Organic Syntheses, Vol. 79, pp. 154-164
Checked by Frédéric Berst and Andrew B. Holmes.
1. Procedure
B. (S)-3-(tert-Butyloxycarbonylamino)-4-phenylbutanoic
acid.
A
500-mL, three-necked flask is equipped with a
nitrogen
gas inlet,
bubble counter, septum and a
magnetic
stirring bar. The flask is carefully wrapped in aluminum foil (to exclude
light during the reaction). The crude diazo ketone from the preceding step is dissolved
in
tetrahydrofuran (380 mL,
Note 10) and added to the flask
under an atmosphere of
nitrogen. De-ionized water (38 mL) is
added, the flask is immersed in a
dry ice-acetone bath, and
the solution is cooled to −25°C (temperature of the
acetone
cooling bath) for 30 min.
Silver trifluoroacetate (2.72
g, 12.3 mmol, Note 11)
is placed in a
50-mL Erlenmeyer flask and quickly dissolved
in
triethylamine (39 mL,
279 mmol, Note 3).
The resulting solution is added to the diazo ketone solution in one portion (via syringe).
The solution is allowed to warm to room temperature overnight. Evolution of
nitrogen
starts at a bath temperature of about −15°C.
The solution is transferred to a
1-L, round-bottomed flask
and the reaction vessel is rinsed with
ethyl acetate
(2 × 10 mL). The solution is evaporated to dryness
with a
rotary evaporator and the residue is stirred for 1 hr
with
saturated aqueous sodium bicarbonate
(NaHCO3) solution (100 mL, Note 12). The black mixture is transferred into a
1-L
separatory funnel with water (150 mL) and
ethyl
acetate (200 mL), and the mixture is shaken
well. The clear aqueous layer is separated and put aside, leaving an organic phase
containing a suspension of black solid.
Brine (30
mL) is added to the organic phase and the resulting mixture is
shaken vigorously.
Saturated, aqueous NaHCO3 solution
(30 mL) is added, the medium is shaken again, and the
layers are separated. The black solid is carried away with the aqueous phase, which
is now combined with the first-separated aqueous phase. The organic layer is washed
with
three additional portions of saturated aqueous NaHCO3
solution (30 mL each) and all the aqueous layers are combined.
The first organic layer is put aside and not used further. The combined aqueous layers
containing a black suspension are extracted with
ethyl
acetate (50 mL) and the
ethyl acetate
layer is then back-extracted with
two portions of saturated
aqueous NaHCO3 solution (25 mL each), which
are combined with the original aqueous layers. The
ethyl
acetate is put aside and not used further. All the combined
aqueous layers are extracted again with
50 mL
of ethyl acetate, which is washed with
saturated
aqueous NaHCO3 solution (2 × 20 mL, Note 13). The organic layer is put aside and
not used further. All the combined aqueous layers are then transferred to a
2-L,
round-bottomed flask equipped with a
magnetic stirring bar
and about
10 drops of Congo Red indicator (Note
14) and
ethyl acetate
(100 mL) are added. The flask is immersed in an
ice-water
bath, the solution is stirred and
5 N (17.5 wt
%) hydrochloric acid is added dropwise through an
addition funnel until the color of the indicator changes from
red to blue (Note
15). The solution is placed in a
1-L
separatory funnel and the organic layer is separated. The aqueous layer
is additionally extracted with
three portions of ethyl
acetate (100 mL each, Note 16). The combined organic layers are dried over
magnesium
sulfate and evaporated on a
rotary evaporator.
Residual
ethyl acetate is
azeotropically removed by adding
dichloromethane
(10 mL) three times and evaporating on the
rotary
evaporator.
Trifluoroacetic acid and
traces of solvent are removed under high vacuum (Note
17). The
product crystallizes slowly to essentially pure material (
16.9-17.1 g,
57.6-61.2
mmol,
61-65%) and
can be recrystallized (
diethyl ether/light
petroleum 1 : 1; about 100 mL) to yield
12.1 g product (
43.3
mmol,
46%, Notes
18,
19).
2. Notes
1.
Boc-phenylalanine
was obtained from Aldrich Chemical Co., Inc. (The submitters
obtained their sample from Bachem).
5.
A short stem, flame-polished funnel of diameter ca.
12.5 cm, free of any scratches or broken edges, was used to prevent spontaneous
decomposition of
diazomethane.
6.
Diazomethane
was prepared by the method described (
de Boer, Th. J.; Backer, H. J. Org. Synth.,
Coll. Vol. IV 1963, 250) using a
special diazomethane generator,
which can be purchased from Aldrich Chemical Company, Inc. (Diazald kit Z10,025-0).
The
diazomethane solution
was prepared by slow distillation of a reaction mixture, which was prepared by adding
first a solution of
21.5 g of N-methyl-N-nitroso-p-toluenesulfonamide
dissolved in
200 mL of ether
to a solution of
6 g of potassium
hydroxide, 10 mL of water,
35
mL of 2-(2-ethoxyethoxy)ethanol and
10 mL of ether, followed by a final addition
of about
30 mL of ether until
the distillate was colorless. All operations involving
diazomethane
were carried out behind a blast shield and special attention should be paid to the
safety instructions made in the above reference.
7. The crude diazo ketone is first obtained as a viscous yellow oil,
which slowly solidifies under high vacuum. The checkers always handled the solid material
behind a safety shield.
9. The checkers purified the diazo ketone (1.5 g) for characterization
purposes by dissolution in the minimum quantity of boiling
diethyl
ether (ca. 2 mL) to which was added boiling
hexane (ca. 40 mL).
The product does not crystallize until the solution is cooled to −20°C.
The crystals are isolated (0.65 g) by filtration under vacuum, washed with
hexane, and then recrystallized to
give the pure diazo ketone (
0.10 g).
The product has the following characteristics:
mp
96°C,
[α]D20
−30.4° (MeOH, c 2.57); IR
(KBr) cm
−1: 699, 1168, 1366,
1498, 1515, 1638, 1702, 2108,
2933, 2979, 3338;
1H NMR (400 MHz, CDCl
3) δ:
1.39 (s, 9 H, C
4H
9), 3.05 (m, 2 H, CH
2Ph),
4.40 (br s, 1 H, CHCH
2Ph), 5.07 (br s, 1 H, NH),
5.20 (br s, 1 H, CHN
2), 7.17-7.31 (m, 5 H, ArH);
13C NMR (100
MHz, CDCl
3) δ: 29.3, 39.6, 55.5,
59.5, 81.1, 128.0, 129.6, 130.4,
137.3, 156.1, 194.3. MS (ES
+) m/z (rel intensity) 312.1320 [(M + Na)
+,
calcd. for C
15H
19N
3O
3Na 312.1324],
290 [70, (M + H)
+]. Anal. Calcd. for
C
15H
19N
3O
3: C, 62.3; H, 6.6; N, 14.5.
Found: C, 62.3; H, 6.6; N, 14.1.
10. The checkers used distilled,
dry tetrahydrofuran
(Note
1), whereas the submitters either distilled the
tetrahydrofuran without drying, or purchased a
pure grade.
11.
Silver trifluoroacetate
was obtained from Fluka Chemika or Aldrich Chemical
Company, Inc., and used as received.
12. At this stage, the material consists of large, black lumps, which
should be broken up with a
spatula.
13. These subsequent re-extractions are essential, since this is
the most convenient method for the complete removal of the side product
Boc-phenylalanine methyl ester.
14.
Solid Congo Red was prepared
as a well-shaken 1% w/w suspension in
ethanol.
15. About
50-60 mL of hydrochloric
acid are used. The color change can be obscured by the presence
of the black solid, which should be allowed to settle from time to time so that the
solution can be clearly viewed. The checkers observed that the pH of the aqueous phase
was between 2-3 as shown by universal pH paper strips.
16. After the second extraction with
ethyl
acetate the pH value of the aqueous layer is shown to be pH
2-3. If necessary more
hydrochloric acid
is added.
17. Drying over a period of 16 hr at a pressure of 10
−3
bar (0.75 mm) is usually sufficient.
18. The submitters obtained
17.4
g (
66%). The product
has the following characteristics:
mp 102-103°C
(the submitters obtained
mp 102-106°C;
Fluka catalog 1999/2000
mp 100-104°C).
[α]D20 −15.7
(MeOH, c 1.84) [Fluka catalog 1999/2000
[α]D20 −17.5° (CH2Cl2,
c 1.00)]; IR (KBr) cm
−1:
3330 (br), 2980, 1712 (br), 1053;
1H NMR (400 MHz,
CDCl
3) δ: 1,40 (s, 9 H, C
4H
9),
2.39-2,60 (m, 2 H, CH
2Ph), 2.79-2.99 (m, 2 H,
CH
2COOH), 4.00-4.25 (br m, 1 H, CHCH
2Ph),
5.02 (br s, 0.66 H, NH), 5.96 (br s, 0.33 H, NH),
7.10-7.35 (m, 5 H, ArH), 7.70 (br s,1 H, -CO
2H);
13C NMR (100
MHz, CDCl
3) δ: 28.7, 37.8, 40.6,
49.1, 80.0, 127.0, 128.9, 129.7,
138.0, 155.6, 176.8. MS (ES
+) m/z (rel intensity) 302.1369 [(M + Na)
+,
calcd. for C
15H
21NO
4Na 302.1368], 280
[65, (M + H)
+], 224 (100), 180 (55).
Anal. Calcd. for C
15H
21NO
4: C, 64.5; H, 7.6; N, 5.0.
Found C, 64.2; H, 7.6; N, 5.2. Owing to the presence of rotamers the NMR spectra measured
at room temperature showed broadened or duplicated signals, and only the more intense
carbon resonances have been listed. The proton and
carbon
spectra of the synthetic sample were identical to those of a commercial (Fluka) sample.
19. The checkers also prepared
(R)-3-(tert-butyloxycarbonylamino)-4-phenylbutanoic
acid
from
Boc-D-phenylalanine
according to the same procedure. The enantiomeric purities of the (S)- and (R)-enantiomers
were checked by courtesy of Mr. Eric Hortense (GlaxoSmithKline, Stevenage) separately
on the corresponding methyl esters, obtained by treatment of the
β-amino
acids (40 mg, 0.14 mmol) with
polymer-supported
carbodiimide (PS-carbodiimide, Argonaut, 250 mg, 0.28 mmol)
and
4-dimethylaminopyridine (8
mg, 0.07 mmol) in
methanol/CH2Cl2
(1.4 v/v, 4 mL) for 18 hr. Subsequent filtration of the
resin and purification of the crude ester by preparative reverse phase HPLC [C18 column,
10-cm × 2-cm, gradient elution, MeCN, H
2O, CF
3CO
2H
95:5 v/v (solvent A), H
2O, CF
3CO
2H 99.9:0.1 v/v (solvent
B) varying from A:B 20:80 to 95:5 A:B over 20 min at a flow rate 6 mL min
−1
afforded, after freeze-drying, the methyl ester as a colorless powder (ca. 40 mg).
Upon chiral HPLC analysis on a Chiralpak AD column (25 cm, solvent
EtOH/heptane
5:95 v/v , flow rate 1.0 mL min
−1), the (S)-enantiomer (retention
time 9.9 min) exhibited an enantiomeric ratio of 99.5:0.5. The retention time of the
(R)-enantiomer was 8.6 min.
All toxic materials were disposed of in accordance with "Prudent Practices in the
Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
The procedure described here has been used for the synthesis of further Boc-protected
β-amino acids:
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved