Expedient synthesis of deuterium-labelled amides within micro-reactors

Article abstract of DOI:10.1002/jlcr.1254

The pharmaceutical industry relies heavily on the synthesis of small quantities (10-500 mg) of stable, isotopically labelled compounds in the evaluation of new drug candidates for metabolism studies. As a result of the phenomenal cost of labelled materials even the preparation of small quantities can be extremely expensive. In this paper, for the first time, we report that micro-reactor technology may be used to prepare stable deuterium-labelled compounds by conducting all optimization experiments using unlabelled precursors and simply substituting the labelled derivatives once the optimization is complete. Here, we wish to present a simple, general procedure for the synthesis of amides containing isotopic labels demonstrated using [C-²H ₃]acetyl chloride 1. The reaction is carried out within a micro-reactor set-up which we believe offers superiority over other reported methods viz requiring stoichiometric quantities of reagents, high containment of the system and generality of the technique, obtaining products in high yields. Copyright

Full text of DOI:10.1002/jlcr.1254

Journal of Labelled Compounds and Radiopharmaceuticals
J Label Compd Radiopharm 2007; 50: 189–196.
Published online in Wiley InterScience
JLCR
(www.interscience.wiley.com). DOI: 10.1002/jlcr.1254
Research Article
Expedient synthesis of deuterium-labelled amides within
micro-reactors
JULIAN HOOPER and PAUL WATTS*
Department of Chemistry, The University of Hull, Cottingham Road, Hull, East Yorkshire HU6 7RX, UK
Received 12 December 2006; Revised 8 January 2007; Accepted 9 January 2007
Abstract: The pharmaceutical industry relies heavily on the synthesis of small quantities (10–500 mg) of stable,
isotopically labelled compounds in the evaluation of new drug candidates for metabolism studies. As a result of the
phenomenal cost of labelled materials even the preparation of small quantities can be extremely expensive.
In this paper, for the first time, we report that micro-reactor technology may be used to prepare stable deuterium-
labelled compounds by conducting all optimization experiments using unlabelled precursors and simply
substituting the labelled derivatives once the optimization is complete. Here, we wish to present a simple, general
procedure for the synthesis of amides containing isotopic labels demonstrated using [C-2H3]acetyl chloride 1. The
reaction is carried out within a micro-reactor set-up which we believe offers superiority over other reported methods
viz requiring stoichiometric quantities of reagents, high containment of the system and generality of the technique,
obtaining products in high yields. Copyright # 2007 John Wiley & Sons, Ltd.
Keywords: micro-reactor; amide; deuterium; label; synthesis
Introduction
spires exclusively by diffusion which, at these small
dimensions, occurs very rapidly. Additionally the small
channel size gives rise to high surface to volume ratios
leading to rapid heat dispersion and efficient tempera-
ture control.4 The overall result is a highly controlled
regime that, when optimized, provides a continual flow
of desired product from the device. The optimization
process for a reaction is commonly focused on mini-
mizing undesirable by-products and maximizing pro-
ductivity, typically achieved by varying concentrations
and flow rates. The end product is an efficient system
which, when coupled with the high containment and
low volumes of chemicals is both safer and produces
less waste. Transferring from laboratory to bulk-scale
production is then a simple process. Since scaling the
apparatus up would alter the unique dimensions of the
reactor, it is far simpler to run many micro-reactors in
parallel. Scaling out in this way retains the original
methodology and reduces lab to plant development
time and cost significantly. Many traditional synthetic
reactions have been successfully performed using
micro-reactors within our laboratories and across the
globe5 and of late potential in the area of PET chemistry
has been demonstrated in the synthesis of nanogram
quantities of 11C and 18F labelled compounds.6 For
Micro-reactors are best described as a network of
interconnecting micron sized (10–500 mm) channels
etched into a solid substrate and a number of typical
designs are illustrated schematically in Figure 1.
Fabrication of such devices in many materials such
as ceramics, metal and polymers is possible but for this
study glass micro-reactors manufactured in house by
photolithography and wet etching have been used.1
Made from this material micro-reactor ‘chips’ are
economical to produce but more importantly they are
inert to all the chemicals and solvents used in
synthesis. One of the simplest micro-reactor designs
is a ‘T’ whereby two separate reagents are moved
through the reactor, either hydrodynamically (using
syringe pumps) or electroosmotically (by applying a
voltage), towards the outlet.2 When the two streams are
brought together within the micron-sized channels they
flow in a laminar fashion, parallel to one another and
are free of turbulence.3 Consequently, mixing tran-
*Correspondence to: Paul Watts, Department of Chemistry, The
University of Hull, Cottingham Road, Hull, East Yorkshire HU6 7RX,
UK. E-mail: p.watts@hull.ac.uk
Contract/grant sponsor: The University of Hull
example a multi-step preparation of the popular
Copyright # 2007 John Wiley & Sons, Ltd.

190 J. HOOPER AND P. WATTS
tumour imaging agent 18F-fluroxydeoxyglucose in a
micro-flow cell has recently been described.7 The
approach of using micro-reactors allows for rapid
radiosynthesis which is a driving factor in the develop-
ment of new methodologies.8 Additionally, for both
labelling with radioactive or stable isotopes, the cost of
specifically labelled precursors is also of importance
and the inherent, well-established effects of a minia-
turized reactor to deliver high yields using only
stoichiometric quantities of reagents make them ideal
candidates for isotopic labelling.
cancerous tumours and in the famously misused but
highly active lysergic acid diethylamide (LSD). Although
diverse in their actions it is clear that the ability to
effectively label amides with isotopes is paramount in
the development and testing of both lead compounds
and known drugs so that their effects and metabolic
fate in situ may be fully understood. Specific examples
of preparing amides containing isotopic labels may be
found in the literature, for example, the labelling of
melatonin via 1.5 equivalents of [C-2H3]acetyl chloride
1 has been reported with a quoted yield of 80% after
1.5 h but the need for excess labelled reagent is clearly
a disadvantage.9 Additionally Courtyn et al.10 pub-
lished a synthesis for [11C]acetylhomotaurine using
The most well-known pharmaceutical to contain the
amide unit is undoubtedly paracetamol, however, it is
also found in many other naturally occurring or
biologically active molecules. For example the basic
core of all penicillin’s contain the amide linkage as does
oseltamivir (Tamiflu) which is currently being stock-
piled due to its inhibitory effects towards certain
strains of influenza. The amide functionality is also
found in paclitaxel (Taxol) which is used to treat
[
11C]acetyl chloride with the focus primarily on biolo-
gical studies using the radiolabelled compound. These
are rare examples though, since, as highlighted by
Shevchenko et al.11 preparing labelled amides using
reagents such as acetic anhydride or acetyl chloride 2
is difficult due to their high cost and volatility. This
problematic issue may be addressed through the use of
micro-reactors which are, for the most part, closed
units thus providing the ability to accurately control
reagent consumption.
Results and discussion
As Figure 2 illustrates, the micro-reactions were (see
Table 1) carried out in
a
first-stage chip
(T1 ¼ 201 mm ꢀ 75 mm ꢀ 2.0 cm) to react the amine and
Figure 1 Typical ‘T’ (left), serpentine (right) micro-reactor
designs.
organic base. The outlet of this chip was connected via
Table 1 General reaction for the acylation of amines
O
O
R1 NH2
+
R1
N
R2
Cl
R2
H
Amine (R1)
Acylating agent (R2)
Amide product
Ph 3
CH3
CH3
CH3
CH3
CH3
2
2
2
2
2
N-Phenyl-acetamide 4
40-MeOPh 5
30-AcOPh 6
N-(4-Methoxy-phenyl)-acetamide 9
N-(3-Acetyl-phenyl)-acetamide 10
N-Benzyl-acetamide 11
PhCH2
PhCH2CH2
7
8
N-Phenethyl-acetamide 12
Ph 3
Ph 13
Ph 13
Ph 13
Ph 13
Ph 13
N-Phenyl-benzamide 14
40-MeOPh 5
30-AcOPh 6
N-(4-Methoxy-phenyl)-benzamide 15
N-(3-Acetyl-phenyl)-benzamide 16
N-Benzyl-benzamide 17
PhCH2
PhCH2CH2
7
8
N-Phenethyl-benzamide 18
Ph 3
CD3
CD3
CD3
CD3
CD3
1
1
1
1
1
N-Phenyl-[C-2H3]acetamide 19
N-(4-Methoxy-phenyl)- [C-2H3]acetamide 20
N-(3-Acetyl-phenyl)- [C-2H3]acetamide 21
N-Benzyl-[C-2H3]acetamide 22
N-Phenethyl-[C-2H3]acetamide 23
40-MeOPh 5
30-AcOPh 6
PhCH2
PhCH2CH2
7
8
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 189–196
DOI: 10.1002.jlcr

SYNTHESIS OF DEUTERIUM-LABELLED AMIDES 191
A = Amine (0.1 M / MeCN)
B = Triethylamine (0.1 M / MeCN)
C = Acyl Chloride (0.05 M / THF)
A
B
500 µl
500 µl
T1
T2
Product
C
1000 µl
Figure 2 Schematic of the micro-reactor set-up used in initial experimentation. Note the differing syringe sizes and reagent
concentrations; the overall effect gives rise to stoichiometric mixing quantities when all syringes are driven at an equal flow rate.
a 2.5 cm length of PEEKTM tubing (360 mm o.d., 150 mm
i.d.) to one inlet of second-stage reactor
two micro-reactors, T1 and T2, joined in a series
arrangement as shown in Figure 2. Since it is necessary
to carry out the reaction in the presence of a base, to
sequester any hydrogen chloride formed, we decided to
investigate the reaction in conjunction with triethyla-
mine due to its ready availability, low toxicity and
known reactivity. The syringe driver was set to the
following flow rates; 20, 10, 5, 2 and 1 ml minꢁ1 and
each reaction repeated 10 times (at each flow rate) to
determine reproducibility. In all cases the relative
standard deviation (RSD) was observed to be less than
5% and the relationship between flow rate and conver-
sion can be clearly seen in Figure 3. At the lowest
a
(T2 ¼ 158 mm ꢀ 54 mm ꢀ 1.5 cm) which allowed the in-
termediate to then react with the acyl chloride. Employ-
ing a modular-type set-up, comprising of two micro-
reactors, allowed for the rapid optimization of reactions
by simple interchange of the second-stage reactor for
one of longer channel length (S1 ¼ 170 mm ꢀ 60 mm ꢀ
23.5 cm) when required. PEEKTM tubing of length 5 cm
was used to connect the micro-reactor to the syringe
pump via PEEKTM HPLC connectors. Glass luer lock
syringes were automatically operated by a syringe
pump capable of simultaneously delivering a maximum
of three solutions. Reactions were operated at
a
investigated flow rate of 1 ml minꢁ1
, the complete
concentration of 0.1 M (amine and base, respectively)
in acetonitrile and 0.05 M (acyl chloride) in THF, which
provided a final concentration of 0.025 M on collection
and correlates to a 1:1:1 ratio of reagents. A mixed
solvent system was selected as the polarity of acetoni-
trile was necessary to avoid precipitation of ammonium
salts whilst THF was found to be a stable solvent for the
daily storage of acyl chloride solutions.
conversion of phenylamine 3 into N-phenyl-acetamide
4 was observed. The acetylation of four other amines
namely 4-methoxy-phenylamine 5, 1-(3-amino-phe-
nyl)-ethanone 6, benzylamine 7 and phenylethylamine
8, to afford N-(4-methoxy-phenyl)-acetamide 9, N-(3-
acetyl-phenyl)-acetamide 10, N-benzyl-acetamide 11
and N-phenethyl-acetamide 12, respectively, was then
investigated and the results summarized in Table 2.
In all cases near quantitative conversions were
achieved, however, for the acetylation of phenylamine
3 and 1-(3-amino-phenyl)-ethanone 6 an increased
residence time (achieved by employing a slower flow
rate) was necessary to allow the reactions enough time
to reach completion. Resonance stabilization of the
former and electron withdrawing effects of the latter’s
acetyl group affect the reactivity of the molecules. In an
effort to improve the productivity of the system for
these particular amines the second stage micro-reac-
tor, T2 which had a channel length of 1.5 cm, was
substituted for one of longer length (23.5 cm). Based on
the flow rates and the channel volume of T2 (0.11 ml) the
relationship between conversions and residence time
was plotted (Figure 4). In this way an appropriate
serpentine micro-reactor, S1, with a channel length of
23.5 cm, was chosen so as to provide an adequate
Working within the given concentration range was
found to effectively avoid precipitation of solids, a
problem which can often hamper any attempts at
optimization and reproducibility. To monitor a parti-
cular reaction 10 ml of product solution was collected
from the reactor outlet into a small vial containing 50 ml
of quench agent (1% water in acetonitrile) and analysed
immediately by HPLC; this enabled us to confirm that
the reaction occurred on chip rather than in the vial.
Percentage conversions were calculated based on the
amount of product (amide) present with respect to the
amine remaining. Retention times and relative re-
sponses of all components were determined using
synthetic standards.
Initially the reaction between phenylamine 3 and
unlabelled acetyl chloride 2 to afford N-phenyl-acet-
amide 4 was investigated using a set-up consisting of
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 189–196
DOI: 10.1002.jlcr

192 J. HOOPER AND P. WATTS
residence time (52.6 s) when the reagents were infused
at a faster flow rate of 20 ml minꢁ1, thus providing a
higher overall throughput. In this way comparable
results to those seen in the first system when running
at lower flow rates were achieved and conversions of
100.0 and 97.7% were obtained for phenylamine 3 and
1-(3-amino-phenyl)-ethanone 6, respectively.
Using the optimized micro-reactor configuration and
conditions the reaction between the same five amines,
3 and 5–8, with benzoyl chloride 13, to form N-phenyl-
benzamide 14, N-(4-methoxy-phenyl)-benzamide 15,
N-(3-acetyl-phenyl)-benzamide 16, N-benzyl-benza-
mide 17 and N-phenethyl-benzamide 18 was investi-
gated. Although benzoyl chloride 13 is less reactive
than the acetyl analogue 2, similar results were
obtained (Table 3). The poor conversions observed in
the case of N-(3-acetly-phenyl)-benzamide 16 can be
attributed to a combination of the decreased reactivity
of benzoyl chloride 13 and delocalization through the
ring between the amino and acetyl group which
stabilizes the molecule. The net result significantly
slows the reaction and consequently a reduction in
conversion is observed.
100
80
60
40
20
0
0
5
10
15
20
Set Flow Rate (µl min-1)
Figure 3 Graph illustrating the effect of varying flow rate on
the conversion of aniline 3 to N-phenyl-acetamide 4.
In order to demonstrate the concept of employing
such a system for deuterium labelling, acetyl chloride 2
was substituted for its deuterated equivalent [C-2H3]a-
cetyl chloride 1. To confirm the synthesis of labelled
amides N-phenyl-[C-2H3]acetamide 19, N-(4-methoxy-
phenyl)-[C-2H3]acetamide 20, N-(3-acetyl-phenyl)-
[C-2H3]acetamide 21, N-benzyl-[C-2H3]acetamide 22
and N-phenethyl-[C-2H3]acetamide 23, the micro-reac-
tor system was typically run for 2.5–3.0 h to prepare
sufficient compound (20 mg) for analysis, following
work-up and recrystallization to remove unwanted
reaction components. As expected, the system pro-
duced analogous results with the labelled reagent and
the reaction efficiency was comparable to the pre-
optimized conditions (Table 2). As can be seen from
100
80
60
Phenylamine 3
1-(3-Amino-phenyl)-ethanone 6
40
20
0
0
0.5
1
1.5
2
2.5
3
3.5
Residence Time (seconds)
Figure
4
The relationship between residence time and
conversion for phenylamine 3 and 1-(3-amino-phenyl)-etha-
none 6 in T2.
Table 2 Acetylation of amines in a double T micro-reactor (n ¼ 10)
Amide product
Set flow rate (ml minꢁ1
)
Conversion (%)
RSD (%)
N-Phenyl-acetamide 4
1
20
1
20
20
100.0
99.3
99.9
95.3
96.4
0.00
0.83
0.83
1.44
1.45
N-(4-Methoxy-phenyl)-acetamide 9
N-(3-Acetyl-phenyl)-acetamide 10
N-Benzyl-acetamide 11
N-Phenethyl-acetamide 12
Table 3 Conversions obtained at optimal conditions for the benzoylation of amines (n ¼ 10)
Amide product
Reactor
Conversion (%)
RSD (%)
N-Phenyl-benzamide 14
T1, S1
T1, T2
T1, S1
T1, T2
T1, T2
90.5
94.0
65.5
97.4
91.9
4.91
5.50
4.70
0.94
2.46
N-(4-Methoxy-phenyl)-benzamide 15
N-(3-Acetyl-phenyl)-benzamide 16
N-Benzyl-benzamide 17
N-Phenethyl-benzamide 18
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 189–196
DOI: 10.1002.jlcr

SYNTHESIS OF DEUTERIUM-LABELLED AMIDES 193
Table 4 Summary of the results obtained for the isotopic labelling of primary amines within micro-reactors
Entry
Amide product
Reactor
Residence time (s)
Throughput (mg hꢁ1
)
Conversion (%)
Yield (%)
H
N
CD3
1
T1, S1
5.54
7.63
95.0
92
O
19
H
N
CD3
2
3
4
5
T1, T2
T1, S1
T1, T2
T1, T2
2.62
5.54
2.62
2.62
9.89
9.95
8.95
9.58
98.0
95.8
98.2
96.4
98
92
98
96
O
20
O
O
H
N
CD3
O
21
O
N
H
CD3
22
H
N
CD3
O
23
Table 4 the deuterium label was incorporated into the
final product and in all cases excellent isolated yields
were obtained after recrystallization. It should be
emphasized that the ability to obtain similar yields
when substituting for the labelled reagent is a sig-
nificant result, as this is often difficult to obtain in
batch.
NMR data; s ¼ singlet, d ¼ doublet, t ¼ triplet, br
s ¼ broad singlet, m ¼ multiplet, sep ¼ septet and C0
¼ quaternary carbon. Infra-red spectra were recorded
on a Perkin Elmer Paragon 1000 FT-IR spectrometer in
the range 4000–600 cmꢁ1 and peaks (nmax) reported in
wavenumbers (cmꢁ1). Mass spectra were recorded on a
Varian GC (CP-3800) coupled to a Varian MS (Saturn
2000) with a CP-Sil 8 (30 m) column manufactured by
Phenomonex (Zebron ZB-5). Ultra-high-purity helium
(99.999% Energas) was used as the carrier gas.
Analysis was carried out using the following method;
Experimental
Materials
injector temperate 2008C, helium flow rate 1 ml minꢁ1
,
All solvents used in micro-reactions were puriss grade
(599.5%) over molecular sieves (H2O40.005%) pur-
chased from Fluka. All other reagents were purchased
from Sigma-Aldrich. 4-Methoxy-phenylamine 5 and 1-
oven temperature 508C for 4 min, and then increased to
2508C at a rate of 308C minꢁ1 with a 3.0 min filament
delay. The HPLC set-up used for experimental analysis
was a Shimadzu (LC-10AD) pump coupled to an in-line
Shimadzu (DGU-14A) degasser and a Prodigy ODS-2
(5 mm, 4.6 ꢀ 250 mm) column manufactured by Pheno-
monex. Analyte detection was carried out at l ¼ 254 nm
(3-amino-phenyl)-ethanone
6
were recrystallized
(DCM/hexane) before use. All other reagents were used
as received without further purification.
using
a Polymer Laboratories (LC 1210) UV/Vis
Instruments
detector. All analyses were carried out at room tem-
perature in isocratic mode using acetonitrile containing
0.1% trifluoroacetic acid (TFA) and water in a 9:1 ratio
as the mobile phase. The borosilicate glass used to
fabricate micro-reactors was crown white glass (TELIC,
USA) pre-coated with layers of chrome and photoresist.
PEEKTM tubing was purchased from Upchurch Scien-
tific, glass luer lock syringes (Hamilton), HPLC fittings
(1/16 in) and female–female luers interfacing the
Nuclear magnetic resonance (NMR) spectra were re-
corded on a Jeol (GX400) spectrometer operating at
room temperature in solutions of deuteriochloroform
(CDCl3) doped with trimethylsilane (TMS, 0.03%) as an
internal standard. Chemical shifts are quoted in parts
per million (ppm) and coupling constants in Hertz (Hz).
The following abbreviations are used when reporting
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 189–196
DOI: 10.1002.jlcr

194 J. HOOPER AND P. WATTS
tubing and syringes were all purchased from Supelco.
The syringe driver (MD-1001) was manufactured by
Bioanalytical Systems Inc. and could operate at set flow
m, Ar) and 7.32 (2 H, m, 2 ꢀ Ar); dC (100 MHz, CDCl3/
TMS) 23.3 (CH3), 35.6 (CH2), 40.6 (CH2), 126.5 (CH),
128.6 (2 ꢀ CH), 128.7 (2 ꢀ CH), 138.8 (C0) and 170.0
(CO); m/z (EI) 164 (Mþ þ 1, 100%), 104 (37) and 65 (6).
N-Phenyl-benzamide (15).16 (0.37 g, 37%) as fine
white crystals; dH (400 MHz, CDCl3/TMS) 7.16 (1 H,
m, Ar), 7.38 (2 H, m, 2 ꢀ Ar), 7.50 (2 H, m, 2 ꢀ Ar), 7.56
(1 H, m, Ar), 7.65 (2 H, m, 2 ꢀ Ar), 7.79 (1 H, br s, NH)
and 7.88 (2 H, m, 2 ꢀ Ar); dC (100 MHz, CDCl3/TMS)
120.2 (2 ꢀ CH), 124.6 (CH), 127.0 (2 ꢀ CH), 128.9
(2 ꢀ CH), 129.1 (2 ꢀ CH), 131.9 (CH), 135.0 (C0), 137.9
(C0) and 165.7 (CO); m/z (EI) 198 (Mþ þ 1, 46%), 197
(100), 105 (68) and 51 (4).
rates ranging between 0.1 and 100 ml minꢁ1
.
General batch preparation methodology
To a stirred solution of pre-weighed amine dissolved in
DCM (150 ml) was added an equimolar amount of
acylating agent (acetyl chloride 2 or benzoyl chloride
13). After 2 h continued stirring at room temperature
any solids were removed by filtration and the reaction
mixture was washed with sat. NH4Cl solution
(3 ꢀ 150 ml). The organic layer was dried over anhy-
drous MgSO4, filtered and concentrated in vacuo.
Subsequent recrystallization (DCM or ether/hexane)
yielded the corresponding amide, confirmed by com-
parison to data in the literature.
N-Phenyl-acetamide (4).12 (0.27 g, 27%) as fine white
needles; dH (400 MHz, CDCl3/TMS) 2.18 (3 H, s, CH3),
7.10 (1 H, m, Ar), 7.31 (2 H, m, 2 ꢀ Ar), 7.36 (1 H, br s,
NH) and 7.50 (2 H, m, 2 ꢀ Ar); dC (100 MHz, CDCl3/
TMS) 24.6 (CH3), 119.9 (CH), 124.3 (2 ꢀ CH), 129.0
(2 ꢀ CH), 137.9 (C0) and 168.3 (CO); m/z (EI) 136
(Mþ þ 1, 30%), 135 (84), 93 (100) and 65 (6).
N-(4-Methoxy-phenyl)-benzamide (16).17 (0.2235 g,
22%) as white crystals; dH (400 MHz, CDCl3/TMS)
3.81 (3 H, s, CH3), 6.91 (2 H, d, J 9.0, 2 ꢀ Ar), 7.47
(2 H, m, 2 ꢀ Ar), 7.54 (3 H, m, 3 ꢀ Ar), 7.76 (1 H, br s,
NH) and 7.86 (2 H, d, J 7.0, 2 ꢀ Ar); dC (100 MHz,
CDCl3/TMS) 55.5 (2 ꢀ CH), 114.2 (2 ꢀ CH), 122.1
(2 ꢀ CH), 127.0 (2 ꢀ CH), 128.7 (2 ꢀ CH), 131.0 (C0),
131.7 (C0), 135.0 (CH), 156.6 (C0) and 165.6 (CO); m/z
(EI) 228 (Mþ þ 1, 15%), 227 (93), 105 (100), 77 (57) and
51 (19).
N-(3-Acetyl-phenyl)-benzamide
(14).18 (0.2770 g,
28%) as faint pink crystals; dH (400 MHz, CDCl3/TMS)
2.61 (3 H, s, CH3), 7.49 (3 H, m, 3 ꢀ Ar), 7.57 (1 H, m,
Ar), 7.73 (1 H, ddd, J 7.9, J 1.7 and J 1.1, Ar), 7.91 (2 H,
m, 2 ꢀ Ar), 8.07 (1 H, ddd, J 8.2, J 2.2 and J 1.1, Ar),
8.17 (1 H, t, J 2.0, Ar) and 8.19 (1 H, br s, NH); dC
(100 MHz, CDCl3/TMS) 26.7 (CH3), 119.6 (CH), 124.4
(CH), 124.8 (CH), 127.1 (2 ꢀ CH), 128.8 (2 ꢀ CH), 129.4
(CH), 132.1 (CH), 134.6 (C0), 137.8 (C0), 138.5 (C0),
165.9 (CO) and 197.9 (CO); m/z (EI) 240 (Mþ þ 1, 10%),
239 (65), 105 (100), 77 (57) and 51 (15).
N-Benzyl-benzamide (17).19 (0.1069 g, 10.7%) as fine
white crystals; dH (400 MHz, CDCl3/TMS) 4.64 (2 H, d,
J 6.0, CH2), 6.46 (1 H, br s, NH), 7.31 (1 H, m, 1 ꢀ Ar),
7.35 (4 H, d, J 4.5, 4 ꢀ Ar), 7.42 (2 H, m, 2 ꢀ Ar), 7.50
(1 H, m, Ar) and 7.79 (2 H, m, 2 ꢀ Ar); dC (100 MHz,
CDCl3/TMS) 44.1 (CH2), 126.9 (2 ꢀ CH), 127.6 (CH),
127.9 (2 ꢀ CH), 128.6 (2 ꢀ CH), 128.8 (2 ꢀ CH), 131.5
(CH), 134.4 (C0), 138.2 (C0) and 167.3 (CO); m/z (EI)
212 (Mþ þ 1, 26%), 211 (100), 105 (96), 77 (80) and
51 (28).
N-Phenethyl-benzamide (18).20 (0.27 g, 27%) as fine
white crystals; dH (400 MHz, CDCl3/TMS) 2.94 (2 H, t, J
7.0, CH2), 3.73 (2 H, m, CH2), 6.15 (H, br s, NH), 7.25
(3 H, m, 3 ꢀ Ar), 7.33 (2 H, m, 2 ꢀ Ar), 7.40 (2 H, m,
2 ꢀ Ar), 7.48 (H, m, Ar) and 7.68 (2 H, m, 2 ꢀ Ar); dC
(100 MHz, CDCl3/TMS) 35.7 (CH2), 41.1 (CH2), 126.6
(CH), 126.8 (2 ꢀ CH), 128.5 (2 ꢀ CH), 128.7 (2 ꢀ CH),
128.8 (2 ꢀ CH), 131.4 (CH), 134.7 (C0), 138.9 (C0) and
167.4 (CO); m/z (EI) 226 (Mþ þ 1, 10%), 134 (20), 105
(100), 77 (46) and 51 (18).
N-(4-Methoxy-phenyl)-acetamide (9).12 (0.21 g, 21%)
as white crystals; dH (400 MHz, CDCl3/TMS) 2.15 (3 H,
s, CH3), 3.78 (3 H, s, CH3), 6.85 (2 H, d, J 9.0, 2 ꢀ Ar),
7.19 (1 H, br s, NH) and 7.38 (2 H, d, J 9.0, 2 ꢀ Ar); dC
(100 MHz, CDCl3/TMS) 24.3 (CH3), 55.5 (CH3), 114.1
(2 ꢀ CH), 121.9 (2 ꢀ CH), 130.9 (C0), 156.4 (C0)
and 168.1 (CO); m/z (EI) 166 (Mþ þ 1, 100%) and
108 (20).
N-(3-Acetyl-phenyl)-acetamide (10).13 (0.12 g, 12%) as
white crystals; dH (400 MHz, CDCl3/TMS) 2.22 (3 H, s,
CH3), 2.61 (3 H, s, CH3), 7.43 (2 H, t and br s, J 7.8, Ar
and NH), 7.70 (1 H, m, Ar), 7.90 (1 H, m, Ar) and 8.00
(1 H, m, Ar); dC (100 MHz, CDCl3/TMS) 24.6 (CH3), 26.7
(CH3), 119.2 (CH), 124.2 (CH), 124.5 (CH), 129.4 (CH),
137.7 (C0), 138.4 (C0), 168.5 (CO) and 197.9 (CO); m/z
(EI) 178 (Mþ þ 1, 48%), 177 (100), 135 (55), 120 (70),
87 (15) and 63 (15).
N-Benzyl-acetamide (11).14 (0.13 g, 13%) as a white
crystalline solid; dH (400 MHz, CDCl3/TMS) 2.02 (3 H,
s, CH3), 4.43 (2 H, d, J 5.6, CH2), 5.85 (1 H, br s, NH)
and 7.28 (3 H, m, 3 ꢀ Ar) and 7.34 (2 H, m, 2 ꢀ Ar); dC
(100 MHz, CDCl3/TMS) 23.3 (CH3), 43.8 (CH2), 127.5
(CH), 127.9 (2 ꢀ CH), 128.7 (2 ꢀ CH), 138.2 (C0) and
169.8 (CO); m/z (EI) 150 (Mþ þ 1, 100%), 106 (90) 91
(20), 77 (6) and 51 (5).
N-Phenethyl-acetamide (12).15 (0.51 g, 51%) as a
white crystalline solid; dH (400 MHz, CDCl3/TMS) 1.93
(3 H, s, CH3), 2.81 (2 H, t, J 7.0, CH2), 3.52 (2 H, m,
CH2), 5.50 (1 H, br s, NH), 7.19 (2 H, m, 2 ꢀ Ar), 7.24 (H,
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 189–196
DOI: 10.1002.jlcr

SYNTHESIS OF DEUTERIUM-LABELLED AMIDES 195
(CD3, sep, J 19.4), 35.6 (CH2), 40.6 (CH2), 126.5 (CH),
128.7 (2 ꢀ CH), 128.8 (2 ꢀ CH), 138.9 (C0) and 170.1
(CO); m/z (EI) 167 (Mþ þ 1, 100%) and 103 (11).
Micro-reaction methodology
For preparing labelled compounds within the micro-
reactor set-up it was necessary to run the system for a
sufficient amount of time (2.5–3.0 h) in order to prepare
20 mg of product for analysis. Post-collection the
reaction products were concentrated in vacuo, dis-
solved in DCM (25 ml) and washed with saturated
NH4Cl solution (3 ꢀ 30 ml). Drying over MgSO4, filtra-
tion, concentration in vacuo and recrystallization
(DCM/hexane) afforded the final product for character-
ization.
Conclusion
We have effectively demonstrated the inherent benefits
of combining a micro-reactor system with traditional
synthetic isotope labelling and a number of deuterium-
labelled amides were produced using the developed
system at
a minimal cost. In addition, close to
quantitative isotope label incorporation into the desired
products was obtained under stoichiometeric condi-
tions. Although the reaction mixtures required purifi-
cation, the use of a phase transfer micro-reactor21 or
coupling to a preparative HPLC system would incorpo-
rate this step into the on-line synthesis. Importantly,
however, the developed system produces a continuous
product stream which makes the rapid synthesis of
isotopically labelled amides (reaction time ꢂ3–6 s) a
reality. For the system used in this article the
throughput is approximately 10 mg hꢁ1. It is clear that
in the realm of isotope chemistry there are significant
cost advantages to pre-optimizing a reaction in such a
system first with unlabelled products. Using micro-
reactors to contain hazardous components of a reac-
tion and control conventional chemistry more effi-
ciently is another important advantage which will
surely see the continued growth of such devices in this
field in the future.
N-Phenyl-[C-2H3]acetamide (19). (18.30 mg, 92%) as
fine white needles. nmax/cmꢁ1 3300, 3196, 3137, 3080,
1667, 1599, 1434, 1320, 759 and 694; dH (400 MHz,
CDCl3/TMS) 7.10 (1H, m, Ar), 7.25 (1H, br s, NH), 7.32
(2H, m, 2 ꢀ Ar) and 7.50 (2H, m, 2 ꢀ Ar); dC (100 MHz,
CDCl3/TMS) 24.0 (CD3, sep, J 19.5), 119.9 (CH), 124.4
(2 ꢀ CH), 129.1 (2 ꢀ CH), 137.9 (C0) and 168.4 (CO); m/
z (EI) 139 (Mþ þ 1, 30%), 138 (63), 94 (100) and 65 (7).
N-(4-Methoxy-phenyl)-[C-2H3]acetamide (20). (19.58 mg,
98%) as white crystals. nmax/cmꢁ1 3266, 2835, 1646,
1605, 1512, 1334, 1246, 1027 and 840; dH (400 MHz,
CDCl3/TMS) 3.78 (3H, s, CH3), 6.83 (2H, d, J 9.0
2 ꢀ Ar), 7.39 (2H, d, J 9.0, 2 ꢀ Ar) and 7.65 (H, br s, NH);
d
C (100 MHz, CDCl3/TMS) 23.5 (CD3, sep, J 19.5), 55.5
(CH3), 114.1 (2 ꢀ CH), 122.0 (2 ꢀ CH), 131.0 (C0), 156.4
(C0) and 168.6 (CO); m/z (EI) 169 (Mþ þ 1, 100%), 124
(33) and 108 (15).
N-(3-Acetyl-phenyl)-[C-2H3]acetamide (21). (18.42 mg,
92%) as white crystals. nmax/cmꢁ1 3351, 2923, 1672,
1593, 1547, 1432, 1273, 792 and 687; dH (400 MHz,
CDCl3/TMS) 2.60 (3H, s, CH3), 7.42 (1H, t, J 8.0, Ar),
7.68 (1H, m, Ar), 7.96 (1H, m, Ar), 7.99 (1H, br s, NH)
and 8.02 (1H, m, Ar); dC (100 MHz, CDCl3/TMS) 23.8
(CD3, sep, J 19.3), 26.7 (CH3), 119.2 (CH), 124.1 (CH),
124.6 (CH), 124.6 (CH), 129.3 (CH), 137.6 (C0), 138.6
(C0), 168.9 (CO) and 198.1 (CO); m/z (EI) 181 (Mþ þ 1,
75%), 180 (100), 136 (30), 121 (57) and 64 (5).
Acknowledgements
The authors would like to thank Charlotte Wiles for
helpful discussion, Steve Clark for micro-reactor
fabrication and The University of Hull for financial
support (J.H.).
REFERENCES
N-Benzyl-[C-2H3]acetamide (22). (19.6 mg, 98%) as a
white crystalline solid. nmax/cmꢁ1 3293, 3087, 2928,
1627, 1552, 1454, 1320, 746 and 694; dH (400 MHz,
CDCl3/TMS) 4.44 (2H, d, J 5.6, CH2), 5.74 (1H, br s,
NH), 7.28 (3H, m, 3 ꢀ Ar) and 7.34 (2H, m, 2 ꢀ Ar); dC
(100 MHz, CDCl3/TMS) 22.6 (CD3, sep, J 19.5), 43.8
(CH2), 127.6 (CH), 127.9 (2 ꢀ CH), 128.7 (2 ꢀ CH),
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N-Phenethyl-[C-2H3]acetamide (23). (19.1 mg, 96%) as
white needles. nmax/cmꢁ1 3291, 3086, 2930, 1646,
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7.32 (2H, m, 2 ꢀ Ar); dC (100 MHz, CDCl3/TMS) 22.6
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