V. Gaja et al.
was anticipated to be not convenient; therefore, the approach
based on direct reaction of the amines with [13N]NO2À in acidic
media was assayed using piperidine as secondary amine. In first
instance, a solution of the amine in aqueous acidic solution was
loaded in L1, and the labelling agent ([13N]NO2À) was loaded in
L3. Different acid concentrations (0.001–1 M) and reaction
temperatures (60–120 ꢀC) were assayed while the flow rate was
maintained at 40 mL/min. Unfortunately, the presence of N-[13N]
nitrosopiperidine could not be detected in any case; thus, a
second approach in which the acid was added to the labelling
agent and the resulting solution was loaded in L3 while the
solution of the amine was loaded in L1 was assayed with
identical negative results, independently of the acid concentration
(0.001–1 M) and reaction temperature (60–120 ꢀC).
Despite that unsuccessful results could be expected,
13N-nitrosation of diisopropylamine was also tested here by using
microfluidics conditions. No N-[13N]nitrosodiisopropylamine could
be detected, irrespective of the experimental conditions used.
These results suggest that the preparation of 13N-labelled nitrosa-
mines under microfluidic conditions might require activation of
the secondary amine via similar strategies to those previously
reported by our research group.
Synthesis of 13N-labelled azo compounds
The synthesis of 13N-labelled azo compounds, which could be
potentially used as in vivo b-amyloid markers, was reported
recently by our research group for the first time.12 The synthetic
approach consisted of a two-step process; the first step involved
the reaction of anion exchange resin trapped [13N]NO2À with
primary aromatic amines in acidic conditions to yield the
corresponding 13N-labelled diazonium salt, which was then
pushed to a reactor pre-charged with a solution of an aromatic
amine or phenol to yield the 13N-labelled azo compound. In
the present work, a parallel strategy was used, although both
steps were performed under microfluidic conditions; the diazo-
nium salt was formed in reactor R1 (Figure 1) and was mixed
with the corresponding aromatic amine or phenol (Scheme 3)
in reactor R2 (Figure 1). The formation of the diazonium salt
takes place under acidic conditions (pH < 2), whereas the
formation of the azo compound gives better yields when
carried out under moderate acidic or basic conditions (pH > 4,
depending on the structure of the final tracer). In order to obtain
comparative results to those previously reported, the same
media and precursor concentrations were used; thus, a solution
of the aromatic amine (precursor A) in 1 M (compounds 8 and
9) or 0.1 M (compound 10) aqueous HCl was loaded in L1, the
labelling agent ([13N]NO2À, aqueous solution) was loaded in L3
and a solution of the aromatic amine or phenol (precursor B,
Scheme 3, see Table 2 for solvent) was loaded in L2.
Figure 2. Radiochemical conversion values for compounds 1 (a) and 5 (b) as a
function of HCl concentration and flow rate.
concentration yielded higher RCCs up to a certain level; in the
case of compound 1 (Figure 2(a)), RCC increased up to
61.8 Æ 2.3% for HCl concentration of 10 mM. However, at higher
HCl concentrations, the RCC reached a plateau (65.3 Æ 3.4% at
HCl concentration of 1 M, result not shown in the figure). A clear
effect of the flow rate on the RCC was observed, independently
of the HCl concentration (lower flow rates yielding higher RCC
values). Interestingly, measurable values of RCC were only
observed for compound
5 when HCl concentration was
≥50 mM (Figure 2(b)). A maximum in RCC was obtained for HCl
concentration of 0.5 M at a flow rate of 20 mL/min (64.1 Æ 4.8%),
which decreased at higher acid concentrations. This result
suggests the decomposition of the S-[13N]nitrosothiol in strong
acidic conditions. For both compounds, the results obtained in
our previous work (solid support synthesis, RCC values of
72.5 Æ 3.9 and 66.2 Æ 5.8% for 1 and 5, respectively) could not
be reached, although lower flow rates at the optimal HCl concen-
tration (0.5 M) might lead to higher RCC values; however, lower
flow rates require longer overall reaction times and could thus
compromise decay-corrected radiochemical yield.
Synthesis of N-[13N]nitrosamines
First assays were performed on compound 8. As expected,
The synthesis of N-[13N]nitrosamines has been also previously because of the aforementioned dependence of reaction rate
reported by our research group.9 In our previous work, our first with pH, the ratio between the volumes of precursors A and B
attempts consisted of the reaction of nitrous acid (generated had a big effect on RCC (Figure 3). Almost quantitative RCC
from nitrite solution and mineral acid in water) with secondary values were obtained when ratios (precursor B)/(precursor A) = 2/1
amines. In this previously reported work, the presence of the were used, with minor effect of flow rate (RCC = 91.6 Æ 1.7,
corresponding N-[13N]nitrosamine could not be detected 93.1 Æ 2.3 and 93.9 Æ 4.1% for flow rates of 40, 60 and 80 mL/min
irrespective of the starting secondary amine. Thus, a second in step 1). When the (precursor B)/(precursor A) ratio was decreased
combined approach (resin-supported NOÀ2 + Ph3P/Br2/amine to 1 and 0.5, lower RCC values (around 35% and 5%, respectively)
strategy) was assayed on a series of aliphatic secondary amines were obtained, with again minor effect due to modifications
with good results (RCC values in the range 45.6 Æ 7.4 to in the flow rate. Interestingly, under the same experimental
53.4 Æ 1.3). In the current work, the use of the latter strategy conditions as described in our previous work12 (same precursor
J. Label Compd. Radiopharm 2012
Copyright © 2012 John Wiley & Sons, Ltd.
Gaja, Vijay
