Synthetic approaches to a challenging and unusual structure—an amino-pyrrolidine guanine core

Article abstract of DOI:10.3390/molecules25040797

The synthesis of an unreported 2-aminopyrrolidine-1-carboxamidine unit is here described for the first time. This unusual and promising structure was attained through the oxidative decarboxylation of amino acids using the pair of reagents, silver(I)/peroxydisulfate (Ag(I)/S2O8²?) followed by intermolecular (in the case of L-proline derivative) and intramolecular trapping (in the case of acyl L-arginine) by N-nucleophiles. The L-proline approach has a broader scope for the synthesis of 2-aminopyrrolidine-1-carboxamidine derivatives, whereas the intramolecular cyclization afforded by the L-acylarginines, when applied, results in higher yields. The former allowed the first synthesis of cernumidine, a natural alkaloid isolated in 2011 from Solanum cernuum Vell, as its racemic form.

Full text of DOI:10.3390/molecules25040797

molecules
Article
Synthetic Approaches to a Challenging and Unusual
Structure—An Amino-Pyrrolidine Guanine Core
Rafael Rippel , Luís Pinheiro, Mónica Lopes, Ana Lourenço, Luísa M. Ferreira * and
Paula S. Branco *
LAQV-REQUIMTE, NOVA School of Science and Technology, 2829-516 Caparica, Portugal;
r.rippel@campus.fct.unl.pt (R.R.); l.pinheiro@campus.fct.unl.pt (L.P.); mam.lopes@campus.fct.unl.pt (M.L.);
ana.lourenco@fct.unl.pt (A.L.)
* Correspondence: lpf@fct.unl.pt (L.M.F.); paula.branco@fct.unl.pt (P.S.B.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Academic Editors: David Díez and María Ángeles Castro
Received: 10 January 2020; Accepted: 10 February 2020; Published: 12 February 2020
Abstract: The synthesis of an unreported 2-aminopyrrolidine-1-carboxamidine unit is here described
for the first time. This unusual and promising structure was attained through the oxidative
2−
decarboxylation of amino acids using the pair of reagents, silver(I)/peroxydisulfate (Ag(I)/S2O8
followed by intermolecular (in the case of l-proline derivative) and intramolecular trapping (in
the case of acyl l-arginine) by N-nucleophiles. The l-proline approach has a broader scope for the
synthesis of 2-aminopyrrolidine-1-carboxamidine derivatives, whereas the intramolecular cyclization
afforded by the l-acylarginines, when applied, results in higher yields. The former allowed the
first synthesis of cernumidine, a natural alkaloid isolated in 2011 from Solanum cernuum Vell, as its
racemic form.
)
Keywords: amino acid decarboxylation; cyclic-guanidine; Cernumidine; radical decarboxylation
1. Introduction
Small molecules are desirable for the pharmaceutical industry and medicinal chemistry where
the search for “hits” with specific effects on cells are essential. The drug discovery community
surely knows the classes of molecules associated with increased clinical success [
portion of the approved drugs are small molecules containing nitrogen-based aromatic heterocycles
such as pyrazoles, imidazoles, benzoimidazoles, triazoles, and 2-amine-pyrimidine moieties [ ].
Being aware of the dimension of the drug-like chemical space able to accommodate an enormous
diversity of small molecules [ ], chemical biology researchers started to look at fragment-based drug
discovery as a different approach [ ]. Without detracting from chemical synthesis, nature is still the
1,2], and a large
3
4
5
best source of diversity, but problems arise when more material is needed to pursue applicability
studies. Chemical modifications to improve therapeutic action, complete synthesis, and semisynthesis
contribute significantly to this purpose [
a detailed analysis of the relevance of natural products in the origin of new drugs from 1984 to
2014 [ ]. About one-third of all approved drugs classified as “small molecules” in this period are
natural products, derived from natural or synthetic products but with a natural pharmacophore.
Guanidine derivatives can be found in several natural products from diverse origin [ 10] and this
structural unit is present as a building block in various pharmaceuticals such as rosuvastatin a
cholesterol biosynthesis inhibitor, [11] guanabenz used clinically as an antihypertensive agent, [12
6,7]. In their 2016 review paper, Newman and Cragg make
8
9,
]
imatinib a tyrosine kinase inhibitor [13] used as an anticancer drug and the blockbuster drug used
to treat peptic ulcers and cimetidine a histamine H2 receptor antagonist that inhibits stomach acid
production [14]. Considered to be one of the strongest organic bases (pKHA = 13.6) guanidine has
the capacity to bind to carboxylates, phosphates and metals with the guanidinium cation engaging
Molecules 2020, 25, 797; doi:10.3390/molecules25040797
www.mdpi.com/journal/molecules

Molecules 2020, 25, 797
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special interactions between ligand/receptor or enzyme/substrate [15]. Because guanidines are super
bases and oxoanion hosts, they are good supporting ligands in organometallic and coordination
chemistry, and can also be used as catalysts [16]. Cernumidine (
in 2011 [17] presenting an unusual cyclic-guanidine core and displaying inhibition of interleukine-8
production by HT-29 colon carcinoma cells [18 21]. Cernumidine and other isomeric compounds
1) (Figure 1) is an alkaloid isolated
–
isolated from the Brazilian native tree used in folk medicine, Solanum cernuum Vell., present a never
reported 2-aminopyrrolidine-1-carboxamidine unit with five consecutive C-N bonds that, although
structurally very simple and with only a labile stereogenic center, presents a major synthetic challenge
to a synthetic chemist [22,23].
Figure 1. Structure of cernumidine (
1); in marked blue is the 2-aminopyrrolidine-1-carboxamidine unit
presenting the cyclic-guanidine core.
Previous efforts on the synthesis of a cyclic-guanidine core were reported in the work of Ascenzi et al.
who described the biological synthesis of the hemiaminal, 2-hydroxopyrrolidin-1-yl carboxamidine
from agmatine ((4-aminobutyl)guanidine) using copper aminoxidase from Pisum sativum L. in an
oxidative deamination process. The formation of the cyclic product N-amidino-2-hydroxypyrrolidine
should occur within the enzyme’s catalytic center since the presence of free 4-guanidobutyraldehyde
(shown within square brackets in Figure 2a) was never detected [24]. Although not precisely to attain
a cyclic-guanidine core, Boto et al. developed a process aimed at the oxidative decarboxylation of a
l-ornithine methyl carbamate derivative using the system (diacetoxyiodo)benzene or iodosylbenzene
at room temperature followed by intramolecular cyclization to a 2-aminopyrrolidine nucleus [25].
In this, and other reported works, they showed that the carboxyl radical evolves by loss of carbon
dioxide to produce a carbon radical which, in turn, undergoes oxidation to an N-acyliminium ion
intermediary demonstrated through inter- and intramolecular trapping with oxygen, nitrogen and
carbon nucleophiles [26–28]. They also showed that the reaction with proline is limited to N-protected
carbamates (Figure 2b) [27]. In a very recent work, Silva et al. [29] on trying to elucidate the metabolic
pathway of crambescins, suggested arginine and fatty acids as precursors for the broad chemical
diversity of this family. Their hypothesis was corroborated with a bio-inspired synthesis of crambescin
A2-448. Starting from arginine, they postulate that the oxidative decarboxylation generates in situ a
highly reactive pyrrolinium salt, and through a Biginelli-like reaction between C-2/C-3 activated fatty
acids and a central guanidinylated pyrrolinium on a cascade sequence the bicyclic system of crambescins
was obtained in a single operation (Figure 2c). There are several procedures available to effect oxidative
decarboxylation of amino acids. Previously we have already mentioned the methodology of Boto et al.
involving the use of the pair of oxidizing reagents diacetoxiiodobenzene/I2 (DIB/I2) [25–28]. Others,
combined this process with a Friedel–Crafts reaction in which they used aromatic rings (benzene,
furan, thiophene, among others) as nucleophiles to react with the acyliminium ion formed after the
oxidative decarboxylation [30,31]. Also using a hypervalent iodine reagent, the o-iodoxybenzoic acid,
Akamanchi et al. [32] pursued the decarboxylation of amino acids to nitriles in aqueous ammonia.
Other methods for amino acid decarboxylation include the use of hypohalides such as hypochlorite
and hypobromite [33], N-bromosuccinimide [34], or the pair of reagents, silver(I)/peroxydisulfate
(Ag(I)/S2O82−) applied for the first time by Stewart et al. [35] for the preparation of aldehydes.

Molecules 2020, 25, 797
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Figure 2. Reported efforts on the synthesis of a cyclic-guanidine core. Works from: (
P., et al. [24], (b) Boto, A., et al. [25,27], and (c) Silva et al [29].
a) Ascenzi,
The formation of the metastable ion Ag(II) generated in situ, can be isolated by chelation with
nitrogen aromatic ligands such as picolinic acid to form the silver picolinate salt (II), Ag(pic)2. The
work of Clarke et al. [36] and Silverman and Zelechonok [37] helped to elucidate the mechanism of
the oxidative decarboxylation reaction with amino acids. Moreover, the synergistic effect of Cu(II)
on this system which can act as a co-catalyst that facilitates the oxidation of the alkyl radical to an
alkene, alcohol, aldehydes, ketones or even imides has been addressed [38
the metal catalysed approaches for the decarboxylative coupling of amino acids can also be performed
with Fe(I) [41], Co(II) [42], Cu(I) [43 44] Ce(IV) [45]. Although several methodologies have been
–40]. In addition to silver,
,
developed in the last ten years, to the best of our knowledge, the decarboxylation of amino acids with
basic residues in the side chain remains elusive. Moreover, the use of nitrogen nucleophiles in metal
catalysed decarboxylative coupling was never reported for intermolecular reactions [46].
Here, we report for the first time the synthesis of the cyclic-guanidine core, the
2-aminopyrrolidine-1-carboxamidine unit (Figure 1) that ultimately allows us to synthesize the
natural product cernumidine (1) and analogues in its racemic form.
2. Results and Discussion
Two complementary strategies were applied for the synthesis of 2-aminopyrrolidine-
1-carboxamidine derivatives (Scheme 1) starting from l-proline and l-arginine through their guanidine
(2) and acyl derivatives (
3), respectively. The delineated synthetic method involves two approaches:
(i) oxidative decarboxylation of carbamimidoyl-l-proline (2) followed by the intermolecular trapping

Molecules 2020, 25, 797
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of the iminium intermediate by nucleophiles and, (ii) oxidative decarboxylation of (
3) followed by
intramolecular trapping of the acyliminium intermediate.
Scheme 1. Retrosynthetic approach to the synthesis of 2-aminopyrrolidine-1-carboxamidine derivatives.
2.1. Studies on the Oxidative Decarboxylation of l-arginine Derivatives (3)
Several l-arginine derivatives were synthesized as described in Table 1. Following the literature
procedure [47] N-benzoylarginine (3a) was obtained in 47% yield by reaction with benzoyl chloride.
Table 1. Procedures used via acyl chloride (Method A) or coupling reagent (Method B) for the
preparation of Nα-l-arginine acyl derivatives (3).
Method A: Acyl chloride (1.1 equiv.), l-arginine (1 equiv.) in water (1.15 M), Na2CO3 2 equiv. (pH
24 h r.t.; Method B: Carboxylic acid (1,2 equiv.) in dry CH2Cl2 (0.5 M), CDI (1.5 equiv.) then l-arginine (1 equiv.)
≈
10) at 0 ◦C then
0.5 M in DMF and DMAP (0.1 equiv.). For each compound the synthetic method used is assigned bellow it.
Other acyl chlorides were used when commercially available (as in the case of the synthesis of
compound 3g) or in situ prepared by reaction of the corresponding carboxylic acid with SOCl2 (Method
A—compounds 3c
, 3e, and 3f were thus prepared). Alternatively, compounds 3b, 3d, and 3h were
prepared using carbonyldiimidazole (CDI) as coupling reagent (Method B) [48]. Next, we turned our

Molecules 2020, 25, 797
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attention to the oxidative decarboxylation of Nα-acyl-l-arginine derivatives (
3) (Scheme 2). Based
on the literature [29], we propose that the oxidative decarboxylation of
intermediate ( ), which goes through an intramolecular cyclization to . Depending on the reaction
conditions, can be trapped by water to the hemiaminal that is further oxidized to the imide ( ) or
3 will generate an iminium
5
4
5
6
cleavage to the aldehyde (7) and the corresponding amide.
Scheme 2. Proposed mechanism for the oxidative decarboxylation of Nα-acyl-l-arginine derivatives
(3) to 2-aminopyrrolidine-1-carboxamidine derivatives (4).
Screening of the reaction conditions and results are presented in Table 2. The first assays for
the oxidative decarboxylation were performed on derivative 3a using the pair of oxidizing reagents
diacetoxyiodobenzene/iodine (DIB/I2) [25,49]. After several attempts, when a mixture of acetic acid
and DCM was used as solvent (Table 2, entry 7) compound 4a was obtained in 59% yield, probably
due to the higher solubility of 3a in this solvent mixture.
Next, we decided to turn our attention to the procedure described by Zelechonok [37] and use
the pair of reagents AgNO3/(NH4)2S2O8. These conditions lead to much faster reactions and have the
advantage that they can be performed in water, which is very suitable for these substrates. It is worth
to mention that since the physical separation of 4a and 6a proved to be challenging, from there on
whenever the formation of 6a was observed only relative amounts of each product will be described in
Table 2. Using AgNO3/(NH4)2S2O8 (0.1:1.2) 39% conversion to compound 4a was observed together
with compound 6a (Table 2, entry 8). The formation of imides in these conditions were already reported
2−
by Huang [40] on the oxidative decarboxylation of N-acyl amino acids induced by Ag+/Cu2+/S2O8
in water. To obviate the formation of 6a the reaction was performed in other solvents, such as DMF,
AcOH or MeOH but it proved to be slower with the formation of complex mixtures (Table 2, entry 9
to 10) or incomplete when MeOH was used (Table 2, entry 11). When the reaction was performed in
AcOH/H2O (Table 2, entry 12) in addition to compound 4a and 6a, compound 7 was also identified
in 26% by NMR (Supplementary Figure S1) [50]. Since the reaction proved to be unsuccessful when
performed in organic solvents, we decided to optimize the results attained in entry 8 by increasing
the amount of catalyst and oxidant (Table 2, entry 13). Despite of the higher conversion observed,
the increased load of persulfate also doubled the conversion to 6a. Being aware of the synergetic
effect of copper in decarboxylation reactions, CuSO4 was applied to our system [38,40]. The use of

Molecules 2020, 25, 797
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catalytic amounts of CuSO4 with an excess of oxidant (3 equiv., Table 2, entry 14) lead to discouraging
results. However, with 1 equivalent of CuSO4 the conversion of the product increased to 76% (89%
based on the recovered starting material, Table 2, entry 15) despite the formation of 6a. Increasing the
temperature to 60 ◦C lead to a complete reaction in 0.5 h as a mixture of compound 4a and 6a (Table 2,
entry 16). Only by inverting the addition of the reagents and making the arginine derivative react first
with CuSO4, the formation of 6a was suppressed and compound 4a was isolated in 72% yield (Table 2,
entry 17).
Table 2. Screening and results for the oxidative decarboxylation of Nα-benzoyl-l-arginine derivatives (3a).
(3a)
(4a)
(6a)
(7)
Additive
(equiv.)
Time
(h)
Relative
Amount
(%) a
Relative
Amount
(%) a
Relative
Amount
(%) a
Relative
Amount
(%) a
Entry
Oxidant
Solvent
T (◦C)
1
2 f
3
DIB/I2 (2:0.5)
DIB/I2 (2:0.5)
DIB/I2 (4:1)
DIB/I2 (4:1)
DIB/I2 (4:1)
DIB/I2 (4:1)
-
-
-
-
-
-
CH2Cl2
DMF
CH3CN
CH2Cl2
CH2Cl2
DMSO
r.t.
r.t.
reflux
r.t.
reflux
r.t.
48
18
56
1.5
48
48
>90
100
>90
100
92
vestigial
-
-
-
-
-
-
-
-
-
-
-
-
-
vestigial
4 f
5
-
8
-
6 g
-
AcOH/CH2Cl2
(3:1)
DIB/I2 (2:0.5)
-
-
-
-
-
-
-
reflux
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
60
72
3
-
59 e
39
-
-
8
-
-
-
7 f
2−
2−
2−
2−
2−
2−
2−
2−
2−
2−
Ag(I)/S2O8
(0.1:1.2)
Ag(I)/S2O8
(0.2:1.2)
8 b
H2O
53
9 b,g
10 b,g
11 b,f
12 b
13 c
14 g
15 c
16
DMF
>24
96
48
20
3
-
-
Ag(I)/S2O8
(0.15:1.2)
Ag(I)/S2O8
(0.15:1.5)
Ag(I)/S2O8
(0.15:1.5)
Ag(I)/S2O8
(0.15:1.5)
Ag(I)/S2O8
(0.15:3)
Ag(I)/S2O8
(0.15:1.5)
Ag(I)/S2O8
(0.15:1.5)
Ag(I)/S2O8
(0.15:1.5)
AcOH
MeOH
-
-
-
-
100
-
-
-
AcOH:H2O
(1:1)
-
-
-
-
-
-
35
69
-
39
17
-
26
11
-
H2O
H2O
H2O
H2O
H2O
CuSO4
(0.2)
CuSO4
(1)
CuSO4
(1)
CuSO4
(1)
3
3
89
83
72 e
7
8
-
4
5
-
0.5
0.5
17 d
60
a
relative amounts of the compounds based on NMR results of the crude mixture; b The concentration of 3a was
0.036 M; c The concentration of 3a was 0.072 M; d Reactions at entries 16 and 17 differ in the order of addition of
reagents; e Pure isolated compound; f No reaction; g Complex mixture.
The reaction was then extended to compounds 3b
–3h. Complex mixtures were obtained with
compounds 3c 3d 3e and 3f all bearing moieties able to be involved in radical reactions. Positive
,
,
results were only achieved for 4b and 4h although, in the latter, the reaction yield was low due to
incomplete reaction (Table 3). As expected, in this last case it was observed by HPLC-ESI-MS the
presence of two peaks, which would correspond to the two diastereomers of 4h as a result of the
racemization at the C-2 position of the pyrrolidine ring being formed (Supplementary Figure S2).
However, for our surprise analysis of 3h also shows two peaks in the HPLC-ESI-MS instead of only one
(Supplementary Figure S3). These peaks co-eluted with the two diastereomers of 4h (Supplementary
Figure S2a). This means that racemization occurs not only during the oxidative decarboxylation but
also during the formation of the dipeptide.
Next we turned our attention to the alternative strategy involving the oxidative decarboxylation
of carbamimidoyl-l-proline (2) using the same oxidative system (AgNO3/(NH4)2S2O8).

Molecules 2020, 25, 797
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Table 3. Scope of substrates for the oxidative decarboxylation of l-arginine derivatives (3) and
carbamimidoyl-l-proline (2).
From l-arginine derivatives (3)
From carbamimidoyl-l-proline (2)
2.2. Studies on the Oxidative Decarboxylation of Carbamimidoyl-l-proline (2)
We believe that this approach proceeds through a mechanism like the one proposed for the
arginine derivatives. As in the latter, the oxidative decarboxylation of generates an iminium ion (
that will be trapped by benzamide to form 4a (Scheme 3). Here, once again, the reaction conditions will
play a crucial role in the reaction outcome. The iminium ( ) can be trapped by water to the hemiaminal
) that can be further oxidized to the pyrrolidone (10) or be cleaved to the aldehyde (11). Screening of
2
8)
8
(9
the reaction conditions is presented in (Table 4). Initially, our attention was devoted to the solubility of
benzamide, and therefore the first assays (Table 4, entries 1–6) were carried out in organic solvents,
but none proved to be suitable to afford the desired product 4a. Moreover, in all these assays it was
possible to observe in the NMR spectra (not isolated and not quantified) the formation of pyrrolidone
(
10) as the major product and hemiaminal (
accordance with literature [24]). Next, considering the solubility of
conditions used for the decarboxylation of by using water as solvent and CuSO4 as a co-catalyst. In
9
) as a minor product (Supplementary Figure S4, data in
2
we decided to fully adopt the
3
these conditions (Table 4, entries 7–8) the formation of 10 was supressed entirely and for the first time,
the formation of the desired product 4a was observed.

Molecules 2020, 25, 797
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Scheme 3. Proposed mechanism for the oxidative decarboxylation of carbamimidoyl-l-proline (2) to
2-aminopyrrolidine-1-carboxamidine (4a).
Curiously, the lower reaction time and formation of product (Table 4, entry 8) contrasts directly
with the result observed in entry 7 where only hemiaminal (9) was formed during the reaction, which
indicates that the temperature is crucial for the reaction outcome. Afterwards, we investigated the
role of copper in the formation of 10 (Table 4, entries 9 and 10). In the absence of any nucleophile
when the reaction is carried out without copper (Table 4, entry 9) compound 10 was observed as the
main product, whereas in the presence of copper the major product was the hemiaminal (
entry 10), which suggests that copper prevents the oxidation of . Once the formation of 10 was
suppressed, we turned our attention to finding strategies that would avoid the formation . We started
9) (Table 4,
9
9
by changing the reaction solvent to MeOH (Table 4, entry 11) and ACN (Table 4, entry 12) but in the
former, the reaction did not occur and in the latter, the yield of 4a was low. Since the reaction proved to
be successful only in water, we investigate if the conversion of the hemiaminal (
was possible. We anticipated that under acidic conditions it should be possible to favour the formation
of from . By adding catalytic amounts of p-toluenesulfonic acid (p-TsOH) to the reaction (Table 4,
9) into the iminium (8)
8
9
entry 13) we only observed the formation of 4a. However, we were not able to separate the desired
product from the p-TsOH. Nonetheless, inspired by the previous result, we decided to use an acid resin
(Amberlyst 15) that was easily removed from the reaction mixture and allowed the isolation of the
desired product in 56% yield (Table 4, entry 14). With these conditions, we were able to observe an
inverse correlation between the yield and reaction time. By increasing the reaction time, the reaction
yield decreases (Table 4, entries 15–17). Reducing the reaction time (to 6 h) the yield also decreased
(Table 4, entry 18) and the product 4a was obtained in only 18% yield. These findings suggest that
the reaction outcome has close relation between the reactivity of the reagents and product stability.
That was confirmed by mixing 4a and Amberlyst 15 in water at 60 ◦C for 24 h, which results in a 1:1
mixture of hemiaminal (9) and 4a. However, despite the product stability, we believed that other
factors might justify the low yield of the reaction. In all the assays that were performed the yield of the
reaction contrasts with the information obtained by the NMR analysis of the reaction mixture. The
NMR always led us to conceive that we were in the presence of a simple mixture of the desired product
and the correspondent benzamide. Thus, knowing already that degradation of product occurs during
the reaction, we decided to analyse the decarboxylation step of our reaction. For that purpose, the
reaction mixture attained (Table 4, entry 10) was analysed by HPLC-ESI-MS (Supplementary Figure
S5). With a retention time of 4.1 min, it was possible to identify compound 9 by the peak at m/z 130
that correspond to the [M + H]+ and the pyrrolidone (10) with the peaks at m/z 128 and m/z 146 that
correspond to the [M + H]+ and [M + H+ H2O]+, respectively. With a retention time of 4.5 min the
peaks at m/z 156 and m/z 313 in negative mode correspond to the ions [M
−
H]- and [2M
−
H]- of the
starting material which are in accordance in positive mode with the peaks at m/z 158 and m/z 315.
2

Molecules 2020, 25, 797
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The LC-MS indicates the presence of other compounds which shows the lability of the intermediates
involved and indicates that, even prior to the formation of our product, the outcome of our reaction is
being affected by a variety of divergent pathways. This factor, combined with the instability of the
products, may explain the low to moderate yields obtained in this approach.
Table 4. Screening and results for the oxidative decarboxylation of carbamimidoyl-l-proline (
2) and
trapping the iminium intermediate with benzamide.
(4a)
Yield (%)
Entry
Oxidant
Additive
Solvent
T (◦C)
Time (h)
(9) a
(10) a
2−
2−
2−
2−
2−
2−
2−
2−
2−
2−
2−
2−
Ag(I)/S2O8
(0.3:3)
Ag(I)/S2O8
(0.3:3)
Ag(I)/S2O8
(0.3:3)
Ag(I)/S2O8
(0.3:3)
Ag(I)/S2O8
(0.3:3)
Ag(I)/S2O8
(0.3:3)
Ag(I)/S2O8
(0.15:1)
Ag(I)/S2O8
(0.15:1)
Ag(I)/S2O8
(0.15:1)
Ag(I)/S2O8
(0.15:1)
Ag(I)/S2O8
(0.15:1)
Ag(I)/S2O8
(0.15:1)
1
2
-
ACN/H2O
ACN/H2O
AcOH
DMA
r.t.
r.t.
5.5
72
72
72
6
X a
X a
X a
X a
X a
-
-
X b
X b
X b
X b
X b
-
-
-
3
-
r.t.
-
4
-
r.t.
-
5
-
ACN/H2O
DMA
80
-
6
-
r.t.
6
-
7
CuSO4
CuSO4
H2O
r.t.
15
2
X a
X a
X b
Xa
-
-
-
X b
-
8
H2O
reflux
reflux
reflux
reflux
reflux
-
H2O
2
X a
-
9 d
10 d
11 e
12
CuSO4
CuSO4
CuSO4
H2O
2
-
MeOH
ACN
16
6
-
-
X b
-
9
CuSO4/
TsOH
(20 mol%)
2−
Ag(I)/S2O8
13
H2O
reflux
16
-
-
X a
(0.15:1)
2−
2−
2−
2−
2−
CuSO4/
Amberlite 15
CuSO4/
Ag(I)/S2O8
(0.15:1)
Ag(I)/S2O8
(0.15:1)
Ag(I)/S2O8
(0.15:1)
Ag(I)/S2O8
(0.15:1)
14
15
16
17
H2O
H2O
H2O
H2O
H2O
60 ◦C
60 ◦C
60 ◦C
60 ◦C
60 ◦C
16
24
48
72
6
-
-
-
-
-
-
-
-
-
-
56 c
50 c
33 c
22 c
18 c
Amberlite 15
CuSO4/
Amberlite 15
CuSO4/
Amberlite 15
CuSO4/
Amberlite 15
Ag(I)/S2O8
(0.15:1)
18
a
Major product; b Minor product; c AgNO3 (0.15 equiv); CuSO4 (1 equiv), (NH4)2S2O8 (1 equiv), Amberlite 15 (20%
w/w) in water (1 M) at 60 ◦C overnight (16–18 h); d Absence of benzamide; e No reaction.
Afterwards, to examine the scope of this decarboxylating coupling reaction, various amides were
reacted under the optimized reaction conditions. Remarkably, at this time we were able to achieve
several 2-aminopyrrolidine-1-carboxamidine derivatives (Table 3) as compounds 4c
were not attainable by the method involving the oxidative decarboxylation of the Nα-acyl-l-arginine
derivatives 3c 3f, and 3g. Gladly, although in its racemic form, we were able to synthesize for the
, 4d, and 4e that
,
first time the cernumidine alkaloid (
isoferulic amide.
1) in 22% yield from reaction of compound 2 with acetylated
To fully explore the scope of the reaction, sulfinate salts were tested as nucleophiles without
success. We believe that the sulfinate salts are even more prone to retro-elimination of the nucleophile,

Molecules 2020, 25, 797
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and therefore the products were not stable under the acidic conditions of the reaction. The use of other
nucleophiles is limited by the stability of the product being formed and the acidic conditions used,
which is only compatible with non-basic nucleophiles.
3. Materials and Methods
All the reagents and solvents were obtained commercially (Merck KGaA, Darmstadt, Germany),
and these were used without further purification. The solvents used were dried using current laboratory
techniques. Thin-layer chromatography (TLC) was carried out on aluminium backed Kieselgel 60
F254 silica gel plates (Merck KGaA, Darmstadt, Germany). Plates were visualized under UV light (254
and/or 366 nm). Reversed phase column chromatography was carried out on silica gel LiChoroprep
RP-18 (40–63
µm, Merck KGaA, Darmstadt, Germany). Ultraviolet spectroscopy (UV) was recorded
on a Thermo Corporation spectrophotometer (Waltham, MA, USA), Helius
γ
, on quartz cell support.
Absorption spectrum measurements were made in the range of 190–320 nm. Infrared spectroscopy (IR)
1
was recorded on a PerkinElmer Spectrum Two (Waltham, MA, USA) in the range of 4000–400 cm−1. H
and 13C NMR spectra were recorded on a Brucker ARX400 (Billerica, MA, USA) at 400 and 101 MHz,
respectively. Chemical shifts are reported in parts per million (ppm,
δ units). The following NMR
abbreviations are used: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, brs = broad singlet.
The relative amount of each compound presented in Table 2 was based on the analysis of the crude
NMR, where the individualized integral of the H-2 proton of each compound was used. HPLC-ESI-MS
was acquired on an Agilent 1200 Series LC (Santa Clara, CA, USA) with a quaternary pump, ALS, TCC,
DAD, and Agilent G6130B LC/MSD (Santa Clara, CA, USA) with API-ES source, in FIA mode or LC
mode with a Zorbax-SB-C18 (4.6
×
150 mm, 5 µm, Agilent, Santa Clara, CA, USA) (Porto University)
and in a QqTOF Impact IITM mass spectrometer (Bruker Daltonics, Billerica, MA, USA) operating in
the high resolution mode. Samples were analyzed by flow injection analysis (FIA) using an isocratic
gradient 50A:50B of 0.1% formic acid in water (A) and 0.1% of formic acid in acetonitrile (B), at a flow
rate of 10
µL
min−1 over 15 min. Calibration of the TOF analyzer was performed with a calibrant
·
solution of sodium formate 10 mM. The full scan mass spectra were acquired over a mass range of
50–1000 m/z, at a spectra rate of 0.2 Hz. Data was processed using Data Analysis 4.2 software (Lisbon
University, Bruker, Billerica, MA, USA, 2015). ESI-HRMS (Electrospray ionization-high resolution mass
spectrometry) was performed in an LTQ Orbitrap XLTM mass spectrometer (Thermo Fischer Scientific,
Bremen, Germany) controlled by LTQ Tune Plus 2.5.5 and Xcalibur 2.1.0. The capillary voltage of the
electrospray ionization (ESI) was set to 3100 V. The capillary temperature was 275 ◦C. The sheath gas
flow rate (nitrogen) was set to 6 (arbitrary unit as provided by the software settings). The capillary
voltage was 49 V and the tube lens voltage 130 V. The description of these spectra follows the format:
mass (m/z); attribution in the molecule. The designation [M + H]+ always corresponds to the molecular
ion of interest.
3.1. General Procedure for the Synthesis of Arginine Derivatives
Method A (3a, 3c, 3e–3g)
To a solution of the corresponding carboxylic acid in toluene (volume to make a 0.5 M solution of
substrate), SOCl2 (5 equiv.) was slowly added and the reaction was stirred at reflux temperature for
1 h. Then, volatiles were removed under reduced pressure to afford the desired acyl chloride.
To a round-bottom flask charged with a solution of l-Arginine in water (volume to make a 0.2 M
solution of substrate) Na2CO3 (2 equiv.) was added. The solution was cooled at 0 ◦C and the acyl
chloride (1.2 equiv.) was added portion wise. The reaction mixture was stirred at 0 ◦C for 10 min and
then at r.t. until complete consumption of the starting material (24 to 72 h). After that, the mixture was
acidified with a solution of HCl 3 M to pH 1, and the aqueous phase was washed 3
resulting aqueous phase was then basified with NaOH 4 M to pH 8, evaporated until dryness and
×
with DMC. The

Molecules 2020, 25, 797
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purified by using RP18 column chromatography. The collected fractions were analyzed by UV-Vis and
were then combined according to the obtained UV-spectra to afford the desired product.
Method B (3b, 3d and 3h)
To a solution of the correspondent carboxylic acid (1.2 equiv.) in dry DCM (volume to make
a 0.5 M solution of substrate) CDI (1.5 equiv.) was added, and the reaction was stirred until
complete consumption of the starting material (TLC, DCM/MeOH 19:1). Then, the reaction mixture
was concentrated under vacuum and to the attained residue a solution of the arginine derivative
(1 equiv.) in DMF (volume to make a 0.5 M solution of substrate) and DMAP (0.1 equiv.) were
added and the reaction was stirred at 80 ◦C until complete consumption of the starting material (TLC,
DCM/MeOH/H2O 13:7:1). After the consumption of the starting material, the solvent was removed
under reduced pressured and the reaction mixture was purified by RP-18 column chromatography.
The collected fractions were analyzed by UV-vis and combined according to the obtained UV-spectra.
Compound 3a
obtained in 47% yield; m.p. 285–288 C. IV (ATR) νmáx (cm−1) 3200, 1677, 1615, 1592. 1H RMN
(400 MHz, D2O)
7.80 (d, J = 7.7 Hz, 2H, H-30 and H-70), 7.62 (t, J = 7.2 Hz, 1H, H-50), 7.54 (t, J = 7.6 Hz,
2H, H-40 and H-60), 4.41 (dd, J = 8.1, 5.1 Hz, 1H, H-2), 3.23 (t, J = 6.8 Hz, 2H, H-5), 2.07–1.92 (m, 1H,
[51]. Following the general procedure A, from 4 g of l-Arginine compound 3a was
◦
δ
H-3a), 1.91–1.77 (m, 1H, H-3b), 1.75–1.63 (m, 2H, H-4). 13C NMR (101 MHz, D2O)
δ 178.22 (C-1), 169.94
(C-10), 156.62 (C-6), 133.42 (C-20), 132.06 (C-50), 128.64 (C-30 e H-70), 126.97 (C-40 e H-60), 55.04 (C-2),
40.58 (C-5), 28.95 (C-3), 24.50 (C-5).
Compound 3b. Following the general procedure B, from 1 g of l-Arginine compound 3b was obtained in
1
59% yield. m.p. 90–93 ◦C. IR (ATR) νmáx (cm−1) 3160, 1604, 1543, 1501, 1393, 1252. H RMN (400 MHz,
D2O)
5.2 Hz, 1H, H-2), 3.88 (s, 3H, OMe), 3.22 (t, J = 6.7 Hz, 2H, H-5), 2.05–1.91 (m, 1H, H-3a), 1.89–1.76
δ
7.79 (d, J = 8.6 Hz, 2H, H-30 e H-70), 7.05 (d, J = 8.7 Hz, 2H, H-40 e H-60), 4.39 (dd, J = 7.8,
(m, 1H, H-3b), 1.74–1.61 (m, 2H, H-4). 13C NMR (101 MHz, D2O) 178.72 (C-1), 169.30 (C-10), 161.91
δ
(C-50), 156.62 (C-6), 129.09 (C-30 e C-70), 125.72 (C-20), 113.88 (C-40 e C-60), 55.41 (OMe), 55.14 (C-2),
40.63 (C-5), 28.87 (C-3), 24.55 (C-4). HRMS (ESI) m/z calculated for C14H20N4O4 [MH+]: 308.14846;
Found: 308.14556.
Compound 3c
[
52]. Following the general procedure
A
, from 2 g of l-Arginine compound 3c was
1
obtained in 55% yield. m.p. 190–192 ◦C. IR (ATR) νmáx (cm−1) 3333, 3161, 3046, 2944, 1667, 1567. H
RMN (400 MHz, CD3OD) δ
7.54–7.44 (m, 3H, H-30, H-50 e H-90), 7.43–7.31 (m, 3H, H-60, H-70 e H-80),
6.73 (d, J = 15.8 Hz, 1H, H-20), 4.46 (dd, J = 7.6, 4.8 Hz, 1H, H-2), 3.23 (tt, J = 13.5, 6.8 Hz, 2H, H-5),
2.04–1.91 (m, 1H, H-3a), 1.89–1.76 (m, 1H, H-3b), 1.75–1.62 (m, 2H, H-4). 13C NMR (101 MHz, CD3OD)
δ
177.61 (C-1), 166.61 (C-10), 157.30 (C-6), 140.14 (C-30), 134.93 (C-40), 129.28 (C-70), 128.45 (C-60 e C-80),
127.48 (C-50 e C-90), 120.83 (C-20), 54.29 (C-2), 40.67 (C-5), 29.80 (C-3), 24.86 (C-4).
Compound 3d. Following the general procedure B, from g of l-Arginine compound 3d was obtained in
1
55% yield. m.p. 112-115 ◦C. IR (ATR) νmáx (cm−1) 3400, 2900, 1638, 1605, 1573, 1510, 1394, 1251. H
RMN (400 MHz, CD3OD)
δ
7.38–7.27 (m, 3H, H-30e, H-50 e H-90), 6.80 (d, J = 8.5 Hz, 2H, H-60 e H-80),
6.48 (d, J = 15.7 Hz, 1H, H-20), 4.36 (dd, J = 8.0, 4.8 Hz, 1H, H-2), 3.72 (s, 3H, OMe), 3.20–3.04 (m, 2H,
H-5), 1.93–1.79 (m, 1H, H-3a), 1.77–1.64 (m, 1H, H-3b), 1.65–1.53 (m, 2H, H-4). 13C NMR (101 MHz,
CD3OD)
δ
179.18 (C-1), 168.46 (C-10), 162.48 (C-70), 158.71 (C-6), 141.32 (C-30), 130.48 (C-50 e H-90),
128.91 (C-40), 119.70 (C-20), 115.26 (C-60 e C-80), 55.83 (OMe), 42.06 (C-5), 31.19 (C-3), 26.32 (C-4). HRMS
(ESI) m/z calculated for C16H22N4O4 [MH]+: 334.16411; Found: 334.16135.
Compound 3e. Following the general procedure A, from 0.5 g of l-Arginine compound 3e was obtained
1
in 63% yield. m.p. 125–128 ◦C. IR (ATR) νmáx (cm−1) 3258, 3160, 2969, 2872, 1633, 1557, 1394. H RMN
(400 MHz, D2O)
J = 8.3, 4.2 Hz, 1H, H-2), 3.04–2.85 (m, 4H, H-5 e H-30), 2,63 (t, J = 6.9 Hz, 2H, H-20), 1.72–1.59 (m, 1H,
H-3a), 1.56–1.43 (m, 1H, H-3b), 1.15 (p, J = 7.5 Hz, 2H, H-4). 13C NMR (101 MHz, D2O)
178.55 (C-1),
δ
7.35 (t, J = 7.4 Hz, 2H, H-60 e H-80), 7.31–7.22 (m, 3H, H-50, H-60 e H-90), 4.05 (dd,
δ

Molecules 2020, 25, 797
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174.90 (C-10), 156.50 (C-6), 140.33 (C-40), 128.58 (C-60 e C-80), 128.49 (C-50 e C-90), 126.35 (C-70), 54.32
(C-2), 40.54 (C-5), 37.24 (C-20), 31.26 (C-30), 28.69 (C-3), 24.12 (C-4). HRMS (ESI) m/z calculated for
C15H23N4O3 [MH]+: 307.17647; Found: 307.17718.
Compound 3f. Following the general procedure A, from 0.5 g of l-Arginine compound 3f was obtained
in 81% yield. m.p. 138–141 ◦C. IR (ATR) νmáx (cm−1) 3262, 3172, 3056, 2864, 1634, 1557, 1495, 1394.
1H RMN (400 MHz, D2O) 7.50–7.29 (m, 5H, H-40, H-50, H-60, H-70 e H-80), 4.19 (dd, J = 8.1, 4.8 Hz,
δ
1H, H-2), 3.68 (d, J = 14.4 Hz, 1H, H-20a), 3.60 (d, J = 14.8 Hz, 1H, H-20b), 3.11 (t, J = 6,9 Hz, 2H, H-5),
1.89–1.77 (m, 1H, H-3a), 1.75–1.61 (m, 1H, H-3b), 1.59–1.43 (m, 2H, H-4). 13C NMR (101 MHz, D2O) δ
178.47 (C-1), 173.97 (C-10), 156.54 (C-6), 135.15 (C-30), 129.06 (C-50 e C-70), 128.89 (C-40 e C-80), 127.25
(C-60), 54.57 (C-2), 42.43 (C-20), 40.49 (C-5), 28.77 (C-3), 24.38 (C-4). HRMS (ESI) m/z calculated for
C14H21N4O3 [MH]+: 293.16082; Found: 293.16134.
Compound 3g. Following the general procedure A, from 0.5 g of l-Arginine compound 3g was obtained
1
in 38% yield. m.p. 108–111 ◦C. IR (ATR) νmáx (cm−1) 3029, 1719, 1632, 1563, 1474, 1364, 1203. H RMN
(400 MHz, CD3OD) δ
7.23 (t, J = 7.8 Hz, 2H, H-40 e H-60), 7.07 (t, J = 7.2 Hz, 1H, H-50), 6.99 (d, J = 7.9 Hz,
2H, H-30 e H-70), 4.01–3.90 (m, 1H, H-2), 3.09 (t, J = 6.6 Hz, 2H, H-5), 1.91–1.74 (m, 1H, H-3a), 1.74–1.54
(m, 3H, H-3b e H-4). HRMS (ESI) m/z calculated for C13H19N4O4 [MH+]: 295.14008; Found: 295.13994.
Compound 3h. [53]. Following the general procedure B, from 0.5 g of l-Arginine compound 3h was
1
obtained in 31% yield. H RMN (400 MHz, D2O)
δ
7.73 (s, 1H, H-60), 6.97 (s, 1H, H-50), 4.37–4.29 (m,
1H, H-20), 4.18 (dd, J = 12.4, 5.0 Hz, 1H, H-2), 3.27–3.12 (m, 2H, H-5), 3.12–3.02 (m, 1H, H-30a), 3.00–2.90
(m, 1H, H-30b), 1.93–1.78 (m, 1H, H-3a), 1.76–1.62 (m, 1H, H-3b), 1.62–1.48 (m, 2H, H-4), 1.38 (s, 9H,
H-30’). HRMS (ESI) m/z calculated for C17H29N7O5 [MH]+: 412.2303; Found: 412.2300.
3.2. Synthesis of Carbamimidoyl-l-proline (2) [54]
To a round-bottom flask charged with a solution l-Proline in a mixture of water (volume to make
a 5.7 M solution of substrate) and methanol (volume to make a 4.3 M solution of substrate) a solution
of cyanamide (1.5 equiv.) in water (volume to make a 4.8 M solution of substrate) was added and the
reaction mixture was stirred at 100 ◦C for 16 h. After complete consumption of the starting material,
the reaction mixture was concentrated under vacuum and washed several times with isopropanol to
1
afford the desired product as a white solid. H NMR (400 MHz, D2O)
δ
4.27 (dd, J = 8.6, 2.9 Hz, 1H),
3.62–3.53 (m, 1H), 3.48 (dd, J = 16.8, 8.7 Hz, 1H), 2.30 (d, J = 10.9 Hz, 1H), 2.18 (dd, J = 6.5, 3.1 Hz, 1H),
2.10–1.89 (m, 2H). 13C NMR (101 MHz, D2O) δ 177.77, 154.73, 62.42, 47.82, 31.02, 23.32.
3.3. General Procedure for the Preparation of Compounds 4 from Decarboxylation of Arginine Derivatives
Following the adapted procedure reported by Huang et al. [40] to a round-bottom flask charged
with a solution of derivatized L-arginine in water (volume to make a 0.4 M solution of substrate) at
60 ◦C, CuSO4.5H2O (1 equiv.) was added, and the solution was stirred for 10 min. Then, AgNO3
(0.15 equiv.) and NH4S2O8 (1.5 equiv.) were added and the reaction was stirred at r.t. for 30 min (the
reaction color evolves from blue to a green solution).
After complete consumption of the starting material, the reaction mixture was concentrated under
vacuum and purified by RP-18 column chromatography. The collected fractions were analyzed by
UV-vis and combined according to the obtained UV-spectra.
Compound 4a. Following the general procedure and starting from 50 mg of 3a compound 4a was
1
obtained in 72% yield. H NMR (400 MHz, D2O)
δ 7.72 (d, J = 7.5 Hz, 1H), 7.61 (t, J = 7.2 Hz, 1H), 7.50
(t, J = 7.5 Hz, 1H), 5.87 (d, J = 6.1 Hz, 1H), 3.60 (t, J = 7.5 Hz, 1H), 3.43 (m, 1H), 2.39–2.07 (m, 1H). 13
NMR (101 MHz, D2O) 179.30, 171.02, 134.50, 133.14, 129.73, 128.06, 56.12, 41.66, 30.03, 25.58. HRMS
(ESI) m/z calculated C12H17N4O [MH]+: 233.13969; Found: 233.13931.
C
δ
Compound 4b. Following the general procedure and starting from 50 mg of 3b compound 4b was
obtained in 80% yield. 1H RMN (400 MHz, D2O) 7.57 (d, J = 8.7 Hz, 2H, H-30 e H-70), 6.87 (d,
δ

Molecules 2020, 25, 797
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J = 8.7 Hz, 2H, H-40 e H-60), 5.75–5.64 (m, 1H, H-2), 3.73 (s, 3H, OMe), 3.54–3.44 (m, 1H, H-5a), 3.38–3.28
(m, 1H, H-5b), 2.27–2.09 (m, 2H, H-3), 2.08–1.94 (m, 2H, H-4). 13C NMR (101 MHz, CD3OD)
179.18,
δ
168.46, 162.47, 158.70, 141.31, 132.28, 130.47, 128.91, 122.49, 119.69, 115.25, 114.90, 114.53, 55.82, 42.06,
31.19, 26.32. HRMS (ESI) m/z calculated C13H19N4O2 [MH]+: 263.15025; Found: 263.15060.
Compound 4h. Following the general procedure and starting from 50 mg of 3h compound 4h was
obtained in 7% yield 1H NMR (400 MHz, D2O)
δ 8.64 (s, 1H), 7.33 (s, 1H), 5.71 (d, J = 5.7 Hz, 1H),
4.46–4.34 (m, 1H), 3.61–3.48 (m, 1H), 3.46–3.37 (m, 1H), 2.17–2.07 (m, 1H), 1.96–1.92 (m, 2H), 1.80–1.70
(m, 1H), 1.68–1.51 (m, 2H), 1.39 (s, 9H) (observed in the crude mixture). ESI-HPLC-MS, RT 12.6 min,
[M + H]+ m/z: 412.2; [M
−
H]+ m/z: 410.2; RT 12.8 min, [M + H]+ m/z: 412.2; [M
−
H]+ m/z: 410.1. HRMS
(ESI) m/z calculated C16H28N7O3 [MH]+: 366.2248; Found: 366.2245.
3.4. General Procedure for the Preparation of Compounds 4 from Decarboxylation of
Carbamimidoyl-l-proline (2)
Following the adapted procedure reported by Huang et al. [40], to a round-bottom flask charged
with a solution of derivatized l-Proline in water (volume to make a 0.8 M solution of substrate) at
r.t., CuSO4 5H2O (1 equiv.), AgNO3 (0.15 equiv.) and NH4S2O8 (1.5 equiv.) were added and the
·
reaction was stirred for 30 min at r.t. Then, the reaction was heated to 60 ◦C and Amberlyst 15 (15%
weight of substrate) and the corresponding nucleophile (1.1 equiv.) were added and the reaction was
stirred for 16–18 h. After that the reaction mixture was diluted with MeOH and filtered under a celite
pad (washed with water and MeOH several times). The filtrate was evaporated until dryness and
purified by using RP18 column chromatography. The collected fractions were analyzed by UV-vis and
combined according to the obtained UV-spectra to afford the desired product.
Compound
compound
ferulic amide was removed during the reaction due to the acidic conditions used. H NMR (400 MHz,
CD3OD) 7.54 (d, J = 15.6 Hz, 1H), 7.11 (d, J = 1.8 Hz, 1H), 7.03 (d, J = 8.3 Hz, 1H), 6.79 (d, J = 8.2 Hz,
1H), 6.47 (d, J = 15.6 Hz, 1H), 5.77 (d, J = 5.2 Hz, 1H), 3.86 (s, 3H), 3.59–3.52 (m, 1H), 3.42–3.35 (m, 1H),
2.33–2.23 (m, 2H), 2.15–2.08 (m, 1H), 2.04–1.99 (m, 1H). 13C NMR (101 MHz, CD3OD)
169.96, 156.27,
1
[17]. Following the general procedure and starting from 100 mg of
2 and 1.1 equiv. of amide,
1
was obtained in 22% yield. It is worth to mention that the acetyl group protecting the
1
δ
δ
151.32, 149.98, 149.55, 149.26, 144.60, 127.80, 126.30, 123.78, 117.06, 111.75, 64.78, 56.13, 32.48, 24.18,
23.80. ESI-MS [M + H]+ m/z: 305.1.
Compound 4a. Following the general procedure and starting from 100 mg of
2 and 1.1 equiv. of
benzamide, compound 4a was obtained in 56% yield. In accordance with spectra data of compound
obtained by method A.
Compound 4c. Following the general procedure and starting from 100 mg of
2
and 1.1 equiv. of amide,
1
compound 4c was obtained in 22% yield. H NMR (400 MHz, (CD3OD)
δ 7.62 (d, J = 15.8 Hz, 1H),
7.60–7.55 (m, 2H), 7.42–7.33 (m, 3H), 6.71 (d, J = 15.7 Hz, 1H), 5.76 (d, J = 6.1 Hz, 1H), 3.63–3.53 (m,
1H), 3.44–3.34 (m, 1H), 2.39–2.20 (m, 2H), 2.15–1.98 (m, 2H). 13C NMR (101 MHz, CD3OD)
δ 169.46,
156.78, 144.14, 135.89, 131.33, 130.04, 129.11, 120.39, 65.27, 48.17, 33.41, 23.77. ESI-MS [M + H]+ m/z:
259.1. HRMS (ESI) m/z calculated C14H19N4O [MH]+: 259.15534; Found: 259,1561.
Compound 4d. Following the general procedure and starting from 100 mg of
2
and 1.1 equiv. of amide,
1
compound 4d was obtained in 32% yield. H NMR (400 MHz, CD3OD)
δ 7.28–7.22 (m, 2H), 7.20–7.13
(m, 3H), 5.59 (d, J = 5.4 Hz, 1H), 3.43 (dd, J = 12.5, 5.3 Hz, 1H), 3.33 (dd, J = 9.0, 6.9 Hz, 1H), 2.90
(t, J = 7.4 Hz, 2H), 2.53 (t, J = 7.4 Hz, 2H), 2.23–2.00 (m, 2H). 13C NMR (101 MHz, CD3OD)
δ 180.48,
175.53, 156.79, 136.18, 130.08, 129.59, 128.06, 65.27, 43.17, 33.28, 24.26, 23.69. ESI-MS [M + H]+ m/z:
261.1. HRMS (ESI) m/z calculated C14H21N4O [MH]+: 261.17154; Found: 261.1718.
Compound 4e. Following the general procedure f and starting from 100 mg of
2
and 1.1 equiv. of amide,
1
compound 4e was obtained in 17% yield. H NMR (400 MHz, CD3OD)
δ 7.31–7.17 (m, J = 11.7, 7.6 Hz,
1H), 5.66 (d, J = 5.2 Hz, 1H), 3.57–3.51 (m, 3H), 3.41–3.33 (m, 1H), 2.31–2.21 (m, 2H), 2.14–2.07 (m, 1H),

Molecules 2020, 25, 797
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2–1.95 (m, 1H). 13C NMR (101 MHz, CD3OD)
δ
176.66, 156.72, 141.66, 129.49, 127.33, 64.96, 48.00, 38.34,
33.08, 32.56, 23.87, 23.55. HRMS (ESI) m/z calculated C13H19N4O [MH]+: 247.15589; Found: 247.1560.
Compound 4f. Following the general procedure and starting from 100 mg of
compound 4f was obtained in 51% yield. 1H NMR (400 MHz, D2O)
6.36–6.29 (m, 2H), 5.80 (d,
J = 2.6 Hz, 1H), 5.79–5.75 (m, 1H), 3.62–3.54 (m, 1H), 3.46–3.38 (m, 1H), 2.33–2.26 (m, 2H), 2.17–2.10 (m,
2 and 1.1 equiv. of amide,
δ
1H), 2.04–1.98 (m, 1H). 13C NMR (101 MHz, D2O)
δ 167.55, 129.37, 127.98, 63.83, 46.81, 31.93, 22.30.
HRMS (ESI) m/z calculated C8H15N4O [MH]+: 183.1240; Found: 183.1236.
Compound 4g. Following the general procedure and starting from 100 mg of
2
and 1.1 equiv. of
amide, compound 4g was obtained in 51% yield. 1H NMR (400 MHz, D2O)
δ 5.66 (d, J = 5.6 Hz,
1H), 3.63–352 (m, 1H), 3.44–3.32 (m, 2H), 2.32–2.06 (m, 4H), 2.02 (s, 3H). 13C NMR (101 MHz, D2O)
δ
173.37, 155.36, 63.69, 46.72, 31.84, 22.27, 21.01. HRMS (ESI) m/z calculated C7H15N4O [MH]+: 171.1240;
Found: 171.1241.
4. Conclusions
The first metal catalysed decarboxylative coupling of amino acids bearing basic residues were
here described. In this process, nitrogen nucleophiles were used for the first time for the iminium
trapping through intermolecular reactions. Excitingly, through this method, novel approaches to a
challenging and unusual amino-pyrrolidin-guaninium core were reported. This privileged structure
is an unexplored motif in natural products and very promising for the design of bioactive synthetic
derivatives. Moreover, efforts are being devoted to establish an enantioselective approach for the
synthesis of compound 4. Also, the promising result obtained with the dipeptide 3h where compound
4h was obtained, definitely makes the preparation of novel peptidomimetics within reach.
Supplementary Materials: The following are available online. 1H, 13C, DEPT 135 and Mass spectra of compounds
1, 2, 3a–h and 4a–h and Figure S1–S4.
Author Contributions: The contributions of the authors to this work is as follows: Conceptualization, P.S.B.
and L.M.F.; methodology, P.S.B. and L.M.F.; formal analysis, A.L.; laboratory work, R.R., L.P. and M.L.;
writing—original draft preparation, P.S.B., L.M.F., R.R., L.P.; writing—review and editing, P.S.B., L.M.F. and A.L.;
project administration, A.L.; funding acquisition, P.S.B., L.M.F. and A.L. All authors have read and agreed to the
published version of the manuscript.
Funding: This work was supported by the Associate Laboratory for Green Chemistry-LAQV, which is financed
by national funds from FCT/MCTES (UIDB/50006/2020). The National NMR Facility and the SCXRD facility are
supported by Fundaç
ão para a Ciência e Tecnologia (RECI/BBB-BQB/0230/2012 and RECI/BBB-BEP/0124/2012,
respectively). R.R. also acknowledgments the grant number SFRH/BD/136692/2018 from FCT/MCTES.
Acknowledgments: We acknowledge the Laboratório de Análises REQUIMTE for the technical support for the
mass spectrometry analyses.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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