Transforming natural amino acids into α-alkyl-substituted amino acids with the help of the HOF·CH3CN complex

Article abstract of DOI:10.1021/jo0709450

(Chemical Equation Presented) α-Alkyl amino acids can be efficiently prepared in high yields from the respective amino acids themselves. The key step is the oxidation of the amine function to create the corresponding α-nitro acid in a fast and very high yield reaction followed by phase-transfer alkylation and finally reduction to the desired α-alkyl amino acid. Several such acids containing aromatic rings or additional carboxylic groups and acids with steric hindrance at the α-position are suitable substrates. Several alkyl halides were examined as alkylating agents.

Full text of DOI:10.1021/jo0709450

Transforming Natural Amino Acids into r-Alkyl-Substituted Amino
Acids with the Help of the HOF‚CH3CN Complex
Tal Harel and Shlomo Rozen*
School of Chemistry, Tel-AViV UniVersity, Tel-AViV 69978, Israel
rozens@post.tau.ac.il
ReceiVed May 10, 2007
R-Alkyl amino acids can be efficiently prepared in high yields from the respective amino acids themselves.
The key step is the oxidation of the amine function to create the corresponding R-nitro acid in a fast and
very high yield reaction followed by phase-transfer alkylation and finally reduction to the desired R-alkyl
amino acid. Several such acids containing aromatic rings or additional carboxylic groups and acids with
steric hindrance at the R-position are suitable substrates. Several alkyl halides were examined as alkylating
agents.
Introduction
as a starting point.7 We describe here a general method for
constructing R-alkylamino acids starting with the relevant amino
acids themselves eliminating the constrains most other methods
impose. The key step of the present synthesis is the use of the
acetonitrile complex of the hypofluorous acid, HOF‚CH3CN,
easily prepared by bubbling dilute fluorine through aqueous
acetonitrile at 0 °C.8
HOF‚CH3CN has established itself as one of the best oxygen
transfer agents organic chemistry has in its arsenal. Earlier
processes developed with this reagent are summarized in two
reviews describing difficult epoxidations, previously impossible
sulfides to sulfone transformations, alkane hydroxylations,
vicinal diamino oxidations, R to carbonyl hydroxylations,
synthesis of the elusive 1,10-phenanthroline N,N′-dioxides, and
much more.9,10 Recently, we have also shown that this reagent
was able to turn thiazoles and oligothiophenes into the corre-
sponding thiazole N-oxides11 and [all]-S,S-dioxide oligo-
thiophenes,12 turn azides into nitro derivatives,13 convert episul-
fides to episulfones,14 quinoxalines to quinoxaline N,N-
Unnatural amino acids, especially R-alkylated ones, are of
great interest for several reasons. They can act as potential
enzyme inhibitors in pharmaceuticals1 and form peptides
resistant to degradation as demonstrated by R-methylphenyl-
alanine, which is much sweeter and more stable than aspartame.2
Lately, members of this family of compounds emerged as key
materials in preventing the ability of amyloidogenic proteins,
responsible for diseases such as Alzheimer’s and Parkinson’s,
from adopting a â-sheet conformation by interfering with the
amyloid self-assembly process.3 Obviously, diverse synthetic
methods for the preparation of amino acids branched at R
position are needed, and indeed, some avenues to this end have
been devised in the past. These methods are usually based on
total syntheses starting with materials such as nitroacetates,4
oxazinones,5 or metalated bis-lactam ethers of 2,5-diketopip-
erazines.6 Only rarely has an original amino acid been utilized
(1) See for example: O’Donnell, M. J. Tetrahedron 1988, 44, 5389-
5401.
(2) Boesten, W. H. J.; Dassen, B. H. N.; Kleijans, J. C. S.; van Agen,
B.; van der Wal, S.; de Vries, N. K.; Schoemaker, H. E.; Meijer, E. M. J.
Agric. Food Chem. 1991, 39, 154-158.
(3) Gilead, S.; Gazit, E. Angew. Chem., Int. Ed. 2004, 43, 4041-4044.
(4) Fu, Y.; Hammarstrom, L. G. J.; Miller, T. J.; Fronczek, F. R.;
McLaughlin, M. L.; Hammer, R. P. J. Org. Chem. 2001, 66, 7118-7124.
(5) Williams, R. M.; Im, M. N. J. Am. Chem. Soc. 1991, 113, 9276-
9286.
(7) O’Donnell, M. Acc. Chem. Res. 2004, 37, 506-517. Guino, M.; Hii,
K. K. Org. Biomol. Chem. 2005, 3, 3188-3193.
(8) Rozen, S.; Brand, M. Angew. Chem., Int. Ed. 1986, 25, 554-555.
(9) Rozen, S. Acc. Chem. Res. 1996, 29, 243-248.
(10) Rozen, S. Eur. J. Org. Chem. 2005, 2433-2447.
(11) Amir, E.; Rozen, S. Chem. Commun. 2006, 2262-2264.
(12) Amir, E.; Rozen, S. Angew. Chem., Int. Ed. 2005, 44, 7374-7378.
(13) Carmeli, M.; Rozen, S. J. Org. Chem. 2006, 71, 4585-4589.
(14) Harel, T.; Amir, E.; Rozen, S. Org. Lett. 2006, 8, 1213-1216.
(6) Schollkopf, U. Pure Appl. Chem. 1983, 55, 1799-1806.
10.1021/jo0709450 CCC: $37.00 © 2007 American Chemical Society
Published on Web 07/18/2007
6500
J. Org. Chem. 2007, 72, 6500-6503

SCHEME 1a
and very high yield reaction. This nitro derivative was alkylated
with ethyl iodide to produce ethyl 2-methyl-2-nitrobutyrate
(3b)18 in 90% yield and then quantitatively reduced to R-ethyl-
alanine ethyl ester (4b).19 Higher alkyl halides such as hexyl
iodide or hexyl bromide could also be used as alkylating agents
although the yield of the alkylation process forming ethyl
2-methyl-2-nitrooctanoate (3c)18 (80%) was somewhat lower
than in the previous case. Eventually 3c was converted to
R-hexylalanine ethyl ester (4c)7 in very high yield.
Alkylation of 2b was carried out with benzyl bromide too,
and the formed 3d20 was again reduced with hydrogen over Pd/C
to form R-methylphenylalanine ethyl ester (4d).7 The same
compound was also obtained from phenylalanine ethyl ester (1e)
itself through its oxidation to 2e,16 followed by methylation with
MeI to 3e (identical to 3d) and reduction to 4e (identical to
4d). Higher alkyl halides such as ethyl iodide could also react
with the nitro derivative 2e under PTC conditions. The ethyl
2-ethyl-2-nitro-3-phenylpropionate (3f) formed in 90% yield was
then reduced to R-ethylphenylalanine ethyl ester (4f)19 in
similarly high yield.
O-Methyltyrosine ethyl ester (1g) successfully followed the
above reaction pathway as well, despite the fact that HOF‚CH3-
CN is capable of transferring oxygen to activated aromatic
rings.21 It turned out that it reacts much faster with the basic
amino group than with the anisole derivatives. The ethyl 2-nitro-
3-(4-methoxyphenyl)propionate (2g) thus obtained was meth-
ylated to form 3g and reduced to R-methyl-O-methyltyrosine
ethyl ester (4g) in nearly quantitative yields.
The oxidation step of the more sterically hindered leucine
methyl ester (1h) did not pose a problem, although the alkylation
reaction took longer than usual. The reaction led to methyl
2-nitro-4-methylpentanoate (2h)16 in 90% yield, but it took 72
h to methylate it under phase-transfer conditions to methyl 2,4-
dimethyl-2-nitro-pentanoate (3h) in 85% yield. It should be
noted that other then MeI alkylating agents did not work nearly
as good due to the steric hindrance exercised by the two
â-methyl groups on the R position. The hydrogenation step
proceeded smoothly and R-methylleucine methyl ester (4h)22
was obtained quantitatively.
The R-position of valine (1i) is also quite hindered. This fact
does not affect the oxidation step forming the nitro derivative
(2i)16 in excellent yield. It affected somewhat the methylation
step which needed 72 h to complete, but eventually the methyl
2,3-dimethyl-2-nitrobutyrate (3i) was formed in 85% yield. The
high steric hindrance on the position R-to the nitro group did
affect alkylation process with alkyl halides other than methyl
iodide. The reaction of 2i with hexyl iodide, for example, took
5 days, and even then, the resulting mixture contained little of
the desired R-hexyl derivative, large proportion of the starting
material and quite a few other byproducts. Following our goal
to produce R-alkyl amino acids 3i was successfully reduced to
form R-methylvaline methyl ester (4i)22 in 80% yield.
a 3d and 3e, as well as 4d and 4e, are identical.
dioxides15 to mention just a few. The ease of transferring oxygen
atoms to basic nitrogens in almost any molecule gave us hope
that we will be able to manipulate amino acids in preparing
some unnatural derivatives as mentioned above.
Results and Discussion
Orthodox oxidation of amino acids invariably result in
destroying the substrate through either decarboxylation, de-
amination, or other pathways mainly because of the harsh
conditions which have to be employed. Since HOF‚CH3CN is
effective even under very mild conditions, it does not destroy
the chemical backbone of the amino acids. Indeed, many natural
amino acids could be transformed to the corresponding nitro
ones in excellent yields and very short reaction times.16 An
R-alkylation of these products and a subsequent reduction of
the nitro moiety back to the parent NH2 group should produce
almost any desired R-alkyl amino acid.
Glycine ethyl ester (1a) served as a starting point for this
project. We were able to oxidize it in excellent yield and in a
matter of seconds to ethyl nitroacetate (2a). After several
experiments, we found that the best strategy for its alkylation,
and for the alkylation of all the R-nitro esters used in this work,
was to employ tetrabutylammonium hydroxide as a base under
phase-transfer conditions. Thus, treating 2a with Bu4NOH and
hexyl iodide produced ethyl 2-nitrooctanoate (3a)17 in 80%
yield. This R-nitro derivative was then successfully reduced by
hydrogenation over Pd/C in higher than 90% yield to form
R-hexylglycine ethyl ester (4a) (Scheme 1).
In order to demonstrate the general nature of the reaction,
we also employed the bifunctional glutamic acid (1j). The
oxygen transfer step performed by the HOF•CH3CN proceeds
Alanine (1b) was also successfully oxidized by the HOF‚
CH3CN complex to give ethyl 2-nitropropionate (2b) in a fast
(18) Reinheckel, H.; Tauber, G. Monatsh. Chem. 1967, 98, 1944-1953.
(19) Liu, W.; Ray, P.; Benezra, S. A. J. Chem. Soc., Perkin Trans. 1
1995, 5, 553-559.
(15) Carmeli, M.; Rozen, S. J. Org. Chem. 2006, 71, 5761-5765.
(16) Rozen, S.; Bar-Haim, A.; Mishani, E. J. Org. Chem. 1994, 59,
1208-1209.
(17) Kornblum, N.; Blackwood, R. K.; Powers, J. W. J. Am. Chem. Soc.
1957, 79, 2507-2509.
(20) Gogte, V. N.; Natu, A. A.; Pore, V. S. Synth. Commun. 1987, 17,
1421-1429.
(21) Rozen, S.; Kol, M. J. Org. Chem. 1992, 57, 7342-7344.
(22) El Achqar, A.; Roumestant, M. L.; Viallefont, P. Tetrahedron Lett.
1988, 29, 2441-2444.
J. Org. Chem, Vol. 72, No. 17, 2007 6501

Harel et al.
smoothly forming diethyl 2-nitroglutarate (2j)23 in 85% yield.
This derivative was exclusively methylated at the R to the nitro
group position forming diethyl 2-methyl-2-nitroglutarate (3j)23
in 90% yield. No alkylation at the 4 position, adjacent to the
second carbonyl was observed. The nitro derivative 3j was
eventually reduced almost quantitatively to the desired R-
methylglutamic diethyl ester (4j).24
It should be noted at this stage that although the nitro
derivatives could be obtained without much racemization, the
present phase-transfer alkylation process results in a full
racemization. We are experimenting now with several ideas to
avoid the loss of the enantiomeric purity, but of course, there
are also well-established orthodox procedures for resolving
amino acid racemates.
General Procedure for R-Alkylated Nitro Acids. The nitro
amino acid ester derivative (usually 2 g, about 10 mmol) dissolved
in 10 mL of CH2Cl2 was slowly added over a period of 30 min to
a stirred solution of 1 molar equiv of tetrabutylammonium
hydroxide, 40 wt % in water. After the mixture was stirred for 10
min, the alkyl halide (5 molar equiv) was added in one portion at
room temperature. The mixture was stirred vigorously for 48 h (72
h for the reactions with leucine and valine derivatives), and then
the organic layer was separated and washed with water and the
aqueous layers were extracted with CH2Cl2 (3 × 50 mL). The
combined organic layers were dried over MgSO4, and the solvent
was evaporated. Dry ether was added to precipitate the tetrabutyl-
ammonium iodide, the filtrate was evaporated, and the desired
products were usually purified by vacuum flash chromatography
using silica gel 60-H (Merck). Data for the new compounds or for
those not well defined in the literature is given below.
General Procedure for R-Alkylated Amino Acids. The nitro
alkylated acids were dissolved in methanol, and a catalytic amount
of Pd/C (5%) was added. The reaction mixture was stirred for 12
h at room temperature under H2 (1 atm). The catalyst was removed
by filtration, the solvent was evaporated, and the residue was
subjected to column chromatography using silica gel Lichroprep
NH2 to afford the corresponding pure amine. Data for some of the
new compounds or those not well defined in the literature is given
below, while the data for the rest of the compounds can be found
in the Supporting Information.
Conclusion
The need for unnatural amino acids is growing quickly due
to their potential biological importance. The method presented
above is unique in the sense that it always starts with the
corresponding amino acid and is of general nature. The
combination of the most reactive element of them all, fluorine,
and these delicate transformations is quite interesting.
Ethyl 2-ethyl-2-nitrobutyrate (3b):18 bright yellow oil; 90%
yield (from 2b); IR 1388, 1553, 1747 cm-1; 1H NMR δ 0.93 (3 H,
t, J ) 7.5 Hz), 1.27 (3 H, t, J ) 7.1 Hz), 1.74 (3 H, s), 2.16-2.29
(2 H, m), 4.25 (2 H, q, J ) 7.1 Hz); 13C NMR δ 8.1, 13.8, 20.7,
29.7, 62.6, 93.2, 167.5; HRMS m/z calcd for C7H13NO4 198.0736
(M + Na)+, found 198.0736.
Experimental Section
General Procedure for Working with Fluorine. This element
is a strong oxidant and very corrosive material. It should be used
only with an appropriate vacuum line whose detailed description
has been documented.25 For the occasional user, however, various
premixed mixtures of F2 in inert gases are commercially available,
simplifying the process. If elementary precautions are taken, work
with fluorine is relatively simple and we had no bad experiences
working with it.
General Procedure for Producing HOF‚CH3CN. Mixtures of
10-20% F2 with nitrogen were used in this work. The gas mixture
was prepared in a secondary container before the reaction was
started. It was then passed at a rate of about 400 mL per minute
through a cold (-15 °C) mixture of 100 mL of CH3CN and 10 mL
of H2O in a regular glass reactor.10 The development of the
oxidizing power was monitored by reacting aliquots with an acidic
aqueous solution of KI. The liberated iodine was then titrated with
thiosulfate. Typical concentrations of the oxidizing reagent were
around 0.4-0.6 mol/L.
R-Ethylalanine ethyl ester (4b):19 colorless oil; 98% yield (from
1
3b); IR 1722 cm-1; H NMR δ 0.87 (3 H, t, J ) 7.4 Hz), 1.28 (3
H, t, J ) 7 Hz), 1.32 (3 H, s), 1.57-1.63 (1 H, m), 1.74-1.78 (1
H, m), 1.97 (2 H, broad s), 4.17 (2 H, q, J ) 7 Hz); 13C NMR δ
8.4, 14.2, 25.8, 33.7, 58.1, 60.9, 177.4; HRMS m/z calcd for C7H15-
NO2 168.0982 (M + Na)+, found 168.0995.
2-Methyl-2-nitrooctanoate (3c):18 yellow oil; 80% yield (from
1
2b); IR 1386, 1553, 1746 cm-1; H NMR δ 0.88 (3 H, t, J ) 7
Hz), 1.23-1.35 (11 H, m), 1.63 (3 H, s), 2.12-2.23 (2 H, m), 4.26
(2 H, q, J ) 7 Hz); 13C NMR δ 13.8, 14.0, 21.2, 22.4, 23.6, 29.0,
31.4, 36.4, 62.6, 92.8, 167.6; HRMS m/z calcd for C11H21NO4
254.1349 (M + Na)+, found 254.1362.
R-Hexylalanine ethyl ester (4c):7 colorless oil; 93% yield (from
1
3c); IR 1718 cm-1; H NMR δ 0.86-0.89 (3 H, m), 1.21-1.35
General Procedure for Working with HOF‚CH3CN. An
appropriate amount of amino acid ester derivative (1-3 g) was
dissolved in CH2Cl2, and the mixture was cooled to 0 °C. The
oxidizing agent was treated with ∼5 g of NaF (a preferable trap
for HF which is a byproduct of the HOF‚CH3CN production) until
nearly neutral, and then 2 molar equiv of the oxidizing agent was
added in one portion to the reaction vessel (1 molar equiv is a source
of one oxygen atom). The reaction was usually quenched after a
few seconds with saturated sodium bicarbonate until neutral and
extracted with CHCl3 (3 × 50 mL), the organic layer was dried
over MgSO4, and the solvent was evaporated. The crude product
was usually purified by vacuum flash chromatography using
increasing portions of EtOAc in PE as an eluent while silica gel
60-H served as the stationary phase. The spectral and physical
properties of the known products were compared with those reported
in the literature. In every case, excellent agreement was obtained.
All known R-nitro esters were referenced throughout this work.
(14 H, m), 1.50-1.57 (1 H, m), 1.65-1.73 (1 H, m), 4.16 (2 H, q,
J ) 7 Hz); 13C NMR δ 14.0, 14.2, 22.5, 24.7, 26.4, 29.5, 31.6,
41.0, 57.7, 60.9, 177.8; HRMS m/z calcd for C11H23NO2 224.1621
(M + Na)+, found 224.1621.
Ethyl 3-(4-methoxyphenyl)-2-methyl-2-nitropropionate (3g):
1
yellow oil; 90% yield (from 2g); IR 1513, 1555, 1748 cm-1; H
NMR δ 1.28 (3 H, t, J ) 7.1 Hz), 1.66 (3 H, s), 3.37 (1 H, d, J )
14 Hz), 3.56 (1 H, d, J ) 14 Hz), 3.78 (3 H, s), 4.27 (2 H, q, J )
7 Hz), 6.80-6.82 (2 H, m), 7.01-7.04 (2 H, m); 13C NMR δ 13.8,
20.8, 41.3, 55.2, 62.9, 93.2, 114.1, 125.0, 131.2, 159.3, 167.4;
HRMS m/z calcd for C13H17NO5 290.1017 (M + Na)+, found
290.0998. Anal. Calcd for C13H17NO5: C, 58.42; H, 6.41; N, 5.24.
Found: C, 58.49; H, 6.30; N, 5.47.
R-Methyl-O-methyltyrosine ethyl ester (4g): yellow oil; 100%
1
yield (from 3g); IR 1725 cm-1; H NMR δ 1.27 (3 H, t, J ) 7.1
Hz), 1.36 (3 H, s), 1.62 (2 H, broad s), 2.73 (1 H, d, J ) 13.3 Hz),
3.07 (1 H, d, J ) 13.3 Hz), 3.78 (3 H, s), 4.15 (2 H, q, J ) 7.1
Hz), 6.80-6.82 (2 H, m), 7.06-7.26 (2 H, m); 13C NMR δ 14.2,
26.6, 45.9, 55.2, 58.7, 61.0, 113.7, 128.6, 131.0, 158.6, 177.2;
HRMS m/z calcd for C13H19NO3 238.1457 (M + H)+, found
238.1437. Anal. Calcd for C13H19NO3: C, 65.80; H, 8.07; N, 5.90.
Found: C, 65.98; H, 8.17; N, 5.91.
(23) Niyazymbetov, M. E.; Evans, D. H. J. Org. Chem. 1993, 58, 779-
783.
(24) Takenishi, T.; Simamura, O. Bull. Chem. Soc. Jpn. 1954, 27, 207-
209.
(25) Dayan, S.; Kol, M.; Rozen, S. Synthesis 1999, 1427-1430.
6502 J. Org. Chem., Vol. 72, No. 17, 2007

Methyl 2,3-dimethyl-2-nitrobutyrate (3i): yellow oil; 85% yield
(from 2i); IR 1398, 1552, 1756 cm-1; 1H NMR δ 0.97-1.01 (6 H,
m), 1.69 (3 H, s), 2.77-2.80 (1 H, m), 3.80 (3 H, s); 13C NMR δ
17.7, 17.8, 18.2, 34.2, 53.6, 96.8, 167.9; HRMS m/z calcd for C7H13-
NO4 198.0741 (M + Na)+, found 198.0736. Anal. Calcd for C7H13-
NO4: C, 47.99; H, 7.48; N, 8.00. Found: C, 47.90; H, 7.42; N,
8.27.
R-Methylglutamic diethyl ester (4j):24 colorless oil; 95% yield
1
(from 3j); IR 1734 cm-1; H NMR δ 1.15-1.24 (9 H, m), 1.78-
1.82 (1 H, m), 2.02-2.07 (1 H, m), 2.21-2.44 (2 H, m), 4.02-
4.15 (4 H, m); 13C NMR δ 14.5, 19.8, 29.1, 29.3, 61.0, 61.5, 65.3,
174.4, 175.2; HRMS m/z calcd for C10H19NO4 240.1195 (M +
Na)+, found 240.1206.
Acknowledgment. This work was supported by the Israel
Science Foundation and by the USA-Israel Binational Science
Foundation (BSF), Jerusalem, Israel. Thanks also to Dr. A.
Sacher, head of Maiman Institute for Proteome Research, Tel
Aviv University, for measuring the MS.
R-Methylvaline methyl ester (4i):22 colorless oil; 80% yield
(from 3i); IR 1733 cm-1; 1H NMR δ 0.86 (3 H, d, J ) 7 Hz), 0.91
(3 H, d, J ) 7 Hz), 1.26 (3 H, s), 1.58 (2H, broad s), 1.94-2.02 (1
H, m), 3.71 (3 H, s); 13C NMR δ 16.3, 17.4, 23.5, 35.7, 51.9, 60.7,
178.3; HRMS m/z calcd for C7H15NO2 146.1164 (M + Na)+, found
146.1175.
Supporting Information Available: General experimental
conditions, as well as complete characterization, including NMR
spectra of all compounds described in this work. This material is
available free of charge via the Internet at http://pubs.acs.org.
Diethyl 2-methyl-2-nitroglutarate (3j):23 brown oil; 90% yield
(from 2j); IR 1387, 1555, 1741 cm-1; 1H NMR δ 1.21-1.30 (6 H,
m), 1.80 (3 H , s), 2.35-2.57 (4 H, m), 4.15 (2 H, q, J ) 7 Hz),
4.28 (2 H, q, J ) 7 Hz); 13C NMR δ 14.2, 14.5, 22.0, 29.3, 31.9,
61.3, 63.4, 92.1, 167.3, 172.0; HRMS m/z calcd for C10H17NO6
248.1146 (M + H)+, found 248.1128.
JO0709450
J. Org. Chem, Vol. 72, No. 17, 2007 6503