Synthesis of All-Z-1,6,9,12,15-Octadecapenten-3-one, A Vinyl Ketone Polyunsaturated Marine Natural Product Isolated from Callysponga sp.

The synthesis of the marine natural product 1,6Z,9Z,12Z,15Z-octadecapentaen-3-one (1) has been achieved by two different routes starting from the ethyl esters of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), respectively. Using EPA ethyl ester as starting material the polyunsaturated vinyl ketone lipid 1 was obtained in 17% overall yield.


Introduction
Over the years numerous polyunsaturated lipids have been isolated and characterized as secondary metabolites from marine sources [1]. Several possess interesting biological properties and are targets for total syntheses. Some years ago 1,6Z,9Z,12Z,15Z-octadecapentaen-3-one (1) was isolated from an Australian marine sponge Callyspongia sp by Urban and Capon [2]. Few fatty acid derived natural products with a vinyl ketone moiety have been described. No biological activity was reported for compound 1 and to best of our knowledge it has not been synthesized yet.
The vinyl ketone 1 attracted our interest as part of an ongoing project utilizing the polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) as starting materials for the OPEN ACCESS syntheses of compounds containing several methylene interrupted Z double bonds [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. A key reaction in this respect is the oxidative degradation of EPA and DHA to aldehydes by way of the corresponding iodolactones. In the present work a simplified iodolactonisation procedure for EPA and DHA is presented as part of the synthesis of vinyl ketone 1.

Results and Discussion
With the use of EPA and DHA in mind three different routes to the target molecule 1 became apparent, as outlined in Scheme 1. Routes A and B proceed through the bromide 2 which is available from EPA ethyl ester by modification of a literature procedure [4,17,18] to improve the yields. The vinyl alcohol 3 is an intermediate in both routes B and C.
Route A seemed promising at first. Katritzky and coworkers have published an elegant one step method for the generation of vinyl ketones by the reaction of the propenoyl anion equivalent N-(ethoxyallyl)benzotriazole with a bromide [19]. Unfortunately, reaction of the bromide 2 with the propenoyl anion equivalent under the reported conditions gave no vinyl ketone and elimination products were the only detectable compounds according to NMR. Scheme 1. Retrosynthetic analysis of the target molecule 1.
We then attempted route B as depicted in Scheme 2. However, transformation of the bromide 2 to the Grignard reagent 4 was accompanied by a high degree of Wurtz coupling; consequently, the reaction with acrolein resulted in a very poor yield of the vinyl alcohol 3. Difficulties with an efficient formation of this Grignard reagent have been observed recently, and attempts to prepare the lithium analogue of 4 by lithium exchange on the corresponding iodide resulted in significant isomerisation of the double bonds [4]. Discouraged by this result we turned our attention to route C that required the C-16 aldehyde 5, available from EPA and DHA as outlined in Schemes 1 and 3. Starting from DHA using the modified iodolactonisation procedure and oxidative cleavage, followed by the DBU-induced isomerisation of the β,-double bond, the conjugated aldehyde 9 was obtained in good overall yield. The corresponding alcohol derived from compound 9 underwent Sharpless epoxidation. Protection of the epoxyalcohol 10, epoxide opening to the diol and oxidative cleavage of this afforded the aldehyde 5 [20].   This route was quite lengthy so we switched to an approach involving chain elongation of aldehyde 6 by a Wittig reaction as depicted in Scheme 3.
Treatment of the aldehyde with the ylide derived from methoxymethyltriphenylphosphonium chloride and potassium t-butoxide followed by hydrolysis of the resulting vinyl ether gave the aldehyde 5. Finally, reaction of 5 with vinylmagnesium bromide followed by oxidation of the allylic alcohol using the Dess-Martin periodinane procedure gave 1 in 21% yield overall from 5. The spectral data compared well with those reported for the natural compound [2].

General
All reactions were performed under nitrogen. EPA ethyl ester was obtained from Pronova Biopharma, Sandefjord, Norway. All other reagents were used as purchased. The NMR spectra were recorded on a Varian Gemini spectrometer. MS (EI) spectra were recorded on an Autospec Ultima instrument and are presented as m/z (% relative intensities). HRMS were recorded on the same instrument. IR spectra were obtained on a reflectance cell on a Perkin Elmer FT-IR instrument. The syntheses of some of the compounds have been previously published. When included here, modifications and/or improvements of yields have been obtained. (11). A solution of the iodolactone (7) [3] (12.09 g, 28.2 mmol) and 5% LiOH . H 2 O in MeOH-H 2 O (19:1, 120 mL) was refluxed for 6 h. Water (120 mL) was added and most of the methanol was removed under reduced pressure. The reaction mixture was cooled (ice bath) and acidified with dilute HCl. Solid NaCl was added to saturation and the reaction mixture extracted with EtOAc (3 × 50 mL). The organic extracts was washed with brine (2 × 50 mL), dried (Na 2 SO 4 ) and solvent was removed under reduced pressure to obtain 11 (9.46 g; 99%) as a yellow oil. Occasionally small amounts of the corresponding hydoxylactone were seen in the NMR spectra. Spectral data were in agreement with those previously reported [21].

5,6-Dihydroxy-(8Z,11Z,14Z,17Z)-icosatetraenoic acid
3Z,6Z,9Z,12Z-pentadecatetraenal (6). A mixture of the dihydroxy acid 11 (9.55 g, 28.3 mmol) and 5% LiOH . H 2 O in MeOH-H 2 O (19:1) (90 mL) was cooled on an ice-bath and left stirring for 30 min. before water (90 mL) was added. A solution of saturated citric acid was added until pH 4 was attained in the reaction mixture. Solid NaIO 4 (7.5 g, 35 mmol) was added in one portion. The reaction mixture was left stirring at room temperature for 1 h. Solid NaCl was added to saturation and the product was extracted with hexane (3 × 50 mL). The extract was washed with brine (2 × 50 mL) and dried (MgSO 4 ). Evaporation of the solvent under reduced pressure gave the unstable aldehyde 6 (4.88 g; 80%) as a colorless oil. Spectral data were in agreement with those previously reported [17]. (8). A mixture of DHA ethyl ester (10.02 g, 28 mmol) and LiOH . H 2 O (5.8 g, 140 mmol) in EtOH-H 2 O (1:1) (60 mL) was left stirring until all the DHA ethyl ester was converted (TLC, CH 2 Cl 2 ). Water (90 mL) were added, the reaction flask was covered with aluminium-foil and cooled to 0 °C. Hydrogen iodide (57%; 20 mL) was added to the reaction mixture, followed successively by saturated KHCO 3 (10 mL) and dropwise addition of a solution of I 2 (21.32 g, 84 mmol) in EtOH (70 mL). The mixture was left stirring at 0-4 °C in the dark for 18 h. The reaction was quenched by adding a saturated aq. solution of Na 2 S 2 O 3 (100 mL). Solid NaCl was added to saturation and the product extracted with hexane (3 × 50 mL). The extract was washed with brine (2 × 50 mL), dried (Na 2 SO 4 ) and evaporated under reduced pressure to give 8 (12.3 g; 97%) as pale yellow oil. Spectral data were in agreement with those previously reported [17]. (7Z,10Z,13Z,16Z,19Z)-pentaenoic acid (12). A solution of the iodolactone 8 (9.53 g, 21 mmol) in dry MeOH (110 mL) was cooled to 0 °C and K 2 CO 3 (5.8 g, 40 mmol) was added. The mixture was left stirring overnight at room temperature. Water (12 mL) was added followed by a solution of 5% LiOH . H 2 O in MeOH-H 2 O (19:1) (90 mL). The mixture was refluxed for 4 h, cooled in an ice bath and acidified with dilute HCl. Solid NaCl was added to saturation and the product extracted with EtOAc (3 × 50 mL). The extract was washed with brine (2 × 50 mL) and dried (Na 2 SO 4 ). Evaporation of the solvent under reduced pressure gave 12 (6.46 g; 85%) as an oil. Spectral data were in accord with the literature [21]. Small amounts of the corresponding hydroxylactone were inevitably present as shown by the NMR spectra [18,22].

4,5-dihydroxydocosa-
2E,6Z,9Z,12Z,15Z-octadecapentaenal (9). A mixture of the crude dihydroxy acid 12 (6.46 g, 18 mmol) and 5% LiOH . H 2 O in MeOH-H 2 O (19:1) (60 mL) was cooled in ice and left stirring for 30 min. Water (60 mL) was added followed by a saturated solution of citric acid until pH 4 was attained. Solid NaIO 4 (5.56 g, 26 mmol) was added in one portion and the reaction mixture left stirring at room temperature for 1 h. Solid NaCl was added to saturation and the reaction mixture was extracted with hexane (3 × 50 mL) The extract was washed with brine (2 × 50 mL) and dried (Na 2 SO 4 ). Evaporation under reduced pressure left a residue that was dissolved in ether (125 mL) and DBU (1 mL) was added. After stirring for 30 min, the organic phase was washed with water to neutral pH, then with brine (2 × 50 mL) and dried (Na 2 SO 4 ). The solvent was removed under reduced pressure furnishing the aldehyde 9 (3.44 g; 74%) as a yellow oil. Spectral data were in agreement with those previously reported [17].

Conclusions
In summary, we have completed the first total synthesis of 1,6Z,9Z,12Z,15Z-octadecapentaen-3-one (1) in 6 steps and 17% overall yield from EPA ethyl ester. The coupling between the C-16 aldehyde 5 and vinyl magnesium bromide is the key step. The synthesis of vinyl ketone analogues of 1 is currently in progress in our laboratory.