First stereoselective synthesis of cytotoxic (-)-kunstleramide

Article abstract of DOI:10.1016/j.tetlet.2013.06.034

The first stereoselective synthesis of the cytotoxic (-)-kunstleramide (1) has been reported from simple and commercially available starting material 3,4-dimethoxyphenylpropanol. The key steps involved are the MacMillan α-hydroxylation, Horner-Wadsworth-E

Full text of DOI:10.1016/j.tetlet.2013.06.034

Accepted Manuscript
First stereoselective synthesis of cytotoxic (-)-kunstleramide
S. Purushotham Reddy, Y. Venkateswarlu
PII:
S0040-4039(13)00990-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.06.034
TETL 43087
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
28 March 2013
6 June 2013
10 June 2013
Please cite this article as: Purushotham Reddy, S., Venkateswarlu, Y., First stereoselective synthesis of cytotoxic
(-)-kunstleramide, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.2013.06.034
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First stereoselective synthesis of cytotoxic (-)-kunstleramide
S. Purushotham Reddy and Y. Venkateswarlu*
Division of Natural Products Chemistry,
CSIR-Indian Institute of Chemical Technology, Hyderabad, 500 007, India
Received………………
________________________________________________________________________
Abstract: The first stereoselective synthesis of the cytotoxic (-)-kunstleramide (1) has
been reported from simple and commercially available starting material 3,4-
dimethoxyphenylpropanol. The key steps involved are the MacMillan α-hydroxylation,
Horner-Wadsworth-Emmons (HWE) olefination, amide-Wittig olefination.
A variety of unsaturated fatty acid amides (alkamides/dienamides) constitutes an
important class of natural products and key intermediates in organic synthesis. These are
existing widely in higher plants, especially well-explored from Genus Piper.1
Interestingly these naturally occurring dienamides were found to express various
biological and pharmacological properties including insecticidal, larvicidal, antidiarrheal,
antiulcer, antioxidative, antimicrobial, antidepressant, activation of TRPV1 and TRPA1,
cell adhesion inhibition, cytotoxic, anti-diabetic, leishmanicidal, ACAT inhibition.2 In
addition some of these are widely used as food flavouring and traditional medicines in
Asia. Most of the naturally occurring dienamides have isobutyl, pyrrolidine and
piperidine as common amine moiety. Recently, Hamid et al isolated a dienamide
kunstleramide (Fig 1, 1) having ethylamine moiety from dichloromethane extract of bark
of Beilschmiedia kunstleri Gamble, along with six known compounds.3 (-) Kunstleramide
(1) exhibited moderate cytotoxic activity against five cancer cell lines3 and attracted our
attention for the synthesis because of its medicinal properties. The structure and
stereochemistry of 1 was established on the basis of spectral data. To the best of our
knowledge, so far no synthesis has been reported for 1.
____
Keywords: (-) Kunstleramide, Beilschmiedia kunstleri, Cytotoxic, MacMillan α-
hydroxylation, Horner-Wadsworth-Emmons (HWE) olefination, Amide-Wittig olefination,
Dienamides.
*Corresponding author. e-mail: luchem@iict.res.in Tel.: +91 40 27193167; fax: +91 40
27160512.

Figure 1. Structure of (-)-kunstleramide
In continuation of our work in the synthesis of biologically active natural
products,4 we became interested in the design of a simple and concise route for synthesis
of (-)-kunstleramide. Here in, we report an efficient approach for the stereoselective
synthesis of (-)-kunstleramide (1) employing MacMillan α-hydroxylation, Horner-
Wadsworth-Emmons (HWE) olefination, amide-Wittig olefination as key steps.
The retro synthetic analysis of 1 revealed that the key intermediate 3 can be
synthesized through MacMillan α-hydroxylation followed by rapid Horner-Wadsworth-
Emmons (HWE) olefination of the aldehyde derived from the IBX oxidation of 2 as
depicted in Scheme 1.
Scheme 1
The sequence of reaction steps in asymmetric synthesis of (-)-kunstleramide
started from 3,4-dimethoxyphenylpropanol (2) which was oxidized to aldehyde using
IBX. Following MacMillan α-hydroxylation5 using nitrosobenzene (PhNO) and 40 mol%
of L-proline in DMSO, and rapid in situ Horner–Wadsworth–Emmons olefination with
triethyl phosphonoacetate and DBU as base resulted in the formation of aminoxyolefinic
ester. The cleavage of the O–N bond in aminoxyolefinic ester6 using 20 mol% of
CuSO4.5H2O in methanol at room temperature gave the γ-hydroxy α, β-unsaturated ester
3 in 60% yield, with an excellent enantioselectivity (97% ee, determined by chiral
HPLC). The absolute stereochemistry of the newly generated stereogenic center in
compound 3 bearing the hydroxyl group was determined by preparing the R and S MTPA
esters by a modified Mosher’s method8 and found to have (R)- configuration (Fig. 2). The
negative chemical shift difference to the left side of the MTPA plane and the positive

chemical shift differences to the right side of the MTPA plane indicated that the hydroxyl
stereochemistry has (R)-configuration (Fig. 2).
The newly generated hydroxyl group in 3 was protected as its silyl ether 4 in 96% yield
with TBSCl and imidazole. The silyl ester was converted into allylic alcohol 5 using
DIBAL-H in 91% yield. Compound 5 was oxidized with IBX (2-iodoxybenzoic acid) to
corresponding aldehyde which was subjected to amide-Wittig olefination7 using
phosphonium salt 9, and KOtBu to afford 6 in 62% yield along with 6% of Z-isomer
(two separable isomers through column chromatography). On the other hand the amide 6
was also prepared from aldehyde obtained from 5 by two carbon Wittig reaction with
Ph3P=CHCO2Et, to afford E, E ester 7 in 85% yield. The ester 7 was hydrolyzed with
LiOH.H2O to give corresponding diene acid 8 in 90% yield. The diene acid 8 was
converted to amide 6 in 93% yield using HATU (O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-
tetramethyluronium hexafluorophosphate), DIPEA (N,N-Diisopropylethylamine) and
ethylamine. Finally, deprotection of TBS ether in compound 6 using p-TSA in MeOH
25
gives target molecule 1 in 95% yield. The physical [α]D -54.2 (C = 0.042, MeOH) and
spectral data (1H NMR and 13C NMR) of synthetically prepared compound 1 are in good
agreement with the reported data of natural product.3
Scheme 2
Figure 2
In conclusion, we have reported the first stereoselective synthesis of the cytotoxic (-)-
kunstleramide (1) in five steps (31% overall yield) from commercially available starting
material 3, 4-dimethoxyphenylpropanol by using MacMillan α-hydroxylation, Horner-
Wardsworth-Emmons (HWE) olefination and amide-Wittig olefination as key steps. We
are currently preparing a broad range of amide analogues of 1 for use in structure activity
relationship studies from the key intermediate 8.
Supplementary data

Typical detailed experimental procedure and analytical data for all the compounds
associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.
Acknowledgements
S. P. R. thanks UGC-CSIR, New Delhi, INDIA, for the financial assistance in the form of
fellowship.
References and notes
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Chem. 2012, 60, 2884-2895; (c) Okumura, Y.; Narukawa, M.; Iwasaki, Y.;
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Huang, Q. –Q.; Xu, J. –J.; Long, C. –L.; Li, J. J. Nat. Prod. 2011, 74, 45–49; (f)
Gosal, S.; Deb, A.; Mishra, P.; Vishwakarma, R. Plantamed. 2012, 78, 906-908;
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H. S.; Kim, Y. K. Phytochemistry 2007, 68, 899–903.
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Synthesis 2011, 19, 3180–3184; (b) Reddy, S. P.; Chinnababu, B.; Shekhar, V.;
Reddy, D. K.; Bhanuprakash, G.V.S.; Velatoor, L. R.; J. Venkateswara Rao,
Venkateswarlu, Y. Bioorg. Med. Chem. Lett. 2012, 22, 4182–4184; (c) Shekhar,

V.; Reddy, D. K.; Suresh, V.; Chanti Babu, D.; Venkateswarlu, Y. Tetrahedron
Lett. 2010, 51, 946–948; (d) Chandra Rao, D.; Reddy, D. K.; Shekhar, V.;
Venkateswarlu, Y. Tetrahedron Lett. 2013, 54, 828–829.
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9. Spectral data for selected compounds:
Compound 3: [α]D25 = +5 (c 0.4, CHCl3); IR (neat): 3492, 2938, 2837, 1713, 1656,
1515, 1462, 1264, 1238, 1158, 1030 and 767 cm-1; 1H NMR (300 MHz, CDCl3): δ =
7.0 (dd, J = 15.6, 4.5 Hz, 1 H), 6.85-6.71 (m, 3 H), 6.05 (d, J = 15.6 Hz, 1 H), 4.55-
4.44 (m, 1 H), 4.20 (q, J = 7.1 Hz, 2 H), 3.86 (s, 6 H), 2.91 (dd, J = 13.6, 4.9 Hz, 1
13
H), 2.72 (dd, J = 13.6, 8.1 Hz, 1 H), 1.28 (t, J = 7.1 Hz, 3 H); C NMR (75 MHz,
CDCl3): δ = 166.4, 148.9, 148.8, 147.8, 129.1, 121.4, 120.4, 112.5, 111.2, 71.6, 60.3,
55.8, 55.7, 42.7, 14.1; ESIMS: [M + Na]+ 303; Compound 4: [α]D25 = +0.6 (c 1.0,
CHCl3); IR (neat): 2954, 2932, 2856, 1719, 1657, 1515, 1465, 1262, 1159, 1115,
1032, 833 and 774 cm-1; 1H NMR (300 MHz, CDCl3): δ = 6.96 (dd, J = 15.1, 4.5 Hz,
1 H), 6.81-6.68 (m, 3 H), 5.96 (d, J = 15.1 Hz, 1 H), 4.46-4.37 (m, 1 H), 4.18 (q, J =
7.5 Hz, 2 H), 3.86 (s, 6 H), 2.83-2.65 (m, 2 H), 1.28 (t, J = 7.5 Hz, 3 H), 0.86 (s, 9 H),
13
-0.07 (s, 3 H), -0.20 (s, 3 H); C NMR (75 MHz, CDCl3): δ = 166.5, 150.2, 148.5,
147.7, 130.1, 121.8, 120.0, 113.2, 111.1, 73.1, 60.2, 55.9, 55.7, 43.8, 25.7, 18.1, 14.2,

-4.9, -5.3; ESIMS: [M + Na]+ 417; Compound 5: [α]D25 = +2.5 (c 1.2, CHCl3); IR
1
(neat): 3481, 2931, 2856, 1514, 1465, 1261, 1030, 834 and 774 cm-1; H NMR (300
MHz, CDCl3): δ = 6.81-6.67 (m, 3 H), 5.78-5.72 (m, 2 H), 4.32-4.23 (m, 1 H), 4.16-
4.07 (m, 2 H), 3.86 (s, 3 H), 3.85 (s, 3 H), 2.74-2.65 (m, 2 H), 0.83 (s, 9 H), -0.10 (s,
3 H), -0.20 (s, 3 H); 13C NMR (75 MHz, CDCl3): δ = 148.3, 147.3, 134.4, 131.1,
128.5, 121.7, 113.1, 110.8, 73.9, 62.8, 55.8, 55.6, 44.5, 25.7, 18.0, -4.7, -5.2; ESIMS:
25
[M + Na]+ 375; Compound 6: [α]D = -18 (c 1.0, CHCl3); IR (neat): 2930, 2856,
1661, 1616, 1515, 1462, 1260, 1150, 834 and 773 cm-1; 1H NMR (300 MHz, CDCl3):
δ = 7.19 (dd, J = 15.1, 11.3 Hz, 1 H), 6.79 (d, J = 8.3 Hz, 1 H), 6.70 (s, 2 H), 6.23
(dd, J = 15.1, 11.3 Hz, 1 H), 6.06 (dd, J = 15.1, 5.2 Hz, 1 H), 5.79 (d, J = 15.8 Hz, 1
H), 5.59 (brs, 1 H), 4.35 (m, 1 H), 3.86 (s, 6 H), 3.37 (q, J = 7.5 Hz, 2 H), 2.72 (m, 2
13
H), 1.17 (t, J = 7.5 Hz, 3 H), 0.85 (s, 9 H), -0.09 (s, 3 H), -0.16 (s, 3 H); C NMR
(125 MHz, CDCl3): δ = 165.9, 148.4, 147.5, 143.7, 140.1, 130.6, 126.9, 123.5, 121.8,
113.2, 111.0, 74.0, 55.8, 55.7, 44.3, 34.4, 25.7, 18.1, 14.8, -4.8, -5.1; ESIMS: [M +
Na]+ 442; Compound 1: [α]D25 = -54.2 (c 0.042, MeOH); IR (KBr): 3432, 2922, 1649,
1
1593, 1536, 1258, 1148, 1027, 991 and 805 cm-1; H NMR (300 MHz, CDCl3): δ =
7.13 (dd, J = 14.3, 9.0 Hz, 1 H), 6.74 (d, J = 8.3 Hz, 1 H), 6.67 (d, J = 9.0 Hz, 2 H),
6.25 (dd, J = 14.3, 11.3 Hz, 1 H), 6.04 (dd, J = 14.3, 5.2 Hz, 1 H), 5.75 (d, J = 14.3
Hz, 1 H), 5.61 (brs, 1 H), 4.36 (m, 1 H), 3.79 (s, 6 H), 3.30 (q, J = 7.5 Hz, 2 H), 2.79
(dd, J = 13.5, 4.5 Hz, 1 H), 2.66 (dd, J = 13.5, 4.5 Hz, 1 H), 2.03 (brs, 1 H), 1.10 (t, J
13
= 7.5 Hz, 3 H); C NMR (75 MHz, CDCl3): δ = 165.9, 148.9, 147.8, 142.3, 139.9,
129.6, 127.6, 124.1, 121.5, 112.7, 111.3, 72.5, 55.8, 43.3, 34.5, 14.8.; HRMS (ESI):
calcd. for C17H23NO4Na [M + Na]+ 328.1519; found 328.1526.

Graphical abstract
S. Purushotham Reddy and Y. Venkateswarlu*
Amide-Wittig olefination
O
MeO
N
H
OH
1
MeO
MacMillan -hydroxylation
a

Figures and Schemes
Figure 1
OMTPA
O
-0.12
(R)
0.03
-0.02
-0.05
O
OMe
OMe
H
-0.03
-0.02
Figure 2. Determination of the absolute configuration and ∆δ values for the (S) and (R)-MTPA
ester derivatives of 3 (∆δ = δS - δR).

Scheme 1: Retrosynthetic analysis of (-) Kunstleramide 1
O
O
MeO
MeO
MeO
MeO
MeO
MeO
a
b
O
OH
O
OTBS
OH
2
3
4
O
e
MeO
MeO
MeO
MeO
c
OH
N
d
1
H
OTBS
OTBS
6
5
f
h
O
O
MeO
MeO
OH
g
MeO
MeO
O
OTBS
8
OTBS
7
Scheme 2: Reagents and conditions: (a) (i) IBX, DMSO, DCM, (ii) nitrosobenzene, L-proline,
o
o
DMSO, 20 C, 25 min, then triethyl phosphonoacetate, DBU, LiCl, 0 C 1 h, then MeOH,
o
CuSO4.5H2O, rt, overnight, 60% (one pot); (b) TBS-Cl, imidazole, DCM, 0 C to rt, 6 h, 96%;
(c) DIBAL-H, DCM, -78 C, 1 h, 91%; (d) (i) IBX, DMSO, DCM, 3 h; (ii) 9, KOtBu, THF, 0
o
oC, 2 h, 62%; (e) P-TSA, MeOH, 0 C, h, 95%; (f) (i) IBX, DMSO, DCM, 3 h; (ii)
o
Ph3P=CHCO2Et, dry benzene, reflux, 1 h, 85%; (g) LiOH.H2O,THF, H2O (1:1), 6 h, 90%; (h)
HATU, DIPEA, EtNH2, dry DMF, 0 oC to rt, over night, 93%.

Graphical abstract
S. Purushotham Reddy and Y. Venkateswarlu*
Amide-Wittig olefination
O
MeO
N
H
OH
1
MeO
MacMillan -hydroxylation
a