Stereoselective synthesis of marine macrolide Aspergillide D

A formal stereoselective synthesis of the naturally occurring 16-membered macrolide aspergillide D is described. The origins of the chiral centers are ribose, lactic acid and the Sharpless asymmetric epoxidation protocol. The foremost reactions involved are Yadav's protocol, the Ohira-Bestmann reaction and alkyl-iodide coupling.


Introduction
Aspergillus is a genus consisting of hundreds of species of fungi found in various climates and causes aspergillosis diseases.They also produce secondary metabolites like aspergillides, aspergillic acid along with many other compounds.Aspergillides A-C are 14-membered macrolides, incorporating a trans-tetra hydropyran ring and aspergillic acid, which is a substituted pyrazine. 1,2Aspergillide D is a 16-membered macrolide, isolated from the extract of a gorgonian associated marine fungal strain Aspergillus sp.SCSGAF0076.The structural confirmation by 1 H, 13 C NMR, NOESY, HRMS-ESI and DEPT spectral analysis was performed by the Qi group. 3The first asymmetric synthesis of aspergillide D was reported by Mohapatra et al. 4 Figure 1

Results and Discussion
As part of our regular research program for the synthesis of biologically active natural products, [5][6][7][8] herein we report the formal stereoselective synthesis of aspergillide D. The retrosynthetic analysis (Scheme 1) reveals that aspergillide D could be synthesized by intramolecular macrolactonisation of fragment 16.The ester (16)  could be derived by the coupling of alkyne (3) and iodide fragments (12).The alkyne fragment could be accomplished from D-ribose and the iodide intermediate from methyl L-lactate (4).

Scheme 1
0][11][12] The alcohol 1 was protected as its benzyl ether 13 with benzyl bromide in presence of NaH in THF to afford compound 2, which on further oxidative cleavage of terminal olefin via Jin's protocol by using OsO 4 , 2,6-lutidine, and NaIO 4 leads to the corresponding terminal aldehyde, 14,15 which was subsequentially subjected to the Ohira-Bestmann reagent 16 to get the terminal alkyne fragment (3) in good yields.

Scheme 2
The synthesis of the iodide fragment 12, was started from commercially available methyl L-lactate 17  (4), by protection of the secondary alcohol as its silyl ether using TBDPS-Cl in the presence of base.This was followed by a controlled reduction of the ester to give the desired aldehyde, which was immediately subjected to a homologation reaction with a two carbon Wittig yilde leading to the α,β-unsaturated ester 5. Reduction of the ester moiety was efficiently carried out using DIBAL-H at -78 o C in CH 2 Cl 2 to afford the allylic alcohol in good yield.Thus obtained double bond was subjected to Sharpless asymmetric epoxidation [18][19][20] using (+)-DIPT as a chiral source in presence of Ti(O i -Pr) 4 and TBHP at -20 o C to furnish the desired epoxide 6 in good yields.Yadav's protocol 21,22 was applied here for a base induced elimination reaction to give the chiral propargyl alcohol 7 in 90% yield overall covering two steps from the epoxy alcohol (6) using CCl 4 -Ph 3 P under reflux conditions followed by reaction with n-BuLi at -20 o C in THF.The hydroxyl functionality in propargyl alcohol 7 was protected as a methoxymethyl ether with MOM-Cl in the presence of DIPEA to afforded compound 8 in excellent yield. 23oupling of (5R,6S)-5-ethynyl-6,9,9-trimethyl-8,8-diphenyl-2,4,7-trioxa-8-siladecane (8) and tert-butyl-(4iodobutoxy)-dimethylsilane was carried out in the presence of n-BuLi 24,25 to afford compound 9 in 70% yield.The selective desilylation of TBS in the presence of TBDPS ether with pyridinium p-toluenesulfonate 26 in methanol at room temperature afforded the primary alcohol 10.Saturation of the triple bond was achieved with Pd/C 27 (10%) under a hydrogen atmosphere in EtOAc to yield alcohol 11.The corresponding alcohol was transformed into the iodide 28 in the presence of iodine-TPP at toluene-acetonitrile (3:1) under reflux conditions to afford compound 12.

Scheme 3
The coupling of iodide 12 and the alkyne compound 3 was carried out in the presence of n-BuLi 24,25 in a THF-HMPA (2:1) mixture at -78 o C to obtain the desired compound 13 in 73% yield.The concomitant removal of the triple bond as well as the benzyl group was achieved via a Pd/C (10%) catalyzed hydrogenation in EtOAc to afford the saturated primary alcohol 29

Scheme 4
Oxidation of the primary alcohol with Dess-martin periodinane in CH 2 Cl 2 gave the corresponding aldehyde and was immediately reacted with the C 2 -ylide in a Wittig reaction resulting in the α,β-unsaturated ester 30 in

Experimental Section
General.All the air and moisture sensitive reactions were carried out under an inert atmosphere (nitrogen or argon).Oven-dried glass ware was used to perform all the reactions.Freshly distilled anhydrous solvents were used for air and moisture sensitive reactions.Commercially available reagents were used as such.The purification of the compounds was carried out via column chromatography using silica gel (60-120 mesh) packed in glass columns. 1 H NMR and 13 C NMR were recorded in CDCl 3 on a 400 MHz and 500 MHz Brucker spectrometer, using TMS as an internal standard.IR spectra were recorded on a Perkin-Elmer FT-IR 240-c spectrophotometer using KBr / Thin Film optics.Mass spectra were recorded on a Finnigan MAT 1020 mass spectrometer operating at 70 eV.Optical rotation values were recorded on a Horiba sepa 300 polarimeter.High resolution mass spectra (HRMS) [ESI+] were obtained using either a TOF or a double focusing spectrometer.
To a stirred solution of the above aldehyde (0.98 g, 3.9 mmol) in methanol (15 mL) was added dimethyl-1diazo-2-oxopropyl phosphonate (0.9 g, 4.7 mmol) at 0 o C and then K 2 CO 3 was added (1.35 g, 9.8 mmol) in portions over 10 minutes.The reaction was allowed to stir at 0 o C for 2 h and then slowly warm up to r.t. and stirred for 1 h.After the completion of the reaction (monitored by TLC), the solvent was evaporated under reduced pressure.The residue was quenched with saturated ammonium chloride and extracted with EtOAc (2×20 mL), the organic layers were washed with brine, dried over Na 2 SO 4 , and evaporated under reduced pressure.Purification was performed by column chromatography using silica gel (60-120 mesh) by eluting with a EtOAc-hexane (1:9) mixture to afford alkyne 3 (0.72 g, 72%) as an oil.
To a stirred solution of the above epoxychloride (2.9 g, 7.75 mmol) in anhydrous THF (20 mL) was added n-BuLi (9.30 mL, 23.3 mmol) dropwise at -20 o C under a nitrogen atmosphere.The reaction mixture was further stirred for 30 min.After completion of the reaction (monitored by TLC), the reaction was quenched by adding saturated NH 4 Cl, was then diluted with EtOAc (30 mL) and extracted with EtOAc (2×20 mL).The combined organic layers were washed with brine, dried over Na 2 SO 4 , then evaporated under reduced pressure.The crude product was purified by column chromatography using silica gel (60-120 mesh) by eluting with a EtOAchexane (1:9) mixture to afford, alkyne compound 7 (2.2 g, 85%) as colorless oil.Optical rotation [α] D 25 13.5 (c =  9).To a stirred solution of alkyne 8 (1.5 g, 4.5 mmol) in a THF-HMPA (2:1, 10 mL) mixture was added dropwise n-BuLi (1.8mL, 4.5 mmol) at -78 o C under a nitrogen atmosphere.While adding n-BuLi, the reaction mixture converted to a brown color and was then stirred for 30 min at the same temperature.Then tertbutyl(4-iodobutoxy)dimethylsilane (1 g, 3.2 mmol), which was dissolved in THF (5 mL), was added.The reaction mixture was stirred for 1 h, and allowed to warm up to room temperature.After completion of the reaction (confirmed by TLC), the reaction waq quenched with saturated NH 4 Cl and the solvent was removed under reduced pressure.The residue was extracted with EtOAc (2x20 mL) and the combined organic layers were washed with brine, dried over Na 2 SO 4 and the solvent was evaporated under reduced pressure.The crude product was purified by column chromatography using silica gel, 60-120 mesh by eluting with a EtOAchexane (5:95) mixture to afford the coupled product

(7R,8S)-8-[(t-Butyldiphenylsilyl)oxy]-7-(methoxymethoxy)non-5-yn-1-ol (10).
To a stirred solution of compound 9 (1.7 g, 3.0 mmol) in anhydrous MeOH (15 mL) was added PPTS (0.82 g, 3.3 mmol) at r.t. and was stirred for 1 h.After completion of the reaction, the solvent was removed and the residue was quenched with NH 4 Cl solution and extracted with EtOAc (2x10 mL), then the organic layers were washed with brine, dried over Na 2 SO 4 and evaporated under reduced pressure.The crude product was purified by column chromatography using silica gel (60-120 mesh) by eluting with EtOAc-hexane (2:8) mixture to afford, alcohol 10 (1.22 g, 90%) as a colourless oil.Optical rotation [α] D 25 -81.0 (c = 0.  To a stirred solution of alkyne 3 (0.56 g, 2.3 mmol) in a THF-HMPA (2:1, 5 mL) mixture was added dropwise n-BuLi (0.78mL, 2.0 mmol) at -78 o C under a nitrogen atmosphere.While adding n-BuLi dropwise, the reaction mass converted to a brown color and was then stirred for a further 30 min at the same temperature.Then, a solution of iodo compound 12 (0.9 g, 1.5 mmol), which was dissolved in THF (5 mL), was added.The reaction mixture was stirred for a further 1 h, and gradually warmed to room temperature.After completion of the reaction, the reaction was terminated with saturated NH 4 Cl and extracted with EtOAc (2x20 mL).The combined organic layers were washed with brine, dried over Na 2 SO 4 and concentrated under reduced pressure.The residue was purified by column chromatography using silica gel (60-120 mesh) by eluting with a EtOAc-hexane (0.5:9.5) mixture to afford 13 (0.76 g, 73%) as a color less liquid.
To a stirred solution of (+)-DIPT (0.58 mL, 2.8 mmol) and molecular sieves (2.0 g, 4 o A) in anhydrous CH 2 Cl 2 at -20 o C were added Ti(O i Pr) 4 (1.13 mL, 3.7 mmol) and TBHP (5.1 mL, 56.4 mmol).After stirring for 20 min at -20 o C the allylic alcohol (3.2 mL, 9.4 mmol) was added, which was dissolved in dry CH 2 Cl 2 (20 mL), then stirred for 12 h at -20 o C. The completion of reaction was detected by TLC, then quenched with a 20% NaOH solution (35 mL).The reaction mixture was stirred for a further 5 h and the reaction mixture was extracted with CH 2 Cl 2 (2x20 mL).The combined organic layers were washed with brine and dried over Na 2 SO 4 .Evaporation of the solvent under reduced pressure gave a crude product that was purified by column chromatography using silica ,2E)ethyl-4-[(tert-butyldiphenylsilyl)oxy]pent-2-enoate, 5 (5.1 g, 92%) as a liquid.Optical rotation [(3×20 mL).The combined organic layers were washed with brine (30 mL) and dried over Na 2 SO 4 .Evaporation of the solvent under reduced pressure led to a crude product that was purified by column chromatography using silica gel (60-120 mesh) by eluting with a EtOAc-hexane (2:8) mixture to afford, (4S,2E)-4-[(tertbutyldiphenylsilyl)oxy]pent-2-en-1-ol (3.4 g, 85%) as colorless liquid.ARKAT USA, Inc To a stirred solution of alcohol 10 (1.1 g, 2.7 mmol) in EtOAc (10 mL) was added Pd/C (10%, 10 mg) at r.t. and was stirred for 10 h, under a hydrogen atmosphere.After completion of the reaction, it was filtered through celite and the filtrate was evaporated under reduced pressure.The crude product was purified by column chromatography using silica gel by eluting with a EtOAc-hexane (2:8) mixture to afford alcohol 11 (0.98 g, 93%) as a colourless oil.The mixture was then stirred for 30 min at r.t. and changed to a thick brown color.After completion of the reaction (confirmed by TLC), the reaction mixture was quenched by adding a saturated Hypo at 0 o C and was stirred for 15 min.The mixture was extracted with EtOAc (2x10 mL) and the combined organic layers were washed with brine, dried over Na 2 SO 4 and the solvent was evaporated under reduced pressure.The crude compound was purified by column chromatography using silica gel (60-120 mesh) by eluting with a EtOAc-hexane (5:95) mixture to afford, iodide compound 12 (1.05g, 95%) as a color less oil.