Preparation of 3. A 303-mg sample of phosphonic acid la7 in 5.8 mL of TFA was heated to'75" C for 5 days, after which'H NMR indicated no starting material and-85% pure 3.5 The burgandy colored solution was rotary evaporated, dissolved in 5 mL of benzene, then rotary evaporated again. The dark semisolid residue was recrystsllized four times from minimum benzene to give 137 mg (42%) of colorless 3, mp 167-168 OC. Its lH NMR spectrum is ...
