Total synthesis of violaceimides A–E and consideration of the reported stereochemistry

Article abstract of DOI:10.1016/j.tetlet.2019.151103

Here we report the first total synthesis of violaceimides A–E, a family of sulfur-containing metabolites from Aspergillus violaceus, a sponge-associated fungus. A concise, convergent and enantioselective synthesis was developed for all five family members

Full text of DOI:10.1016/j.tetlet.2019.151103

Journal Pre-proofs
Total synthesis of Violaceimides A-E and consideration of the reported stereo-
chemistry
Charles K. Perry, Mark G. Fulton, Craig W. Lindsley
PII:
S0040-4039(19)30866-4
DOI:
https://doi.org/10.1016/j.tetlet.2019.151103
TETL 151103
Reference:
Tetrahedron Letters
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Received Date:
Revised Date:
Accepted Date:
7 August 2019
21 August 2019
1 September 2019
Please cite this article as: Perry, C.K., Fulton, M.G., Lindsley, C.W., Total synthesis of Violaceimides A-E and
consideration of the reported stereochemistry, Tetrahedron Letters (2019), doi: https://doi.org/10.1016/j.tetlet.
2019.151103
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Total synthesis of Violaceimides A-E and
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consideration of the reported stereochemistry
Charles K. Perry, Mark G. Fulton and Craig W. Lindsley

1
Tetrahedron Letters
Total synthesis of Violaceimides A-E and consideration of the reported
stereochemistry
ǂ,
¥,
¥,ǂ
⊥
Charles K. Perry, Mark G. Fulton ⊥ and Craig W. Lindsley *
¥Department of Chemistry, Vanderbilt University, Nashville, TN 37232-6600, U.S.A.
ǂDepartment of Pharmacology, Vanderbilt Center for Neuroscience Drug Discovery, Vanderbilt University, Nashville, TN 37232-6600, U.S.A.
⊥These authors contributed equally
ARTICLE INFO
ABSTRACT
Here we report the first total synthesis of violaceimides A-E, a family of sulfur-containing
metabolites from Aspergillus violaceus, a sponge-associated fungus. A concise, convergent and
enantioselective synthesis was developed for all five family members, from a common advanced
intermediate. However, while the NMR spectral data matched that of the reported natural
products, the optical rotations were of opposite sign. This result prompted the enantioselective
synthesis of all four diastereomeric pairs of violaceimide E, and suggests that the stereochemistry
might have been misassigned.
Article history:
Received
Received in revised form
Accepted
Available online
Keywords:
violacemide
.
enantioselective
total synthesis
natural product
2019 Elsevier Ltd. All rights reserved.
Sulfur-containing natural products are uncommon from marine
sources, and compounds harboring disulfide linkages represent an
even smaller fraction of reported natural products.1 Recently, we
reported on the first total synthesis of gombamide A (1),2,3 a 17-
membered macrocyclic peptide possessing a disulfide linkage, which
displayed modest cytotoxic activity. Last year, Lin4 and co-workers
reported on a family of sulfur-containing metabolites, violaceimides
A-E (2-6) from the sponge-associate fungus Aspergillus violaceus. Of
these new natural products, violaceimide E (6) included the disulfide
linkage of interest to our lab. Moreover, 2 and 3 displayed selective
growth inhibition against leukemia U937 and colorectal HCT-8 cells,
with low cytotoxicity in non-transformed cells.4 Lin and co-workers
employed 1D and 2D NMR, Mosher’s and Snatzke’s methods as well
as calculated ECD and chemical degradation to assign the
stereochemistry of 2-6.4 Based on our interest in sulfur-containing
marine natural products, and the therapeutic index of these compounds
in tumor cell lines, we launched an effort to synthesize violaceimides
A-E (2-6), and further profile their biological activity.
Scheme 1 highlights our retrosynthetic plan for violaceimides A-
E (2-6) affording a concise entry, via a common synthetic intermediate
7, to efficiently construct all five members of this marine natural
product family starting from commercially available, unnatural D-
cysteine methyl ester 85 and (R)-methylsuccinic acid 9.6
For the convergent total synthesis of 2-6, we first focused our
attention on the construction of common intermediate 7 (Scheme 2).
Trityl protection of D-cysteine methyl ester 8 afforded thioether 10 in
60% yield (i.e., the trityl group was chosen as we anticipated the
disulfide bond in 6 could be directly formed from the S-Tr moiety).8
In parallel, (R)-methylsuccinic acid 9, upon exposure to acetyl
chloride,9 generates the key cyclic anhydride 11 in 97% yield.
Condensation of 10 and 11 in warm toluene, followed by cyclization
with NaOAc in Ac2O at 90 oC provides protected 12 in 82% isolated
yield.10 Standard trityl deprotection (Et3SiH, TFA, CH2Cl2, rt)
afforded common intermediate 7 in 82% yield. Overall, common
intermediate
7
(methyl
(S)-3-mercapto-2-(R)-3-methyl-2,5-
dioxopyrrolidin-1-yl)propanoate) is accessed in five steps (4 steps
LLS) and in 39.1% overall yield.
Figure 1. Structures of sulfur-containing marine natural products gombamide
A (1) and violaceimides A-E (2-6).

2
Tetrahedron
Scheme 1. Retrosynthetic plan for the violaceimides A-E (2-6).
the natural products, and 4 and 5 were prepared in five steps and in
13.5% and 27.3% overall yields, respectively (Scheme 4).
Scheme 3. Synthesis of the reported structures of violaceimides A (2) and B
(3).
Scheme 4. Synthesis of the reported structures of violaceimides C (4) and D
(5).
Scheme 2. Synthesis of common intermediate 7.
With violaceimides A-D (2-5) in hand, we now turned our attention
to the disulfide bond-containing congener violaceimide E (6). In this
case, the S-trityl protected intermediate 12 was perfectly suited for an
I2-mediated deprotection and disulfide bond forming reaction. In the
event (Scheme 5), treatment of 12 with molecular iodine in a
CH2Cl2:MeOH mixture provided the desired violaceimide E (6) in
91% yield. As described in the isolation paper, 6 is reported to possess
the 2/2’-(S), 3/3’-(R) stereochemistry, and thus our synthetic efforts
afforded the desired structure of the reported natural product. As with
2-5, the NMR data were in agreement with those reported for the
Scheme 5. Synthesis of the reported structure of violaceimide E (6).
With common intermediate
7 in hand, the synthesis of
violaceimides A-D (2-5) was straightforward. For violaceimides A (2)
and B (3), exposure of 7 to either (S)-oxirane-2-carboxylate 13 or
methyl (S)-oxirane-2-carboxylate 14, respectively, affords the
reported structures of the natural products in acceptable (29-35%)
yields (Scheme 3). Here, 2 and 3 were prepared in five steps and in
13.6% and 11.9% overall yields, respectively, and the NMR data were
in agreement with those reported for the natural products.11 In a similar
fashion, treatment of common intermediate 7 with either 2-
bromoacetic acid 15 or methyl-2-bromoacetate 1612, provided the
reported structures of violaceimides C (4) and D (5), respectively.
Once again, the NMR data were in agreement with those reported for
natural product; however,5,11 when we recorded the optical rotation,
we obtained a value of opposite sign [α]2D3 = +181 (c 0.5, CHCl3), as
compared to the natural product, [α]2D3 = -90 (c 0.5, CHCl3). This trend
was seen in all 5 family members, as all violaceimides displayed

3
negative rotations in the isolation paper; and in contrast, all of our
stereochemical congeners also failed to match the optical rotation
synthetic congeners showed positive rotations (Table 1).5,11
reported in the isolation paper. Thus, the actual stereochemistry for
violaceimides A-E (2-6) is not conclusive, but the stereochemistry is
likely not as assigned in the isolation paper. This may be potentially
due to a misassignment of the stereocenter from the cysteine core
(reported as D-cysteine in the natural products), as the synthetic L-
cysteine congeners 6b and 6c matched, at least in sign, the negative
rotation reported for violaceimide E (6). Further studies to determine
the stereochemistry of the violaceimides A-E (2-6), as well as
biological evaluation of the synthetic material, are underway and
results will be reported in due course.
Table 1. Optical rotations of natural and synthetic violaceimides 2-6.a
Compound
Reported
rotation
2
3
4
5
6
[α]2D3= -52
[α]2D3 = -88
[α]2D3= -102
[α]2D3= -208
[α]2D3= -90
from
isolation
Author Contributions
Rotation of
synthetic
The manuscript was written through contributions of all authors. All
authors have given approval to the final version of the manuscript.
[α]2D3
=
[α]2D3
=
[α]2D3
=
[α]2D3
=
[α]2D3= +47.9
+44.6
+80.4
+103.1
+181
material
Notes
a(c = 0.5, CHCl3)
The authors declare no competing financial interest.
It is possible that the stereochemistry may have been misassigned
via the analytical techniques used. Did the acidic hydrolysis used to
determine stereochemistry of the 3-methyl succinimide in the isolation
work epimerize one of the chiral centers, or did ECD misassign the
stereochemistry of the serine fragment? We have encountered this
previously within other classes of natural products where
stereochemical assignment was based, at least partially, on acidic
hydrolysis and calculated ECD.13,14 Thus, utilizing the natural L-
serine methyl ester and (S)-methylsuccinic acid, and following
Scheme 2, three additional stereochemical congers of 12, 12a-c, were
readily prepared. By constructing the disulfide bond across all
possible combinations of these building blocks, we prepared the three
other stereoisomers of 6 2/2'-(S), 3/3'-(R): the 2/2'-(S), 3/3'-(S) isomer,
6a, the 2/2'-(R), 3/3'-(R) isomer, 6b and the 2/2'-(R), 3/3'-(S) isomer,
6c. As shown in Table 2, the 1H NMR spectra were conserved across
ACKNOWLEDGMENTS
C.W.L. thanks the William K. Warren Family and Foundation for
funding the William K. Warren, Jr. Chair in Medicine and support of
our programs.
REFERENCES
1. Feng, M.; Tang, B.; Liang, S.H.; Jiang, X. Curr. Top. Med. Chem. 2016,
16, 1200-1216.
2. Timmermans, M.L.; Paudel, Y.P.; Ross, A.C. Mar. Drugs 2017, 15, E235.
3. Garcia-Barrantes, P. M.; Lindsley, C. W. Org Lett 2016, 18 (15), 3810-
3813.
4. Woo, J.K.; Jeon, J.E.; Kim, C.K.; Sim, C.J.; Oh, D.C.; Oh, K.B.; Shin, J. J.
Nat. Prod. 2013, 76, 1380-1383.
5.Yin, J.; Zhang, C.; Huang, J.; Zhang, J.; Liu, D.; Huang, J.; Proksch, P.;
Lin, W. Tet. Lett. 2018, 59, 3157-3160.
6.https://www.sigmaaldrich.com/catalog/product/aldrich/410209?lang=en&re
gion=US; (25g @$58.60).
7. https://www.sigmaaldrich.com/catalog/search?term=%28R%29-
methylsuccinic+aci&interface=All&N=0&mode=match%20partialmax&lang
=en&region=US&focus=product; (5g @$107).
Scheme 6. Synthesis of the remaining stereoisomers of violaceimide E (6), 6a-
c.
8. Sieber, P.; Kamber, B.; Riniker, B.; Rittel, W. Helevtica 1980, 63, 2358-
2363.
9. Ismail, K. A.; Bergmeier, S. C. Eur. J. Med. Chem. 2002, 37, 460-474.
10. Bennett, D. J.; Blake, A. J.; Cooke, P. A.; Godfrey, C. R. A.; Pickering, P.
L.; Simpkins, N. S.; Walker, M. D.; Wilson, C. Tetrahedron 2004, 60, 4491-
4511.
11. See Supporting Information for full details.
12. Ma, M.; Paredes, A.; Bong, D. J. Amer. Chem. Soc. 2008, 130, 14456-
14458.
13. Childress, E.S.; Garrison, A.T.; Sheldon, J.R.; Skaar, E.P.; Lindsley, C.W.
J. Org. Chem. 2019, 84, 6459-6464
14. Daniels, R.N.; Melancon, B.J.; Wang, E.A.; Crews, B.S.; Marnett, L.M.;
Sulikowski, G.A.; Lindsley, C.W. J. Org. Chem. 2009, 74, 8852-8855.
the four compounds (as were the 13C NMR spectra); however, isomers
6b and 6c showed negative rotations, akin to what was reported by
Yin, but the magnitude of rotation was ~2x greater. These data are
inconclusive as to what the actual stereochemistry of natural
violaceimides E (6), as well as 2-5, should be (but the reported
stereochemistry has opposite rotation, suggesting a misassignment);
however, the core structures/atom connectivity of violaceimides A-E
(2-6) are correct.5,11 Thus, we have completed the first total synthesis
of the proposed structures of violaceimides A-E (2-6), and data from
the synthetic material, of known stereochemistry, raises
considerations as to the original assignment.
ASSOCIATED CONTENT
Supporting Information
1
Experimental procedures, characterization data, H and 13C NMR spectra for
new compounds.
AUTHOR INFORMATION
Corresponding Author
In conclusion, we have completed the first total synthesis of the
proposed structures of violaceimides A-E (2-6), with NMR spectral
data identical to those reported. The convergent route rapidly
assembles the proposed structures of violaceimides A-E (2-6) in high
yields. However, concerns have been raised regarding the original
stereochemical assignment as the reported natural products and
synthetic congeners show opposite rotations. A closer examination of
the disulfide congener violaceimide E (6) showed that three other
* E-mail: craig.lindsley@vanderbilt.edu
Phone: 615-322-8700, fax: 615-343-3088

4
Tetrahedron
Table 2. 1H NMR Comparison of natural violaceimide E to synthetic
violaceimide E and corresponding diastereomers (6, 6a-c).
Yin’s Work
Violaceimide E (6)
2/2'-(S), 3/3'-(R)
This Work
This Work
This Work
This Work
Violaceimide E (6)
2/2'-(S), 3/3'-(R)
Violaceimide E (6a)
2/2'-(S), 3/3'-(S)
Violaceimide E (6b)
2/2'-(R), 3/3'-(R)
Violaceimide E (6c)
2/2'-(R), 3/3'-(S)
Assignment
δH (mult., J/Hz)
δH (mult., J/Hz)
δH (mult., J/Hz)
δH (mult., J/Hz)
δH (mult., J/Hz)
1/1’
2/2’
3/3’
2.98, ddq (2.3, 6.8)
2.45, dd (2.3, 13.8)
3.03, dd (6.8, 13.8)
3.04-2.94, m
2.45-2.38, m
3.04-2.94, m
3.03-2.96, m
2.44, dd (8.0, 21.0)
3.03-2.96, m
3.02-2.95, m
2.43, dd (8.1, 21.8)
3.02-2.95, m
3.04-2.94, m
2.42, dd (2.8, 16.2)
3.04-2.94, m
4/4’
5/5’
6/6’
7/7’
8/8’
1.21, d (6.8)
1.22-1.20, m
1.21, d (6.8)
1.21, d (6.5)
1.21, d (7.1)
4.97, dd (5.0, 10.9)
3.20, dd (10.9, 14.0)
3.37, dd (5.0, 14.0)
3.66, s
4.96, sextext (4.8)
3.24-3.17, m
3.45-3.38, m
3.66, s
4.98, dd (4.7, 10.3)
3.21, dd (10.4, 14.4)
3.41, dd (4.7, 14.4)
3.66, s
4.97, dd (4.6, 10.3)
3.20, dd (10.4, 14.3)
3.40, dd (4.6, 14.4)
3.66, s
4.97, dd (4.8, 10.2)
3.21, dd (10.3, 14.4)
3.40, dd (4.8, 14.4)
3.66, s
OMe
[α]2D3
-90
+181
+196.5
-197.2
-167.5

5
Highlights



First total synthesis of violacemides A-E
Synthesis of unnatural diastereomers
Consideration and revision of stereocheimcal
assignment