A concise and efficient route to the total synthesis of bacillamide A and its analogues

Article abstract of DOI:10.24820/ark.5550190.p010.362

The synthesis of bacillamide A, a tryptamide alkaloid of marine origin, and its analogues from L-cysteine ethyl ester hydrochloride through an efficient and convergent synthetic approach is described in this work. The present two-step protocol involves th

Full text of DOI:10.24820/ark.5550190.p010.362

The Free Internet Journal
for Organic Chemistry
Paper
Archive for
Organic Chemistry
Arkivoc 2018, part v, 0-0
A concise and efficient route to the total synthesis of bacillamide A and its
analogues
a,b
a
Sunil Kumar and Ranjana Aggarwal *
a
Department of Chemistry, Kurukshetra University, Kurukshetra- 136 119, India
b
Department of Chemistry and Centre of Advanced Studies in Chemistry, Panjab University,
Chandigarh- 160 014, India
Email: ranjanaaggarwal67@gmail.com
Received 10-13-2017
Accepted 02-14-2018
Published on line 05-29-2018
Abstract
The synthesis of bacillamide A, a tryptamide alkaloid of marine origin, and its analogues from L-cysteine ethyl
ester hydrochloride through an efficient and convergent synthetic approach is described in this work. The
present two-step protocol involves the use of iodobenzene diacetate, a versatile oxidising agent, to synthesize
the key intermediate ethyl 2-differently substituted-1,3-thiazole-4-carboxylates in one step. In this work, 4,6-
dimethylpyrimidin-2-ol was used as a catalyst for solvent-free aminolysis of esters to achieve the title
compounds by taking advantage of its property of simultaneously donating and accepting a hydrogen bond.
Keywords: Bacillamide A; thiazole; 4,6-dimethylpyrimidin-2-ol; tryptamine; hypervalent iodine reagent; amide
bond formation.
DOI: https://doi.org/10.24820/ark.5550190.p010.362
Page 1
©ARKAT USA, Inc

Arkivoc 2018, v, 0-0
Kumar, S. et al.
Introduction
Numerous marine natural products exhibit wide range of biological activities and thus provide good lead in
1-3
drug discovery but quite often availability of such bioactive compounds is very low from natural resources.
Hence there is need to synthesize them to obtain requisite quantities and build diversity around their scaffold
to further explore their therapeutic potential. The study of natural products, their isolation, characterization
and synthesis, have been motivated by a quest for some benefit to mankind. Recently, from marine natural
products bioprospecting afforded a significant number of drug candidates. Some of the commercialized
products from marine organisms include the antibiotic cephalosporin-C from marine fungi, keramidine from
marine sponge having antagonistic activity against serotonergic and cholinergic receptors, the anthelmintic
insecticide 3-indolacrylamide from red alga, which have proved to be excellent bioactive compounds (Figure
4-8
1).
Figure 1. Bioactive compounds from marine organisms.
Bacillamide A (1a) is a natural tryptamide alkaloid isolated in 2003 from marine bacterium, Bacillus species
9
SY-I during termination of blooms of Cochlodinium polykrikoides. Most interestingly, it was found to be the
first compound to show excellent algicidal activity against the harmful dinoflagellate Cochlodinium
polykrikoides but it does not kill or inhibit the growth of bacteria, fungi, yeast and useful microalgae of other
phyla such as diatoms, green algae and cyanobacteria. This dinoflagellate is responsible for seafood poisoning
and mass mortality of cultured fishes. Rowley and co-workers isolated bacillamide B (enol form of bacillamide
10
A) from a Bahamian hypersaline microbial mat in 2007 as an antimicrobial agent. Bray revised the proposed
11
stereochemistry of the natural bacillamide B from R- to S- configuration. Sun et al described its total
synthesis via a cascade disulfide cleavage/thiocarbonylation/Staudinger reduction/aza-Wittig reaction using β-
12
azido disulfide and carboxylic acid as substrates in 30% overall yield.
Outbreaks of seafood poisoning are usually sporadic and unpredictable because fish or shellfish do not
produce toxins themselves, but acquire them from marine microorganisms such as dinoflagellate and marine
bacteria that they eat. High selectivity exhibited by bacillamide A 1a, makes it a promising algicidal agent for
regulating blooms of harmful dinoflagellate species. However, because of low yields of bacillamide A obtained
from natural sources there is a necessity to gain access to large quantities of this substance for further
biological screening through improved total synthesis.
Page 2
©ARKAT USA, Inc

Arkivoc 2018, v, 0-0
Kumar, S. et al.
The first synthesis of bacillamide A (1a) developed by Figueira et al. involved a five-step linear approach;
13
starting from 4-methyl-1,3-thiazole, the desired product was obtained in only 14% overall yield. This
multistep approach requires tedious workup and purification steps. Formation of by-products, particularly
during benzylic bromination of 4-methyl-2-acetyl-1,3-thiazole to synthesize key intermediate 2-acetyl-1,3-
thiazole-4-carboxylic acid and coupling of amide bond through mixed anhydride approach, affected the yields
adversely. Most importantly, the potential of this method is limited as specified structural variation cannot be
introduced in structure of bacillamide to achieve better biological active analogues.
In view of these observations and due to our interest in the synthesis of biological active thiazole
compounds,1
4-16
we developed a concise route for the total synthesis of bacillamide A. After retrosynthetic
analysis of 1a, we aimed to synthesize it in only two steps from the commercially available L-cysteine ethyl
ester hydrochloride, pyruvaldehyde and tryptamine, in a sequence of thiazole ring formation via [4+1]
approach in the first step, and amide bond formation by aminolysis of esters in the second (Figure 2). Based on
this diversity-driven approach, we also envisaged the synthesis of six analogues of bacillamide having variation
at position-2 of the thiazole ring.
Figure 2. Retrosynthetic analysis of bacillamide A.
Results and Discussion
The key intermediate ethyl 2-acetyl-1,3-thiazole-4-carboxylate (4a) was synthesized earlier through [4+1]
17
approach by Hughes et al. It was reported that stirring L-cysteine ethyl ester hydrochloride and pyruvic
aldehyde in the presence of mild base (KHCO ) under nitrogen atmosphere for 18 h led to the formation of 1:1
3
diastereomeric mixture of ethyl 2-acetyl-1,3-thiazolidine-4-carboxylate which was subsequently oxidised in the
next step with manganese dioxide in 18 h in 55% overall yield. A perusal of the literature revealed that
1
8-19
although MnO
agents e.g. NBS-benzoyl peroxide and BrCCl
2
is the reagent of choice for the oxidation of thiazolidine to thiazole,
alternative oxidising
20
21
3
-DBU have also been employed. However, these methods
require two steps, carefully controlled reaction conditions, longer reaction periods and low overall yields.
Recently there is an increasing interest in hypervalent iodine reagents mainly due to their remarkable oxidising
properties combined with their environmental benign nature and commercial viability. Our research group
and others have successfully accomplished the synthesis of a wide variety of heterocyclic compounds utilising
22
iodine(III) reagents through in situ oxidative cyclisation or dehydrogenation. This encouraged us to develop a
one pot protocol for the synthesis of 4 utilising iodobenzene diacetate (IBD) as an oxidising agent.
In the present protocol, pyruvaldehyde was stirred with L-cysteine ethyl ester hydrochloride dissolved in
o
EtOH-water (50:50) in the presence of sodium bicarbonate at 60 C. The reaction was monitored by TLC at
regular intervals. When it indicated the consumption of starting materials, the solvent was distilled off in
o
vacuo and the solution was stirred for additional 4 h at 40 C after the addition of a solution of IBD in
dichloromethane. The workup of the reaction afforded 4a in 62% yield (Scheme 1).
Page 3
©ARKAT USA, Inc

Arkivoc 2018, v, 0-0
Kumar, S. et al.
Scheme 1. Total synthesis of bacillamide A (1a).
The final step in the synthesis required aminolysis of ethyl 2-acetyl-1,3-thiazole-4-carboxylate with
tryptamine. Though being a potentially attractive approach, limited examples of the direct transformation of
esters to amides have been reported in literature, involving catalytic ester-amide exchange utilising
2
3
24
zirconium(IV) tert-butoxide with activator 1-hydroxy-7-azabenzotriazole,
2-pyridone,
DBU (1,8-
25
26
diazabicyclo[5.4.0]undec-7-ene), diethylaluminium dimethylamide, etc. However, in most of the cases
yields were not satisfactory and stoichiometric additives or tailored substrates were necessary. We thought of
exploring the potential of pyrimidin-2-one, which may exist and aggregate with tautomeric form pyrimidin-2-
ol, as catalyst for the amide synthesis due to its catalytic efficiency based on the property of simultaneously
donating and accepting a hydrogen bond. Pyrimidin-2-ol has also been used as ligand to generate structure of
II
high complexity with trans-a
2
Pt (a = NH
3
, CH
3
NH ) by taking advantage of its H-bonding and metal bonding
2
27
properties. This reagent leads to an efficient solid phase synthesis of bacillamide A with better yield (51%
overall yield) and without giving any side product in this reaction. It has been noted that during the reaction
no trans-esterification was observed. The identity of the final compound was confirmed by melting point and
1
13
H NMR and the results are consistent with the reported natural product.
Table 1. Substrate scope for the synthesis of bacillamide analogues
%
Yield
(
Obs. M.pt.
(Lit.) oC
%Yield
(1)b
Entry
3
1
4)a
1
(
44-145
b
78
82
80
145)14
1
(
62-164
164)14
c
80
Page 4
©ARKAT USA, Inc

Arkivoc 2018, v, 0-0
Kumar, S. et al.
%
Yield
(
Obs. M.pt.
(Lit.) oC
%Yield
(1)b
Entry
3
1
4)a
1
1
1
1
82-183
(
d
e
f
75
84
183)14
40-141
80
78
78
81
75
78
(
141)14
68-170
(
170)14
92-193
193)14
g
(
a
Percentage yields of thiazole (4) from L-cysteine ethyl ester hydrochloride (2)
Percentage yields from thiazole (4) to bacillamide analogues (1b-g)
b
A similar synthetic approach was used to prepare various bacillamide analogues having variation at
position-2 of the thiazole ring by the reaction of differently substituted aldehydes with L-cysteine ethyl ester
hydrochloride (2). Further, synthesized thiazoles reacted with tryptamine in the presence of 4,6-
dimethylpyrimidin-2-ol (6) through a solvent-free reaction. After the usual work up the corresponding analogs
1
(
1b-g) were obtained in good yield. All structures were confirmed by comparing their melting points and H
14
NMR data with the literature. Structures, melting points and yields of bacillamide analogues are given in
Table 1. These derivatives 1b-g had been synthesized earlier by us employing an alternative [3+2] approach,
14
Hantzsch thiazole synthesis. However, the present protocol is more promising as it not only provided
improved yields and cleaner reactions but also avoided the synthesis of precursors, use of the lachrymatory
ethyl bromopyruvate and thioamides.
Scheme 2. Plausible mechanism for the synthesis of bacillamide A and its analogues.
Page 5
©ARKAT USA, Inc

Arkivoc 2018, v, 0-0
Kumar, S. et al.
A plausible mechanism for the oxidation of thiazolidine to thiazole and amide coupling is outlined in
Scheme 2. The dehydrogenation of thiazolidine to thiazole involves the electrophilic attack of IBD to generate
intermediates A and B which undergo reductive loss of iodobenzene along with elimination of acetic acid. The
possible reaction mechanism for amide bond formation is analogous to that described for 4,6-
dimethylpyrimidin-2-ol involving an eight-membered transition state between ester and amine through
simultaneously donating and accepting a hydrogen bond.
Conclusions
In conclusion, our short sequence leads to bacillamide A in only two steps and 51% overall yield from
commercially available L-cysteine ethyl ester hydrochloride. The efficiency of our environmental-benign
method enabled the synthesis of bacillamide A and its additional derivatives on a milligram scale for the
purpose of evaluation in multiple biological assays.
Experimental Section
General. Melting points were determined in open capillaries in electrical apparatus and are uncorrected. IR
1
spectra were recorded on a Buck Scientific IR M500 instrument. The H NMR spectra were recorded on a
Bruker instrument at 300 MHz. Mass spectra were measured in EI mode on a Kratos MS-50 spectrometer at
MS Facilities at CIL-SAIF, Panjab University, Chandigarh, India. The starting materials L-cysteine ethyl ester
hydrochloride and tryptamine were purchased from HiMedia.
One-pot two-step procedure for the preparation of ethyl 2-acetyl-1,3-thiazole-4-carboxylate (4a). Sodium
bicarbonate (0.68 g, 8.1 mmol) was added to L-cysteine ethyl ester hydrochloride (1 g, 5.4 mmol) in water (25
mL). A solution of pyruvaldehyde (0.43 g, 5.9 mmol) in ethanol (25 mL) was added to the reaction mixture. The
o
reaction mixture was stirred at 60 C for 8 h and concentrated to evaporate the solvent under vacuum after
the consumption of starting materials (checked by TLC). The reaction mixture was extracted with
dichloromethane (2 × 30 mL). The combined organic extracts were dried over anhydrous sodium sulfate and
concentrated to reduce the solvent to half (30 mL). Iodobenzene diacetate (3.65 g, 11.3 mmol) was added and
0
stirred the reaction mixture at 40 C for 4 h. After completion of the reaction (monitored by TLC) the solvent
was evaporated. The product was purified by column chromatography (5% Ethyl acetate/hexane).
o
o
14
-1
Ethyl 2-acetylthiazole-4-carboxylate (4a): mp 65-67 C (lit mp 68 C) ; yield 62%; IR (cm ): 1728 (CO, ester),
1
1695 (CO, acetyl); H NMR (300 MHz, CDCl
H, thiazole 5-H).
3
, δ): 1.45 (t, 3H, J 7.2 Hz), 2.79 (s, 3H), 4.49 (q, 2H, J 7.2 Hz), 8.44 (s,
1
General procedure for the synthesis of bacillamide A, 2-acetyl-N-[2-(1H-indol-3-yl)ethyl]-1,3-thiazole-4-
carboxamide (1a) and analogues. A mixture of ethyl-2-acetyl-1,3-thiazole-4-carboxylate (4a, 0.40 g, 2 mmol),
tryptamine (5, 0.40 g, 2.5 mmol) and 4,6-dimethylpyrimidin-2-ol (6, 0.12 g, 2 mmol) was ground vigorously
o
using a pestle and mortar. The contents were transferred to a round bottom flask and heated to 100-105 C
for 6 h. After completion of the reaction (monitored by TLC), water (25 ml) was added. Compound was
extracted with ethyl acetate. The organic layer was washed thoroughly with 10% HCl solution (2 × 25 ml) and
the combined organic extracts were dried over anh. sodium sulfate. The reaction mixture was concentrated
and the crude product crystallized from ethanol.
Page 6
©ARKAT USA, Inc

Arkivoc 2018, v, 0-0
Kumar, S. et al.
o
The contents were transferred to a round bottom flask and heated to 100-105 C for 6 h. After completion of
the reaction (monitored by TLC), water (25 ml) was added. Compound was extracted with ethyl acetate. The
organic layer was washed thoroughly with 10% HCl solution (2 × 25 ml) and the combined organic extracts
were dried over anh. sodium sulfate. The reaction mixture was concentrated and the crude product
crystallized from ethanol.
Similarly other compounds (1b-g) are prepared using the same procedure.
Bacillamide A (1a). Brown solid; mp 165-168 C (lit mp 169 C); yield 82%; IR (cm ): 3271 (NH), 1697 (CO),
o
10
o
-1
1
1
659 (CONH); H NMR (300 MHz, DMSO-d
6
, δ): 2.72 (s, 3H, CH
-NH), 7.05 (t, 1H, J 7.2 Hz, 5′-H), 7.16 (t, 1H, J 7.5 Hz, 6′-H), 7.20 (s, 1H, 2′-H), 7.37 (d, 1H, J
.5 Hz, 7′-H), 7.63 (d, 1H, J 7.5 Hz, 4′-H), 8.66 (bs, 1H, thiazole-5H), 10.84 (s, indole-NH, exchangeable with
3
), 2.95 (t, 2H, J 7.2 Hz, CH
2
-indole), 3.61 (dd, 2H,
J 6.6 Hz, 7.8 Hz, CH
2
7
D
13
2
O); C NMR (75 MHz, DMSO-d , δ): 18.93, 25.47, 40.82, 111.34, 112.07, 118.45, 118.72, 121.42, 122.29,
6
122.45, 127.22, 136.49, 149.73, 160.99, 165.75, 190.96.
Acknowledgements
Authors are thankful to UGC, New Delhi, India for providing financial assistance in the form of UGC Major
Research Project (F.No. 33-300/2007; 2008-2011).
Supplementary Material
1
Supplementary data [ H NMR data of the compounds (1a and 4a)] associated with this article can be found in
the online version.
References
1
2
3
4
5
6
7
. Harvey, A. L.; Edrada-Ebel, R. A.; Quinn. R. J. Nat. Rev. Drug Discov. 2015, 14, 111.
http://dx.doi.org/10.1038/nrd4510
. Cragg, G. M.; Newman, D. J. Biochim. Biophys. Acta 2013, 1830, 3670.
http://dx.doi.org/10.1016/j.bbagen.2013.02.008
. Blunt, J. W.; Copp, B. R.; Keyzers, R. A.; Munro, M. H.; Prinsep, M. R. Nat. Pro. Rep. 2013, 30, 237.
http://dx.doi.org/10.1039/c2np20112g
. Nightingale, C. H.; Greene, D. S.; Quintiliani, R. J. Pharm. Sci. 1975, 64, 1899.
https://doi.org/10.1002/jps.2600641202
. Newton, G. G. F.: Abraham, E. P. Nature 1955, 175, 548.
http://dx.doi.org/10.1038/175548a0
. Nakamura, H.; Ohizumi, Y.; Kobayashi, J. Tetrahedron Lett. 1984, 25, 2475.
https://doi.org/10.1016/S0040-4039(01)81208-9
. Hossfeld, D. K.; Weh, H. J.; Kleeberg, U. R. Leuk. Res. 1985, 9, 329.
https://doi.org/10.1016/0145-2126(85)90096-7
Page 7
©ARKAT USA, Inc

Arkivoc 2018, v, 0-0
Kumar, S. et al.
8. Davyt, D.; Entz, W.; Fernandez, R.; Mariezcurrena, R.; Mombru, A. W.; Saldana, I. Dominguez, L.; Coil, J.;
Manta, E. J. Nat. Prod. 1998, 61, 1560.
https://doi.org/10.1021/np980114c
9
1
1
1
1
1
. Jeong, S.; Ishida, K; Ito, Y; Okada, S.; Mukrakami, M. Tetrahedron Lett. 2003, 44, 8005.
https://doi.org/10.1016/j.tetlet.2007.09.052
0. Socha, A. M.; Long, R. A.; Rowley, D. C. J. Nat. Prod. 2007, 70, 1793.
http://doi/10.1021/np070126a
1. Bray, C. D.; Olasoji, J. Synlett 2010, 599.
http://doi/10.1055/s-0029-1219153
2. Sun, X.; Liu, Y.; Liu, J.; Gu, G.; Du, Y. Org. Biomol. Chem. 2015, 13, 4271.
http://doi/10.1039/c5ob00093a
3. Figueria, V. B. C.; Prabhakar, S.; Lobo, A. M. Arkivoc 2005, (xiv), 14.
http://dx.doi.org/10.3998/ark.5550190.0006.e02
4. Kumar, S.; Aggarwal, R.; Kumar, V.; Sadana, R., Patel, B., Kaushik, P.; Kaushik, D. Eur. J. Med. Chem. 2016,
123, 718.
https://doi.org/10.1016/j.ejmech.2016.07.033
1
1
1
1
1
2
2
5. Kumar, S.; Aggarwal, R.; Sharma, C. Synth. Commun. 2015, 2022.
http://dx.doi.org/10.1080/00397911.2015.106986
6. Aggarwal, R.; Kumar, S.; Kaushik, P.; Kaushik, D.; Gupta, G. K. Eur. J. Med. Chem. 2013, 62, 508.
https://doi.org/10.1016/j.ejmech.2012.11.046
7. Hughes, R. A.; Thompson, S. P.; Alcaraz, L.; Moody, C. J. J. Am. Chem. Soc. 2005, 127, 15644.
https://doi.org/10.1021/ja0547937
8. Credico, B. D.; Reginato, G.; Gonsalvi, L.; Peruzzini, M.; Rossin, A. Tetrahedron 2011, 67, 267.
https://doi.org/10.1016/j.tet.2010.09.104
9. Sugiyama, H.; Yokokawa, F.; Shioiri, T. Org. Lett. 2000, 2, 2149.
http://10.1021/ol000128l
0. Gududuru, V.; Hurh, E.; Dalton, J. T.; Miller, D. D. J. Med. Chem. 2005, 48, 2584.
https://doi.org/10.1021/jm049208b
1. Lu, Y.; Li, C. M.; Wang, Z.; Ross II, C. R.; Chen, J.; Dalton, J. T.; Li, W.; Miller, D. D. J. Med. Chem. 2009, 52,
1701.
https://doi.org/10.1021/jm801449a
2
2
2
2
2. Tangella, Y.; Manasa, K. L.; Sathish, M.; Alarifi, A.; Kamal, A. Chemistry Select. 2016, 1, 2895.
https://doi.org/10.1002/slct.201600772
3. Han, C.; Lee, J. P.; Lobkovsky, E.; Porco, J. A. J. Am. Chem. Soc. 2005, 127, 10039.
https://doi.org/10.1021/ja0527976
4. Fischer, C. B.; Steininger, H.; Stephenson, D. S.; Zipse, H. J. Phys. Org. Chem. 2005, 18, 901.
http://dx.doi.org/10.1002/poc.914
5. Lima, E. C.; deSouza, C. C.; Soares, R. O.; Vaz, B. G.; Eberlin, M. N.; Dias, A. G.; Costa, P. R. R. J. Braz. Chem.
Soc. 2011, 22, 2186.
http://dx.doi.org/10.1590/S0103-50532011001100023
6. Basha, A.; Lipton, M.; Weinreb, S. M. Tetrahedron Lett. 1977, 18, 4171.
https://doi.org/10.1016/S0040-4039(01)83457-2
7. Navarro, J. A. R.; Freisinger, E.; Lippert, B. Inorg. Chem. 2000, 39, 1059.
https://doi.org/10.1021/ic9903634
2
2
Page 8
©ARKAT USA, Inc