Convergent synthesis of Carpatamide-A: Cytotoxic arylamine derivative from marine derived Streptomyces sp.

Article abstract of DOI:10.1080/14786419.2018.1460837

First total synthesis of carpatamide-A 7a, cytotoxic arylamine derivative isolated from marine derived Streptomyces sp., was achieved in twelve steps with overall yield of 24% with seven longest linear steps. In the penultimate step, dienoic acid 13 and a

Full text of DOI:10.1080/14786419.2018.1460837

Natural Product Research
Formerly Natural Product Letters
ISSN: 1478-6419 (Print) 1478-6427 (Online) Journal homepage: http://www.tandfonline.com/loi/gnpl20
Convergent synthesis of Carpatamide-A: Cytotoxic
arylamine derivative from marine derived
Streptomyces sp.
Rohani Prasad Burman, Shantanu Gupta, Jyoti Bhatti, Krishan Verma,
Devendra Rajak & Manjinder Singh Gill
To cite this article: Rohani Prasad Burman, Shantanu Gupta, Jyoti Bhatti, Krishan Verma,
Devendra Rajak & Manjinder Singh Gill (2018): Convergent synthesis of Carpatamide-A: Cytotoxic
arylamine derivative from marine derived Streptomyces sp., Natural Product Research, DOI:
10.1080/14786419.2018.1460837
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Natural Product research, 2018
https://doi.org/10.1080/14786419.2018.1460837
Convergent synthesis of Carpatamide-A: Cytotoxic arylamine
derivative from marine derived Streptomyces sp.
Rohani Prasad Burman, Shantanu Gupta, Jyoti Bhatti, Krishan Verma, Devendra Rajak
and Manjinder Singh Gill
department of Pharmaceutical technology (Process chemistry), National Institute of Pharmaceutical
education and research (NIPer), Punjab, India
ABSTRACT
ARTICLE HISTORY
received 28 december 2017
accepted 31 March 2018
First total synthesis of carpatamide-A 7a, cytotoxic arylamine derivative
isolated from marine derived Streptomyces sp., was achieved in twelve
steps with overall yield of 24% with seven longest linear steps. In
the penultimate step, dienoic acid 13 and an amino-phenylpropionic
acid methyl ester core 21 were coupled to synthesize methylated
derivativative of carpatamide-A 22 followed by demethylation of
the intermediate with BBr3 to accomplish carpatamide-A 7a. Both
precursors 13 and 21 were synthesized from readily available starting
materials i.e. isovaleraldehyde 8 and 2, 4-dihydroxy benzaldehyde 14.
KEYWORDS
convergent synthesis;
arylamine; carpatamide-a;
isovaleraldehyde; horner-
Wadsworths-emmons
reaction
1. Introduction
Marine natural products are an important source of lead compounds in medicine because
of their variety and novel architecture. The isolation of cytotoxic arylamine derivatives car-
patamide-A from marine derived Streptomyces sp (Fu et al. 2014) also accompanied the
isolation of two other derivatives namely carpatamide-B and carpatamide-C. These com-
pounds possess a novel core structure consisting of an amino-phenylpropionic acid, and an
unsaturated fatty acid chain i.e. dienoic acid chain joined to the amine moiety of amino-phe-
nyl propionic acid via an amide bond. Such aromatic substitution pattern has previously
been encountered only in manumycin 1 (Kohno et al. 1996) and derivatives (Hwang et al.
1996) from Streptomyces parvulus. There are a large number of phenylpropionic acid-
containing natural products reported in the literature (Figure 1). Compound 2, a major
CONTACT Manjinder singh Gill
msingh@niper.ac.in
supplemental data for this article can be accessed at https://doi.org/10.1080/14786419.2018.1460837.
© 2018 Informa uK limited, trading as taylor & Francis Group

2
R. P. BURMAN ET AL.
Figure 1. structural analogues of carpatamides.
metabolite from the crude extract of Bacillus licheniformis SAB1, showed significant antifun-
gal activity against Rhodotorula sp. and moderate antifungal activity against Candida albicans
and Aspergillus niger. Bioactive compound 3 (Apteniol-A), an oxyneolignan from the Leaves
of Aptenia cordifolia a terrestrial perennial herb belonging to the family Aizoaceae also pro-
duced by marine bacterium B. licheniformis SAB1, showed significant antifungal activity
against Aspergillus fumigates and moderate antibacterial activity against Vibrio cholerae and
Salmonella typhi (Devi et al. 2010). Compounds 4 and 5, noroxyneolignan isolated from
Oxalis pescaprae (Greca et al. 2009) showed phytotoxic activity on germination, root elon-
gation and shoot elongation of Lactuca sativa. Compound 6 (Piperlotine-E), isolated from
the leaves of Piper lolot, showed potent inhibitory activity on platelet aggregation induced
by arachidonic acid (AA) and platelet activating factor (PAF). Compounds 7a, 7b and 7c
(Carpatamides-A, B and C), respectively, were recently isolated and evaluated for cytotoxic
activity. 7a and 7c have shown moderate cytotoxicity against non-small-cell lung cancer
cell lines HCC366, A549, and HCC44 but no activity against H2122 (Fu et al. 2014). These were
also evaluated for their cytotoxic activity specially against NSCLC cell lines HCC366, A549,
and HCC44 with IC50 values ranging from 2.2 to 8.4 μM, but 7b did not show any activity
against cancer cell lines due to its inability to penetrate cancer cells, as the studies have
demonstrated that methyl ester of 7a acts as a prodrug and is cleaved in cells to give the
active pharmacophore (Fu et al. 2014).
To the best of our knowledge, this intriguing molecular architecture of carpatamide-A
and related compounds comprising of unsaturated fatty acid with arylamine propionic acid
esters have not been synthesized in laboratory to date. The low natural abundance of these
compounds with impressive biological properties prompted us to explore concise and scal-
able total synthesis of these molecules. In continuation of our program involving synthesis
of natural products of therapeutically importance, herein a concise approachtowards total

NATURAL PRODUCT RESEARCH
3
synthesis of carpatamides-A is described. This flexible synthetic strategy could be utilized
to synthesize other members of carpatamides.
A retrosynthetic route to carpatamide-A has been depicted in Scheme 1. As shown, amide
linkage could be accomplished by coupling of the corresponding unsaturated fatty acid 13
with amine derivative 20 or 21. The unsaturated fatty acid can be synthesize from readily
available isovaleraldehyde 8, and arylamine derivative 20 or 21 from commercially available
2, 4-dihydroxy benzaldehyde 14 in a set of reactions involving nitration, Horner-Wadworth-
Emmons, catalytic hydrogenation, oxidation and hydrolysis.
2. Results and discussion
As envisaged in Scheme 2, synthesis of unsaturated fatty acid 13 was commenced with HWE
(Ando and Yamada 2011) reaction of isovaleraldehyde 8 with triethylphosphonoacetate
readily produced corresponding selective (E) α,β-unsaturated ester 9 with an isolated yield
of 89% in the presence of DBU and K2CO3 under solvent free reaction conditions.
Retrosynthetically reduction of 9 with DIBAL-H (Weiß et al. 2012) produced desired alcohol
10 in an yield of 79% at −78 °C addition followed by warming up to room temperature. The
metallic oxidation of 10 with MnO2 (Sharma et al. 2008) conferred the corresponding alde-
hyde 11 in an isolated yield of 68% after column chromatography. The aldehyde 11 was
further subjected to HWE (Ando andYamada 2011) reaction with triethylphosphonoacetate
to yield the selective (E) α,β- unsaturated fatty acid ester 12 as a clear oil with an isolated
yield of 87% under solvent free reaction conditions. The (E) α,β- unsaturated fatty acid ester
12 has proved to be low boiling point, which required attentive operation during work-up
in order to prevent loss on evaporation. The ester 12 was further hydrolyzed in aqueous
NaOH solution (4 N) (Ueno et al. 2005) at reflux condition to obtain desired dienoic acid 13
quantitatively as monitored by TLC, which was also a low boiling clear oil.
With unsaturated fatty acid 13 secured, we then focused on the synthesis of other pre-
cursor the pivotal arylamines (Scheme 3), 20 and 21. Treatment of commercially available
2,4-dihydroxybenzaldehyde 14 with concentrated HNO3 (Selvaraju et al. 2015) regioselec-
tively produced 5-nitro 2,4-dihydroxybenzaldehyde 15 in 84% yield. Retrosynthetically we
needed to protect free hydroxyls of 15 by using benzyl bromide and K2CO3 at refluxed
conditions to produce 2,4-bis (benzyloxy)-5-nitrobenzaldehyde 16 in 65% isolated yield.
Aldehyde 16 underwent HWE olefination (Ando and Yamada 2011) to yield the (E) α,β-
unsaturated ester 18 with a yield of 89% under solvent free reaction conditions. Using 18
then we performed catalytic hydrogenation (Wilford et al. 1996) at refluxed conditions led
to three synthetic transformation i.e. reduction of nitro group to desired amino group, reduc-
tion of the conjugated double bond and deprotection of benzyl ethers to give pivotal 20 as
an orange fluoresce compound in an isolated yield of 72%.
With two desired fragments 13 and 20 retrosynthetically, we then aimed to furnish car-
patamide-A by coupling of 13 and 20 using EDCI (Hausler et al. 2012) as a coupling agent
with a catalytic amount of DMAP under reflux conditions. Unfortunately, the reaction mix-
tures obtained during number of attempts showed multiple close moving spots on thin
layer chromatography, which were difficult to purify. The reason for this outcome could very
well be due to the presence of free hydroxyl group on the intermediate 20 and these can
also participate in coupling leading to ester formation, and ester are prone to hydrolysis, the
presence of corresponding acids can also not be ruled out.

4
R. P. BURMAN ET AL.
Scheme 1. retrosynthetic approach to carpatamide-a.
Scheme 2. synthesis of unsaturated fatty acid 13.
Consequently, we diverted the benzyl to methyl group. Following the preparation of the
corresponding compound 21, We prepared 2,4-dimethoxy-5-nitrobenzaldehyde 17 with
84% yield using 15, methyl iodide and K2CO3 (Gudipudi et al. 2014) in acetone at refluxed
conditions. The HWE (Ando andYamada 2011) reaction of aldehyde 17 with triethylphospho-
noacetate readily produced corresponding selective (E) α,β-unsaturated ester 19. Upon
catalytic hydrogenation (Wilford et al. 1996) of 19 using palladium on charcoal under refluxed
condition compound 21 was afforded as brown oil in 77% yield (Scheme 3).

NATURAL PRODUCT RESEARCH
5
Scheme 3. synthesis of amino-phenyl propionic acid methyl esters 20 and 21.
Scheme 4. synthesis of carpatamide-a, 7a.
Upon successfully synthesis of two retrosynthetically desired fragments 13 and 21 were
subjected to coupling reaction using EDCI, catalytic amount of DMAP (Hausler et al. 2012)
under refluxed to yield the desired methyl precursor 22 of carpatamide-A with an isolated
yields of 68%, which was also synthesized and reported by Fu et al. as derivative of carpat-
amide-A, but they have synthesized compound 22 in a biosynthetic manner, in which they
have firstly isolated the carpatamide-A 7a followed by methylation of carpatamide-A 7a
was carried out using TMS-CHN2 to yield compound 22.
Finally demethylation of 22 with boron tribromide (Bergman et al. 1999) at −78 °C resulted
target molecule carpatamide-A 7a with an isolated yield of 90% (Scheme 4). Spectroscopic
data of 7a and 22 were in agreement with those reported for the natural and synthetic
product respectively (Fu et al. 2014).
3. Experimental section
3.1. Preparation of dienoic acid fragment
3.1.1. Synthesis of ethyl (E)-5-methylhex-2-enoate (9)
To a stirred suspension of triethyl phosphonoacetate (10.90 mL, 55 mmol), potassium car-
bonate (13.82 g, 100 mmol) and DBU (2.23 mL, 1.5 mmol), isovaleraldehyde 8 (5.48 mL,

6
R. P. BURMAN ET AL.
50 mmol) was added at room temperature and reaction mixture was allowed to stirred at
same temperature and monitored by TLC, after completion of reaction (5 h) to a yellow mass
of reaction mixture was added DCM (3 × 50 mL). The combined organic extract were washed
with brine, dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure.
Purification of the residue by silica gel column chromatography (9:1 petroleum ether/EtOAc)
gave ester 9 as a clear oil: (6.9 mL, 89%); 1H NMR (400 MHz, DMSO): δ 0.93 (6H, d, J = 6.64 Hz),
1.28 (3H, t, J = 7.12 Hz), 1.78 (1H, m), 2.10 (2H, t, J = 6.92 Hz), 4.19 (2H, q, J = 7.16 Hz), 5.78
(1H, d, J = 16.24 Hz), 6.94 (1H, q, J = 7.88 Hz); 13C NMR (100 MHz DMSO) 14.20, 22.27, 27.73,
41.39, 60.03, 122.25, 148.12, 166.58; IR (KBr) νmax 2959, 2879, 1691, 1641, 1469, 1381, 1361,
1280 cm-1; HRMS (ESI): m/z: calcd for C9H16NaO2: [M + Na]+ 179.1048, Found 179.1041.
3.1.2. (E)-5-methylhex-2-en-1-ol (10)
To a stirred solution of ester 9 (6.24 mL, 40 mmol) in dry DCM (40 mL) was added slowly
DIBAL-H (1 M in THF) (100 mL, 100 mmol) over 15 min dropwise under argon at −78 °C. The
reaction mixture was gradually warmed to room temperature for 12 h and cooled again to
0 °C and quenched by the cautious addition of MeOH. A saturated solution of potassium
sodium tartrate (Rochelle salt) was added and mixture left to stirr for several hours until the
organic and aqueous layers had completely separated. The combined organic extract were
washed with brine, dried over anhydrous Na2SO4, filtered, and concentrated under reduced
pressure. Purification of the residue by silica gel column chromatography (8:2 petroleum
ether/EtOAc) gave alcohol 10 as clear viscous oil, (3.6 mL, 79%); 1H NMR (DMSO, 400 MHz):
δ 0.85 (6H, d, J = 6.68 Hz), 1.59 (1H, m), 1.89 (2H, t, J = 6.48 Hz), 3.89 (2H, t, J = 5.20 Hz), 4.62
(1H, t, J = 5.48 Hz), 5.54 (2H, m); 13C NMR (DMSO, 100 MHz) 27.29, 33.03, 46.30, 66.67, 133.69,
136.92; IR (KBr) νmax 3614, 3441, 2899, 1469, 1389, 1361 cm−1; HRMS (ESI): m/z calcd for
C7H14NaO: [M + Na]+ 137.0942, Found 137.0936.
3.1.3. (E)-5-methylhex-2-enal (11)
To a stirred solution of alcohol 10 (2.85 mL, 25 mmol) in CHCl3 (30 mL) was added MnO2
(17.38 g, 200 mmol). The mixture was heated at 50 °C overnight. The mixture was filtered
through a bed of Celite, solvent was carefully removed under reduced pressure and purified
1
by silica gel chromatography to give the product 11 as clear volatile oil (1.9 mL, 68%), H
NMR (DMSO, 400 MHz): δ 0.92 (6H, d, J = 6.68 Hz), 1.79 (1H, m), 2.21 (2H, dt, J = 1.08 Hz), 6.12
(1H, m), 7.01 (1H, m), 9.50 (1H, d, J = 8 Hz); 13C NMR (DMSO, 100 MHz) 22.03, 22.08, 27.16,
41.12, 133.59, 158.43, 194.43; IR (KBr) νmax 3606, 3491, 2910, 1772, 1469, 1389, 1361 cm−1;
HRMS (ESI): m/z calcd for C7H12NaO: [M + Na]+ 135.0786, Found 135.0774.
3.1.4. Ethyl (2E,4E)-7-methylocta-2,4-dienoate (12)
To a stirred suspension of triethyl phosphonoacetate (3.05 mL, 15.4 mmol), potassium car-
bonate (3.86 g, 28 mmol) and DBU (0.63 mL, 0.42 mmol) was added (E)-5-methylhex2-enal
11 (1.57 mL, 14 mmol) at room temperature and reaction mixture was allowed to stirred at
room temperature and monitored by TLC, after completion of reaction (5 h) reaction mixture
was partitioned between DCM and water, organic layer was separated and allowed to con-
centrate at reduced pressure carefully and purified by silica gel column chromatography by
using 5% ethyl acetate in petroleum ether to give ester 12 (1.94 mL, 87%), as a clear oil, 1H
NMR (DMSO, 400 MHz): d 0.85 (6H, d, J = 6.60 Hz), 1.20 (3H, t, J = 7.08 Hz), 1.66 (1H, m), 2.02
(2H, t, J = 6.36 Hz), 4.09 (2H, t, J = 7.12 Hz), 5.82 (1H, d, J = 15.32 Hz), 6.23 (2H, m), 7.20 (1H,

NATURAL PRODUCT RESEARCH
7
m); 13C NMR (DMSO, 100 MHz) 14.52, 22.48, 28.15, 42.17, 60.05, 119.50, 129.74, 143.87, 145.22,
166.62; IR (KBr) νmax 2955, 2869, 1721, 1641, 1514, 1475, 1376, 1355, 1320 cm−1; HRMS (ESI):
m/z: calcd for C9H16NaO2: [M + Na]+ 205.1204, Found 205.1225.
3.1.5. (2E,4E)-7-methylocta-2,4-dienoic acid
To a stirred solution of ester 12 (1.50 mL, 8.25 mmol) inTHF (10 mL) was added NaOH solution
(4 N) (5 mL) slowly and allowed to stirred at room temperature and monitored by TLC, upon
completion of reaction (5 h), reaction mixture was partitioned between DCM and water,
organic layer was separated and allowed to concentrate under reduced pressure to obtained
dienoic acid 13 (Quantitatively, on the basis of conversion in TLC) which require additional
care in order to avoid evapouration due to its low boiling point and moved to the coupling
step without further purification. HRMS (ESI): m/z calcd for C9H14NaO2: [M + Na]+ 177.0891,
Found 177.0893.
3.2. Preparation of amino aryl propionic acid methyl ester fragment
3.2.1. 2,4-dihydroxy-5-nitrobenzaldehyde (15)
To a stirred nitric acid (0.834 mL, 20 mmol) at ice bath 2,4-dihydroxybenzaldehyde 14 (1.38 g,
10 mmol) was added slowly in period of 15 min and reaction mixture allowed to stirred at
room temperature and monitored by TLC, upon completion of reaction (3 h) ice cold water
was added with stirring to precipitate the titled compound 15 as creamish crystalline solid,
(1.53 g, 84%); 1H NMR (DMSO, 400 MHz): δ 6.63 (1H, s), 8.29 (1H, s), 10.09 (1H, s), 11.87 (2H,
s); 13C NMR (DMSO, 100 MHz) δ 104.83, 116.08, 129.90, 130.21, 159.72, 166.14, 190.46; IR (KBr)
νmax 3426, 1764, 1469, 1389, 1361 cm−1; HRMS (ESI): m/z calcd for C7H4NNaO5: [M + Na]+
206.0065, Found 206.0071.
3.2.2. 2,4-bis(benzyloxy)-5-nitrobenzaldehyde (16)
To a stirred suspension of 2,4-dihydroxy-5-nitrobenzaldehyde 15 (1.062 g, 5.8 mmol), potas-
sium carbonate (2.004 g, 14.5 mmol) in acetone (20 mL), benzyl bromide (1.55 mL,
13.05 mmol) was added dropwise and allowed to refluxed and monitored by TLC, workup
was done with ethyl acetate and water, organic layer was concentrated under reduced pres-
sure to obtained crude which was purified by recrystallizaion to obtain 16 (65%), yellow
1
soild H NMR (DMSO, 400 MHz): δ 5.43 (2H, s), 5.44 (2H, s), 7.23 (1H, s), 7.50 (10H, m), 8.28
(1H, s), 10.18 (1H, s); 13C NMR (DMSO, 100 MHz) δ 71.05, 71.41, 100.43, 117.24, 126.77, 127.50,
127.73, 128.33, 128.35, 128.61, 128.65, 132.76, 135.15, 135.34, 157.91, 164.59, 186.69; IR (KBr)
νmax 3426, 3410, 2829, 2810, 1764, 1690 1469, 1389, 1361 cm−1; HRMS (ESI): m/z: calcd for
C21H17NNaO5 [M + Na]+ 386.1004, Found 386.1003.
3.2.3. 2,4-dimethoxy-5-nitrobenzaldehyde (17)
To a stirred suspension of 2,4-dihydroxy 5-nitrobenzaldehyde 15 (2.76 g, 20 mmol) and
potassium carbonate (6.91 g, 50 mmol) in acetone (30 mL) at ice cooled condition, methyl
iodide (2.73 mL, 44.4 mmol) was added and reaction mixture allowed to stirred at room
temperature and monitored by TLC, upon completion of reaction (6 h), workup was done
using ethyl acetate and water, organic layer was concentrated under reduced pressure to
obtain 17, (2.79 g, 84%), brown crystalline solid, 1H NMR (CDCl3, 400 MHz): δ 4.07 (3H, s), 4.08
(3H, s), 6.55 (1H, s), 8.50 (1H, s), 10.28 (1H, s); 13C NMR (CDCl3, 100 MHz) δ 56.49, 56.97, 95.93,

8
R. P. BURMAN ET AL.
127.99, 159.53, 165.83, 186.54; IR (KBr) νmax 2981, 2970, 2892, 1702, 1671, 1580, 1420,
1302 cm−1; HRMS (ESI): m/z: calcd for C9H9NNaO5 [M + Na]+ 234.0378, Found 234.0376.
3.2.4. Methyl (E)-3-(2,4-bis(benzyloxy)-5-nitrophenyl)acrylate (18)
To a stirred suspension of trimethylphosphono acetate, potassium carbonate (3.31 g,
24 mmol) and DBU (0.54 mL, 0.36 mmol) was added 4-bis (benzyloxy)-5-nitrobenzaldehyde
16 (7.20 mL, 66 mmol) at room temperature and reaction mixture was allowed to stirred at
room temperature and monitored by TLC, after completion of reaction (5 h) reaction mixture
was partitioned between DCM and water, organic layer was separated and allowed to con-
centrate at reduced pressure carefully and purified by silica gel column chromatography by
using 5% ethyl acetate in petroleum ether to give titled compound 18 (1.94 mL, 89%), yellow
1
solid, H NMR (CDCl3, 400 MHz): δ 3.81 (3H, s), 5.18 (4H, s), 6.54 (1H, d, J = 16.12 Hz), 6.58
(1H, s), 7.43 (10H, m), 7.93 (1H, d, J = 16.16 Hz), 8.24 (1H, s); 13C NMR (CDCl3, 100 MHz) δ 51.79,
71.20, 71.43, 99.47, 116.71, 118.93, 126.86, 127.11, 127.15, 128.48, 128.69, 128.87, 129.01,
133.08, 134.96, 134.98, 137.50, 155.33, 165.50, 167.46; IR (KBr) νmax 3468, 3422, 1761, 1456,
1469, 1332, 1261 cm−1; HRMS (ESI): m/z calcd for C24H21NNaO6 [M + Na]+ 442.1267, Found
442.1237.
3.2.5. Methyl (E)-3-(2,4-dimethoxy-5-nitrophenyl)acrylate (19)
To a stirred suspension of trimethylphosphonoacetate (1.43 mL, 8.8 mmol), potassium car-
bonate (2.21 g, 16 mmol) and DBU (0.32 mL, 0.24 mmol) was added 2,4-dimethoxy-5-ni-
trobenzaldehyde 17 (1.68 g, 8 mmol) at room temperature and reaction mixture was allowed
to stirred and monitored by TLC, after completion of reaction (5 h), reaction mixture was
partitioned between DCM and water, organic layer was separated and allowed to concentrate
at reduced pressure carefully and purified by silica gel column chromatography by using
5% ethyl acetate in petroleum ether to give titled compound 19 (1.73 g, 81%), as yellow
solid; 1H NMR (CDCl3, 400 MHz): δ 3.81 (3H, s), 4.01 (3H, s), 4.03 (3H, s), 6.51 (1H, d, J = 16.12 Hz),
6.52 (1H, s); 7.85 (1H, d, J = 16.12 Hz), 8.21 (1H, s); 13C NMR (CDCl3, 100 MHz) δ 51.76, 56.29,
56.74, 95.96, 116.07, 118.72, 127.27, 132.40, 137.52, 156.78, 162.92, 167.47; IR (KBr) νmax 3020,
2972, 2810, 1700, 1610, 1590, 1440, 1320 cm−1: HRMS (ESI): m/z: calcd for C12H13NNaO6
[M + Na]+ 290.0641, Found 290.0602.
3.2.6. Methyl 3-(5-amino-2,4-dihydroxyphenyl)propanoate (20)
To a stirred Suspension of methyl (E)-3-(2,4-bis(benzyloxy)-5-nitrophenyl)acrylate 18 (2.30 g,
5.5 mmol) and ammonium formate (1.47 g, 23.27 mmol) in methanol, Pd/C (20 mol %) was
added and reaction mixture was allowed to refluxed and monitored by TLC, after completion
of reaction (3 h), reaction mixture was passed through the celite bed and washed with meth-
anol and filtrate was concentrated under reduced pressure to obtain crude, which was puri-
fied by column chromatography using 15% EtOAc: hexane to obtain desired methyl
3-(5-amino-2,4-hydroxyphenyl)propanoate 20 (832 mg, 72%), light brown oil; 1H NMR (CDCl3,
400 MHz): δ 2.74 (2H, t, J = 6.88 Hz), 2.96 (2H, t, J = 7.00 Hz), 3.71 (3H, s), 6.56 (1H, s), 7.39
(1H, s); 13C NMR (CDCl3, 100 MHz) δ 22.71, 25.04, 52.17, 104.70, 113.44, 122.40, 122.69, 125.64,
131.59, 146.91, 148.23, 148.75, 174.60; IR (KBr) νmax 3420, 2994, 2872, 2860, 1740, 1691,
1540 cm−1: HRMS (ESI): m/z calcd for C10H13NNaO4 [M + Na]+234.0742, Found 234.0753.

NATURAL PRODUCT RESEARCH
9
3.2.7. Methyl 3-(5-amino-2,4-dimethoxyphenyl)propanoate (21)
To a stirred solution of methyl (E)-3-(2,4-dimethotxy-5-nitrophenyl)acrylate 19 (1.33 g,
5 mmol) and ammonium formate (1.33 g, 21.25 mmol) in methanol, Pd/C (20 mol %) was
added and reaction mixture was allowed to refluxed and monitored by TLC, after completion
of reaction (3 h), reaction mixture was passed through the celite bed and washed with meth-
anol and filtrate was concentrated under reduced pressure to obtain crude, which was puri-
fied by column chromatography using 10% EtOAc: hexane to obtain desired methyl
1
3-(5-amino-2,4-dimethoxyphenyl)propanoate 21 (861 mg, 72%), light brown oil; H NMR
(CDCl3, 400 MHz): δ 2.50 (2H, t, J = 7.96 Hz), 2.76 (2H, t, J = 7.64 Hz), 3.59 (3H, s), 3.70 (3H, s),
3.76 (3H, s), 6.36 (1H, s), 6.48 (1H, s); 13C NMR (CDCl3, 100 MHz) δ 24.38, 33.51, 50.47, 54.76,
55.28, 95.83, 116.20, 119.88, 128.00, 145.38, 149.43, 172.93; IR (KBr) νmax 3480, 2980, 2972,
2960, 2820, 1980, 1720, 1690, 1580, 1410, 1390, 1220 cm−1: HRMS (ESI): m/z calcd for
C12H17NNaO4 [M + Na]+ 262.1055, Found 262.1038.
3.3. Coupling of two fragments
3.3.1. Methyl 3-(2,4-dimethoxy-5-((2E,4E)-7-methylocta-2,4-dienamido)phenyl)
propanoate 22
To a stirred suspension of methyl 3-(5-amino-2,4-dimethoxyphenyl)propanoate 21
(478.54 mg, 2 mmol), EDCI.HCl (341.55 mg, 2.2 mmol) and DMAP (24.4 mg, 0.2 mmol) in
dichloromethane (5 mL), (2E,4E)-7-methylocta-2,4-dienoic acid 13 (339.25 mg, 2.2 mmol)
was added and reaction mixture was allowed to stirred at refluxed condition and monitored
by TLC, after completion of reaction (5 h), work-up was done using dichloromethane and
water, organic layer was concentrated under reduced pressure to obtain crude, which was
purified by column chromatography using 15% EtOAc: hexane to yield title compound 22,
(510 mg, 68%), as yellow oil. 1H NMR (CDCl3, 400 MHz): δ 0.93 (6H, d, J = 6.64 Hz), 1.76 (1H,
m), 2.09 (2H, t, J = 6.76 Hz), 2.61 (2H, t, J = 8.32 Hz), 2.92 (2H, t, J = 7.52 Hz), 3.69 (3H, s), 3.82
(3H, s), 3.89 (3H, s), 5.95 (1H, d, J = 14.92 Hz), 6.22 (2H, m), 6.46 (1H, s), 7.30 (1H, m), 7.57 (1H,
s), 8.25 (1H, s); 13C NMR (CDCl3, 100 MHz) δ 22.33, 25.53, 28.31, 34.44, 42.33, 51.49, 55.71,
55.92, 95.08, 114.06, 120.72, 121.81, 126.48, 129.35, 141.83, 142.38, 147.57, 153.79, 163.88,
173.80; IR (KBr) νmax 3410, 2969, 2929, 1729, 1610, 1390, 1220, 1148, 1102, 1037 cm−1: HRMS
(ESI): m/z calcd for C21H29NNaO5 [M + Na]+ 398.1943, Found 398.1936.
3.3.2. Carpatamide-A (7a)
To a stirred solution of compound 22 (375.46 mg, 1 mmol), in dried dichloromethane at
−78 °C, boron tribromide (2.5 mL, 2.5 mmol) in DCM (1 M) was added dropwise and reaction
mixture was warmed to room temperature slowly and monitored by TLC, after completion
of reaction (4 h), work-up was done using dichloromethane and water, organic layer was
concentrated under reduced pressure to obtain crude, which was purified by column chro-
matography using 15% EtOAc: petroleum ether to yield title compound carpatamide-A 7a,
(315 mg, 90%), as yellow oil.1H NMR (CDCl3, 400 MHz): δ 0.94 (6H, d, J = 6.64 Hz), 1.77 (1H,
m), 2.10 (2H, t, J = 6.40 Hz), 2.67 (2H, t, J = 6.60 Hz), 2.81 (2H, t, J = 6.40 Hz), 3.69 (3H, s), 5.99
(1H, d, J = 14.84 Hz), 6.20 (2H, m), 6.53 (1H, s), 6.75 (1H, s), 7.39 (1H, m), 7.67 (1H, s); 13C NMR
(CDCl3, 100 MHz) δ 22.34, 22.66, 22.69, 24.29, 28.30, 34.80, 42.39, 52.17, 107.49, 114.07, 119.83,
123.89, 129.10, 139.29, 144.06, 144.27, 148.82, 153.43, 165.77, 175.73; IR (KBr) νmax 3610, 3440,

10
R. P. BURMAN ET AL.
2996, 2920, 2890, 2862, 2866, 1740, 1680, 1590, 1485, 1392, 1248, 1182, 1037 cm−1: HRMS
(ESI): m/z: calcd for C19H25NNaO5 [M + Na]+ 370.1630, Found 370.1628.
4. Conclusions
In summary, a potentially scalable, an efficient and convergent approach to carpatamide-A
(7a) has been accomplished in twelve steps with overall yield of 24% with seven longest
linear steps. The salient features of this synthetic approach are including Horner-Wadworth-
Emmons reaction for the construction of the required E-double bond, metallic oxidation,
nitration and incorporation of multiple transformations in catalytic hydrogenation step.
Acknowledgements
The authors wish to express their thanks to NIPER, SAS Nagar, Mohali, Punjab for providing necessary
facilities to carry out this research work. The authors would also thank to Ministry of Chemical and
Fertilizers, Govt. of India for NIPER Research fellowship Scheme.
Disclosure statement
No potential conflict of interest was reported by the authors.
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