Synthesis of the novel amino acid 4-amino-3-(aminomethyl)benzoic acid (AmAbz) and its protected derivatives as building blocks for pseudopeptide synthesis

Article abstract of DOI:10.1002/1099-0690(200011)2000:22<3755::aid-ejoc3755>3.0.co;2-i

4-Amino-3-(aminomethyl)benzoic acid (1) (AmAbz) is a novel unnatural amino acid with promise in applications as a building block for the synthesis of peptidomimetics and as a scaffold for combinatorial chemistry. It was efficiently synthesized in three st

Full text of DOI:10.1002/1099-0690(200011)2000:22<3755::aid-ejoc3755>3.0.co;2-i

FULL PAPER
Synthesis of the Novel Amino Acid 4-Amino-3-(aminomethyl)benzoic Acid
(AmAbz) and Its Protected Derivatives as Building Blocks for Pseudopeptide
Synthesis
[a]
[a]
[a]
[a]
´
´ ´
´ `
Robert Pascal,* Regine Sola, Frederic Labeguere, and Patrick Jouin
Keywords: Amidoalkylation / Combinatorial chemistry / Peptidomimetics / Scaffold / Solid-phase synthesis
4-Amino-3-(aminomethyl)benzoic acid (1) (AmAbz) is
a
noticed in coupling experiments using the BOP reagent and
building block 8b. This made protection of the arylamino
group unnecessary either for peptide bond formation at the
carboxyl group, or for subsequent elongation of a peptide
chain at the benzylamino group. Finally, the arylamino group
could be acylated under base-free, carbodiimide-mediated
coupling conditions. These properties are illustrated by the
novel unnatural amino acid with promise in applications as
a building block for the synthesis of peptidomimetics and as
a scaffold for combinatorial chemistry. It was efficiently syn-
thesized in three steps (63% overall yield) from 4-aminoben-
zoic acid, by means of regioselective amidomethylation with
hydroxymethylphthalimide. AmAbz (1) contains three dis-
tinct functionalities which could be discriminated from one solid-phase synthesis of the AmAbz-containing branched
another. Firstly, Boc2O or Fmoc−OSu reacted selectively with
pseudopeptide
the benzylamino group to give the monoprotected derivat- Fmoc−Ala−Phe−AmAbz(H−Lys−Leu)−Val−Gly−NH2 (15). The
ives AmAbz(Boc) (8a) or AmAbz(Fmoc) (8b), respectively.
synthesis of Fmoc−AmAbz(Boc) (10) is also described.
The absence of acylation at the arylamino group was also
Introduction
synthesis of AmAbz, starting from 4-aminobenzoic acid (2)
and introducing the aminomethyl group onto the aromatic
ring by amidomethylation.[10] The preparation of some Am-
Abz-derived building blocks designed for applications in so-
lution or solid-phase synthesis is also described, as well as
their reactivities in solid-phase elongation of branched pep-
tides.
The availability of new tools and building blocks both for
combinatorial organic chemistry and for peptidomimetic
construction is of considerable interest in drug design.[1Ϫ3]
In this context, rigid equivalents of dipeptides with distin-
guishable functional groups are promising targets. 4-
Amino-3-(aminomethyl)benzoic acid (1) (AmAbz)[4] fulfils
these criteria, but surprisingly, despite its simple structure,
its synthesis and reactivity have not yet been investigated.[5]
We propose here a synthetic route to this new amino acid.
Pertinent features of AmAbz are: (i) the aromatic cyclic sys-
tem, which is useful for mimicking a dipeptide residue ex-
Results and Discussion
Synthesis of AmAbz (1)
The aminomethyl group was introduced at the position
hibiting strongly reduced conformational mobility, (ii) two ortho to the arylamino group after conversion of 4-amino-
amino groups with a large difference in basicities, allowing benzoic acid (2) into the methylcarbamate derivative 3
the preparation of cyclic or branched peptide analogues, (Scheme 1). Protection of the arylamino group was required
and (iii) a scaffold structure permitting presentation of because amidomethylation was performed, as usual,[10] in
three independent sets of building blocks, with potential ap- strong acid. Under these conditions, the unprotected aryla-
plications in combinatorial chemistry.[3,6] Reported syn- mino group would be fully protonated, leading to substitu-
theses of compounds containing the AmAbz core deal ex- tion at the undesired meta position.[11] We selected a ureth-
clusively with N-alkyl derivatives and involve multi-step re- ane-type protection which, compared with acyl protection,
actions through nucleophilic substitution on halomethyl increases both the rates of electrophilic aromatic substitu-
aromatic precursors,[7] intramolecular sulfonylamidome- tions and their regioselectivities for the ortho position,[11,12]
thylation of 4-aminobenzoic acid derivatives[8] or thanks probably to the more difficult protonation of the
FriedelϪCrafts acylation of an o-aminobenzylamine cyclic carbamate group.[13] The favourable meta-directing effect of
urea.[9] Here we describe a straightforward and efficient the carboxyl group present in reactant 3 was also expected
to increase the regioselectivity towards the right position.
[a]
´
´
CNRS
UPR
9023,
Mecanismes
Moleculaires
des
Carbamate 3 was prepared in high yield (97%) by treating
4-aminobenzoic acid (2) with methyl chloroformate, using
an unusual procedure involving solid Na2SO4 in dioxane.
These acidic conditions are therefore a good alternative to
SchottenϪBaumann-type procedures, which can give rise to
Communications Cellulaires, Centre de Pharmacologie-
Endocrinologie,
Rue de la Cardonille, 34094 Montpellier Cedex 5, France
Fax: (internat.) ϩ 33-4/67542432
E-mail: rpascal@ccipe.montp.inserm.fr
Supporting information for this article is available on the WWW
under http://www.wiley-vch.de/home/eurjoc or from the author. side reactions of carboxylate with chloroformate, as some-
Eur. J. Org. Chem. 2000, 3755Ϫ3761
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ193X/00/1122Ϫ3755 $ 17.50ϩ.50/0
3755

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R. Pascal, R. Sola, F. Labeguere, P. Jouin
FULL PAPER
Deblocking of both amino groups of compound 4 by
quantitative hydrolysis under strongly alkaline conditions
afforded AmAbz (1).[18] It was isolated either by means of a
tedious separation on sulfonic acid ion exchange resin (89%
yield) or, alternatively, obtained as its phthalic acid salt,
which readily precipitated upon acidification of the reaction
medium (67% yield). Both forms could be used efficiently
for further preparation of protected derivatives. It should
be noted that hydrolysis of 4 in weakly alkaline carbonate
buffers gave the substituted phthalamic acid 7 in high yield,
confirming that the alkaline hydrolysis occurs by ring-open-
ing of the phthalimide. Urea 6b was also isolated in signific-
ant yield after incomplete hydrolysis with NaOH, indicating
that hydrolysis occurs, at least in part, through an intramo-
lecular pathway, which is consistent with the ease of cycliza-
tion of 2-aminobenzylamine carbamates.[19] The relative po-
sitions of substituents in AmAbz (1) were confirmed by its
ability to undergo the intramolecular reaction to give the
cyclic urea 6a[20] (Scheme 1). This urea was identified by
comparison of its NMR spectrum with the published data
for the 3-methyl derivative 6c.[21]
Protection of AmAbz
The difference in nucleophilicities between the aryl amine
and the benzyl amine of AmAbz (pKA values of 2.38[15] and
9.33[22] for 4-aminobenzoic acid and benzylamine, respect-
ively) was used in the protection strategy to discriminate
between the two amino groups. In alkaline dioxane/water,
Boc2O or FmocϪOSu reacted with high regioselectivity to
give the monoprotected benzylamines 8a or 8b, respectively
(Scheme 2). The amounts of bis-protected side products 9a
and b were lower than 1% (by HPLC analysis of the reac-
Scheme 1. Synthesis of AmAbz, 1: (a) MeOCOCl, Na2SO4, diox-
ane, 70 °C, 97%; (b) N-hydroxymethylphthalimide, 96% H2SO4/
H2O (9:1), 50 °C, 73%; (c) 5  NaOH, reflux, 48 h; (d) HCO2H,
purification on cation exchange resin, 89%; (e) SOCl2, MeOH;
(f) N,NЈ-carbonyldiimidazole, DIEA, dioxane
1
tion medium or H NMR analysis of the crude product),
provided that 1 was maintained in slight excess. We noticed,
however, that significant amounts (ഠ 4%) of the bis-Fmoc
side product 9b were obtained when FmocϪCl was used
instead of FmocϪOSu under identical conditions (data not
given). In view of this reactivity, protection of the arylam-
ino group to obtain 10 then proved feasible, by treatment
of 8a with FmocϪCl in dioxane at 70 °C in the presence
of Na2SO4. In contrast, however, introduction of the Boc
protecting group onto AmAbz(Fmoc) 8b proved to be
much more difficult. It remains the case though that coup-
ling of AmAbz derivatives was possible without any protec-
tion of the arylamino group (vide infra).
times observed during protection of amino acids with ur-
ethane-type blocking groups.[14] Such side reactions might
be favoured by the low nucleophilicity of the arylamine in
amino acid 2. Under the moderately acidic conditions se-
lected, the reactivity of the arylamino group (pKA ϭ 2.38
in water[15]) was preserved, while that of the carboxyl group
was suppressed, with no need to start from the methyl es-
ter[16] or to convert the carboxyl group into a trimethylsilyl
ester[17] as previously described in some syntheses of 4-ami-
nobenzoic acid carbamates.
Peptide Bond Formation Involving AmAbz Derivatives
Amidomethylation of aromatic compound 3 with hydro-
xymethylphthalimide was carried out in 96% H2SO4/H2O
We first studied the acylation of an amino acid methyl
(9:1) at 50 °C and took place exclusively at the position ester by AmAbz(Fmoc) 8b. When 8b (0.1  in DMF) was
ortho to the carbamate group, as unambiguously confirmed allowed to react with BOP reagent (1 equiv.) in the presence
by subsequent preparation of the urea 6a (vide infra). The of HOBt (1 equiv.) and DIEA (2 equiv.), HPLC monitoring
only side product (5 ഠ 8%) was identified as an ortho,or- detected the formation of only one species (Supporting In-
thoЈ-disubstituted compound with two aromatic hydrogen formation):
presumably
the
active
ester
1
atoms in a symmetrical structure, as attested by H NMR AmAbz(Fmoc)ϪOBt which suffered no degradation even
analysis on the isolated substance. This side product was after 2 h in DMF at room temperature. This species was
efficiently removed by recrystallization, and the desired capable of subsequently acylating AlaϪOMe to give the de-
product 4 was obtained in 73% yield.
sired dipeptide ester AmAbz(Fmoc)ϪAlaϪOMe 11 in a
3756
Eur. J. Org. Chem. 2000, 3755Ϫ3761

Synthesis of the Novel Amino Acid 4-Amino-3-(aminomethyl)benzoic Acid
FULL PAPER
Scheme 2. Protection of AmAbz: (a) Boc2O, NaOH, dioxane/H2O,
0
°C, 86%; (b) FmocϪCl, Na2SO4, dioxane, 70 °C, 69%;
(c) FmocϪOSu, Na2CO3, NaOH, dioxane/H2O, 0 °C, 91%
clean reaction, as shown by HPLC analysis of the reaction
medium. These results demonstrated that protection of the
arylamino group is not necessary for acylation with AmAbz
derivatives, which is consistent with results obtained in the
related 3,5-diaminobenzoic acid series.[6] They also sug-
gested the possibility of continuing peptide elongation from
the benzylamino group without protection of the arylam-
ino group.
We took advantage of these results for the solid-phase
synthesis of the AmAbz-containing branched pseudopep-
tides 13 and 15 (Scheme 3), starting from aminomethylpoly-
styrene resin derivatized with the Rink amide linker.[23]
Coupling reactions were carried out using the BOP/HOBt/
DIEA methodology, with the exception of that of
FmocϪPhe. As anticipated, Leu and Lys residues could be
introduced without protecting the arylamino group of Am-
Abz. The arylamino group was then acylated with
FmocϪPhe using DIC activation in CH2Cl2 in the absence
of base:[24] conditions under which any racemization of ac-
tivated amino acid derivatives was expected to be prevented.
This method was therefore preferred to the use of Py-
Brop[25] in DMF, recently recommended for acylation of the
arylamino group of 3,5-diaminobenzoic acid residue,[26] but
which requires the presence of a base. DIC activation was
Scheme 3. Solid-phase synthesis of peptides 13 and 15 from amino-
methyl polystyrene resin: (a) Fmoc-Rink amide linker (1.5 equiv.),
BOP (1.5 equiv.), DIEA (2.25 equiv.), DMF, 90 min, then Ac2O
(10 equiv.), DIEA (3.0 equiv.), DMF, 15 min; (b) piperidine/DMF
(1:4)
1 ϫ 1 min ϩ 3 ϫ 3 min; (c) protected amino acid
(FmocϪGlyϪOH, FmocϪValϪOH, HϪAmAbz(Fmoc)ϪOH,
FmocϪLeuϪOH, BocϪLys(Boc)ϪOH or FmocϪAlaϪOH) (3.3
equiv.), HOBt (3.0 equiv.), BOP (3.0 equiv.), DIEA (4.5 equiv.),
DMF, 60 min; (d) FmocϪPhe (3.3 equiv.), DIC (3.0 equiv.),
selected on the basis of preliminary experiments[27] that CH2Cl2, 2 ϫ 3 h (with DMF addition for the second coupling);
(e) TFA/H2O/triisopropylsilane (95:2.5:2.5), 3 h
showed that acylation of the arylamino group of AmAbz
was remarkably fast and complete when a base-free, car-
bodiimide-mediated coupling reaction in CH2Cl2 was
used,[28] contrasting sharply with the inadequacy mentioned
above of BOP activation in DMF. This efficiency is in agree-
ment with literature data reporting that the activation of
carboxylic acids with carbodiimides is highly solvent-
dependent[29Ϫ31] and remarkably fast in dichlorome-
thane;[31,32] addition of bases is also generally unfavour-
able.[32,33]
HϪLysϪAmAbz(HϪLysϪLeu)ϪValϪGlyϪNH2 (m/z ϭ
691.5) or HϪLysϪLeuϪAmAbz(HϪLysϪLeu)ϪValϪ
GlyϪNH2 (m/z ϭ 804.5). Additionally, acylation of the ary-
lamine with FmocϪPhe was shown to be quantitative, since
no pentapeptide HϪAmAbz(HϪLysϪLeu)ϪValϪGlyϪ
NH2 (m/z ϭ 563.4) was detected.
The high purity of crude pseudopeptides 13 and 15 as
assessed by HPLC and MALDI-MS analyses (Supporting
Information) demonstrates the efficiency of this synthetic
Conclusion
We have shown experimentally that AmAbz is an easily
method. In particular, the MALDI mass spectra displayed accessible new dipeptide mimic that can be converted into
the expected masses at m/z ϭ 932.5 and 1003.3, respectively, building blocks for branched peptide synthesis using mild
with no trace of the side products that would have been coupling methods. Further applications of this compound
formed by premature acylation of the arylamine: as a scaffold in combinatorial chemistry are under study. In
Eur. J. Org. Chem. 2000, 3755Ϫ3761
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R. Pascal, R. Sola, F. Labeguere, P. Jouin
FULL PAPER
[s, 2 H, CH2], 7.62 [d, J5,6 ϭ 8.4 Hz, 1 H, 5-H], 7.68 [d, J2,6
ϭ
addition, our background in the design of safety-catch link-
ers for solid-phase synthesis[34,35] had led us to anticipate
that derivatives of AmAbz might have useful applications
in this field, thanks to their potential to be converted into
highly reactive cyclic N-acylureas.[35] We have therefore un-
dertaken studies aimed at evaluating the reactivity of such
1.9 Hz, 1 H, 2-H], 7.83 [dd, J5,6 ϭ 8.4 Hz, J2,6 ϭ 1.9 Hz, 1 H, 6-
H], 7.84Ϫ7.93 [m, 4 H, aromatic H], 9.38 [s, 1 H, NH], 12.88 [s, 1
H, COOH].
Ϫ
MS (FAB): m/z 355 [M
ϩ
Hϩ].
Ϫ
C18H14N2O6 · 0.5 H2O (363.32): calcd. C 59.52, H 4.16, N 7.71;
found C 59.82, H 4.22, N 7.79.
ureas with various nucleophiles for further applications in Isolation of 5: The filtrate remaining after recrystallization of 4
from DMF/AcOH was concentrated under reduced pressure. A
solid precipitated on addition of water. After filtration, the solid
was recrystallized from DMF/AcOH (1:1) to give compound 5
(1.05 g, 4%) as a white solid, m.p. 283 °C (dec.). Ϫ TLC: Rf ϭ 0.24
convergent synthesis of peptides or in peptide ligation.
Experimental Section
1
(toluene/dioxane/AcOH, 95:25:4). Ϫ H NMR ([D6]DMSO): δ ϭ
3.62 [s, 3 H, OCH3], 4.79 [s, 4 H, CH2], 7.66 [s, 2 H, aromatic H],
7.86Ϫ7.94 [m, 8 H, aromatic H], 9.31 [s, 1 H, NH], 13.10 [s, 1 H,
COOH]. Ϫ MS (FAB): m/z 514 [M ϩ Hϩ].
General Remarks: TLC: SDS precoated plates (0.2 mm), silica gel
60, F254; detection: UV and/or spraying with a solution of ninhyd-
rin (1 g) in 95% EtOH (100 mL) and heating. Ϫ Melting points:
Electrothermal IA9200. Ϫ Elemental analyses were performed by
the ‘‘Service Central d’Analyse’’, CNRS, Vernaison (France). Ϫ 1H
NMR: Bruker Avance DPX 200 (200 MHz). [D6]DMSO as solvent
δH ϭ 2.50. Ϫ MS: Bruker Biflex III (MALDI-TOF); Jeol SX 102
(FABϩ). Ϫ Analytical HPLC: Buffer A 0.1% aqueous TFA, B
CH3CN (0.1% TFA); System A: Beckman System Gold Chromato-
graph (Programmable Solvent Module 126, Diode Array Detector
Module 168, column Kromasil C8 5µm 4.6 ϫ 150 mm, linear gradi-
ent 50 to 100% buffer B over 10 min, flow 1.5 mL/min); System B:
Merck chromatograph (655AϪ11 pump, 655A UV/Vis detector
(λ ϭ 214 nm), L-5000 LC Controller, D-2000 Chromato Integrator,
column LiChrospher 100 RP-18, linear gradient 20 to 60% buffer
B over 30 min, flow 2 mL/min).
4-Amino-3-(aminomethyl)benzoic Acid (1): A mixture of compound
4 (21.3 g, 60 mmol), NaOH (16.8 g, 420 mmol) and water (60 mL)
in a Teflon flask was stirred and heated at reflux. The reaction
was complete after 48 h, as shown by TLC analysis (butyl alcohol/
AcOH/water, 2:1:1) indicating the presence of the hydrolysis prod-
ucts only: 1 (Rf ϭ 0.70) and phthalic acid (Rf ϭ 0.75). The solution
was cooled to room temperature and then partially neutralized with
AcOH (10.7 mL), which caused precipitation. The filtrate and the
solid (which dissolved in a large volume of water) were introduced
onto a column filled with Dowex 50 (Hϩ-form) ion exchange resin.
The resin was washed with water to remove phthalic acid, and the
product was eluted with 0.2  ammonia. Removal of the solvent
under reduced pressure and drying in vacuo gave amino acid 1
(9.4 g, 89%) as a hemihydrate, m.p. Ͼ 250 °C (dec.). Ϫ TLC: Rf ϭ
0.70 (butyl alcohol/AcOH/water, 2:1:1). Ϫ 1H NMR (D2O): δ ϭ
3.99 [s, 2 H, CH2], 6.69Ϫ6.74 [m, 1 H, aromatic H], 7.54Ϫ7.60 [m,
2 H, aromatic H]. Ϫ MS (FAB): m/z (%) 167 (100) [M ϩ Hϩ], 150
(85) [M ϩ Hϩ Ϫ NH3]. Ϫ C8H10N2O2 · 0.5 H2O (175.18): calcd. C
54.85, H 6.33, N 15.99; found C 54.66, H 6.25, N 15.81.
4-(Methoxycarbonylamino)benzoic Acid (3): A mixture of 4-amino-
benzoic acid 2 (13.7 g, 100 mmol) and Na2SO4 (28.4 g, 200 mmol,
added to combine with the liberated HCl) in dioxane (100 mL) was
heated to 70 °C with vigorous stirring (heating ensured complete
dissolution of 2). A solution of MeOCOCl (7.8 mL, 100 mmol) in
dioxane (20 mL) was slowly added over 2 h. The mixture was fur-
ther stirred for 15 min at 70 °C. After cooling to room temperature,
water (50 mL) was added and the mixture was concentrated under
reduced pressure. A solid precipitated upon addition of water
(100 mL). It was collected by filtration, washed repeatedly with
Phthalic Acid Salt of 1: To the alkaline solution obtained after a
similar hydrolysis of compound 4 (7.08 g, 20 mmol) with NaOH
(4.80 g, 120 mmol) in 25 mL of water was added H2SO4 (2.5 ,
50 mL). The precipitate was filtered, washed with water, EtOH, and
water and dried in vacuo to give crude 3 (18.9 g, 97%), which was Et2O, and dried in vacuo to give 1 as a phthalic acid salt (4.48 g,
used in the next step without further purification. White crystals
were obtained by recrystallization from EtOH/water (1:4). Ϫ M.p.
67%), m.p. Ͼ 300 °C (dec.). Ϫ TLC: Rf ϭ 0.70 and Rf ϭ 0.75
1
(butyl alcohol/AcOH/water, 2:1:1). Ϫ H NMR ([D6]DMSO): δ ϭ
204.5 °C (ref.[36] 207 °C,[16] 195Ϫ196 °C for 3 · 0.25 H2O). Ϫ TLC: 3.94 [s, 2 H, CH2], 6.12 [broad s, 2 H, NH2], 6.69 [d, J5,6 ϭ 8.5 Hz,
Rf ϭ 0.65 (CH2Cl2/MeOH/AcOH, 10:1:0.15). Ϫ For 1H NMR
1 H, 5-H], 7.45Ϫ7.54 [m, 2 H, aromatic H], 7.65 [dd, J5,6 ϭ 8.5 Hz,
J2,6 ϭ 2.0 Hz, 1 H, 6-H], 7.79 [d, J2,6 ϭ 2.0 Hz, 1 H, 2-H],
8.11Ϫ8.19 [m, 2 H, aromatic H].
spectrum see ref.[16]
4-Methoxycarbonylamino-3-(phthalimidomethyl)benzoic Acid (4): A
mixture of 3 (9.76 g, 50.0 mmol), water (5 mL) and concentrated
H2SO4 (45 mL) was heated to 50 °C with stirring. Solid N-hydroxy-
methylphthalimide (8.86 g, 50.0 mmol) was added in portions over
30 min. The solution was stirred for 2 h at 50 °C, cooled to room
Methyl 1,2,3,4-Tetrahydro-2-oxo-6-quinazolinecarboxylate (6a):
SOCl2 (0.88 mL, 12.1 mmol) was added carefully to MeOH
(50 mL) at 0 °C with vigorous stirring. Amino acid 1 (1.00 g,
5.7 mmol) was added to the resulting solution and the mixture was
temperature, and slowly poured into EtOH/water (1:3, 400 mL) heated at reflux for 5 h, then concentrated under reduced pressure.
with vigorous stirring. The product was collected by filtration, re-
peatedly washed with water to neutrality and dried in vacuo to give
Recrystallization of the residue from MeOH gave the methyl ester
dihydrochloride (1.49 g), m.p. 209 °C (dec.). Ϫ 1H NMR (D2O):
a solid (16.3 g) containing 4 (85% molar ratio by NMR), residual δ ϭ 3.72 [s, 3 H, OCH3], 4.05 [s, 2 H, CH2], 6.90 [d, J5,6 ϭ 8.4 Hz,
carbamate 3 (7%) and bis-adduct 5 (8%). The product was recrys-
tallized from DMF/AcOH (1:5), washed with AcOH, EtOH, and
Et2O and dried in vacuo to give white crystals (13.5 g) containing
residual AcOH (4.6% by weight), corresponding to a 73% yield.
This product was used in the next step. Recrystallization from
damp MeOH gave a pure sample of the hemihydrate 4 · 1/2 H2O,
1 H, 5-H], 7.74 [dd, J5,6 ϭ 8.0 Hz, J2,6 ϭ 2.0 Hz, 1 H, 6-H], 7.80
[d, J2,6 ϭ 2.0 Hz, 1 H, 2-H]. This salt (0.200 g, 0.79 mmol) was
allowed to react in dioxane (10 mL) with N,NЈ-carbonyldiimidazole
(0.253 g, 1.56 mmol) and DIEA (0.27 mL, 1.56 mmol) for 2.5 h at
80 °C. The solution was cooled to room temperature and poured
into water. The precipitate was filtered, washed with water and
dried in vacuo. Recrystallization from isopropyl alcohol gave 6a as
m.p. 263 °C (dec.). Ϫ TLC: Rf ϭ 0.39 (toluene/dioxane/AcOH,
1
95:25:4). Ϫ H NMR ([D6]DMSO): δ ϭ 3.71 [s, 3 H, OCH3], 4.81 a white solid (0.080 g), m.p. 259 °C. Ϫ TLC: Rf ϭ 0.35 (CH2Cl2/
3758
Eur. J. Org. Chem. 2000, 3755Ϫ3761

Synthesis of the Novel Amino Acid 4-Amino-3-(aminomethyl)benzoic Acid
FULL PAPER
1
MeOH, 10:1). Ϫ H NMR ([D6]DMSO): δ ϭ 3.78 [s, 3 H, OCH3],
4.36 [s, 2 H, CH2], 6.82 [d, J5,6 ϭ 8.9 Hz, 1 H, 5-H], 7.02 [broad s,
1 H, CH2ϪNH], 7.70Ϫ7.74 [m, 2 H, 2,6-H], 9.45 [broad s, 1 H,
concentrated to ca. 5 mL. The solid which precipitated was col-
lected by filtration, washed with a small volume of cold EtOAc and
then with pentane. It was dried to give 8b as a white solid (0.514 g,
NH]. Ϫ MS (FAB): m/z 207 [M ϩ Hϩ]. Ϫ C10H10N2O3 (206.20): 66%), m.p. Ͼ 200 °C (dec.). An additional fraction of 8b (0.184 g,
calcd. C 58.25, H 4.89, N 13.59; found C 58.26, H 4.49, N 13.41.
25%, HPLC purity ca. 95%) was obtained from the filtrate after
removal of the solvent under reduced pressure, dissolving of the
residue in EtOH and precipitation by addition of water. Ϫ TLC:
Rf ϭ 0.4 (EtOAc). Ϫ HPLC: tR ϭ 4.1 min (system A). Ϫ 1H NMR
([D6]DMSO): δ ϭ 4.02 [d, J ϭ 6.0 Hz, 2 H, CH2], 4.2Ϫ4.35 [m, 3
H, CHϪCH2], 5.79 [s, 2 H, NH2], 6.62 [d, J ϭ 8.4 Hz, 1 H, aro-
matic H], 7.28Ϫ7.90 [m, 11 H, aromatic H and CH2NH], 12.03 [s,
1 H, COOH]. Ϫ MS (FAB): m/z 389 [M ϩ Hϩ]. Ϫ C23H20N2O4
(388.42): calcd. C 71.12, H 5.19, N 7.21; found C 71.20, H 5.29,
N 7.18.
1,2,3,4-Tetrahydro-2-oxo-6-quinazolinecarboxylic Acid (6b): A solu-
tion of 4 (0.354 g, 1.00 mmol) in NaOH (1 , 3.5 mL) was heated
at 100 °C for 24 h. The solid formed upon acidification to pH 1
with concentrated HCl was collected by filtration, washed with
water and dried in vacuo to give 32 mg (16%) of compound 6b,
m.p. Ͼ 275 °C (dec.). Ϫ TLC: Rf ϭ 0.80 (butyl alcohol/AcOH/
water, 2:1:1). Ϫ 1H NMR ([D6]DMSO): δ ϭ 4.35 [s, 2 H, CH2], 6.80
[d, J5,6 ϭ 8.0 Hz, 1 H, 5-H], 6.98 [s, 1 H, CH2ϪNH], 7.67Ϫ7.72 [m,
2 H, 2,6-H], 9.38 [s, 1 H, NH], 12.56 [broad s, 1 H, COOH].
3-(2-Carboxybenzoylaminomethyl)-4-(methoxycarbonylamino)-
benzoic Acid (7): A mixture of 4 (0.177 g, 0.50 mmol), Na2CO3
(0.212 g, 2.00 mmol) and water (2 mL) was heated at 60 °C for 3 h.
The medium was acidified to pH 1 with concentrated HCl, the
solid was collected by filtration, washed with water and dried to
give 0.17 g (91%) of compound 7. M.p. Ͼ 220 °C (dec.). Ϫ 1H
NMR ([D6]DMSO): δ ϭ 3.70 [s, 3 H, OCH3], 4.42 [d, J ϭ 6.1 Hz,
2 H, CH2], 7.39Ϫ7.94 [m, 7 H, aromatic H], 9.11 [t, J ϭ 6.1 Hz, 1
H, CH2NH], 9.75 [s, 1 H, NH], 12.9 [broad s, 2 H, COOH]. Ϫ MS
(FAB): m/z 373 [M ϩ Hϩ].
Fmoc؊AmAbz(Boc) (10): Na2SO4 (0.763 g, 5.37 mmol) was added
to a solution of 8a (0.715 g, 2.68 mmol) in dioxane (5 mL). The
mixture was vigorously stirred and heated to 70 °C. Next, a solu-
tion of FmocϪCl (0.695 g, 2.68 mmol) in dioxane (0.7 mL) was
added dropwise over 60 min. The mixture was cooled to room tem-
perature and allowed to react further for 3 h. Complete precipita-
tion was accomplished on addition of water with vigorous stirring.
The precipitate was collected by filtration, washed and triturated
with water, then dried in vacuo. The solid was dissolved in MeOH,
and the solution was concentrated under reduced pressure. A pre-
cipitate was obtained, collected by filtration and dried to give 10
(0.907 g, 69%). Ϫ M.p. Ͼ 200 °C (dec.). Ϫ TLC: Rf ϭ 0.67
AmAbz(Boc) (8a): A solution of Boc2O (1.244 g, 5.70 mmol) in di-
oxane was prepared (3 mL). A suspension of 1 (hemihydrate,
1.051 g, 6.00 mmol) in water (6 mL) and dioxane (12 mL), stirred
and cooled to 0 °C, was subjected five times to the following treat-
ment: addition of NaOH (2.5 , 0.93 mL) immediately followed by
addition of a 0.6 mL aliquot of the Boc2O solution, with a 10 min
interval between each Boc2O addition. The mixture was further
allowed to react for 1 h at room temperature, then diluted with
water and washed with tBuOMe. The aqueous layer was acidified
with formic acid (0.6 mL, 2.5 mmol) and extracted with EtOAc.
The organic layer was washed with water and with brine, and then
was dried (Na2SO4) and concentrated to ca. 5 mL. Addition of
pentane gave a solid, which was collected by filtration, washed with
pentane and dried in vacuo to give 1.315 g (86%) of pure 8a. M.p.
Ͼ 190 °C (dec.). Ϫ TLC: Rf ϭ 0.62 (CH2Cl2/MeOH/AcOH,
1
(CH2Cl2/MeOH/AcOH, 10:1:0.15). Ϫ H NMR ([D6]DMSO): δ ϭ
1.42 [s, 9H, C(CH3)3], 4.15 [d, J ϭ 6.0 Hz, 2 H, CH2], 4.31 [t,
J ϭ 6.8 Hz, 1 H, CHϪCH2], 4.44 [d, J ϭ 6.8 Hz, 2 H, CHϪCH2],
7.29Ϫ7.47 [m, 4 H, aromatic H], 7.5Ϫ7.7 [m, 2 H, aromatic H and
CH2NH], 7.75Ϫ7.93 [m, 6 H, aromatic H], 9.65 [s, 1 H, NH], 12.81
[s, 1 H, COOH]. Ϫ MS (FAB): m/z (%) 489 (3) [M ϩ Hϩ], 433 (4)
[M ϩ Hϩ Ϫ tBu], 389 (28) [M ϩ Hϩ Ϫ Boc], 179 (100) [dibenzoful-
venylium cation]. Ϫ C28H28N2O6 · 0.25 CH3OH (496.54): calcd. C
68.33, H 5.89, N 5.64; found C 67.86, H 5.84, N 5.63.
Reaction of 8b with BOP Reagent and Formation of
AmAbz(Fmoc)؊Ala؊OMe (11):
A solution of BOP (88 mg,
0.20 mmol) in DMF (1.8 mL) was poured into a test tube con-
taining AmAbz(Fmoc) 8b (78 mg, 0.20 mmol) and HOBt · H2O
(31 mg, 0.20 mmol). After addition of DIEA (70 µL, 0.40 mmol),
the mixture was allowed to react at room temperature. The reaction
was monitored by HPLC analysis (System A) of samples (50 µL)
withdrawn from the reaction medium and diluted with 2.5 mL
CH3CN/water (4:1). Compound 8b (tR ϭ 4.1 min) was quickly con-
verted (Ͼ 95% at 2 min, Ͼ 99% at 30 min) into the activated ester
(tR ϭ 6.8 min); after 2 h, no further change was observed, and
AlaϪOMe · HCl (28 mg, 0.20 mmol) with DIEA (35 µL,
0.20 mmol) were added to the mixture, resulting in the formation
of one product (tR ϭ 4.4 min, 85% at 60 min); the reaction was
then forced to completion by addition of further AlaϪOMe · HCl
(5 mg) and standing overnight at room temperature. The reaction
medium was diluted with EtOAc (20 mL), washed repeatedly with
water, 5% Na2CO3, 5% KHSO4, water and brine, then dried
(Na2SO4) and concentrated under reduced pressure to give 11. Ϫ
HPLC: tR ϭ 4.4 min (system A). Ϫ 1H NMR ([D6]DMSO): δ ϭ
1.37 [d, J ϭ 7.3 Hz, 3 H, CHϪCH3], 3.62 [s, 3 H, OϪCH3], 4.01
1
10:1:0.15). Ϫ H NMR ([D6]DMSO): δ ϭ 1.39 [s, 9 H, C(CH3)3],
3.95 [d, J ϭ 6.1 Hz, 2 H, CH2], 5.79 [broad s, 2 H, NH2], 6.59 [d,
J ϭ 8.3 Hz, 1 H, aromatic H], 7.31 [t, J ϭ 6.1 Hz, 1 H, CH2NH],
7.51Ϫ7.57 [m, 2 H, aromatic H], 12.0 [broad s, 1 H, COOH]. Ϫ
MS (FAB): m/z 267 [M ϩ Hϩ]. Ϫ C13H18N2O4 (266.29): calcd. C
58.63, H 6.81, N 10.52; found C 58.36, H 6.78, N 10.23.
Alternatively, a suspension of the phthalic acid salt of 1 (1.66 g,
5.0 mmol) in dioxane (10 mL), NaOH (2.5 , 4 mL), and water
(1 mL) was treated similarly to give 8a (1.024 g, 81%).
AmAbz(Fmoc) (8b): Amino acid 1 (2.00 mmol) was allowed to react
with FmocϪOSu (1.90 mmol) as follows. A suspension of 1
(hemihydrate, 0.350 g), Na2CO3 (0.265 g, 2.50 mmol) in dioxane
(10 mL), and water (5 mL) was vigorously stirred, then cooled to
0 °C. Addition of NaOH (2.5 , 0.16 mL), immediately followed
by addition of solid FmocϪOSu (0.128 g), was performed five
times, with a 10 min interval between each FmocϪOSu addition.
The mixture was further allowed to react for 1 h at room temper-
ature, then diluted with water and washed with tBuOMe (40 mL); [d, J ϭ 5.8 Hz, 2 H, CH2], 4.19Ϫ4.50 [m, 4 H, αCH and CHϪCH2],
the organic layer was extracted with 5% (aq.) NaHCO3 (ca. 10 mL).
The combined aqueous phases were acidified to pH 1 with concen-
trated H2SO4, then extracted with EtOAc. The organic layer was
washed with 5% KHSO4, water and brine, then dried (Na2SO4) and
5.60 [broad s, 2 H, NH2], 6.64 [d, J5,6 ϭ 8.3 Hz, 1 H, 5-H],
7.28Ϫ7.89 [m, 11 H, aromatic H and NHϪCH2], 8.33 [d, J ϭ
7.0 Hz, 1 H, NHϪCH]. Ϫ MS (MALDI): m/z (%) 474 (14) [M ϩ
Hϩ], 496 (100) [M ϩ Naϩ], 512 (100) [M ϩ Kϩ].
Eur. J. Org. Chem. 2000, 3755Ϫ3761
3759

´
`
R. Pascal, R. Sola, F. Labeguere, P. Jouin
FULL PAPER
Resin 12: Solid-phase syntheses were carried out using a glassware
manual apparatus involving a reaction vessel equipped with a frit-
ted disc, a solvent inlet and a device to apply low nitrogen pressure
used to fill the vessel with DMF or CH2Cl2 and to remove liquid
waste. Continuous ‘‘stirring’’ was performed during all the washing
and coupling steps, either by gently rocking the vessel on a shaker
or by bubbling nitrogen through the fritted plate. The efficiency
of the coupling reactions was monitored using the ninhydrin test.
Commercial aminomethylpolystyrene resin (1.00 g, 0.56 mmol/g,
NovaBiochem) was introduced into the reaction vessel and sub-
jected to the following washes: DMF (2 ϫ 1 min ϩ 1 ϫ 30 min);
CH2Cl2 (3 ϫ 1 min); TFA/CH2Cl2 (1:1) (1 ϫ 10 min); DIEA/
CH2Cl2 (1:20) (2 ϫ 10 min); CH2Cl2 (4 ϫ 1 min); DMF (4 ϫ
1 min). Then, FmocϪRink amide linker (0.45 g, 0.84 mmol, Nova-
Biochem) was allowed to react with the resin for 90 min, using BOP
(0.37 g, 0.84 mmol), DIEA (0.22 mL, 1.26 mmol), and DMF. Unre-
acted amino groups were acetylated by reaction with Ac2O
(0.53 mL, 5.6 mmol) and DIEA (0.29 mL, 1.68 mmol) in DMF for
15 min. Next, amino acids were incorporated as follows: (i) Fmoc
removal: piperidine/DMF (1:4) (1 ϫ 1 min ϩ 3 ϫ 3 min); DMF
washes (4 ϫ 1 min); (ii) coupling reaction: the protected amino acid
(1.85 mmol, 3.3 equiv.) and HOBt (0.26 g, 1.68 mmol) were dis-
solved in the minimum possible volume of DMF and then intro-
duced into the reaction vessel, DIEA (0.44 mL, 2.52 mmol) and
solid BOP (0.743 g, 1.68 mmol) were added and stirring was con-
tinued for 60 min; DMF washes (4 ϫ 1 min). An acetylation step
was inserted after the FmocϪGly coupling reaction because of a
slightly positive ninhydrin test. A different procedure was used for
the FmocϪPhe coupling reaction: DMF washes (4 ϫ 1 min);
CH2Cl2 washes (4 ϫ 1 min); FmocϪPhe (0.716 g, 1.85 mmol) was
dissolved in CH2Cl2 and then introduced into the reaction vessel;
coupling was initiated by addition of DIC (0.263 mL, 1.68 mmol).
CH2Cl2 was added 5 min later because gelation of the medium was
observed, making it necessary to resuspend the resin. The suspen-
sion was allowed to react for 3 h. Since no test for detecting un-
changed arylamino groups was available (the ninhydrin test is inef-
ficient), a second coupling was performed using a similar procedure
except that DMF was added after 5 min and CH2Cl2 was evapor-
ated off by nitrogen bubbling during the reaction. Finally, the resin
was washed repeatedly with DMF, CH2Cl2, MeOH, 100% EtOH
and Et2O, and dried, first by flushing nitrogen through the vessel
(10 min) and then under vacuum, yielding resin 12 (1.475 g, theor-
etically 1.79 g based on the loading of the starting support).
Acknowledgments
We thank Dr. S. L. Salhi for editorial assistance.
[1]
E. M. Gordon, R. W. Barrett, W. J. Dower, S. P. A. Fodor, M.
A. Gallop, J. Med. Chem. 1994, 37, 1385Ϫ1401.
[2]
J. A. Ellman, Acc. Chem. Res. 1996, 29, 132Ϫ143.
[3]
M. Royo, M. del Fresno, A. Frieden, W. Van Den Nest, M.
Sanseverino, J. Alsina, S. A. Kates, G. Barany, F. Albericio,
React. Funct. Polym. 1999, 41, 103Ϫ110 and references therein.
[4]
Abbreviations for AmAbz derivatives are constructed accord-
ing to the recommendations of the IUPAC-IUB Commission
on Biochemical Nomenclature (Eur. J. Biochem. 1984, 138,
9Ϫ37), 4-amino and 3-aminomethyl groups being defined as
main-chain and side-chain groups, respectively. Additional ab-
breviations used are: Ϫ Boc: tert-butyloxycarbonyl; BOP:
benzotriazol-1-yloxy-tris(dimethylamino)-phosphonium hexa-
fluorophosphate; DIC: N,NЈ-diisopropylcarbodiimide; DMF:
N,N-dimethylformamide; DIEA: N-ethyldiisopropylamine;
HOBt: N-hydroxybenzotriazole; Pht: phthaloyl; OSu: succini-
midyloxy; TFA: trifluoroacetic acid.
[5]
For examples of dipeptide mimics based on amino or aminoal-
kyl benzoic acid, see K. S. Para, T. K. Sawyer, in: Methods in
Molecular Medicine, Vol. 23: Peptidomimetics Protocols (Ed.:
W. M. Kazmierski), Humana Press Inc., Totowa, NJ, 1999,
pp. 513Ϫ526.
[6]
B. R. Neustadt, E. M. Smith, T. Nechuta, Y. Zhang, Tetrahed-
ron Lett. 1998, 39, 5317Ϫ5320.
[7] [7a]
G. Decodts, M. Wakselman, Eur. J. Med. Chem. Ϫ Chim.
[7b]
Ther. 1983, 18, 107Ϫ111. Ϫ
W. H. Miller, T. W. Ku, F. E.
Ali, W. E. Bondinell, R. R. Calvo, L. D. Davis, K. F. Erhard,
L. B. Hall, W. F. Huffman, R. M. Keenan, C. Kwon, K. A.
Newlander, S. T. Ross, J. M. Samanen, D. T. Takata, C.-K.
Yuan, Tetrahedron Lett. 1995, 36, 9433Ϫ9436. Ϫ [7c] T. W. Ku,
W. H. Miller, W. E. Bondinell, K. F. Erhard, R. M. Keenan,
A. J. Nichols, C. E. Peishoff, J. M. Samanen, A. S. Wong, W.
F. Huffman, J. Med. Chem. 1995, 38, 9Ϫ12. Ϫ [7d] J. Keck, H.
Pieper, G. Krueger, S. Pueschmann, K. R. Noll, German Pa-
tent 73Ϫ2318636 (1973); Chem. Abstr. 1975, 82, 98715.
R. G. Pews, J. Org. Chem. 1979, 44, 2032Ϫ2034.
[8]
[9]
H. Ogawa, S. Tamada, T. Fujioka, S. Teramoto, K. Kondo, S.
Yamashita, Y. Yabuuchi, M. Tominaga, K. Nakagawa, Chem.
Pharm. Bull. 1988, 36, 2253Ϫ2258.
[10] H. E. Zaugg, Synthesis 1984, 85Ϫ110.
[11] J. Rosevear, J. F. K. Wilshire, Aust. J. Chem. 1990, 43, 339Ϫ353.
[12] J. Rosevear, J. F. K. Wilshire, Aust. J. Chem. 1985, 38, 723Ϫ733.
[13] R. B. Homer, C. D. Johnson, in: The Chemistry of Amides (Ed.:
J. Zabicky), Interscience Publishers, London, 1970; pp.
187Ϫ245.
[14] D. R. Bolin, I.-I. Sytwu, F. Humiec, J. Meienhofer, Int. J. Pept.
Protein Res. 1989, 33, 353Ϫ359 and references therein.
[15] G. Kortüm, W. Vogel, K. Andrussow, Dissociation Constants
of Organic Acids in Aqueous Solution, Butterworths, London,
1961, p. 381.
Peptide 13: This was released into solution by shaking a suspension
of resin 12 (50 mg) in TFA/water/triisopropylsilane (95:2.5:2.5,
2 mL) for 3 h. The suspension was filtered and the resin was
washed with TFA (2 ϫ 1 mL). The filtrates were concentrated, then
diluted with Et2O (100 mL) and extracted twice with water (25 mL
then 10 mL). The combined aqueous phases were washed with
Et2O (3 ϫ 50 mL), then concentrated and freeze-dried to give 13
(7.6 mg). Ϫ HPLC: tR ϭ 18.7 min (system B). Ϫ MS (MALDI):
m/z 932.5 (100) [M ϩ Hϩ], 954.5 (60) [M ϩ Naϩ], 970.4 (31) [M
ϩ Kϩ].
[16] P. Chand, Y. S. Babu, S. Bantia, N. Chu, L. B. Cole, P. L.
Kotian, W. G. Laver, J. A. Montgomery, V. P. Pathak, S. L.
Petty, D. P. Shrout, D. A. Walsh, G. M. Walsh, J. Med. Chem.
1997, 40, 4030Ϫ4052.
[17] H. R. Kricheldorf, Synthesis 1970, 649Ϫ651.
[18] In preliminary experiments, acid hydrolysis turned out to be
impracticable due to a very sluggish process even in hot con-
centrated acid.
[19] S. Papot, C. Bachmann, D. Combaud, J.-P. Gesson, Tetrahed-
ron 1999, 55, 4699Ϫ4708.
[20] Prior conversion of carboxyl group into methyl ester was
achieved in order to avoid any side reaction with carbonyldiim-
idazole.
Peptide 15: The Fmoc group was removed from resin 12 (200 mg)
and then FmocϪAla (65 mg, 0.21 mmol) was coupled with the re-
sulting resin using the BOP/HOBt/DIEA method described above.
The resin was washed and dried to give resin 14 (209 mg). A sus-
pension of 14 in TFA/water/triisopropylsilane (95:2.5:2.5, 8 mL)
was shaken for 3 h, then treated as described above to yield crude
peptide 15 (19.7 mg ). Ϫ HPLC: tR ϭ 18.6 min (system B). Ϫ MS
(MALDI): m/z (%) 1003.3 (100) [M ϩ Hϩ], 1025.3 (25) [M ϩ Naϩ],
1041.2 (5) [M ϩ Kϩ].
6c: 1H NMR ([D6]DMSO): δ ϭ 2.82 [s, 3 H], 4.41 [s, 2 H],
[21]
6.79 [d, J ϭ 8.2 Hz, 1H], 7.48Ϫ7.76 [m, 2 H], 9.54 [s, 1 H],
12.56 [broad s, 1 H]; from ref.[9]
[22] D. D. Perrin, Dissociation Constants of Organic Bases in Aque-
ous Solution, Butterworths, London, 1965, p. 116.
[23] [23a]
[23b]
H. Rink, Tetrahedron Lett. 1987, 28, 3787Ϫ3790. Ϫ
M. S. Bernatowicz, S. B. Daniels, H. Köster, Tetrahedron Lett.
1989, 30, 4645Ϫ4648.
[24] During acylation with FmocϪPhe in CH2Cl2 to give peptide
3760
Eur. J. Org. Chem. 2000, 3755Ϫ3761

Synthesis of the Novel Amino Acid 4-Amino-3-(aminomethyl)benzoic Acid
FULL PAPER
resin 12, solvent (DCM or DMF) was added 5 min after car-
tonated in the medium, it is able to react in the absence of base.
[29] H. S. Bates, J. H. Jones, W. I. Ramage, M. J. Witty, in: Peptides
1980. Proc. 16th Eur. Pept. Symp. (Ed.: K. Brunfeldt), Scriptor,
Copenhagen, 1981, pp. 185Ϫ190.
bodiimide addition for technical reasons (gelation of the reac-
tion medium, probably due to precipitation of FmocϪPhe
symmetrical anhydride).
[25] J. Coste, E. Frerot, P. Jouin, B. Castro, Tetrahedron Lett. 1991,
32, 1967Ϫ1970.
[30] D. F. DeTar, R. Silverstein, J. Am. Chem. Soc. 1966, 88,
1013Ϫ1019.
[26] ‘‘Typical carbodiimide conditions’’, without experimental de-
tails, were reported to be inefficient for acylation with
BocϪVal (ref.[6]).
[31] B. J. Balcom, N. O. Petersen, J. Org. Chem. 1989, 54,
1922Ϫ1927.
[32] M. Beyermann, P. Henklein, A. Klose, R. Sohr, M. Bienert,
[27] The reaction of BocϪPhe (2 equiv.) with the arylamino group
of a soluble AmAbz derivative was completed within 25 min
after dicyclohexylcarbodiimide activation in CH2Cl2.
[28] An explanation of this efficiency is that the effective acylating
species may be the initially formed O-acylisourea intermediate,
since the arylamine is a stronger nucleophile than neutral carb-
oxylic acid and, thus, acylation may have occurred before the
symmetrical anhydride had formed. This interpretation is con-
sistent with the proposal of direct formation of acylpyridinium
from O-acylisourea and pyridine (D. F. DeTar, R. Silverstein,
J. Am. Chem. Soc. 1966, 88, 1020Ϫ1023). In addition, since the
poorly nucleophilic arylamino group of AmAbz is not pro-
Int. J. Pept. Protein Res. 1991, 37, 252Ϫ256.
[33] L. A. Carpino, A. El-Faham, Tetrahedron 1999, 55,
6813Ϫ6830.
[34] R. Sola, P. Saguer, M. L. David, R. Pascal, J. Chem. Soc.,
Chem. Commun. 1993, 1786Ϫ1788.
[35] R. Pascal, D. Chauvey, R. Sola, Tetrahedron Lett. 1994, 34,
6291Ϫ6294.
[36] H. Najer, P. Chabrier, R. Giudicelli, Bull. Soc. Chim. Fr.
1955, 1189Ϫ1192.
Received March 23, 2000
[O00155]
Eur. J. Org. Chem. 2000, 3755Ϫ3761
3761