A highly regio- and stereoselective synthesis of ?±-fluorinated imides via fluorination of chiral enamides

Article abstract of DOI:10.1021/ol503591d

A highly ??-facial selective and regioselective fluorination of chiral enamides is described. The reaction involves an enantioselective fluorination exclusively at the electron-rich enamide olefin with N-F reagents such as Selectfluor and N-fluoro-benzenesulfonimide [NFSI] accompanied by trapping of the ?2-fluoro-iminium cationic intermediate with water. The resulting N,O-hemiacetal could be oxidized using Dess-Martin periodinane, leading to an asymmetric sequence for syntheses of chiral ?±-fluoro-imides and optically enriched ?±-fluoro-ketones.

Full text of DOI:10.1021/ol503591d

Letter
pubs.acs.org/OrgLett
A Highly Regio- and Stereoselective Synthesis of α‑Fluorinated
Imides via Fluorination of Chiral Enamides
Yan-Shuang Xu,† Yu Tang,*,† He-Jing Feng,† Ji-Tian Liu,† and Richard P. Hsung*,‡
†School of Pharmaceutical Science and Technology, Key Laboratory for Modern Drug Delivery and High-Efficiency, Collaborative
Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin, 300072, P. R. China
‡Division of Pharmaceutical Sciences, School of Pharmacy, and Department of Chemistry, University of Wisconsin, Madison,
Wisconsin 53705, United States
S
* Supporting Information
ABSTRACT: A highly π-facial selective and regioselective
fluorination of chiral enamides is described. The reaction
involves an enantioselective fluorination exclusively at the
electron-rich enamide olefin with N−F reagents such as
Selectfluor and N-fluoro-benzenesulfonimide [NFSI] accom-
panied by trapping of the β-fluoro-iminium cationic inter-
mediate with water. The resulting N,O-hemiacetal could be
oxidized using Dess-Martin periodinane, leading to an
asymmetric sequence for syntheses of chiral α-fluoro-imides
and optically enriched α-fluoro-ketones.
Scheme 1. Stereoselective Synthesis of α-Fluoro-imides
he importance of organo-fluorine compounds has been
Tabundantly validated through a broad range of applications
in medicinal chemistry and drug development1 as well as
material2 and agrochemical sciences.3 The incorporation of
fluorine and/or fluorine-containing groups into an organic
compound has often provided agents and materials with unique
chemical, physical, and biological properties.4 One of the major
synthetic challenges in fluoro-organic chemistry is to asymmetri-
cally construct fluorinated stereogenic carbon centers. Differd-
ing5 demonstrated the first stoichiometric asymmetric fluorina-
tion of β-ketoester enolates with a chiral N−F (N-fluoroamine)
reagent in 1988. Given the number of impressive examples of
enantioselective fluorinations being reported over the past
decade,6 asymmetric fluorinations represent an area of immense
interest from the synthetic community. In relevance to our work,
Davis7 reported an elegant synthesis of α-fluoro-imides using
enolate derived from chiral imide 1 substituted with chiral
oxazolidinone auxiliary8 (1 → 2 in Scheme 1). Our longstanding
interest in the chemistry of enamides has drawn us to develop
stereoselective fluorination methods using chiral enamides,9
which has remained elusive. Such an approach in combination
with an oxidative process (3 → 4) would represent a
complementary approach to Davis’ asymmetric approach to α-
fluoro-imides.
epoxidation,15 and halo-etherification.16 We wish to report
here a highly regio- and stereoselective fluorination of chiral
enamides for the synthesis of chiral α-fluoro-imides and optically
enriched α-fluoro-ketones.
Our efforts commenced with exploring the right fluorination
conditions using chiral enamide 3a as the testing enamide, and
with Selectflur,17 N-fluoropyridinium triflate [Py-F],18 and N-
fluoro-benzenesulfonimide [NFSI]19 as the fluorinating agent.
When reactions were carried out in CH3CN at 40 °C, we found
that it was faster with Selectflur (entry 1 in Table 1) and much
slower with Py-F (entry 2), and that NFSI was the best N−F
reagent20 in terms of diastereoselectivity, leading to α-fluoro-
imides 4a and 4a′ as an isomeric mixture in 57% yield after DMP
oxidation (entry 3).21 We note here that attempts to directly
With the nitrogen atom being substituted with an electron-
withdrawing functionality, chiral enamides possess superior
stability to their enamine counterparts, while maintaining
excellent reactivities. This is evident by their recent emergence
as another powerful chiral building block in organic syn-
thesis.10,11 In particular, chiral enamides have been employed
in highly regio- and/or stereoselective Diels−Alder cyclo-
additions,12 [2 + 2] cycloadditions,13 cyclopropanation,14
Received: December 13, 2014
Published: January 12, 2015
© 2015 American Chemical Society
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DOI: 10.1021/ol503591d
Org. Lett. 2015, 17, 572−575

Organic Letters
Letter
a
fluorinations of 3d and ent-3d with NFSI at 40 °C in 2% H2O in
CH3CN led to 4d and ent-4d, respectively, in good yields and
essentially as a single diastereomer (entries 3 and 4). X-ray
structures of α-fluoro-imide 4b and ent-4d allowed for
unambiguous assignment of both stereochemical and structural
integrity (Figure 1).
Table 1. Optimization of the Fluorination Conditions
b
time
[h]
yield [%]
[dr]
a
c
entry
solvent
reagent
1
2
3
4
5
6
7
8
CH3CN
CH3CN
CH3CN
Selectfluor
Py-F
0.5
24
9.5
8
29 [57:43]
26 [61:39]
57 [89:11]
<10 [85:15]
43 [89:11]
62 [91:9]
NFSI
NFSI
NFSI
NFSI
NFSI
NFSI
NFSI
d
anhyd CH3CN
2.0 equiv H2O in CH3CN
1% H2O in CH3CN
2% H2O in CH3CN
5% H2O in CH3CN
2% H2O in CH3CN
8
12
12
31
6
68 [91:9]
29 [91:9]
e
9
22 [71:29]
a
In all reactions, 1.1 equiv of N−F reagent was added. Unless
otherwise noted, reactions were run at 40 °C. DMP: Dess-Marin
Periodinane; NSFI: N-fluoro-benzenesulfonimide; Py-F: N-fluoro-
Figure 1. X-ray structures of α-fluoro-imides 4b and ent-4d.
b
pyridinium triflate. Yields were determined by 1H NMR analysis
c
Success in finding chiral amides that can provide a high level of
diastereoselectivity allowed us to broaden the scope of this
fluorination significantly as shown in Table 3. For all examples in
using mesitylene as the internal standard. Diastereomeric ratios [dr]
d
were determined using H or/and 19F NMR spectroscopy. CH3CN
1
e
distilled over CaH2. The reaction temperature is 80 °C.
Table 3. Stereoselective Fluorination of Chiral Enamides
work up the crude fluorination mixture without any oxidation
protocols failed to give us the desired N,O-hemiacetal products,
or the corresponding α-fluoro-aldehydes via hydrolysis.
Most notably, H2O plays a significant role in this reaction. The
use of anhydrous CH3CN did afforded comparable diaster-
eoselectivity, but the yield is very low (entry 4). The addition of
2.0 equiv, 1%, 2%, and 5% H2O in the CH3CN appeared to
improve the efficiency (entries 5−8) with 2% H2O being the
most optimal (entry 7). On the other hand, when we increased
the temperature from 40 to 80 °C, neither the dr ratio nor the
yield was satisfactory.
To improve the diastereoselectivity, we examined a range of
chiral auxiliaries as shown in Table 2. Enamide 3b with the i-Pr-
substituted Evans chiral auxiliary22 gave α-fluoro-imide 4b in
58% yield with a 93:7 dr ratio. Fluorination of chiral enamide 3c
substituted with the Sibi auxiliary23 afforded 4c in 51% yield
(entry 2). When using the Ph-substituted Evans auxiliary,
a
b
entry
α-fluoro-imides: R =
time [h]
yield [%] [dr]
1
2
3
4
5
6
7
8
4-MeC6H4 (6a)
4-MeOC6H4 (6b)
4-NO2C6H4 (6c)
4-FC6H4 (6d)
10
12
140
12
12
14
12
11
67 [94:6]
42 [92:8]
71 [>95:5]
61 [>95:5]
65 [>95:5]
69 [95:5]
59 [>95:5]
53 [>95:5]
4-ClC6H4 (6e)
4-BrC6H4 (6f)
2-ClC6H4 (6g)
n-Pent (6h)
a
b
Isolated yields. The diastereomeric ratio [dr] was determined by 1H
or/and 19F NMR spectroscopy.
a
Table 2. Effect of the Chiral Auxiliary on Selectivity
which a variety of different R substituents were evaluated,
excellent diastereoselectivities of >92:8 and moderate-to-good
yields were obtained. It is noteworthy that while electron-
donating and -withdrawing substitution groups at the aryl does
not influence diastereoselectivity, with a strong electron-
withdrawing group, reactions indeed took a much longer time
(see entry 3). Lastly, although predictable, these asymmetric
fluorinations can indeed be highly regioselective in favor of the
electron-rich enamide-olefin as demonstrated with fluorinations
of chiral enamides 7 and 9 (Scheme 2).
While synthetically this method represents a stereoselective
approach for constructing chiral α-fluoro-imides, mechanisti-
cally, this fluorination provides some insight into the chemistry of
chiral enamides. On the basis of the observed stereochemical
outcome, a proposed mechanistic model is shown in Scheme 3.
We had calculated that the most stable conformation of these
chiral enamides has the chiral oxazolidinone ring essentially
coplanar with the alkene to maximize the delocalization of the
nitrogen lone pair into the olefin.24 With this conformational
preference, the chiral oxazolidinone auxiliary plays a distinct role
a
Reaction condition: 1.1 equiv of NFSI, 2% H2O in CH3CN, 40 °C;
b
c
and then, DMP, NaHCO3, CH2Cl2, rt. Isolated yields. Diastereo-
meric ratios [dr] were determined by 1H or/and 19F NMR
spectroscopy.
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DOI: 10.1021/ol503591d
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Organic Letters
Letter
N,O-hemiacetal intermediate from fluorinations of disubstituted
enamides.
Scheme 2. A Regioselective Fluorination
We have reported an asymmetric synthesis of α-fluoro-imides
via fluorinations of chiral enamides. The reaction involves
selectively fluorinating the electron-rich enamide olefin using
Selectfluor and N-fluoro-benzenesulfonimide [NFSI] followed
by trapping of a β-fluoro-iminium cationic intermediate with
water and oxidation of the resulting N,O-hemiacetal. From a
practical perspective, the reaction is operationally simple,
requires inexpensive reagents and mild conditions, and provides
chiral α-fluoro-imides and optically enriched α-fluoro-ketones in
good yields and high selectivities.
ASSOCIATED CONTENT
* Supporting Information
■
Scheme 3. A Proposed Stereochemical Model
S
Experimental procedures as well as NMR spectra, character-
izations, and X-ray structural files (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: yutang@tju.edu.cn.
*E-mail: rhsung@wisc.edu.
Notes
The authors declare no competing financial interest.
in providing a key facial bias for the fluorine to approach from the
Si-face as represented in ent-3d. Subsequent trapping of the N-
acyl iminium ion A with water can in principle be stereoselective,
but this stereochemical information is lost in the ensuing
oxidation.
On the other hand, for Z-enamides such as 3e, the predicted
selectivity would be poor based on this conformational model.
To alleviate the allylic strain shown in 3e, the chiral
oxazolidinone ring needs to rotate away and can no longer be
coplanar with the alkene, thereby leading much less differentiated
π-faces [see C]. To support this model, we prepared Z-enamide
3e and found that not only is the yield of its fluorination inferior
to those of E-enamides but also the diastereoselectivity is
diminished significantly.
ACKNOWLEDGMENTS
■
R.P.H. thanks the NIH (GM66055) for funding. Y.T. thanks the
National Natural Science Foundation of China (Nos. 21172169
and 21172168) and The National Basic Research Project (No.
2014CB932201) for generous funding. We also thank Dr. Hai-
Bin Song of the Nankai University for providing X-ray structure
and data analysis.
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