A short practical synthesis of 2′-deoxymugineic acid

Article abstract of DOI:10.1016/j.tetlet.2005.01.030

A short and practical synthesis of 2′-deoxymugineic acid (DMA) has been developed via reductive alkylation with the aldehyde intermediates. No protection of azetidine-2-carboxylic acid was required and the presence of free carboxylic acid function facilitated purification by simple acid and base extractions. Furthermore, the intermediates were conveniently purified by HPLC due to the presence of chromophoric benzyl ester protecting group(s). Hydrogenolysis of the benzyl protecting groups in the final step furnished DMA in overall good yield.

Full text of DOI:10.1016/j.tetlet.2005.01.030

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 1419–1421
A short practical synthesis of 20-deoxymugineic acid
Satendra Singh,* George Crossley, Saswati Ghosal,
Yann Lefievre and Michael W. Pennington
BACHEM Bioscience, Inc., 3700 Horizon Drive, King of Prussia, PA 19406, USA
Received 7 December 2004; revised 7 January 2005; accepted 10 January 2005
Abstract—A short and practical synthesis of 20-deoxymugineic acid (DMA) has been developed via reductive alkylation with the
aldehyde intermediates. No protection of azetidine-2-carboxylic acid was required and the presence of free carboxylic acid function
facilitated purification by simple acid and base extractions. Furthermore, the intermediates were conveniently purified by HPLC due
to the presence of chromophoric benzyl ester protecting group(s). Hydrogenolysis of the benzyl protecting groups in the final step
furnished DMA in overall good yield.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
version to thioamide and subsequent desulfurization
with Raney-nickel.7,8 Reductive alkylation has been car-
ried out with a protected carboxylate aldehyde3–5 or
with a masked carboxylate functionality, such as p-
methoxyphenyl group, which is then oxidized to afford
a carboxylic acid.6
20-Deoxymugineic acid (DMA) is an important phyto-
siderophore. It is isolated fromthe roots of gramina-
ceous plants, for example, barley, oats, rice, wheat,
etc. It is produced when these plants are kept under
iron-deficient conditions. Its main function is to chelate
FeIII fromthe soil and assist in its transportation in the
plants.1
DMA is a polar molecule and has to be purified using
H+ ion-exchange chromatography, because it does not
contain any chromophore to allow purification by re-
versed phase HPLC. In this communication, we report
a short and practical synthesis of DMA via reductive
alkylation. The important aspect of the synthesis is the
choice of the protecting groups that permitted purifica-
tion of the intermediates by reversed phase HPLC. Fur-
thermore, we would like to point out that the use of a
base in the final step of DMA synthesis should be
avoided because it leads to the formation of lactam,
The importance of DMA in plant physiology has led
several groups to develop its synthesis.2 DMA (1) mole-
cule contains two reduced peptide bonds and one a-hy-
droxy acid. As shown in Scheme 1, it can be assembled
fromthree appropriately functionalized building blocks.
There are mainly two strategies for its synthesis: (1)
reductive alkylation with the protected aldehydes,3–6
and recently, (2) reduction of the amide bond via con-
CO2R
CO2H
CO2R
NH
CO2R
NHR
CO2R
OR
N
CO2R
O
N
CO2H
OHC
OHC
+
+
O
N
CO2R
OR
N
CO2H
H
H
OH
1
Scheme 1.
Keywords: DMA; 20-Deoxymugineic acid; Reductive alkylation; Reduced peptide bond.
*
Corresponding author. Tel.: +1 610 239 0300; fax: +1 610 239 0800; e-mail: ssingh@usbachem.com
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.01.030

1420
S. Singh et al. / Tetrahedron Letters 46(2005) 1419–1421
which is difficult to separate. In earlier reports, molecu-
lar ion peak for lactamhas been presumed to be the
fragment ion peak for DMA [M+HꢀH2O]+.8
Dowex 50 W · 8 (H+) ion-exchange resin with 1 N aque-
ous ammonia as the eluting agent as described earl-
ier.7,13 However, MS analysis revealed significant
contamination from the lactam 7, which perhaps formed
during the base-mediated hydrolysis of the ester func-
tions in contrast to earlier reports.4,7,8 Base-catalyzed
lactam formation is similar to aspartimide formation
in peptides containing Asp-Gly motif. The problem is
minimized by employing a bulky protecting group
group, such as t-butyl or using a milder base.14 Alterna-
tively, the secondary amine function could be protected
with a suitable protecting group and should be cleaved
only in the last step after deprotection of the carboxy
function of malic acid moiety.3
2. Results and discussion
Synthesis of DMA appeared to be rather straightfor-
ward using either of the above-described approaches.
In our first attempt, we followed the strategy of thio-
amide reduction,7 because the preparation of the
building blocks was convenient. Besides, protection of
the carboxylic acid groups as methyl esters further
improved the overall synthesis.
As shown in Scheme 2, azetidine-2-carboxylic acid (2)
was reacted with Z-Asp(OSu)-OMe (3), which after
esterification with MeOH/HCl(g) and hydrogenation
afforded the dipeptide 4 in 85% yield. Use of OSu ester
3 eliminated the need for protection of the carboxylic
acid function of 2, which has been synthesized in three
steps.9 Treatment of the dipeptide 4 with the protected
malic acid OSu ester 5 in the presence of diisopropyleth-
ylamine (DIEA) gave the tripeptide 6 in good yield
(83%).10 Reduction of the amide bond was accom-
plished in two steps: thioamidation with LawessonÕs
reagent11 and hydrogenolysis with Raney-nickel.12 The
final step involved the base-mediated hydrolysis of the
ester groups. The crude product was purified by using
Therefore, another strategy was designed, which
avoided the use of alkaline reagents during synthesis
as well as purification. Furthermore, the protecting
groups were chosen in such a way as to allow monitor-
ing and purification by HPLC.
As shown in Scheme 3, reductive amination of Boc-
Asp(H)-OBzl (8) with the unprotected azetidine-2-carb-
oxylic acid (2) afforded 9 in 51% yield.15,16 The reaction
was conducted in 1% AcOH/DMF as solvent and uti-
lized solid NaBH3CN as the reducing agent. Use of
unprotected amino acid 2 was based on two reasons:
(1) reductive alkylation can be performed in DMF con-
taining 1% AcOH;17 thereby, unprotected –COOH
CO2Me
SuO
CO2Me
NH-Z
CO2H
NH
1)
DMF/
DIEA
SuO
CO2Me
OAc
O
N
O
CO2Me
3
+
2) MeOH/HCl(g)
3) H2; Pd/C
O
NH2
2
5
4
CO2Me
N
O
CO2Me
CO2H
N
1) Lawesson's reagent
N
CO2Me
OAc
CO2H
H
1
+
2) Raney-Ni
3) 1%KOH/MeOH
4) Dowex 50W x 8(H+)
O
N
6
O
HO
7
Scheme 2.
CO2Bzl
NH-Boc
OHC
CO2H
NH
CO2H
CO2H
N
1)
8
TFA
N
CO2Bzl
NH-Boc
CO2Bzl
2) NaBH3CN/DMF
NH2
9
2
10
CO2Bzl
CO2H
OHC
1)
OTHP
N
CO2Bzl
11
1) HPLC purification
2) H2; Pd/C, MeOH
1
2) NaBH3CN
3) 90% TFA/H2O
N
CO2Bzl
OH
H
12
Scheme 3.

S. Singh et al. / Tetrahedron Letters 46(2005) 1419–1421
1421
group in 2 was not considered a hindrance, and (2) the
presence of free carboxylic acid would allow isolation
of the intermediate(s) by simple acid base extractions.
Cleavage of the Boc protecting group with TFA/DCM
(1:1) afforded 10 in quantitative yield.18 Reductive alkyl-
ation of 10 with THP protected malic acid half aldehyde
11 in 1% AcOH/DMF gave THP/benzyl protected
DMA in 52% yield. Unreacted acid 10 was quantita-
tively recovered fromthe reaction mixture by extraction
with 5% aqueous sodiumcarbonate solution, followed
by acidification with HCl. The crude THP/benzyl pro-
tected DMA was treated with 90% TFA/H2O at ambient
temperature for 30 min, which afforded dibenzyl pro-
tected DMA 12 in 96% yield. Partially protected
DMA 12 was purified by reversed phase HPLC to
95% purity using standard 0.1% TFA/H2O/MeCN
buffers in 48% isolated yield.19 Hydrogenolysis of the
benzyl protecting groups using 10% Pd/C and H2 gas
in MeOH at 60 psi occurred quantitatively and without
any lactam formation. Removal of the solvent and crys-
tary data is available online with the paper in ScienceDi-
rect. Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/
j.tetlet.2005.01.030.
References and notes
1. Ro¨mheld, V. Plant Soil 1991, 130, 731–740.
2. Shioiri, T.; Hamada, Y.; Matsuura, F. Tetrahedron 1995,
51, 3939–3958.
3. Ohfune, Y.; Tomita, M.; Nomoto, K. J. Am. Chem. Soc.
1981, 103, 2409–2410.
4. Fushiya, S.; Sato, Y.; Nakatsuyama, S.; Kanuma, N.;
Nozoe, S. Chem. Lett. 1981, 909–912.
5. Shioiri, T.; Irako, N.; Sakakibara, S.; Matsuura, F.;
Hamada, Y. Heterocycles 1997, 44, 519–530.
6. Matsuura, F.; Hamada, Y.; Shioiri, T. Tetrahedron 1994,
50, 9457–9470.
7. Klair, S. S.; Mohan, H. R.; Kitahara, T. Tetrahedron Lett.
1998, 39, 89–92.
8. Miyakoshi, K.; Oshita, J.; Kitahara, T. Tetrahedron 2001,
57, 3355–3360.
9. Matsuura, F.; Hamada, Y.; Shioiri, T. Tetrahedron 1993,
49, 8211–8222.
ˆ
10. Henrot, S.; Larcheveque, M.; Petit, Y. Synth. Commun.
1986, 16, 183–190.
tallization fromH O/EtOH afforded DMA as a white
2
solid in >12% overall yield, which exhibited same phys-
ical and spectrometric characteristics as the reference
DMA sample.
The aldehyde 11 was prepared from the commercially
available (S)-(ꢀ)-a-hydroxy-c-butyrolactone via protec-
tion of the hydroxyl function with DHP, hydrolysis of
the lactone with aqueous KOH, protection of the carb-
oxylic acid function with benzyl bromide, and Swern
oxidation of the primary alcohol.3 We also tried TBS
protecting group for the a-hydroxyl group of c-butyro-
lactone.5 However, TBS group was less stable to alkaline
conditions, which were used in the next step to hydro-
lyze the lactone.
11. Clausen, K.; Thorsen, M.; Lawesson, S.-O. Tetrahedron
Lett. 1981, 37, 3635–3639.
12. Kornfeld, E. C. J. Org. Chem. 1950, 16, 131–138.
13. Beck, A. K.; Blank, S.; Job, K.; Seebach, D.; Sommerfeld,
Th. Org. Synth. Coll. 1998, Vol. IX, 626–632.
14. Mergler, M.; Dick, F.; Sax, B.; Sta¨helin, C.; Vorherr, T. J.
Pept. Sci. 2003, 9, 518–526.
15. Borch, R. F.; Bernstein, M. D.; Durst, H. D. J. Am. Chem.
Soc. 1971, 93, 2897–2904.
1
16. Compound 9. H NMR (DMSO-d6): 7.52–7.28 (m, 5H),
5.23–5.01 (m, 2H), 4.80–4.69 (m, 1H), 4.11–4.04 (m, 1H),
3.80–3.60 (m, 2H), 3.20–3.11 (m, 1H), 3.05–2.95 (m, 1H),
2.50–2.41 (m, 1H), 2.38–2.26 (m, 1H), 2.00–1.85 (m, 1H),
1.85–1.72 (m, 1H), 1.46 (s, 9H).
In summary, a short and practical synthesis of DMA
was developed. The new synthesis has several advanta-
ges. For example, synthesis involves fewer steps than re-
ported in literature. The reactions are easy to monitor
by HPLC. The intermediates could be purified simply
by acid and base extractions or by reversed phase
HPLC. Overall yield using above strategy was >12%.
17. Sasaki, Y.; Coy, D. H. Peptides 1987, 8, 119–121.
1
18. Compound 10. H NMR (DMSO-d6): 7.52–7.31(m, 5H),
5.25 (s, 2H), 4.79–4.64 (m, 1H), 4.24–4.01 (m, 1H), 3.81–
3.72 (m, 1H), 3.63–3.52 (m, 1H), 3.30–3.20 (m, 1H), 3.16–
3.04 (m, 1H), 2.50–2.42 (m, 1H), 2.41–2.36 (m, 1H), 2.17–
2.04 (m, 1H), 2.01–1.93 (m, 1H).
19. Compound 12. 1H NMR (DMSO-d6): 7.40–7.30 (m, 10H),
5.42–5.33 (m, 1H), 5.15–5.05 (m, 4H), 4.89–4.85 (m, 1H),
4.75–4.68 (m, 1H), 4.29–4.24 (m, 1H), 4.18–4.10 (m, 1H),
3.35–3.30 (m, 1H), 3.30–3.21 (m, 1H), 2.74–2.65 (m, 1H),
2.30–2.22 (m, 2H), 2.10–2.04 (m, 1H), 2.00–1.95 (m, 1H),
1.85–1.75 (m, 2H), 1.67–1.62 (m, 1H).
Supplementary data
Mass spectrometric analysis of DMA samples obtained
fromtwo different methods is available. The supplemen-