Stereoselective synthesis of fully functionalized acyclic core of Tianchimycin A

A highly convergent synthetic approach towards the macrolactone polyketide tianchimycin A is described. Notable features of our synthetic approach include highly stereoselective Myers alkylation, substrate controlled anti aldol reaction, and Masamune-Roush olefination.


Introduction
Actinobacteria are important sources of bioactive secondary metabolites. 1In 2013, Deng and co-workers isolated two new 16-membered macrolactone type polyketides from the rare actinomycete Saccharothrix xinjiangensis B-24321 and named them as tianchimycin A and tianchimycin B (Figure 1). 2 Structures of tianchimycins A-B were determined based on detailed NMR and MS spectroscopy.Architecturally tianchimycin A is quite interesting.It is a macrocyclic lactone adorned with six stereogenic centers and three olefinic moieties.Out of three double bonds, two are part of 1,4-butadiene system and the third one is a part of α,βunsaturated lactone moiety.Initial biological studies revealed that they do not have antibacterial activity.However modification of the structure might provide good antibacterial lead.Thus with this intention we initiated a program for the total synthesis of tianchimycin A and its analogs to unveil the full biological potential.In 2015 Sabitha et al. reported the synthetic study of tianchimycin A. They synthesized the entire acyclic C1-C16 framework of the molecule.However the macrolactonization under different conditions was unsuccessful. 3Thus we thought an alternate approach to construct 1 could be intramolecular Heck-cyclization for the formation of macrocyclic ring. 4Recently the employment of intramolecular Heck reaction for macrocyclization has flourished in natural product synthesis. 4,16,17,18,19It is interesting to note that this reaction is applicable to variation depending upon the macrocyclic ring size (16-24 size macrocycles), which requires optimization of variety of reaction parameters.

Results and Discussion
Retrosynthetically, we dissected 3 into building blocks 4 and 5 (Scheme 1).Heck coupling of substrate 3 was envisaged as a key step to close the macrocycle, while connection of the cyclization precursor 3 was planned to arise from Masamune-Roush olefination of aldehyde 4 and ketophosphonate 5. 5,6 The aldehyde 4 would be acquired from the known compound 7 7 using Myers asymmetric alkylation and the ketophosphonate 5 might be synthesized from known compound 9 8 using Paterson's anti aldol reaction.

Scheme 1. Retrosynthetic analysis of tianchimycin A.
From the synthetic perspective, synthesis of 4 (Scheme 2) began with the diastereoselective alkylation of the known iodide 7 7 with Myers pseudoephedrine derived auxiliary 9.10 to yield amide 10 as a single diastereomer in 95% yield.Reduction of 10 with BH3.NH3 gave primary alcohol 6 in 90% yield.Compound 6 on oxidation under Dess-Martin Periodinane conditions provided an aldehyde, to which addition of (Z)-enolate, generated from 11, using Crimmins's protocol 11 afforded 12 with excellent diastereoselectivity (98:2 dr), which are separated by standard silica gel column chromatography to obtain the required single isomer 12 in 96% yield.Reductive removal of the chiral auxiliary with LiBH4 in ether furnished the 1,3-diol compound 13, which on protection as PMP-acetal followed by TBDPS deprotection with TBAF gave a primary alcohol 15.Oxidation of 15 with DMP gave an aldehyde which on reaction with vinylmganesium bromide yielded diastereomerically mixture of alcohols 16, which on TBS protection with TBSOTf in presence of 2,6-lutidine gave globally protected compound 17.At this stage the p-methoxybenzylidine acetal of 17 was opened regioselectively with DIBAL-H to give a primary alcohol 4a, which was oxidized with DMP to give required aldehyde 4 in 90% yield.
Synthesis of phosphonate fragment 5 commenced from known keto compound 9 8 (Scheme 3), which on reaction with acetaldehyde under Paterson's anti-aldol conditions 12,13 using dicyclohexylborane chloride afforded β-keto alcohol 8, in 96% yield with excellent diastereoselectivity which was protected as its TBS ether to give compound 18 in 92% yield.Reduction of the keto as well as benzoate group in 18 with LiBH4 afforded diastereomerically mixture of diols 19 in 92% yield. 7Oxidative cleavage of the diol with NaIO4 furnished an aldehyde, which on Takai olefination gave vinyl iodide 20 (E:Z ratio 19:1) in 80% yield over two steps. 14,15TBS deprotection from compound 20 furnished a secondary alcohol, which on acylation with diethyl phosphonoacetic acid under EDCI/DMAP conditions afforded the phosphonate 5 in 85% yield.Having both the fragments in our hand, the Horner-Wadsworth-Emmons reaction 5 under Masamune-Roush conditions was carried out between aldehyde 4 and the phosphonate 5 in presence of DBU and LiCl in acetonitrile to give key acyclic precursor 3 (Scheme 4) for intramolecular Heck-cyclization.At this stage the crucial intramolecular Heck-cyclization under assorted conditions (Table 1) was not successful leaving the total synthesis still elusive. 4,16,17,18,19heme 4. Reagents and conditions: (a) LiCl, DBU, CH3CN, 0 o C-rt, 12 h, 77%.

Conclusions
The synthesis of acyclic precursor of macrocyclic tianchimycin A was achieved by employing Myers asymmetric alkylation, Crimmins's aldol reaction, Paterson's aldol reaction, Takai olefination and Masamune-Roush olefination as key steps.Currently we are working to develop a diverse strategy to circumvent the problem of macrocyclization, which might help us to achieve the total synthesis of tianchimycin A and its potential analogs and for biological screening, which will be reported in due course.

Experimental Section
General.All the reactions were performed in oven-dried glass apparatus under nitrogen or argon atmosphere under magnetic stirring.Standard methods were used to make anhydrous solvents.Unless otherwise noted, commercially available reagents were used without further purification.Glass columns packed with silica gel (60-120 or 100-200 mesh) were used for column chromatography.)-(-)-pseudoephedrine propionamide (6.32 g, 38.30 mmol) in THF (20 mL) was then added to the reaction mixture very slowly using syringe pump.After being stirred at -78 o C for 30 min the reaction mixture was warmed to -20 °C and treated with a solution of the iodide 7 (2.0 g, 9.12 mmol) in THF (10 mL).The reaction mixture was stirred at -20 o C for 24 h and quenched with saturated aqueous solution of NH4Cl (5 mL).The reaction mixture was extracted with EtOAc (2 x 20 mL).The combined organic extracts were washed with water (5 mL), brine (5 mL), dried over Na2SO4, filtered and concentrated under vacuo.The residue was purified by column chromatography (SiO2, 25% EtOAc/hexane) to afford 10 (2.40 g, 95%) as a colorless liquid.Rf 0.

(2S,4R)-5-(tert-Butyldiphenylsilyloxy)-2,4-dimethylpentan-1-ol (6).
To a stirred solution of diisopropylamine (4.81 mL, 33.76 mmol) in THF (20 mL) at 0 o C was added n-BuLi (12.54 mL of 2.5 M solution in hexanes, 31.36 mmol) drop wise and the solution was stirred at 0 o C for an additional 15 min.Then Borane-ammonia complex (90%, 0.96 g, 31.36 mmol) was added to the solution.After stirring for 30 min at 0 o C, the solution was warmed to rt and stirred for additional 30 min.Then the reaction mixture was cooled to 0 o C and it was treated with a solution of amide 10 (2.1 g, 3.94 mmol) in 10 mL of THF.After stirring for 4 h at room temperature, the reaction mixture was quenched with saturated aqueous NH4Cl (5 mL) solution and extracted with EtOAc (2x50 mL).The combined organic extracts were washed with brine (20 mL) and dried over Na2SO4.
To a stirred solution of crude aldehyde in dry THF (10 mL), vinyl magnesium bromide (1M solution in THF 1.94 mL, 1.94 mmol) was added at 0 o C. Reaction mixture was warmed to room temperature and stirred for 1 h.It was quenched with saturated aqueous NH4Cl solution (5 mL), extracted with EtOAc (2 x 10 mL).The combined organic layers were washed with brine (10 mL), dried over Na2SO4 and concentrated in vacuo.To a stirred solution of primary alcohol (0.1 g, 0.22 mmol) in CH2Cl2 (3 mL), NaHCO3 (0.37 mg, 0.44 mmol) was added at 0 o C, followed by Dess-Martin periodinane (0.18 mg, 0.44 mmol) under nitrogen atmosphere.The reaction mixture was allowed to attain room temperature and stirred for 2 h.Saturated Na2S2O3 (10 mL) and NaHCO3 (2 mL) were added to quench the reaction mixture.After stirring 15 min the reaction mixture was extracted with EtOAc (2 x 20mL).The organic phase was washed with water (20 mL), brine (20 mL), dried over Na2SO4 and concentrated in vacuo.The aldehyde 4 thus obtained was directly used, after passing through a short pad of silica, for the next reaction without any further characterization.(2S, 4R, 5R)-5-Hydroxy-4-methyl-3-oxohexan-2-yl benzoate (8).To a stirred solution of c-Hex2BCl (14.54 mL, 14.54 mmol) in Et2O (20 mL) at -78 o C was added Me2NEt (1.68 mL, 19.4 mmol), followed by ketone 9 (2 g, 9.7 mmol) in Et2O (10 mL).The reaction mixture was warmed to -10 o C for first 30 min and then to 0 o C for next 1 h before being cooled to -78 o C. The commercially available acetaldehyde (2.70 mL, 48.48 mmol) was added and stirred for further 2 h at the same temperature.Then the reaction mixture temperature was raised to -20 o C and stirred for 14 h.The reaction was quenched at 0 o C by addition of MeOH (10 mL) and p H 7 buffer (10 mL), H2O2 (5 mL, 30%) was then added and the stirring continued for 1 h and extracted with EtOAc (3x50mL).
The combined organic extracts were washed with water (10 mL), brine (5 mL) and dried over Na2SO4 and concentrated in vacuo.The residue was purified by column chromatography (SiO
To a stirred solution of anhydrous CrCI2 (1.9 g, 31.98 mmol) in THF (20 mL) under argon atmosphere was added a solution of crude aldehyde and iodoform (4.19 g, 10.66 mmol) in THF (15 mL) at 0 o C and the reaction mixture was stirred at 0 °C for 1 h before being quenched with water (15 mL).The reaction mixture was extracted with EtOAc (2x20 mL).The combined organic extracts were washed with water (10 mL), brine (10 mL) and dried over Na2SO4 and concentrated in vacuo.The residue was purified by column chromatography (SiO2, hexanes) to afford 20 (1.45 g, 80% yield) as a colourless viscous liquid.Rf 0.9 (5% EtOAc in hexanes); [α]D

Table 1 .
Attempts to Intramolecular Heck cross Coupling for Macrocyclization.