α,α-Dialkylglycines obtained by solid phase Ugi reaction performed over isocyanide functionalized resins

Article abstract of DOI:10.1016/j.tet.2013.08.002

The multicomponent Ugi reaction is a straightforward method that can be used for the synthesis of highly hindered C-tetrasubstituted amino acids by reacting an amine, a ketone or aldehyde, a carboxylic acid and an isocyanide. In the present work, the synthesis of several α,α-dialkylglycines (α,α-diethylglycine, Deg; α,α-dipropylglycine, Dpg; 1-amino-1-cyclohexanecarboxylic acid, Ac₆c) was achieved by solid phase Ugi reaction using resins functionalized with the isocyanide group. Since no resins with these features were available commercially, the functionalization of an aminomethylated resin started by the use of glycine (Gly), β-alanine (β-Ala) and γ-aminobutyric acid (GABA) as spacers. After spacer N-formylation, followed by dehydration, isocyanide functionalised resins were obtained. The resins were then used in solid phase Ugi reaction, using phenylacetic acid as the acid component, 4-methoxybenzylamine as the amine component and different ketones, to afford the desired N-acylated α,α-dialkylglycines in good overall yields (60-80%), after acidolytic cleavage from the resin, thus proving the feasibility of this approach.

Full text of DOI:10.1016/j.tet.2013.08.002

Accepted Manuscript
α,α-Dialkylglycines obtained by solid phase Ugi reaction performed over isocyanide
functionalized resins
Nádia R. Aguiam, Vânia I. Castro, Ana I. Ribeiro, Rui D.V. Fernandes, Carina M.
Carvalho, Susana P.G. Costa, Sílvia M.M. A. Pereira-Lima
PII:
S0040-4020(13)01240-4
10.1016/j.tet.2013.08.002
TET 24688
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 29 June 2013
Revised Date: 30 July 2013
Accepted Date: 1 August 2013
Please cite this article as: Aguiam NR, Castro VI, Ribeiro AI, Fernandes RDV, Carvalho CM, Costa
SPG, Pereira-Lima SMMA, α,α-Dialkylglycines obtained by solid phase Ugi reaction performed over
isocyanide functionalized resins, Tetrahedron (2013), doi: 10.1016/j.tet.2013.08.002.
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ACCEPTED MANUSCRIPT
GRAPHICAL ABSTRACT
α,α-Dialkylglycines obtained by solid phase Ugi reaction performed
over isocyanide functionalized resins
Nádia R. Aguiam, Vânia I. Castro, Ana I. Ribeiro, Rui D. V. Fernandes, Carina M.
Carvalho, Susana P. G. Costa, Sílvia M. M. A. Pereira-Lima*
OH
O
OCH3
+
CN
+
+
C
O
H2N
R
R
solid phase Ugi reaction
followed by acidolytic cleavage
O
H
N
C
R = -CH2CH3
,
C
O
C
OH
-(CH2)2CH3
-(CH2)5-
,
R
R

ACCEPTED MANUSCRIPT
α,α-Dialkylglycines obtained by solid phase Ugi reaction performed
over isocyanide functionalized resins
Nádia R. Aguiam, Vânia I. Castro, Ana I. Ribeiro, Rui D. V. Fernandes, Carina M.
Carvalho, Susana P. G. Costa, Sílvia M. M. A. Pereira-Lima*
Centro de Química, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal;
email: silviap@quimica.uminho.pt
Abstract: The multicomponent Ugi reaction is a straightforward method that can be
used for the synthesis of highly hindered C-tetrasubstituted amino acids by reacting an
amine, a ketone, an acid and an isocyanide. In the present work, the synthesis of several
α,α-dialkylglycines (α,α-diethylglycine, Deg; α,α-dipropylglycine, Dpg; 1-amino-1-
cyclohexanecarboxylic acid, Ac6c) was achieved by solid phase Ugi reaction using
resins functionalized with the isocyanide group. Since no resins with these features were
available commercially, the functionalization of an aminomethylated resin started by the
use of glycine (Gly), β-alanine (β-Ala) and γ-aminobutyric acid (GABA) as spacers.
After spacer N-formylation, followed by dehydration, isocyanide functionalised resins
were obtained. The resins were then used in solid phase Ugi reaction, using
phenylacetic acid as the acid component, p-methoxybenzylamine as the amine
component and different ketones, to afford the desired N-acylated α,α-dialkylglycines in
good yields (60-80%), after acidolytic cleavage from the resin, thus proving the
feasibility of this approach.
Keywords: α,α-dialkylglycines, Ugi reaction, isocyanide resin, solid phase synthesis
1. Introduction
α,α-Dialkylglycines are amino acids sterically constrained around the central carbon
atom and, for this reason, are useful building blocks in the assembly of peptides with
* Tel: + 351 253 604375. Fax: + 351 253 604382. Email: silviap@quimica.uminho.pt
2

ACCEPTED MANUSCRIPT
specific conformations.1–3 Additionally, these non-coded amino acids are not recognized
by proteolytic enzymes, increasing the resistance of the peptides to physiological
biodegradation. However, due to steric hindrance, most of these amino acids are
difficult to synthesize4,5 and to use in peptide synthesis by conventional methods.6 In
fact, the same structural features that make these amino acids interesting and unique for
the development of peptide pharmaceuticals, are also responsible for the challenge
associated with their synthesis and application. Therefore there is an interest in
developing straightforward methodologies for the preparation and incorporation of these
amino acids into peptides.
In 1971 Ivar Ugi proposed a condensation reaction of four components as an alternative
to the classical methods of amino acid synthesis based on isocyanide chemistry, by
reacting an amine, a carbonyl compound, an acid and an isocyanide.7–9 Our group has
been involved in the application of this reaction to the synthesis of symmetrical α,α-
dialkylglycines and model di- to pentapeptides containing these highly constrained
amino acids. By taking advantage of an unusual acid lability of the C-terminal amide
bond, it was possible to overcome drawbacks that prevented a broad application of this
multicomponent reaction to the synthesis of α,α-dialkylglycines bearing peptides.10–12
Despite this innovative contribution, the synthetic protocols used were carried out in
solution, with the usual problems associated with purification procedures. Therefore,
bearing the above facts in mind, it was envisaged to adapt this synthetic methodology to
solid phase synthesis to simplify purification processes. Moreover, in the case of
peptide synthesis, it would allow the in situ assembly of α,α-dialkylglycines, to
overcome the remarkably low reactivity of the α-amino and carboxyl groups in α,α-
dialkylglycines, associated to steric hindrance, which difficult classical coupling
methods. There have been reports in the literature on solid phase Ugi reaction with
amide resins as the amine component,13–15 but only few relating to isocyanide
functionalized resins as examples for the synthesis of other types of molecules such as
heterocycles,16–18 or involving coupling of isocyanocarboxylic acids prior conducting
the Ugi reaction.19
As a result, we now report the preparation of resins bearing an isocyanide functionalized
spacer unit, to decrease steric hindrance at the functional group in order to facilitate the
multicomponent reaction. After unsuccessful preliminary results with a directly
functionalized isocyanide resin, it was decided to use variable length unbranched amino
acids as spacers (glycine, β-alanine and γ-aminobutyric acid) in order to maintain
3

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chemical identity without increasing steric hindrance. The isocyanide resins were used
in Ugi reactions involving symmetrical ketones and appropriate acid and amine
components, to evaluate the efficacy of this approach to the straightforward synthesis of
α,α-dialkylglycine derivatives.
2. Results and Discussion
Having in mind the aim of obtaining highly hindered amino acids using a solid phase
Ugi reaction with an isocyanide functionalized resin in conditions that would represent
a reduction of the processing time of the previously developed strategy11,12,20 without a
severe increase in costs, a simple and inexpensive resin was chosen as the subject of our
study, an aminomethylated polystyrene resin.
Taking into account previous reports,16,18 the resin was formylated with formic acid in
the presence of N,N’-diisopropylcarbodiimide (DIC) (scheme 1). The reactions
proceeded as expected yielding a resin 2a where the presence of the carbonyl group
could be detected by IR spectroscopy performed on KBr disc at 1694 cm-1(figure 1).
The resin was then dehydrated in the presence of POCl3 (scheme 1) affording the
desired resin 3a, as could be verified by the disappearance of the carbonyl band and the
appearance of the isocyanide band in the IR spectra at 2147 cm-1 (figure 1).
Scheme 1 Preparation of the isocyanide functionalized resins 3a-d.
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Figure 1. IR spectra of resins 2a and 3a obtained in KBr disc.
This resin was used as the isocyanide component in a Ugi reaction where the carbonyl
component chosen was 3-pentanone 4A, to achieve the synthesis of a α,α-diethylglycine
derivative, and the amine was 4-methoxybenzylamine, since the resulting 4-
methoxybenzyl group in the Ugi adduct 5Aa (scheme 2) is cleavable with trifluoroacetic
acid (TFA).21 The chosen acid was phenylacetic acid, because we have used it
previously in Ugi reactions and it afforded the desired products in good yields.
Moreover, the corresponding NMR signals in the Ugi adduct are distinctive from the
signals of the remaining molecule.20,21 The reaction was carried out using a 10
equivalent excess of the reactants relatively to the initial degree of functionalization of
the resin. Ugi reactions performed over this resin didn’t afford the desired products 5Aa,
as the intensity of the isocyanide IR band didn’t decrease significantly after 2 weeks
reacting. Despite the unpromising result, the resin was treated with 25 % TFA
affording an oily trace of compound 6A as suggested by the NMR of the obtained
residue.
5

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OH
O
OCH3
+
+
+
CN
(3a-d)
O
H2N
R
R
(4A-C)
DCM/MeOH
H3CO
O
N
N
H
R
R
O
(5)
25% TFA in DCM
A R = CH2CH3
B R = (CH2)2CH3
C R = -(CH2)5-
O
H
N
OH
R
R
O
(6A-C)
Scheme 2. Preparation of N-phenylacetyl-α,α-dialkylglycines 6A-C.
Given this result, it was decided to introduce a spacer between the site of the
multicomponent reaction and the polymer chains of the resin. Amino acids of variable
carbon chain length were chosen as spacers, namely glycine (Gly), β-alanine (β-Ala)
and γ-aminoisobutyric acid (GABA), as with these unbranched compounds the
distancing from the polymer chain could be achieved without increasing the
stereochemical hindrance, while maintaining chemical character.
The amino acids were formylated prior to their attachment to the resin, by reaction with
formic acid in the presence of acetic anhydride. The reaction with Gly and β-Ala
proceeded smoothly affording the desired compounds 1b-c in good yields (table 1). The
reaction with GABA yielded the desired compound 1d in lower yields due to a side
reaction of internal cyclisation under the reaction conditions, affording pyrrolidin-2-one,
1
proven by the appearance of the corresponding signals in the H-NMR spectra of the
crude reaction mixture.
Table 1. Synthesis and characterization data of amino acids 1b-d bearing a formyl
group (For).
Compound
n
1
2
3
Yield (%) δC (C=O) (ppm) ν (C=O, NH) (cm-1)
For-Gly-OH
For-β-Ala-OH
For-GABA-OH
74
93
58
161.47
161.40
164.27
1631, 3373
1634, 3370
1633, 3385
1b
1c
1d
6

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The formyl-amino acids 1b-d were coupled to the resin (scheme 1) using DIC as
coupling reagent. In order to assure that all the amine groups of the resin had reacted
and no loss of functionalization had occurred, 2,4,6-trinitrobenzenesulphonic acid
(TNBS) tests were performed and, whenever any coloured beads were detected, the
coupling procedure was repeated. The coupling of For-Gly-OH 1b proceeded well
under these conditions; however, coupling of For-β-Ala-OH 1c and For-GABA-OH 1d
did not proceed as smoothly, requiring a second coupling reaction cycle. The two latter
compounds were successfully coupled to the resin in only one coupling cycle when
Oxyma was added to the coupling reaction cocktail.
Dehydration of the obtained resin formamides 2b-d to the corresponding isocyanide
was achieved with POCl3 in the presence of N,N-diisopropylamine (DIPA) (Scheme 1).
The modification of the functional group present was confirmed by IR spectroscopy by
following the appearance of the characteristic band of the isocyanide at 2145-2155 cm-1
(Figure 2- 2b and 3b).
Figure 2. Example of the resin IR spectra obtained in KBr disc after spacer coupling
(resin 2b), dehydration reaction (resin 3b) and Ugi reaction (resin 5Bb).
Resins 3b-d were then subjected to Ugi reaction using phenylacetic acid as acid
component and 4-methoxybenzylamine as amine component, in the same way as the
7

ACCEPTED MANUSCRIPT
Ugi reaction performed over the 3a resin. Three different ketones (4A-C) were used in
order to achieve the synthesis of phenylacetyl-α,α-dialkylglycines with different side
chain length, namely phenylacetyl-α,α-diethylglycine 6A, phenylacetyl-α,α-
dipropylglycine 6B and 1-N-phenylacetylamino-1-cyclohexenecarboxylic acid 6C. A 10
equivalent excess of the different components with respect to the initial degree of
functionalization of the resin was used and the reaction followed by IR spectroscopy
until disappearance of the isocyanide band (Figure 2 – 5Bb). The desired
phenylacetylamino acids (6A-C) were obtained after separation of the product from the
resin, by acidolysis with 25% TFA in DCM at room temperature for 4 hours (Scheme
2).
Compound 6A was obtained in yields ranging from 58 to 69%, 6B from 70 to 77% and
6C from 77 to 82%. These values were considered quite satisfactory, as they refer to the
initial degree of functionalization of the resin after a process of four sequential reactions
and they are better than the results obtained when similar compounds bearing a N-acetyl
group were synthesized in solution phase in two sequential reactions, with overall yields
between 52-67% for Deg and 46-71% for Dpg.11 The obtained N-phenylaceyl-amino
acids 6A-C can be converted to the corresponding free amino acids, suitable for peptide
synthesis, by a straightforward acid hydrolysis.11,21 Resin 3d afforded the best results
although Ugi reaction on this resin required a longer reaction time.
Resins 2b-d proved to be resistant to degradation during the period of this study, as they
were prepared in large scale and used frequently several months later without significant
decrease in the overall yields observed. Dehydrated resins 3b-d were not as resistant,
since some colouring of the resins beads was observed after a few weeks from
preparation.
Table 2. Conditions used in the Ugi reactions performed
Solid phase Ugi reaction*
Reaction time
TFA acidolysis
m (g)
Isolated
product
Yield**
(%)
traces
58
Ketone
Resin
Resin adduct
(d)
15
3
3a
3b
3c
3d
3b
5Aa
5Ab
5Ac
5Ad
5Bb
-
0.3211
0.3712
0.3852
0.4366
4A
4B
6A
6B
4
67
5
69
4
70
8

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3c
3d
3b
3c
3d
5
6
3
4
5
5Bc
5Bd
5Cb
5Cc
5Cd
0.4612
0.4748
0.4521
0.4587
0.4810
74
77
77
79
82
4C
6C
* All reactions were performed with 2 g of resin and a 10 equiv excess of reactants relative to
the isocyanide resin.
** yields were calculated relatively to resin initial degree of functionalization (1.12 mmol/g).
3. Conclusions
The results herein presented prove that it is possible to synthetize α,α-dialkylglycines by
solid phase Ugi reaction performed over isocyanide functionalized resins, when a spacer
is placed between the polymer support and the reaction site. The yields presented are all
above 58% which was considered very satisfactory, as they represent the overall
procedure, including the coupling of the N-formylamino acid, the dehydration, the Ugi
reaction and the separation from the resin.
The comparison of the different results obtained when different spacers where used led
us to conclude that, the grater the distance from the polymer chain, the better de
resulting overall yield and thus N-formyl-GABA 1d was considered the best spacer in
our study. However, the differences were not very substantial and N-formyl-Gly 1b,
considerably less expensive, can also be used satisfactorily.
As expected, the yields for the synthesis of products 6A-C were different due to the
different nature of the α,α-dialkylglycines side chains. This effect proved to be as
significant as the spacer size in the overall process.
For further incorporation into peptides, a straightforward acid hydrolysis of the obtained
N-phenylaceyl-amino acids 6A-C will afford the corresponding fully deprotected amino
acids. Although this methodology was tested for symmetrical α,α-dialkylglycines, it can
easily be extended to chiral amino acids of this family (with formation of the
corresponding racemates).
4. Experimental Section
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4.1 General
All melting points were measured on a Stuart SMP3 melting point apparatus. TLC
analyses were carried out on 0.25 mm thick precoated silica plates (Merck Fertigplatten
Kieselgel 60F254) and spots were visualised under UV light. Chromatography on silica
gel was carried out on Merck Kieselgel (230-240 mesh). IR spectra were determined on
a BOMEM MB 104 spectrophotometer and the samples prepared in KBr discs. NMR
spectra were obtained on a Varian Unity Plus Spectrometer at an operating frequency of
1
300 MHz for H and 75.4 MHz for 13C or a Bruker Avance III 400 at an operating
frequency of 400 MHz for 1H and 100.6 MHz for 13C using the solvent peak as internal
reference at 25 ºC. All chemical shifts are given in ppm using δH Me4Si = 0 ppm as
reference and J values are given in Hz. Assignments were supported by spin
decoupling-double resonance and bidimensional heteronuclear correlation techniques.
Mass spectrometry analyses were performed at the “C.A.C.T.I. - Unidad de
Espectrometria de Masas”, at University of Vigo, Spain. All commercial reagents were
used as received, except methanol, dichloromethane and N,N-dimethylformamide, that
were dried as described in the literature.22 The aminomethylated polystyrene resin used
was purchased from AAPPTec with a degree of functionalization of 1.12 mmol/g. All
glass and metallic material used in Ugi reactions was dried in an incubator at 60ºC.
4.2 Synthesis of formylated resins 2a-d
4.2.1. N-formyl-aminomethylated polystyrene resin (2a).
Aminomethylated
polystyrene resin (2 g, 1.12 mmol/g of resin) was swollen in dry dimethylformamide
(DMF, 10 mL) for 10 min. The solvent was removed and a mixture of formic acid (5
equiv, 0.17 mL) and N,N’-diisopropylcarbodiimide (DIC) (5 equiv, 0.69 mL) in DMF
(20 mL) was added. The mixture was left stirring for 24 h. After washing the resin with
DMF (3×5 mL) the 2,4,6-trinitrobenzenesulphonic acid (TNBS) test was performed to
evaluate if there were still some free NH2 groups in the resin. If the resin turned red the
above procedure was repeated until no free NH2 groups were detected. IR (KBr): ν =
509, 537, 697, 755, 822, 841, 906, 942, 964, 1004, 1028, 1069, 1114, 1154, 1182, 1384,
1451, 1493, 1583, 1601, 1694, 1745, 1803, 1873, 1943, 2337, 2852, 2921, 3025, 3081,
3323, 3406, 3646, 3851 cm-1.
4.2.2. N-formylglycyl-aminomethylated polystyrene resin (2b). This compound was
prepared by the procedure described for 2a, using N-formylglycine 1b (5 equiv, 0.58 g)
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in the place of the formic acid. IR (KBr): ν = 504, 519, 529, 695, 755, 819, 841, 907,
944, 977, 1028, 1069, 1114, 1155, 1182, 1245, 1384, 1453, 1494, 1538, 1582, 1601,
1651, 1667, 1804, 1874, 1945, 2311, 2336, 2603, 2852, 2925, 3025, 3059, 3082, 3293,
3645 cm-1.
4.2.3.
N-formyl-β-alaninyl-aminomethylated
polystyrene
resin
(2c).
Aminomethylated polystyrene resin (2 g, 1.12 mmol/g of resin) was swollen in dry
DMF (10 mL) for 10 min. The solvent was removed and a mixture of N-formyl-β-
alanine 1c (3 equiv, 0.39 g), DIC (3 equiv, 0.52 mL) and ethyl 2-cyano-2-
(hydroxylimino)acetate (Oxyma, 3 equiv, 2.74×10-3 mol) in DMF (20 mL) was added.
The mixture was left stirring for 24 h. After washing the resin with DMF (3×5 mL) the
TNBS test was performed to evaluate if there were still some free NH2 groups in the
resin. If the resin turned red the above procedure was repeated until no free NH2 groups
were detected. IR (KBr): ν = 514, 533, 696, 757, 819, 841, 907, 843, 965, 980, 1028,
1069, 1095, 1114, 1155, 1196, 1240, 1266, 1384, 1451, 1493, 1538, 1582, 1601, 1651,
1805, 1874, 1945, 2311, 2317, 2603, 2632, 2851, 2921, 3002, 3025, 3059, 3081, 3296,
3403, 3645 cm-1.
4.2.4. N-formyl-GABA-aminomethylated polystyrene resin (2d). This compound
was prepared by the procedure described for 2c using N-formyl-γ-aminobutyric acid 1d
(3 equiv, 0.88 mL). IR (KBr): ν = 509, 537, 697, 755, 822, 841, 906, 942, 964, 1004,
1028, 1069, 1114, 1154, 1182, 1384, 1451, 1493, 1583, 1601, 1694, 1745, 1803, 1873,
1943, 2337, 2852, 2921, 3025, 3081, 3323, 3406, 3646, 3851 cm-1.
4.3 General procedure for the preparation of N-formyl amino acids 1c-d.
Formic acid (140 mL, 3.71 mol) and acetic anhydride (47 mL, 0.50 mol) were stirred at
45ºC for 1 hour. After the addition of the appropriate amino acid (0.05 mol), the mixture
was left stirring at room temperature for 24 h, and evaporated until dryness.
4.3.1. N-formylglycine (1b). The compound was obtained as described in the general
procedure above using 5 g of glycine, yielding a white solid (5,06 g, 74 %). mp = 128.8
– 130.5 ºC. δH (400 MHz, DMSO-d6) 3.78 (2H, d, J 6.0 Hz, CH2), 8.06 (1H, s, CHO),
8.27 (1H, s, NH). ). δC (100.6 MHz, DMSO-d6) 40.46 (CH2), 161.47 (CHO), 170.92
(COOH). IR (KBr): ν = 515, 623, 656, 684, 786, 889, 931, 947, 993, 1017, 1041, 1065,
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1128, 1215, 1236, 1367, 1443, 1631, 1713, 1923, 1944, 2112, 2169, 2198, 2305, 2345,
2471, 2534, 2632, 2926, 3373 cm-1.
4.3.2. N-formyl-β-alanine (1c). The compound was obtained as described in the
general procedure above using 1 g of β-alanine, yielding an yellow oil (1.22 g, 93 %).
δH (300 MHz, DMSO-d6) 2.37 (2H, t, J 6.9 Hz, CH2COOH), 3.25 (2H, q, J 7.5 Hz,
NHCH2), 7.95 (1H, s, CHO), 8,18 (1H, s, NH). δC (75.4 MHz, DMSO-d6) 34.72
(NHCH2), 35.22 (CH2COOH), 161.40 (CHO), 171.50 (COOH). IR (KBr): ν = 509, 658,
753, 843, 874, 921, 943, 988, 1014, 1031, 1060, 1156, 1220, 1361, 1453, 1554, 1634,
1717, 1934, 1983, 2063, 2081, 2155, 2190, 2312, 2351, 2470, 2509, 2612, 2891, 2930,
3370 cm-1.
4.3.3. N-formyl-γ-aminobutyric acid (1d). The compound was obtained as described
in the general procedure above using 1 g of γ-aminobutyric acid (GABA), yielding a
yellow solid (0.742 g, 58 %). mp = 85.1 – 87.0 ºC. δH (300 MHz, DMSO-d6) 1.67-1.76
(2H, m, CH2CH2CH2), 2.30 (2H, t, J 9 Hz, 6 Hz, CH2COOH), 2.76 (2H, t, J 6 Hz, 6 Hz,
NHCH2), 8.27 (2H, s, CHO and NH). δC (75.4 MHz, DMSO-d6) 22.79 (CH2CH2CH2),
31.38 (CH2COOH), 38.42 (NHCH2), 164.28 (CHO), 174.17 (COOH). IR (KBr): ν =
506, 636, 765, 845, 879, 953, 979, 1064, 1128, 1195, 1281, 1333, 1395, 1411, 1461,
1501, 1603, 1726, 1929, 1954, 2053, 2088, 2159, 2195, 2303, 2340, 2468, 2504, 2622,
3030, 3385 cm-1.
4.4. Dehydration of resins 2a-d.
Dry dichloromethane (DCM) (40 mL) and 2 g of the appropriate resin 2, previously
swollen in dry DCM, were placed into a 2 neck 200 mL round bottom flask, under
nitrogen atmosphere. After the addition of N,N-diisopropylamine (DIPA) (15 equiv,
4.74 mL), the mixture was placed in an ice bath and POCl3 (5 equiv, 1.04 mL) were
added dropwise. The mixture was stirred for 2 h in an ice bath followed by 2 h at room
temperature. NaHCO3 1M (20 mL) was added and the mixture stirred for 1 hour. The
resin was separated from the reaction mixture by filtration and washed successively
with DCM (10 mL), MeOH (10 mL), DCM (10 mL), MeOH (10 mL) and diethyl ether
(10 mL). The resin was stored under vacuum and the presence of the isocyanide group
confirmed by IR spectroscopy.
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Isocyanide resin 3a. IR (KBr): ν = 501, 511, 538, 619, 659, 697, 752, 818, 843, 907,
940, 963, 1028, 1089, 1114, 1153, 1180, 1247, 1302, 1349, 1383, 1452, 1493, 1512,
1583, 1601, 1681, 1716, 1745, 1784, 1874, 1946, 2147(NC), 2237, 2603, 2691, 2850,
2926, 2973, 3025, 3059, 3082, 3102, 3337, 3646 cm-1.
Isocyanide resin 3b. IR (KBr): ν = 504, 526, 538, 638, 666, 697, 754, 817, 836, 907,
942, 965, 1003, 1028, 1070, 1115, 1154, 1181, 1218, 1277, 1317, 1360, 1422, 1451,
1492, 1513, 1526, 1583, 1601, 1674, 1802, 1873, 1944, 2148 (NC), 2312, 2337, 2631,
2850, 2923, 3003, 3025, 3060, 3082, 3101, 3323, 3583, 3644 cm-1.
Isocyanide resin 3c. IR (KBr): ν = 510, 517, 532, 696, 759, 842, 907, 942, 1028, 1071,
1115, 1154, 1182, 1267, 1350, 1380, 1454, 1494, 1556, 1602, 1681, 1715, 1805, 1874,
1945, 2148 (NC), 2337, 2853, 2915, 2973, 3026, 3060, 3082, 3307, 3403, 3645 cm-1.
Isocyanide resin 3d. IR (KBr): ν = 508, 528, 692, 754, 816, 841, 907, 944, 963, 1003,
1028, 1069, 1114, 1155, 1181, 1266, 1372, 1424, 1452, 1593, 1512, 1538, 1583, 1601,
1693, 1747, 1804, 1873, 1944, 2147 (NC), 2337, 2850, 2915, 3026, 3082, 3405 cm-1
4.5 General procedure for solid phase Ugi reaction and separation of the Ugi
adducts (6A-C) from the solid support
The appropriate ketone (10 equiv) and 4-methoxybenzylamine (10 equiv) were
dissolved in a mixture of dry DCM (12 mL) and dry MeOH (5 mL) and stirred for 30
minutes. Phenylacetic acid (10 equiv) was dissolved in dry MeOH (2 mL), added to the
mixture and stirred for 30 minutes. This mixture was poured into a reaction vessel
containing the appropriate resin 3 (2 g) previously swollen and suspended in dry DCM
(6 mL). The mixture was left stirring for 1-3 days at room temperature. After separation
from the reaction mixture by filtration, the resin was washed with DMF (2×10 mL),
DCM (2×10 mL) and MeOH (2×10 mL) and dried in vacuum. The nonexistence of the
isocyanide group in the resin was proven by the disappearance of the isocyanide signal
from the IR spectrum. If the signal was still present the previous procedure was
repeated.
The resin obtained after Ugi reaction (2.50 g) was swollen in DCM and suspended in 25
% trifluoroacetic acid (TFA) in DCM (5 mL). The mixture was stirred for 4 hours and
the resin removed by filtration. The filtrate was concentrated till dryness under reduced
pressure, the crude suspended in 2 mL of water and the pH raised till 12 by the addition
of NaOH 6M. The suspension was stirred overnight and the solid removed by filtration.
The filtrate pH reduced to 2 with HCl 6M and the solid that separated isolated by
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filtration and washed with water and light petroleum. The exact reaction conditions and
yields obtained using the different isocyanide functionalized resins 3 are summarized in
table 1.
4.5.1. N-Phenylacetyl-α,α-diethylglycine (6A). Obtained by using 3-pentanone (4A)
and the above general procedure. mp = 220.1 – 221.6 ºC. δH (400 MHz, DMSO-d6)
0.65 (6H, t, J 7.3 Hz, 2×CH3), 1.69-1.74 (2H, m, β-CH2), 1.89-1.94 (2H, m, β-CH2),
3.47 (2H, s, CH2CO), 7.19-7.30 (5H, m, Ph), 7.60 (1H, s, NH), 12.56 (1H, br s, COOH).
δC (100.6 MHz, DMSO-d6) 7.89 (2×CH3), 25.90 (2×β-CH2), 42.46 (CH2CO), 63.00 (α-
C), 126.29 (Ph-C4), 128.17 (Ph-C3,5), 129.00 (Ph-C2,6), 136.56 (Ph-C1), 169.23
(CONH), 174.47 (COOH). HRMS (ESI): m/z [M++H] calc. for C14H20NO3 250.14440,
found 250.14452.
4.5.2. N-Phenylacetyl-α,α-dipropylglycine (6B). Obtained by using 4-heptanone (4B)
and the above general procedure. mp = 156.3 – 157.4 ºC. δH (400 MHz, DMSO-d6) 0.78
(6H, t, J 7.2 Hz, 2×CH3), 1.02-1.10 (4H, m, 2×γ-CH2), 1.65-1.68 (2H, m, β-CH2), 1.87-
1.90 (2H, m, β-CH2), 3.46 (2H, s, CH2CO), 7.20-7.30 (5H, m, Ph), 7.60 (1H, s, NH),
12.56 (1H, br s, COOH). δC (100.6 MHz, DMSO-d6) 14.09 (2×CH3), 16.57 (2×β-CH2),
35.95 (2×γ-CH2) 42.49 (CH2CO), 62.14 (α-C), 126.31 (Ph-C4), 128.17 (Ph-C3,5),
128.98 (Ph-C2,6), 136,52 (Ph-C1), 169.23 (CONH), 174.70 (COOH). HRMS (ESI): m/z
[M++H] calc. for C16H24NO3 278.17572, found 278.17587.
4.5.3. 1-(N-Phenylacetyl)amino-1-cyclohexanecarboxylic acid (6C). Obtained by
using cyclohexanone (4C) and the above general procedure. mp = 155.6 – 156.4 ºC. δH
(400 MHz, DMSO-d6) 1.14-1.23 (1H, m, δ-CH2), 1.29-1.51 (5H, m, 2×γ-CH2 and δ-
CH2), 1.61 (2H, dt, J 13.4 and 2.4 Hz, β-CH2), 1.92 (2H, dt, J 13.2 and 2.4 Hz, β-CH2),
3.45 (2H, s, CH2CO), 7.18-7.29 (5H, m, Ph), 8.01 (1H, s, NH), 12.04 (1H, br s, COOH).
δC (100.6 MHz, DMSO-d6) 21.01 (2×γ-CH2), 24.96 (δ-CH2), 31.64 (2×β-CH2), 42.00
(CH2CO), 57.68 (α-C), 126.16 (Ph-C4), 128.07 (Ph-C3,5), 128.93 (Ph-C2,6), 136.65
(Ph-C1), 169.75 (CONH), 175.53 (COOH). HRMS (ESI): m/z [M++H] calc. for
C15H20NO3 262.14440, found 262.14423.
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Acknowledgements
Thanks are due to the Foundation for Science and Technology (FCT-Portugal) for
financial support through projects PTDC/QUI-BIQ/118389/2010 (FCOMP-01-0124-
FEDER-020906) and PEst-C/QUI/UI0686/2011 (F-COMP-01-0124-FEDER-022716),
FEDER-COMPETE. The NMR spectrometer Bruker Avance III 400 is part of the
National NMR Network and was purchased with funds from POCI 2010 (FEDER) and
FCT.
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CAPTIONS
Scheme 1. Preparation of the isocyanide functionalized resins 3a-d.
Scheme 2. Preparation of N-phenylacetyl-α,α-dialkylglycines 6A-C.
Figure 1 IR spectra of resins 2a and 3a obtained in KBr disc.
Figure 2. Example of the resin IR spectra obtained in KBr disc after spacer coupling
(resin 2b), dehydration reaction (resin 3b) and Ugi reaction (resin 5Bb).
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Table 1. Synthesis and characterization data of formyl-amino acids 1b-d.
Table 2. Conditions used in the Ugi reactions performed
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