839 mg (1.62 mmol) of cis-1,3-dibenzyl-4-(4-(2,4,10-trioxaadamantyl)-butylidene)-hexahydro-1H-thieno[3,4-d]imidazol-2-one (prepared in accordance with Example 3) are treated with 4.6 ml of dioxan, 4.6 ml of 0.2N sulphuric acid as well as 20 mg of p-toluenesulphonic acid and the mixture is heated to boiling under an argon atmosphere for 2.5 hours. The mixture is subsequently made alkaline with 1.54 ml of 2N sodium hydroxide solution and boiled under reflux for a further 45 minutes. After cooling to room temperature the solution is extracted with ethyl acetate and the extract is discarded. The aqueous phase is acidified with dilute hydrochloric acid and again extracted with a total of 200 ml of ethyl acetate. The combined extracts are washed once with saturated sodium chloride solution and dried over sodium sulphate. After removing the solvent there are obtained 622 mg (89% of theory) of cis-2-oxo-1,3-dibenzyl-hexahydro-1H-thieno[3,4-d]imidazol-4-ylidenepentanoic acid.
