One-Pot Synthesis of 5-Hydroxy-4H-1,3-thiazin-4-ones: Structure Revision, Synthesis, and NMR Shift Dependence of Thiasporine A

Article abstract of DOI:10.1021/acs.orglett.6b01166

An annulation of arylthioamides with 3-bromopyruvic acid chloride to 5-hydroxy-4H-1,3-thiazin-4-ones has been developed. The initial condensation affords two regioisomeric thiazolinone intermediates in a temperature-dependent manner. The synthesis of the 2-aminophenylthiazinone derivative led to the revision of the previously proposed structure of thiasporine A. Synthesis of the revised structure and NMR analysis revealed that thiasporine A had been isolated as a carboxylate.

Full text of DOI:10.1021/acs.orglett.6b01166

Letter
pubs.acs.org/OrgLett
One-Pot Synthesis of 5‑Hydroxy‑4H‑1,3-thiazin-4-ones: Structure
Revision, Synthesis, and NMR Shift Dependence of Thiasporine A
†
‡
†
†
†
†
Tobias Seitz, Peng Fu, Franz-Lucas Haut, Lutz Adam, Marija Habicht, Dieter Lentz,
,
‡
,†
John B. MacMillan,* and Mathias Christmann*
†
Institute of Chemistry and Biochemistry, Freie Universitat
̈
Berlin, Takustraße 3, 14195 Berlin, Germany
‡
Department of Biochemistry, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75390, United States
*
S Supporting Information
ABSTRACT: An annulation of arylthioamides with 3-
bromopyruvic acid chloride to 5-hydroxy-4H-1,3-thiazin-4-
ones has been developed. The initial condensation affords two
regioisomeric thiazolinone intermediates in a temperature-
dependent manner. The synthesis of the 2-aminophenyl-
thiazinone derivative led to the revision of the previously
proposed structure of thiasporine A. Synthesis of the revised
structure and NMR analysis revealed that thiasporine A had been isolated as a carboxylate.
1
n 2015, the MacMillan laboratory reported the isolation of
Scheme 1. Reactions between Thioamides and Pyruvates
I
thiasporines A−C from a marine-derived Actinomycetospora
chlora (Figure 1). While thiasporines B and C had been isolated
Figure 1. Structures of the thiasporines and anithiactins.
outpace competing thiazole formation in favor of the desired
thiazinones (Scheme 1b), we deemed it necessary for the
anticipated [3 + 3]-annulation to activate C1 in pyruvate (e.g.,
2
previously as anithiactins by Kang, Nam and co-workers,
6
−10
11,12
as acyl chloride 3). While preparations
and reactions
of
thiasporine A (1a) appeared to be clearly different. Its structure
was assigned to contain a 5-hydroxy-4H-1,3-thiazin-4-one
4
H-1,3-thiazin-4-ones have been reported, a direct synthesis
3
from arylthioamides and pyruvates is not known to the best of
our knowledge.
moiety which was the first example of such a ring system in
a natural product. Motivated by the unusual heterocycle and its
activity against the nonsmall-cell lung cancer cell line H2122
The synthesis of 1a commenced with the preparation of
thioamide 5 from commercially available Boc-protected amino-
benzonitrile 4 using sodium thiolate in the presence of
4
,5
(
IC50 5.4 μM), the Christmann laboratory completed a
synthesis of 5-hydroxy-4H-1,3-thiazin-4-ones from arylthioa-
13
1
13
magnesium chloride (Scheme 2). The arylamino group was
protected in order to inhibit amide formation with the acyl
halide. Treatment of 5 with freshly prepared 3-bromopyruvic
acid chloride 3 in dichloromethane at room temperature led,
quite unexpectedly, to S-acylation and hemiaminal formation to
mides. Comparison of the H and C NMR spectra for isolated
thiasporine A and synthetic 1a indicated a problem with its
structural assignment. Herein, we report the synthesis of the
originally proposed structure for thiasporine A (1a) and the
determination and synthesis of the revised structure (1b).
These joint efforts underscore the indispensable role of
chemical synthesis for validating structural assignments.
1
4
subsequently yield 1,3-thiazolin-5-one (±)-6 in 66% yield.
Lowering the temperature to −78 °C completely reversed the
regioselectivity in favor of the desired N-acylation to furnish
Our annulation strategy for the synthesis of the proposed
structure of thiasporine A (1a) was focused on using a
thioamide and a pyruvate derivative as readily available coupling
partners. In the classical Hantzsch thiazole synthesis, α-halo
ketones (2) and thioamides are joined (Scheme 1a). In order to
1
,3-thiazolin-4-one (±)-7. This material could be isolated, but
Received: April 21, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01166
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Letter
Scheme 2. Synthesis of 1a
Scheme 3. Substrate Scope of the One-Pot Synthesis of 5-
Hydroxy-4H-1,3-thiazin-4-ones 10
a
a
Yields refer to isolated material. For detailed reaction conditions, see
the Supporting Information.
the hypothesis that the carboxylate ion might cause significant
changes to the 13C shifts (Scheme 4).
Scheme 4. Synthesis of 1b
partial conversion to 1,3-thiazinone-4-one 8 was observed
during warming. Heating of 7 in DMF in the presence of 2,6-
lutidine afforded 8 in good yield. The structures of
intermediates 6−8 were confirmed by X-ray crystallographic
1
5
analysis. Extensive experimentation revealed conditions that
allowed telescoping the latter two transformations into an
16
efficient one-pot process. Accordingly, thiazinone 8 could be
obtained in 80% yield from 5. In the final step, the Boc-
protecting group was removed with trifluoroacetic acid in
dichloromethane to give target compound 1a in 94% yield.
As shown in Scheme 3, the above-described one-pot, two-
step synthesis of 4H-1,3-thiazin-4-ones 10a−g was demon-
strated to be tolerant of a small set of arylthioamides 9
containing both electron-withdrawing and -donating substitu-
ents. Moreover, thioamides with bulky alkyl residues were
tolerated and provided the corresponding thiazinone products
2
,13
1
0h−i in good yield.
By adapting known
protocols, 2-aminobenzonitrile was
Comparison of the H and 13C NMR data of synthetic 1a
1
treated with sodium hydrosulfide in the presence of magnesium
chloride to give thioamide 11, which was subsequently
condensed with methyl bromopyruvate 2b. Comparison of
and isolated thiasporine A showed clear differences in the
chemical shifts on the heterocyclic moiety (Figure S1). This
discrepancy coupled with the coisolation of thiazole-4-
carboxylic acid containing thiasporines B and C led us to
reconsider the structure of thiasporine A. The original
assignment of the 5-hydroxy-4H-1,3-thiazin-4-one moiety was
in part motivated by the significant differences of the
1
1
the H NMR of synthetic 12 with the H NMR of methylated
thiasporine A showed they were identical (Figure S2),
suggesting that thiasporine A contains the thiazole-4-carboxylic
acid moiety. Hydrolysis of 12 with NaOH gave 1b in an 88%
yield. Anticipating future SAR studies, we also demonstrated
that it is possible to streamline the synthesis of 1b to a one-pot
operation albeit with a reduced yield of 37%. Surprisingly,
comparison of the NMR data for synthetic 1b with isolated
1
3
C chemical shifts compared to the thiazole-4-carboxylic acid
moieties found in thiasporine B and C. Thus, we decided to
carry out a synthesis of the thiazole-4-carboxylic acid 1b with
B
DOI: 10.1021/acs.orglett.6b01166
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Figure 2. Comparison of the H and 13C NMR of synthetic 1b and its sodium salt with that of isolated thiasporine A in DMSO-d6.
1
thiasporine A revealed significant differences of up to 9.5 ppm
In summary, we have revised the structure of thiasporine A
from 1a to 1b by chemical synthesis using a newly developed
[3 + 3]-annulation between 3-bromopyruvic acid chloride and
arylthioamides. The unusual NMR shifts in isolated thiaspor-
ine A were attributed to its isolation as a carboxylate salt as
shown by comparison with the NMR spectra of the free
carboxylic acid.
13
in the C NMR spectrum of the heteroaromatic region
Figure 2). In particular, carbons C-7, C-9, C-10, and C-12
(
suffer substantial changes in the chemical shift (see the
Supporting Information for a titration experiment and the
effect of different counterions). This result prompted us to
consider the possibility that thiasporine A was isolated as a
carboxylate. In order to probe this hypothesis, the salts of 1b
1
13
ASSOCIATED CONTENT
Supporting Information
were prepared. As shown in Figure 2, the H and C NMR
spectra of the sodium salt compare nicely with isolated
thiasporine A. The dramatic chemical shift differences in the
carboxylate also explain the difference between isolated
thiasporine A and thiasporines B and C. In 4-thiazolecarboxylic
acid (A), mesomeric representations B and C can be used to
reflect a relative decrease in electron density (deshielding) at C-
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b01166.
Crystallographic data (CIF, CIF, CIF)
Procedures and spectroscopic data (PDF)
9
and increased electron density at C-10 (Figure 3). In the case
of the corresponding carboxylate A′, mesomeric forms B′ and
C′ become less favorable (compared to B and C) due to
electron repulsion. Similar shift dependences have been
reported with coumaric acids and the corresponding alkali
AUTHOR INFORMATION
Corresponding Authors
■
1
7
*E-mail: John.MacMillan@UTSouthwestern.edu.
E-mail: mathias.christmann@fu-berlin.de.
metal coumarates.
*
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the following grants for funding this project:
Welch Foundation I-1689 (J.B.M.) and NIH R01CA1499833
(
J.B.M.).
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