solution of 1 52 g 4 5 mmol of cis 1 3 dibenzyl hexahydro 1h thieno 3 4 d imidazole 2 4 dione in 25 ml of tetrahydrofuran the mixture is stirred at room temperature for a further 15 hours then diluted with 300 ml of ethyl acetate and treated with 100 ml of 1n hydrochloric acid the organic phase is separated washed with 10 wt bicarbonate solution with water and with saturated sodium chloride solution and dried over sodium sulphate the residue remaining after removing the solvent is chromatographed on silica gel elution with toluene toluene ethyl acetate 2 1 and toluene ethyl acetate 1 2 gives firstly 0 5 g of a white waxy mass there are subsequently eluted 1 75 g 73 of theory of cis 1 3 dibenzyl 4 hydroxy 4 4 2 4 10 trioxaadamantyl butyl hexahydro 1h thieno 3 4 d imidazol 2 one in the form of a white powder melting point 186 u00b0 192 u00b0 c 1 61 g 3 mmol of cis 1 3 dibenzyl 4 hydroxy 4 4 2 4 10 trioxaadamantyl butyl hexahydro 1h thieno 3 4 d imidazol 2 one prepared in accordance with example 1 are dissolved in 50 ml of toluene the solution is treated with 5 mg of p toluenesulphonic acid and heated to boiling 20 ml of toluene are distilled off within 30 minutes after this time starting material can no longer be detected in a thin layer chromatogram the solution is now evaporated to dryness the residue is taken up with 8 5 ml of dioxan the solution is treated with 8 5 ml of 0 02n sulphuric acid and boiled under reflux for 2 hours thereafter the mixture is made alkaline with 2 85 ml of 2n sodium.