Structural equilibrium and ring-chain tautomerism of aqueous solutions of 4-aminobutyraldehyde

Article abstract of DOI:10.3987/com-03-9802

NMR spectroscopy of aqueous solutions of 4-aminobutyraldehyde in the range 0 < pH < 13 established the occurrence of protonated and hydrated amino aldehydes in equilibrium with pyrrolines, and pyrrolinium salts.

Full text of DOI:10.3987/com-03-9802

HETEROCYCLES, Vol. 60, No. 8, 2003, pp. 1907 - 1914
Received, 9th May, 2003, Accepted, 1st July, 2003, Published online, 7th July, 2003
STRUCTURAL EQUILIBRIUM AND RING-CHAIN TAUTOMERISM OF
AQUEOUS SOLUTIONS OF 4-AMINOBUTYRALDEHYDE
Casper Struve and Carsten Christophersen*
Marine Chemistry Section, Department of Chemistry, University of Copenhagen,
The H. C. Ørsted Institute, Universitetsparken 5, DK-2100 Copenhagen, Denmark
Fax: +45 3532 0212; Tel: +45 3532 0157; E-mail: carsten@kiku.dk
Abstract – NMR spectroscopy of aqueous solutions of 4-aminobutyraldehyde in
the range 0 < pH < 13 established the occurrence of protonated and hydrated
amino aldehydes in equilibrium with pyrrolines, and pyrrolinium salts.
INTRODUCTION
4-Aminobutyraldehyde (1) is potentially an important synthon. Most literature routes to 1 in aqueous
solution involve hydrolysis of the diethyl acetal preferably with strong acids.1 The crude product is usually
used without further purification. However, the simultaneous occurrence of the amino and formyl function
gives ample possibilities for the formation of by-products and the exact nature of the reacting species is not
known. Biosynthetically, 4-aminobutyraldehyde (1) is an intermediate in the formation of several alkaloids.
R
3
N
+N
2
N
5
2
5
2
4
1/3
N
N
a
4
3
4
3
5
c
b
1
2
3
R = H
H2O
H
R = (CH2)4NH2
R = n-Bu
R
R
R
+H2N
+H2N
+N
5
H2O
5
H2O
O
OH
OH
5
2
4
3
4
3
2
H
2
OH
4
3
e
f
d
Scheme 1 The equilibrium structures of 4-aminobutyraldehyde at different pD´s.

Thus, 1 (or the stable cyclic condensation product ∆1-pyrroline (1b)) has been shown to be an
ornithine-derived precursor of retronecine2 and contributes to the production of hygrine and tropinone in
Hyoscamus albus.3 Moreover it emerges from important substrates, e.g. putrescine and spermine, by action
of several types of mono- and polyamine oxidases containing topaquinone as well as flavin adenine
dinucleotide as cofactors.4 Directly or indirectly, 1 is a possible oxidation product of at least eight common
metabolites derived from amino acids.5 However, 1 is difficult to detect and quantify especially in aqueous
solution due to the inherent reactivity. Usually 1 is converted to the stable and non-volatile 4-aminobutyric
acid (GABA) by aldehyde dehydrogenase.1f,6 Alternatively it may be isolated and characterized as a
derivative1b,1d or the intramolecular condensation products monomer1e,5a,7 (1b) or trimer8 (1a) ∆1-pyrroline
easily available by synthesis.9
The possibility of ring-chain tautomerism of 1 in aqueous solution has never been investigated. Recently it
was shown† that 4-(N-aminobutyl)aminobutyraldehyde (2) in aqueous solution at different pH gave rise to
at least six species characterised by NMR spectroscopy. Moreover it is known that amino ketones in
aqueous buffers exist in equilibrium with pyrrolinium forms while hemiaminals were absent.10 The present
paper describes the equilibria of 1 in aqueous solution as detected by 1H NMR spectroscopy.
RESULTS AND DISCUSSION
Scheme 1 depicts the species involved in the case of 1 consistent with our results and with the results
obtained from bimolecular formation of imines from aldehydes and amines.11
Figure 1 Product distribution at different pD´s.

The equilibrium involves two neutral (the pyrroline (1b) and the corresponding trimer (1a)) and four
protonated species (pyrrolinium ion (1c), hemiaminal (1d), aldehyde (1f) and hydrate (1e)).
1
The identification is primarily based on assignments of the H NMR spectral data listed in Table 1
corroborated by spin decoupling, COSY experiments, etc.
When leaving 4-aminobutyraldehyde (1) for some time in basic solution (7 < pH < 13) an equilibrium is
established between monomeric ∆1-pyrroline (1b) together with small amounts of the trimeric (1a). Both 1a
and 1b have been the subject of extensive investigations8,9a establishing the solvent dependence of the
equilibrium, their structure and spectroscopic properties. Monomeric 1b appear by dilution of solutions12 or
in gas phase.7b,9b
In the interval 6 < pH < 7 the intensity of signals from 1a and 1b disappear due to protonation of the imine
functionality (pKB = 7.3).5b Concomitantly, a complicated pattern of signals develop, which proves to
originate from protonated species in the approximate ratio 1c:1d:1e:1f = 20:50:20:10 as illustrated in
figure 1. On the NMR spectral time-scale there is a sufficiently slow exchange and/or interconversion
between the different species in the entire region 1 < pH < 6 to allow separate observation. The open-chain
species (1e) and (1f) as well as 1d and the conformationally rigid 1c13 gave satisfactory NMR spectra in this
Table 1 1H NMR Chemical Shifts.
Compound H-2
H-3
H-4
H-5
1.70, 1H, m
1.95, 1H, m
1.70, 1H, m
1.95, 1H, m
2.43, 1H, dt, Jd 6.6, Jt 9.2
2.91, 1H, m
1aa
3.18, 1H, t, J 6.4
1ba
1c
7.67, 1H, br s
2.57, 2H, br t, J 8.0 Hz
1.80, 2H, q, J 8.0
3.74, 2H, tq, Jt 7.5, Jq 2.2
8.81, 1H, br s
3.03, 2H, dt, Jd 7.3, Jt 7.1
2.29, 2H, quintet, J 8.0
4.13, 2H, tq, Jt 8.0
1.96, 1H, m
2.16, 1H, m
1.75, 2H, m
1.96, 1H, m
2.16, 1H, m
1.68, 2H, m
3.29, 1H, dt, Jd 11.7, Jt 7.1
3.39, 1H, dt, Jd 11.7, Jt 7.1
3.26, 2H, br t, Jt 7.9
1d
1e
5.49, 1H, t, J 4.5
5.08, 1H, t, J 5.3
9.69, 1H, s
1f
2.03, ?H, m
?
2.71, 2H, t, J 7.1
2cb
3cc
8.73, 1H, m, J 1.5
8.76, 1H, m, J 1.4
3.22, 2H, br t, J 7.0
3.28, 2H, br t, J 7.3
2.35, 2H, quintet, J 8.1
2.42, 2H, quintet, J 8.0
4.21, 2H, tq, Jt 8.1, Jq 1.8
4.27, 2H, br t, J 7.3
aThe carbon chemical shifts in CDCl3 are: 1a – 20.19 (C-4), 27.77 (C-3), 45.78 (C-5), 81.85 (C-2) and 1b – 20.41 (C-4), 36.54
(C-3), 61.10 (C-5), 166.88 (C-2).
bThe proton chemical shifts for the aminobutyl substituent of 2c are: 1.74 (2 H, m, H-3’), 1.93 (2 H, m, H-2’), 3.06 (2 H, br t,
J 7.7, H-4’), 3.98 (2 H, br t, J 7.5, H-1’).
cThe proton chemical shifts for the n-butyl substituent of 3c are: 1.01 (3 H, t, J 7.3, H-4´), 1.45 (2 H, sextet, J 7.4, H-3´),
1.88 (2 H, quintet, J 7.5, H-2´), 3.99 (2 H, br t, J 7.3, H-1´).

pH range. Concentrated solutions were avoided due to the rapid emergence of acid catalyzed aldol
condensation products easily recognized from signals in the range 5 < δ < 6 with allylic couplings from
groupings such as -CH=CR-CH(OH)2 or -CH=CR-CHOH-N+HR1R2.
Similar results have been obtained in the intermolecular reaction between isobutyraldehyde and
methylamine.11a Thus, in basic solutions, where neither amines nor imines are protonated, isobutyraldehyde
is transformed largely (but not completely) to N-isobutylidenemethylimine by excess methylamine. In the
case of 1 the tendency for formation of cyclic species increases due to the entropy factor and complete
conversion to 1b/1a is observed. The unprotonated hemiaminal is, as expected from similar reactions, only
present in a very low steady-state concentration.14 In acid solution, N-isobutylidenemethylimine is
protonated and transformed by attack of water to a mixture of ammonium salt, isobutyraldehyde, and its
hydrate via intermediate protonated carbinolamine. Again, in the case of 1 the entropy factor is responsible
for the increased stability of the cyclic species (1c) and (1d) relative to the aldehyde (1f) and its hydrate (1e)
as evidenced by the observation of signals from all 4 species in the NMR spectra. The unusual15 dominance
of protonated hemiaminal (1d) in the equilibrium mixture in acid solution can be attributed to the presence
of the easily accessible and strongly electrophilic -CH=NH+- group in 1c.
It is interesting to compare these results with those reported for N-(4-aminobutyl)aminobutyraldehyde (2).
In contrast to 1, only signals from the quaternary pyrrolinium ion (2c) were observed, while the
hemiaminal (2d) and the open chain forms (2e) and (2f) were absent. The N-(4-aminobutyl)-substituent in
2c leads to increased steric hindrance for formation of 2d by attack of water, and displaces the equilibrium
towards 2c. Provided the open-chain forms (1e) and (1f) are stabilised by intramolecular hydrogen bonding,
N-substitution with the lipophilic aminobutyl group decreases the stabilisation and would explain the
absence of 2e and 2f. This explanation is supported by the observation, that 4-butylaminobutyraldehyde (3)
in acid solution exclusively exists as N-butylpyrrolinium ion (3c).
EXPERIMENTAL
General
All reagents and solvents were obtained from commercial suppliers and used without further purification.
4-Aminobutyraldehyde dimethyl acetal is 98 % pure from Tokyo Chemical Industry (TCI) America. NMR
spectra are reported in δ units (ppm) relative to tetramethylsilane. The largest solvent peak is used as
reference, except for spectra in D2O where 0.5 mM DSS (2,2-dimethyl-2-silapentane-5-sulfonate) is used
as reference. Values of coupling constants J are given in Hz. Mass spectra are recorded on a JEOL
JMS-HX/HX 110A Tandem Mass spectrometer. Microanalyses were obtained on a Flash EA 1112 series
elemental analyzer.

∆1-Pyrroline (1b)
4-Aminobutyraldehyde dimethyl acetal (13.33 g, 0.100 mol) is cooled to 0 °C, before addition of ice-cold
2 M HCl (200 mL, 0.400 mol). The reaction mixture is agitated by a magnetic stirrer for 25 min at 0 °C
before slowly adding a solution of 2.67 M K2CO3 (300 mL, 0.800 mol) through a dropping funnel. The
alkaline mixture is allowed to return to rt, before extraction with CH2Cl2 (3 × 200 mL). The combined
extracts were dried (MgSO4) and evaporated under reduced pressure to leave the cyclized aminoaldehyde
1b (5.93 g, 86 %) as a light yellow oil. Destillation of the crude product (bp 91-105 °C) yields a colourless
stable oil (4.70 g, 68 %). The destilled product contains approximately 10 moles of water pr. mol
∆1-pyrroline (1H NMR), and about 5 % CH2Cl2 (1H NMR). The CH2Cl2 can be removed by flushing the
destilled oil with nitrogen, while placed in an ultrasonic waterbath at 45 °C for 1 h. The water can be
removed by addition of 6 molecular sieves (4 Å) to the oil followed by treatment in an ultrasonic waterbath
for 48 h. ∆1-Pyrroline may be used without further purification or re-destilled mixed with powdered KOH
to yield analytically pure ∆1-pyrroline. In our hands purification of the product by eluting the oil with ether
through a column of neutral alumina as proposed by Nomura et al.,8 yielded the product (1b) and an
impurity of 15 % ether upon evaporation. (Found: C, 69.2; H, 10.1; N, 20.2. C4H7N requires C, 69.5;
H, 10.2; N, 20.3 %); δH(400 MHz, D2O) and δC(100 MHz, CDCl3) see Table 1. NMR assignments of
monomeric (1b) and trimeric (1a) pyrroline have been further substantiated by COSY, HETCOR, DEPT
and spin decoupling experiments; m/z (EI) 69 (M+, 89%), 68 (31), 67 (3), 54 (6), 42 (42, M - HCN),
41 (100, M - C2H4), 40 (15), 39 (28). m/z (FAB, 3-nitrobenzyl alcohol + 1 % TFA) 70.1 ([M + H]+),
206.1 (trimeric pyrroline, [M + H - H2]+).
Purification of ∆1-pyrroline from its bis(∆1-pyrroline)diiodozinc complex proved unsuccessful. Liberation
of ∆1-pyrroline from the complex was not possible, which was ascribed to the low vapour pressure of
complex. The zinc-complex was prepared from purified ∆1-pyrroline (1b) by a method reported by
Baxter et al..16
ZnI2 (1.192 g, 3.73 mmol) is added to a solution of ∆1-pyrroline (1b) (0.517 g, 7.48 mmol) in dry ether
(100 mL), and stirred for 2 h at 30 °C, before being cooled to 0 °C. The beige precipitate was filtered off
(1.32 g, 40 %), washed with ether and recrystalized from CHCl3/Heptane to give the pyrroline complex as
colourless crystals mp 203–205 °C (from CHCl3/n-C7H16); (Found: C, 21.05; H, 2.9; N, 5.9. C8H14N2ZnI2
requires C, 21.0; H, 3.1; N, 6.1 %); δH(400 MHz, CDCl3) 2.08 (4 H, quintet, J 7.9, H-4), 2.88 (4 H, br t,
J 7.9, H-3), 4.02 (4 H, br t, J 6.6, H-5), 8.29 (2 H, br s, H-2); δC(100 MHz, CDCl3) 20.1 (C-4), 36.7 (C-3),
59.2 (C-5), 177.5 (C-2).

N-Butyl-4-aminobutyraldehyde diethyl acetal
A solution of 4-chlorobutyraldehyde diethyl acetal (4.60 g, 0.025 mol) in butylamine (40 mL, 0.4 mol) is
refluxed (77 °C) for 24 h. Cyclohexane (200 mL) is added, and excess butylamine is removed as the
azeotrope with cyclohexane (60/40) until the bp of the mixture rises to 81 °C (after 150 - 200 mL destillate
has been collected). After cooling, the resulting slurry of precipitated hydrochloride in cyclohexane is
added 2.5 M NaOH (20 mL) and extracted with cyclohexane (2 × 25 mL). The combined organic extracts
are washed successively with 5 % NaHCO3 (2 × 25 mL), brine (2 × 25 mL) and finally dried over MgSO4.
Evaporation of the solvent followed by destillation of the residue (75 °C / 0.4 mbar) yields the desired
product (4.83 g, 87 %) as a colourless oil (Found: C, 66.0; H, 12.4; N, 6.5. C12H27NO2 requires C, 66.3;
H, 12.5; N, 6.4 %); δH(400 MHz, CDCl3) 0.84 (3 H, J 7.3, H-9), 1.13 (6 H, t, J 7.1, CH3), 1.22 (1 H, br s,
NH), 1.27 (2 H, m, J 7.4, H-8), 1.40 (2 H, m, H-7), 1.50 (2 H, m, H-3), 1.57 (2 H, m, H-2), 2.53 (2 H, t, J 7.1,
H-6), 2.55 (2 H, t, J 7.1, H-4), 3.42 and 3.57 (4 H, dq, J 9.5 and 7.1, CH2), 4.42 (1 H, t, J 5.5, H-1);
δC(100 MHz, CDCl3) 13.8 (C-9), 15.1 (CH3), 20.3 (C-8), 25.2 (C-3), 31.3 (C-2), 32.1 (C-7), 49.5 and 49.6
(C-6 and C-4), 60.7 (CH2), 102.6 (C-1); m/z (FAB, 3-nitrobenzyl alcohol) 218.2 (M + H+); m/z (EI) 188
(95%, M – C2H5), 172 (28, M – C2H5O), 126 (86), 103 (25), 99 (30), 86 (73, M – CH2CH2CH(OEt)2),
84 (100), 71 (35).
N-Butyl-4-aminobutyraldehyde hydrochloride (3)
N-Butyl-4-aminobutyraldehyde diethyl acetal (0.505 g, 2.32 mmol) was hydrolysed in water (20 mL), with
1 M HCl (2.53 mL, 2.53 mmol, 9 % excess) for 25 min at rt, before the entire content was lyophilized for
two days to give hygroscopic white amorphous crystals, which were used without further purification. The
aminoaldehyde analysed as C8H16NCl with varying amounts of water. (Found: C, 57.4; H, 10.1; N, 8.4.
C8H16NCl + 3.5 % H2O requires C, 57.35; H, 10.0; N, 8.4%); δH(400 MHz, D2O) see entry 3c in table 1;
δC(100 MHz, D2O): 12.5 (C-4´), 18.7 and 19.1 (C-4 and C-3´), 27.9 (C-2´), 36.4-(C-3), 53.6 (C-1´), 58.6
(C-5), 180.2 (C-2); m/z (FAB, 3-nitrobenzyl alcohol) 126.1 (M+).
The pH equilibrium of ∆1-pyrroline investigated by 1H NMR spectroscopy
The samples (0.6 mL) were prepared in NMR tubes using the following procedure, which ensures
uniformity. A stock solution was prepared by dissolving ∆1-pyrroline (30 mg, 0.434 mmol) in
D2O (9.7 mL) and adding DSS in D2O (50 mM, 0.1 mL). Half of the sample (4.9 mL) is transferred to a
beaker fitted with the pH electrode and DCl is added (20 %, 0.1 mL) to give a total volume of 5.0 mL. Next,
1
the pD (close to 1) is measured accurately, 0.6 mL is placed in a NMR tube, and the H NMR spectrum
recorded. D2O (0.1 mL) is added to the remaining stock solution to a total volume of 5.0 mL. Transferring
0.6 mL of the stock solution to the beaker leaves the concentration unhanged and restores the volume to

5.0 mL. The pD can now be adjusted to a higher value with 12 M NaOD in D2O using capillary tubes
1
without changing the volume significantly. Again, 0.6 mL is placed in a NMR tube, and the H NMR
1
spectrum recorded. By repeating this procedure, H NMR spectra covering the pD range 1-12 may be
recorded. 1H NMR spectra of the samples were recorded within 3 hours after preparation using 40 scans per
spectrum. DSS (0.5 mM) is used as internal standard and the largest peak arising from DSS is referenced at
0.00 ppm. DSS is preferred to the readily available solvent peak HDO because of the narrow linewidth and
chemical shift insensitivity to pH in the range 2 – 11.17 pD values are calculated by adding 0.40 units18 to the
pH meter reading, obtained from a standard pH-meter calibrated to measure pH in aqueous solutions. One
of the alkaline samples was titrated to an acidic pD. The spectrum proved identical to that recorded for the
acidic range demonstrating that the equilibrium is reversible.
REFERENCES AND NOTES
†
G. Houen, C. Struve, R. Søndergaard, T. Friis, U. Anthoni, P. H. Nielsen, and C. Christophersen
(2003) Manuscript in preparation.
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