Synthesis of obscuraminol A using an organocatalyzed enantioselective Henry reaction

Article abstract of DOI:10.1016/j.tet.2016.08.070

The first synthesis of the polyunsaturated amino alcohol natural product obscuraminol A is reported. This stereoselective synthesis was based on an anti- and enantioselective organocatalyzed Henry reaction followed by a chemoselective SmI₂-mediated reduction that affected only the nitro-group of the Henry product. These efforts yielded obscuraminol A where the configuration of the all-Z skipped double bonds was conserved from the starting material, i.e. the ethyl ester of (all-Z)-eicosa-5,8,11,14,17-pentaenoic acid. Our synthesis confirmed the reported structure of obscuraminol A.

Full text of DOI:10.1016/j.tet.2016.08.070

Accepted Manuscript
Synthesis of obscuraminol A using an organocatalyzed enantioselective Henry
reaction
Liudmila Filippova, Simen Antonsen, Yngve Stenstrøm, Trond Vidar Hansen
PII:
S0040-4020(16)30855-9
10.1016/j.tet.2016.08.070
TET 28051
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 23 June 2016
Revised Date: 8 August 2016
Accepted Date: 23 August 2016
Please cite this article as: Filippova L, Antonsen S, Stenstrøm Y, Hansen TV, Synthesis of obscuraminol
A using an organocatalyzed enantioselective Henry reaction, Tetrahedron (2016), doi: 10.1016/
j.tet.2016.08.070.
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Synthesis of Obscuraminol A using an organocatalyzed enantioselective Henry reaction
Liudmila Filippova, Simen Antonsen, Yngve Stenstrøm, Trond Vidar Hansen

ACCEPTED MANUSCRIPT
Synthesis of Obscuraminol A using an organocatalyzed enantioselective Henry reaction
Liudmila Filippova,a Simen Antonsen,a Yngve Stenstrøma and Trond Vidar Hansena,b*
aDepartment of Chemistry, Biology and Food Science, Norwegian University of Life
Sciences, PO Box 5003, N-1432 Ås, Norway
bSchool of Pharmacy, Department of Pharmaceutical Chemistry, University of Oslo, PO Box
1068, Blindern, N-0316 Oslo, Norway
E-mail: t.v.hansen@farmasi.uio.no
Abstract: The first synthesis of the polyunsaturated amino alcohol natural product
obscuraminol A is reported. This stereoselective synthesis was based on an anti- and
enantioselective organocatalyzed Henry reaction followed by a chemoselective SmI2-
mediated reduction that affected only the nitro-group of the Henry product. These efforts
yielded obscuraminol A where the configuration of the all-Z skipped double bonds was
conserved from the starting material, i.e. the ethyl ester of (all-Z)-eicosa-5,8,11,14,17-
pentaenoic acid. Our synthesis confirmed the reported structure of obscuraminol A.
Keywords: obscuraminol A; sphingolipids; anti-selective Henry reaction; organocatalysis;
Wang catalyst; SmI2 reduction
1. Introduction
Sphingosines are common biomembrane constituents as the backbone of sphingolipids.
Sphingolipids are involved in many biological functions.1 The sphingolipid class of natural
products derived from long-chain vicinal amino alcohols with the 1-deoxygenated
sphingosines base are widely distributed among marine invertebrates, especially in marine
ascidian.1 The wide spectrum of bioactivities reported for these substances, such as
cytotoxicity, antimicrobial and antifungal effects, makes them interesting targets in marine
bioprospecting efforts towards the development of novel pharmaceuticals.2-7 For example,
(2S,3R)-2-amino-3-octadecanol (1), isolated from the mollusk Spisula polinima, also known
as spisulosine (Figure 1),8 showed activity against several tumor cell lines. The structural
simple saturated natural product 1 has been subjected to clinical trial development programs
against solid tumors.9 However, the related compounds with a polyunsaturated Z-skipped side
chain have been less explored. Examples of this class of the sphingoids are obscuraminol A
(2) and crucigasterin 277 (3) (Figure 1).
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Figure 1. Structures of spisulosine (1), obscuraminol A (2) and crucigasterin 277 (3).
The vicinal anti-amino alcohol 2 was assigned the same (2S,3R)-configuration as spisulosine
(1) by application of Mosher’s method.10 However, this has not yet been established by
matching data obtained from total synthesis efforts, which is very often necessary for
determination of absolute configurations of vicinal amino alcohols. Obscuraminol A (2) was
isolated by Garrido et al. in 2001 from the chloroform extracts of the marine ascidian
Pseudodistoma obscurum.4 Shortly after, Clark et al. reported the isolation of the N-acetyl
derivative of 2 from the sponge Haliclona sp. 1031, and named it halaminol D.3 Structurally,
compound 2 is related to the cytotoxic and antimicrobial polyunsaturated amino alcohol
crucigasterin 277 (3), the latter has been isolated from the Mediterranean tunicate
Pseudodistoma crucigaster.5 Recently we reported a total synthesis of 3 that proved its
structure and afforded sufficient amount of material for biological studies.11 This synthesis, as
well as those of several other polyunsaturated natural products with a Z-skipped moiety,
utilized the commercially available polyunsaturated fatty acid (all-Z)-eicosa-5,8,11,14,17-
pentaenoic acid (EPA, 4), Scheme 1, as a convenient starting material.12,13 For most of these
synthetic studies, four of the five Z-skipped double bonds in 4 were conserved in the final
targets. Therefore, the formation of multiple skipped Z-double bonds from either repeated,
steroselective reduction reaction of internal alkynes,13,14 or using multiple Z-selective Wittig
reactions,13,15 is omitted. These advantages were also taken into consideration in our
retrosynthetic analysis of obscuraminol A (2). To date, there are no reports on the synthesis
of this highly unsaturated anti-amino alcohol. Herein, we report our efforts towards a
stereoselective synthesis of obscuraminol A (2), employing 4a as a convenient starting
material.
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2. Results and discussion
As outlined in Scheme 1, the retrosynthetic analysis suggests cleavage of the C2-C3 bond in
the target molecule. Towards the synthesis it was contemplated that the two stereogenic
centers at C2 and C3 will be formed in an anti-selective Henry reaction between nitroethane
and the C16 polyenoic aldehyde 5 followed by a selective reduction of the nitro group. The
ethyl ester 4a can be readily converted into aldehyde 6, and then a one-carbon homologation
should give the C16 unsaturated aldehyde 5. The anti- and concomitant enantioselective
Henry reaction between nitroethane and the chemical sensitive aldehyde 5 was considered to
be the biggest challenge toward the preparation of 2. As of today, very few protocols have
been reported with linear aldehydes to give enantioselectively anti-nitro aldol products. This is
particular true for γ,δ-unsaturated aldehydes, such as 5.16
Scheme 1. Retrosynthetic analysis of obscuraminol A (5).
The synthesis of aldehyde 5 started from the aldehyde 6, the latter was available in 75%
overall yield using a degradation protocol of the ethyl ester 4a (See Supporting Information).
The aldehyde 6 was then reduced with sodium borohydride to yield the corresponding C15
alcohol 7 that was transformed into its mesylate 8. This mesylate was immediately converted
into the nitrile 9 that was subjected to a DIBAL-H reduction, see Scheme 2. Overall, aldehyde
5 was obtained in seven steps in an overall yield of 30% from the ethyl ester 4a.
5

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Scheme 2. Synthesis of aldehyde 5.
The organocatalyzed Henry reaction was then to be investigated. Recently, several examples
of enantioselective versions of the Henry reaction have appeared using metal-based
catalysts,17 enzymatic protocols18 or organocatalysts.19 Organoatalysts for both syn- and anti-
diastereoselective variations of the Henry reaction have been developed as well.20 However,
most of the anti-diastereoselective catalysts reported give high selectivity only towards
aromatic aldehydes.20 Recently, we reported an asymmetric Henry reaction using a pinane-
derived ligand copper (II) complex that worked reasonably well for aliphatic aldehydes.21
Unfortunately, when this system was applied for the reaction between 5 and nitroethane, the
expected nitroaldol product was obtained in very low yields and with poor selectivity towards
the undesired syn-diastereomer. So we turned our attention to literature protocols for the
generation of aliphatic anti-nitro alcohols. The phenol-proline derived ligand 10 reported by
Wang and co-workers20e seemed the most promising one, and was prepared as previously
described (See Supporting Information).20e Applying 10 to the nitroaldol reaction using the
original conditions (THF, ̶ 15 °C) resulted in a sluggish reaction. However, after a prolonged
reaction time, the (2S,3R)-nitroaldol adduct 11 was isolated in 94% yield and in good
1
diastereoselectivity (anti/syn 11.5:1) as determined by H NMR analysis (Scheme 3). The
enantiomeric excess was determined to be ee = 83% by HPLC analyses (See Supporting
Information). The absolute configuration of the isolated major product of the nitro-alcohol 11
was not determined at this stage. However, based on NMR data and by comparison with
analogs reported by Wang and co-workers, the relative configuration was tentatively assigned
as anti.20e Raising the reaction temperature to
̶
8 °C significantly reduced the reaction time,
6

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but resulted in a diminished diastereoselectivity (anti/syn 5.7:1). Lowering the temperature
to ̶20 °C did not improve the selectivity.
Scheme 3. Synthesis of obscuraminol A (2) and derivatives.
The most common way for the reduction of vicinal nitroaldols into their corresponding amino
alcohols with retention of configuration is by hydrogenation using Pd/C.20b,c,e,h Obviously, this
method is not applicable towards substrates with a high degree of unsaturation, such as 11.
Transfer hydrogenation protocols have also been reported,22 but in our hands only complex
reaction mixtures were returned. The conventional reduction of the nitro-group using zinc
powder in acidic media (HCl/AcOH)20a,23 afforded the reduced product, but in addition to a
low yield of the reaction, these attempts were accompanied by partial elimination of the
hydroxyl group. Attempts to protect the hydroxyl functionality as its TBDPS-ether24 gave a
complex mixture of products. Reduction protocols employing nickel boride, which has
previously been utilized in nitroaldol reduction reactions,25 affected the double bonds. When
7

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using the NaBH4-ZnCl2 reductive system, the retro-nitroaldol reaction was observed to take
place.26 Finally, we tried a previously reported SmI2-mediated reduction protocol27 on the
nitroaldol product 11. Gratifyingly, this resulted in the isolation of the desired amino-alcohol 2
in 60% yield. In addition, the hydroxylamine 12 was detected as a by-product (Scheme 3). The
structure of 12 was assigned based on NMR- and MS-analyses of mixed fractions with
unreacted 11, 2 and 12. Full conversion to the target molecule 2 was not achieved even when
increasing the molar equivalents of SmI2 to 10 equivalents or using prolonged reaction times.
After purification by column chromatography, this reduction protocol afforded the target
aminoalcohol 2 with a lower diastereoselectivity than expected from the starting material. The
diastereomeric ratio was determined to be 5.2:1 in favor of the desired anti-isomer (See
Supporting Information). The origin of the lowered diastereomeric ratio is currently under
investigation. Unfortunately, the resulting mixture of the two diastereomers of the target the
amino-alcohol 2 was inseparable at this stage. In order to enhance the separation of these
isomers, as well as to remove compound 12, the diasteromeric mixture of 2 and with minor
amounts of 12 present, was treated with 1,1’-carbonyldiimidazole to afford two cyclic
oxazolidinones where the major product was the cis-isomer 13, that was isolated after
chromatography in 61% yield. The configuration of 13 was confirmed by NOESY and
ROESY experiments (See Supporting Information), which showed a clear correlation for the
protons of C3 and C4. Similar observations have been reported by Garrido et al..4 Moreover,
these experiments provided evidence for the initial assignment of the relative configuration of
11 also as anti.
The isolated oxazolidinone 13 was hydrolyzed28 under alkaline conditions that provided
access to the target amino alcohol 2 (dr > 20:1 determined by 1H NMR). Unfortunately, there
was a discrepancy in the specific optical rotation values and the NMR data of the synthetic
product with data reported for naturally occurring obscuraminol A (2) (See Supporting
Information).4 Examination of the literature data revealed that 2 was isolated as its
hydrochloride salt. So, the amino-alcohol was converted to its hydrochloride salt 14 after
treatment with excess hydrogen chloride in MeOH. The spectral data for 14 were in agreement
4
ꢀ ꢂꢄꢅ
with those reported earlier. Moreover, the specific optical rotation data of synthetic 14 ( ꢁ
ꢃ
ꢀ ꢂꢄꢅ
= +2.0, c = 0.14, MeOH) was dextrorotary, as previously reported ( ꢁ = +4.8, c = 0.14,
ꢃ
MeOH).4 These data supported that the absolute configuration of 2 is 2S,3R, but a significant
difference in the rather low figures for the specific optical rotation data was observed. In order
to build more confidence in establishing the absolute configuration at C2 and C3, the diacetate
8

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15 was also prepared (See Supporting Information).4 To our delight, the specific optical
rotation data of the synthetic diacetate of 2 showed similar values as reported in the literature,4
23.3 (c = 0.65, CHCl3), respectively.29 Hence,
ꢀ ꢂꢄꢅ
23.8 (c = 0.65, CHCl3) and ꢁ = ̶
ꢃ
ꢀ ꢂꢄꢅ
ꢁ
=
̶
ꢃ
the absolute configuration of the stereogenic centers in 2 is indeed 2S,3R as originally
assigned by Garrido et al. The enantiomeric excess of 15 was determined by GLC-analyses to
be ee = 88% (see Supporting Information).
3. Conclusions
To summarize, the first synthesis of obscuraminol A (2) has been presented using an
organocatalyzed anti- and enantioselective Henry reaction using the Wang catalyst 10. Several
synthetic strategies have been used for the stereoselective synthesis of vicinal amino-
alcohols,31 but organocatalysis offers many advantages.32 An important part for advancing this
important field of environmentally benign asymmetric synthesis is to apply such methodology
in the total synthesis of natural products, now extended to polyunsaturated sphingolipids. Of
note, the challenging chemoselective reduction of the nitro group in 11 was achieved by
employing a mild and rather underutilized SmI2-mediated reduction protocol.27 These efforts
yielded the naturally occurring obscuraminol A (2) in 6% overall yield over 11 steps from the
ethyl ester of eicosapentaenoic acid (4). Noteworthy, the stereochemistry of the four Z-skipped
double bonds has been conserved from the starting material 4a. The synthetic work presented
confirmed the assigned structure of the natural product 2 and provided sufficient material for
biological testing. However, a highly stereoselective preparation of obscuraminol A (2) is still
elusive. Such efforts are ongoing and will be reported in due time.
4. Experimental section
4.1 General methods.
All reactions were performed under a nitrogen atmosphere protected from light exposure. All
reagents and solvents were commercial grade and used without further purification. Thin layer
chromatography (TLC) was performed using aluminium backed silica gel 60 F254 plates and
flash chromatography utilized silica gel 60 (40-63 µm) from Merck. 1H NMR (400 MHz) and
13C NMR (100 MHz) were recorded in CDCl3 on a Bruker AscendTM 400 spectrometer.
Chemical shifts are measured in ppm relative to residual solvent peak as internal standard set
to δ 7.26 and 77.0. HRMS was performed using EI method of ionization. IR spectra (4000-600
cm-1) were recorded on Perkin-Elmer Specrtum BX series FT-IR spectrophotometer using a
reflectance cell (HATR). Optical rotations were measured using a 1 mL cell with 1.0 dm path
9

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length on Perkin Elmer 341 polarimeter in dedicated solvent. HPLC analyses were performed
on Agilent 1200 Series instrument using an AD-H column with a chiral stationary phase. The
GC analyses were performed on an Agilent GC system using Agilent J1W HP-5 GC column
(20 m, i.d. = 0.18 mm) with FID detector or a CP Chirasil 7502 column with FID detector.
Diastereomeric ratios or yields reported in this paper have not been validated by calibration,
please see reference Hudlicky and Wernerova for discussions and guidelines.33
4.2 (4Z,7Z,10Z,13Z)-hexadeca-4,7,10,13-tetraenal (5)
The aldehyde 5 was prepared from 4a as previously reported.12l The overall yield of 5 is 30%.
All spectroscopic data were in full agreement with the literature.12f,l 1H NMR (400 MHz,
CHCl3): δ 9.76 (t, J = 1.5 Hz, 1H), 5.47-5.23 (m, 8H), 2.89-2.75 (m, 6H), 2.56-2.44 (m, 2H),
2.44 -2.33 (m, 2H), 2.12-1.99 (m, 2H), 0.95 (t, J = 7.5 Hz, 3H). 13C NMR (100 MHz, CDCl3):
δ 201.9, 132.0, 129.4, 128.6, 128.4, 127.8, 127.8, 127.6, 127.0, 43.7, 25.6, 25.6, 25.5, 20.6,
20.0, 14.3.
4.3 (R)-2-((2-(hydroxydiphenylmethyl)pyrrolidin-1-yl)methyl)-6-(trifluoromethyl)phenol
Catalyst 10 was prepared according to the procedure by Wang et al.20e A white solid resulted
(64%), mp: 125-127 °C, with spectroscopic data in agreement with the literature.20a 1H NMR
(400 MHz, CHCl3) δ 7.59-7.46 (m, 4H), 7.34-7.18 (m, 6H), 7.12-7.08 (m, 1H), 6.92 (d, J = 7.4
Hz, 1H), 6.68 (t, J = 7.7 Hz, 1H), 5.20 (s, 1H), 3.91 (dd, J = 9.5, 4.7 Hz, 1H), 3.47 (d, J = 13.6
Hz, 1H), 3.30 (d, J = 13.6 Hz, 1H), 2.96 (ddd, J = 10.0, 6.3, 3.4 Hz, 1H), 2.34 (td, J = 9.8, 6.5
Hz, 1H), 2.17-2.00 (m, 1H), 1.88 (ddt, J = 12.6, 8.2, 4.3 Hz, 1H), 1.74-1.64 (m, 2H); 13C NMR
(100 MHz, CDCl3): δ 154.7, 144.9, 144.3, 130.6, 127.4, 127.3, 126.0, 125.9, 125.0, 124.8,
123.1, 116.8, 78.8, 71.2, 59.4, 54.5, 28.3, 23.1.
4.4 (2S,3R,6Z,9Z,12Z,15Z)-2-Nitrooctadeca-6,9,12,15-tetraen-3-ol (11)
To a solution of 10 (25 mg, 0.058 mmol), CuBr2 (13 mg, 0.058 mmol) and Cs2CO3 (28 mg,
0.087 mmol) in 1.2 mL of THF, nitroethane (5.9 mmol, 440 µL) was added. The mixture was
left stirring for the next four hours at room temperature until a white precipitate appeared. The
tube was centrifuged for 5 min (6000 rpm), and the supernatant was transferred to the test
tube containing pre-cooled to
̶15 °C aldehyde 5 (137 mg, 0.59 mmol). The mixture was
reacted at 15 °C until completion as monitored by TLC (120 h). Then the volatiles were
̶
removed under reduced pressure and the residue was then directly subjected to silica gel
column eluting with hexane:EtOAc (10:1) to afford 170 mg (94%) of the nitroaldol 11 as a
ꢀ ꢂꢄꢅ
colorless oil; ꢁ = -11.5 (c = 0.3, CHCl3, anti/syn 11.5:1); νmax (liquid film) 3665-3250
ꢃ
(br), 3011, 2963, 2933, 2874, 1547, 1453, 1392, 1020, 797, 705 cm-1; ee = 83% (HPLC,
10

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Chiracel AD-H column, hexane:isopropanol 98:2, 1.0 mL/min, 25 °C, 215 nm): tR (anti
minor) = 10.98 min, tR (anti major) = 12.05 min, tR (syn major) = 13.93 min, tR (syn minor) =
14.47 min; 1H NMR (400 MHz, CDCl3): δ 5.49-5.23 (m, 8H), 4.47 (qd, J = 6.9, 3.0 Hz, 1H),
4.16 (dq, J = 7.5, 3.6 Hz, 1H anti), 2.86-2.75 (m, 6H), 2.32-2.12 (m, 3H), 2.05 (td, J = 7.4, 1.3
Hz, 2H), 1.61-1.37 (m, 3H), 1.53 (d, J = 7.5 Hz 3H), 0.95 (t, J = 7.5 Hz, 3H); 13C NMR (100
MHz, CDCl3): δ 132.1, 129.6, 128.7, 128.4, 128.3, 127.9, 127.8, 127.0, 86.4, 71.4, 32.7, 25.6,
25.6, 23.4, 20.6, 14.3, 12.6.
4.5 Obscuraminol A (2) obtained by SmI2-mediated reduction
To a stirred 0.1 M solution of SmI2 in THF (1.5 mmol, 15 mL) a solution of nitro-alcohol 11
as mixture of syn- and anti-isomer (62 mg, 0.2 mmol) in THF:MeOH 2:1 was added. The
mixture was left stirring at ambient temperature until full conversion of 11 (TLC). After six h
the reaction was quenched with 10% aqueous solution of Na2S2O3 and extracted with EtOAc
(3 × 10 mL). The combined organic phase was dried (Na2SO4) and evaporated. The residue
was subjected to silica gel column chromatography eluting with CHCl3:MeOH (9:1) to afford
1
33 mg (60%) of the amino-alcohol 2 as a colorless oil; Rf = 0.13; dr (determined by H
NMR) 5.2:1 ofanti/syn-isomers . The crude reaction product consisting of 2 and 12 was
transformed into the corresponding oxazolidinone in the subsequent step.
4.6 Cis-(4S,5S)-4-methyl-5-((3Z,6Z,9Z,12Z)-pentadeca-3,6,9,12-tetraene-1-yl)oxazolidin-
2-one (13)
To a solution of the amino-alcohol 2 (16 mg, 0.058 mmol) in dry THF (2 mL) was 1,1´-
carbonyldiimidazole (14 mg, 0.087 mmol) added. The solution was refluxed for 12 h. The
solvent was evaporated and the crude product was purified by silica gel chromatography
eluting with hexane:EtOAc 7:3 to 1: 1 to obtain 10.3 mg (61%) of 13 as a colourless oil; Rf =
0.48 (hexane:EtOAc 1:1, KMnO4); 1H NMR (400 MHz, CDCl3) δ 5.57-5.53 (br s, 1H), 5.49-
5.24 (m, 8H), 4.56 (ddd, J = 10.1, 7.4, 3.8 Hz, 1H), 3.89 (pentet, J = 6.6 Hz, 1H), 2.89-2.73
(m, 6H), 2.34-2.14 (m, 2H), 2.10-2.01 (m, 2H), 1.82 (m, 1H), 1.53 (m, 1H), 1.15 (d, J = 6.5
Hz, 3H), 0.95 (t, J = 7.5 Hz, 3H); 13C NMR (100 MHz, CDCl3): δ 159.3, 132.0, 129.5, 128.6,
128.4, 128.1, 127.9, 127.8, 127.0, 79.4, 51.0, 29.2, 25.6, 25.58, 25.5, 23.5, 20.5, 16.0, 14.3.
4.7 Obscuraminol A (2) obtained by hydrolysis of 13
A solution of the cis-carbamate 13 (6.9 mg, 0.023 mmol) in a 1N aqueous solution of KOH in
EtOH-H2O 1:1 (1 mL) was refluxed for four hours. After cooling to room temperature, the
mixture was diluted with water (1 mL) and was extracted with EtOAc (3 x 2.5 mL). The
combined organic phase was dried (MgSO4) and evaporated to afford 4 mg (63%) of 2 as a
1
yellow oil; νmax (liquid film) 3680-3200 (br), 3011, 1621, 1435, 1024, 801, 704 cm-1; H
11

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NMR (400 MHz, CDCl3): δ 5.45-5.28 (m, 8H), 3.61-3.58 (m, 1H), 3.08-3.00 (m, 1H), 2.85-
2.76 (m, 6H), 2.75 (br s, 4H), 2.28-2.24 (m, 1H), 2.19-2.16 (m, 1H), 2.07 (m, 2H), 1.51-1.40
13
(m, 2H), 1.09 (d, J = 6.6 Hz, 2H), 0.97 (t, J = 7.5 Hz, 3H); C NMR (100 MHz, CDCl3): δ
132.0, 129.5, 128.6 (2 x), 128.2 (2 x), 127.9, 127.0, 73.1, 50.8, 32.3, 25.6, 25.5, 23.9, 20.6,
15.7, 14.3.
4.8 (2R,3S,6Z,9Z,12Z,15Z)-2-aminooctadeca-6,9,12,15-tetraene-3-ol hydrochloride (14)
The amino-alcohol 2, obtained by hydrolysis of 13, (5 mg, 0.017 mmol) in 0.5 mL of dry
MeOH at 5 °C was immediately treated with a five-fold excess of HCl-gas in MeOH for 30
min. The solvent was removed by evaporation flushing with nitrogen to afford 5 mg (95%) its
ꢀ ꢂꢄꢅ
4 ꢀ ꢂꢄꢅ
hydrochloride salt 14 as a pale yellow oil;
ꢁ
ꢃ
= +2.0 (c = 0.14, CH3OH), lit. ꢁ = +4.8
ꢃ
(c = 0.14, CH3OH);4 1H NMR (400 MHz, CDCl3): δ 7.93 (br s, 3H), 5.40-5.26 (m, 8H), 4.00
(m, 1H), 3.43 (m, 1H), 2.81-2.76 (m, 6H), 2.27-2.20 (m, 1H), 2.15-2.00 (m, 3H), 1.59-1.53
(m, 1H), 1.42-1.40 (m, 1H), 1.30 (d, J = 6.7 Hz, 3H), 0.95 (t, J = 7.5 Hz, 3H); 13C NMR (100
MHz, CDCl3): δ 132.0, 129.0, 128.6, 128.4, 128.1, 127.8, 127.0, 70.1, 52.0, 32.8, 25.6, 25.5,
23.6, 20.6, 14.3, 12.0; HRMS (EI+): Exact mass calculated for C18H32NO (M-Cl)+: 278.2474,
found 278.2484.
4.9 (2S,3R,6Z,9Z,12Z,15Z)-2-acetamidooctadeca-6,9,12,15-tetraen-3-yl acetate (15,
diacetate of obscuraminol A)
The amino-alcohol 2 (13 mg, 0.047 mmol) was dissolved in pyridine (0.2 mL) and Ac2O (510
mg, 0.47 mmol was added at ambient temperature. The mixture was left stirring for two hours,
then the solvents were removed by evaporation flushing with nitrogen and the residue was
purified using column chromatography eluting with hexane:EtOAc 1:1 to obtain 15 mg (90%)
ꢀ ꢂꢄꢅ
4 ꢀ ꢂꢄꢅ
of 15 as a yellow oil in a 14:1 diasteromeric ratio; ꢁ
=
̶
23.8 (c = 0.65, CHCl3), lit. ꢁ
ꢃ
ꢃ
= ̶
23.3 (c = 0.65, CHCl3); 1H NMR (400 MHz, CDCl3): δ 5.84 (d, J = 8.6 Hz, 1H), 5.43-5.22
(m, 8H), 4.83 (ddd, J = 9.3, 4.3, 3.2 Hz, 1H), 4.14 (dqd, J = 10.1, 8.4, 6.9, 3.2 Hz, 1H), 2.88-
2.69 (m, 5H), 2.14-1.98 (m, 8H), 1.93 (s, 3 H), 1.71-1.48 (m, 2H), 1.07 (d, J = 6.8 Hz, 3H),
13
0.95 (t, J = 7.5 Hz, 3H); C NMR (100 MHz, CDCl3): δ 171.6, 169.3, 132.0, 129.0, 128.6,
128.4, 128.3, 128.0, 127.8, 127.0, 76.4, 47.7, 31.2, 25.6, 25.6, 25.5, 23.5, 23.4, 21.1, 20.6,
14.9, 14.3; HRMS (EI+): Exact mass calculated for C22H35NO3: 361.2617, found 361.2637; ee
= 88% (GLC, CP Chirasil 7502, 25m, film 0.25 mm, i.d. 0.25 mm, 80 °C (45 min.), then 2
°C/min, 150 °C (10 min.)).
Acknowledgment
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We are grateful to Pronova Biopharma AS, Sandefjord, Norway and Olav Thorstad, for a kind
gift of the EPA ethyl ester 4a. We also want to thank Anne Helene Bjerke, Department of
Pharmaceutical Chemistry, the School of Pharmacy, University of Oslo and professor Dag
Ekeberg, the Norwegian University of Life Sciences, IKBM, for help with the GLC- and MS-
analyses, respectively. We are indebted to the Norwegian University of Life Sciences (Quota
Scheme) and the Norwegian University of Life Sciences, IKBM, to scholarships to L. F. and
S. A., respectively. The Norwegian Research Council is also acknowledged for funding to T.
V. H (FRIPRO-FRINATEK 230470) and Y. S. (ISP 209335), respectively.
Supplementary data
Supplementary data include experimental procedures and characterization data of starting
materials for making catalyst 10, copies of IR-spectra, 1H- and 13C-NMR spectra, as well as
copies of chromatograms of GLC and HPLC analyses.
References and notes:
1. (a) Bartke, N; Hannun, Y. J. Lipid Res. 2009, 50, S91–S96; (b) Pruett, S. T.; Bushnev,
A.; Hagedorn, K.; Adiga, M.; Haynes, C. A.; Sullards, M. C.; Liotta, D. C.; Merrill, A.
H. J. Lipid Res. 2008, 49, 1621-1639; (c) Won, T. H.; You, M.; Lee, S.-H.; Rho, B. J.;
Oh, D.-C.; Oh, K.-B.; Shin, J. Mar. Drugs, 2014, 12, 3754-3769.
2. Jimenez, C.; Crews, P. J. Nat. Prod. 1990, 53, 978-982.
3. Clark, R. J.; Garson, M. J.; Hooper, J. N. J. Nat. Prod. 2001, 64, 1568-1571.
4. Garrido, L.; Zubia, E.; Ortega, M. J.; Santiago, N.; Salva, J. Tetrahedron 2001, 57,
4579-4588.
5. Jares-Erijman, E. A.; Bapat, C. P.; Lithgow-Bertelloni, A.; Rinehart, K. L.; Sakai, R.
J. Org. Chem. 1993, 58, 5732-5737.
6. Giavatta, M. L.; Manzo, E.; Nuzzo, G.; Villani, G. Tetrahedron. 2010, 66, 7533-7538.
7. Aiello, A.; Fattorruso, E.; Giordano, A.; Menna, M.; Navarrete, C.; Munoz, E. Bioorg.
Med. Chem. 2007, 15, 2920-2926.
8. Cuadros, R.; Montejo de Garcini, Wandosell, F.; Faircloth, G; Fernandez-Sousa,
Avila, J. Cancer Lett. 2000, 152, 23-29.
9. (a) Massard, C.; Salazar, R.; Armand, P.; Majem, M.; Deutsch, E.; Garcia, M.;
Oaknin, A.; Fernandez-Garcia, E. M.; Soto, A.; Soria, J. C. Invest. New Drugs 2012,
30, 2318-2326; (b) Zubia, E. Ciencias Marinas 2003, 29, 251-260.
10. Dale; D. L. Dull; H. S. Mosher J. Org. Chem. 1969, 34, 2543-2549.
11. (a) Flock, S.; Antonsen, S.; Gallantree-Smith, H.; Langseter, A. M.; Skattebøl, L.;
Stenstrøm, Y. Tetrahedron 2016, 72, 4518-4522; (b) For unsuccessful attempts
13

ACCEPTED MANUSCRIPT
towards the synthesis of crucigasterin 277, see Estermeier, D. PhD. Dissertation.
Ludwig Maximilian University, Munich, 2005.
12. (a) Primdahl, K. G.; Stenstrøm, Y.; Hansen, T. V.; Vik, A. Chem. Phys. Lipids 2016,
196, 1-4; (b) Filippova, L.; Aarum, I.; Ringdal, M.; Dahl, M.K.; Hansen, T. V.;
Stenstrøm, Y. Org. Biomol. Chem. 2015, 13, 4680-4685; (c) Langseter, A. M.;
Skattebøl, L.; Stenstrøm, Y. Molecules 2014, 19, 3804-3812; (d) Jacobsen, M. G.;
Vik, A.; Hansen, T. V. Tetrahedron Lett. 2014, 55, 2842-2844; (e) Yasser M. A.; Vik,
A.; Hofer, T.; Hammer Andersen, J.; Hansen, T. V. Chem. Phys. Lipids, 2013, 170-
171, 41-45; (f) Langseter, A. M.; Skattebøl, L.; Stenstrøm, Y. Tetrahedron Lett. 2012,
53, 940-941; (g) Jacobsen, M. G.; Vik, A.; Hansen T. V. Tetrahedron Lett. 2012, 53,
5837-5839; (h) Vik, A.; Hansen, T. V. Tetrahedron Lett. 2011, 52, 1060-1062; (i) Vik,
A.; Hansen, T. V.; Holmeide, A. K.; Skattebøl, L. Tetrahedron Lett. 2010, 51, 2852-
2854; (j) Anwar, H. F.; Hansen, T. V. Org. Lett. 2009, 11, 587-589; (k) Hansen, T. V.;
Skattebøl. L. Tetrahedron Lett. 2004, 45, 2809-2811; (l) Flock, S.; Skattebøl, L. J.
Chem. Soc., Perkin Trans. 1, 2000, 3071-3076; (m) Flock, S.; Lundquist, M.;
Skattebøl, L. Acta Chem. Scand. 1999, 436-445.
13. Vik, A., Hansen, T. V. “Fatty Acids and their Derivatives.” In Natural Products:
From Biosynthesis to Total Synthesis. Ed.: A. L. Zografos, Wiley, ISBN 978-1-118-
75173-2, 2016, 130-161.
14. (a) Oger, C.; Balas, L.; Durand, T.; Galano, J.-M. Chem. Rev. 2013, 113, 1313-1350;
(b) Yasser M. A.; Hansen, T. V. Tetrahedron 2013, 69, 3872-3877; (c) Hansen, T. V.;
Stenstrøm, Y. Tetrahedron:Asymmetry 2001, 12, 1407-1411; (d) Hansen, T. V.;
Stenstrøm, Y. Synth. Comm. 2000, 30, 2549-2557.
15. (a) Maryanoff, B. E.; Reitz, A. B. Chem. Rev. 1989, 89, 863-927; (b) Primdahl, K. G.;
Aursnes, M.; Tungen, J. E.; Hansen, T. V.; Vik, A. Org. Biomol. Chem. 2015, 13,
5412-5417; (c) Tungen, J. E.; Aursnes, M.; Hansen, T. V. Tetrahedron Lett. 2015, 56,
1843-1845; (d) Tungen, J. E.; Aursnes, M.; Dalli, J.; Arnardottir, H.; Serhan, C. N.;
Hansen, T. V. Chem. Eur. J. 2014, 20, 14575-14578.
16. (a) Boruwa, J.; Gogoi, N.; Saikia, P. P.; Barua, N. C. Tetrahedron: Asymmetry 2006,
17, 3315-3326; (b) Palomo, C.; Oirbide, M.; Laso, A. Eur. J. Org Chem. 2007, 2561-
2574.
17. Chelucci, G. Coord. Chem. Rev. 2013, 257, 1887-1932.
18. Milner, A. E.; Moody, T. S., Maguire, A. R. Eur. J. Org. Chem. 2012, 3059-3067.
19. Alvarez-Casao, Y.; Marques-Lopez, E.; Herrera, R. P. Symmetry 2011, 3, 220-245.
14

ACCEPTED MANUSCRIPT
20. For selected examples of anti-selective Henry reaction, see: (a) Arai, T.; Joko, A.;
Sato, K. Synlett 2015, 26, 209-214; (b) Uraguchi, D.; Nakamura, S.; Ooi, T. Angew.
Chem. Int. Ed. 2010, 49, 7562-7565; (c) Blay, G.; Hernandez-Olmos, V.; Pedro, J. R.
Synlett 2011, 9, 1195-1211; (d) Lang, K.; Park, J.; Hong, S. Angew. Chem. Int. Ed.
2012, 51, 1620-1624; (e) Xu, K.; Lai.G.; Zha, Z.; Pan, S.; Chen, H; Wang, Z. Y.
Chem. Eur. J. 2012, 18, 12357-12362; (f) Ogawa, T.; Kumagai, N.; Shibasaki, M.
Angew. Chem. Int. Ed. 2013, 52, 6196-6201; (g) Handa, S.; Nagawa, K.; Sohtome, Y.;
Matsunaga, S.; Shibasaki, M. Angew. Chem. Int. Ed. 2008, 47, 3230-3233.
21. Filippova, L.; Stenstrøm, Y.; Hansen, T. V. Molecules 2015, 20, 6224-6232.
22. Barret, A. G. M.; Spilling, C. D. Tetrahedron Lett. 1988, 29, 5733-5734.
23. (a) Arai, T.; Wasai, M.; Yokoyama, N. J. Org. Chem. 2011, 76, 2909-2912; (b)
Nitabaru, T.; Kumagai, N.; Shibassaki, M. Angew. Chem. Int. Ed. 2012, 51, 1644-
1647; (c) Blay, G.; Domingo, L. R.; Hernandez-Olmo, V.; Pedro, J. R. Chem. Eur. J.
2008, 14, 4724-4730.
24. Downham, R.; Ng, F. W.; Overman, L. E. J. Org. Chem. 1998, 63, 8096-8097.
25. (a) Osby, J. O.; Ganem, B. Tetrahedron Lett. 1985, 26, 6413-6416; (b) Handa, S.;
Gnanadesikan, V.; Matsunaga, S.; Shibasaki, M. J Am. Chem. Soc. 2007, 129, 4900-
4901.
26. Behzad, Z.; Karam, Z. J. Chin. Chem. Soc. 2003, 50, 267-271.
27. (a) Kende, A. S.; Mendoza, J. S. Tetrahedron Lett. 1991, 32, 1699-1702; (b) Brady, E.
D.; Clark, D. L.; Keogh, W.; Scott, B. L.; Watkin, J. G. J. Am. Chem. Soc. 2002, 124,
7007-7015; (c) Ankner, T.; Hilmersson, G. Tetrahedron Lett. 2007, 48, 5707-5710.
28. Abad, J. L.; Nieves, I.; Rayo, P.; Casas, J.; Fabrias, G.; Deldago, A. J. Org. Chem.
2013, 78, 5858-5866.
29. The dr of the diacetate 12 was determined by 1H NMR to be 14:1 in favor of the anti-
isomer. Hence, an acyl migration might occur. Such processes are known, see
reference 30. An analytical pure sample for GLC analyses was obtained by
purification using silica gel chromatography.
30. Skwarczynski, M.; Kiso, Y. Curr Med. Chem. 2007, 14, 2813-2823.
31. (a) Karjalainen, O. K.; Koskinen, A. M. P. Org. Biomol. Chem. 2012, 10, 4311- 4326;
(b) Das P.; Kundooru, S.; Shaw, A. S. RSC Adv. 2016, 6, 14505-14511; (c) Silveira-
Dorta, G.; Donadel, O. J.; Martín, S. M.; Padrón, J. M. J. Org. Chem. 2014, 79, 6775-
6782; (d) Kobyashi, S.; Ishitani, S.; Ueno, M. J. Am. Chem. Soc. 1998, 120, 431-432
32. MacMillan, D. C. Nature 2008, 455, 304-308.
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33. Wernerova, M.; Hudlicky, T. Synlett 2010, 18, 2701-2707.
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