Provided is a process for preparing a compound of formula 1 or its salt. The process includes reacting a compound of formula 4 with acetaldehyde in a mixed solvent including water, isopropanol, and methylenechloride in a volume ratio of 1:3-6:11-14 in the presence of a first base to stereospecifically prepare a compound of formula 3 and reacting the compound of the formula 3 with an anhydrous compound of formula 2 in the presence of a second base.