A short synthesis of an acetylenic alcohol from the sponge cribrochalina vasculum

Article abstract of DOI:10.1081/SCC-120006040

A short synthesis of the acetylenic alcohol from cribrochalina vasculum is described. No protecting groups are used in this synthesis. The key step is an enzymatic resolution of a racemic mixture of the title compound.

Full text of DOI:10.1081/SCC-120006040

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
A SHORT SYNTHESIS OF AN ACETYLENIC ALCOHOL
FROM THE SPONGE CRIBROCHALINA VASCULUM
Benjamin W. Gung , Hamilton D. Dickson , Sara Seggerson & Kristin Bluhm
To cite this article: Benjamin W. Gung , Hamilton D. Dickson , Sara Seggerson & Kristin Bluhm
(2002) A SHORT SYNTHESIS OF AN ACETYLENIC ALCOHOL FROM THE SPONGE CRIBROCHALINA
VASCULUM , Synthetic Communications, 32:17, 2733-2740, DOI: 10.1081/SCC-120006040
To link to this article: http://dx.doi.org/10.1081/SCC-120006040
Published online: 20 Aug 2006.
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 17, pp. 2733–2740, 2002
A SHORT SYNTHESIS OF AN
ACETYLENIC ALCOHOL FROM THE
SPONGE CRIBROCHALINA VASCULUM
Benjamin W. Gung,* Hamilton D. Dickson,
Sara Seggerson, and Kristin Bluhm
Department of Chemistry and Biochemistry,
Miami University, Oxford, OH 45056, USA
ABSTRACT
Ashort synthesis of the acetylenic alcohol from cribrochalina
vasculum is described. No protecting groups are used in this
synthesis. The key step is an enzymatic resolution of a racemic
mixture of the title compound.
Natural acetylenic compounds are known to occur in terrestrial plants.
However, biologically active polyacetylenes having unusual structural fea-
[
1–7]
tures have been reported recently from marine organisms.
Arelatively
simple member of these acetylenic alcohols, (S)-(þ)-1, is isolated from the
marine sponge, cribrochalina vasculum. Despite their biological activities, as
far as we know, synthetic studies are limited to a few relatively lengthy
[8–12]
routes.
*
Corresponding author. E-mail: gungbw@muohio.edu
733
2
DOI: 10.1081/SCC-120006040
Copyright & 2002 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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2
734
GUNG ET AL.
The unique structure feature of 1 is the attachment of one acetylenic
and one vinyl group to the stereocenter. Chiral reagents introducing an allyl
[
13]
group are generally available. However, few synthetic methods can pro-
[
14]
vide propargyl alcohols or vinyl alcohols in high enantiomeric purity.
Arecent synthesis of 1 derives the stereocenter from (D)-glucolactone neces-
[8]
sitating a lengthy journey.
Our initial plan involves an asymmetric synthesis using ethynylzinc
reagent to introduce the stereocenter. The starting material for this synthesis
is the commercially available hexadecanol. Oxidation of 2 using PCC
afforded the corresponding aldehyde 3 in 88% yield. The next step is the
Wittig olefination using the stabilized ylide. This reagent is relatively unreac-
ꢀ
tive and the reaction took several days at 40 C to provide the ꢀ,b-unsatu-
rated aldehyde 4.
Carreira reported an asymmetric synthesis of propargylic alcohols by
[
15]
direct addition of terminal alkynes to aldehydes. The procedure involves a
mix of an alkyne and an aldehyde in the presence of Zn(OTf) , Et N, and
2
3
N-methylephedrine. Prior metalation of the terminal alkyne is not necessary.
Thus it appeared this method would serve our purpose in the synthesis of 1.
Unfortunately, numerous attempts under various conditions did not
produce any addition product. It becomes apparent to us that this method is
substrate sensitive. Triethylsilylacetylene does not appear to add to aldehyde
4
under the reported conditions. Next we turned our attention to enzymatic
resolution of racemic acetylenic alcohol 1 in order to avoid using protecting
groups and to keep the synthesis short. Ethynyl magnesium bromide was
allowed to react with aldehyde 4 to afford the racemic acetylenic alcohol 1 in
83% yield.
Previously, Burgess has reported an enzymatic resolution of secondary
[
alcohols by the lipase from Pseudomonas sp. Asimple model for predicting
16]
enantioselectivity was proposed to rationalize the results. This model predicts
Scheme 1.

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ACETYLENIC ALCOHOL FROM CRIBROCHALINA VASCULUM
2735
Scheme 2.
that the alcohols that are resolved most effectively have one small and one
relatively large group attached to the hydroxylmethine functionality. For
most secondary alcohols, the rate of acylation is faster for the alcohols
with (R)-configuration than those of (S)-configuration. However, for
the acetylenic alcohol 1 the opposite is true because the small acetylenic
group has a higher priority in the nomenclature system. Therefore the
naturally occurring enantiometer of 1 should be acylated faster than the
(R)-enantiomer. This was indeed observed in our experiments.
Kinetic resolution of racemic 1 mediated by lipase AK from Amano
was performed by stirring the propargyl alcohol in hexanes with half the
mass of the enzyme and 4 eq. of vinyl acetate. The reaction was followed
1
quantitatively by H NMR, and qualitatively by TLC. The hydroxymethine
proton is characteristic of the change between the starting material 1 and the
acylated product 5. Molecular sieves were added to remove traces of water
in the enzyme mediated reaction. Workup of the reaction consisted of a
simple filtration, rotary evaporation, and flash column separation of the
acetate from the alcohol.
Acylation of the racemic alcohols was allowed to proceed to 25% and
5% conversion respectively in two separate experiments. After separation
4
from the alcohol, the acetate 5 was treated with K CO in methanol to
2
3
provide (S)-1. The optical rotations for (S)-1 from these two experiments
ꢀ
ꢀ
were found to be [ꢀ]
D
¼ þ 19.5 and [ꢀ]
D
¼ þ 21.2 .
The naturally occurring alcohol 1 was first reported to have a rotation
ꢀ
ꢀ
[2,6]
of [ꢀ]
D
¼ þ 3.8 and later revised to be [ꢀ]
D
¼ þ 18.3 .
The synthetic 1 was
ꢀ
ꢀ
[8]
reported to have [ꢀ]
prepared the O-methylmandelate esters of 1, which were enantiomerically
D
¼ ꢁ19.5 for (R)-1 and [ꢀ]
D
¼ þ 21.8 for (S)-1. We
[
17]
enriched in either enantiomer.
1
Judging from the peak area of the vinyl
protons in the H NMR spectra, the samples from our two experiments
should have an optical purity around 90% ee for 45% conversion and
98% ee for 25% conversion, respectively.

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2
736
GUNG ET AL.
In summary, a synthesis of the naturally occurring acetylenic alcohol 1
has been completed in 5 steps. Although the enantioselective addition of
Et SiCꢂCH to aldehyde 4 did not give any desirable product, an enzymatic
3
resolution of racemic 1 using the lipase from Pseudomonas sp. turns out to
be a convenient method of obtaining optically active acetylenic alcohols.
This synthesis of (S)-1 reduces the synthetic steps by more than 10 opera-
[
8]
tions from the previous report.
EXPERIMENTAL SECTION
Methylene chloride was freshly distilled from calcium hydride. THF
was freshly distilled from sodium benzophenone under N . Routine
2
1
H NMR spectra (for characterization) were obtained on a Bruker AC-200
spectrometer. Commercially obtained starting materials were used without
further purification. Column chromatography was carried out using up to
5-psi air pressure with 230–400-mesh silica gel from VWR Scientific. Columns
eluted with EtOAc in hexanes were slurry-packed after the slurry was stirred
ꢀ
with 5% ethyl acetate in hexanes. All glassware was dried in an oven at 120 C.
Hexadecanal (3): Hexadecanol (2.0 g, 8.26 mmol) was dissolved in
2
0 mL anhydrous dichloromethane. The mixture was added to a stirred
suspension consisting of 2.68 g (12.4 mmol) pyridinium chlorochromate
PCC) and 2.68 g Celite in 24.8 mL anhydrous dichloromethane. The result-
(
ing suspension was stirred for 2 h at room temperature, and the progress of
the reaction was monitored by TLC. When the starting material disap-
peared, the reaction mixture was diluted with 500 mL of ethyl ether and
filtered through a pad of Florisil, resulting in a clear yellow solution. The
filter cake was rinsed with an additional 200 mL ethyl ether and the filtrates
were combined. The solvent was removed under reduced pressure to afford
1
hexadecanal as an oil (1.74 g, 88%). H NMR (200 MHz, CDCl ) d 0.84
3
(
2
t, J ¼ 6.2 Hz, 3H, CH ), 1.19 (m, 26H, CH ), 2.35 (dt, J ¼ 1.84, 7.24 Hz,
3
2
H, -CH CHO), 9.69 (d, J ¼ 7.8 Hz, 1H, CHO).
2
2-Octadecenal (4): Apressure-resistant tube was charged with a stirring
bar, 1.74 g (7.24 mmol) hexadecanal and 17 mL anhydrous dichloromethane.
After addition of 3.48 g (11.4 mmol) the Wittig reagent Ph P ¼ CHCHO, the
3
ꢀ
tube was flushed with nitrogen and sealed. The mixture was heated to 65 C
in an oil bath and progress of the reaction monitored by TLC. After three
days, most of the aldehyde had reacted and the reaction mixture was diluted
with 300 mL hexanes and filtered through a pad of Celite. The solvent was
removed under reduced pressure to give the crude product. Purification was
effected by column chromatography on silica gel, with 5–20% EtOAc/hexane
1
as eluents affording pure 2-octadecenal as an oil (832 mg, 43%). H NMR

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ACETYLENIC ALCOHOL FROM CRIBROCHALINA VASCULUM
2737
(
(
6
1
200 MHz, CDCl ) d 0.88 (t ¼ 5.8 Hz, 3H, CH ), 1.26 (m, 26H, CH ), 2.31
3
3
2
m, 2H, -C¼C-CH -), 6.13 (ddt, J ¼ 1.2, 6.9, 15.6 Hz, 1H, -CH¼CH-CHO),
2
.85 (dt, J ¼ 7.7, 15.6 Hz, 1H, -CH¼CH-CHO), 9.52 (d, J ¼ 7.8 Hz,
H, CHO).
Eicos-4-(E)-en-1-yn-3-ol (11): Asolution of 320 mg (1.20 mmol) of
-octadecenal in 7 mL anhydrous THF was cooled to 0 C. To this solution,
2
ꢀ
2
4
.0 mL (1.44 mmol, 0.5 M solution in THF) ethynylmagnesium bromide was
ꢀ
added and the reaction mixture was stirred at 0 C. Progress of the reaction
was monitored by TLC. After 2.5 h, the reaction was complete and the
mixture was quenched with 20 mL saturated NH Cl, and the organic layer
4
extracted with ether (3 ꢃ 20 mL). The organic layers were combined,
neutralized with 20 mL saturated NaHCO , separated and dried over
3
anhydrous MgSO . The solution was filtered and the solvent removed
4
under reduced pressure to afford eicos-4-(E)-en-1-yn-3-ol as a yellow solid
1
(
1
4
289 mg, 83%). H NMR (200 MHz, CDCl ) d 0.90 (t, J ¼ 6.1 Hz, 3H, CH ),
3
3
.29 (m, 26H, CH ), 2.09 (m, 2H, -C¼C-CH -), 2.60 (s, 1H, H-CꢂC-),
2
2
.88 (broad d, 1H, CH-OH), 5.63 (ddt, J ¼ 1.2, 6.1, 15.3 Hz, 1H,
-
CH=CH-CHO), 5.85 (dt, J ¼ 8.6, 15.4 Hz, 1H, -CH¼CH-CHOH).
R)-Eicos-(4E)-en-1-yn-3-ol (– 1) and (S)-Eicos-(4E)-en-1-yn-3-ol acet-
ate (5): A50 mL round bottom flask was charged with 150 mg lipase AK
‘Amano’’ 20 and 300 mg molecular sieves. A solution of 300 mg of the
(
‘
racemic alcohol in 5.5 mL hexane and 0.4 mL (4.0 mmol) Vinyl acetate
was added to the flask and the reaction mixture stirred at room temperature.
1
Progress of the reaction was monitored by TLC and H NMR. When the
reaction reached 45% conversion, the reaction mixture was filtered through
a pad of Celite and the filter cake washed with 30 mL hexanes. The solvent
was removed and the solvent was removed under reduced pressure to give a
crude oil. Separation of the alcohol from the acetate was effected by column
chromatography on silica gel, with 5–20% EtOAc/hexanes as eluents afford-
ing 45 mg of the acetate, and 44 mg of the alcohol.
ꢀ
ꢀ
(
R)-Eicos-(4E)-en-1-yn-3-ol: [ꢀ]
D
¼ ꢁ 11.8 (c ¼ 0.0044, MeOH, 22 C).
3
1
H NMR (200 MHz, CDCl ) d 0.90 (t ¼ 6.4 Hz, 3H, CH ), 1.29 (m, 26H,
3
CH ), 2.10 (m, 2H, C¼C-CH -), 2.60 (s, 1H, H-CꢂC-), 4.88 (broad d, 1H,
2
2
CH-OH), 5.64 (ddt, J ¼ 1.2, 5.4, 15.3 Hz, 1H, -CH¼CH-CHOH), 5.96
1
3
(
1
1
dt, J ¼ 7.8, 15.6 Hz, 1H, -CH¼CH-CHOH); C NMR (200 MHz, CDCl )
3
4.1, 22.6, 28.8, 29.2, 29.3, 29.4, 29.6, 31.9, 62.6, 73.8, 76.4, 77.0, 77.6, 83.4,
–
28.4, 134.8. IR (thin film) cm : 3431, 2117, 3012, 2927.
1
1
(
S)-Eicos-(4E)-en-1-yn-3-yl acetate: H NMR (200 MHz, CDCl ) d 0.90
3
(
(
-
-
t ¼ 6.8 Hz, 3H, CH ), 1.30 (m, 26H, CH ), 2.09 (m, 2H, -C¼C-CH -), 2.14
3
2
2
s, 3H, CO-CH ), 2.60 (s, 1H, H-CꢂC-), 5.57 (ddt, J ¼ 1.2, 6.9, 16.6 Hz, 1H,
3
CH¼CH-CHOAc), 5.87 (d, 1H, -CH-OAc), 6.06 (dt, J ¼ 7.5, 15.4 Hz, 1H,
CH¼CH-CHOAc).

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2
738
GUNG ET AL.
(S)-Eicos-(4E)-en-1-yn-3-ol (Q1): 45 mg (0.135 mmol) of the acetate (5)
and 2.0 mg (0.013 mmol) K CO was dissolved in 2.0 mL methanol. The
2
3
reaction was allowed to proceed at room temperature and the progress
monitored by TLC. When the starting material disappeared (3 h), the reac-
tion mixture was quenched with 1.0 M HCl and the organic layer extracted
three times with diethyl ether. The organic layers were combined, washed
with saturated sodium chloride and dried over anhydrous MgSO4.
The solvent was removed under reduced pressure to give the crude product.
Purification was effected by chromatography on silica gel with 5–20%
EtOAc/hexanes as eluents affording pure (S)-Eicos-(4E)-en-1-yn-3-ol as an
ꢀ
ꢀ
1
oil (31 mg, 80%). [ꢀ]
D
¼ þ 19.4 (c ¼ 0.0039, MeOH, 22 C). H NMR
(
(
200 MHz, CDCl ) d 0.90 (t ¼ 6.1 Hz, 3H, CH ), 1.29 (m, 26H, CH ), 2.09
3
3
2
m, 2H, -C¼C-CH -), 2.60 (s, 1H, H-CꢂC-), 4.88 (broad d, 1H, CH-OH),
2
5
1
2
1
.63 (ddt, J ¼ 1.2, 6.1, 15.3 Hz, 1H, -CH¼CH-CHO), 5.85 (dt, J ¼ 8.6,
1
3
5.4 Hz, 1H, -CH¼CH-CHOH). C NMR (200 MHz, CDCl ), 14.1, 22.6,
3
8.8, 29.2, 29.3, 29.4, 29.6, 31.9, 62.6, 73.8, 76.4, 77.0, 77.6, 83.4, 128.4,
–
34.8. IR (thin film) cm : 3431, 2117, 3012, 2927; HRMS m/z 292.2756
1
(
Calcd for C H O m/z 292.2766).
20 36
Preparation of the O-Methylmandelate Esters
A25 mL round bottom flask was charged with a stirring bar, a sample
from the enzymatic resolution (59 mg, 0.2 mmol) and 2 mL anhydrous
dichloromethane. After addition of 34 mg (0.2 mmol) (R)-(–)-ꢀ-methoxyphe-
nylacetic acid and 42 mg (0.2 mmol) 1,3 dicyclohexylcarbodiimide, 2.5 mg
(
0.02 mmol) of 4-dimethylaminopyridine was also added to the flask. The
reaction was stirred at room temperature and progress monitored by TLC.
After 2 h, when all the starting material was consumed, the solution was
diluted with 50 mL hexanes and poured into a separatory funnel. The mixture
was washed once with 20 mL 1 N HCl, 20 mL saturated sodium bicarbonate,
20 mL brine, and dried over anhydrous magnesium sulfate. The solution was
filtered through a pad of Celite and the solvent removed under reduced press-
ure. Purification was effected by chromatography on silica gel with 5%
1
EtOAc/hexanes producing the diasteriomeric esters. H NMR (200 MHz,
CDCl ) d 0.85 (t ¼ 6.2 Hz, 3H, CH ), 1.95 (m, 26H, CH ), 2.09 (m, 2H,
3
3
2
-
C¼C-CH -), 2.43 (s, 1H, H-CꢂC-, minor diastereomer), 2.53 (s, 1H,
2
H-CꢂC-, major diastereomer), 3.40 (s, 3H, OMe), 4.78 (s, 1H,
PhCHOMe), 5.34 (dd, J ¼ 6.4, 15.1 Hz, 1H, -CH¼CH-CH-O-C¼O, major
diastereomer), 5.51 (dd, J ¼ 6.5, 15.1 Hz, 1H, -CH¼CH–CH-O-C¼O, minor
diastereomer), 5.86 (d, J ¼ 6.1 Hz, 1H, CH-O-C¼O), 5.74–5.95

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ACETYLENIC ALCOHOL FROM CRIBROCHALINA VASCULUM
2739
(
7
m, 1H, -CH¼CH-CHOH, major and minor diastereomers overlap),
.24–7.66 (m, 5H, Ph).
ACKNOWLEDGMENT
This research is supported in part by the NSF-REU program (SS) and
the Miami University Summer Scholarship program (KB). We thank
Amano Enzyme Inc. for free samples of lipases. Technical assistance from
Sara Burlage is acknowledged.
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Received in the USASeptember 14, 2001