the american chemical society in the examples that follow 34 mg 34 means milligram s 34 m 34 means molar 34 ml 34 means milliliter s 34 mmol 34 means millimole s 34 l 34 means liter s 34 mol 34 means mole s 34 g 34 means gram s 34 tlc 34 means thin layer chromatography 34 hplc 34 means high pressure liquid chromatography 34 mm 34 means millimole s 34 mp 34 means melting point 34 mhz 34 means megaherz 34 1 h nmr 34 means proton nuclear magnetic resonance 34 hz 34 means hertz 34 h 34 means hour s and 34 n 34 means normal unless otherwise noted all starting materials were obtained from commercial suppliers and used without further purification all reactions involving oxygen or moisture sensitive compounds were performed under a dry n 2 atmosphere all reactions and chromatography fractions were analyzed by thin layer chromatography on silica gel plates visualized with uv light and i 2 stain a compound of formula vi wherein r 5 is hydrogen and r 6 is methyl a 250 ml one neck round bottom flask is charged with 4 ethyl 8 methoxy 1h pyrano 3 4 c pyridine 10 g 52 4 mmol prepared as described in u s patent application ser no 08 237 081 fang et al journal of organic chemistry 59 21 6142 6143 1994 pct us95 05425 and pct.