Iodine(III)-Mediated Oxy-fluorination of Alkenyl Oximes: An Easy Path to Monofluoromethyl-Substituted Isoxazolines

Article abstract of DOI:10.1021/acs.orglett.5b01646

A highly regioselective intramolecular oxy-fluorination of alkenyl oximes was achieved. This new transformation represents an efficient method for the preparation of monofluoromethyl-substituted isoxazolines. The synthetic application of the oxy-fluorinat

Full text of DOI:10.1021/acs.orglett.5b01646

Letter
pubs.acs.org/OrgLett
Iodine(III)-Mediated Oxy-fluorination of Alkenyl Oximes: An Easy
Path to Monofluoromethyl-Substituted Isoxazolines
Weidong Kong,§,†,‡ Quanping Guo,§,‡ Zhaoqing Xu,*,‡ Guoqiang Wang,‡ Xianxing Jiang,*,†
and Rui Wang*,†,‡
†School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006, China
‡Key Laboratory of Preclinical Study for New Drugs of Gansu Province, School of Basic Medical Sciences, Lanzhou University,
Lanzhou 730000, China
S
* Supporting Information
ABSTRACT: A highly regioselective intramolecular oxy-fluorination of alkenyl oximes was achieved. This new transformation
represents an efficient method for the preparation of monofluoromethyl-substituted isoxazolines. The synthetic application of the
oxy-fluorination product was demonstrated by a one-step synthesis of monofluoromethyl-substituted β-hydroxyl ketone
derivatives.
esearch in the field of organofluorine chemistry has
increased dramatically in recent years due to the unique
omethyl-substituted isoxazolines. Difunctionalization of an
alkene through an oxy-fluorination reaction is the most
straightforward way to construct a vicinal oxygen fluorine
moiety.11 We envisioned that by using an intromoleculer oxy-
fluorination reaction, an alkenyl oxime might be cyclized to
form monofluoromethyl-substituted isoxazolines.12 However,
despite the significant attention received by the synthetic
community, there are still some limitations in alkene oxy-
fluorination reactions:13 (i) the oxy parts in previous studies
were mainly restricted to carbons connected to an −OH group,
such as simple alcohols and carbonate acids; (ii) electrophilic
fluorination reagents, such as Selectfluor (1-chloromethyl-4-
fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate))
or NFSI (N-fluorobenzenesulfonamide) are mostly used,
whereas the nucleophilc fluorine resources were seldom
studied.
R
electronic properties of fluorine and its ability to induce
conformational and electronic changes in organic molecules.1
In drug discovery, the introduction of fluorine for drug
candidate modification has become an important strategy.2
Compounds containing C−F bonds also play important roles
in material3 and agrochemical sciences.4 Therefore, it is not
surprising that studies focused on the synthesis of fluorine-
containing organic compounds are attracting more and more
attention. Recently, the introduction of monofluoromethyl
groups (−CH2F) into drug candidates had been utilized as an
important strategy in drug discovery. Indeed, −CH2F motifs
play important roles in many biologically active compounds,
such as afloqualone, fluticasone propionate, and the anesthetic
sevoflurane (Figure 1).5 Compared with the numerous reports
Hypervalent iodine complexes are well-known as versatile
oxidizing reagents for mediating organic reactions involving the
formation of new C−C and C−heteroatom bonds.14 Because of
their low toxicity, high stability, ease of handling, and now
commercial availability, they have served as useful synthetic
oxidants in organic synthesis and have become highly welcome
both in the chemical industry and academic laboratories. In the
last three decades, hypervalent iodine reagents have been
employed to mediate the difunctionalization of alkenes without
the use of metal catalysts.15 In 2000, Hara and Yoneda reported
the oxy-fluorination of unsaturated alcohols or carboxylic acids
by using difluoroiodotoluene (4-CH3-PhIF2) as the promoter,
and the yields were between 40% to 60%.13d However, the
preparation of ArIF2 usually requires relatively harsh conditions,
Figure 1. Representative bioactive molecules containing monofluor-
omethyl groups.
that exist on the preparation of trifluoromethyl6 and
difluoromethyl-containing7 molecules, research on the synthesis
of monofluoromethyl substituted molecules is very limited.8
The isoxazolines skeleton is found in several biologically
active agents and also plays a versatile role in organic synthesis.9
As part of our research on the synthesis of isoxazolines,10 we
recently became interested in the preparation of monofluor-
Received: June 5, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01646
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
such as the use of F2,16a XeF2,16b or toxic HgO and Cl2,13d
which restricted its further application. Very recently, Li17a and
Nevado17b described the intramolecular aminifluorination of
alkenes by employing iodine(III) as a mediator, respectively.
In this paper, we report the first example of hypervalent
iodine-promoted intromolecular oxy-fluorination of unsatu-
rated oximes. By this method, the biologically interesting
monofluoromethyl-substituted isoxazolines were obtained in
good yield through fluorocylization. In the reaction, benign
PIDP (PhI(OPiv)2) was used as the oxidant and the cost-
effective HF−Py (HF−pyridine) was employed as the F-source.
Furthermore, a synthetically useful building block, namely,
monofluoromethyl-substituted β-hydroxyl ketone derivatives,
can now be synthesized via a simple ring-opening reaction from
the isoxazoline product.
We initiated our study with oxime 1a as a substrate, Et3N−
3HF (10 equiv) as a F-source, and 1.2 equiv of PhIO as an
oxidant. The reaction was carried out in dry CH2Cl2 under an
argon atmosphere at room temperature. However, only 15% of
the desired oxy-fluorination product 2a was obtained (Table 1,
entry 1). Reaction temperature screening revealed that −20 °C
was the most efficient for this reaction (entries 2−5). In all of
the above-mentioned cases, some oxy-hydroxylation byproduct
2a′ was also detected in the system. Various fluorination
reagents were also tested, and we found that AgF or KF were
completely ineffective for the reaction (entries 6 and 7).
Fortunately, when HF−Py was used, no 2a′ was observed in
the reaction and the yield increased to 69% (entry 8).
Replacement of PhIO by other iodine(III) reagents, namely,
PIDA (PhI(OAc)2, entry 9) and PIFA (PhI(O2CCF3)2, entry
10), led to the formation of byproducts 2a″ and 2a‴,
respectively. We envisioned that a large substituent, with
weak nucleophility, on the iodine atom might be able to
suppress the generation of this byproduct. To our delight, the
bulky I(III) reagent, PIDP, promoted a clean transformation
without any side product, and the yield increased to 77% (entry
11). Finally, a solvent survey indicated that the mixed solvent
(CH2Cl2/tol = 1:1) was optimal, whereafter an 84% yield of 2a
was obtained (entries 12−20).18
With the optimal reaction conditions in hand, we next
investigated reaction generality (Scheme 1). Aromatic oximes
bearing an −CH3 or −OCH3 substituent at the o-, m-, or p-
a,b
Scheme 1. Investigation of Substrate Scope
a
Table 1. Optimization of Reaction Conditions
b
entry iodine(III)
F source
solvent
CH2Cl2
T (°C) yield (%)
1
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PIDA
PIFA
PIDP
PIDP
PIDP
PIDP
PIDP
PIDP
PIDP
PIDP
PIDP
PIDP
Et3N−3HF
Et3N−3HF
Et3N−3HF
Et3N−3HF
Et3N−3HF
AgF
rt
0
15
30
56
55
50
0
2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
tol.
3
−20
−40
−78
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
−20
4
5
6
7
KF
0
8
HF−Py
HF−Py
HF−Py
HF−Py
HF−Py
HF−Py
HF−Py
HF−Py
HF−Py
HF−Py
HF−Py
HF−Py
HF−Py
69
58
32
77
76
68
trace
71
72
trace
55
66
84
9
10
11
12
13
14
15
16
17
18
19
20
Et2O
THF
CHCl3
ClCH2CH2Cl
CH3CN
Cl-Ph
xylene
c
CH2Cl2/tol
a
a
All reactions were carried out by using 0.2 mmol of 1a, 2 mmol of F
source, 0.24 mmol of iodine(III), and 2 mL of solvent with stirring for
All reactions were carried out by using 0.4 mmol of 1, 10 equiv of
HF−Py, 1.2 equiv of PIDP, and 4 mL of solvent (CH2Cl2/toluene =
b
c
b
2 h. Isolated yields. The ratio of CH2Cl2 and toluene is 1:1.
1/1) at −20 °C for 2 h. Yields refer to isolated yields.
B
DOI: 10.1021/acs.orglett.5b01646
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
position on the aryl ring performed well under the standard
conditions (2b−f). Substrates with a 2-, 3-, or 4-chlorine-
substituted phenyl also reacted smoothly in good yield (2g−i).
When disubstituted aromatic oximes were employed, different
reactivities were observed. Oximes with a 3,4-dimethylphenyl
group or a 2,4-dichlorophenyl showed good results (2j and 2l),
whereas a 3,5-dimethoxylphenyl-substituted oxime only led to a
40% yield (2k). Substrates with other types of substituents,
such as p-F-phenyl, p-Br-phenyl, p-NO2-phenyl, p-CN-phenyl,
or 2-naphthalenyl, all worked well and gave the desired
products in good yields (2m−q). Under the stated conditions,
aliphatic oximes provided moderate conversions to the desired
products (2r, 42%). We were pleased to find that the present
method could be successfully applied to construct a
monofluoromethyl-substituted isoxazoline containing a quater-
nary carbon center (2s, 83%). Finally, an oxime with an internal
alkene was tested and gave the corresponding cyclization
product 2t in 86% isolated yield.
In conclusion, we have developed a highly regioselective
metal-free method for the intramolecular oxidative oxy-
fluorination of unactivated terminal alkenes, in which HF−Py
is employed as the fluorine source in the presence of PIDP.
This new transformation represents an efficient method for the
preparation of monofluoromethyl-substituted isoxazolines.
Moreover, the synthetic utility of the product was demon-
strated by a one-step synthesis of monofluoromethyl-
substituted β-hydroxyl ketone derivatives in good yield.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, characterization data, and NMR spectra.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b01646.
AUTHOR INFORMATION
Corresponding Authors
■
To further illustrate the synthetic utility of the oxy-
fluorination product, isoxazoline 2a was applied to a reductive
ring-opening reaction. The reaction proceeded smoothly and
gave the monofluoromethyl substituted β-hydroxyl ketone 3 in
84% yield (Scheme 2, eq 1). Moreover, the oxy-fluorination
reaction could be conducted on a 10 mmol scale without
affecting the reactivity of the process (eq 2).
*E-mail: zqxu@lzu.edu.cn.
*E-mail: xianxingjiang@163.com.
*E-mail: wangrui@lzu.edu.cn.
Author Contributions
§W.K. and Q.G. contributed equally to the work.
Notes
Scheme 2. Synthetic Utility of Current Method
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the grants from the NSFC (No. 21432003
and 21202072), the Fundamental Research Funds for the
Central Universities (Nos. 860976 and 861966), and Major
Scientific and Technological Project of Guangdong Province
(No. 2011A08030001)
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