Patent ID: 7622577

Claim:
A process for preparing a compound represented by the following formula 1 or its salt, which comprises: reacting a compound represented by the following formula 4 with acetaldehyde in a mixed solvent comprising 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 represented by the following formula 3′; and reacting the compound of the formula 3′ with an anhydrous compound represented by the following formula 2 in the presence of a second base: wherein R 1 is a hydrogen or an amino protecting group, R 2 is a hydrogen or a carboxyl protecting group, and R 3 is a hydrogen or an amino protecting group consisting of phenylacetyl group, wherein when R 3 is a phenylacetyl group, R 2 is not a hydrogen in formula 3′; and wherein, when at least one of R 2 and R 3 is a protecting group, all such protecting groups are removed from formula 3′, thereby producing a compound represented by formula 6 prior to reacting it with the compound of formula 2.