.5°C (sometimes 4 does not solidify); [α]D −264° (CHCl3, c 3.3); IR (CHCl3) cm−1: 3320 (w), 3005 (m), 2950 (m), 1740 (s), 1600 (w), 1440 (s), 1365 (m), 1340 (m), 1170 (m), 1010 (m), 1000 (m), 900 (w), 690 (m), 610 (w); UV (EtOH) λ (ε): 310 (9,600), 298 (8,700), 276 (22,100), 239 (43,700), 231 (50,700), 211 (66, 200); 1H NMR δ: 2.35 (dd, 1 H, J = 15, 5.4), 2.52 (dd, 1 H, J = 15, 6.8), 3.01 (m, 1 H), 3.3 (br s, 1 H), 3.34 (s, 3 H), 3.65 (s, 3 H), 7.15–7.4 (m, 11 H), 7.7–7.8 (m, 2 H); 13C NMR δ: 39.7, 51.5, 51.8, 52.7, 72.7, 119.7, 119.9, 125.4, 125.8, 125.9, 127.2, 127.4, 127.7, 128.2, 128.3, 139.7, 141.1, 144.4, 148.3, 148.5, 170.8, 174.6 (Note: Three 13C NMR signals appear to be missing, possibly due to overlapping of signals.); TLC (1/3 EtOAc/hexane, aluminum backed silica) Rf 0.52. Anal. Calcd for C25H23NO4: C, 74.8; H, 5.8; N, 3.5. Found: C, 74.6; H, 5.8; N, 3.4.
Handling and Disposal of Hazardous Chemicals
The procedures in this article are intended for use only by persons with prior training in experimental organic chemistry. All hazardous materials should be handled using the standard procedures for work with chemicals described in references such as "Prudent Practices in the Laboratory" (The National Academies Press, Washington, D.C., 2011 www.nap.edu). All chemical waste should be disposed of in accordance with local regulations. For general guidelines for the management of chemical waste, see Chapter 8 of Prudent Practices.
These procedures must be conducted at one's own risk. Organic Syntheses, Inc., its Editors, and its Board of Directors do not warrant or guarantee the safety of individuals using these procedures and hereby disclaim any liability for any injuries or damages claimed to have resulted from or related in any way to the procedures herein.
3. Discussion
Procedures A and B illustrate the two current methods for preparation of N-9-phenylfluoren-9-yl derivatives of amino acids and amino acid esters. Free carboxylate (as in alanine in Step A) or free hydroxyl (e.g., serine8) functions can be blocked for the duration of the reaction as trimethylsilyl (TMS) esters or ethers, respectively, by treatment with chlorotrimethylsilane and triethylamine. The TMS group(s) are then removed by methanolysis from carboxylic acids (as in Step A) and mild acidic hydrolysis from hydroxyl groups, both being accomplished during product isolation. In addition to 2, the N-9-phenylfluoren-9-yl derivatives of serine,8 glutamic acid γ-methyl ester,9 and aspartic acid β-methyl ester4,10 have been prepared in this manner.
Substrates whose only reactive nucleophile is an amino group can be alkylated with 9-bromo-9-phenylfluorene using the method described in Step B. In addition to 4, the N-9-phenylfluoren-9-yl derivatives of glutamate diesters,9 aziridines,11 and N-alkyl aspartate diesters4,12,13 have been prepared by this method.
Exclusion of moisture from the reagents and apparatus is most important, as 9-bromo-9-phenylfluorene hydrolyzes to 9-phenyl-9-fluorenol, which is unreactive toward free amines. Accordingly, all glassware, K3PO4, Pb(NO3)2 (which scavenges bromide ion), and substrates must be dried thoroughly (as described above) and kept under an inert atmosphere prior to use. In addition, solvents must be distilled immediately before use and transferred to the reaction vessel under an inert atmosphere.
The methanol quench in Step B facilitates chromatographic purification by forming the known 9-methoxy-9-phenylfluorene,7 which is less polar than 9-phenyl-9-fluorenol, from 9-bromo-9-phenylfluorene (but not from 9-phenyl-9-fluorenol). Both 9-methoxy-9-phenylfluorene and 9-phenyl-9-fluorenol can be converted to 9-bromo-9-phenylfluorene by treatment with 48% hydrobromic acid (HBr).5
Both the stability14 and rigid steric bulk of the 9-phenylfluoren-9-yl group have increased significantly the utility of amino acids N-protected in this way as chiral educts for asymmetric synthesis. N-(9-Phenylfluoren-9-yl)-α-amino aldehydes maintain configurational stability at the αcarbon during treatment with silica gel or triethylamine and on treatment with Wittig and organometallic reagents.3,8,15,16 N-(9-Phenylfluoren-9-yl)-α-amino ketones and esters behave similarly under these conditions, and they can also be regioselectively enolized and subsequently alkylated with a variety of electrophiles in good to excellent yield with modest to excellent diastereoselectivity, no detectable racemization, and no detectable alkylation on nitrogen or at the carbon corresponding to the αcarbon of the starting amino acid.4,9,10,11,12,13,15,16 Consequently, these N-(9-phenylfluoren-9-yl)-α-amino carbonyl compounds have enabled the enantiospecific syntheses of many important compounds, including cyclosporin's unique amino acid MeBmt,16 other unusual amino acids,9,10 α-amino aldehydes,3,8,15,16 vinca alkaloids,4,12 (−)-vindoline,13,17 α-alkyl branched carboxylic acids,15 and the core nuclei of two antineoplastic agents.11
Removal of the 9-phenylfluoren-9-yl group has been accomplished by three different procedures: acidolysis with trifluoroacetic acid,3,9,11,12,13 catalytic hydrogenolysis,4,8,10,15,16 and dissolving metal reduction.16
This preparation is referenced from:

References and Notes
  1. Current address: Laboratorium für Organische Chemie, Eidgenössische Technische Hochschule-Zentrum, CH-8092, Zürich, Switzerland;
  2. Department of Chemistry, University of California, Berkeley, CA 94720.
  3. Lubell, W. D.; Rapoport, H. J. Am. Chem. Soc. 1987, 109, 236.
  4. Gmeiner, P.; Feldman, P. L.; Chu-Moyer, M. Y.; Rapoport, H. J. Org. Chem. 1990, 55, 3068.
  5. Jamison, T. F.; Lubell, W. D.; Dener, J. M.; Krisché, M. J.; Rapoport, H. Org. Synth., Coll. Vol. IX 1998, 103.
  6. Grassmann, W.; Wünsch, E. Chem. Ber. 1958, 91, 449.
  7. Kajigaeshi, S. Nippon Kagaku Zasshi 1963, 84, 185.
  8. Lubell, W.; Rapoport, H. J. Org. Chem. 1989, 54, 3824.
  9. Koskinen, A. M. P.; Rapoport, H. J. Org. Chem. 1989, 54, 1859.
  10. Wolf, J.-P.; Rapoport, H. J. Org. Chem. 1989, 54, 3164.
  11. Jones, R. J.; Rapoport, H. J. Org. Chem. 1990, 55, 1144.
  12. Christie, B. D.; Rapoport, H. J. Org. Chem. 1985, 50, 1239.
  13. Feldman, P. L.; Rapoport, H. J. Org. Chem. 1986, 51, 3882.
  14. Bolton, R.; Chapman, N. B.; Shorter, J. J. Chem. Soc. 1964, 1895.
  15. Lubell, W. D.; Rapoport, H. J. Am. Chem. Soc. 1988, 110, 7447.
  16. Lubell, W. D.; Jamison, T. F.; Rapoport, H. J. Org. Chem. 1990, 55, 3511.
  17. Feldman, P. L.; Rapoport, H. J. Am. Chem. Soc. 1987, 109, 1603.

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

silica gel

silica

L-Aspartic acid, N-(9-phenyl-9H-fluoren-9-yl-, dimethyl ester

acetic acid (64-19-7)

ethyl acetate (141-78-6)

methanol (67-56-1)

ether,
ethyl ether (60-29-7)

acetonitrile (75-05-8)

sodium hydroxide (1310-73-2)

citric acid (77-92-9)

chloroform (67-66-3)

sodium chloride (7647-14-5)

alanine,
L-alanine (56-41-7)

HYDROBROMIC ACID (10035-10-6)

barium oxide

nitrogen (7727-37-9)

aluminum (7429-90-5)

acetone (67-64-1)

carbon (7782-42-5)

2-propanol (67-63-0)

lead nitrate (10099-74-8)

hexane (110-54-3)

serine (56-45-1)

triethylamine (121-44-8)

calcium hydride (7789-78-8)

trifluoroacetic acid (76-05-1)

CHLOROTRIMETHYLSILANE (75-77-4)

phosphorus oxide (1314-56-3)

9-Bromo-9-phenylfluorene (55135-66-5)

9-Phenyl-9-fluorenol (25603-67-2)

potassium phosphate,
tripotassium phosphate (7778-53-2)

(S)-N-(9-Phenylfluoren-9-yl)alanine,
L-Alanine, N-(9-phenyl-9H-fluoren-9-yl)- (105519-71-9)

(S)-Dimethyl N-(9-phenylfluoren-9-yl)aspartate (120230-62-8)

9-methoxy-9-phenylfluorene

(S)-Dimethyl aspartate hydrochloride