A novel synthesis of N-methyl asparagine, arginine, histidine, and tryptophan

Article abstract of DOI:10.1021/ol026799w

(graph presented) R = CH₂(3-indolyl) tryptophan R = CH₂CONH₂ asparagine R = CH₂(2-imidazolyl) histidine R = CH₂CH₂CH₂(guanidyl) arginine N-Methyl amino acid residues in peptides

Full text of DOI:10.1021/ol026799w

ORGANIC
LETTERS
2002
Vol. 4, No. 21
3767-3769
A Novel Synthesis of N-Methyl
Asparagine, Arginine, Histidine, and
Tryptophan
Luigi Aurelio, Robert T. C. Brownlee, and Andrew B. Hughes*
Department of Chemistry, La Trobe UniVersity, Victoria 3086, Australia
a.hughes@latrobe.edu.au
Received August 27, 2002
ABSTRACT
N-Methyl amino acid residues in peptides modify several pharmacologically useful parameters, but synthesis of alkylated peptides is hampered
by unavailability of N-methylated monomers. The syntheses of four N-methyl amino acids with basic side chains are presented. The side
chains of these basic amino acids needed to be specially protected or constructed. This completes the set of 20 common L-amino acid
N-methyl derivatives prepared via 5-oxazolidinone intermediates by our group.
N-Methyl amino acid containing peptides are increasingly
recognized as potentially useful therapeutics.1 N-Methylation
has been shown to improve pharmacological parameters such
as membrane permeability, conformational rigidity, and
proteolytic stability.2
The ability to construct libraries of compounds including
N-methyl amino acids is dependent on the ready availability
of these building blocks. To date, there has been no method
describing the synthesis of the common 20 amino acids in
their N-methyl form with defined stereochemistry and in high
yields by a common route for formation of the N-methyl
functionality. The majority of syntheses concerning N-methyl
amino acids focus predominantly on aliphatic amino acids3
and in some cases the reaction conditions result in racem-
ization.4 Though the majority of naturally occurring N-methyl
amino acids in peptides are of the aliphatic type (MePhe,
MeAla, MeGly, MeVal, MeIle, MeLeu), there are a number
of others that contain basic, acidic, and alcoholic side chains.
The syntheses of these N-methyl amino acids are often not
straightforward or have not been described. The unavail-
ability of these optically active derivatives impedes the
synthesis of peptides that possess them.
Herein, we describe the syntheses of the four basic
N-methyl amino acids tryptophan, asparagine, histidine, and
arginine by way of intermediate 5-oxazolidinones. By
successfully completing the syntheses of these four N-methyl
amino acids, using methods similar to those in our previous
report,5 we have exploited the 5-oxazolidinone methodology
to the point that it is the first method for preparing the
N-methyl derivatives of all 20 of the common L-amino acids
(Figure 1).
Although the synthetic methods for the four basic N-
methyl amino acids use a common 5-oxazolidinone-type
(1) Fairlie, D. P. Curr. Med. Chem. 1995, 2, 654. Angell, Y. M.; Thomas,
T. L.; Flentke, G. R.; Rich, D. H. J. Am. Chem. Soc. 1995, 117, 7279 and
references therein.
(2) Cody, W. L.; He, J. X.; Reily, M. D.; Haleen, S. J.; Walker, D. M.;
Reyner, E. L.; Stewart, B. H.; Doherty, A. M. J. Med. Chem. 1997, 40,
2228. Haviv, F.; Fitzpatrick, T. D.; Swenson, R. E.; Nichols, C. J.; Mort,
N. A.; Bush, E. U.; Diaz, G.; Bammert, G.; Nguyen, A.; Nellans, H. N.;
Hoffman, D. J.; Johnson, E. S.; Greer, J. J. Med. Chem. 1993, 36, 363.
Vitoux, B.; Aubry, A.; Cung, M. T.; Marraud, M. Int. J. Pept. Protein Res.
1986, 27, 617.
(3) For example: McDermott, J. R.; Benoiton, N. L. Can. J. Chem. 1973,
51, 2555. O’Donnell, M. J.; Bruder, W. A.; Daugherty, B. W.; Liu, D.;
Wojciechowski, K. Tetrahedron Lett. 1984, 25, 3651. Wisniewski, K.;
Kolodziejczyk, A. S. Tetrahedron Lett. 1996, 38, 483. Dorow, R. L.;
Gingrich, D. E. J. Org. Chem. 1995, 60, 4986.
(4) See references in refs 3 and 5.
(5) Aurelio, L.; Brownlee, R. T. C.; Hughes, A. B.; Sleebs, B. E. Aust.
J. Chem. 2000, 53, 425.
10.1021/ol026799w CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/19/2002

Scheme 2. Synthesis of N-Methyl Arginine 10a
Figure 1. 5-Oxazolidinone strategy for the synthesis of N-methyl
R-amino acids via reductive cleavage.5
intermediate, each of them required a different strategy of
protection and/or construction of the side chain that was not
compromised by the chemistry required for the generation
of the N-methyl group.
a (a) DCC, EtSH, 92%; (b) Et3SiH, CF3CO2H, 87%; (c) CH2N2,
100%; (d) Et3SiH, 10% Pd/C; (e) MeOH, NH4OAc, NaCNBH3;
(f) di-boc-triflylguanidine, DIPEA, 49% from 7.
Asparagine. It has been shown that carbamoylation of the
side chain of glutamine allows its conversion to N-methyl
glutamine.5 This protection strategy was not possible with
asparagine and so an alternative was sought. Tritylation (Trt)
of the asparagine amide side chain was achieved under acidic
conditions (Scheme 1).6 Carbamoylation using N-(benzyl-
Reductive cleavage then proceeded smoothly to give the
N-methyl amino acid 6 (87%). The carboxylic acid was
protected as the methyl ester 7 by diazomethylation,8 in
quantitative yield. The resulting thioester was converted into
the aldehyde 8 by treatment with palladium catalyst in the
presence of triethylsilane.9 This material was not purified
and reductive amination with ammonium acetate then af-
forded the N-methyl ornithine 9. Reaction of this with the
guanylating reagent di-boc-triflylguanidine gave the desired
N-methyl arginine 10 in 49% yield from the methyl ester 7.
Tryptophan. Carbamate protection of tryptophan is an
obvious approach to N-methyl tryptophan (Abrine), but
attempted oxazolidination of the tryptophan carbamate results
in decomposition. This is presumably due to side reactions
of the indole nitrogen. An electron-withdrawing protecting
group was anticipated to solve this problem, and accordingly,
N-formyl tryptophan10 was prepared in quantitative yield
from L-tryptophan. Carbamoylation then gave the precursor
11 for oxazolidination (Scheme 3). The oxazolidination
proceeded in good yield (86%), and 12 was isolated as an
oil. The following reductive cleavage did not proceed as
planned, and two products were isolated. The minor product
Scheme 1. Synthesis of N-Methyl Asparagine 3a
a (a) Ph3COH, H2SO4, AcOH, Ac2O; (b) Et3N, DMF, BnOCO2-
Succ; (c) C6H6, (CH2O)n, DMF, CSA, vV, 83%; (d) Et3SiH,
CF3CO2H, 75%.
oxycarbonyl-oxy)succinimide (BnOCO2Succ) then gave 1,6
and subsequent oxazolidination afforded 2 (83%). The
solubility of the asparagine carbamate 1 was not high, and
a minimal amount of DMF was included in the reaction
protocol to improve substrate solubility and reaction yield.
Reductive cleavage of the oxazolidinone 2 gave a 75% yield
of the desired N-methyl product 3.7 In this reaction the
N-methyl group forms with concomitant removal of the trityl
group under the acidic conditions. The low solubility of the
N-methyl intermediate 3 necessitated workup by concentra-
tion of the reaction mixture and column chromatography of
the residue rather than the normal aqueous procedure.
Arginine. The reactions depicted in Scheme 2 offer a
synthesis of N-methyl arginine by direct and less demanding
transformations. The oxazolidinone 4 was converted to the
thioester 5 (92%) by DCC coupling with ethanethiol.
Scheme 3. Synthesis of N-Methyl Tryptophan 13a
a (a) C6H6, (CH2O)n, CSA, vV, 86%; (b) Et3SiH, CF3CO2H, 13
22%, 14 69%; (c) Et2O, tBuNH2.
(6) Sieber, P.; Riniker, B. Tetrahedron Lett. 1991, 32, 739.
(7) Sokolov, V. V.; Kozhushkov, S. I.; Nikolskaya, S.; Belov, V. N.;
Es-Sayed, M.; De Meijere, A. Eur. J. Org. Chem. 1998, 777.
3768
Org. Lett., Vol. 4, No. 21, 2002

was the expected N-methyl tryptophan 13 (22%). The major
product was the â-carboline 14 (69%), which arises by
reaction of the intermediate iminium ion with the indole in
an intramolecular electrophilic aromatic substitution. The
resulting carboxylic acid 14 was isolated as its tert-
butylammonium salt 15.
Scheme 5. Synthesis of the N-Methyl Histidine 24a
Dihydrotryptophan. To further substantiate the role of
the indole in the intramolecular interception of the iminium
intermediate that leads to both 13 and 14, tryptophan was
converted to dihydrotryptophan.11 This material underwent
bis-carbamoylation to give the precursor 16 (Scheme 4).
Scheme 4. Synthesis of the N-Methyl Dihydrotryptophan 18a
a (a) BnOCO2Succ, Et3N, CH3CN, 72%; (b) (i) PrNH2, (ii) Et3N,
CH3CN, 2,4-dinitrofluorobenzene, 84%; (c) AcOH, 2 M HCl, 3 d,
rt, 92%; (d) CH3CO2H, Ac2O, (CH2O)n, CSA 66%; (e) Et3SiH,
CF3CO2H, 81%.
hydrochloric acid resulted in hydrolysis of the methyl ester
to afford the acid 22,12 the precursor for formation of the
oxazolidinone. However, standard conditions for its forma-
tion could not be used as a result of the insolubility of 22.
This was overcome by dissolving the hydrochloride 22 in
acetic acid and acetic anhydride in the presence of cam-
phorsulfonic acid catalyst. Treatment of this mixture with
paraformaldehyde afforded the required oxazolidinone 23 in
66% yield. Reductive cleavage then gave the N-methyl
histidine carbamate 24 with the side chain imidazole still
protected with the dinitrophenyl group.
In conclusion, this application of 5-oxazolidinone inter-
mediates to the problematic R-amino acids arginine, histidine,
tryptophan, and asparagine for generating N-methyl groups
has been highly successful. It has generated novel amino
acid derivatives for inclusion in wide-ranging target synthesis
projects, and further results in these areas will be reported
in due course. Elaboration of the chemistry to encompass
new conditions for the oxazolidination and reductive cleavage
that allows the generation of novel lipoamino acids, esters,
peptide coupling chemistry, surfactants, â-amino acid deriva-
tives and target syntheses is underway.
a (a) C6H6, (CH2O)n, CSA, vV, 68%; (b) Et2SiH, CF3CO2H, 83%.
Oxazolidination proceeded smoothly to afford the mixture
of diastereoisomers 17. The key reductive cleavage now
proceeded as expected to afford the N-methyl dihydrotryp-
tophan 18 in 83% yield.
Histidine. Again, the basic and highly nucleophilic nature
of the histidine side chain caused problems in the initial
attempts to form N-methyl histidine. Selective formation of
the R-amino carbamate is also difficult, and so the following
sequence (Scheme 5) was adopted. Histidine methyl ester
dihydrochloride salt 19 was carbamoylated with 2 equiv of
BnOCO2Succ to give the bis-carbamate 20. Treatment of this
with propylamine effected removal of the imidazole car-
bamate. The reaction mixture was then evaporated under
reduced pressure, and the residue in acetonitrile was treated
with 2,4-dinitrofluorobenzene; nucleophilic aromatic substi-
tution afforded the dinitrophenyl imidazole 21. Treatment
of this compound with a mixture of acetic acid and 2 M
Acknowledgment. We thank La Trobe University for the
provision of a Post-Graduate scholarship. This research was
supported by an Australian Research Council Small Grant.
Supporting Information Available: Experimental pro-
cedures and product characterization data for compounds 2,
3, 5-7, 10, 12, 13, 15, 17, 18, and 20-24. This material is
available free of charge via the Internet at http://pubs.acs.org.
(8) De Boer, T. J.; Backer, H. J. Organic Syntheses; Wiley: New York,
1963; Collect. Vol. IV, p. 250.
(9) Fukuyama, T.; Liu, G.; Linton, S. D.; Lin, S. C.; Nishino, H.
Tetrahedron Lett. 1993, 34, 2577. Fukuyama, T.; Lin, S. C.; Li. L. J. Am.
Chem. Soc. 1990, 112, 7050.
(10) Previero, A.; Coletti-Previero, M. A.; Cavadore, J.-C. Biochim.
Biophys. Acta 1967, 147, 453.
OL026799W
(11) Daly, J. W.; Mauger, A. B.; Yonemitsu, O.; Antonov, V. K.; Takase,
K.; Witkop, B. Biochemistry 1967, 6, 648.
(12) Siepmann, E.; Zahn, H. Biochim. Biophys. Acta 1964, 82, 412.
Org. Lett., Vol. 4, No. 21, 2002
3769