Isobutyric anhydride

1D and 2D NMR spectra were acquired using a 500 or 600 MHz Bruker AVANCE spectrometer (Billerica, MA, USA) using a cryoprobe and CDCl₃ as solvent. Mass spectra were measured with a LC/MS Waters Alliance mass spectrometer (Malford, MA, USA) equipped with a 2695 HPLC module, 996 photodiode array detector, and Micromass Quattro triple quadrupole mass spectrometer with ESI under the positive mode. High-resolution electrospray ionization mass spectroscopy (HRESIMS) was recorded on an Agilent Technologies (Santa Clara, CA, USA) 6224 TOFLC/MS mass spectrometer. TLC was carried out on pre-coated silica gel plates (silica gel 60 F-254, Merck KGaA, Darmstadt, Germany). Spots were visualized by spraying with 20% sulfuric acid in ethanol followed by heating. Preparative HPLC was performed on a Waters 1528 binary pump and 2487 diode array detector system using a Phenomenex (Torrance, CA, USA) Kinetex 5 μm C₁₈ 250 × 21 mm and a Phenomenex Luna 5 μm C₁₈ 250 × 21 mm columns. Isobutyric anhydride, pivalic anhydride, isovaleric anhydride, cyclopropane chloride, formic acid, 4-dimethylaminopyridine (DMAP), and N,N-diisopropylethylamine (Hünig’s base) were obtained from Sigma-Aldrich (St. Louis, MO, USA). Anthraquinone-2-carbonyl chloride was purchased from TCI America (Portland, OR, USA).