Hydrolysis of cyclic ureas under microwave irradiation: Synthesis and characterization of 7,8-diaminopelargonic acid

Article abstract of DOI:10.1080/00397910701464951

A simple and efficient method for the synthesis of 7,8-diaminopelargonic acid, a key intermediate in the biotin biosynthesis pathway, is reported. The d-desthiobiotin powder was dissolved in concentrated hydrochloric acid, and the solution was exposed to microwave radiation of 2.45 GHz for varying lengths of time ranging from 60 s to 2 min. The product thus obtained was characterized by spectroscopic techniques and confirmed through bioassay. Further, the protocol was extended to the synthesis of several diamines from their corresponding cyclic ureas. The results show that the method is generally applicable and not only accelerates the hydrolysis reaction but also offers excellent yields. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701464951

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Hydrolysis of Cyclic Ureas under Microwave
Irradiation: Synthesis and Characterization of
7,8‐Diaminopelargonic Acid
Ganga Ramu Vasanthakumar a , Vikrant M. Bhor a & Avadhesha Surolia a
a Molecular Biophysics Unit, Indian Institute of Science, Bangalore, India
Version of record first published: 14 Aug 2007.
To cite this article: Ganga Ramu Vasanthakumar , Vikrant M. Bhor & Avadhesha Surolia (2007): Hydrolysis
of Cyclic Ureas under Microwave Irradiation: Synthesis and Characterization of 7,8‐Diaminopelargonic Acid,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
37:16, 2633-2639
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Synthetic Communicationsw, 37: 2633–2639, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701464951
Hydrolysis of Cyclic Ureas under
Microwave Irradiation: Synthesis and
Characterization of 7,8-Diaminopelargonic
Acid
Ganga Ramu Vasanthakumar, Vikrant M. Bhor, and
Avadhesha Surolia
Molecular Biophysics Unit, Indian Institute of Science, Bangalore, India
Abstract: A simple and efficient method for the synthesis of 7,8-diaminopelargonic
acid, a key intermediate in the biotin biosynthesis pathway, is reported. The d-desthio-
biotin powder was dissolved in concentrated hydrochloric acid, and the solution was
exposed to microwave radiation of 2.45 GHz for varying lengths of time ranging
from 60 s to 2 min. The product thus obtained was characterized by spectroscopic tech-
niques and confirmed through bioassay. Further, the protocol was extended to the
synthesis of several diamines from their corresponding cyclic ureas. The results
show that the method is generally applicable and not only accelerates the hydrolysis
reaction but also offers excellent yields.
Keywords: acid hydrolysis, cyclic urea, d-desthiobiotin, 7,8-diaminopelargonic acid,
microwave irradiation
INTRODUCTION
Biotin (vitamin H) plays an important role in a number of carboxylation and
transcarboxylation reactions, which are essential for the growth of plants,
microorganisms, and animals. The biosynthesis of biotin, however, occurs
only in plants and microorganisms, in four enzyme-catalyzed reactions
Received December 15, 2006
Address correspondence to Avadhesha Surolia, Molecular Biophysics Unit, Indian
Institute of Science, Bangalore 560 012, India. E-mail: surolia@mbu.iisc.ernet.in
2633

2634
G. R. Vasanthakumar, V. M. Bhor, and A. Surolia
following the initial synthesis of pimeloyl-CoA. 7-Keto-8-aminopelargonic
acid (KAPA), 7,8-diaminopelargonic acid (DAPA), and desthiobiotin are
the intermediates in the biotin biosynthesis pathway.[1] DAPA is produced
by the DAPA synthase catalyzed transfer of an amino group from S-
adenosyl methionine (SAM) to KAPA. It is taken up by dethiobiotin synthe-
tase, the next enzyme in the pathway, which converts it to desthiobiotin by
the addition of CO2 in an ATP-dependent reaction.[2] In the literature, very
few methods are available for the synthesis of DAPA. Nudelman et al.
described detailed synthesis of KAPA, racemic DAPA methyl ester, and
their analogues starting from L-alanine as a chiral substrate.[3] However, we
were unable to reproduce their results for DAPA synthesis. du Vigneaud
et al. have reported synthesis of DAPA by hydrolyzing the cyclic
urea moiety of desthiobiotin ester with minimum characterization data.[4]
There are a few other methods available for the hydrolysis of cyclic ureas,
but these are either specific or the conditions employed are very harsh
requiring long times (2.5–19.5 h) and resulting in poor yields.[5,6] The import-
ance of DAPA and the lack of a generally applicable method for the hydrolysis
of cyclic ureas prompted us to explore an efficient and diastereospecific
synthesis.
Microwave irradiation can dramatically increase reaction rates and
thereby decrease the reaction time. Moreover, it has also been an attractive
tool for enhancing the selectivity.[7] Additionally, modern microwave
instruments allow precise control of temperature and pressure in sealed
reaction tubes. Therefore, over the past two decades, organic chemists
have explored its utility for various types of organic reactions. Here, we
report the direct hydrolysis of cyclic urea to its corresponding diamine
hydrochloride salt.
RESULTS AND DISCUSSION
Our preliminary approach involved hydrolysis of d-desthiobiotin using con-
centrated HCl under conditions developed by du Vigneaud et al. The high
temperature (2008C) employed, along with the long duration (2.5 h) of conven-
tional heating, results in partial charring of the product. This is however not sur-
prising, as the melting point of DAPA is 1808C. In view of the need for
improvements and our prior results in the synthesis of amino acid esters, b-
amino acids, isocyanates, and antimicrobial 2-hydroxy diaryl ethers[8]
employing microwave irradiation for similar thermal reactions, we decided
to reinvestigate this reaction. Initial efforts focused on optimizing
microwave conditions for the formation of 7,8-diaminopelargonic acid based
on prior investigations of conventional thermal conditions. The above
reaction was successfully carried out using microwave irradiation. Thus the
method became very reproducible and gave the desired product in excellent

Hydrolysis of Cyclic Ureas
2635
Scheme 1. Synthesis of 7,8-diamino pelargonic acid by microwave irradiation.
purity and good yield (98%). For the synthesis of 7,8-diaminopelargonic acid,
d-desthiobiotin (50 mg) and concentrated HCl (5 mL) were placed sequentially
under stirring in a sealed tube (CEM designed 20-mL pressure-rated reaction
vial), and the reaction mixture was exposed to microwave irradiation for
60 s to 2 min at 1008C (Scheme 1). To confirm the purity of 2, it was made
to react with phthalic dicarboxaldehyde and b-mercaptoethanol in sodium
carbonate solution at pH 10 and stored at rt for 1 h. The complex mixture
thus obtained was injected onto an RP-HPLC with C18 column and run isocra-
tically in 43% acetonitrile and 1% acetic acid with a Waters HPLC system at a
flow rate of 0.5 mL/min. The DAPA derivative, which absorbs at 420 nm
eluted at 17.06 min as a single peak. Further, DAPA was characterized by
NMR and mass.
The biological activity of DAPA synthesized by the present protocol was
tested by a bioassay using the E. coli bioA109 (MEC1) strain (E. coli Genetic
Stock Center, Yale University) originally described by Eisenberg and
Stoner[9] and recently reported by us.[10] Since the biotin biosynthesis
pathway in E. coli bioA109 (MEC1) strain is blocked at the level of the
synthesis of DAPA, its growth in minimal M9 medium is dependent on the
exogenous provision of DAPA. The synthesized DAPA supported
the growth of the bacterial cells as evident by the observation of a zone of
growth around the paper discs loaded with the compound. The semilogarith-
mic (concentration) plot of the diameters of the growth zone against the con-
centration of DAPA used was linear (Fig. 1). These results confirm that the
synthesized DAPA is biologically active.
Encouraging results in the synthesis of DAPA under microwave
conditions prompted us to evaluate whether the procedure can be extended
to prepare simple diamines. A general method was desired, although
primarily we were interested in making 2 from 1 for further use as a
substrate for biotin biosynthesis pathway enzymes. Surprisingly, by
following the above protocol, it was possible to hydrolyze other cyclic
ureas to their corresponding diamines (Scheme 2) in very short periods of
time. The desired products were obtained in pure form and in high yields
by evaporating the obtained reaction mixture in vacuo; the residue was crystal-
lized using ethanol/ether (Table 1).

2636
G. R. Vasanthakumar, V. M. Bhor, and A. Surolia
Figure 1. Growth response of E. coli bioA109 to DAPA. Ten-mL aliquots of
known concentrations of DAPA were loaded on paper discs over the agar plate, and
the diameter of the zone of growth were manually determined after 15 h of incubation
at 378C.
EXPERIMENTAL
Hydrolysis of Desthiobiotin
The d-desthiobiotin (50 mg, 0.23 mmol) powder was dissolved in concen-
trated HCl (5 mL), and the solution was stirred in a sealed tube (CEM
designed 20-mL pressure-rated reaction vial) and then exposed to
microwave irradiation for 60 s to 2 min. The resulting mixture was evaporated
to dryness and then thrice concentrated with 5 mL of water to remove excess
HCl. The residue was dissolved in minimum ethanol and recrystallized by
adding dry ether. Yield 96%; mp, 180–1818C; [a]2D5 ¼ þ4.12 (c ¼1 in
MeOH).
Scheme 2. Synthesis of diamines by microwave irradiation.

Table 1. Examples for hydrolysis of cyclic urea using microwave conditions
Conditions
Time
Yield (%)
Entry 4
R1
R2
C6H5
Lit.
Exptl.
Lit.
Exptl.
3 min
Lit.
Exptl.
94
a
C6H5
47% aq. HBr5c in reflux
AcOH
Conc. HCl
1008C
—
24 h
98
b
c
CH3
CH2C6H5
H
CO2H
2 min
3 min
—
100
98
96
2M Hcl59 reflux
—
2.5 h
34 h drying
—
19.5 h
d
e
H
R1-R255CH2-CH55CH-
CH2
H
—
Ba(OH)52d H2SO4 NH3-
CHCl3 1408C
—
2 min
5 min
—
72
94
93
—

2638
G. R. Vasanthakumar, V. M. Bhor, and A. Surolia
Hydrolysis of Cyclic Ureas
The cyclic urea (1 mmol) was dissolved in conc. HCl (6 mL), and the solution
was sequentially stirred in a sealed tube (CEM designed 20-mL pressure-rated
reaction vial) and then exposed to microwave irradiation for 60 s to 2 min. The
resulting mixture was evaporated to dryness and then thrice concentrated with
5 mL of water to remove excess HCl. The products were dissolved in
minimum ethanol and recrystallized by dry ether.
CONCLUSION
In summary, we have demonstrated the direct hydrolysis of d-desthiobiotin to
7,8-diaminopelargonic acid in concentrated HCl under microwave conditions.
The synthesized DAPA was characterized both chemically and biologically.
Further, the conditions reported here are also generally applicable to the
hydrolysis of other cyclic ureas. Thus, the presently developed method is
very useful because the reaction proceeds smoothly. Moreover, it has a high
selectivity, faster rate, and offers good yields.
ACKNOWLEDGMENTS
We thank the Department of Biotechnology, Government of India, for spon-
soring A. S., and Bejoy Thomas and Subramanyam Prakash for analytical
support.
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