Experimental Part

6.1 General methods

All non-aqueous reactions were carried out under argon or nitrogen atmosphere using Schlenk technique unless noted otherwise. Flasks were dried in an oven or manually with a heat gun under vacuum. In these cases only freshly dried and readily distilled solvents (abs.) were used. In cases when no shielding gas is required, solvents with the purification grade p.a. or freshly distilled technical solvents were employed.

Solvents/chemicals: Standard solvents for extraction and column chromatography (cyclohexane, n-hexane, dichloromethane, chloroform, ethyl acetate, diethyl ether, methyl tert-butyl ether, tetrahydrofuran, ethanol, methanol) were distilled from technical sources prior to their use.

If dry solvents/reagents were used they were dried according to literature procedures²⁶² and freshly distilled before use. Dichloromethane, dimethylformamide, dimethyl sulfoxide and triethylamine were dried over calcium hydride, diethyl ether and tetrahydrofuran over sodium/benzophenone and methanol over magnesium.

Acetonitrile was dried over potassium carbonate and phosphorus pentoxide.

Chemicals were obtained by various suppliers (ABCR, Acros Organics, AK Scientific, Alfa Aesar, Carbolution, Fisher Scientific, Fluka, Grüssing, Merck, Sigma-Aldrich, Roth, VWR) and used without further purification unless noted otherwise.

Analytical TLC: TLC was carried out using silica gel plates (either TLC silica gel 60 F254 foil plates from Merck or TLC silica gel 60 RP-18 F254s foil plates from Merck).

Besides UV-light (254 and 366 nm), for visualisation a variety of stains were used:

• iodine on fine silica gel;

In the case of 3-acyltetramic acid, two Rf -values might by reported, representing two tautomers of the same compound.

Column chromatography: Column chromatography was performed using silica gel with the particle size of 63 – 200 µm or flash silica gel with the particle size of 25 – 40 µm from Macherey-Nagel. The column length was 30 cm and the diameter of the column was adjusted to the amount of silica gel needed. For a standard separation a mass ratio of 20:1 (silica gel/substance) is applied, for a more challenging separation a mass ratio of 100:1 (silica gel/substance) is applied.²⁶²

HPLC: Analytical HPLC measurements were carried out with a Beckman System Gold Programmable Solvent Module 126 either using a Kinetex^TM C18-HPLC-column (250 × 4.6 mm, pore size 100 Å, particle size 5 µm) by Phenomenex or a Prontosil^TM RP 200-5-C18 column (250 × 4.0 mm, particle size 5 µm) by Bischoff. Detection was performed by the use of a Diode Array Detection Module 168 from Beckman Instruments.

Preparative HPLC was carried out with a Knauer WellChrom K-1800 either using a Kinetex^TM C18-HPLC-column (250 × 21.2 mm, pore size 100 Å, particle size 5 µm) by Phenomenex or a Prontosil^TM RP 200-5-C18 column (250 × 20.0 mm, particle size 5 µm) by Bischoff. Detection was performed by the use of a WellChrom UV-detector K-2600 from Knauer.

NMR spectroscopy: All ¹H-NMR, ¹³C-NMR and ³¹P-NMR spectra were recorded on a Bruker DRX 300 or DRX 500. Coupling constants are recorded in [Hz] and abbreviations are used: s (singlet), d (doublet), t (triplet), q (quartet), quin (quintet), m (multiplet), br (broad). The assignment of the signals to the corresponding atoms was conducted with the help of advanced ¹³C-NMR spectra (JMOD) and 2D-NMR spectra (HSQC, HMBC).

If signals are interchangeable, and therefore cannot be assigned with certainty, the atoms are marked with "†". If multiple stereoisomers are visible, resulting in multiple signals for one atom, the major signal is marked with "*". If one multiplet can

be assigned to multiple atoms, the atoms concerned are separated with ",". If an assignment cannot be made, all atoms concerned are separated with "/".

Gas chromatography: All GC spectra are recorded on a Carlo Erba HRGC 5160 carried out using 70 eV of ionisation energy (positive ion mode). The measurements were carried out by direct insertion probe-mass spectroscopy (DIP-MS).

GC-MS spectra were obtained on a Thermo Finnigan MAT 95 spectrometer coupled to a GC-unit (Hewlett Packard 5890 Series II).

HRMS spectra were obtained on a Thermo Scientific Q Exactive Orbitrap mass spectrometer connected to a Dionex UltiMate 3000 RS UHPLC unit. The measurements were carried out applying electrospray ionisation (ESI).

The peak values represent the mass-to-charge ratio (m/z) and their relative intensity [%] is reported in brackets in reference to the base peak which represents 100%. Furthermore, when dealing with halogens, the respective m/z values are marked to which isotope they belong (e.g. ⁷⁹Br vs. ⁸¹Br).

IR-spectroscopy: The IR spectra were recorded on a PerkinElmer Spectrum One FT-IR-spectrometer. The absorption wavelength (λ) of the bands is reported as their spectroscopic wavenumbers ( ) in [cm⁻¹]. The intensity of the bands is labelled with s (strong), m (medium) or w (weak).

Melting point measurements: The melting points of solids were measured with a BÜCHI M-565 melting point apparatus and remain uncorrected.

Polarimetry: The specific rotation ([α]D) of substances was measured with a PerkinElmer polarimeter 343 at 24 °C and/or with a Jasco P-1020 polarimeter (λ = 589 nm, c = 1.0 ≡ 1 mg/mL) (OC1/2).

X-Ray: Crystal structures were obtained on a STOE-IPDS II diffractometer and were measured and calculated by M.Sc. Johannes Obenauf from the group of Prof. R.

Kempe (inorganic chemistry II).

6.2 Macrocidin A and analogues

6.2.1 Synthesis of an allyl-protected

-tyrosine derived tetramic acid

6.2.1.1 General method to prepare tetramic acids from protected amino acids

According to procedures of Jouin et al.⁴⁸ and Hosseini et al.,⁵³ to a 0.15M solution of a protected amino acid (1.0 eq.) in CH2Cl2 abs. at 0 °C are added Meldrum's acid (35) (1.1 eq.), EDC · HCl (1.2 eq.) and DMAP (1.4 eq.) and the mixture is stirred for 14 h at room temperature. The mixture is diluted with EtOAc (20 mL/mmol) and the organic phase is washed with brine (2 × 10 mL/mmol), 5% citric acid (3 × 10 mL/mmol) and brine (10 mL/mmol). It is dried over Na2SO4, filtered and heated to reflux for 2 h before the solvent is evaporated. After drying in vacuo the crude tetramic acid is obtained.

6.2.1.2 Preparation of N-(tert-butoxycarbonyl)-(5S)-5-((4-allyloxy) benzyl)-pyrrolidine-2,4-dione (116)

Compound 116 is prepared applying the general method of 6.2.1.1. The use of bisprotected L-tyrosine derived amino acid 224 (prepared from L-tyrosine over two steps according to literature procedures of Barnickel¹⁵², Bose et al.⁹⁹ and Kane et al.¹⁰⁰) (10.2 g, 31.9 mmol) yields the product as a yellow foam (9.8 g, 28.5 mmol, 89%) without further purification.

Rf 0.38 (33% EtOAc in ^cHex + two drops of acetic acid)

1H-NMR (300 MHz, CDCl3): tautomeric ratio: (keto : enol) 90% : 10% (calculated from NMR) (only keto form recorded, because enol form to weak and indistinct), assignment differs from literature.¹⁵²

1.62 (s, 9H, H-16), 2.27 (dd, J = 22.3, 1.7 Hz, 1H, H-3a), 2.85 (d, J = 22.3 Hz, 1H, H-3b), 3.15 (dd, J = 14.1, 3.1 Hz, 1H, H-6a), 3.33 (dd, J = 14.1, 5.1 Hz, 1H, H-6b), 4.48 (ddd, J = 5.4, 1.5, 1.4 Hz, 2H, H-11), 4.60 (ddd, J = 5.1, 3.1, 1.7 Hz, 1H, H-5), 5.28 (ddt, J = 10.6, 1.6, 1.4 Hz, 1H, H-13Z), 5.39 (ddt, J = 17.3, 1.6, 1.5 Hz, 1H, H-13E), 6.02 (ddt, J = 17.3, 10.6, 5.4 Hz, 1H, H-12), 6.82 (d, J = 8.7 Hz, 2H, H-9), 6.92 (d, J = 8.7 Hz, 2H, H-8).

13C-NMR (75 MHz, CDCl3): assignment differs from literature.¹⁵²

28.2 (CH3, C-16), 35.8 (CH2, C-6), 43.5 (CH2, C-3), 68.6 (CH, C-5), 69.0 (CH2, C-11), 84.4 (Cquart, C-15), 115.3 (CH, C-9), 117.9 (CH2, C-13), 125.9 (Cquart, C-7), 131.0 (CH, C-8), 133.2 (CH, C-12), 149.2 (Cquart, C-14), 158.2 (Cquart, C-10), 167.6 (Cquart, C-2), 204.6 (Cquart, C-4).

DIP-MS The MS spectrum of 116 is in good agreement with literature.¹⁵²

mp 68.4 °C

[α]D24

+77.4 ° (c = 1.0, CHCl3), [lit.: +75.3 ° (c = 1.0, CHCl3)]¹⁵²

IR (cm^-1) The IR spectrum of 116 is in good agreement with literature.¹⁵²

6.2.2 Synthesis of unbranched and α -methylated bromo acids

6.2.2.1 General method to prepare monoethyl esters from