An amino acid N-derivatising group that can be coloured on demand

Article abstract of DOI:10.1016/S0040-4039(02)00625-1

A method is presented whereby an amino acid is reacted with 5-formyl-1H-pyrrole-2-carboxylic acid to give an N-derivatised tag that has a latent ability to be coloured. A characteristic red pyrrolizin-3-one (coloured tag) is then revealed on treatment with hydrocinnamoyl chloride. This sequence has been carried out on amino acids in solution, and on solid phase, and also on dipeptides.

Full text of DOI:10.1016/S0040-4039(02)00625-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 3673–3675
An amino acid N-derivatising group that can be coloured on
demand
Andrew D. Abell,* Derek C. Martyn, Barnaby C. H. May and Brent K. Nabbs
Department of Chemistry, University of Canterbury, Christchurch, New Zealand
Received 19 December 2001; revised 21 March 2002; accepted 28 March 2002
Abstract—A method is presented whereby an amino acid is reacted with 5-formyl-1H-pyrrole-2-carboxylic acid to give an
N-derivatised tag that has a latent ability to be coloured. A characteristic red pyrrolizin-3-one (coloured tag) is then revealed on
treatment with hydrocinnamoyl chloride. This sequence has been carried out on amino acids in solution, and on solid phase, and
also on dipeptides. © 2002 Elsevier Science Ltd. All rights reserved.
We have recently embarked on a program to develop
methods for the solution and solid phase labelling of
compounds with a molecular tag that can be unmasked
on specific and controlled chemical reaction. The idea is
to produce a ‘built-in’ chemical label that has an ability
to be revealed and hence identified on demand, a
property that is unique amongst existing tagging sys-
tems.1 This idea offers a number of potential applica-
tions and advantages over existing methods. It allows
specific identification and/or isolation of tagged deriva-
tives, or key intermediates, from synthetic mixtures or
libraries of compounds on demand and subsequent to
the initial step of chemical attachment of the tag pre-
cursor. A potential advantage of latency is that it
removes the possibility of the process of identification,
e.g. fluorescence, interfering with the particular study,
or synthetic sequence, being undertaken. A set of such
tags would also provide a method to encode the pro-
gress of a combinatorial synthetic sequence and the
potential to sequence a peptide or the like.2
coloured molecular tags, i.e. they give rise to the corre-
sponding red pyrrolizin-3-ones 4, on reaction with
hydrocinnamoyl chloride under mild conditions.
The latent tags, 3, were synthesised as detailed in
Scheme 1. The pyrrole acid 2, readily prepared by
In this paper we demonstrate this principle with a
method that releases
a characteristic and highly
coloured tag from a precursor that is conveniently
attached to the free amino group of an amino acid,
either in solution or on a solid support. The result is a
new N-derivatising group that possesses a latent ability
to be coloured. The method presented here is based on
the N-derivatisation of an amino acid with 5-formyl-
1H-pyrrole-2-carboxylic acid 2 to give N-(5-formyl-1H-
pyrrole-2-carbonyl)amino acids, e.g. 3 (Scheme 1).
These compounds are convenient precursors to highly
Scheme 1. (a) KOH, H2O, 40–50°C (82%); (b) EDCI, HOBt,
DIPEA and amino acid·HCl (3a, 79%; 3b, 43%; 3c, 90%; 3d,
53%; 3e, 91%); (c)
L-LeuOMe·HCl, DCC, HOBt, DIPEA
(3c); (d) DMAP, DIPEA, CH2Cl2 then Ph(CH2)2COCl (4a,
58%; 4b, 21%; 4c, 55%; 4d, 35%; 4e, 58%); (e) MeONa,
MeOH (5a, 85%), (5c, 80%), (5e, 82%).
* Corresponding author.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00625-1

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A. D. Abell et al. / Tetrahedron Letters 43 (2002) 3673–3675
hydrolysis of 1,3 was coupled with a range of amino
acid esters, using standard 1-(3-dimethylaminopropyl)-
3-ethylcarbodiimide hydrochloride (EDCI) peptide cou-
pling methodology,4 to give 3a–e in good yields.
Compound 3c was also prepared in 89% yield using
standard DCC–peptide coupling methodology. Next,
separate dichloromethane solutions of each of 3a–e,
containing diisopropylethyl amine (DIPEA) and a cata-
lytic amount of 4-dimethylaminopyridine (DMAP),
were treated with hydrocinnamoyl chloride. The red
pyrolizin-3-ones 4 were isolated after stirring at rt for
24 h (Scheme 1). The structures of these derivatives
were fully characterised by one- and two-dimensional
NMR spectroscopy, mass spectrometry, ultraviolet
spectroscopy and in the case of 4a, X-ray crystallogra-
phy. Independent conformation of the structures of the
red tags 4a, 4c and 4e was provided by treating each
with sodium methoxide in methanol to give the colour-
less pyrrole acrylic esters 5a, 5c and 5e in yields of 85,
80 and 82%, respectively (Scheme 1).† This reaction also
provides a means to remove the red coloration of the
tag.
Scheme 2. (a) BOP, DIPEA, either
L-LeuGlyOMe·HCl (45%)
or -PheGlyOMe·HCl (49%); (b) DMAP, DIPEA, CH2Cl2
L
then Ph(CH2)2COCl (7a, 33%), (7b, 34%).
Next, we demonstrated that the method could be
applied to dipeptides.
-phenylalanylglycine methyl ester were separately
L
-Leucylglycine methyl ester and
L
reacted with
2
to give the corresponding N-
pyrroloylpeptides 6a and 6b, respectively (Scheme 2).
Reaction of each of these with hydrocinnamoyl chlo-
ride, as described for 3 above, gave the dipeptide-based
pyrolizin-3-ones 7 which were fully characterised.
Scheme 3. (a) 20% piperidine in DCM; (b) 2, EDCI, HOBt,
DIPEA; (c) DMAP, DIPEA, CH2Cl2 then Ph(CH2)2COCl;
(d) 20% TFA/DCM; (e) CH2N2; (f) glycine methyl ester
hydrochloride, EDCI, BOP, DIPEA.
Finally, we investigated whether the method could be
used to develop a red tagging system for resin bound
amino acids (Scheme 3).5 To this end, FMOC-glycine
Wang resin 8 was deprotected by reaction with pipe-
ridine and the resulting free amine 9 was coupled with
the pyrrole acid 2 to give the N-pyrroloyl protected
resin bound amino acid 10. Reaction of this sample
with hydrocinnamoyl chloride, as per the formation of
3 in Scheme 1, gave the red resin beads, 11 (see Fig. 1
for a photograph). The resin bound intermediate 10
was characterised by cleaving it from the resin on
treatment with TFA. Methylation of the resulting car-
boxylic acid 12, with diazomethane, gave 13, which was
shown to be identical to a sample independently synthe-
sised from 2 (see step f in Scheme 3).
The method reported here for the preparation of
pyrrolizin-3-ones is much milder than existing literature
methods.6 The reaction is assumed to occur via N-
acylation of 3 with subsequent intramolecular Knoeve-
† It is interesting to note that in the case of the leucine and phenylala-
nine examples 5c and 5e, only the (Z)-isomer was observed by 1H
NMR. However, in the case of the glycine example 5a, a mixture of
the (Z)- and (E)-isomers was obtained in a ratio of 3:1 (by 1H
NMR). Subsequent recrystallization of this mixture gave a pure
sample of the (E)-isomer which was fully characterized. The assign-
ment of a (Z)-configuration for the alkene 5c (and by analogy for
5a and 5e) was based on an observed strong positive NOE enhance-
ment between the acrylic proton and the methylene protons of the
benzyl group.
Figure 1. Resin bound 11.
nagel-type condensation of the resulting N-acylated
formylpyrrole intermediates. The nature of the acid
chloride used in these reactions would appear to be
important. An equivalent reaction of 3c, using acetyl
chloride in place of hydrocinnamoyl chloride, gave 14
rather than the corresponding pyrrolizin-3-one.

A. D. Abell et al. / Tetrahedron Letters 43 (2002) 3673–3675
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In conclusion, we present a method whereby a latent
coloured molecular tag can be conveniently attached to
the free amino group of an amino acid in either solu-
tion or the solid phase. A characteristic red tag can
then be revealed, on demand, upon treatment with
hydrocinnamoyl chloride. Ongoing work is centred on
identifying milder methods for introducing the N-
derivatising group and also developing a set of tags.
This chemical sequence presented here offers a conve-
nient method to tag on demand, and hence identify
compounds containing an amino group.
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Acknowledgements
The work was supported by a Royal Society of New
Zealand Marsden grant. D.C.M. was supported by a
Bright Futures Top Achiever Doctoral Scholarship.