A mixture of 80 g (0.182 mol) of (6) in 160 ml glacial acetic acid at 90° C. is added dropwise to a suspension of 11.7 g (0.378 mol) of red phosphorus in 150 ml of hydriodic acid (57%, 1.134 mol). The mixture is refluxed for 4 hours, 5 g of activated charcoal are added, the mixture is then filtered and the residue is washed with glacial acetic acid and water. The filtrate is added dropwise, with vigorous stirring, to a solution of 270 ml water, 270 ml of methanol and 270 ml of sodium hydroxide solution (50%). The mixture is stirred for a further 30 minutes at 15° C. and the solid precipitated is filtered off. This is washed twice with 500 ml of water, then suspended in 500 ml of water, filtered off and dried in vacuo at 45° C. Yield: 65.2 g (91.9%)
