Cycloaddition of Δ 2 -Thiazolines and Acyl Ketenes under Acidic Conditions results in Bicyclic 1,3- Oxazines not 6-Acylpenams as earlier reported. Nils Pemberton † , Hans Emtenäs † , Dan Boström ‡ , Peter J. Domaille, William A. Greenberg, Michael D. Levin, Zuolin Zhu § , and Fredrik Almqvist † * Organic Chemistry and Inorganic Chemistry, Umeå University, S-901 87 Umeå, Sweden and Diversa Corporation, 4955 Directors Place, San Diego, CA 92121 USA fredrik.almqvist@chem.umu.se Supporting Information Table of contents S1-6: experimental procedures S7-10: C 13 -C 13 Inadequate data General. All reactions were carried out under an inert atmosphere with dry solvents under anhydrous conditions, unless otherwise stated. CH 2 Cl 2 were distilled from calcium hydride and THF from potassium, respectively. Benzene and toluene were distilled from sodium. TLC was performed on Silica Gel 60 F 254 (Merck) with detection by UV light and staining with a solution of anisaldehyde (26 ml), glacial acetic acid (11 ml) and concentrated sulphuric acid (35 ml) in 95 % ethanol (960 ml). Flash column chromatography (eluents given in brackets) was performed on silica gel (Matrex, 60 Å, 35-70 µm, Grace Amicon). The 1 H and 13 C NMR spectra were recorded on a Bruker DRX-400 for solutions in CDCl 3 [residual CHCl 3 (δ H 7.26 ppm) or CDCl 3 (δ C 77.0 ppm) as internal standard] or CD 3 OD [residual CD 3 OD (δ H 4.87 ppm) at 298 K. First-order chemical shifts and coupling constants were obtained from one-dimensional spectra and proton resonances were assigned from appropriate combinations of COSY. IR spectra were recorded on an ATI Mattson Genesis Series FTIR TM spectrometer. Optical rotations were measured with a Perkin-Elmer 343 polarimeter. Positive fast atom bombardment mass spectra (FABMS) were recorded on a JEOL SX102 A mass spectrometer. Ions for