A process for making a compound of formula: wherein: R a and R b together with the pyrimidine ring carbon atoms to which they are attached form a cyclopentyl ring; R 1 is phenyl, unsubstituted or substituted by 1 to 3 fluoro groups; R 2 is C alkyl substituted by NR 5 R 6; or R 2 is Het-C alkyl in which Het is a 5- to 7- membered heterocyclic ring containing N and in which N is substituted by C alkyl; R 3 is phenyl; R 4 is phenyl unsubstituted or substituted by C alkyl or mono to perfluoro- C alkyl; and R 5 and R 6 which may be the same or different are C alkyl; the process comprising: (a) treating a C alkyl 2-oxocyclopentanecarboxylate with an alkali metal salt of glycine to form a compound of formula (b) cyclising a compound of said formula to form the hexahydro-1 H -cyclopenta[ d]pyrimidin-1-yl)acetic acid of formula by treating a compound of said formula with either: (i) a thiocyanate salt and a) a haloalkylsilane and a proton source, with heating, or b) an anhydrous acid, with heating; or (ii) trimethylsilylisothiocyanate, with heating; (c) forming a thio-4-oxo-4,5,6,7-tetrahydro-1 H -cyclopenta[ d]pyrimidin-1-yl)acetic acid of formula where n is 0 to 3, by treating a compound of said formula with a thio-alkylating reagent which is a benzyl derivative of formula where n is 0 to 3 and X is a leaving group, in the presence of an alkali metal base and/or an alkali metal carbonate; and (d) forming a compound of formula by treating a compound of said formula with carbonyldiimidazole and the secondary amine of formula and heating the mixture. R a and R b together with the pyrimidine ring carbon atoms to which they are attached form a cyclopentyl ring; R 1 is phenyl, unsubstituted or substituted by 1 to 3 fluoro groups; R 2 is C alkyl substituted by NR 5 R 6; or R 2 is Het-C alkyl in which Het is a 5- to 7- membered heterocyclic ring containing N and in which N is substituted by C alkyl; R 3 is phenyl; R 4 is phenyl unsubstituted or substituted by C alkyl or mono to perfluoro- C alkyl; and R 5 and R 6 which may be the same or different are C alkyl; the process comprising: (a) treating a C alkyl 2-oxocyclopentanecarboxylate with an alkali metal salt of glycine to form a compound of formula (b) cyclising a compound of said formula to form the hexahydro-1 H -cyclopenta[ d]pyrimidin-1-yl)acetic acid of formula by treating a compound of said formula with either: (i) a thiocyanate salt and a) a haloalkylsilane and a proton source, with heating, or b) an anhydrous acid, with heating; or (ii) trimethylsilylisothiocyanate, with heating; (c) forming a thio-4-oxo-4,5,6,7-tetrahydro-1 H -cyclopenta[ d]pyrimidin-1-yl)acetic acid of formula where n is 0 to 3, by treating a compound of said formula with a thio-alkylating reagent which is a benzyl derivative of formula where n is 0 to 3 and X is a leaving group, in the presence of an alkali metal base and/or an alkali metal carbonate; and (d) forming a compound of formula by treating a compound of said formula with carbonyldiimidazole and the secondary amine of formula (a) treating a C alkyl 2-oxocyclopentanecarboxylate with an alkali metal salt of glycine to form a compound of formula (b) cyclising a compound of said formula to form the hexahydro-1 H -cyclopenta[ d]pyrimidin-1-yl)acetic acid of formula by treating a compound of said formula with either: (i) a thiocyanate salt and a) a haloalkylsilane and a proton source, with heating, or b) an anhydrous acid, with heating; or (ii) trimethylsilylisothiocyanate, with heating; (i) a thiocyanate salt and a) a haloalkylsilane and a proton source, with heating, or b) an anhydrous acid, with heating; or a) a haloalkylsilane and a proton source, with heating, or b) an anhydrous acid, with heating; or (ii) trimethylsilylisothiocyanate, with heating; (c) forming a thio-4-oxo-4,5,6,7-tetrahydro-1 H -cyclopenta[ d]pyrimidin-1-yl)acetic acid of formula where n is 0 to 3, by treating a compound of said formula with a thio-alkylating reagent which is a benzyl derivative of formula where n is 0 to 3 and X is a leaving group, in the presence of an alkali metal base and/or an alkali metal carbonate; and (d) forming a compound of formula by treating a compound of said formula with carbonyldiimidazole and the secondary amine of formula and heating the mixture.