Syntheses of fluorooxindole and 2-fluoro-2-arylacetic acid derivatives from diethyl 2-fluoromalonate ester

Article abstract of DOI:10.3762/bjoc.10.119

Diethyl 2-fluoromalonate ester is utilised as a building block for the synthesis of 2-fluoro-2-arylacetic acid and fluorooxindole derivatives by a strategy involving nucleophilic aromatic substitution reactions with ortho-fluoronitrobenzene substrates followed by decarboxylation, esterification and reductive cyclisation processes.

Full text of DOI:10.3762/bjoc.10.119

Syntheses of fluorooxindole and 2-fluoro-2-arylacetic
acid derivatives from diethyl 2-fluoromalonate ester
Antal Harsanyi1, Graham Sandford*1, Dmitri S. Yufit2
and Judith A.K. Howard2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1213–1219.
1Department of Chemistry, Durham University, South Road, Durham,
DH1 3LE, UK and 2Chemical Crystallography, Department of
Chemistry, Durham University, South Road, Durham, DH1 3LE, UK
doi:10.3762/bjoc.10.119
Received: 10 January 2014
Accepted: 16 May 2014
Published: 22 May 2014
Email:
Graham Sandford* - graham.sandford@durham.ac.uk
This article is part of the Thematic Series "Organo-fluorine chemistry III".
Guest Editor: D. O'Hagan
* Corresponding author
Keywords:
fluorinated building blocks; fluoroarylacetic acid; fluoromalonate;
fluorooxindole; organo-fluorine; selective fluorination
© 2014 Harsanyi et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Diethyl 2-fluoromalonate ester is utilised as a building block for the synthesis of 2-fluoro-2-arylacetic acid and fluorooxindole
derivatives by a strategy involving nucleophilic aromatic substitution reactions with ortho-fluoronitrobenzene substrates followed
by decarboxylation, esterification and reductive cyclisation processes.
Introduction
Since 1954, when Fried and Sabo observed that the incorpor- the construction of carbon–fluorine bonds using a fluorinating
ation of a fluorine atom into a corticosteroid derivative led to agent (‘late-stage’ fluorination) or the application of polyfunc-
valuable enhanced biological activity [1], a growing number of tional fluorine-containing small molecule building blocks
commercially significant life science products, which owe their (‘early stage’ fluorination) which may be employed in further
activity to the presence of fluorine atoms within their structures, transformations involving all the reactions and techniques avail-
have developed. Fluorine incorporation can lead, for example, able to synthetic organic chemists [10-13]. Of course, the
to enhanced bioavailability, metabolic stability and lipophilicity success of an ‘early stage’ fluorination approach depends on the
of the organic system and these properties are exploited in availability of a range of appropriately functionalised, fluori-
a number of commercially valuable drugs including nated building blocks and the establishment of corresponding
Ciprofloxacin, Lipitor and Voriconazole [2-6].
reactivity profiles [14]. However, it does not necessarily follow
that reactions for which regio- and stereoselectivity profiles are
Given the very small number of fluorinated systems available well established for hydrocarbon systems will be similar to
from nature [7-9], in essence all organic molecules bearing those for corresponding selectively fluorinated systems and,
carbon–fluorine bonds are ‘man-made’. Syntheses rely either on indeed, this is often not the case [15].
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The use of 1,3-diketone, 1,3-ketoester and 1,3-diester deriva- to provide diester 3 in good yield (Scheme 1). Displacement of
tives in retrosynthetic planning is widespread in general organic fluorine from ortho-fluoronitrobenzene was quantitative as
chemistry and numerous terpenes, heterocycles and steroids measured by 19F NMR spectroscopy of the crude reaction mix-
originate from such simple yet synthetically versatile substrates ture and the structure of isolated diester 3 was confirmed by
[16-19]. In contrast, despite the availability of synthetic pro- X-ray crystallography (Figure 1).
cedures for the preparation of various 2-fluoro-1,3-dicarbonyl
systems [20-27], there is, surprisingly, only a relatively limited
number of publications that report the use of such potentially
useful fluorinated building blocks for the synthesis of more
structurally sophisticated selectively fluorinated systems. For
example, 2-fluoromalonate esters have been used for the prepar-
ation of various α-fluorocarboxylic acids [28-32], heterocycles,
such as fluoropyrimidine [33] and quinolone [34] derivatives,
alkylated [35] and Michael addition [36-40] products, providing
an indication of the potential uses and opportunities available
for the synthesis of fluoro-organic products from fluoroma-
lonate precursors.
As part of a wider research programme aimed at developing
routes for the synthesis of selectively fluorinated molecules
using elemental fluorine for the key construction of the
carbon–fluorine bond by complementary direct selective direct
fluorination [41-44], continuous flow [45-49] and building
Figure 1: Molecular structure of 3.
block [50] strategies, in this paper, we describe nucleophilic
aromatic substitution reactions of carbanions derived from
diethyl 2-fluoromalonate ester as the first stage in the synthesis In initial experiments, decarboxylation of 3 by reaction with
of fluoroacetic acid and fluoroxindole systems. While related potassium hydroxide gave good yields of the corresponding
palladium catalysed coupling processes between aryl bromides 2-fluoro-2-arylacetic acid 4a. However, in subsequent experi-
and diethyl 2-fluoromalonate have been described [51], reac- ments, we found that further purification of the diester 3 after
tions involving nucleophilic aromatic substitution between fluo- the initial SNAr step was not necessary and decarboxylation of
romalonate systems [52] and appropriate aryl substrates have crude diester 3 gave 4a very efficiently. Consequently, in all
not been reported previously. Recently, various routes to fluo- analogous experiments (Table 1), crude product diesters of type
rooxindoles have been discussed involving enantioselective 3 were isolated and used without further purification, allowing
fluorination of appropriate oxindole substrates by electrophilic the ready synthesis of a range of arylfluoroacetic acid deriva-
fluorinating agents [53-62] or DAST [63] providing an indica- tives 4a–f (Table 1). Structures 4a–f were confirmed by NMR
tion of the importance of fluorooxindoles for medicinal chem- techniques and, in particular, a doublet located at −190 ppm
istry applications.
(2JHF = 50 Hz) in the 19F NMR spectra assigned to the CFH
resonances and the corresponding doublets observed at ~6 ppm
in the 1H NMR spectra, are diagnostic for the structures
Results and Discussion
Reactions of carbanions generated by the addition of sodium proposed.
hydride to a solution of diethyl 2-fluoromalonate (1) in DMF
with ortho-fluoronitrobenzene (2a) led to the efficient displace- A nitro group ortho to a fluorine atom on the aryl ring is neces-
ment of fluorine by a nucleophilic aromatic substitution process sary under the present conditions to achieve full conversion of
Scheme 1: SNAr reaction of 2-fluoronitrobenzene (2a) with diethyl 2-fluoromalonate (1).
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Beilstein J. Org. Chem. 2014, 10, 1213–1219.
Table 1: SNAr reactions using fluoromalonate derivatives.
Fluoronitroaryl 2
Fluoroarylacetic acid 4
Yield
62%
2a
2b
2c
2d
2e
2f
4a
4b
4c
4d
4e
4f
77%
83%
56%
60%
86%
the starting fluoroarene. In related experiments, we found that a Attempts to prepare 2-fluoro-2-(2,4-dinitrophenyl)acetic acid by
para-trifluoromethyl group is not sufficiently activating for an analogous process led to the isolation of a benzyl fluoride
reaction to occur whilst para-fluoronitrobenzene gave a com- derivative 5, after evaporation of toluene and purification by
plex mixture of unidentified products, most probably derived column chromatography in 61% yield. The two consecutive
from competing benzyne formation.
decarboxylation reactions reflect the greater stability of the
benzylic carbanion formed on loss of carbon dioxide from this
This efficient methodology complements reported processes for system (Scheme 2).
the synthesis of various biologically active 2-fluoro-2-phenyl-
acetic acids [64] which may be prepared using electrophilic With the series of 2-fluorophenylacetic acids in hand, we
fluorination of enolate esters [64-66], deoxofluorination [67-69] attempted the reduction of the nitro group in 4a using sodium
nucleophilic [70] and electrochemical fluorination [71,72] dithionite, adapting reaction conditions similar to those
strategies.
described in the literature for the synthesis of biologically active
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Beilstein J. Org. Chem. 2014, 10, 1213–1219.
was confirmed unambiguously by X-ray crystallography
(Figure 2) and all other methyl esters 6b–e were characterised
by comparison with appropriate NMR data obtained for 6a.
Scheme 2: Synthesis of benzyl fluoride derivative 5.
system MaxiPost [63]. However, very low isolated yields of the
cyclised product were obtained, presumably because of the high
solubility of the amino acid intermediate in the aqueous reac-
tion mixture and the well-established difficulty of direct amide
bond formation processes. Consequently, before carrying out
the nitro group reduction and amide forming cyclisation reac-
tions, the acids 4a–e were transformed to the corresponding
methyl esters 6a–e by stirring a mixture of the acid in
hydrochloric acid and methanol (Table 2). The structure of 6a
Table 2: Synthesis of methyl ester derivatives.
Figure 2: Molecular structure of methyl ester 6a.
However, corresponding attempted esterification of the salt 4f
with HCl in methanol gave 2-fluoromethyl-3-nitropyridine (7)
in 68% yield (Scheme 3) after purification of the crude material
by column chromatography and the structure was confirmed by
X-ray analysis (Figure 3). In this case competing decarboxyla-
tion, rather than esterification, reflects the greater stabilisation
of the carbanion system formed upon decarboxylation for this
system.
Fluoroacetic acid 4
Methyl ester 6
Yield
88%
4a
4b
4c
4d
4e
6a
6b
6c
6d
6e
98%
97%
65%
98%
Scheme 3: Synthesis of pyridyl fluoride 7.
Reductive cyclization of methyl esters 6a–e using sodium
dithionite provided fluorooxindoles 8a–e in acceptable yield
after isolation by column chromatography (Table 3). In the
1H NMR spectrum, the characteristic CHF doublet located at
5.7 ppm (2JHF = 51 Hz) for the fluorooxindole systems 8 are
0.9 ppm upfield from the corresponding CHF resonances of the
arylfluoroacetic esters 6a–e and, additionally, a broad NH
singlet was detected at 9.0 ppm. The chemical shift of the
doublet (−194.8 ppm) in the 19F NMR spectrum of fluorooxin-
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doles 8a–e is also observed 10 ppm upfield from the fluorine
resonance of the starting esters 6a–e.
Conclusion
Diethyl 2-fluoromalonate ester can be used as a highly effec-
tive fluorinated building block for the synthesis of various poly-
functional 2-fluoroacetic acid and 3-fluorooxindole systems.
Fluorooxindoles are relatively rare fluorinated heterocyclic
systems, even though several derivatives have useful biological
activity, and current literature syntheses only involve fluorin-
ation of appropriate hydroxy and oxindole substrates. The
strategy described here provides complementary building block
syntheses from readily available fluorinated starting materials,
further demonstrating the viability of using fluorinated dicar-
bonyl systems for the synthesis of more structurally sophisti-
cated fluorinated derivatives.
Figure 3: Molecular structure of 7.
Table 3: Synthesis of 3-fluorooxindoles.
Supporting Information
Supporting Information File 1
Experimental procedures.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-119-S1.pdf]
Methyl ester 6
Fluorooxindole 8
Yield
32%
Supporting Information File 2
NMR spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-119-S2.pdf]
Supporting Information File 3
X-ray crystallographic data.
6a
6b
6c
6d
6e
8a
8b
8c
8d
8e
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-119-S3.cif]
82%
57%
0%
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