New synthesis of nematocidal natural products dithiocynates thiocyanatin A and 1,8,16-trihydroxyhexadecane

Article abstract of DOI:10.1080/14786410802587073

A new and short synthesis of nematocidal natural products, thiocyanatin A and 1,8,16-trihydroxyhexadecane, from readily available starting compounds 1,7-heptanediol and 1,9-nonanediol in six steps is described.

Full text of DOI:10.1080/14786410802587073

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Natural Product Research: Formerly
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New synthesis of nematocidal natural
products dithiocynates thiocyanatin A
and 1,8,16-trihydroxyhexadecane
Ashima Singh a , M.L. Sharma a & Jasvinder Singh a
a Department of Chemistry and Centre of Advanced Studies in
Chemistry , Panjab University , Chandigarh, India
Published online: 29 Oct 2009.
To cite this article: Ashima Singh , M.L. Sharma & Jasvinder Singh (2009) New synthesis of
nematocidal natural products dithiocynates thiocyanatin A and 1,8,16-trihydroxyhexadecane,
Natural Product Research: Formerly Natural Product Letters, 23:11, 1029-1034, DOI:
10.1080/14786410802587073
To link to this article: http://dx.doi.org/10.1080/14786410802587073
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Natural Product Research
Vol. 23, No. 11, 20 July 2009, 1029–1034
New synthesis of nematocidal natural products dithiocynates
thiocyanatin A and 1,8,16-trihydroxyhexadecane
*
Ashima Singh, M.L. Sharma and Jasvinder Singh
Department of Chemistry and Centre of Advanced Studies in Chemistry, Panjab University,
Chandigarh, India
(Received 25 May 2008; final version received 29 October 2008)
A new and short synthesis of nematocidal natural products, thiocyanatin A and
1,8,16-trihydroxyhexadecane, from readily available starting compounds
1,7-heptanediol and 1,9-nonanediol in six steps is described.
Keywords: nematocidal; natural products; microwave
1. Introduction
The unique structural features and nematocidal properties of thiocyanatin continue to
attract the attention of synthetic organic chemists (Capon et al., 2001, 2004). Marine
natural products possessing a thiocyanate functionality are rare, with the earliest reports
by He, Faulkner, Shumsky, Hong and Clardy (1989) describing sesquiterpene
monothiocyanates from a Palauan sponge. The thiocyanatins are acyclic lipids with
twin terminal thiocyanate moieties (He, Salva, Catalos, & Faulkner, 1992; Pham et al.,
1991; Okino, Yoshimura, Hirota, & Fusetani, 1996; Simpson, Garson, Hopper, Cline, &
Angerhofer, 1997; Simpson & Garson, 1998). In addition to featuring an unprecedented
dithiocyanate functionality, thiocyanatins possess an unusual 1,16-difunctionalised
hexadecane carbon skeleton and are revealed as a hitherto unknown class of nematocidal
agent. Marine metabolites hold considerable promise as a source for bioprospecting new
generation antiparasitics. Marine thiocyanates have been attributed a range of biological
properties, including cytotoxicity (Pham et al., 1991), antifouling (Okino et al., 1996),
antimalarial (Simpson et al., 1997) and antifungal activity (Fusetani et al., 1992).
Australian marine sponge, i.e. Oceanapia have yielded thiocyanatin A or B or C. Out of
them, thiocyanatin-A (2) is bioactive and plays very important role as a nematocidal agent.
1,8,16-Trihydroxyhexadecane (1) has been isolated from the stem cutin of Psilotum nudam
(Caldicott, Simoneit, & Eglinton, 1975).
2. Results and discussion
In continuation of our work utilising microwave energy as a source of heat in organic
synthesis (Gupta, Sonu, & Singh, 2007; Kad, Kaur, Bhandari, Singh, & Kaur, 2003; Kad,
*Corresponding author. Email: jsbrar_pu@yahoo.com
ISSN 1478–6419 print/ISSN 1029–2349 online
ß 2009 Taylor & Francis
DOI: 10.1080/14786410802587073
http://www.informaworld.com

1030
A. Singh et al.
a
OH
Br
HO
5
HO
3
4
b
Br
O
O
c
OH
HO
6
OH
O
O
O
7
d
H
O
O
8
OH
e
O
O
O
O
9
g
f
SCN
OH
NCS
HO
OH
2
OH
1
1,8,16-Trihydroxyhexadecane
Thiocyanatin A
Scheme 1. Synthesis of 1,8,16-trihydroxyhexadecane and thiocyanatin A. Reagents and conditions:
(a): 48% HBr, Bu4NþIꢁ, MWI, 355W, 5 min, 80% (b): DHP, (bmim)HSO4, MWI, 160 W, 2 min,
94% (c): DHP, (bmim)HSO4/hexane, MWI, 160 W, 2 min, 97% (d): NaNO2, (CH3CO)2O, 87%
(e): Mg, dry ether, 0ꢀC, 70% (f): NaHSO4.SiO2, 320 W, 3 min, 96% (g): Ph3P(SCN)2, CH3CN, 88%.
Kaur, Bansal, & Singh, 1996; Kad, Singh, Kaur, & Singh, 1997; Singh, Gupta, Kad, &
Kaur, 2006; Singh, Sharma, & Singh, 2008; Singh, Sharma, Kad, & Chhabra, 1997), we
have accomplished a simple synthesis of 1,8,16-trihydroxyhexadecane (1) and thiocyanatin
A (2) (Scheme 1).
Microwave-assisted bromination (Kad et al., 2003) of 1,7-heptanediol (3) with 48%
HBr in the presence of a catalytic amount of tetrabutylammonium iodide afforded
7-bromoheptan-1-ol (4) in 80% yield. 7-Bromoheptan-1-ol (4) was tetrahydropyranylated
in the presence of acidic ionic liquid 1-butyl-3-methylimidazolium hydrogensulphate under
microwave irradiation (Singh et al., 2006) to obtain 2-(7-bromo-heptyloxy)-tetrahydro-
pyran (5) in 94% yield. Microwave-assisted tetrahydropyranylation (Singh et al., 2006) of
1,9-nonanediol (6) using 3,4-dihydro-2H-pyran and 1-butyl-3-methylimidazolium hydro-
gensulphate/hexane (biphasic conditions) furnished 9-(tetrahydro-pyran-2-yloxy)-nonan-
1-ol (7) in 97% yield. Rapid oxidation (Bandgar, Sadavarte, & Uppalla, 2000) of
9-(tetrahydro-pyran-2-yloxy)-nonan-1-ol (7) with NaNO2–acetic anhydride under mild
and solvent-free conditions gave compound 9-(tetrahydro-pyran-2-yloxy)-nonanal (8) in
87% yield. The reaction of 9-(tetrahydro-pyran-2-yloxy)-nonanal (8) with Grignard
reagent (Kochi & Tamura, 1971) prepared from 2-(7-bromo-heptyloxy)-tetrahydro-pyran
(5) in dry ether at 0ꢀC gave 1,16-bis-(tetrahydro-pyran-2-yloxy)-hexadecan-8-ol (9).

Natural Product Research
1031
Deprotection of 1,16-bis-(tetrahydro-pyran-2-yloxy)-hexadecan-8-ol (9) with catalytic
amount of silica supported sodium hydrogen sulphate in microwave furnished pure 1,8,16-
trihydroxy hexadecane (1). The spectral data of the synthesised product was found to be
compatible with that reported in literature (Caldicott et al., 1975). Conversion of 1,16-bis-
(tetrahydro-pyran-2-yloxy)-hexadecan-8-ol (9) into 1,16-bis-thiocyanato-hexadecan-8-ol
(2) was done with Ph3P(SCN)2 in the presence of CH3CN as solvent (Iranpoor,
Firouzabadi, & Shaterian, 2005). The spectral data were in good agreement with those
reported in literature (Capon et al., 2001, 2004).
3. Experimental
Melting points were determined with a Sunbim melting point apparatus. IR spectra were
1
recorded on a Perkin Elmer model 1430 spectrometer. H NMR and 13C NMR spectra
were recorded using Joel 300 MHz spectrometer. Chemical shifts (ꢀ) are reported in ppm
with tetramethylsilane as internal standard. Microwave-induced reactions were carried out
in a domestic microwave (MG-396 W A, 2450 MHz, 800W).
7-Bromoheptan-1-ol (4). A mixture of 1,7-heptanediol (3, 0.5 g, 3.78 mmol), 48% aqueous
hydrogen bromide (0.64 g, 7.9 mmol) and tetrabutylammonium iodide (0.28 g, 0.76 mmol)
was taken in a 100 mL beaker covered with a watch glass. The mixture was exposed to
microwave irradiation for 5 min at 355 W. The contents were cooled, extracted with DCM
(3 ꢂ 10 mL), washed with saturated sodium bicarbonate solution (2 ꢂ 5 mL), water
(2 ꢂ 5 mL), brine and dried. Removal of the solvent under vacuum followed by silica gel
column chromatography eluting with (EtOAc/hexane: 2 : 8) furnished pure 7-bromohep-
tan-1-ol (4, 0.59 g) in 80% yield.
1
IR(CHCl3)/ꢁmax cmꢁ1: 3450, 2980, 1054, 540. H NMR (CCl4, 300 MHz) ꢀ: 1.0–1.8
(m, 10H, saturated methylene protons), 3.2–3.6 (m, 4H, –CH2CH2OH, –CH2CH2Br), 4.8
(bs, 1H, -OH, exchangeable with D2O). 13C NMR (CDCl3, 300 MHz) ꢀ: 26.6, 27.7, 29.3,
32.2, 32.5, 32.9, 61.7.
2-(7-Bromo-heptyloxy)-tetrahydro-pyran (5). A mixture of 7-bromoheptan-1-ol (4, 0.5 g,
2.56 mmol), 3,4-dihydro-2H-pyran (0.21 g, 2.5 mmol) and (bmim)HSO4 (0.08 g,
0.35 mmol) was put in a 100 mL conical flask, covered with watch glass, and exposed to
MW radiations (160 W) in a domestic microwave oven for 2 min. The mixture was cooled
and extracted with (2 ꢂ 10 mL) diethyl ether, and the combined ethereal layer was passed
through a short pad of silica. Evaporation of the solvent under reduced pressure afforded
pure 2-(7-bromo-heptyloxy)-tetrahydro-pyran (5, 0.67 g, 94%).
IR(CHCl3)/ꢁmax cmꢁ1: 2900, 1220, 1150, 890, 815. 1H NMR (CCl4, 300 MHz) ꢀ:
1.2–1.9 (m, 16H, saturated methylene protons), 3.2–3.8 (m, 6H, –CH2O–, –OCH2–,
–CH2CH2Br), 4.6 (t, J ¼ 4Hz, 1H, –OCHO). 13C NMR (CDCl3, 300 MHz) ꢀ: 19.0, 26.0,
27.8, 29.3, 29.5, 30.2, 32.5, 32.6, 34.1, 61.5, 66.7, 97.7.
9-(Tetrahydro-pyran-2-yloxy)-nonan-1-ol (7). To a mixture of 1,9-nonanediol (6, 0.5 g,
3.12 mmol), 3,4-dihydro-2H-pyran (0.26 g, 3.12 mmol) in a 100 mL conical flask was added
(bmim)HSO4 (0.21 g, 0.88 mmol) and hexane (1 mL). The conical flask was covered with a
watch glass and irradiated at 160 W for 2 min. On cooling, the hexane layer was decanted,
the remaining ionic liquid was extracted with ether (2 ꢂ 10 mL), and the combined organic
layer was evaporated in vacuo to yield crude product, which was purified by column

1032
A. Singh et al.
chromatography using 2% ethyl acetate in hexane as eluent to furnish pure 9-(tetrahydro-
pyran-2-yloxy)-nonan-1-ol (7, 0.73 g, 97%).
IR(CHCl3)/ꢁmax cmꢁ1: 3450, 2960, 1210, 1120, 720. 1H NMR (CCl4, 300 MHz) ꢀ: 1.2–
1.8 (m, 18H, saturated methylene protons), 2.1–2.3 (m, 2H, –CH2CH2OH), 3.3–4.0 (m,
6H, –CH2CH2OH, –CH2CH2O–, –OCH2CH2–), 4.2 (bs, 1H, –OH, D2O exchangeable),
4.6 (t, J ¼ 4Hz, 1H, –OCHO–). 13C NMR (CDCl3, 300 MHz) ꢀ: 14.0, 19.0, 26.3, 26.6, 28.1,
30.3, 31.3, 33.1, 33.2, 61.5, 97.8.
9-(Tetrahydro-pyran-2-yloxy)-nonanal (8). To a stirred mixture of NaNO2 (2.07 g,
30 mmol) and 9-(tetrahydro-pyran-2-yloxy)-nonan-1-ol (7, 2.44 g, 10 mmol) in a 100 mL
round-bottomed flask was added acetic anhydride (0.40 g, 4 mmol). A spontaneous
exothermic reaction took place, which was controlled by cooling the mixture in ice. After
completion of the reaction (checked by TLC), the product was extracted with ether
(2 ꢂ 10 mL). Removal of the solvent under reduced pressure afforded crude product,
which was purified by column chromatography (petroleum ether–ethyl acetate, 9 : 1) to
furnish pure 9-(tetrahydro-pyran-2-yloxy)-nonanal (8, 2.10 g, 87%).
IR(CHCl3)/ꢁmax cmꢁ1: 2920, 2720, 1730, 1180, 720. 1H NMR (CCl4, 300 MHz) ꢀ: 1.2–
1.8 (m, 18H, saturated methylene protons), 2.3 (t, J ¼ 7Hz, 2H, –CH2CH2CO–), 3.2–3.9
(m, 4H, –CH2CH2O–, –OCH2CH2–), 4.4 (t, J ¼ 4Hz, 1H, –OCHO–), 9.9 (s, 1H, –CHO).
13C NMR (CDCl3, 300 MHz) ꢀ: 19.0, 23.1, 26.6, 27.5, 29.8, 30.0, 30.3, 31.3, 33.2, 42.8,
61.5, 64.3, 97.5, 200.1.
1,16-Bis-(tetrahydro-pyran-2-yloxy)-hexadecan-8-ol (9). A flame-dried, nitrogen gas
flushed 100 mL three-necked round-bottomed flask, fitted with a condenser and an
addition funnel was charged with magnesium turnings (0.19 g, 8.0 mmol), dry diethyl ether
(7 mL) and a small crystal of iodine. To this stirred mixture was added a few drops of 2-(7-
bromo-heptyloxy)-tetrahydro-pyran (5, 2.23 g, 8.0 mmol) in dry diethyl ether (15 mL). The
flask was warmed in a hot water bath and on discharge of the violet colour of iodine, the
remaining compound (5) was added dropwise over a period of 0.5 h. On complete
consumption of magnesium, the reaction mixture was cooled to 0ꢀC using an ice-salt bath
and 9-(tetrahydro-pyran-2-yloxy)-nonanal (8, 2.03 g, 8.36 mmol) in anhydrous diethyl
ether (15 mL) was added dropwise over a period of 1h. Stirring was continued overnight
and the reaction mixture was quenched with saturated ammonium chloride solution. The
organic layer was separated and washed with water (2 ꢂ 10 mL), brine and dried. Removal
of solvent under reduced pressure followed by purification using silica gel column eluting
10% ethyl acetate in n-hexane, furnished pure 1,16-bis-(tetrahydro-pyran-2-yloxy)-
hexadecan-8-ol (9, 2.47 g, 70%).
1
IR(CHCl3)/ꢁmax cmꢁ1: 3410, 2935, 1230, 1115. H NMR (CCl4, 300 MHz) ꢀ: 1.2–1.6
H
H
H
H
H
H
H
H
H
(m, 26H, saturated methylene envelope), 1.6–1.8 (m, 12H,
,
), 3.3
H
H
O
), 3.7 (m, 1H, –CH(OH)–), 3.9 (s, 1H, –OH, D2O
O
O
O
H
H
(m, 4H,
,
O
O
O
O
H
H
exchangeable), 4.6 (t, J ¼ 4.2 Hz, 2H, –OCHO–, –OCHO–). 13C NMR (CDCl3, 300 MHz)
ꢀ: 19.5, 25.4, 27.9, 29.4, 30.3, 30.7, 31.3, 33.6, 39.4, 61.9, 67.2, 72.2, 98.6.
1,8,16-Trihydroxyhexadecane (1). A mixture of 1,16-bis-(tetrahydro-pyran-2-yloxy)-
hexadecan-8-ol (9, 2.21 g, 5 mmol) and a catalytic amount of silica supported sodium
hydrogen sulphate were put in a 50 mL Erlenmeyer flask. The flask was covered with

Natural Product Research
1033
a watch glass and subjected to MWI at 320 W for 3 min. The mixture was cooled, taken in
diethyl ether (2 ꢂ 10 mL) and filtered. The filtrate was washed with 5% NaHCO3 solution,
water, brine and dried over sodium sulphate. Evaporation of the solvent in vacuo furnished
pure 1,8,16-trihydroxyhexadecane (1, 1.31 g) in 96% yield.
IR(CHCl3)/ꢁmax cmꢁ1: 3364, 2932, 1257. 1H NMR (CCl4, 300 MHz) ꢀ: 1.2–1.6
(m, methylene envelope), 3.5 (m, 1H, –CH(OH)–), 3.64 (t, J ¼ 7 Hz, 4H, 2 ꢂ –CH2OH).
13C NMR (CDCl3, 300 MHz) ꢀ: 25.0, 28.0, 29.4, 29.7, 32.7, 39.5, 63, 71.9.
Thiocynatin A (2). A three-necked flask equipped with a dropping funnel, stirrer, drying
CaCl2 tube and N2 gas inlet was charged with Ph3P (0.57 g, 2.2 mmol) and dry CH3CN
(5 mL), then Br2 (0.35 g, 2.2 mmol) was added dropwise to the solution at room
temperature under nitrogen atmosphere. After complete addition of Br2, a solution of
NH4SCN (0.33 g, 4.4 mmol) in CH3CN (5 mL) was added dropwise. Upon addition of
1,16-bis-(tetrahydro-pyran-2-yloxy)-hexadecan-8-ol (9) (0.88 g, 2 mmol) to the resulting
mixture, an spontaneous reaction occurred. To the resulting mixture silica gel was added
and the solvent was evaporated on a rotary evaporator. The resulting solid was purified by
silica gel column chromatography using petroleum ether 60–80ꢀC/EtOAc (9/1) to get the
desired thiocyanatin A (2, 0.62 g) in 88% yield.
IR(CHCl3)/ꢁmax cmꢁ1: 3490, 2150. 1H NMR (CCl4, 300 MHz) ꢀ: 1.2-1.4 (m, methylene
envelope), 1.82 (m, 4H, 2 ꢂ –CH2CH2SCN), 2.94 (t, J ¼ 7.2 Hz, 4H, 2 ꢂ –CH2–SCN), 3.6
(m, 1H, –CH(OH)–). 13C NMR (CDCl3, 300 MHz) ꢀ: 25.3, 25.4, 28.7, 28.8, 29.2, 29.3,
29.4, 29.8, 33.9, 34.1, 37.3, 37.4, 71.8, 111.8.
Acknowledgements
The authors thank UGC New Delhi for financial assistance.
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