Utilisation of Cashew Nut Shell Liquid from Anacardium occidentale as Starting Material for Organic Synthesis : A Novel Route to Lasiodiplodin from Cardols

Como parte de nosso programa envolvendo a utilização do LCC (Líquido da Casca da Castanha), obtido a partir Anacardium occidentale, como matéria-prima para preparação de compostos de maior valor agregado, descrevemos a conversão dos cardóis (6-alquenilresorcinóis) na lasiodiplodina, um macrolídeo tipo orcelínico de 12 membros, de ocorrência natural, que exibe propriedade reguladora do crescimento de plantas e atividade antileucêmica.z


Introduction
The cashew tree, Anacardium occidentale L, is a botanical species native of eastern Brazil and was introduced into other tropical countries such as India, Africa, Indonesia and South East Asia in the 16 th century 1 .The true fruit of cashew is the nut, a kidney shaped structure of approximately 2-3 cm in length which is attached to the end of a fleshy bulb, generally called the cashew apple.The shell comprises some 50% of the weight of the raw nut, the kernel represents 25% and the remaining 25% consists of the natural cashew nut shell liquid (CNSL), a viscous reddish brown liquid.Recently, Brazilian raw cashew nut production has increased greatly in relation to other countries.The estimated production of raw cashew nuts in Brazil for the end of this century is 290.000tons 2 .The CNSL is traditionally obtained as a by-product during the isolation of the kernel by roasting the raw nuts, making Brazil one of the leading producers of this material.
Crude cashew nut shell liquid represents one of the major and cheapest sources of naturally occurring non-isoprenoid phenolic lipids such as anacardic acids (1), cardols (2), cardanols (3), methylcardols (4) (Fig. 1) and polymeric materials.CNSL has found important commercial usage as the phenolic raw material for the manufacture of certain resins and plastics having unusual electric and frictional properties 3 .
The interesting chemical characteristics of cardols, such as the presence of a double bond at the 8-position of the long-chain in the monoene, diene and triene components and a convenient aromatic orcinol system led us to search for a strategy to convert these materials into lasiodiplodin (5) (Fig. 2).The latter compound is a naturally occurring 12-membered orsellinic acid type macrolide, isolated from the culture broth of the fungus Botrydiplodia theobromae (formally Lasiodiplodia theobromae), and exhibits plant growth regulating properties 4 .This macrolide has also been found in the stems and leaves of Euphorbia splendens and shows antileukemic activity 5 .Several total synthesis of racemic lasiodiplodin have been carried out during the last 20 years 6 , while some asymmetric synthesis of the R-lasiodiplodin have been recently published 7 .
Although a considerable number of procedures for the synthesis of lasiodiplodin have been disclosed, none of them have dealt with the utilisation of CNSL, a very attractive potential raw material.This study using CNSL constituents is an original contribution to the synthesis of the macrolide lasiodiplodin.
The route reported herein combined the well-established synthetic procedures (ozonolysis of double bounds; regioselective carboxylation of the aromatic ring; selective methylation of one of the free phenolic groups; chemioselective oxidation of the aromatic ω-hydroxy-aldehyde and macrolactonization of ω-hydroxy-acid) for construction of lasiodiplodin from cardols.

Results and Discussion
The key reaction in our initial proposal for the synthesis of lasiodiplodin was the conversion of alkenylresorcinols 2 into the corresponding substituted salicylic acids.Attempts at direct carboxylation of 2, via the Kolbe-Schmitt and related methods failed, resulting in the recovery of unreacted 2. Then we turned our attention to the use of the modified Gattermann reaction for indirect carboxylation, however the maximum yield obtained was not synthetically significant.The sterically hindered imide hydrochloride intermediates obtained from these phenols were not readily hydrolysed by water or dilute hydrochloric acid, but required refluxing with 10-20% hydrochloric acid.This fact suggested that the harsh conditions could chemically alter the side chains, resulting in low yields of the desired product.
We were much more successful in carrying out the desired transformation by the modified Gattermann reaction 8 , using zinc cyanide and anhydrous hydrogen chloride in anhydrous THF-ethyl ether on phenol 7 furnishing the salicylic aldehyde 8 in 85% yield (Scheme 1).The compound 7 was previously prepared by ozonolysis of the acetyl derivatives 6 on multigram scales, followed by the reductive cleavage of the derived ozonides with sodium borohydride 9 .
Next, we thought to carry out the indiscriminate alkylation of both phenolic functions in 8 to prepare the dimethylether 9, followed by its transformation into diester 10, the key intermediate used by Solladié et al. 7b in the total asymmetric synthesis of both antipodes of lasiodiplodin via the asymmetric reduction of the corresponding β-ketosulfoxides.
The dimethylether 9 was prepared by treatment of phenol 8 with potassium carbonate and methyl iodide in acetone under reflux for 5 h.To our surprise several attempts at conversion of the compound 9 into the diester 10, by treatment with potassium permanganate under many different reaction conditions, were unsuccessful, giving the desired ester in poor overall yields (15-30%).In addition, appreciable over oxidation was observed.Both potassium permanganate at room temperature and Jones reagent under normal conditions were unsatisfactory for the oxidation of 9 to the corresponding diacid, since they attacked the alcohol function without affecting the aldehyde group.Searching next for an alternative oxidative procedure for converting the hindered aldehyde group into acid group, we found that it could be readily done by using the inexpensive pair sodium chlorite-DMSO, 10 however the alcohol group was quite stable under these conditions.Since the direct oxidation of 9 to the corresponding diacid did not proceed satisfactorily, we decided to do it in a two-step process.
The effective oxidative procedure was achieved by treatment of compound 9 with PCC in methylene chloride at room temperature 11 , followed by treatment with sodium chlorite-DMSO.The crude diacid was then readily esterified by methyl iodide under reflux to form the desired diester 10 in 87% overall yield (Scheme 2).Compared to the use of sodium permanganate, the two-step oxidative sequence using mild reaction conditions resulted in better overall yield.The spectral data for 10 were identical to the published data 7b , thus our preparation of 10 constitutes a formal synthesis of lasiodiplodin.
The recent synthetic routes to lasiodiplodin 7a-d were achieved via the methyl ether (10), which has been converted into the macrolide in a multistep protocol (demethylation-benzylation-methylation-debenzylation).As we were committed to developing a simplified synthesis of lasiodiplodin, we reasoned that it should be possible to selectively protect the hydroxyl group in 8 based on an internal hydrogen-bonding effect (Scheme 3).
Application of the usual benzylation [PhCH 2 X-K 2 CO 3 (X = Cl, Br or I)] in acetone under several conditions did not lead to monoalkylation of phenol 8, but rather gave a complex mixture of both mono-and dialkylated products along with starting material.However, when compound 8 was treated with BnCl/K 2 CO 3 /KI in acetone at 40 °C for 30 h, the desired monobenzylated compound 11 was afforded exclusively.The use of more drastic conditions decreased the reaction time, but also led to a decrease in selectivity.
The residual phenolic hydroxyl group was methylated as described above to give 12, which was oxidised (PCC/sodium chlorite-DMSO) and methylated to afford in high yield the unknown methyl 2-methoxy-4-benzyloxy-6-(7methoxycarbonylheptyl) benzoate 13 (Scheme 3), a compound that we expect to be a more suitable acyclic precursor for the synthesis of both antipodes of lasiodiplodin.
Alternatively (Scheme 4), treatment of 12 with sodium chlorite-DMSO, followed by methylation furnished the ester 14 in 83% yield.The conversion into aldehyde 15 was achieved in 95% yield by treatment with PCC in methylene chloride.Treatment of the aldehyde 15 with methylmagnesium iodide afforded the hydroxyester 16.Hydrolysis of the ester 16 required rather severe conditions, the hydroxyacid 17 being formed in 85% yield by using potassium hydroxide (10 M) in ethylene glycol at 165 °C.
In order to complete the synthesis, the hydroxyacid 17 was converted, under high dilution conditions, into lasiodiplodin benzylether 18 in 65% yield by using Mukai- yama's procedure 12 .In the last stage, lasiodiplodin benzylether 18 was submitted to hydrogenolysis to yield quantitatively racemic macrolide, which had spectral properties identical with those reported by Gerlach 6a (Scheme 4).The above methodology not only constitutes a highly efficient synthesis of (±) lasiodiplodin from cardols (2), abundant and inexpensive natural products, but also has additional advantages over the earlier routes because of the simplicity of the chemistry involved.

Experimental
The oxidative cleavages were performed using a Welsbach T-408 ozonizer.All reactions were monitored by thin layer chromatography on silica gel (60F 254 / 0.2 mm) plates.UV light and 5% phosphomolybdic acid in ethanol, followed by heating, being used as developing agents.The purification was performed by dry-column flash chromatography on silica gel (60, particle size 0.040-0.063mm).Melting points were determined on a Köffler apparatus and are uncorrected.IR spectra were recorded on Perkin Elmer 283-B and Nicolet 5ZDX-FT IR spectrometers. 1 H-NMR spectra were recorded at 300, 200 or 90 MHz on Bruker spectrometers or a Varian EM 290 model. 13C-NMR spectra were recorded at 75 and 50 MHz on Bruker spectrometers.Mass spectra were recorded on a Perkin Elmer Q-Mass 910 mass spectrometer.

Cardols
The shells (500 g) of cashew nuts from Anacardium occidentale (Ceará, Brazil) were extracted in a Soxhlet extractor with commercial ethanol (2.0 L), yielding a crude extract (CNSL, 157 g, 31% by weight).Anacardic acids were removed from CNSL (25 g batches) by precipitation as lead anacardates according to protocols described in the literature 13 .The resulting filtrate was concentrated to give a brown oil, which was chromatographed on silica gel by eluting with hexane-ethyl acetate mixtures of increased polarity.Cardanols ( 1.5%) and cardols (6.15 g, 24.6%) were isolated sequentially.

1,3-Diacetylcardols (6)
To a solution of cardols (3.2 g, ~10 mmol) in acetic anhydride (20 cm 3 ) was added pyridine (20 cm 3 ).The mixture was stirred at room temperature until the reaction was shown to be complete by thin layer chromatography (hexane-ethyl acetate, 4:1, 2 h).Water was added and then the mixture was extracted with ethyl acetate.The combined extracts were washed with a cool 5% hydrochloric acid solution, brine, dried over sodium sulphate and evaporated.The residue was chromatographed on silica gel (hexaneethyl acetate, 4:1) to furnish 6 as a colourless oil [3.

2,4-Dihydroxy-6-(8-hydroxyoctyl) benzene (7)
A solution of 6 (3.9 g, ~9.6 mmol) in methylene chloride (50 cm 3 ) and methanol (50 cm 3 ) at -70 °C was treated with ozone until the reaction was shown to be complete by thin layer chromatography (hexane-ethyl acetate, 4:1).The reaction mixture was purged with nitrogen, sodium borohydride (~4.0 g) was added, and the resulting mixture was stirred at room temperature overnight.After addition of water, the reaction mixture was hydrolysed with 10% hydrochloric acid and then extracted with ethyl acetate.The combined extracts were washed with brine, dried over sodium sulphate and evaporated.The pure alcohol 7 was obtained as a white solid by recrystallization from acetonemethylene chloride [

2,4-Dihydroxy-6-(8-hydroxyoctyl)benzaldehyde (8)
A 150 cm 3 three necked round-bottom flask was fitted with an adapter provided with a reflux condenser, a gas inlet tube extending nearly to the bottom of the flask and an exit tube leading to an aqueous sodium hydroxide trap.The flask was charged with the phenol 7 (1.0 g, 4.2 mmol) dissolved in THF (~5 cm 3 ), dry ether (100 cm 3 ) and powdered zinc cyanide (1.24 g, 10.5 mmol).The mixture was stirred vigorously and gaseous hydrogen chloride was passed rapidly to dissolve the zinc cyanide (20 -30 min).Hydrogen chloride was slowly bubbled into the mixture until the reaction was shown to be complete (about 1.5 h).The solid imide intermediate was separated from the solvent by filtration and the imide hydrochloride was hydrolysed by heating for 2 h with 10-20% hydrochloric acid in methanol.After cooling, the reaction mixture was extracted with ethyl acetate.The combined extracts were washed with brine, dried over sodium sulfate and evaporated.The crude mixture was chromatographed on silica gel (hexaneethyl acetate, 3:1) to give 8 as a white solid [0.95 g, 85%, mp 68

Methyl 2,4-dimethoxy-6-(7-methoxycarbonylheptyl) benzoate (10)
To a suspension of PCC (110 mg, 0.51 mmol) in anhydrous methylene chloride (15 cm 3 ) was added hydroxyaldehyde 9 (100 mg, 0.34 mmol) in anhydrous methylene chloride (5 cm 3 ).The mixture was stirred at room temperature until the reaction was shown to be complete (about 2 h).The reaction mixture was quenched with anhydrous ether and filtered.The filtrate was washed with brine, dried over sodium sulphate and evaporated.To the crude residue in DMSO (5 cm 3 ) was added a solution of sodium chlorite (154 mg, 1.7 mmol) in water (1.5 cm 3 ) followed by a solution of NaH 2 PO 4 .H 2 O (235 mg, 1.7 mmol) in water (1.5 cm 3 ) and the mixture was stirred at room temperature overnight.After addition of water the mixture was extracted with ethyl acetate.The combined extracts were washed with brine, dried over sodium sulphate and evaporated.To the crude product in acetone (10 cm 3 ) was added potassium carbonate (470 mg, 3.4 mmol), an excess of methyl iodide (1 cm 3 ) and the resulting mixture was stirred under reflux for 2 h.After usual work-up, the product was chromatographed on silica gel (hexane-ethyl acetate, 9:1) to furnish 10 as a colourless oil [  (10).

Methyl 2-methoxy-4-benzyloxy-6-(7-methoxycarbonylheptyl) benzoate (13)
To a suspension of PCC (110 mg, 0.51 mmol) in anhydrous methylene chloride (15 cm 3 ) was added the hydroxyaldehyde 12 (126 mg, 0.34 mmol) in anhydrous methylene chloride (5 cm 3 ).The mixture was stirred at room temperature for about 2 hours.The reaction mixture was quenched with anhydrous ether and filtered.The filtrate was washed with brine, dried over sodium sulphate and evaporated.To the crude residue in DMSO (5 cm 3 ) was added a solution of sodium chlorite (154 mg, 1.7 mmol) in water (1.5 cm 3 ) followed by a solution of NaH 2 PO 4 .H 2 O (235 mg, 1.7 mmol) in water (1.5 cm 3 ) and stirred at room temperature overnight.After addition of water, the mixture was extracted with ethyl acetate.The combined extracts were washed with brine, dried over sodium sulphate and evaporated.To the crude product in acetone (10 cm 3 ) was added potassium carbonate (470 mg, 3.4 mmol) and an excess of methyl iodide (1 cm 3 ).The resulting mixture was stirred under reflux for 2 h.After usual work-up the product was chromatographed on silica gel (hexane-ethyl acetate, 9:1) to furnish 13 as a colourless oil [

(±) Methyl 2-methoxy-4-benzyloxy-6-(8-hydroxynonanyl) benzoate (16)
A solution of methylmagnesium iodide (~2.2 mmol) in anhydrous ether (5 cm 3 ) was slowly added at 0 °C to a solution of the preceding aldehyde 15 (750 mg, 1.88 mmol) in anhydrous ether (20 cm 3 ).At the end of addition, the reaction mixture was refluxed for 2 h.After the addition of ether, the reaction mixture was hydrolysed with a 5% hydrochloric acid solution and extracted with ether.The combined extracts were washed with a 5% sodium thiosulphate, sodium bicarbonate and brine.The organic phase was dried over sodium sulphate and after evaporation of the solvent, the residue was chromatographed on silica gel (hexane-ethyl acetate, 3: To a solution of the hydroxyester 16 (450 mg, 1.08 mmol) in ethylene glycol (15 cm 3 ) was added a 10 M potassium hydroxide solution (2 cm 3 ).After stirring at 165 °C for 2 h under a nitrogen atmosphere, the reaction mixture was acidified with 10% hydrochloric acid and extracted with ethyl acetate.The combined extracts were washed with brine, dried over sodium sulphate and evaporated.After chromatography on silica gel (hexane-ethyl acetate, 1:1), hydroxyacid 17 was obtained as a viscous oil (370 mg, 85%