First stereoselective total synthesis of an anti-fouling agent, C 2 -symmetric natural macrolide trichobotryside A

A stereoselective total synthesis of 16-membered macrodiolide trichobotryside A has been accomplished. The key features are: regioselective epoxide opening, Reetz anti -allylation, Sharpless asymmetric epoxidation, cross metathesis and Yamaguchi protocols for esterification followed by macrolactonization.


Introduction
2][3] The macropolylides, essentially 16-membered macrodiolides such as pyrenophorol, pyrenophorin, and vermiculin (Figure 1) have strong antifungal and anthelmintic activity.Trichobotryside A is C 2 -Symmetric 16-membered macrodiolide isolated by the Shu-Hua Qi group from deep sea sediment of the South China Sea fungal strain Trychobotrys effuse DFFSCS021.These compounds were screened against cancer cell lines as well as the HSV-1 virus.5][6] The structure of trichobotryside A was determined by 1 H and 13 C NMR data along with 2D-NMR experiments such as 1 H-1 H COSY, HSQC and HMBC spectra.[9] Figure 1 The biological activity and the structural fascination of trichobotryside A attracted us to carry out its total synthesis.As part of our regular research program in synthesis of biologically active natural and synthetic compounds, herein we demonstrate the first stereoselective total synthesis of the 16-membered macrodiolide trichobotryside A.

Results and Discussion
As envisaged in the retrosynthetic analysis (Scheme 1) trichobotryside A could be reached from the monomer 18 after Wittig, Sharpless epoxidation and cross metathesis.The intermediate 8 could be obtained after the Reetz allylation and hydroboration of commercially available propylene oxide (1) and dithiane 2.

Scheme 1
The synthesis commenced with regioselective ring opening of enantiomerically enriched (S)-2methyloxirane (1) with anion of 1,3-dithiane (2) using n-BuLi, HMPA, followed by protection of the secondary hydroxyl group with benzyl bromide to obtain (S)-2-[2-(benzyloxy)propyl]-1,3-dithiane (4) in quantitative yield. 10 Oxidative cleavage of dithiane 4 was performed smoothly with [bis(trifluoroacetoxy)iodo]benzene and SrCO 3 to afford aldehyde 5 in 65% yield. 11,124][15] The relative stereochemistry of 1,3-anti-diols (major isomer) was confirmed by chemical shift correlations of the acetonide carbons in 13 C NMR, as described by the Rychnovsky and Skalitzky protocol [deprotection of the benzyl group with Li-metal in liquid ammonia, followed by protection of the 1,3-diol as its acetonide with 2,2-DMP].Examination of the 13 C spectrum of the acetonide showed chemical shift values of the acetonide methyl groups at δ 24.75 and 24.83 ppm and the ketal carbon at 100.1 ppm.These chemical shift values correlate with a twist boat conformation of a 1,3-anti-diol. 16,17The secondary alcohol 6 was protected as its tert-butyldimethylsilyl ether and the terminal olefin was converted to primary alcohol 8 by hydroboration. 182][23] The E/Z ratio was determined by 1 H NMR analysis of the crude product.Chemoselective reduction of ester 10 with DIBAL-H at -40 o C gave the allylic alcohol 11 in excellent yield. 24,257][28] The free alcohol 12 was converted into iodide 13 in 77% yield by using molecular iodine, triphenylphosphine and imidazole in Et 2 O/CH 3 CN (3:1) mixture.The iodo compound 13 was subjected to immediate reductive elimination using zinc in ethanol under reflux to achieve the rearranged allylic alcohol 14 in 83% yield. 29The resulting secondary hydroxyl group in 14 was protected as its MOM ether by treating with methoxymethylchloride and diisopropylethylamine to furnish 15 in 92% yield.Cross metathesis of 15 with ethyl acrylate in the presence of Grubbs 2 nd generation catalyst in dry CH 2 Cl 2 led to the formation of trans-olefinic ester 16 in 84% yield [30][31][32] as confirmed by 1 H NMR spectroscopic data.The ester 16 on hydrolysis with 2 N NaOH in MeOH gave acid 17 in 92% yield and subsequent deprotection of the silyl group with TBAF in THF under reflux conditions furnished monomer seco-acid 18) in 95% yield as shown in Scheme 2.
Macrodilactonization of seco-acid 18 was attempted as displayed in Scheme 3, by using Yamaguchi, Shiina and DMC conditions, 33,34 but the required dimer product 22 was not obtained and the starting material also could not be recovered.Whereas, Yamaguchi conditions at high dilution, yielded the dimer product 22, but only in 5% yield and the starting material was decomposed.

Scheme 3
After the above experiments, the acid group in seco-acid 18 was converted into its methyl ester using N-nitroso-N-methylurea salt in presence of KOH and the afforded compound 19 was allied with compound 17 under Yamaguchi conditions to furnish 20 in 85% yield.][37] Thus obtained seco-acid 21 was subjected to Yamaguchi macrolactonization to afford bis-MOM, bis-benzyl protected trichobotryside A 22 in 42% yield.Global deprotection of MOM and benzyl groups in 22 was achieved in a single step by using excess TiCl 4 in dry CH 2 Cl 2 to obtain trichobotryside A in 50% yield, as shown in Scheme 4. 38 The optical rotation of the synthesized trichobotryside A was [α] D 25 +65.7 (c = 0.12, CH 3 OH) and that of the natural product is lit [α] D 20 +50.09 (c = 1.58,CH 3 OH). 7

Conclusions
The first stereoselective total synthesis of the naturally occurring, 16-membered macrodiolide, trichobotryside A has been accomplished in 16 steps with 0.54% overall yield.The key reactions involved are regioselective epoxide opening, Reetz anti-allylation, Sharpless asymmetric epoxidation, cross metathesis and Yamaguchi protocols for esterification followed by macrolactonization.

Experimental Section
General.All the air and moisture sensitive reactions were carried out under an inert atmosphere (nitrogen or argon).Oven-dried glass apparatus 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.Purification of compounds was carried out via column chromatography by using silica gel (60-120 mesh) packed in glass columns. 1 H NMR and 13 C NMR were recorded in CDCl 3 and DMSO-d 6 solvents on 400 MHz and 500 MHz spectrometer respectively, using TMS as an internal standard.IR spectra were recorded on a Perkin-Elmer FT-IR 240-c Spectrophotometer using KBr / Thin Film optics.Optical rotation values were recorded on a Horiba sepa 300 polarimeter using a 2 mL cell with a 10 mm path length.Mass spectra were recorded on a Finnigan MAT 1020 mass spectrometer operating at 70 eV.High resolution mass spectra (HRMS) (ESI + ) were obtained using either a TOF or a double focusing spectrometer. (S)-1-(
[α] D 25 +72.  .To a stirred solution of N-Nitroso-N-Methylurea salt (53 mg, 0.06 mmol) in dry ether was added KOH solution (0.2 mL, 2 N, 0.05 mmol) at 0 o C and stirred for 10 minutes, the pale yellow color results the formation of diazomethane.The ether layer was separated, passed through Na 2 SO 4 , at cooling condition immediately poured into seco-acid 18 (20 mg, 56.8 μmol), which was dissolved in dry ether (2 mL) and stirred for 10 minutes.After completion of the reaction (monitored by TLC), organic layers were separated, washed with brine, dried over Na 2 SO 4 and concentrated under vacuum.The crude product was purified by column chromatography using silica gel (60-ARKAT USA, Inc 120 mesh) by eluting with hexane/EtOAc (6:4)    (20).To a stirred solution of compound 17 (50 mg, 142 μmol) in dry CH 2 Cl 2 (5 mL) was added compound 19 (19.6 mg, 54 μmol) and the mixture was concentrated under reduced pressure and dried at high vacuum.The resulting thick paste was dissolved in toluene (0.6 mL) and under stirring was added Et 3 N (43 μmL, 426 μmol) at rt.The resulting mixture was stirred for 5 minutes and added DMAP (68 mg, 568 μmol) followed by TCBC (44.4 μmL 284 μmol) and continued stirring for 30 minutes then the solution becomes sticky.At this moment was added toluene (1 mL) and shaken manually then stirred for 1 h.After completion of reaction (monitored by TLC) the reaction mixture concentrated under vacuum.The crude product was purified by column chromatography using silica gel (60-120 mesh) by eluting with hexane/EtOAc (8:2) mixture to afford compound 20 (57.
The above acid compound was dissolved in dry THF (2 mL) and added HF.pyridine (0.01 mL, 337.4 μmol, 70% HF.30% pyridine) at 0 o C. Then slowly warmed to rt and stirred for 6 h.After completion of the reaction (monitored by TLC), the mixture was cooled to 0 o C and quenched with saturated NaHCO 3 (5 mL), followed by 0.05 N HCl (5 mL).The reaction mixture was extracted with EtOAc (2x10 mL), the combined organic layers were washed with brine and dried over Na 2 SO 4 concentrated under vacuum.The crude product was purified by column chromatography using silica gel (60-120 mesh) by eluting with EtOAc/ MeOH (9:1) mixture to afford, compound 21