Silver-promoted (radio)fluorination of unsaturated carbamates via a radical process

Article abstract of DOI:10.1039/c7cc01393k

The intramolecular fluorocyclization of unsaturated carbamates is described here using a hypervalent iodine reagent in the presence of a silver catalyst. Both (hetero)aryl-substituted olefins and acrylamides can be utilized as effective substrates. Preliminary mechanistic investigations suggest that the reaction proceeds via a cyclization/1,2-(hetero)aryl migration/fluorination cascade involving an unusual radical process. Furthermore, starting from no-carrier-added [¹⁸F]TBAF, a simple one-pot, two-step cascade method was developed for the generation of ¹⁸F-labeled heterocycles with high radiochemical purity.

Full text of DOI:10.1039/c7cc01393k

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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DOI: 10.1039/C7CC01393K
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Silver-Promoted (Radio)fluorination of Unsaturated Carbamates
via a Radical Process
Received 00th January 20xx,
Accepted 00th January 20xx
Bin Yanga‡, Kantapat Chansaenpakb,c‡, Hongmiao Wua, Lin Zhua, Mengzhe Wangb, Zibo Li*b and
Hongjian Lu*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Intramolecular fluorocyclization of unsaturated carbamates is the construction of aliphatic fluorine-containing bioactive
described here using a hypervalent iodine reagent in the presence compounds.1 Among the different methods that have been
of silver catalyst. Both (hetero)aryl-substituted olefins and developed, fluorocyclization using electrophilic fluorinating
acrylamides can be utilized as effective substrates. Preliminary reagents is one of most important approaches.5,6 However, it is
mechanistic investigations suggest the reaction proceeds via a costly and often involves the use of fluorine gas to prepare the
cyclization/1,2-(hetero)aryl
migration/fluorination
cascade electrophilic fluorinating reagents, which restricted its
involving an unusual radical process. Furthermore, starting from applications in industry and laboratory especially in 18F
no-carrier-added [18F]TBAF, a simple one-pot, two-step cascade radiolabeling.3,4 Additionally, electrophilic fluorination
method was developed to generate 18F-labeled heterocycles with reactions generally suffer from low specific activity, which
high radiochemical purity.
limits their applications in radiofluorination.2 Recently,
hypervalent iodine reagents have been used as very attractive
fluorine sources in fluorocyclization reactions,7 as they exhibit
unique reactivity and can often be prepared from easily
obtained, cheap fluorides. Although the reaction mechanism
has not been elaborated in detail, an ionic pathway has been
proposed to be involved in most of the reactions.8 Alkyl olefins
and styrene derivatives were utilized as efficient nucleophilic
reagents in these reactions, but fluorocyclization of the
substrates bearing electron-poor double bonds has not been
reported.7
The chemical, biological and physical properties of organic
molecules can be significantly changed by introduction of
fluorine atoms and thus organofluorine compounds are widely
used in almost all aspects of chemical industry from
pharmaceutical, medical, agrochemical, to material sectors.1
Additionally, 18F-labeled organic compounds enjoy broad use
as positron emission tomography (PET) agents in diagnostic
medicine and clinical pharmaceutical research.2 Therefore,
there is a significant need for the development of novel
synthetic strategies providing easy access to complex
fluorinated compounds.3 In addition, due to the short half-life
of 18F, a late stage fluorination strategy would greatly simplify
the synthesis of complex radiotracers from radiolabeling point
of view.4
In this paper, we report a silver-promoted cyclization/1,2-
(hetero)aryl migration/fluorination cascade using (hetero)aryl-
substituted olefins and acrylamides as substrates and
proceeding through
a
novel radical mechanism.9 This
methodology provides a straightforward approach to the
preparation of various fluorinated or radiofluorinated
oxazolidin-2-ones, oxazolidin-2,4-diones and 1,3-oxazinan-2-
one under mild conditions.
The intramolecular fluorocyclization of alkenes is an
attractive transformation that constructs multiple bonds
including a C−F bond in one step to form fluorinated cyclic
structures,5 which may serve as versatile building blocks for
Our initial test focused on using the air and moisture-stable
10-12
fluoroiodine compound
Compound
of ionic fluorocyclization reactions of styrene and alkyl olefin
1 as the fluorine source (Table 1).
1
has recently been exploited for the development
a.Institute of Chemistry & BioMedical Sciences, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093, China.
Email: hongjianlu@nju.edu.cn
derivatives.11 Preparation of
1 involves fluoride and its
b.Radiology and Biomedical Research Imaging Center, University of North Carolina
at Chapel Hill, Chapel Hill, NC 27599, USA.
chloro(OTs) analogue10a,b making it potentially applicable for
no-carrier-added 18F-radiofluorination. As shown in Table 1,
Email: ziboli@med.unc.edu
c. National Nanotechnology Center (NANOTEC), National Science and Technology
Development Agency (NSTDA), 111 Thailand Science Park, Pathum Thani,
Thailand 12120.
starting from unsaturated carbamates (2), fluorinated
oxazolidin-2-ones could be obtained, whose non-fluorinated
analogues demonstrate very important bioactivities on
treating bacterial infection.13,14 For example, N-aryl-oxazolidin-
2-ones are considered as a last resort treatment against gram-
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
‡ These two authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 20xx
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Table 1. Selected Results of Conditions Optimization
Table 2. Substrate Scope of (Hetero)Aryl-Substituted Olefinsa
View Article Online
DOI: 10.1039/C7CC01393K
entry
1
variation from the standard conditions
none
Yield [%]a
90 (79b)
2
3
4
5
6
7
8
9
12 h instead of 3.5 h
AgBF4 instead of AgSbF6
AgOTf instead of AgSbF6
AgF2 instead of AgSbF6
[Cu] or [Fe] instead of AgSbF6
10 μL H2O was added
AgOTf instead of AgSbF6c
4Å MS was added
89 (74)
84
42
trace
<30
<10
-
-
10
11
without AgSbF6
1.0 equiv of AgSbF6
-
18
a
b
1H NMR yield with dibromomethane as internal standard, PMP = 4-OMeC6H4.
Isolated yield. c With additional KF (0.5 equiv).
positive bacterial pathogens which are resistant to other
antibiotics, including methicillins and vancomycins. After
screening different conditions (Table S1 in the Supporting
Information (SI)), the styrene-derived carbamate (2a) was
found to partake in cyclization in combination with AgSbF6 as
the catalyst (entry 1, Table 1), providing fluorinated N-aryl-
oxazolidin-2-one (3a) in 79% isolated yield. Other metal
a
The conditions： 2a (0.2 mmol), 1 (0.3 mmol), AgSbF6 (0.02 mmol) and DCM (2
mL), 55 °C , 3.5 h, isolated yield, dr was determined from 19F NMR spectrum.
3
3
b
equiv 1 for 12 h. c 5 h. d 12 h, dr = 1.3/1. e R1 = C6H5. f 3 equiv 1 at 35 °C for 5 h. g
equiv 1 at 0 °C for 24 h. h dr = 2.3/1. i dr = 1.5/1. j 18 h, dr = 1.4/1. k 12 h. l 7 h.
catalysts, such as AgBF4, AgOTf and AgF2 provided reduced ring formation, the 6-membered cyclic product (3y), a key
yields (entries 3-6) and small amount of water reduced the
reactivity dramatically (entry 7). When additional fluoride
anion was added, compound 3a was not observed in present
of AgOTf (entry 8). No desired product 3a was formed in
absence of Ag catalysts (entries 9-10), even in the presence of
molecular sieves.11 The yield was reduced when one
equivalent of AgSbF6 was used, due to the elimination reaction
of product 3a in the presence of AgSbF6 (entry 11 and Scheme
S1 in SI).
Using the optimal conditions, the scope of substrates was
investigated (Table 2). We first examined the reagents with
different substituted phenyl groups on the nitrogen (3a-f),
both electron-donating and -withdrawing phenyl substituents
were well tolerated, leading to moderate to good yields of N-
aryl-oxazolidin-2-ones. Functional groups on phenyl group,
structure found in novel antiretroviral and antibacterial
drugs,15 could also be generated smoothly by this method.
In order to reveal some details of the catalytic mechanism,
the side products in the reaction of
identified. In addition to the product 2-(2-iodophenyl)propan-
2-ol (4, 85% yield), 1-(2-iodophenyl)ethanone ( , 8% NMR yield)
was observed (Scheme S2 in SI). In addition, the deuterated
compound (Z)- 2a was subjected to the reaction conditions
(Scheme 1A), and the resulting compound 3a was observed
1 and 2a were isolated and
5
D
-
D
-
with a diastereomeric ratio of 1:1 and no H-D scrambling.
Along with the low diastereoselectivity achieved in the
reactions of substrates 2u-2w (Scheme 3), these results are
interesting and contrast with the known hypervalent
iodine(III)-mediated ionic fluorinations in which good to high
diastereoselectivity can be generally obtained,5,11 suggesting
such as halogens (3c, e), ester (3d) and methoxyl groups (3a, f
)
were compatible with these reaction conditions. Additionally,
alkyl substituted agents (3g-i) performed well in this process
giving 66-78% yields. The nature of the aromatic migrating
group (3j-t) was also investigated. Phenyl groups substituted
with both electron-donating and -withdrawing groups were
found to migrate efficiently (3j-p). Although 9H-carbazole was
found to be a poor migration group (3t), other heteroaryl
groups, such as thienyl and furyl groups (3q-s), were used for
the first time as efficient migrating groups in fluorocyclization
induced by hypervalent iodine reagents.11 The substituted
group R2 did not affect the efficiency of the reaction (3u-v).
The internal olefin (Z)-2w could also be used as a substrate,
providing the desired product (3w) in moderate yield. With an
alkyl olefin as the substrate, fluorinated oxazolidin-2-one (3x
)
was formed without migration. In addition to the 5-membered
Scheme 1. Radical Experiments
2 |Chem. Commun., 2017, 00, 1-3
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reactions; the reaction must take place rapidly due to the 110
View Article Online
18
min half-life time of the F-radioisotoDpOeI;: 1a0n.1d039th/Ce7CcCo0n1c3e93rnK
associated with radiation exposure.4c,d Encouraged by our
promising fluorination results,18 we explored radiolabeling of
alkene precursors (2a-c) by means of a one-pot synthesis to
minimize the radiosynthetic reaction time (See Scheme S3 in
Scheme 2. Proposed Radical Mechanisms of Aryl Transfer Step
that this reaction may proceed through an unknown SI). Unfortunately, the [18F]-fluorine-containing heterocyclic
mechanism.
products could not be successfully produced by this protocol.
To study the mechanism further, several radical This could be the result of AgCl formation due to the reaction
experimentals were performed. When the radical trapping between chloroiodane (10) and AgOTf which impedes the
reagent 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was radio-fluorination process.
added to the reaction, it resulted in a decrease in the yield of
3a and the formation of the radical trapping product, 1- pot labeling of the alkene precursor (2a-c) and the synthetic
methoxy-2,2,6,6-tetramethylpiperidine which was parameters were varied in a search for a suitable condition for
To solve this problem, we decided to attempt two-step one-
(6)
identified by GCMS (Scheme 1B), suggesting that a methyl radiofluorination (Table S3). As the radiosynthetic method
radical may be generated during the catalytic process. Upon consists of two radiochemical steps, 18F-Cl exchange and 18F-
addition of 2,6-di-tert-butyl-4-methylphenol (BHT), the radical fluorination, we first varied the reaction temperatures in both
trapping product (
When compound 2z was prepared and subjected to the effect on the radiochemical yield (RCY), while the 18F-Cl
reaction conditions, the ring opening products and were exchange temperature has a minimal impact (entries 1-3). The
identified as reaction products (Scheme 1D). These two results 18F-Cl exchange temperature and 18F-fluorination temperature
strongly indicate that a methylene radical (Scheme 2) in the were therefore set at 60 °C and 80 °C respectively. The next
5-position of oxazolidin-2-one may be formed in this catalytic parameters that have a significant effect are the concentration
process. The radical rearrangement of may take place, of the reactants, the solvent, and the silver salt. In the case of
overcoming
low barrier,16 and forming the radical the concentration and the solvent, the radioactive heterocyclic
intermediate (III) via a three-membered ring cyclic structure compound [18F]-3a could be prepared successfully only at a
II). Then the fluorine atom transfers from Ag(II)−F to the high concentration (0.24 μM) of 2a in acetonitrile solution. We
methylene radical (III) to form the desired product (
).9,17 An failed to observe any radio signals of the product when either
7
) was isolated in 45% yield (Scheme 1C). steps. We found that the 18F-fluorination temperature has an
8
9
I
I
a
(
3
alternative explanation, involving Ag(II) oxidizes the alkyl a low concentration or the DMSO/DMF solvent systems were
radical (III) to a carbocation which is subsequently captured by used in the radiofluorination reaction (entries 5-7). In addition,
a fluoride anion, cannot be excluded.
we observed that the Ag salt is required in this reaction and
Benefitting from the novel radical mechanisms, this reaction the type of silver salt has an impact on the RCY of the reaction
may be tolerated by other olefins in place of activated styrene (entries 9, 11, 12, and 15). AgSbF6 yielded the highest RCY.
derivatives. Finally, we found that phenyl substituted However, the radiofluorination failed when the amount of
acrylamide was compatible with this catalytic system affording AgSbF6 was diminished to 0.5 or 0.05 equivalents (entries 13-
the desired product under modified reaction conditions (Table 14). With using AgOTf instead of AgSbF6 as catalyst to avoid
S2 in SI). A brief investigation of acrylamides (2′) was adding additional 19F fluoride, the specific activity of [18F]-3a
conducted, producing the results shown in Scheme 3. Similar was determined to be 34.2±2.1 GBq/µmol (entry 2). Using the
to styrene derivatives, phenyl rings with electron-withdrawing conditions, we prepared various 18F-oxazolidin-2-ones with
or -donating groups and heteroaryl rings are good migration high radiochemical purity and the radiochemical yields (RCY)
groups, all providing the desired oxazolidine-2,4-diones (3′) in are shown in Scheme 4.
acceptable yields. This is particularly interesting because
To confirm that our synthetic 18F-oxazolidin-2-ones are
fluorocyclization mediated by hypervalent iodine reagents stable, we evaluated the stability of [18F]-3a in vitro and in vivo
using acrylamides as nucleophilic reagents has not been
reported previously.7
Although different fluorination techniques have been
developed, the successful translation of these approaches to
practical radiofluorination syntheses remains a challenge.3,4
The unique obstacles for radiofluorination include the fact that
only trace amounts of [18F]fluoride are available for synthetic
Scheme 4. Radiosynthesis of [18F]-3 (n=3). aDecay corrected RCY is
calculated by dividing the activity of isolated product with the total
amount of 18F-activity used in each reaction (decay corrected).
Scheme 3. Substrate Scope of Acrylamides 2′
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D. Gee, Angew. Chem., Int. Ed., 2008, 47, 8998; (d) L. Cai, S.
and found that [18F]-3a is extremely stable in aqueous solution
displaying negligible decomposition up to 3 h after incubation
in phosphate buffer solution at pH 7.5 (Figure S1, SI). In term
of in vivo stability, we observed no bone uptake signals due to
tracer defluorination in the small animal PET/CT images of
nude mice obtained at 1 h after the injection of [18F]-3a (Figure
S2, SI). The observed C−F bond stability supports its application
in biomedical research in vivo.19
In summary, we have developed an efficient synthesis of
fluorinated oxazolidin-2-ones, oxazolidine-2,4-diones and 1,3-
oxazinan-2-ones via an intramolecular fluorocyclization
strategy. The transformation exhibits broad substrate scope
with tolerance of diverse functional groups, and provides
versatile building blocks for the construction of related
bioactive molecules. Preliminary mechanistic studies suggest
that the unique reactivity and selectivity profile of this
synthetic reaction can be attributed to the involvement of an
unusual radical-mediated pathway. Additionally, the modified
one-pot, two-step process using no-carrier-added 18F-
nucleophilic agents can be used to construct multiple bonds
and generate 18F-labeled heterocycles rapidly, and this enables
the synthesis of previously inaccessible PET radiotracers for
use in biomedical research. Further mechanistic research and
applications in PET is in progress in our group.
View Article Online
Lu, V. W. Pike, Eur. J. Org. Chem., 200D8,OI: 10.1039/C7CC01393K
2008, 2853.
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We gratefully acknowledge National Natural Science
Foundation of China (21472085, 21332005), Qin Lan Project,
UNC LCCC pilot grant and UNC Radiology Department and
Biomedical Research Imaging Center.
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4 |Chem. Commun., 2017, 00, 1-3
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