Stereoselective Oxidative Cyclization ofN-Allyl Benzamides to Oxaz(ol)ines

Article abstract of DOI:10.1021/acs.orglett.1c01607

This study presents an enantioselective oxidative cyclization ofN-allyl carboxamides via a chiral triazole-substituted iodoarene catalyst. The method allows the synthesis of highly enantioenriched oxazolines and oxazines, with yields of up to 94% and enantioselectivities of up to 98% ee. Quaternary stereocenters can be constructed and, besidesN-allyl amides, the corresponding thioamides and imideamides are well tolerated as substrates, giving rise to a plethora of chiral 5-memberedN-heterocycles. By applying a multitude of further functionalizations, we finally demonstrate the high value of the observed chiral heterocycles as strategic intermediates for the synthesis of other enantioenriched target structures.

Full text of DOI:10.1021/acs.orglett.1c01607

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Letter
Stereoselective Oxidative Cyclization of N‑Allyl Benzamides to
Oxaz(ol)ines
Ayham H. Abazid, Tom-Niklas Hollwedel, and Boris J. Nachtsheim*
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ABSTRACT: This study presents an enantioselective oxidative cyclization of N-allyl
carboxamides via a chiral triazole-substituted iodoarene catalyst. The method allows
the synthesis of highly enantioenriched oxazolines and oxazines, with yields of up to
94% and enantioselectivities of up to 98% ee. Quaternary stereocenters can be
constructed and, besides N-allyl amides, the corresponding thioamides and
imideamides are well tolerated as substrates, giving rise to a plethora of chiral 5-
membered N-heterocycles. By applying a multitude of further functionalizations, we finally demonstrate the high value of the
observed chiral heterocycles as strategic intermediates for the synthesis of other enantioenriched target structures.
Scheme 1. Known Approaches for the Synthesis of 5-
Oxazolines
artially hydrogenated N-heterocycles (azolines), in partic-
ular 2-oxazolines, 2-thiazolines, and 2-imidazolines, are
P
important synthetic targets, not only due to their abundance in
biologically active compounds1,2 but also due to their high
value as useful synthetic building blocks.3−5 Enantiopure
derivatives substituted at C4, at C5, or at both saturated
carbons are of particular importance due to the stereochemical
requirements of the desired products or the chiral building
blocks made by them. Enantiopure 4-oxazolines are also widely
applied as core structural motifs, for example in chiral ligands
for enantioselective transition-metal-catalyzed reactions.6
Enantiopure 5-oxazolines are found in biologically active
compounds such as shahidinethe parent compound of the
strong antioxidant aegilin,7 nagelamide alkaloids,8 and the
tubulin-binder A289099 (Figure 1).2 While synthetic ap-
proaches for 4- and 4,5-disubstituted chiral 2-azolines are well
established,4,5,9 the synthesis of enantiopure monosubstituted
5-azolines is underdeveloped (Scheme 1).10,11 The latter can
be synthesized by Ru-catalyzed hydrogenations of oxazoles
(Scheme 1a)12 or by Pd-catalyzed cyclizations of allene-
substituted aryl amides (Scheme 1b).13 Organocatalytic
approaches have also been established whereby a hydrogen-
bonding donor in the presence of a halonium source results in
enantioselective halocyclizations of N-allyl amides (NAAs,
Scheme 1c).14
A more general approach for the cyclization of NAAs
involves their treatment with a chiral hypervalent iodine
compound. This induces an oxidative cyclization via a π-
complexed iodane A, similar to halonium sources. Iodine(III)
activates π-complexed NAA to produce B through an
intramolecular attack by the amide oxygen (Scheme 2). The
Received: May 11, 2021
Published: June 17, 2021
Figure 1. Examples of natural products containing a chiral C5-
substituted oxazoline unit.
© 2021 The Authors. Published by
American Chemical Society
https://doi.org/10.1021/acs.orglett.1c01607
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Scheme 2. General Mechanism for an Iodane-Mediated N-
Table 1. Optimization of the Reaction Conditions
Oxidative Cyclization of NAAs
a
entry
catalyst
yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
1a
1b
1c
1d
1e
1f
1g
1h
1i
67
61
62
72
71
75
83
81
91
68
80
65
48
85
74
64
67
74
87
70
85
93
84
84
93
92
1j
1k
1i
b
c
1i
a
Reaction conditions: 2a (0.40 mmol, 1.00 equiv), cat (0.04 mmol, 10
mol %), TFA (0.60 mmol, 1.50 equiv), selectfluor (0.40 mmol, 1.00
emerging alkyl-substituted iodane B then reacts rapidly with
additives, usually added Brønsted acids HX, to form compound
C under reduction of the iodane to the iodoarene D.
Depending on the nature of C, rapid substitution often follows
with external nucleophiles NuH, for example water or
hydroxide anions, to generate the substituted oxazoline E.
The chiral aryl iodide can then be reoxidized by an external
oxidant and thus can be used in catalytic amounts.15−17 This
general method has been applied by Moran and co-workers
using a well-established resorcinol-based chiral iodoarene
catalyst,18−20 but unfortunately with only moderate results,
particularly regarding stereoselectivity and substrate scope,
with a focus on 6-membered N-heterocycles.11
Our group recently established chiral triazole-substituted
iodoarenes 1, where the triazole has a role as a direct stabilizing
donor of the hypervalent iodine center through dative N−I
interaction, and applied them in a wide range of
enantioselective oxidative transformations (Figure 2).16,17,21
In this letter we report their further application to the as-yet
underexplored oxidative cyclization of N-allyl amides.
b
c
equiv), CH3CN (0.12 M). 5 mol % of 1i was used. The reaction was
performed at 0 °C and took 36 h with 70% conversion of 2a.
a co-oxidant and TFA as an acid additive. While using catalyst
1a, the 5-substituted oxazoline 3a was isolated with a 67% yield
and 85% ee. Under these conditions, a fluorinated intermediate
was not detected via MS. However, formation of the
corresponding methyl 2,2,2-trifluoroacetate derivative of C
(Scheme 2, X = OTFA) is likely since the reaction must be
quenched with aq. NaOH, to give 3a as the final reaction
product.
We then systematically investigated the influence of both
ether substituents (R1 and R2) in catalysts 1 on the reaction
performance. Switching the TIPS group to other alkyl groups,
such as catalysts 1b−1e, was found to be counterproductive
and resulted in diminished enantioselectivities (see entries 2−
5). We also varied the alkyl ether at R1 and decided to
introduce greater sterical bulk at this position. With R2 being
TIPS again, replacement of the methyl with an ethyl ether for
R1 (catalyst 1f) resulted in a small but less than significant
improvement of the enantioselectivity.
Introduction of an isopropyl group (catalyst 1i) resulted in
the most significant improvements, as 3a was now isolated with
a 91% yield and 93% ee. By contrast, the corresponding use of
benzyl ether (1j) or benzoyl ester (1k) was less effective (see
entries 10 and 11).
Decreasing the catalyst loading to 5 mol % had a negative
effect on the yield. Reduction of the reaction time or
performing the reaction at 0 °C did not improve the
enantioselectivity but only led to lower reaction rates and
incomplete conversions (see entries 12 and 13). Increasing the
amount of the additive TFA had no effect on the yield of
compound 3a.
With the optimized conditions in hand, we elaborated the
substrate scope of the oxidative cyclization (Scheme 3). The
NAA derivatives 2b−f, with additional substituents at the 2-
aryl group, provided the corresponding oxazolines 3b−f in
yields and enantioselectivities comparable to those of the
parent compound 3a (84−95%), regardless of the electronic
nature of the substituent. Comparison of the rotary power of
the 4-nitro derivative 3f with a known literature value allowed
Figure 2. Structure of the chiral triazole catalysts 1.
We started our investigations using N-allyl benzamide, 2a, as
the model substrate (Table 1). It was established in our early
studies that an additional substituent ortho to the central
iodine atom is crucial for high reactivity and stereoselectivity in
reactions performed with these catalysts. Therefore, we started
with the o-OMe-substituted iodoarene catalyst 1a, the most
successful to be used to date. During a preliminary
optimization we had already established acetonitrile as the
best solvent for this reaction, in combination with selectfluor as
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a
Scheme 3. Substrate Scope
Scheme 4. Derivatization of Oxazoline 5a
us to determine the absolute configuration of the products to
be (S).22 Cyclic aliphatic substituents (R = cyclohexyl) were
tolerated as well, giving 3g with an 84% yield and 93% ee.
Next, we wondered whether quaternary stereocenters could
be constructed.23 We therefore decided to apply our method
for the synthesis of quaternary 2-oxazolines starting from
various N-(but-3-en-2-yl) benzamides 2h−2k (R1 = Me, R2 =
H). The desired products 3h−3k could be observed in high
yields of up to 94% and excellent enantioselectivities (91−98%
ee). The enantioenriched thiazoline 3l and the imidazoline 3m
were prepared for the first time by this method, although yields
and enantioselectivities were significantly lower in direct
comparison with their oxa derivatives (71% ee and 74% ee).
It is nonetheless worth mentioning that these N-heterocycles
are very important core structural motifs found in many
biologically active compounds and this method provides a to
date undescribed means of accessing them.24
a
Reaction conditions: (a) I2 (1.20 equiv), PPh3 (1.30 equiv), Pyridine
(0.95 M), rt, 24 h. (b) NaN3 (3.00 equiv), BF3·Et2O (1.50 equiv)
Dioxane, rt, 24 h. (c) Acetic anhydride (2.00 equiv) DCM, rt, 4 h. (d)
TF2O (1.20 equiv), Pyridine (1.10 equiv), DCM, rt, 16 h. (e) MnO2
(12.0 equiv), CHCl3, 60 °C. 4 h. (f) 1-Phthalimide (1.10 equiv), PPh3
(1.10 equiv), DIAD (1.30 equiv), THF (0.42 M), rt, 7 h, 2-Hydrazine
(1.50 equiv), EtOH (5 mL), 80 °C, 0.5 h. (g) 1-NaBH4 (1.20 equiv),
I2 (1.00 equiv), 2- HCl in MeOH, 24 h. (h) TsCl (1.20 equiv)
MeCN, rt, 6 h.
We subsequently focused on the synthesis of 6-membered
N-heterocycles. Application of N-homoallyl benzamide 2n and
2o (n = 2) provided the oxazines 3n and 3o in yields of 95%
and 93% and enantioselectivities of 89% ee and 90% ee,
respectively. The mesityl-substituted derivative gave the
corresponding oxazine 3p in comparable yields but with a
diminished enantioselectivity (69% ee).
Aliphatic homoallyl amides 2q and 2r were subsequently
investigated, giving the desired 2-cyclohexyl- and 2-n-butyl-
substituted derivatives 3q and 3r in respective yields of 81%
and 67% and enantioselectivities of 93% and 81% ee for each.
Applications of similar compounds have been reported for the
preparation of poly(2-oxazoline) gels for delayed drug delivery
systems.25 Again, our method provides unique access to these
enantioenriched N-heterocycles.
Since 5-oxazolines are potentially useful intermediates for
the synthesis of other chiral building blocks, we finally
elaborated further synthetic transformations of 3a (Scheme
4). The OH group could be replaced by iodine under
Mitsunobu conditions to afford 4a in 68% yield without a
significant loss of enantiomeric excess.26 Azidation was
achieved using a method devised by Kumar and co-workers,
treating 3a with NaN3 and BF3·Et2O to give 4b in 83% yield
and 90% ee.27 Protection of the OH group to the
corresponding acetates and triflates (4c and 4d) was successful
as well. In addition, the corresponding aldehyde could be
obtained by treatment of 3a with MnO2, to give 4e in 95%
yield and 93% ee. Interestingly, no overoxidation of the
oxazoline was observed. A Mitsunobu reaction was also utilized
to prepare the primary amine 4f in a moderate yield (54%) but
sustaining the stereochemistry (92% ee).28 Lastly, we prepared
3-aminopropane-1,2-diol by a reduction of 3a followed by
acid-mediated ring opening to give the amino diol 4g. Since
this compound could not be analyzed by HPLC, it was directly
transformed into the N-tosylated derivative 4h. 4h was isolated
in 42% yield but with a diminished selectivity of 78% ee.
Racemization of 4h could occur via intermediate oxirane
formation by intramolecular attack of an activated form of the
chiral secondary alcohol by the primary alcohol and a
subsequent terminal ring opening by water.
In summary, we have established a practical method for the
enantioselective oxidative cyclization of N-allyl amides by using
an improved triazole-substituted iodoarene catalyst.29 This
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Med. Chem. 2014, 14, 941−951. (f) Gaumont, A.-C.; Gulea, M.;
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method is characterized by a broad substrate scope which
allows the construction of highly enantioenriched 5-oxazolines,
thiazolines, and imidazolines. Quaternary stereocenters can be
constructed with high efficiency as well, and the method was
further extended to oxazines. Many of the constructed
compounds can serve as chiral building blocks for the synthesis
of interesting chiral target structures. This was demonstrated in
a variety of further functionalizations, in particular of the
terminal OH group.
In further investigations, we aim to apply C1-symmetric
triazole-based iodoarenes in similar oxidative cyclizations to
generate other useful 5- and 6-membered heterocycles.
Additionally, cascade reactions in which the reactive hyper-
valent iodine intermediate is trapped directly by nucleophiles
other than OH will also be part of future investigations.
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All experimental procedures and characterization data
(PDF)
AUTHOR INFORMATION
Corresponding Author
■
Boris J. Nachtsheim − University of Bremen, Institute of
Organic and Analytical Chemistry, 28359 Bremen,
Germany; orcid.org/0000-0002-3759-2770;
Email: nachtsheim@uni-bremen.de
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(8) Appenzeller, J.; Tilvi, S.; Martin, M.-T.; Gallard, J.-F.; El-bitar,
H.; Dau, E. T. H.; Debitus, C.; Laurent, D.; Moriou, C.; Al-Mourabit,
A. Org. Lett. 2009, 11, 4874−4877.
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Hamburger, M. J. Nat. Prod. 2017, 80, 459−470. (b) Inahashi, Y.;
Iwatsuki, M.; Ishiyama, A.; Namatame, M.; Nishihara-Tsukashima, A.;
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Ayham H. Abazid − University of Bremen, Institute of Organic
and Analytical Chemistry, 28359 Bremen, Germany
Tom-Niklas Hollwedel − University of Bremen, Institute of
Organic and Analytical Chemistry, 28359 Bremen, Germany
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01607
Author Contributions
The manuscript was written by B.J.N. and A.H.A. A.H.A. and
T.-N.H. performed the experiments.
Notes
The authors declare no competing financial interest.
(12) Kuwano, R.; Kameyama, N.; Ikeda, R. J. Am. Chem. Soc. 2011,
133, 7312−7315.
ACKNOWLEDGMENTS
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(13) Gromova, M. A.; Kharitonov, Y. V.; Bagryanskaya, I. Y.; Shults,
E. E. ChemistryOpen 2018, 7, 890−901.
A.H.A. acknowledges the Scholars at Risk organization for
personal funding.
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2015, 17, 3710−3713. (b) Qin, T.; Jiang, Q.; Ji, J.; Luo, J.; Zhao, X.
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