in dichloromethane under nitrogen, 41°C, 48 h, some decompn. obsd.; TLC monitoring, solvent removed (vac.); residue washed (warm petroleum ether; purifn. chromy. (silica, CH2Cl2/petroleum ether 2/1, v/v), crystn. from eluant solvent; mixt. of two isomers (by (19)F-NMR);