The current invention discloses preparation of acid addition salts of 4-[2-[5-(2-Fluoro-3-methoxyphenyl)-3-[2-fluoro-6-(trifluoromethyl)benzyl]-4-methyl-2,6-dioxo-1,2,3,6-tetrahydro-1-pyrimidinyl]-1(R)-phenylethylamino]butyric acid by treatment of an acid-sensitive ester of , 4-[2-[5-(2-Fluoro-3-methoxyphenyl)-3-[2-fluoro-6-(trifluoromethyl)benzyl]-4-methyl-2,6-dioxo-1,2,3,6-tetrahydro-1-pyrimidinyl]-1 (R)-phenylethylamino]butyric acid with a sufficiently strong acid and subsequent isolation by filtration. The process of the invention prevents formation of (R)-5-(2-fluoro-3-methoxyphenyl)-1-(2-fluoro-6-(trifluoromethyl)benzyl)-6-methyl-3-(2-(2-oxopyrrolidin-1-yl)-2-phenylethyl)pyrimidine-2,4(1H,3H)-dione (Va).
