tetrabutyl ammonium fluoride in N,N,N',N',N'',N''-hexamethylphosphoric triamide mixt. is heated at 70°C for 24 h; mixt. is poured into water and extd. with hexane, combined organic extracts are passed through a bed of SiO2, eluting with hexane and concd., sublimation (70°C, 5E-2 mbar);