Improved synthesis of amino acid and dipeptide chloromethyl esters using bromochloromethane

Article abstract of DOI:10.1081/SCC-120018930

Peptide chloromethyl esters are important compounds in prodrug synthesis. A simple, mild and efficient method for the synthesis of chloromethyl esters of N-blocked amino acids and dipeptides using exclusively bromochloromethane is reported. These N-blocke

Full text of DOI:10.1081/SCC-120018930

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Improved Synthesis of Amino Acid and Dipeptide
Chloromethyl Esters Using Bromochloromethane
Paula Gomes , Maria Isabel Santos , Maria Joaquina Trigo , Raquel
Castanheiro & Rui Moreira
To cite this article: Paula Gomes , Maria Isabel Santos , Maria Joaquina Trigo , Raquel
Castanheiro & Rui Moreira (2003) Improved Synthesis of Amino Acid and Dipeptide
Chloromethyl Esters Using Bromochloromethane, Synthetic Communications, 33:10,
1683-1693, DOI: 10.1081/SCC-120018930
To link to this article: http://dx.doi.org/10.1081/SCC-120018930
Published online: 17 Aug 2006.
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 10, pp. 1683–1693, 2003
Improved Synthesis of Amino Acid and Dipeptide
Chloromethyl Esters Using Bromochloromethane
2
2
Paula Gomes,1, Maria Isabel Santos, Maria Joaquina Trigo,
*
Raquel Castanheiro,2 and Rui Moreira3
1Centro de Investigacao em Quımica da Universidade
do Porto, and 2Departamento de Quımica da Faculdade
de Ciencias do Porto, R. Campo Alegre, Porto, Portugal
3Centro de Estudos de Ciencias Farmaceuticas,
Faculdade de Farmacia, Universidade de Lisboa,
Lisboa, Portugal
ABSTRACT
Peptide chloromethyl esters are important compounds in prodrug
synthesis. A simple, mildandefficient methodfor the synthesis of
chloromethyl esters of N-blocked amino acids and dipeptides using
exclusively bromochloromethane is reported. These N-blocked
amino acid and dipeptide chloromethyl esters react readily with the
carboxylic acidgroup of aspirin andwith the sulfonamido group of
the antimalarial sulfamethazine, to give the corresponding prodrugs.
*Correspondence: Paula Gomes, Centro de Investigacao em Quımica da
Universidade do Porto, R. Campo Alegre, 687P-4169-007 Porto, Portugal;
Fax: þ351-226082959; E-mail: pgomes@fc.up.pt.
1683
DOI: 10.1081/SCC-120018930
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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Gomes et al.
Key Words: Antimalarial; Aspirin; Bromochloromethane; Chloro-
methyl esters; Peptides; Prodrugs.
RESEARCH REPORT
The search for suitable prodrugs as chemical drug delivery systems
is a high-priority task to overcome serious problems relatedto drug
[1,2]
solubility, oral absorption, toxicity andmetabolic inactivation.
Amino acids are promising carriers in prodrug design, as they can
improve water solubility and, thus, oral absorption of the drug.[3]
Dipeptides have also received attention as potential drug carriers to
improve oral absorption, targeted to the dipeptide transporters present
in the gastrointestinal tract. In this context, peptide chloromethyl esters
are suitable coupling agents, capable of reacting with several nucleophilic
[4–6]
groups frequently foundin rdugs,
functionality.
such as the carboxylic acid
In 1979, Wheeler andco-workers edscribedthe synthesis of
N-blockedamino acidchloromethyl esters [ 1] using triethylamine and
chloroiodomethane (Fig. 1).[7] However, they obtainedrather low
global yields (9–25%) of [1], andhadto face the difficulties of separating
the desired ester from large amounts of the gem-diester [2], also formedas
a by-product.[7] Recent work from Tsujihara’s group describes an
alternative synthetic route to N-blockedamino acidchloromethyl esters
[1] using the reagent chloromethyl chlorosulfate, with goodto excellent
yields (79–100%).[8] Even though chloromethyl chlorosulfate is an
excellent reagent for this reaction step, its synthesis occurs in modest
yields (ca. 30%) and involves hazardous reagents (chlorosulfonic acid)
[9]
andproducts.
We now wish to report an alternative methodfor the synthesis of
N-blockedamino acidchloromethyl esters [ 4], basedon the reaction
between the corresponding N-blockedamino acidcaesium salts [ 3] and
bromochloromethane (Fig. 2 andTable 1). Moreover, we have success-
fully extended this method to the synthesis of N-blocked dipeptide
[7]
Figure 1. Synthetic route to chloromethyl esters by Wheeler andco-workers.

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Peptide Chloromethyl Esters
1685
Figure 2. Synthetic route to potential sulfamethazine andaspirin prodrugs:
(i) preparation of N-blocked amino acid and dipeptide caesium salts; (ii) synthesis
of N-blocked amino acid and dipeptide chloromethyl esters; (iii) derivatization of
sulfamethazine andaspirin by reaction with amino acidanddipeptide chloro-
methyl esters.
chloromethyl esters [5] (Table 1). To our best knowledge, this is the first
report on the synthesis of N-blocked dipeptide chloromethyl esters.
The methodnow describedinvolves the previous preparation of the
caesium salts of the N-protected amino acids and dipeptides, which were
formedquantitatively from the corresponding N-protectedamino acids
andpeptides. The use of caesium salts is known to improve the esterifica-
tion rates andyields of amino acidchloromethylation when comparedwith
the corresponding potassium, sodium, calcium or quaternary ammonium
salts.[10,11] The overall yields of [4] and[ 5] herein described range from
50 to 74% (Table 1), representing a significant improvement over the
previous methodusing chloroioodmethane as the chloromethylating
agent.[7] The formation of the gem-diester [2] was almost completely
suppressed, and thus it was easily separated from [4] or [5] by column
chromatography.
The applicability of chloromethyl esters [4] and[ 5] in prodrug
synthesis was assessedby preparing sulfamethazine andaspirin deriva-
tives (Fig. 2). Chloromethyl esters [4] and[ 5] react readily both with the
carboxylic acidgroup of aspirin andthe sulfonamiod group of the
antimalarial sulfamethazine, to give the corresponding prodrugs with
quite reasonable yields (ca. 40%, Table 2). Interestingly, no cyclization
of the N-blocked dipeptide chloromethyl esters [5] was observedin the
presence of the weak base sulfamethazine.

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1690
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EXPERIMENTAL SECTION
Synthesis of the N-Blocked Amino Acid/Dipeptide
Caesium Salts
The appropriate N-blocked amino acid or dipeptide (5 mmol) was
suspended in ethanol/water 7:3 v/v (25 mL) with magnetic stirring. The
electrode of a pH-meter was dipped into the solution and aqueous
caesium carbonate (1 M) was added dropwise until pH 6.5 was reached.
The solvents were then evaporatedunder low pressure andthe resulting
white caesium salt was allowedto dry in vacuum.
Synthesis of the N-Blocked Amino Acid/Dipeptide
Chloromethyl Esters
Bromochloromethane (35 mL) was slowly added to a solution of the
appropriate N-blocked amino acid/dipeptide caesium salt (5 mmol) in dry
N,N0-dimethylformamide (DMF, 15 mL), and the reaction was allowed
to proceedin the dark at room temperature. After 20 h, the caesium
bromide formed was removed by suction filtration and filtrate was
evaporatedto dryness. The resulting mixture presentedthree components
(thin layer chromatography, TLC), by increasing order of Rf, ceasium
bromide, the gem-diester by-product and the main product. This main
product was isolated by low pressure liquid chromatography (LPLC) on
silica, using dichloromethane (DCM)/ethyl ether or DCM/acetone
(in varying proportions) as eluents. Solidcompounds were recrystallized
from 1:1 ethyl ether/petroleum ether (40–60ꢀ) prior to analysis.
The identity and purity of the isolated chloromethyl esters were
checkedby 1H and 13C NMR, FAB/Ion Trap or MALDI-TOF MS
and/or elemental analysis.
Synthesis of the N-Blocked Amino Acid/Dipeptide
Sulfamethazine Derivatives
The sodium salt of sulfamethazine (1 mmol) was slowly added to a
solution of the appropriate N-blocked amino acid/dipeptide chloromethyl
ester (1 mmol) in dry DMF (4 mL). The reaction was allowed to proceed
under constant stirring at room temperature, in the dark. After two days,
the mixture was evaporated to dryness and the residue was redissolved
in acetone (2 mL) upon heating. The solution was storedat 4 ꢀC overnight

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Peptide Chloromethyl Esters
1691
to promote the precipitation of sodium chloride, which was then removed
by filtration under reduced pressure. The filtrate was evaporated to dryness
andthe resulting mixture revealedthe presence of five to seven components
(TLC), the main of which was isolatedby LPLC on silica, using
DCM/ethyl ether 1:1 as eluent. Isolatedcompounds were recrystallized
from 1:1 ethyl ether/petroleum ether (40–60ꢀ) prior to analysis.
The structure andpurity of the isolatedcompounds were confirmed
1
by H and 13C NMR, MALDI-TOF MS and/or elemental analysis.
Synthesis of the N-Blocked Amino Acid Acetylsalicilic Acid
(‘‘Aspirin’’) Derivatives
The N-blockedamino acidchloromethyl ester (1 mmol) was dissolved
in dry DMF (2 mL) and acetylsalicilic acid (1 mmol) was added.
Triethylamine (1 mmol) was then added dropwise. The reaction was
allowedto proceedunder constant stirring at room temperature, in the
dark. After three days, the triethylammonium chloride formed was
removedby vacuum filtration andthe filtrate was evaporatedto dryness.
The residue was redissolved in acetone (2 mL) upon heating and left to
ꢀ
standat 4 C overnight for additional precipitation of the triethylammo-
nium salt. After a new cycle of salt filtration andfiltrate evaporation to
dryness, the mixture was submitted to LPLC on silica, with ethyl acetate
1
as eluent. The target product was isolated and characterized by H and
13C NMR andby MALDI-TOF MS.
ABBREVIATIONS
Boc, tert-butyloxycarbonyl; DCM, dichloromethane; DMF, N,N0-
dimethylformamide; FAB-MS, fast atom bombardment mass spectrome-
try; LPLC, low pressure liquidchromatography; MALDI–TOF MS,
matrix-assistedlaser esdorption–time-of-flight mass spectrometry;
NMR, nuclear magnetic resonance; PG, protecting group; TLC, thin
layer chromatography; Z, benzyloxycarbonyl.
ACKNOWLEDGMENTS
The authors thank Fundacao para a Ciencia e Tecnologia for finan-
cial support (POCTI/FCB/39218/2001). PG thanks Dr. Adelina Macedo
(Chemistry Department, Faculty of Sciences, University of Porto) for

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1692
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FAB/Ion Trap MS spectra. Special thanks to Drs. Eliandre de Oliveira,
Judit Villen andMiquel Vila (Peptide Research Laboratory, University
of Barcelona) for MALDI-TOF MS analyses.
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Receivedin the UK May 14, 2002

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