First total synthesis of acetylenic alcohol 15-methyltricosa-2,4-diyne-1, 6-diol (strongylodiol-G) derived from marine sponge

Article abstract of DOI:10.1080/14786419.2013.867855

The first total and efficient synthesis of a naturally occurring acetylenic alcohol 15-methyltricosa-2,4-diyne-1,6-diol (strongylodiol-G) derived from marine sponge involving nine steps has been described. 1-Bromo-9- methyloctadecane (5) and hex-6-tetrahydropyranyloxyhex-2,4-diyn-1-al (9) which were initially synthesised separately starting from 1,8-octanediol (1) and propargyl alcohol (6), respectively, have been used as the final intermediates to obtain the title compound. The key steps in the synthesis involved ionic liquid-mediated bromination of 1,8-octanediol (1), tetrahydropyranylation of 8-bromooctan-1-ol (2) using acidic ionic liquid [bmim]HSO₄ and monotetrahydropyranylation of hex-2,4-yn-1,6-diol (7) using ultrasonic energy.

Full text of DOI:10.1080/14786419.2013.867855

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First total synthesis of acetylenic
alcohol 15-methyltricosa-2,4-diyne-1,
6-diol (strongylodiol-G) derived from
marine sponge
Neeraj Guptaa, Shallua, Goverdhan Lal Kada & Jasvinder Singha
a Department of Chemistry and Centre of Advanced Studies in
Chemistry, Panjab University, Chandigarh, India
Published online: 15 Jan 2014.
To cite this article: Neeraj Gupta, Shallu, Goverdhan Lal Kad & Jasvinder Singh (2014) First total
synthesis of acetylenic alcohol 15-methyltricosa-2,4-diyne-1, 6-diol (strongylodiol-G) derived from
marine sponge, Natural Product Research: Formerly Natural Product Letters, 28:7, 424-430, DOI:
10.1080/14786419.2013.867855
To link to this article: http://dx.doi.org/10.1080/14786419.2013.867855
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Natural Product Research, 2014
Vol. 28, No. 7, 424–430, http://dx.doi.org/10.1080/14786419.2013.867855
First total synthesis of acetylenic alcohol 15-methyltricosa-2,4-diyne-
1, 6-diol (strongylodiol-G) derived from marine sponge
Neeraj Gupta, Shallu, Goverdhan Lal Kad and Jasvinder Singh*
Department of Chemistry and Centre of Advanced Studies in Chemistry, Panjab University,
Chandigarh, India
(Received 15 July 2013; final version received 18 November 2013)
The first total and efficient synthesis of a naturally occurring acetylenic alcohol 15-
methyltricosa-2,4-diyne-1,6-diol (strongylodiol-G) derived from marine sponge
involving nine steps has been described. 1-Bromo-9-methyloctadecane (5) and hex-
6-tetrahydropyranyloxyhex-2,4-diyn-1-al (9) which were initially synthesised separ-
ately starting from 1,8-octanediol (1) and propargyl alcohol (6), respectively, have been
used as the final intermediates to obtain the title compound. The key steps in the
synthesis involved ionic liquid-mediated bromination of 1,8-octanediol (1),
tetrahydropyranylation of 8-bromooctan-1-ol (2) using acidic ionic liquid [bmim]
HSO4 and monotetrahydropyranylation of hex-2,4-yn-1,6-diol (7) using ultrasonic
energy.
Keywords: acetylenic alcohol; strongylodiol; bromination; tetrahydropyranylation;
microwave irradiation; ultrasonic energy; ionic liquid
1. Introduction
Marine flora and fauna are highly prized, as many new compounds with diverse structural
features and biological activity have been isolated from them. Straight-chain polyacetylenes are
secondary metabolites of sponges of the order Haplosclerida (Diaa et al. 2003) and includes five
families named Callyspongiidae, Petrosiidae, Chalinidae, Oceanapiidae and Niphatidae.
Polyacetylenes vary in chain length and degree of oxygenation. The biological activities
(Gunasekera & Faircloth 1990; Fusetani et al. 1993) reported for polyacetylenes in the literature
are diverse and include antifungal, antimicrobial, HIV protease inhibitory, HIV reverse
transcriptase inhibitory, Hþ, Kþ-ATPase inhibitory, antifouling, a-glucosidase inhibitory,
immunosuppressant and anti-tumour effects.
One such class of acetylenic alcohols is ‘Strongylodiol’, because of the fact that they were
isolated for the first time from the marine sponge belonging to genus ‘Strongylophora’. Various
strongylodiols are known in the literature, and few of them have also been synthesised.
The isolation and characterisation of seven new long-chain acetylenic alcohols have also been
reported, which have been named as strongylodiols ‘D–J’ (Watanabe et al. 2005).
The first synthesis of cytotoxic strongylodiol-A, a first compound belonging to this series,
has been reported in the literature (Yadav & Mishra 2002). The asymmetric synthesis of the
cytotoxic sponge metabolites (R)-strongylodiols ‘A’ and ‘B’ and its analogues has also been
reported (Kirkham et al. 2004). Besides this, the enantioselective total synthesis of (R)-
strongylodiols ‘A’ and ‘B’ has also been described (Reber et al. 2003). Due to our interest
towards the synthesis of various biologically active compounds, we report herein the first
*Corresponding author. Email: jsbrar_pu@yahoo.com
q 2014 Taylor & Francis

Natural Product Research
425
synthesis of acetylenic alcohol strongylodiol-G (Figure 1), via the maximum use of various ionic
liquid-mediated synthetic methodologies (Kad et al. 1997, 2003; Singh et al. 2006; Gupta et al.
2007) developed by us during the course of our research work.
2. Results and discussion
Retrosynthetic analysis revealedthe need of two starting materials (5) and (9), which were initially
synthesised separately starting from 1,8-octanediol (1) and propargyl alcohol (6), respectively.
The first step was to monobrominate octan-1,8-diol to get 8-bromooctan-1-ol (2) (Scheme 1). The
conversion was done using both conventional and non-conventional techniques. The compound
was synthesised by using hydrobromic acid in benzene and by refluxing the resulting solution.
Alternatively, acidic ionic liquid 1-butyl-3-methylimidazolium hydrogen sulphate ([bmim]
HSO4) which has been identified as an efficient and recyclable reagent for various acid-catalysed
reactions (Singh et al. 2006; Gupta et al. 2007) was used to carry out aforementioned conversion
with sodium bromide in conjunction with microwave irradiation (Gupta et al. 2009).
Compound (2) prepared was subjected to tetrahydropyranylation using ionic liquid [bmim]
HSO4 to get the corresponding tetrahydropyranyl ether (3) (Singh et al. 2006). The coupling
reaction of 8-bromooctanyl tetrahydropyranyl ether (3) with 2-bromodecane leads to the
desirable product 9-methylheptadecyl tetrahydropyranyl ether (4). Tetrahydropyranyl ether (4)
was converted directly to bromide by using sodium bromide and [bmim]HSO4 to get 1-bromo-9-
methyloctadecane (5). The conversion was also accomplished conventionally by using
hydrobromic acid and refluxing the resulting solution. Simultaneously, the Glaser coupling of
propargyl alcohol (6) leads to the formation of hex-2,4-yn-1,6-diol (7) (Scheme 2). Diol (7)
obtained was further monoprotected in the form of tetrahydropyranyl ether to get 6-
tetrahydropyranyloxyhex-2,4-diyn-1-ol (8) using biphasic media of hexane and ionic liquid
[bmim]HSO4. Then monotetrahydropyranyl ether (8) of the diol obtained was oxidised with
pyridinium chlorochromate (PCC) to yield aldehyde 6-tetrahydropyranyloxyhex-2,4-diyn-1-al
(9). The reaction was continuously observed via thin-layer chromatography (TLC) analysis, and
the progress of the reaction was arrested after 7 h.
Further, aldehyde (9) and bromide (5) were combined via Grignard reaction to afford the
monotetrahydropyranyl ether of strongylodiol-G (10) (Scheme 3). Finally, the monotetrahy-
dropyranyl ether of the title compound was deprotected by using acidic ionic liquid [bmim]
HSO4 to afford the title compound strongylodiol-G (11). The conversion was achieved by both
MWI and thermal heating.
Spectral data of the synthesised compound were compatible with that reported in the
literature (Watanabe et al. 2005).
3. Experimental
IR spectra were recorded on a Perkin Elmer model 1430 spectrometer (Perkin Elmer Instruments,
1
Switzerland). H NMR spectra were recorded using FT JEOL 300 MHz spectrometer (JEOL,
Tokyo, Japan). Chemical shifts (d) are reported in ppm with tetramethylsilane as an internal
standard. Mass spectra (MS) were recorded at 70 eV using VG Analytical 11-250 J70S
HO
OH
Strongylodiol-G
Figure 1. The structure of strongylodiol-G.

426
N. Gupta et al.
i)
ii)
HO
HO
Br
OH
(1)
(2)
65%
iii)
Br
O
O
O
O
(4)
82%
(3)
94%
iv)
Br
(5)
58%
Scheme 1. (i) HBr, benzene, reflux or [bmim]HSO4, NaBr, MWI/808C, (ii) DHP, [bmim]HSO4, sonication,
(iii) Mg, 2-bromodecane, CuI, diethyl ether, (iv) [bmim]HSO4, NaBr, 808C or HBr, reflux.
spectrometer (VG Analytical Instruments, Tokyo, Japan). Microwave-assisted reactions were
carried out in LGMS-194A (800 W) domestic microwave oven (LG Electronics, India).
3.1. 8-Bromooctan-1-ol (2)
3.1.1. Macroscale
In a flame-dried assembly, [bmim]HSO4 (2.83 g, 12 mmol) and NaBr (1.54 g, 15 mmol) were
stirred at 808C for half an hour, followed by the addition of octan-1,8-diol (1, 1.46 g, 10 mmol).
The reaction mixture was heated at the same temperature for 15 h. After the completion of the
reaction (indicated by TLC), the upper organic layer was decanted, and the residue was extracted
with diethyl ether (20 mL). Both the organic layers were combined and evaporation of the
solvent under reduced pressure afforded the crude product, which was purified by
column chromatography using ethyl acetate and hexane (1:9) to give the product in good
yield (2, 1.21 g, 58%).
3.1.2. Microscale
In a vial (10 mL), a mixture of [bmim]HSO4 (0.283 g, 1.2 mmol) and NaBr (0.154 g, 1.5 mmol)
was taken and irradiated in microwave oven [160 W in LGMS-194A (800 W)] for 1 min,
i)
HO
OH
HO
(6)
(7)
ii)
84%
iii)
CHO
THPO
OH
THPO
(8)
(9)
82%
38%
Scheme 2. (i) CH3COONa, Cucl2, SiO2, 608C (ii) DHP, [bmim]HSO4, sonication, (iii) PCC, SiO2, MWI.

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427
+
CHO
Br
THPO
(9)
(5)
i)
THPO
OH
(10)
ii)
52%
HO
OH
(11)
80%
Scheme 3. (i) Mg, ether, (ii) [bmim]HSO4, MWI or heat.
followed by cooling period of 1 min. Alcohol (1 mmol) was added to vial, sealed properly and
irradiated at 160 W for 7 min with the mechanical shaking done after each 30 sec irradiation
time. The resulting mixture was allowed to cool, followed by extraction with diethyl ether (3 £
10 mL) and worked up as before to get the product (2, 0.135 g, 65%).
IR (neat)/nmax cm21: 3390, 2928, 1670, 1310, 602. 1H NMR (CDCl3, 300 MHz) d: 3.38 (t,
2H, J ¼ 6.2 Hz, ZCH2OH), 3.20 (t, 2H, J ¼ 6.4 Hz, ZCH2Br), 1.93–1.88 (m, 4H, saturated
methylene protons), 1.48–1.16 (m, 12H, saturated methylene protons).
3.2. 8-Bromooctanyl tetrahydropyranyl ether (3)
A mixture of 8-bromooctan-1-ol (2, 6.27 g, 30 mmol), dihydropyran (DHP) (2.52 g, 3 mmol) and
[bmim]HSO4 (1.0 g, 4.2 mmol) was taken in round-bottomed flask (50 mL), and exposed to
ultrasonic waves using Branson [B-2200 EI (60 W, 47 kHz)] Ultrasonic Cleaning bath for 7 min.
The mixture was cooled, extracted with diethyl ether (2 £ 10 mL) and combined ethereal layers
were passed through a short pad of silica. Evaporation of solvent under reduced pressure
afforded the pure product (3, 8.26 g, 94%).
IR (neat)/nmax cm21: 2941, 2860, 1466, 1353, 1208, 1138, 590. 1H NMR (CDCl3, 300 MHz)
d: 4.58 (bs, 1H, ZOCHOZ), 3.66–3.51 (m, 4H, 2 £ ZCH2O), 3.25 (t, 2H, J ¼ 6.0 Hz,
ZCH2Br), 1.76–1.62 (m, 10H, saturated methylene protons), 1.30–1.18 (m, 8H, saturated
methylene protons).
3.3. 9-Methylheptadecyl tetrahydropyranyl ether (4)
To a well-stirred solution of Grignard reagent prepared from activated magnesium powder
(0.23 g, 9.65 mmol) and 8-bromo-octanyl tetrahydropyranyl ether (3, 2.83 g, 9.65 mmol) in
anhyd. diethyl ether was added dropwise 2-bromodecane (18.3 g, 8.26 mmol) in dry diethyl ether
(200 mL). Stirring was continued at room temperature, and after 6 h CuI (10 mol%, 0.191 g,

428
N. Gupta et al.
1 mmol) was added and stirring was continued for another half an hour. Finally, the reaction
mixture was refluxed for 4 h, cooled and decomposed with ammonium chloride solution, then
filtered and the ethereal layer was separated. The aqueous layer was further extracted with ether.
The combined organic layer was dried and evaporated to afford the crude product that was
purified by column chromatography using hexane as eluent to afford the product in good yield
(4, 2.80 g, 82%).
IR (neat)/nmax cm21: 2930, 1210, 1150, 1020. 1H NMR (CDCl3, 300 MHz) d: 4.50 (bs, 1H,
ZOCHOZ), 3.42–3.36 (m, 4H, 2 £ ZCH2O), 1.58–1.32 (m, 8H, saturated methylene protons),
1.28–1.19 (m, 27H, saturated methylene protons), 1.02 (d, 3H, J ¼ 6.6 Hz, ZCH3), 0.97 (t, 3H,
J ¼ 6.6 Hz, ZCH3). D.E.P.T. (145): 98.1 (inverted), 67.6, 67.1, 41.2, 38.8, 33.0, 32.8, 31.7, 30.6,
29.8, 29.4, 28.8, 28.2, 27.7, 26.5 (inverted), 26.3, 25.7, 22.6, 19.3, 15.4 (inverted), 14.2 (inverted).
3.4. 9-Methylheptadecyl bromide (5)
In a flame-dried assembly, [bmim]HSO4 (2.83 g, 12 mmol) and NaBr (1.54 g, 15 mmol) were
stirred at 808C for half an hour, followed by the addition of 9-methylheptadecyl
tetrahydropyranyl ether (4, 3.54 g, 10 mmol). The reaction mixture was heated at the same
temperature for 22 h. After the completion of the reaction (indicated by TLC), the upper organic
layer was decanted and residual ionic liquid was extracted with diethyl ether (20 mL). Both the
organic layers were combined and evaporation of the solvent under reduced pressure afforded
the crude product that was purified by column chromatography using ethyl acetate and hexane
(1:9) to give the pure product (5, 1.93 g, 58%).
1
IR (neat)/nmax cm21: 2926, 2850, 1430, 618. H NMR (CDCl3, 300 MHz) d: 3.24 (t, 2H,
J ¼ 6.0 Hz, ZCH2Br), 1.64–1.59 (m, 2H, saturated methylene protons), 1.43–1.12 (m, 27H,
saturated methylene protons), 1.07 (d, 3H, J ¼ 6.8 Hz, ZCH3), 0.98 (t, 3H, J ¼ 6.5 Hz, ZCH3).
3.5. Hex-2,4-yn-1,6-diol (7)
A mixture of CuCl2 (1.35 g, 10 mmol), sodium acetate (0.82 g, 10 mmol) and activated silica
(40 g) was grinded firmly in pestle and mortar. Then propargyl alcohol (6, 0.56 g, 10 mmol) was
added slowly to it, grinded for 15 min and shifted to a conical flask, fitted with a guard tube. The
reaction mixture was kept at 508C in water bath for 6 h (till the green colour did not discharge)
and finally extracted with DCM (3 £ 20 mL). The organic layer was washed with water and
dried over sodium sulphate. Evaporation of the solvent under reduced pressure afforded the
crude product that was purified by column chromatography using hexane and ethyl acetate (9:1)
as eluent to get the pure product (7, 0.929 g, 84%).
IR (neat)/nmax cm21: 3384, 2940, 2845, 2130. 1H NMR (CDCl3, 300 MHz) d: 4.01 (s, 4H, 2 £
ZCH2), 2.5 (bs, 2H, 2 £ ZOH, D2O exchangeable).
3.6. 6-Tetrahydropyranyloxyhex-2,4-diyn-1-ol (8)
Hex-2,4-diyn-1,6-diol (7, 0.33 g, 3 mmol) and DHP (0.25 g, 3 mmol) were taken in a 10 mL
round-bottomed flask followed by the addition of [bmim]HSO4 (0.2 g, 0.85 mmol) and hexane
(1 mL). The reaction mixture was exposed to ultrasonic waves for 7 min. The hexane layer was
decanted, the remaining ionic liquid was extracted with ether (2 £ 10 mL) and the combined
organic layer was evaporated in vacuo to yield the crude product, which was purified by column
chromatography using 2% ethyl acetate in hexane as eluent, to furnish the pure monoprotected
alcohol in excellent yield (8, 0.477 g, 82%).
IR (neat)/nmax cm21: 3422, 2938, 2860, 1466, 1353, 1121, 1070. 1H NMR (CDCl3,
300 MHz) d: 4.67 (bs, 1H, ZOCHOZ), 4.29 (s, 2H, ZOCH2), 4.14 (s, 2H, ZOCH2), 3.51 (t, 2H,
J ¼ 5.8 Hz, ZOCH2), 1.71–1.43 (m, 6H, saturated methylene protons).

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3.7. 6-Tetrahydropyranyloxyhex-2,4-diyn-1-al (9)
A mixture of 6-tetrahydropyranyloxy-2,4-diyn-1-ol (8, 0.61 g, 3.16 mmol), PCC (1.02 g,
4.72 mmol) and activated silica (1 g) was grinded firmly in pestle and mortar and shifted to a
conical covered with polypropylene film, and was exposed to MWI at 160 W for 2.5 min. On
cooling, the reaction mixture was diluted with DCM (20 mL), filtered through a short pad of
neutral alumina eluting with DCM (eluted fractions were collected in different conicals to avoid
contamination with unreacted alcohol). The solvent was evaporated under reduced pressure to
yield 6-tetrahydropyranyloxy-2,4-diyn-1-al.(9, 0.230 g, 38%).
1
IR (neat)/nmax cm21: 2938, 2850, 2110, 1680, 1210, 1079. H NMR (CDCl3, 300 MHz) d:
9.82 (s, 1H, ZCHO), 4.64 (bs, 1H, ZOCHOZ), 4.23 (s, 2H, uCCH2OZ), 3.42 (t, 2H,
J ¼ 5.8 Hz, ZOCH2), 1.68–1.41 (m, 6H, saturated methylene protons).
3.8. 15-Methyl-1-tetrahydropyranyloxy-tricosa-2,4-diyn-6-ol (10)
3.8.1. Monotetrahydropyranyl ether of strongylodiol-G
Flame-dried nitrogen flushed 250 mL three-necked round-bottomed flask, fitted with a condenser
and addition funnel, was charged with magnesium turnings (0.24, 9.87 mmol), anhyd. diethyl
ether (10 mL) and a small crystal of iodine. To this stirred mixture were added a few drops of 9-
methylheptadecyl bromide (5, 3.25 g, 9.76 mmol) in dry diethyl ether (30 mL). The flask was
warmed on a hot water bath and on discharge of violet colour of iodine, the remaining 9-
methylheptadecyl bromide was added dropwise over a period of 0.5 h. On complete
consumption of magnesium metal, the reaction mixture was cooled to 08C and compound (9) in
dry diethyl ether (25 mL) was added dropwise over a period of 1 h. Stirring was continued
overnight, and the reaction mixture was quenched with saturated aq. ammonium chloride
solution. The organic layer was separated and washed with water (3 £ 10 mL), brine and dried.
Removal of solvent under reduced pressure followed by purification over silica gel column using
ethyl acetate and hexane (1:9) as eluent furnished pure product (10, 2.26 g, 52%).
1
IR (neat)/nmax cm21: 3375, 2120, 1348, 1205, 1150, 1080. H NMR (CDCl3, 300 MHz) d:
4.64 (bs, 1H, ZOCHOZ), 4.18 (t, 1H, J ¼ 6.9 Hz, uCHCOH), 4.01 (s, 2H, ZOCH2Cu), 3.53–
3.49 (m, 2H, ZCH2O), 3.2 (bs, 1H, ZOH, D2O exchangeable), 1.45–1.40 (m, 9H, saturated
methylene and methine protons), 1.38–1.22 (m, 28H, saturated methylene protons), 1.05 (d, 3H,
J ¼ 6.0 Hz, ZCH3), 0.96 (t, 3H, J ¼ 6.2 Hz, ZCH3).
3.9. 15-Methyltricosa-2,4-diyne-1,6-diol (strongylodiol-G) (11)
15-Methyl-1-tetrahydropyranyloxy-tricosa-2,4-diyn-6-ol (10, 0.446 g, 1 mmol), water (0.018 g,
1 mmol) and [bmim]HSO4 (2 mmol) were mixed together in a sealed conical. The vessel was
then irradiated with MWI at 320 W for 2 min, cooled and finally eluted with DCM (3 £ 10 mL).
The organic layer was washed with brine to afford the crude product that was purified by column
chromatography using ethyl acetate and hexane (0.8:9.2) as eluent (11, 0.649 g, 80%).
IR (neat)/nmax cm21: 3380, 2860, 2128. 1H NMR (CDCl3, 300 MHz) d: 4.21 (s, 2H,
HOCH2Cu), 4.00 (t, 1H, J ¼ 6.2 Hz, uCCHOHZ), 2.4 (bs, 2H, 2 £ OH, D2O exchangeable),
1.51–1.45 (m, 3H, saturated methylene and methine protons), 1.40–1.12 (m, 28H, saturated
methylene protons), 1.04 (d, 3H, J ¼ 7.8 Hz, ZCH3), 0.97 (t, 3H, J ¼ 8.0 Hz, ZCH3). MS: 363,
362, 284, 221, 110.
4. Conclusions
In conclusion, we have achieved the first total and efficient synthesis of a naturally occurring
acetylenic alcohol, 15-methyltricosa-2,4-diyne-1,6-diol (strongylodiol-G) derived from marine

430
N. Gupta et al.
sponge, starting from 1-bromo-9-methyloctadecane (5) and hex-6-tetrahydropyranyloxyhex-
2,4-diyn-1-al (9) which were initially synthesised separately from 1,8-octanediol (1) and
propargyl alcohol (6), respectively. The key steps in the synthesis involved ionic liquid mediated
bromination of 1,8-octanediol (1), tetrahydropyranylation of 8-bromooctan-1-ol (2) using acidic
ionic liquid [bmim]HSO4 and monotetrahydropyranylation of hex-2,4-yn-1,6-diol (7) using
ultrasonic energy.
Supplementary material
Supplementary figures S1–S3 relating to this article are available online.
Acknowledgements
The authors are thankful to CSIR and UGC-New Delhi (India) for providing financial assistance.
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