Synthesis of N-protected N-methyl serine and threonine

Article abstract of DOI:10.1016/S0040-4039(01)00606-2

Two efficient and convenient syntheses of N-Cbz and N-Fmoc N-methyl serine and threonine are described. The amino acid side-chain alcohol can be protected as a TBDMS ether in very good yield or left free, followed by the formation and subsequent reduction of the corresponding oxazolidinone.

Full text of DOI:10.1016/S0040-4039(01)00606-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3807–3809
Synthesis of N-protected N-methyl serine and threonine
Yue Luo, Ghotas Evindar, Dan Fishlock and Gilles A. Lajoie*
Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Received 12 April 2001
Abstract—Two efficient and convenient syntheses of N-Cbz and N-Fmoc N-methyl serine and threonine are described. The amino
acid side-chain alcohol can be protected as a TBDMS ether in very good yield or left free, followed by the formation and
subsequent reduction of the corresponding oxazolidinone. © 2001 Published by Elsevier Science Ltd.
N-Methyl amino acids are important constituents of
many biologically active peptides1 and are often critical
for their biological activity. The assembly of such pep-
tides depends on the availability of N-methyl amino
acid precursors as direct N-methylation of peptide
bonds is not selective.
tion step is required precluding the use of Cbz as the
N-amino protecting group.
We report here the preparation of N-protected N-
methyl serine and threonine by an analogous scheme
but with the t-butyl dimethyl silyl (TBDMS) protection
of the side-chain alcohol.7 The (O-TBDMS) N-pro-
tected serine and threonine are reacted with formalde-
hyde and reduction of the corresponding oxazolidinone
gives the desired N-methyl derivative with simultaneous
removal of the TBDMS protecting group. We also
describe an improved procedure to prepare the O-
TBDMS derivatives of serine and threonine, giving
excellent yields with both N-Cbz and N-Fmoc protect-
ing groups.
There are a number of methods to prepare N-methyl
amino acids with unfunctionalized side chains.2 How-
ever, these methods are not adequate for the N-methyl-
ation of the b-hydroxy amino acids serine and
threonine, giving mixtures of products including O-
methylated and/or dehydrated derivatives and partial
racemization.2,3 Freidinger and co-workers reported a
method in which N-Fmoc amino acids were reacted
with paraformaldehyde in the presence of catalytic p-
TSA to form oxazolidinones4 that are subsequently
reduced5 to N-methylated amino acids with triethyl-
silane–TFA.6 The O-benzyl derivative of Fmoc N-
methyl serine was prepared in excellent yields by this
method. However, the O-benzyl protection scheme
requires commercially expensive N-protected O-benzyl-
serine or threonine, and if further elaboration of the
side-chain function is desired an additional deprotec-
Our first attempt to obtain N-methyl b-hydroxy amino
acids involved the formation of the oxazolidinone with
paraformaldehyde under standard conditions without
protection of the hydroxyl function (Scheme 1). The
desired N-Cbz oxazolidinone 1a and 1b is indeed
obtained in 25–30% yield but the major product is the
corresponding oxazoline 2a and 2b (60–65%), corre-
Scheme 1. (i) (CH2O)n, p-TsOH (0.1 equiv.), reflux in toluene.
* Corresponding author. E-mail: glajoie@uwo.ca
0040-4039/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0040-4039(01)00606-2

3808
Y. Luo et al. / Tetrahedron Letters 42 (2001) 3807–3809
Scheme 2. (i) TBDMS-Cl (1.1 equiv.), imidazole (2 equiv.), DMF; (ii) (for 3c–d) Fmoc-OSu (1 equiv.), Na2CO3, dioxane/H2O
(1:1) (*yield for two steps); (iii) (CH2O)n, TsOH (0.1 equiv.), reflux in toluene, (iv) Et3SiH (3–10 equiv.), TFA/CHCl3, rt, 3–5 days.
sponding to recently reported results.3 With the N-
Fmoc derivative the yield of the oxazolidinone 1c and
1d is better (40–45%). In each case the mixture of
products can be separated by simple base/acid extrac-
tion. The oxazolidinones 1a–d can be reduced in 90–
95% yield using the triethylsilane/TFA conditions
reported by Freidinger and co-workers.6
Two alternative methods to the Freidinger procedure
are described for the generation of N-protected N-
methyl b-hydroxy amino acids. The first method, with-
out protection of the hydroxyl group, provides a
shorter route to the N-Fmoc protected substrates while
the second method with O-TBDMS protection works
best for the N-Cbz group, and provides the highest
yields overall. In each case, purification is convenient at
each step simplifying scale-up. The choice between the
two methods will thus be dependent on the nature of
the b-hydroxy amino acid and which N-protecting
group is desired.
We have examined a similar reaction scheme with
protection of the hydroxyl group as O-TBDMS ether.
The O-TBDMS protecting group is introduced to Cbz-
serine and Cbz-threonine to give the corresponding
products 3a (90%) and 3b (82%) via reaction with
TBDMS-Cl and imidazole in DMF for 2–3 days8–10
(Scheme 2). The products are readily separated by
simple base/acid extraction, and no chromatography is
needed.11,12
Acknowledgements
We wish to thank NSERC for financial support.
In contrast, Fmoc-serine and Fmoc-threonine react
extremely slowly under the same conditions and give a
very low yield of product (20–40%) along with partial
loss of Fmoc protecting group.10 The crude products
are difficult to purify by acid/base extraction and chro-
matography is necessary. Increasing the amount of
TBDMS-Cl (5 equiv.) and imidazole (3 equiv.) does not
improve the yield. The protection sequence was
reversed such that the TBDMS group was first intro-
References
1. For example: MeBMT in cyclosporin A.1a b-OH-N-Me-
L
-valine in aureobasidins:1b (a) Dreyfus, M.; Harri, E.;
Hofmann, H.; Kobel, H.; Pache, W.; Tscherter, H. Eur.
J. Appl. Microb. 1976, 3, 125; (b) Takesako, K.; Ikai, K.;
Haruna, F.; Endo, M.; Shikamanka, K.; Sono, E.; Naka-
mura, T.; Kata, I.; Tamaguchi, H. J. Antibiot. 1991, 44,
919.
duced to
L
-serine and
L
-threonine, followed by addition
-serine
of the Fmoc group (Scheme 2). The reaction of
L
with TBDMS-Cl (1.1 equiv.) and imidazole (2.2 equiv.)
2. N-Methylation of N-tosylated amino acids with MeI
occurs in basic solutions at 65°C, but deprotection of the
Tos group results in racemization in acidic conditions
unless carried out in Na/NH3.2a N-Benzyl protection is
introduced via reductive amination of amino acids, then
reductive alkylation with paraformaldehyde yields N-
methyl amino acids,2b but Schiff base intermediates tau-
tomerize causing racemization.2e N-Boc or Cbz protected
amino acids react with methyl iodide and silver oxide to
give N-methyl amino acid methyl esters2c which racemize
during saponification of the methyl ester.2d,e The reaction
with Boc-Ser and Boc-S-benzyl cysteine gave b-elimina-
tion by-products.2c Boc or Cbz amino acids have been
N-methylated with NaH/CH3I in THF at room tempera-
ture.2d–i Boc-Ser(Bzl)-OH undergoes b-elimination to
room temperature under this condition:2g (a) Fisher, E.;
Lipschitz, W. Ber 1915, 48, 360; (b) Quitt, P.; Hellerbach,
J.; Vogler, K. Helv. Chim. Acta 1963, 46, 327; (c) Olsen,
R. K. J. Org. Chem. 1970, 35, 1912; (d) McDermott, J.
in DMF gives the ether in excellent yield (90%),13 while
L
-threonine gives a very good yield (77%) with
TBDMS-Cl (2 equiv.) and imidazole (3 equiv.). The
Fmoc protecting group was subsequently introduced in
a standard procedure to give the corresponding (O-
TBDMS) protected Fmoc-serine 3c and Fmoc-
threonine 3d with high yields (90–98%). This approach
avoids column purification and provided higher overall
yield than the literature method.10
The O-TBMDS amino acids 3a–d were refluxed with
paraformaldehyde and catalytic p-TsOH in toluene
using a Dean–Stark apparatus to give the oxazolidi-
none 5a–d in very good yields.14 Deprotection of the
hydroxyl groups and reduction of the oxazolidinone
5a–d is achieved in one-step using TFA and triethyl-
silane giving protected N-methyl amino acids 6a–d in
excellent yields.15

Y. Luo et al. / Tetrahedron Letters 42 (2001) 3807–3809
3809
R.; Benoiton, N. L. Can. J. Chem. 1973, 51, 2555; (e)
Cheung, S. T.; Benoiton, N. L. Can. J. Chem. 1977, 55,
916; (f) McDermott, J. R.; Benoiton, N. L. Can. J. Chem.
1973, 51, 1915; (g) Cheung, S. T.; Benoiton, N. L. Can. J.
Chem. 1977, 55, 906; (h) Coggins, J. R.; Benoiton, N. L.
Can. J. Chem. 1971, 49, 1968; (i) Chen, F. M. F.;
Benoiton, N. L. Can. J. Chem. 1971, 55, 1433; (j) Chen,
F. M. F.; Benoiton, N. L. J. Org. Chem. 1979, 44, 2299.
3. Aurelio, L.; Brownlee, R. T. C.; Hughes, A. B.; Sleebs, B.
E. Aust. J. Chem. 2000, 53, 425.
4. Ben-Ishai, D. J. Am. Chem. Soc. 1957, 79, 5736.
5. Auebach, J.; Azmore, M.; Weinreb, S. M. J. Org. Chem.
1976, 41, 725.
6. (a) Freidinger, R. M.; Hinkle, J. S.; Perlow, D. S.;
Arison, B. H. J. Org. Chem. 1983, 48, 77; (b) Groth, U.;
Beulshausen, T. Ger. Offen. DE-91-4122940A1 1992.
7. This material previously presented at the 16th American
Peptide Symposium, Minneapolis, Minnesota, June 26–
July 1, 1999.
4.00–4.10 (br m, 1H), 3.80–3.87 (br m, 1H), 0.84 (2, 9H),
0.02 (s, 6H).
13. Sample procedure: L-Ser (10.6 g, 0.1 mol) was suspended
in DMF (100 mL). Imidazole (2 equiv.) and TBDMSCl
(1.1 equiv.) were added and the mixture stirred at rt for
20 h. DMF was evaporated to give an oily residue. This
was stirred with 1:1 H2O:hexane for 4 h and the resulting
white solid filtered off, rinsed with hexane and air dried
to give
L
-Ser(OTBDMS)-OH (20.0 g, 91% yield). 1H
NMR (CD3OD, 250 MHz): l 3.84–3.98 (m, 2H), 3.49–
3.55 (m, 1H), 0.82 (s, 9H), 0.01 (s, 6H).
14. Sample procedure: 3a (10.16 g, 28.74 mmol),
paraformaldehyde (5.08 g) and p-TsOH (0.27 g, 0.05
equiv.) were suspended in toluene (550 mL) and slowly
heated to dissolve most of the starting material, then
refluxed for 1 h using a Dean–Stark water removal
apparatus. The reaction was cooled to rt and washed
with 5% NaHCO3, brine, dried over MgSO4, and evapo-
rated to yield 5a as a yellow non-viscous oil (7.60 g, 72%
yield). 1H NMR (CDCl3, 250 MHz): l 7.24–7.39 (m, 5H),
5.47–5.54 (bs, 2H), 5.11–5.31 (m, 3H), 4.15–4.30 (bs, 1H),
3.90–4.10 (bs, 1H), 0.82 (s, 9H), −0.01 (s, 3H), −0.03 (s,
3H).
15. Sample procedure: 5a (7.13 g, 19.51 mol) was dissolved in
CHCl3 (40 mL) under argon at rt in which TFA (40 mL)
was added followed by Et3SiH (3 equiv.). The mixture
was stirred at rt and monitored by TLC (95:5
CH2Cl2:MeOH). Additional Et3SiH (up to 10 equiv.) was
periodically added. After 3 days the reaction was concen-
trated and re-dissolved in CH2Cl2 several times to remove
most of the TFA and Et3SiH. The crude oil was dissolved
in hexane and extracted with 5% NaHCO3. The aqueous
fraction was acidified to pH 3 using 1 M KHSO4, and
extracted with EtOAc. The combined EtOAc fractions
were washed with brine and dried over MgSO4, and
evaporated to yield 6a as thick colorless oil (96% yield).
1H NMR (CDCl3, 250 MHz): l 7.20–7.45 (m, 5H),
5.10–5.20 (br, 2H), 4.55–4.63 (m, 1H), 4.05–4.15 (m, 1H),
3.85–4.05 (m, 1H), 3.04 (s, 3H).
8. Corey, E. J.; Venkateswarlu, A. J. Am. Chem. Soc. 1972,
94, 6190.
9. Bohnstedt, A. C.; Vara Prasad, J. V. N.; Rich, D. H.
Tetrahedron Lett. 1993, 34, 5217.
10. Fischer, P. M. Tetrahedron Lett. 1992, 33, 7605 (No
yields reported; our attempts under conditions described
provided very low yields).
1
11. All compounds were characterized by H and 13C NMR,
ES-MS and elemental analysis.
12. Sample procedure: Cbz-Ser (15 g, 62.7 mmol), TBDMSCl
(1 equiv.) and imidazole (2 equiv.) were dissolved in dry
DMF (150 mL) and stirred at rt for 48 h under argon.
The reaction was concentrated, suspended in hexane and
extracted with 5% NaHCO3. The aqueous fraction was
acidified to pH 3 using 1 M KHSO4 then extracted into
EtOAc. The combined EtOAc fractions were washed with
brine, dried over MgSO4 and evaporated to dryness.
After 4 h under high vacuum 3a was obtained as a white
1
solid (20.50 g, 90% yield). H NMR (CDCl3, 250 MHz):
l 9.65–10.00 (bs, 1H), 7.23–7.34 (m, 5H), 5.58–5.61 (d,
1H, J=8.1 Hz), 5.10–5.13 (br m, 2H), 4.42–4.46 (bs, 1H),
.