A chemoselective route to β-enamino esters and thioesters

Article abstract of DOI:10.1021/ol5005643

Conditions were developed for syntheses of β-enamino esters, thioesters, and amides. These reactions involve hydroxybenzotriazole derivatives in buffered media. Illustrative syntheses of some heterocyclic systems are given, including some related to protein-protein interface mimics.

Full text of DOI:10.1021/ol5005643

Letter
pubs.acs.org/OrgLett
A Chemoselective Route to β‑Enamino Esters and Thioesters
Dongyue Xin and Kevin Burgess*
Texas A & M University, Chemistry Department, P.O. Box 30012, College Station, Texas 77842, United States
S
* Supporting Information
ABSTRACT: Conditions were developed for syntheses of β-
enamino esters, thioesters, and amides. These reactions involve
hydroxybenzotriazole derivatives in buffered media. Illustrative
syntheses of some heterocyclic systems are given, including
some related to protein−protein interface mimics.
β-Enamino esters A are versatile synthons for many hetero-
cycles and benzenoid compounds. There are too many
applications of these building blocks to cite them all, but
illustrative ones include syntheses of amino-furanones, indoles,1
oxazoles,2 pyrazoles,3 pyridines4 and dihydropyridines,5 pyr-
imidineones,6 pyrroles,7 and quinolines8 (Figure 1). Existing
Apparently the literature on formation of β-enamino esters
from β-keto esters contains few, if any, references to conditions
most commonly associated with amide bond couplings. We
envisaged these types of conditions might be effective, and data
from exploratory experiments to test this hypothesis are shown
in Table 1 (a full description is in the Supporting Information).
These pilot reactions were performed on phenolic esters to
allow the opportunity for the amines to displace phenoxide, ie a
more stringent chemoselectivity test than if less reactive esters
were used. In the event, such acylation reactions did not
account for significant product formation under the best
conditions identified.
Figure 1. Illustrative uses of β-enamino esters.
Together, entries 1−5 of Table 1 indicate HOAt (1-hydroxy-
7-azabenzotriazole)14 is preferred as an additive over HOBt or
DMAP. Entry 7, compared with all the others, strongly
indicates that a weak base is required. Chloroform seems to be
a better solvent for this reaction type than dichloromethane or
DMF (cf. entries 8 and 12 vs 5 and 9). The best yield was
obtained using EDC·HCl (N-(3-dimethylaminopropyl)-N′-
ethylcarbodiimide hydrochloride) in entry 8, but the PyBOP
((benzotriazol-1-yloxy)tripyrrolidinophosphonium hexafluoro-
methods to prepare β-enamino carbonyl derivatives were
summarized in two papers.9 These methods tend to involve
acids, elevated temperatures, and oxidizing agents, i.e. character-
istics that limit the range of products that can be formed.
Our interest in β-enamino derivatives arose when pursuing
the interface mimics B,10 C,11 and D.12 Scaffolds B−D have
several β-enamino amide functionalities, and overall yields in
their syntheses are directly related to efficient formation of the
bonds indicated. Consequently, we accumulated experience
with methods for efficient formation of β-enamino derivatives
under mild conditions. This Letter summarizes the most
important of those findings.
Received: February 20, 2014
Published: March 31, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
(via multistep procedures,16 or by adding ammonia under
buffered conditions17) or via condensations of tert-butyl β-
ketothioesters in the presence of ceric ammonium nitrate
(CAN) where the large S-alkyl group attenuates the reactivity
of the thioester.9b
Table 1. Pilot Reactions for Formation of β-Enamino
Derivatives
Inverted chemoselectivity relative to the reaction above was
achieved by modifying the Table 1, entry 12 conditions to
include the acid HOAt and hydrogen carbonate buffer. This
enabled syntheses of the β-enamino thioesters shown in Figure
3.
entry activating reagent
base
TEA
solvent
additive yield (%)
1
EDC·HCl
EDC·HCl
EDC·HCl
EDC·HCl
EDC·HCl
EDC·HCl
EDC·HCl
EDC·HCl
EDC·HCl
HBTU13
PyBOP
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
DMF
HOBt
−
11
12
17
74
85
71
33
93
13
60
80
87
2
KHCO3
KHCO3
KHCO3
KHCO3
imidazole
−
KHCO3
KHCO3
KHCO3
KHCO3
imidazole
3
DMAP
HOBt
HOAt
HOAt
HOAt
HOAt
HOAt
−
4
5
6
7
8
9
10
11
12
CHCl3
CHCl3
CHCl3
−
−
PyBOP
phosphate)15 conditions in entry 12 were only marginally
inferior and sometimes, depending on the substrate, have
advantages with respect to workup simplicity. Figure 2 shows
products and yields for other β-enamino esters formed via the
method illustrated in entry 12.
Figure 3. Illustrative uses of β-enamino thioesters.
Figure 3 demonstrates that both aliphatic and aromatic
amines can be used as the nucleophile in these reactions.
Formation of the β-enamino thioesters 2j and 2p indicates that
acetal and S-trityl groups are tolerated, whereas they
presumably would not be in the presence of acids. Products
2k, 2l, and 2m demonstrate primary alcohols and phenols are
compatible with the featured condensation reaction. Phenylthio
ester products 2r−t were formed, even though PhS− is a better
leaving group than in the other reactions.
Figure 2. β-Enamino esters formed using the conditions shown for
entry 12 in Table 1, with the following exceptions: a 36 h; b 2 equiv of
amine and 24 h.
The next step in this study was to investigate β-ketothioesters
as substrates. Reaction 1 illustrates how, in the absence of an
additive, an amine is preferentially acylated by a β-ketothioester,
rather than condensing with it. It is potentially valuable to be
able to invert this chemoselectivity because that would enable
preparation of β-enamino thioesters. Previous syntheses of β-
enamino thioesters are limited to the N-unsubstituted forms
Figure 4. Illustrative Nenitzescu indole syntheses.
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Three Nenitzescu indole syntheses18 were performed to
illustrate how β-enamino thioesters can be used (Figure 4).19
All three reactions did not perturb the thioester functionality,
even when this involved a more reactive phenylthiolate leaving
group, i.e. for indole 3b. Formation of the regioisomers shown
was confirmed via NOESY experiments.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and spectroscopic data for com-
pounds 1−6. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
S
An isolable intermediate azabenzotriazole ether was formed
when 1,3-cyclohexanedione was allowed to react under the
conditions used under typical coupling conditions (Figure 5). It
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: burgess@tamu.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support for this work was provided by The National
Institutes of Health (GM087981) and The Robert A. Welch
Foundation (A-1121).
DEDICATION
■
In honor of Alan R. Katritzky, 18 Aug. 1928−10 Feb. 2014.
REFERENCES
■
(1) (a) Jia, Z.; Nagano, T.; Li, X.; Chan, A. S. C. Eur. J. Org. Chem.
2013, 858. (b) Neumann, J. J.; Rakshit, S.; Droege, T.; Wuertz, S.;
Glorius, F. Chem.Eur. J. 2011, 7298. (c) Nguyen, H. H.; Kurth, M. J.
Org. Lett. 2013, 362. (d) Kramer, S.; Dooleweerdt, K.; Lindhardt, A.
T.; Rottlander, M.; Skrydstrup, T. Org. Lett. 2009, 4208.
(2) Zheng, Y.; Li, X.; Ren, C.; Zhang-Negrerie, D.; Du, Y.; Zhao, K. J.
Org. Chem. 2012, 10353.
Figure 5. Intermediate formation in HOAt-mediated reactions.
seems probable that this intermediate is more stable than the
corresponding one in the thioester reactions. If similar
intermediates are formed throughout, this reaction indicates a
two-step conjugate addition−elimination process is operative.
This would also explain why HOAt is superior to HOBt in
these transformations: because the aza-derivative can “lever”
proton transfer in the amine addition step as indicated in
transition state E.
Finally, Figure 6 illustrates how the conditions developed for
formation of β-enamino amides were applied in reactions
typical of those used to make the interface mimics B−D. Good
yields were obtained, and the near-neutral conditions indicate
there is likely to be a broad substrate scope.
(3) Suri, M.; Jousseaume, T.; Neumann, J. J.; Glorius, F. Green Chem.
2012, 2193.
(4) Yamamoto, S.-i.; Okamoto, K.; Murakoso, M.; Kuninobu, Y.;
Takai, K. Org. Lett. 2012, 3182.
(5) Noole, A.; Borissova, M.; Lopp, M.; Kanger, T. J. Org. Chem.
2011, 1538.
(6) Chun, Y. S.; Xuan, Z.; Kim, J. H.; Lee, S.-g. Org. Lett. 2013, 3162.
(7) (a) Meng, L.; Wu, K.; Liu, C.; Lei, A. Chem. Commun. 2013,
5853. (b) Ke, J.; He, C.; Liu, H.; Li, M.; Lei, A. Chem. Commun. 2013,
7549. (c) Toh, K. K.; Wang, Y.-F.; Ng, E. P. J.; Chiba, S. J. Am. Chem.
Soc. 2011, 13942. (d) Zhao, M.; Wang, F.; Li, X. Org. Lett. 2012, 1412.
(8) Toh, K. K.; Sanjaya, S.; Sahnoun, S.; Chong, S. Y.; Chiba, S. Org.
Lett. 2012, 2290.
(9) (a) Zhang, Z.-H.; Yin, L.; Wang, Y.-M. Adv. Synth. Catal. 2006,
184. (b) Sridharan, V.; Avendano, C.; Menendez, J. C. Synlett 2007,
881.
(10) (a) Raghuraman, A.; Ko, E.; Perez, L. M.; Ioerger, T. R.;
Burgess, K. J. Am. Chem. Soc. 2011, 12350. (b) Raghuraman, A.; Xin,
D.; Perez, L. M.; Burgess, K. J. Org. Chem. 2013, 4823. (c) Ko, E.;
Raghuraman, A.; Perez, L. M.; Ioerger, T. R.; Burgess, K. J. Am. Chem.
Soc. 2013, 167.
(11) Fedoseyenko, D.; Raghuraman, A.; Ko, E.; Burgess, K. Org.
Biomol. Chem. 2012, 921.
(12) Xin, D.; Perez, L. M.; Ioerger, T. R.; Burgess, K. Angew. Chem.,
Int. Ed. 2014, DOI: 10.1002/anie.201400927.
(13) Knorr, R.; Trzeciak, A.; Bannwarth, W.; Gillessen, D.
Tetrahedron Lett. 1989, 1927.
Figure 6. Application of the featured conditions for syntheses of
(14) Carpino, L. A. J. Am. Chem. Soc. 1993, 4397.
(15) Coste, J.; Le-Nguyen, D.; Castro, B. Tetrahedron Lett. 1990, 205.
(16) Vitale, P.; Di Nunno, L.; Scilimati, A. Synthesis 2010, 3195.
(17) Roh, E. J.; Keller, J. M.; Olah, Z.; Iadarola, M. J.; Jacobson, K. A.
Bioorg. Med. Chem. 2008, 9349.
(18) (a) Allen, G. R. Org. React. 2011, 337. (b) Ketcha, D. M.;
Wilson, L. J.; Portlock, D. E. Tetrahedron Lett. 2000, 6253. (c) Patrick,
J. B.; Saunders, E. K. Tetrahedron Lett. 1979, 4009.
(19) Suryavanshi, P. A.; Sridharan, V.; Menendez, J. C. Org. Biomol.
Chem. 2010, 3426.
interface mimic fragments.
In summary, β-enamino derivatives can be formed via
carefully buffered conditions featuring activation agents such as
EDC·HCl and PyBOP. These conditions are sufficiently mild
to facilitate synthesis of a wide range of examples, including
relatively delicate synthons like β-enamino thioesters.
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