Preparation of 18O-labelled nicotinamide

Article abstract of DOI:10.1002/jlcr.617

A method of preparing ¹⁸O-labelled nicotinamide, involving cyanopyridine, H₂ ¹⁸O, and 1,1′3,3′-tetramethylguanidine as a catalyst is described. The desired product is produced in 91% chemical yield and 97.5% isotopic incorporation. Copyright

Full text of DOI:10.1002/jlcr.617

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd Radiopharm 2002; 45: 1005–1010.
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.617
Research Article
18
Preparation of O-labelled nicotinamide
!
Magdalena Ko"odziejska-Huben, Zbigniew Kaminski and
Piotr Paneth*
’
! !
Department of Chemistry, Technical University of Łodz, Zeromskiego 116,
! !
90-924 Łodz, Poland
Summary
A method of preparing 18O-labelled nicotinamide, involving cyanopyridine, H2
18O, and 1,103,30-tetramethylguanidine as a catalyst is described. The desired
product is produced in 91% chemical yield and 97.5% isotopic incorpor-
ation. Copyright # 2002 John Wiley & Sons, Ltd.
Key Words: nicotinamide; Vilsmeier reaction; alkaline hydrolysis; iminium
ions; propyl benzenecarboximidoate
Introduction
Niacin, commonly known as nicotinamide, plays an important role in
the biosynthesis of pyridine nucleotides. This nitrogen heterocyclic
compound combines in vivo with the nucleotide adenosine to form
nicotinamide adenine dinucleotide (NAD+), which serves as a soluble
electron carrier in biochemical reactions. Clinically, niacin maintains the
normal function of the digestive system, reducing cholesterol levels, and
even reducing dizziness and ringing in the ears. Other symptoms
associated with niacin deficiency include general weakness and fatigue,
loss of appetite, dermatitis, skin lesions, a swollen tongue, and
dementia.
*Correspondence to: Professor P. Paneth, Department of Chemistry, Technical University of Łodz,
Zeromskiego 116, 90-924, Łodz, Poland. E-mail: paneth@ck-sg.p.lodz.pl
Contract/grant sponsor: Maria Sklodowska–Curie Joint Funds II
Copyright # 2002 John Wiley & Sons, Ltd.
Received 4 April 2002
Revised 23 May 2002
Accepted 3 June 2002

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Oxygen-labelled nicotinamide can be obtained by two synthetic
strategies. The first of these is based on the exchange of the oxygen atom
of the nicotinamide with 18O-enriched water. In this approach a large
excess of the labelled oxygen source is needed to assure high enrichment
of the nicotinamide. The second strategy relies on the incorporation of
the labelled oxygen from a readily accessible 18O-enriched source into a
mono-oxygenated precursor of nicotinamide. This enables more
efficient use of the enriched substrate and high oxygen incorporation
into the precursor. The synthesis of 14C-nicotinamide,1 (6-2H)
nicotinamide,2,3 (1-15N) nicotinamide,4 4-deutero-3-pyridi-necarboxa-
mide,5,6 (carboxyl 11C) nicotinamide,7 (6-13C) nicotinamide,8 and
(4-13C) nicotinamide9 have been presented in the literature.
Results and discussion
Several approaches to the synthesis of nicotinamide have been presented
in the literature.1–19 Preparation of 18O-labelled nicotinamide by
treatment of unlabelled precursor with 18O-labelled water in alkaline
medium is possible due to the much faster oxygen exchange than
hydrolysis.10,11 In the reaction scheme presented in Figure 1, the
intermediate [I] is assumed to give products or generate reactants and,
provided that the oxygen atoms in I become equivalent by virtue of
rapid proton transfers, this reverse step should lead to oxygen
exchange.10–13
Under alkaline conditions the rate constants for the exchange and
hydrolysis are then related to kꢀ1 and k2 by Equation (1):
ke=kh ¼ kꢀ1=2k2
ð1Þ
In acidic solution, the yield of labelled nicotinamide was negligible.14 In
the presence of hydrogen peroxide only a 19% yield was achieved.10
The synthesis of nicotinamide from thionicotinamide has been
reported.15 The superoxide group, generated by K2O with the crown
Figure 1. The mechanism of amide hydrolysis
Copyright # 2002 John Wiley & Sons, Ltd.
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PREPARATION OF 18O-LABELLED NICOTINAMIDE
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Figure 2. The Vilsmeier reaction scheme
ether as the catalyst, was effective for the desulfurization of the
thioamide group of thioisonicotinamide, its 2-ethyl derivative, and
thionicotinamide to give the appropriate amides in 69, 61 and 71%
yields, respectively. This yield is, however, too low to apply this
synthetic route for labelling purposes.
The conversion of nicotinonitrile into nicotinamide has been reported
in several publications. A good yield was reported by the use of
ammonia under pressure, with 73% of the nitrile being converted into
nicotinamide and the rest into nicotinic acid.16 The inherent disadvan-
tage associated with the use of alkali is the formation of acids as by-
products with a corresponding decrease in yield and the necessity of
separating the products formed. Our attempts to diminish the formation
of acid were unsuccessful.
The Vilsmeier reaction (Figure 2) has long been used to convert
amides to highly electrophilic iminium ions, which are susceptible
towards attack of various nucleophiles including those as weak as
aromatic rings.17
Iminium ions may be generated also in the reaction of nitriles with
HCl and with oxalyl chloride. R1, R2 and R3 could be different alkyl or
aryl groups. We expected that such intermediates might provide a useful
alternative route of activating nitriles towards nucleophilic attack by
water. The reaction of amide and oxalyl chloride 1:1 gave the product in
less than 20% yield.18 Much better yields (70%) have been obtained
from the reaction of 3-cyanopyridine with HCl.19 We have studied the
applicability of these procedures to oxygen-labelled nicotinamide. We
used benzonitrile and HCl, as described by Veale et al. (Figure 3).
The model experiments involving reaction of benzonitrile with HCl in
alcohol gave the required imidoyl ester.
Moreover, only unreacted substrate was isolated in experiments
involving methylene chloride instead of expected imidoyl chloride.
Copyright # 2002 John Wiley & Sons, Ltd.
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M. KOŁODZIEJSKA-HUBEN ET AL.
Figure 3. The synthesis of nicotinamide via an iminium ion
Figure 4. The hydrolysis of nicotinonitrile
It is well known that the action of KOH or NaOH on nitriles leads
directly to the formation of the corresponding acids. The amides,
postulated as intermediate products in this reaction, are obtained efficiently
when hydrogen peroxide is added to the mixture. For example, the use of
amonium hydroxide gave nicotinamide in 70% yield, but the high excess of
the labelled water necessary for efficient hydrolysis strongly diminishes the
isotopic efficiency of the method.16 In order to reduce the requirement for
labelled water we improved the procedure through the use 1,10,3,30-
tetramethyl-guanidine (TMG) as catalyst (Figure 4).
TMG was chosen as a base because it is volatile and can be easily
removed from the final product by evaporation or lyophilization.
Moreover, the absence of any exchangable oxygen atom does not lower
the enrichment of the labelled water. Overall the chemical and isotopic
yields in our method were 91 and 97.5%, respectively, making this
procedure an excellent way for preparing 18O–labelled nicotinamide.
Experimental
Alkaline hydrolysis of nicotinamide
Four samples each containing nicotinamide (100 mg, 0.83 mM) in water
(1 ml) and 10, 20, 40 and 100 ml of Ba(OH)2, respectively, were heated in
Copyright # 2002 John Wiley & Sons, Ltd.
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a water bath for 1 h. After cooling the water was evaporated off. The
products were dried over P2O5 and recrystallized from ethyl acetate. 50,
39, 21 and 0 mg of amide were produced, respectively. 1H-NMR
(250 MHz, 2H2O): d ¼ 4:89 ppm (s, 2H) R–NH2, 7.53–8.87 ppm (m, 4H)
ring [H-2 8.87 ppm; H-4 8.2 ppm; H-5 7.6 ppm; H-6 8.7 ppm]. 13C-NMR
2
(62.9 MHz, H2O): d ¼ 151:60 ppm C-2, 128.89 ppm C-3, 136.19 ppm
C-4, 124.40 ppm C-5, 147.40 ppm C-6, 170.16 ppm –C ¼ O.
Propyl benzenecarboximidoate
A solution of benzonitrile (3 g, 29 mM) in propanol (3 ml) was cooled
to 08C and saturated with dry HCl gas for 2 h. The reaction mixture
was allowed to warm to room temperature and stand for 16 h. The
solvent was then evaporated off. The white solid product was dried
1
over P2O5. 4.37 g of product (27 mM, 92%) was obtained. H-NMR
(250 MHz, 2H2O): d ¼ 1:15 ppm (t, 3H, J ¼ 7 Hz) R–CH3,
1.95–2.09 ppm (m, 2H) R–CH2–Me, 4.84–4.89 ppm (t, 2H, J ¼ 7 Hz)
RO–CH2–Et, 7.32–8.43 ppm (m, 5H) ring, 11.94 and 12.66 ppm (2 ꢁ s,
1H) H–N.
18O-labelled nicotinamide
A solution of 3-cyanopyridine (Aldrich) (270 mg, 2.6 mM) and 1,10,3,30-
tetramethylguanidine (Aldrich) (13.8 mg, 15 ml) in H128O (40% enrich-
ment, Technabexport, Russia) (300 ml) was heated in water bath for 1 h.
Water and 1,103,30-tetramethylguanidine were then removed by lyophili-
zation. The residue was crystallized from benzene, yielding 245 mg (91%)
of 18O-nicotinamide, m.p. 129–1308C (39% 18O from FAB mass
spectrometry). 1H-NMR (250 MHz, 2H2O): d ¼ 4:89 ppm (s, 2H)
R–NH2, 7.53–8.87 ppm (m, 4H) ring [H-2 8.87 ppm; H-4 8.2 ppm;
2
H-5 7.6 ppm; H-6 8.7 ppm]. 13C-NMR (62.9 MHz, H2O): d ¼ 151:69
ppm C-2, 129.16 ppm C-3, 136.33 ppm C-4, 124.02 ppm C-5, 147.51 ppm
C-6, 170.58 ppm –C ¼ O.
Acknowledgements
This work was supported by grants from the Maria Sk"odowska–Curie
Joint Funds II (P. Paneth).
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