Attempts towards the synthesis of mupirocin-H

The stereoselective synthesis of segments C1-C6 ( 3 ), C7-C12 ( 4 ) of mupirocin-H has been achieved. The synthetic procedure for the C1-C6 segment includes the zinc mediated allyl Grignard reaction with R - glyceraldehyde, Swern oxidation/Witting olefination reactions and followed by Sharpless asymmetric epoxidation. The C7-C12 segment was synthesized using again Sharpless asymmetric epoxidation on mono PMB protected 2-butene-1,4-diol, followed by regioselective opening of this epoxide with trimethyl aluminium. Both segments C1-C6 ( 3 ) and C7-C12 ( 4 ) possesses the five new stereogenic centers along with trans -olefin, but in various attempts condensation of 3 and 4 segments to give C-C bond forming parent segment ( 2 ) not affirmed, hence this work constitutes the synthesis of fragments C1-C6 ( 3 ) and C7-C12 ( 4 ) of mupirocin-H.


Introduction
Mupirocin is a polyketide, found to be possess a wider spectrum of antibacterial activity against both grampositive, -negative bacteria, including methicillin-resistant staphylococcus aureus (MRSA) and it is used clinically for the treatment of bacterial skin infections. 1 It is a mixture of pseudomonic acids produced by Pseudomonas fluorescens, a soil isolate reported to possess antibacterial activity as early as 1887, 2,3 while the mixture of pseudomonic acids was found to be the active component in the 1960s, 4 the major constituent was characterized later and named pseudomonic acid A (mupirocin). 5,6It is prescribed for treating skin infections such as cuts, burn wounds, candidiasis and impetigo.Besides mupirocin inhibits the bacterial isoleucyl tRNA synthetase enzyme responsible for loading the amino acid isoleucine onto its cognate tRNA required for ribosomal protein synthesis.Aminoacyl tRNA synthetases belong to a super family of nucleotidyl transferase enzymes related to other ATP-binding proteins such as dehydrogenases and photolyase. 7,8Consequently, mupirocin-H 1, is a novel metabolite belongs to the family of mupirocin and it is resulted mutation of the βhydroxy-β-methylglutaryl coenzyme A (HMG-CoA) synthase encoding mup H gene in Pseudomonas fluorescens and providing in vivo evidence for the roles of mup H and cognate genes found in several "AT-less" and other bacterial PKS gene clusters responsible for the biosynthesis of diverse metabolites containing acetate/propionate derived side chains, as well as possess anti bacterial activity akin mupirocin. 3,9,10[12][13][14][15]  The amenable biological importance and fascinating structure of the mupirocin-H attracted the attention of chemists for the total synthesis.To date, five total synthesis of mupirocin-H were reported, sequentially are in 2011, the Chakraborty group reported the first enantioselective total synthesis of mupirocin-H in 19 steps with 5% overall yield by utilising D-glucose as the chiral source and Julia-Kocienski reaction for construction of the E-olefinic bond. 12In 2012 the Willis group also reported a convergent total synthesis of mupirocin H in 11 steps with 6.9% overall yield using a functionalized lactone transformation strategy. 10In 2014, She et al. have reported the total synthesis in 7 steps with 39% overall yield by utilizing Suzuki-Miyaura coupling reaction as key step. 14Again in 2014, T. Sim and co-worker reported the consise synthesis of mupirocin-H in 17 steps with 10.1% overall yield by using Grubbs reaction as key step. 15n present work our goal is to target the synthesis of mupirocin-H.In a convergent synthesis, the target molecule mupirocin H 1 was devided into two fragments C1-C6 (3) and C-7-C12 (4).Both the fragments are synthesized from commericially available starting materials.The detailed retro synthetic approach for the synthesis of mupirocin-H is depicted in scheme 1.

Results and Discussion
The retro synthetic analysis revealed that the target compound 1 (Scheme 1) could be obtained from alcohol 2, which on further conversion of resulting primary alcohol to methyl/C5-hydroxy protection/selective deprotection MOM ether, followed by oxidation to carboxylic acid and subsequent lactonization with deprotected C4-PMB ether.Compound 2 in turn could be obtained from two building blocks 3 and 4, while, epoxide 3 C-1 to C-6 segment could be obtained from (R)-glyceraldehyde derivative 5 (Scheme 2 & 3).Similarly, 4 C-7 to C-12 segment could be derived from allyl alcohol 20, which was derived from commercially available 2-butyne-1,4-diol (Scheme 4 & 5).

Synthesis of C-1 to C-6 segment (3).
Synthesis of fragment 3 is achieved as shown in Scheme 2. Allylation of (R)-Glyceraldehyde with allyl bromide in THF gave the isomeric mixture of compounds 5 and 5a. 16The major isomer 5 isolated by colomn chromatography and further treated with benzyl bromide and NaH in THF to give the benzyl ether 7 in 94% yield.Then compound 7 was subjected to 70% aq.acetic acid at room temperature furnished diol 8 in 87% yield (Scheme 2).Diol 8 was selectively silylted using TBSCl and imidazole in dichloromethane at 0 o C to room temperature to give silyl ether 9 in 77% yield.Repeatedly, secondary alcohol in 9 was O-benzylated with p-methoxybenzyl bromide and NaH in THF to afford the PMB ether 10 in 91% yield.Dihydroxylation of terminal olefine of compound 10 in presence of OsO4 in acetone, water (9:1) using NMO as cooxidant gave diol 11 in 62% yield. 17 For the synthesis of one of the fragmet 3 from compound 14, first deprotected the TBS ether in compound 14 using TBAF in THF to give primary alcohol 15 in 85% yields.The obtained alcohol 15 was subjected for Swern oxidation in CH2Cl2 at -78 o C afforded aldehyde 16, 18,19 which on subsequent Wittig olefination with (ethoxycarbonylmethylene)triphenyl phosphorane in benzene gave mixture of 17a and 17b in 1:19 ratio (75%). 20,21,22Then using colomn chromatography technique purified major isomer 17b on selective reduction of ester with LAH and AlCl3 in ether at 0 o C furnished trans-allylic alcohol 18 in 78% yield (Scheme 3).The allylic alcohol 18 was subjected to enantioselective epoxidation under Sharpless asymmetric epoxidation reaction conditions using (+)-DIPT, Ti(i-OPr)4 and cumene hydroperoxide at -20 o C furnished the desired chiral epoxide 3 in 70% yield. 23cheme 3. Synthesis of C-1 to C-6 segment 3.

Synthesis of C-7 to C-12 segment (4).
To acieve the synthesis of fragment 4 with required stereochemistry, we have used the commercially available achiral 2-butyn-1,4-diol as starting material as shown in Scheme 4. Accordingly, 2-butyne-1,4-diol was treated with PMB-Br, NaH and TBAI in THF to give PMB-ether 19, which on further reaction with Red-Al in dry ether afforded the trans-alcohol 20 in 62% yield.Alcohol 20 was subjected to enantio selective epoxidation under Sharpless epoxidation reaction conditions using (+)-DIPT, Ti(i-OPr)4 and cumene hydroperoxide at -20 o C furnished the chiral epoxide 21 in 69% yield. 23The newly generated chiral epoxide alcohol 21 on oxidation under Swern reaction conditions 18,19 gave aldehyde 22, which on subsequent Wittig olefination with (ethoxycarbonylmethylene)triphenyl phosphorane in benzene afforded 6 and 6a in 9:1 ratio (72%). 20,21,22Epoxy ester 6 was treated with trimethylaluminium in CH2Cl2 at -40 o C to give regioselective compound 23 in 85% yield an exclusively. 24Then, α,β-unsaturated ester 23 on selective reduction of ester with LAH and AlCl3 in ether gave the diol 24 in 58% yield.The primary alcohol in 24 was selectively silylated using TBSCl and imidazole in CH2Cl2 to give silyl ether 25 in 82% yield.Confirmation of newly generated stereocenter in compound 23, first deprotection of PMB ether in compound 25, followed by protection of resulted diol 27 with 2,2-dimethoxy propane and PPTS (cat.) in CH2Cl2 afforded 28 in 73% yield.The stereochemistry of 26 was established by 1 H NMR (300 MHz, CDCl3) data of compound 28, which reveals that the adjacent vicinal proton coupling constants (J = 10.1 Hz) of methyl and hydroxyl substituents are in anti-substitution pattern in 26.
For taking the intermediate compound 25 to the target compound 4, first protection of secondary alcohol (25) with benzyl bromide and NaH in THF at room temperature afforded the benzyl ether 26 in 76% yield (Scheme 5).Finally, desylilation of 26 with TBAF in THF gave alcohol 29 in 88% yield, which on reaction with triphenylphosphine, and NaHCO3 in CCl4 at 80 o C afforded allyl halide 4 in 90% yield.Attempts towards the synthesis of segment 2. In above obtained both segments C-1 to C-6, 3 (Scheme 3) and C-7 to C-12, 4 (Scheme 5) were further attempted to condense through epoxide opening procedure to furnish the main fragment 2 as shown in scheme 6. Hence the treatment of allyl halide 4 with Mg in ether at room temperature followed by reaction with 3 at -78 o C, few other attempts -40 o C, -20 o C and even at room temperature in dry ether/tetrahydrofuran reaction conditions were met with failure to furnish 2 (Scheme 6). 25,26heme 6. Coupling of segments 3 and 4.

Conclusions
A route was developed for the synthesis of C1 to C6 (3), C7 to C12 (4) segments of mupirocin-H (1).In further attempts towards the synthesis of mupirocin-H 1 to form C-C bond forming main segment (2) by using both segments C1-C6 (3) and C7-C12 (4) was not affirmed.So this is a synthetic protocol for the synthesis of C1 to C6 segment (3), which has three chiral centers, and C7 to C12 segment (4), which has two chiral centers along with trans-olefin.Further work on the synthesis of mupirocin H (1) is in progress in our laboratories, with modified protocols.

Experimental Section
General.Analytical thin layer chromatography (TLC) was carried out using silica gel 60 F254 pre-coated plates.Visualization was accomplished with UV lamp or I2 stain.All products were characterized by their NMR and HRMS spectra.The 1 H NMR (300 MHz) spectra were recorded on Bruker Avance spectrometer and 13 C NMR (75 MHz) spectra were recorded on Bruker Avance spectrometers using TMS as an internal standard, chemical shifts were reported in parts per million (ppm, δ) downfield from tetramethylsilane.ESI, HRMS were recorded on 'High Resolution QSTAR XL hybrid MS/MS system, Applied biosystems' under Electron Spray Ionization conditions preparing sample solutions in MeOH.IR spectra were recorded on Perkin-Elmer Infrared-683 spectrometer.