Total synthesis of elansolids B1 and B2

The elansolids A1–A3, B1, and B2 are secondary metabolites formed by the gliding bacterium Chitinophaga sancti. They show antibacterial activity against Gram-positive bacteria. A second generation total synthesis of the antibiotic elansolid B1 (2) and the first synthesis of elansolid B2 (3) are reported. In contrast to previous work, the (Z,E,Z)-triene at C10–C15 was assembled by using an optimized C–C cross-coupling sequence with a Suzuki cross-coupling reaction as key step.

For the first generation total synthesis of elansolid B1 (2) we utilized an endo-selective intramolecular Diels-Alder (IMDA) cycloaddition as key step to construct the tetrahydroindane unit (Scheme 1) [7].An enone, derived from allylic alcohol 8 served as precursor to yield tetrahydroindane 9 with excellent diastereocontrol at −25 °C.The major drawback of our first total synthesis of elansolid B1 (2) was the installation of the side chain at C1-C13.The synthesis relied on two consecutive Sonogashira-Hagihara cross-coupling reactions that provided the ene-diyne system (C10-C15) 10 in good yield.However, partial hydrogenation (only the zinc-copper couple worked) furnished the desired (Z,E,Z)-triene 11 in only low yield (35%) and overreduction was difficult to control.Practically, the reduction was stopped when still substantial amounts of monoreduced product (the alkyne at C10-C11 is reduced pref-  erentially) were present.Consequently, the hydrogenation yielded a mixture of products, which in any case made the separation and isolation a very challenging task.
As continuation of our synthetic investigations on the elansolids, we report an improved second generation approach for generating the carbon chain C1-C13 [8] and for preparing elan- solid B1 (2).Furthermore, we also describe the first synthesis of elansolid B2 (3).The key for improvement was to abandon the two Sonogashira reactions along with the syn-reductions of the two alkynes.Instead, we planned to utilize the Suzuki-Miyaura and the Stille reactions and two Z-configured vinyl iodides to assemble the (Z,E,Z)-triene unit.

Results and Discussion
The improved synthesis utilizes the Suzuki-Miyaura cross-coupling reaction to merge the western fragment derived from ketone 9 with the newly designed eastern building block 13.This fragment was obtained in very good yield from vinyl iodide 12 [9] by a Stille protocol using doubly functionalized alkene 14 which is suited for a sequential cross-coupling strategy (Scheme 2).Under the catalytic conditions, we did not encounter isomerization of the alkene and diene configurations in vinyl boronate 13.
The preparation of the newly modified western fragment started from known IMDA product 9 [7], which was first reduced at C-25 (Scheme 3).The two diastereoisomers could be separated by chromatography and the stereochemical assignment of the major isomer was based on X-ray crystallographic analysis [7].Next, Tamao-Fleming oxidation [10] yielded phenol 15.The alkyne was transformed into vinyl iodide 17 after O-acylation, iodination of the terminal alkyne and finally diimide-mediated syn-reduction [11].
Next, DDQ-mediated removal of the PMB protecting group yielded vinyl iodide 18.The synthesis of both fragments 13 and 18 set the stage for the Suzuki-Miyaura coupling which delivered the desired (Z,E,Z)-configured triene 19.Again, we did not encounter formation of stereoisomers in the triene unit.The configuration of the triene was unequivocally assigned by analysis of coupling constants (J) and by measuring nuclear Overhauser effects (nOe).Finally, desilylation and global saponification of all ester groups in the presence of isopropanol success-fully yielded elansolid B1 (2).When isopropanol was exchanged by methanol, elansolid B2 (3) was generated.Its formation can be rationalized by formation of the intermediate p-methide quinone which selectively trapped methanol, exclusively yielding the R-isomer at C25.This excellent facial selectivity has been demonstrated, e.g., for anilines as nucleophiles before.It is due to the preferred conformation around the bond at C24-C25 which leads to the efficient shielding of the si-face by the two germinal methyl groups at C22 [4,5].The NMR data determined for both synthetic products were identical with those of authentic samples of elansolid B1 (2) and elansolid B2 (3) (copies of spectra, see Supporting Information File 1).

Conclusion
In conclusion, we describe an improved second generation synthesis of the highly active antibiotic elansolid B1 (2).The improvements are mainly associated with the preparation of the triene unit at C10-C15 by utilizing the Stille and the Suzuki-Miyaura cross-coupling reactions as well as the highly versatile difunctionalized building block 14.In principal, the synthesis sheds light on how such (Z,E,Z)-configured triene units are ideally be constructed, clearly demonstrating that enediynes are less preferred precursors for such structural elements.It has to be noted that there is precedence in the literature for the use of the Suzuki-Miyaura cross-coupling reaction as key step to assemble differently configured trienes present in polyketides [12][13][14][15].Furthermore, we show how the intermediate p-methide quinone can be exploited to also prepare elansolid B2 (3).The improved synthesis allows more easily preparing analogues of the elansolids for further biological evaluation.

Experimental General information:
1 H NMR spectra were recorded at 400 MHz or 500 MHz, respectively, and 13 C NMR spectra were recorded at 100 MHz or 125 MHz, respectively, with a Bruker Avance 400, DPX 400 or DRX 500.Chemical shift values of NMR data are reported as values in ppm relative to the (residual undeuterated) solvent signal as internal standard.Multiplicities for 1 H NMR signals are described using the following abbreviations: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet; where appropriate with the addition of b = broad.Mass spectra were obtained with a type LCT (ESI) (Micromass) equipped with a lockspray dual ion source in combination with a Waters Alliance 2695 LC system, or with a type QTOF premier (Micromass) spectrometer (ESI mode) in combination with a Waters Acquity UPLC system equipped with a Waters BEH C18 1.7 μm (SN 01473711315545) column (solvent A: water + 0.1% (v/v) formic acid, solvent B: MeOH + 0.1% (v/v) formic acid; flow rate = 0.4 mL/min; gradient (t [min]/solvent B [%]): (0:5) (2.5:95) (6.5:95) (6.6:5) (8:5)).Ion mass signals (m/z) are reported as values in atomic mass units.Optical rotations were measured on a Perkin-Elmer polarimeter type 341 or 241 in a quartz glass cuvette at l = 589 nm (Na D-line).The optical rotation is given in [° mL•g −1 •dm −1 ] with c = 1 corresponding to 10 mg mL −1 .Preparative HPLC was operated at a Merck Hitachi LaChrome HPLC (Pump L7150 or L7100, Interface D-7000, Diode Array Detector L-7450), respectively, at a Beckmann system Gold HPLC (Solvent Module 125, Detector 166).Solvents, columns, operating procedures and retention times are given with the corresponding experimental and analytical data.
All reactions were performed under an argon atmosphere unless otherwise stated.Glassware was dried by heating under vacuum followed by flushing with argon gas prior to use.Dry solvents were obtained after filtration through drying columns on a M. Braun solvent purification system or purchased form commercial providers.The synthesis of building blocks 9 [7] and 12 [9] was reported before.

Reduction of ketone 9 and formation of benzyl alcohol
A solution of ketone 9 (466.2mg, 0.79 mmol, 1.0 equiv) in THF (3 mL) was added to a suspension of LiAlH 4 (599.2mg, 15.79 mmol, 20 equiv) in THF (12 mL) at −70 °C.After stirring for 3 d at this temperature the reaction was terminated by slow addition of a saturated potassium sodium tartrate solution.

Synthesis of phenol 15
To a solution of the benzyl alcohol described above (124.0mg, 0.21 mmol, 1.0 equiv) in THF (0.75 mL) was added a solution of TBAF (c = 1.0 M in THF, 0.84 mL, 4.0 equiv) and the mixture was stirred for 15 min.Methanol (2.23 mL), KHCO

Synthesis of elansolid B1 (2)
Polyene 19 (2.65 mg, 2.79 µmol, 1.0 equiv) was dissolved in THF (0.5 mL) and cooled to 0 °C.A solution of hydrogen fluoride pyridine complex (0.5 mL) prepared by mixing hydrogen fluoride pyridine (2 mL; hydrogen fluoride ≈70 %) with pyridine (5.6 mL) in THF (9.8 mL) at 0 °C.The reaction mixture was stirred for 1 h at this temperature and the reaction was terminated by addition of a saturated bicarbonate solution.The aqueous solution was extracted with Et 2 O for three times.The combined organic phases were dried over Na 2 SO 4 , filtered and concentrated under reduced pressure to afford the corresponding diol suitably pure for directly being employed in the next step.An aqueous solution of LiOH (1 M, 0.3 mL, 107 equiv) was added to crude diol in iPrOH (0.3 mL) and THF (0.