MATERIALS AND METHODS Spectroscopy. Proton nuclear magnetic resonance (NMR) spectra were recorded at 250 MHz with a Bruker AM-260 spectrometer for samples dissolved in deuteriochloroform. Mass spectra (MS) after chemical ionization (methane at 0.5 Torr) were recorded on a Finnigan 4600 mass spectrometer connected to the output of a gas-phase chromatography column (DB17, 15-m capillary).